Advances in Agronomy, Volume 36

ADVANCES IN AGRONOMY VOLUME 36 CONTRIBUTORS TO THIS VOLUME C . AZCON-AGUILAR J. M. BAREA P. BEMIS WILLIAM E. B. A...

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ADVANCES IN

AGRONOMY

VOLUME 36

CONTRIBUTORS TO THIS VOLUME C . AZCON-AGUILAR

J. M. BAREA P. BEMIS WILLIAM E. B. A. BISDOM JEAN-MARC BOLLAG C. M. DONALD

IANB. EDWARDS

KEITHW. T. GOULDING J. HAMBLIN

KRITONK. HATZIOS LEMOYNEHOGAN J. LOLL MICHAEL J. NEILRUTGER

K. L. SAHRAWAT S. S. VIRMANI

ADVANCES IN

AGRONOMY Prepared in cooperation with the AMERICAN SOCIETY OF AGRONOMY

VOLUME 36 Edited by N. C. BRADY Science and Technology Agency for International Development Department of Srate Washington, D . C .

ADVISORY BOARD H . J. GORZ.CHAIRMAN

E. J . KAMPRATH T. M. STARLING

J. B. POWELL J . W. BIGGAR M. A . TABATABAI M . STELLY. EX

OFFICIO,

ASA Headquarters I983

ACADEMIC PRESS, INC. (Harcourt Brace Jovanovich, Publishers)

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COPYRIGHT @ 1983, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER.

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This volume is dedicated to Dr. Arthur Geoffrey Norman, editor of the first 20 volumes of Advances in Agronomy.

This Page Intentionally Left Blank

CONTENTS CONTRIBUTORS .................................................

xi

PREFACE ....................................................... IN MEMORIAM.................................................

Xlll

...

xv

MYCORRHIZAS AND THEIR SIGNIFICANCE IN NODULATING NITROGEN-FIXING PLANTS

J . M . Barea and C. Azc6n-Aguilar I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Mycorrhizas .................................................

1 4 23

111. Mycorrhizas in Legumes ...................................... IV . Mycorrhizas in Nodulating Nitrogen-Fixing Nonlegume Plants . . . . . . . V. Conclusions and Perspectives .................................. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

44 45

46

SUBMICROSCOPIC EXAMINATION OF SOILS

E . B . A . Bisdom I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Submicroscopic Techniques .................................... 111. Applications of Electron Microscopy ............................ IV . Applications of Ion Microscopy ................................ V . Applications of Other Forms of Submicroscopy .................... VI . Conclusions ................................................. References .................................................

55 57

65 88 89 90 91

THE CONVERGENT EVOLUTION OF ANNUAL SEED CROPS IN AGRICULTURE

C. M . Donald and J . Hamblin I. Introduction .................................................

II. Selection in Domesticated Crops ............................... I11. IV . V. VI .

Ekotypic Parallelism in Crop Plants ............................ Selection, Evolution, and Crop Yield ........................... Progress and Prospects in the Development of Annual Seed Crops . . . A Basic Ideotype for All Annual Seed Crops ..................... References ................................................

vii

97 100 111 112 121 134 139

...

Vlll

CONTENTS CURRENT STATUS AND FUTURE PROSPECTS FOR BREEDING HYBRID RICE AND WHEAT

S. S . Virmani and Ian B . Edwards I . Introduction ................................................ Heterosis in Rice and Wheat .................................. Advantages of Hybrids over Conventionally Bred Varieties . . . . . . . . . Cytoplasmic-Genetic Male Sterility Systems in Rice and Wheat . . . . . Fertility Restoration ......................................... Use of Chemical Pollen Suppressants in Hybrid Production ......... Factors Affecting Cross-Fertilization ............................ Seed Production ............................................ Ix. Quality of Hybrids .......................................... X . Economic Considerations ..................................... XI . Problems .................................................. XI1. Conclusion ................................................ References ................................................

11. I11. IV . V. VI. VII. VIII.

146 147 155 157 169 180 183 191 196 198 200 202 206

THERMODYNAMICS AND POTASSIUM EXCHANGE IN SOILS AND CLAY MATERIALS

Keith W . T. Goulding

I. Introduction ................................................ 11. The Thermodynamics of Ion-Exchange Equilibria . . . . . . . . . . . . . . . . . 111. Calorimetry in Ion-Exchange Studies ........................... IV . Thermodynamics Applied to Potassium Exchange in Soils and Clay Minerals ............................................ V. Exchange Equilibrium and the Kinetics of Potassium Exchange . . . . . . VI . Summary and Conclusions .................................... VII. Appendix: List of Symbols ................................... References ................................................

215 217 228 233 256 258 259 260

HERBICIDE ANTIDOTES: DEVELOPMENT. CHEMISTRY. AND MODE OF ACTION

Kriton K . Hatzios I . Introduction ................................................ 11. Development of Herbicide Antidotes ........................... III. Chemistry of Herbicide Antidotes .............................. IV . Field Performance of Herbicide Antidotes ....................... V . Mode of Action of Herbicide Antidotes .........................

265 270 292 296 301

ix

CONTENTS

VI . Degradation of Herbicide Antidotes in Plants . . . . . . . . . . . . . . . . . . . . . VII . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

309 310 310

BUFFALO GOURD AND JOJOBA: POTENTIAL NEW CROPS FOR ARID LANDS

LeMoyne Hogan and William P. Bemis I. 11. 111. IV .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Buffalo Gourd: Cucurbitu foeridissirnu HBK ..................... Jojoba: Simmondsiu chinensis (Link) Schneider . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

317 319 332 346 347

PROTEIN TRANSFORMATION IN SOIL

Michael J . Loll and Jean-Marc Bollag I. I1. 111. IV . V. VI . VII .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Protein Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proteolytic Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characteristics of Proteolytic Enzymes in Soils . . . . . . . . . . . . . . . . . . . Environmental Factors Affecting Proteolysis ..................... Transformation and Binding of Protein in Soil .................... Ecological and Agronomic Importance of Protein Transformation . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

352 352 354 361 364 370 376 377

APPLICATIONS OF INDUCED AND SPONTANEOUS MUTATION IN RICE BREEDING AND GENETICS

J . Neil Rutger I . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I1. Breeding Applications of Semidwarf Mutants . . . . . . . . . . . . . . . . . . . . 111. Breeding Applications of Early Maturity Mutants . . . . . . . . . . . . . . . . . IV . Breeding Applications of Other Types of Mutants . . . . . . . . . . . . . . . . . V . Genetic Applications of Mutants . . . . . . . . . . . .................... VI . Future Uses of Mutation in Rice Improvement . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

383 385 396 399 404

408 410

X

CONTENTS

NITROGEN AVAILABILITY INDEXES FOR SUBMERGED RICE SOILS

K . L . Sahrawat I. I1. 111. IV .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Mineralization of Organic Nitrogen . . . . . . . . . . . . . . Biological Indexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chemical Indexes ........................................... V . Simple Models of Nitrogen-Supplying Capacity Based on Biological and Chemical Indexes ......................................... VI . A Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII . Electro-Ultrafiltration ........................................ VIII . Plant Analyses ............................................. IX . Nitrogen-Supplying Capacity and Fertilizer Recommendations . . . . . . . X . Perspectives ............................................... References ................................................

415 417 421 428

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

453

435 439 441 442 443 445 447

CONTRIBUTORS Numbers in parentheses indicate the pages on which the authors’ contributions begin

C. AZCON-AGUILAR (l), Unidad de Microbiologia, Estacibn Experimental del Zaidin, Granuda, Spain J. M. BAREA (l), Unidad de Microbiologia, Estacibn Experimental del Zaidin, Granada, Spain WILLIAM P. BEMIS (3 17), Plant Sciences Department, University of Arizona, Tucson, Arizona 85721 E. B. A. BISDOM ( 5 3 , Netherlands Soil Survey Institute, 6700 AB Wageningen, The Netherlands JEAN-MARC BOLLAG (35 1 ) , Department of Agronomy, Pennsylvania State University, University Park, Pennsylvania I6802 C. M. DONALD (97), Waite Agricultural Research Institute, The University of Adelaide, Glen Osmond, South Australia 5064 IAN B. EDWARDS (145), Pioneer Hi-Bred Institute, Inc., Glyndon, Minnesota 56547 KEITH W. T. GOULDING (215), Soils and Plant Nutrition Department, Rothamsted Experimental Station, Harpenden, Herifordshire AL5 2JQ, United Kingdom J . HAMBLIN (97), Department of Agriculture, Geraldton District Office, Marine Terrace, Geraldton, West Australia KRITON K . HATZIOS (265), Department of Plant Pathology and Physiology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 LEMOYNE HOGAN (3 17), Plant Sciences Department, University of Arizona, Tucson, Arizona 85721 MICHAEL J. LOLL (35 l), Department of Agronomy, Pennsylvania State University, University Park, Pennsylvania 16802 J . NEIL RUTGER (383), U.S. Department of Agriculture, Agricultural Research Service, and Department of Agronomy and Range Science, University of California, Davis, California 95616 K. L. SAHRAWAT* (4 1 3 , Soil Science Department, ICRISAT, Patancheru P. O., Andhra Pradesh 502324, India S. S . VIRMANI ( 1 4 3 , International Rice Research Institute, Manila, Philippines

*Present address: Soil Science Department, University of Wisconsin, Madison, Wisconsin 53706

xi


PREFACE Two events occurred in the past year which have special significance for this review publication. First, the American Society of Agronomy (ASA) celebrated its 75th anniversary. The first 30 volumes of Advances in Agronomy were prepared under the auspices of this scientific society, and the remaining volumes have been developed in cooperation with it. This long association has been most fruitful for the series and has likewise been beneficial to the society. Most of the articles, particularly in the early years, have been authored by ASA members. An advisory committee chosen by the society has provided advice and guidance from the first to the present volume. The American Society of Agronomy has made great progress during the last three-quarters of a century and is to be congratulated on its 75 years of service. It has grown from the handful of dedicated soil and crop scientists, who met in Chicago in 1907 to form the society, to a membership of more than 12,000 today. It publishes four major research and education journals whose articles make up a fair share of those reviewed in Advances in Agronomy. More than 20 major monographs and 45 special publications have been published by the ASA. The second event of the past year which has special significance to Advances in Agronomy is a sad one. Dr. Geoffrey Norman, who was the founding editor and who continued as editor for the first 20 volumes, passed away on November 14, 1982. Crop and soil scientists throughout the world owe a debt of gratitude to Dr. Norman: He was not only a world-renowned soil microbiologist in his own right, but also an intellectual leader who stimulated biological science in general. We are pleased to publish a brief but meaningful tribute to him in this volume. The articles in Volume 36 reflect the advice given to me by Dr. Norman when 1 became editor. They each focus on a timely topic of wide interest to agronomists. They are written by scientists and educators from seven countries, illustrating the growing internationality of crop and soil science. And they repregent balance in subject matter among crops and soils and among basic and applied research. Advances in Agronomy continues to play a important role in keeping crop and soil scientists abreast of major research findings around the world. We are all challenged to maintain the energy and quality which so characterized Dr. Norman. The 15 scientists who prepared reviews for this volume have set excellent examples for us to follow. N. C. BRADY

...

Xlll


IN MEMORIAM ARTHURGEOFFREYNORMAN 1905-1982

A. G. Norman, who was the first editor of Advances in Agronomy, and who continued in that capacity for a period of 20 years, died November 14, 1982, after a distinguished career spanning more than half a century. In the Preface to Volume 1, he wrote, “This volume, Advances in Agronomy, has as its objective the survey and review of progress in agronomic research and practice. The editors . . . will be guided in their choice more by what information may be of use to agronomists than by what constitutes agronomy. The central theme must be soil-crop relationships, for soils without crops are barren, and field crops cannot be considered without reference to the soil on which they are produced.” The broad range of subjects covered by Advances over the years and the status which it has attained attest to the wisdom of this policy which he initiated. At the close of his 20 years of editorial service for this publication, he wrote in the preface to Volume 20, “Those who had a part in what seemed to be an uncertain venture in 1948 can take some pride in its acceptance . . .” “In the next 20 years one may confidently expect the accretion of new knowledge about the characteristics of soils and crop plants and of their interactions to proceed at an accelerating rate. These developments will find their way into later volumes and serve the agronomists of the world in their great task of providing sufficient food for all men.” Dr. Norman’s capacity for well-organized expression, both in speaking and writing, served the American Society of Agronomy well in other editorial efforts. Shortly after World War I1 he initiated the Monographs Series of the society and served as editor of the first six volumes. Other service to the society included a term as President in 1957, a year in which its 50th Anniversary was commemorated by an outstanding program in meetings at Atlanta, Georgia. A. G. Norman was born in Birmingham, England in 1905. He received the B.Sc. degree from the University of Birmingham in 1925 and the Ph.D. in Biochemistry from the same institution in 1928. This was at a time when biochemistry was just emerging as a separate discipline. Dr. Norman’s training there kindled a lifelong interest in plant biochemistry. From Birmingham, Norman went to the Rothamsted Experimental Station, where he began biochemical and microbiological studies on the decomposition of plant materials with special emphasis on cell wall substances. He did some of the first quantitative work on nitrogen transformations in the decomposition of plant materials, which has xv

xvi

IN MEMORIAM

subsequently had a major impact on crop residue management in practical agriculture. He came to the United States in 1930 as a Rockefeller Fellow at the University of Wisconsin, where he studied the microbiology of hemicelluloses and the structure of some fungal polysaccharides. Returning to Rothamsted in 1932, he became head of the Biochemistry Section there in 1933. In 1937 Dr. Norman moved to Iowa State College at Ames as Professor of Soils, where he directed a broad-ranging research program dealing with microbial thermogenesis, biochemistry of the major plant constituents and their decomposition processes in soil, fundamental studies on the chemistry of soil organic matter, and on carbon-nitrogen transformationsduring decomposition of organic materials. His application of biochemical techniques to studies in soil microbiology represented a significant departure from the older traditional techniques, which had their roots in medical bacteriology. Dr. Norman pioneered the application of stable isotope techniques to soil research and was possibly the first to use both "N- and I3C-labeled plant materials in decomposition studies. As early as 1943 he published an article indicating the potential of stable tracer technology in agronomic research. One of his significant early contributions was the use of "N in greenhouse experiments to measure N2 fixation by legumes. Field application of this type of methodology is now being made on a worldwide scale. During World War I1 Dr. Norman left the Iowa State campus to serve with the Chemical Corps for 2 years, directing a research program at Camp Detrick, Maryland. After a brief return to Iowa State, he left late in 1946 to accept a civilian position with the Chemical Corps dealing with basic studies on plant growth regulators and inhibitors. In 1952 he went to the University of Michigan as Professor of Botany and director of a research project in plant nutrition and root physiology as part of a university program promoting the use of radiation and radionuclides in the biological sciences. During this period he became much interested in rhizosphere microorganisms and in metabolic products which, when excreted, modified root growth and root physiology. Working with a number of graduate and postdoctoral students, he developed a research program concerned also with geotropic responses, factors limiting microbial activities in soils, influence of organisms on nutrient uptake, and artificial microbial environments. Other responsibilities at the University of Michigan included Directorship of the Botanical Gardens, a facility providing support for instruction and research in several university departments. In 1964 he was appointed Vice President for Research at the University of Michigan, in which capacity he continued to serve until his retirement. During 1963-1964 he was a Staff Advisor to the National Academy of Sciences, being involved, among other things, with initial organization of U.S. participation in the International Biological Program. From 1965 to 1969 he was Chairman of the Division of Biology in Agriculture of the National Research Council.

IN MEMORIAM

xvii

One of Dr. Norman’s significant contributions to the field of agronomy was through the students who came under his direction. His clear, well-organized lectures and his precise and articulate expression had a lasting influence upon his students. His insistence on clear and concise writing made a valuable contribution to the training of those who took his classes. A. G. Norman was that rare combination of brilliant research scientist, stimulating and effective teacher, and superbly organized and efficient administrator. Advances in Agronomy salutes the memory of a man who not only served this publication well as its editor for 20 years, but who in addition brought great credit to himself and to his profession.

FRANCES BROADBENT


ADVANCES IN AGRONOMY. VOL. 36

MYCORRHIZAS AND THEIR SIGNIFICANCE IN NODULATING NITROGEN-FIXING PLANTS J. M. Barea and C. Azcon-Aguilar Unidad de Microbiologia, Estaci6n Experimental del Zaidin Granada, Spain

I. Introduction ............................................ A. Root Microorganisms in the Ecosystem ...................... B. Mycorrhizas and Root Nodules. . . . . . . . . . . . . . . .

............................................ 11. Mycorrhizas . A. General ................................... B. Physiology of Mycorrhizas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Factors Affecting Mycorrhiza Establishment, Development, and Function.. . . . . . . . . . . . . D. Applications of Mycorrhizal P ...................... Horticulture, and Forestry . . . 111. Mycorrhizas' in Legumes . . . . . . . . . . . . . . . . . . . . . . . . . . A. Introduction ...................................................... B. Occurrence . . .............. ...................... C. Interactions be cies of Rhizobiu and Legumes.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Interactions between Added Fertilizers and Myconhizas in Legume-Rhizobium sp. Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Ecological Significance of Vesicular-Arbuscular Mycorrhizas in Legumes. . , F. Practical Field Application of Mycorrhizal Effects on Legume Production IV. Mycorrhizas in Nodulating Nitrogen-Fixing Nonlegume Plants . . . . . . . . . . . . . . . . , A. Occurrence and Distribution ........................................ ical Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V.

Conclusions

............................................ ......................

1 1 3 4 4 4 10 15 19 23 23 24 25 31 37 41 44 44

44 45 45 46

1. INTRODUCTION A. ROOT MICROORGANISMS IN THE

ECOSYSTEM

Microorganisms, which are known to play vital roles in physiological processes in the ecosystem, are invariably present in the root region, the rhi1

Copyright 8 by Academic F'ress, Inc. All righu of reproduction in any form reserved. ISBN 0-12600736-3

2

J. M. BAREA AND C. AZC6N-AGUILAR

zosphere, of plants growing in soil. Actually, rhizosphere bacteria (including actinomycetes) and fungi carry out a range of activities (e.g., the breakdown of organic matter, nitrogen fixation, secretion of growth substances, increase of the availability of mineral nutrients, and immobilization of those assimilable) of great relevance to plant growth; they also cause plant disease or protect the plant from pathogens. The extent of microbial activity depends, in most cases, on the supply of organic substrates from the root. Hence, the abundance and activity of soil microorganisms in general diminish with increasing distance from the root (Newman, 1979). From the point of view of their relationships with the plant, microorganisms can be classified into three groups: (1) saprophytes, usually opportunists but benefactors in some situations; (2) parasitic syrnbionts or pathogens, potentially harmful to the plant; and (3) mutualistic symbionts, usually called symbionts in the literature, which develop activities beneficial to plant growth (for reviews, see Brown, 1975; Dommergues, 1978; Newman, 1979). It is widely assumed that one of the most beneficial contributions of soil microorganisms to plant development is the supply of nutrients essential to plant growth, particularly those involved in nitrogen (N) and phosphorus (P) cycling. Among these, the organisms concerned with N fixation and the enhancement of P uptake by the plant are especially relevant. As it is well known, N and P are two major elements in plant nutrition that commonly limit plant growth; thus, they are usually added to soil as industrial fertilizers. However, in addition to the energy-intensive technology, implied in the synthesis of chemical fertilizers, most of these compounds are lost when they are added to the soil because they are not readily used by the plant. Actually, no more than 30% of the N fertilizer (Postgate and Hill, 1979) and only about 25% of the P fertilizer (Hayman, 1975a) are taken up by the crop in the year of its application. The rest of the N is lost either in the soil water, causing pollution problems (Bolin and Arrhenius, 1977), or to the atmosphere as a result of denitrification; most of the P fertilizer added is quickly fixed by some soil components and converted into forms which are not readily available to plants. Consequently, N fixation, which cycles N to the biosphere from the atmosphere, is an important factor in biological productivity; it is accepted that more than 60% of the N input to the plant community through fixation has a biological origin (Postgate and Hill, 1979; Brill, 1979). The activities of the N-fixing bacteria either convert N into bacterial proteins (in free-living systems) or make it directly available to plants as NH, in symbiotic associations which occur in root nodules. Many common soil microorganisms can release soluble phosphate from sparingly soluble inorganic and/or organic phosphates known to occur in soil. Several problems inherent with the lack of energy sources in the rhizosphere, micro-

MYCORRHIZAS IN NODULATING N-FIXING PLANTS

3

bial antangonism, difficulties in the translocation of the phosphate ions to the absorption places at root surface, and other factors make the microbial solubilization of phosphates a minor contribution to the P nutrition of plants (Hayman, 1975a). However, mycorrhizas, mutualistic symbioses between plant roots and certain soil fungi, play an unquestionable role in P cycling and in the uptake of phosphate by the plant. Because the known world reserves of P could be depleted in a few decades (Rhodes, 1980), the contribution of this symbiosis to the reduction of fertilizer requirements is of increasing interest. B. MYCORRHUAS AND ROOT NODULES

All but a few vascular plants are able to form mycorrhizas. Under natural circumstances, the mycorrhizal condition is the norm for most of the higher plants. The mycorrhizal fungus has an ecologically protected niche inside the plant root; the products of photosynthesis arrive here, furnishing abundant energetic substrate for the fungi which by means of their network of hyphae or mycelial strands extend the mycelium to the surrounding soil, take up nutrients (mainly phosphate) from the soil solution, and translocate these ions to the host plant (Tinker, 1975; Hayman, 1978). Mycorrhizas therefore have a worldwide recognized value for plant survival and nutrient cycling in the ecosystem. They contribute significantly to plant productivity both in arable and in plantation crops. Several types of mycorrhizas occur; their characteristics will be described later. Three different types of microorganisms (bacteria) are able to induce nodules on roots of higher plants and to inhabit them by establishing mutualistic symbioses. As a consequence of these associations, the microsymbionts are able to fix N. The energy requirements for these processes are satisfied by the photosynthate which is directly received by the bacteria at the plant roots (Hardy and Havelka, 1976). The microorganism exports NH4+ to the plant, avoiding transport and dispersal problems. The bacterial genera and the corresponding host plants involved are (1) Rhizobium, which nodulates, with one exception, on legume roots; (2) Frunkiu, actinomycetes that fix N in nodules they form on nonlegume, often woody, angiosperms; and (3) Nosfoc and Anubuenu, cyanobacteria (formerly blue-green algae), which form N-fixing nodules on the roots of plants of the family Cycadaceae (gymnosperms). Legume-Rhizobium sp. associations are the most important for the incorporation of N into pasture and agricultural ecosystems, whereas the nodulated angiosperms are similarly important in forest ecosystems. Plants bearing N-fixing nodules are usually mycorrhizal when grown in soil. This fact has great ecological relevance because nodulation and nitrogen fixation depend on a balanced mineral nutrition of the host plant (in particular, plants

4

J.

M.BAREA AND C. AZC6N-AGUILAR

have high phosphate requirements), and the mycorrhiza can satisfy these demands. Thus, mycorrhizal fungi not only help the plant itself but also aid the bacterial symbiont to fix N in the nodular tissues. Nodulate and mycorrhizal plants are therefore adapted to cope with nutrient-deficient situations (Harley, 1973).

The intent of this article is the comprehensive study of the role of mycorrhizas in the growth and nutrition of N-fixing nodulated plants. As an introduction for a better understanding of mycorrhizal effects, we will present a brief review of some general, well-established principles on mycorrhizal types, morphology, physiology, and function. Current information will be condensed to achieve an up-to-date presentation of this sllbject and to create a conceptual background for nonspecialist readers. This will constitute a quantitatively and qualitatively important part of the article. Then, the interactions between nodular and mycorrhizal endophytes related to the formation and effects of these dual symbioses, which greatly enhance the development of the common host plant, will be discussed. This part of the article will be concerned not only with conceptual principles but also with the rationally stated hypotheses and the current trends in basic and applied research on this subject. Attention will be given to the ecological significance of plants bearing the two types of symbioses, with emphasis on the possibilities of harnessing them to increase crop yield.

II. MYCORRHIZAS A. GENERAL

The previous statements on the concept and function of mycorrhizas, although concise, may allow us to envisage these widespread associations as the most metabolically active parts of the absorbing organs of almost all land plants. Both the autotrophic host plant and the heterotrophic fungal associate derive, in most cases, physiological and ecological benefits from one another. Furthermore, the “mycorrhiza-dependent’’ plants cannot develop adequately without their mycorrhizal partner. However, the general term mycorrhiza, broadly considered, is of little significance. The taxonomic diversity in the fungi and plants involved and the differences in the morphological, structural, and nutritional features of mycorrhizal associations require a subdivision to reflect the different physiological relationships that are now recognized.


5

I . Mycorrhizal Types and Their Structural and Nutritional Features Five types of mycorrhizas can be recognized. These and the main groups of host plants on whose roots they are formed are recorded in Table I, as summarized from Smith (1980) and Azc6n-Aguilar and Barea (1980). The first type, ectotrophic mycorrhizas (ECM),is characterized by a lack of intracellular penetration of the fungus into the cortical cells of the root. A network of fungal mycelia, the Hartig net, is formed by hyphal growth among the host cells. This in turn establishs a close contact between fungus and root-cell plasmalemma, which is critical for nutrient exchange in mycorrhizal associations. In most cases the fungus will develop a mantle or sheath of interwoven hyphae growing around the feeder roots. The fungal mantle is extended some distance into the surrounding soil by mycelial strands or rhizomorphs (only rarely by extramatrical hyphae) (Harley, 1978). The fungi involved are mostly higher basidiomycetes (Boletus, Suillus, Amanita, Lactarius, Tricholoma, Pisolithus, Scleroderma, Rhizopogon, etc.), some ascomycetes (Tuber), and zygomycetes (Mam and Krupa, 1978). The second group, vesicular-arbuscular mycorrhizas (VAM), is by far the most widespread type of mycorrhiza. The nomenclature refers to the formation of vesicles and arbuscules, typical morphological structures that will be considered later. As with ericoid, arbutoid, and orchidaceous mycorrhizas, the VA fungus penetrates into the cortical host cells, but the invading mycelium usually lives only a short time intracellularly (Smith, 1980); lysis of intracellular struc-

Table I Mycorrhizal Types and the Main Groups of Host Plants Involved Nomenclature Traditional

Actual

Ectotrophic

Ectotrophic or sheathing

Endotrophic

Vesicular-arbuscular

Ericoid Arbutoid Orchidaceous

Typical host plants Pinaceae, Fagaceae, Betulaceae,*Eucalyptus, Rosaceae,a Leguminosae" (woody), Cupressaceae Four-fifths of all land plants including agronomically important crops such as woody and herbaceous legumes" (pasture, forage, and grain) and Gramineae Calluna, Vaccinium, Erica, Epacris Arbutus, Monotropa Orchidaceae

"Groups of plants also bearing nitrogen-fixing root nodules.

6

J. M. BAREA AND

c. AZCON-AGUILAR

tures (the arbuscules in VAM) then occurs, but the host cell survives and can be colonized again by the fungus. Vesicular-arbuscular fungi do not form sheaths around the root, but a network of extramatrical hyphae usually develops. This grows into the soil and can extend the mycelium several centimeters beyond the root surface. The total hyphal length can reach more than 1 m of hyphae per centimeter of infected root (see Smith, 1980; Hayman, 1982). These VA fungi are members of the family Endogonaceae that are placed in the genera Glomus, Sclerocystis, Gigaspora, and Acaulospora (Gerdemann and Trappe, 1974). Because they cannot be successfully subcultured axenically, they must be considered ecologically obligate symbionts (i.e., they do not complete their life cycle unless they can colonize a suitable host plant) (Lewis, 1973). An ascomycete (Pezizella ericae) has proved to be a fungal partner of the third type of mycorrhiza, namely, the ericoid, which occurs on roots of some autotrophic shrubs in the families Ericaceae, Epacridaceae, and Empetraceae (Read and Stribley, 1975; Read, 1983). Intracellular coils and extramatrical hyphae are typical structures of these mycorrhizas. The structure of arbutoid mycorrhizas, the fourth type, is characterized by the formation of a sheath but not a Hartig net, and they also form intracellular haustoria. Their nutritional features are not yet fully understood. Confined to the family Orchidaceae, the fifth group of mycorrhizas shows unique characteristics; they infect protocorms and rhizomes, but rarely the terrestrial roots. Their hosts are temporarily or permanently achlorophyllic, and the mycorrhizal fungi (Rhizoctonia spp. and Armillaria melea), which are pathogens for nonorchidaceous hosts, aid the heterotrophic orchid in assimilation of carbohydrates, probably from a simultaneous association with another true autotrophic host plant (Mosse, 1978). The major types of mycorrhizas and the groups of plants on which they occur having been described, the discussion may now be limited to ECM and VAM, the only mycorrhizal types formed on plant families also bearing N-fixing root nodules (Table I). Emphasis will be placed on VA mycorrhizas because these are the commonest type occurring on nodulated plants and also because these mycorrhizas, as deduced from their near omnipresence, play an integral role in most crop-production systems. 2 . Occurrence and Distribution Mycorrhizas, mainly VAM, can be found in most plant species growing in most plant habitats under tropical, temperate, and even arctic conditions (Hayman, 1982). To understand the worldwide distribution and ecological implications of this symbiosis, it is interesting to go back 400 million years and consider the role played by a fairly similar mutualistic association-the “ancestral mycorrhiza”-in the evolution of terrestrial plants (Pirozynski and Malloch, 1975). As


7

pointed out by these authors and by Malloch et al. (1980), the SilurianDevonian colonization of the land by “plants” seems to have been facilitated by the development of a mutualistic partnership between a semiaquatic ancestral green alga and a certain aquatic fungus. The ancestral mycorrhiza probably equipped the plants to cope with the problems of starvation and desiccation resulting from the colonization of a nonaquatic habitat, the soil. The earliest of the land plants preserved in a petrified form is the Rhynie fossil dated to 370 million years ago, and this possessed in its “roots” a form of mycorrhiza remarkably similar to the modern VAM (Nicolson, 1975). In this context it can be assumed that mycotrophy (Lewis, 1973) and mycorrhizas are as old as plants that seem to have depended on such mycorrhizas to thrive early in their evolution. The symbiosis followed the course of evolution as a component of the plants and, in such a way, it has been perpetuated as an adaptation for the more efficient absorption of phosphorus. The claim of Pirozynski and Malloch (1975) is that “land plants never had any independence (from mycorrhizal fungi), for if they had, they could never have colonized the land.” On these bases it can be concluded that mycorrhizas have occupied, from the Middle Cretaceous on, a crucial role in the evolution, ecology, growth, and nutrition of the plant cover of the surface of the Earth. They can be found in tropical rain forests, open woodlands, grasslands, savannas, heaths, sand dunes, and other habitats (Safii, 1980; Hayman, 1982). In spite of some descriptions (Sondergaard and Laegaard, 1977), plants growing in aquatic habitats appear to lack mycorrhizas, and they also seem to be rare in the families Cruciferae, Polygonaceae, Chenopodiaceae, Cyperaceae, and others (Gerdemann, 1975). According to Meyer (1973), only about 3% of the higher plants have sheathing mycorrhizas (see Table I). These occur mostly in temperate timber trees (Marx and Krupa, 1978; Fogel, 1980; Trappe, 1981). Vesicular-arbuscular mycorrhizas are formed in most angiosperms, some gymnosperms, and in pteridophytes and briophytes. These are the mycorrhizas of most of the economically important crops [e.g., legumes, maize, wheat, barley, rice, temperate fruit trees, many tropical timber trees, woody shrubs, tropical plantation crops (cocoa, coffea, tea, rubber, etc.), cotton, tobacco, olive, citrus, and grapevine] (Hayman, 1982). Finally, some plants may form both sheathing and VA mycorrhizas [e.g., apple, oak, alder, hazel, juniper, certain woody legumes, and members of the family Populaceae (Trappe, 1977), and members of the genus Eucalyptus (Malajczuk et al., 1981)l. 3 . The Process of Mycorrhiza Formation

The establishment of mycorrhizal status occurs in a sequence of phases involving interactions between the host, fungus, and environment. Because ECM can

8


be synthesized in vitro, the details of the process of their formation can be accurately studied. Nylund and Unestam (1982) have well illustrated the interactions that are occurring. Taking into consideration many previous descriptions, these authors extrapolate the findings to provide a generalizable sequence of events. According to them, the process is controlled mainly by the host, but this does not preclude an active participation of the fungus (i.e., different mycorrhizal structurescan be originated in the same host by different endophytes). The process is initiated by the germination and development of propagules (spores or hyphae) of the fungi living in proximity to the feeder roots of the host. The host releases certain substances that produce a remote and selective stimulation of the tentative mycosymbionts. This enhances the growth of these fungi to an extent that is dependent on the species. Only mycorrhizal fungi have the ability to respond, because only they recognize the host signal that is meaningless to the other rhizosphere inhabitants. Fungal growth is stimulated, hyphae aggregate around the root establishing a close contact between both mycorrhizal partners, and a hyphal envelope forms, induced by host substances. This envelope structure apparently is essential for the infection. The penetration between root epidermal cells seems to be mechanical, and the host apparently does not resist although it controls the fungal lytic enzymes. During further development of the mycorrhiza, the fungus is more protagonistic and more interactions occur. At the labyrinthine Hartig net formation phase, the morphogenetic changes of the fungus are again a response to host factors although released by fungal induction. The formation of the mantle then takes place, the fungus induces some morphological changes in the host, and the colonization of all suitable root tissue by the fungus completes the mycorrhiza. The mycelial strands, sclerotia, and fruiting structures are developed later (PichC and Fortin, 1982). Because VA fungi have not yet been successfully cultured axenically, studies on the development of VA infections are difficult. Some hyphal growth can be obtained in vitro from germinated spores or infected root pieces, but the growth ceases when the hyphae are excised from the parent spore or when the root piece dies. It is therefore said that these fungi do not grow saprophytically. Nevertheless, the assays carried out by Hepper (1979) on the germination and growth of surface-sterilized Glomus sp. spores indicated that the protein synthesis required for germination is programmed by stored mRNA, but that the spores also have the ability to synthesize the new mRNA that is required for hyphal growth. The fungi possess an Embden- Meyerhof-Pamas system, a tricarboxylic acid cycle, and a hexose monophosphate shunt (MacDonald and Lewis, 1978). These results suggest that the endophytes resemble saprophytic fungi more than obligate biotrophs. Moreover, a certain independent spread of VAM fungi in soil has been reported (Warner and Mosse, 1980), suggesting some saprophytic ability of these fungi.


9

Studies on the development of VAM infection (Powell, 1976a) showed that neither spore germination nor the initial direction of hyphal growth was influenced by the presence of host roots. Hyphae from spores were not attracted to the roots until they approached them closely. First, the stimulated germ tube formed a fanlike structure of mainly septate hyphae, from which the infective aseptate ones developed later. Because hyphae from root segments did not form preinfection structures to infect a new root, it was suggested that these fanlike structures have the function of absorbing nutrients or hormones from root exudates. Studies of mycorrhizal infection in root organ cultures (Mosse and Hepper, 1975) also indicated the lack of apparent attraction of germ tubes to the root until they grow very close to it. Once a fungal hyphae is attached to the root surface, root penetration may or may not occur. Young lateral roots seemed to exert a greater stimulatory effect on the fungus. In summarizing the previous statements we have four key facts in VAM formation: (1) spore germination and mycelia development; (2) a stimulation of the germ tubes when they approach the roots closely; (3) attachment of the infective hyphae to the root surface; and (4) root penetration. With respect to the stimulation of the hyphae in the rhizosphere, it is obvious that root exudates are significant, and a positive correlation between VAM infection and the degree of root exudation, which in turn is correlated with an increased permeability of root membranes, has been found (Ratnayake et al., 1978; Azc6n and Ocampo, 1981). However, there is another important factor that distinguishes rhizosphere from nonrhizosphere soil, namely, the presence of active populations of microorganisms. They probably play a role in the development of VA fungi and VAM infection. This is supported by studies on the influence of free-living rhizosphere microorganisms on VA fungi in pure culture. The preliminary results have shown that several fungi stimulate the “growth” of Glomus mosseae in culture. The rate of spore germination, the length of the hyphae, and the number of vegetative spores per resting spore were increased by the action of common rhizosphere inhabitants (C. Azc6n-Aguilar and J. M. Barea, unpublished). Once the infective hypha arrives at the root surface an appresorium is usually produced on cortical cells or on root hairs, and hyphal penetration occurs into or between these cells. When the first successful entry point is established, the root becomes more prone to further penetration. This behavior could be because the fungus is invigorated and/or because of changes in the root as a consequence of the infection. The fungus then colonizes the root cortex and the hyphae multiply both inter- and intracellularly, although they never invade the endodermis, stele, or root meristems. Shortly after infection, a hypha growing into a single cell may show repeated dichotomic branching, and a treelike structure, the arbuscule, is formed. The function of the arbuscules is the biotrophic bidirectional transfer of nutrients, the mechanism of which requires living fungus (Cox and Tinker, 1976). Fine-struc-

10

J. M.BAREA AND C.AZC6N-AGUILAR

ture studies (Cox and Sanders, 1974; Scannerini and Bonfante, 1983; Scannerini et al., 1975; Dexheimer et al., 1979) revealed that the arbuscules are surrounded by the intact host-cell plasmalemma. The cytological changes that occur during arbuscule formation have been well documented by Rhodes and Gerdemann (1980). There is an increase in host-cell cytoplasm, the starch within the invaded cells disappears, and the nuclei become enlarged and at times divide. The cell organelles (mitochondria, ribosomes, etc.) also increase in number. When an individual arbuscule degenerates (they exist for 4-13 days), the cell and its structures return to their normal stage. This cell is then ready for the formation of a new arbuscule (Hayman, 1982). When the mycorrhiza is well established, the fungi may form vesicles. These are oval-to-spherical structures containing oil droplets that can develop inter- or intracellularly. They may have a temporary storage function, after which they remain thin walled or become thick walled as chlamydospore-like structures. When the internal infection has been consolidated, the penetration hyphae ramify externally. These external hyphae may grow along the root surface forming more penetration points and also grow through the surrounding soil forming an extensive tridimensional network of mycelium. A typical feature of the VAM is the dimorphic nature of the external hyphae: the coarse, thick-walled (20-30 pm in diameter) hyphae bearing resting spores are the permanent basis of the mycelium, and the fine, thin-walled hyphae (2-7 pm) are more ephemeral and have absorption functions. The density, geometry, and size of the external mycelium, and the number of entry points per unit of root length (1-25 per millimeter), are of great relevance in the functioning of VAM. When the mycorrhiza matures the external mycelium usually produces large resting spores and smaller secondary spores, or external vesicles. Some VA fungal species do not form spores, and some develop sporocarps (Gerdemann, 1975). B. PHYSIOLOGY OF MYCORRHIZAS

Current literature on mycorrhizal research records progress toward a better understanding of many physiological features of these symbioses, particularly of the mechanisms that account for the mycorrhizal effects on plant growth and nutrition. Some of these mechanisms, however, remain unexplained or poorly understood. The review by Smith (1980) is a detailed and illustrating study on this subject. Her qualitative model of the interactions between fungus, host, and environment is a comprehensive summary of the available information relating the flow of materials.and the feedback controls in mycorrhizal associations. This review and those by Tinker (1978, 1980), Hayman (1975a, 1978, 1982, 1983), Bowen and Bevege (1976), Mosse (1978), Rhodes and Gerdemann (1980), Safir (1980), and Gianinazzi-Pearson and Gianinazzi (1981) thoroughly cover the published knowledge on the nutritional relationships between the mycorrhizal partners. Hence only the main points will be summarized here.

MYCORRHEAS IN NODULATING N-FIXING PLANTS

11

1 . Effect of Mycorrhizas on Plant Growth There is considerable published information indicating that mycorrhizas enhance plant growth, which can be understood to be the result of an improved mineral nutrition of the host plant, for which evidence has been provided using isotopic tracers. An increased concentration and/or content of phosphorus in plants is by far the response to mycorrhizas most often described. However, the concentration and content of other nutrients can also increase (sometimes as a consequence of a better P uptake), and there might also be some nonnutritional effects. Mycorrhizas not only increase plant biomass but also influence the partitioning of this material between shoot and root. The enhanced nutrient uptake and the subsequent translocation to the aerial part of the plant increases the utilization of photosynthate in the shoot, hence relatively fewer photosynthesis products are transferred to the root. Consequently, the root/shoot ratio is usually lower in mycorrhizal plants than in the corresponding nonmycorrhizal controls (Smith, 1980). A change in the hormonal status as induced by mycorrhizal infection also can be involved (Allen et al., 1980, 1982). In some instances adverse effects on plant growth in response to VAM have been found (Smith, 1980;Buwalda and Goh, 1982). In most cases the depression is merely transitory and caused by a fungus-plant competition for available photosynthate at the early infection stages or under suboptimal photosynthetic conditions (e.g., shading or low temperatures). Persistent depressions take place when supraoptimal P concentrations are reached in the plant tissues or when the soil phosphate concentration is such that fungus maintenance becomes expensive. 2 . Eflect of Mycorrhizas on Phosphorous Nutrition a. Source of Phosphorus for Mycorrhizas. Because approximately 95-99% of soil P occurs in forms that are not directly available to plant roots (Bieleski, 1973), the possibility that the fungus could solubilize unavailable P was a tantalizing hypothesis to explain the mechanism for the increased P supply by mycorrhizas. In addition, certain sparingly soluble P compounds seemed to be utilized by VAM as a source of P. This possibility was investigated by experiments in which the labile phosphate pool was labeled with 32P (Sanders and Tinker, 1971; Mosse and Hayman, 1971; Powell, 1975). The specific activity (32P/31P)of P in plant tissues was similar for mycorrhizal and nonmycorrhizal plants although the former take up more phosphate. If mycorrhizal plants did utilize nonlabile (unlabeled) sources of P, the specific activity in these plants would be expected to be lower than in nonmycorrhizal controls. These facts show that the plants used the same soluble phosphate pool irrespective of whether they were mycorrhizal. Consequently, it is widely assumed that mycor-

12

J. M.BAREA AND C. AZC6N-AGUILAR

rhizal plants draw most of the phosphate from the soluble pool, although more efficiently than nonmycorrhizal plants. This is in agreement with the finding that VA infection does not modify the root surface-bound phosphatase activity. Moreover, there is not an increase in the exudation of hydroxyacids that could solubilize phosphate either by their chelating activity or merely by reducing the pH in the mycorrhizosphere. The apparent solubilization of poorly soluble sources of P, such as rock phosphate, can be attributed to the greater contact between the network of external hyphae and the surfaces of phosphate particles in soil where phosphate is being physicochemically or biologically dissociated (see Hayman 1975a, 1978). However, further research on this subject is needed to clarify some of the points previously discussed. b. Phosphate Uptake by External Mycelium. The rate of nutrient absorption by roots or mycorrhizas is known to depend on the rate of nutrient supply to the rhizosphere, this being influenced by the mobility of the ion and its concentration in the soil solution. These facts are of great relevance in P nutrition (Chapin, 1980). Phosphate ions, which are in low concentration in the labile pool (Bieleski, 1973), move by diffusion very slowly because they are readily adsorbed to the soil colloids. Plants take up phosphate much faster than these ions can diffuse to the root surface; consequently, phosphate-depleted zones normally develop around the absorbing organs of the plant. These zones, which are 1-2 mm wide, coincide with the rhizosphere and can be visualized by autoradiography (Owusu-Bennoah and Wild, 1979). Vesicular-arbuscular mycorrhizas enhance P uptake in two different ways. One mode of fungal action is merely physical and is based on the increased number of sites for absorption achieved by the external mycelium. The hyphae growing through soil pore spaces are able to effect phosphate absorption beyond the depletion zone up to 8 cm from the root (Rhodes and Gerdemann, 1975). Thus, mycorrhizal roots explore a much greater volume of soil to take up phosphate. A correlation has been found between the size of the external mycelium and the flux of phosphate into mycorrhizal roots (Sanders et al., 1977). Obviously, once inside the hyphae, phosphate ions are protected against absorption by soil components. On the other hand, the kinetic analyses carried out by Cress et al. (1979) demonstrated that mycorrhizas have a lower apparent Michaelis constant (K,) of phosphate uptake than nonmycorrhizal roots, suggesting as the second mode of fungal action the existence of a pathway of greater affinity for P in mycorrhizal roots. This reinforces the results of Mosse ef al. (1973) which suggest that VAM reduce the threshold value for effective phosphate absorption from soil. In spite of the activity of surface phosphatases in ECM, the bulk of P gained by them also comes from the labile pool, and in this case the mycelial strands, which may reach 12 cm in length, are responsible for the increased absorption of phosphate (Bowen, 1973; Harley, 1978).


13

c. Translocation of Phosphate from External to Internal Mycelia. There is now experimental evidence to show that P is translocated to internal fungal structures as polyphosphate granules contained inside vacuoles (Ling-Lee et. al., 1975; Cox and Tinker, 1976; Callow et al., 1978). These are propelled through the hyphal lumen by cytoplasmic streaming to the arbuscules, although bulk flow may also contribute to the translocation (Cooper and Tinker, 1981). The specific mechanism for polyphosphate loading, translocation, and unloading is active and very efficient, and the calculated inflow of phosphate through external hyphae is approximately 1000-fold faster than the phosphate diffusion rate through soil (Bieleski, 1973). Mycorrhiza-specific phosphatase activity has been described in VA infections (for reviews see Gianinazzi-Pearson and Gianinazzi, 1981; Capaccio and Callow, 1982), suggesting that these phosphatases may play a key role in the active phosphate translocation and/or transfer mechanisms in VAM. d. Phosphate Transferfrom Fungus to Host. The main site of phosphate transfer to the host, which occurs by an active mechanism across the membrane of both partners, seems to be the arbuscule (Cox and Tinker, 1976; Cox et al., 1980). This hypothesis is strengthened by the finding that the plasmalemmabound ATPase activity of the host is concentrated around the arbuscules when the VAM infection develops (Marx et al., 1982). It is now accepted that the breakdown of the arbuscules can account for only 1% of the P inflow to the host cells. Phosphate release by other structures such as hyphae or vesicles might also be involved, but the extensive increase of contact surface area makes the arbuscules the more probable sites for nutrient transfer between mycorrhizal symbionts. In ECM, the host tissues are sealed off from the soil when the fungal mantle is well developed (Nylund and Unestam, 1982); all transport between soil and host therefore must pass through the fungal mycelium. The nutrient transfer between fungus and host takes place across the hyphae of the Hartig net. A characteristic feature for the functioning of ECM is the storage of phosphate in the sheath and its slow release into host cells. Polyphosphate granules contribute to the storage pool in these mycorrhizas (Chilvers and Harley, 1980; Strullu, 1982). 3. Absorption of Other Nutrients by Mycorrhizas

It has often been reported that mycorrhizal infection also increases the concentration of nutrients other than phosphorus in plant tissues, but it is unclear if this enhancement of nutrient uptake is merely a consequence of improved P supply. The general pattern of uptake, translocation, and transfer of a nutrient into host cells is similar to that for phosphate, but the role and the relative contribution of the external hyphae will differ with the nutrient involved. Mycorrhizal hyphae will help the plant to overcome uptake limitations in the case of nutrients that diffuse slowly and give way to depleted zones around roots.

14


Conversely, mycorrhizas will confer little additional advantage for the uptake of ions such as sulfate which may move through soil by mass flow with no ratelimiting step in their way to the root surface. Increased N concentrations in VAM plants have been reported, and because zones of depletion in nitrate are not usually formed, mycorrhizas would improve uptake only when ammonium, which is relatively immobile, is the source of N (Smith, 1980). Ectotrophic mycorrhizas, as well as ericoids, have the ability to take N from organic matter (Raven et al., 1978). A direct effect of VAM in improving Zn and Cu uptake has also been found (Bodes and Gerdemann, 1980). Some rather limited hyphal translocation of S can account for some increases in S uptake by mycorrhizal roots. Mycorrhizal infection may also decrease resistance to water transport; thus mycorrhizal plants recover faster from water stress than do nonmycorrhizal ones. However, the hyphal translocation of water has not been fully demonstrated yet (Safir, 1981). Hence, the role of VAM in relation to drought is actually a topic with many unclear aspects (see Allen and Boosalis, 1983; Levy et al., 1983). 4 . Nonnutritionul Effects of Mycorrhizas

Mycorrhizas can affect plant growth and vigor by mechanisms other than improved host nutrition. The production of substances with hormonal activity is involved in the effects of ECM fungi on root morphology (Slankis, 1974; Ng et al., 1982). The ability to synthesize compounds like auxins, gibberellins, and cytokinins has also been described for VA fungi “growing” in vitro (Barea and Azc6n-Aguilar, 1982b). These substances can alter plant morphology and physiology, and some mycorrhizal effects may be mediated by changes in the hormonal balance. A role of mycorrhizas in improving soil structure also has been suggested (Nicolson, 1960; Sutton and Sheppard, 1976; Koske and Halvorson, 1981). This is significant in eroded soils, sand dunes, and the like; mycorrhizal hyphae can bind soil particles into more stable aggregates, helped by the cementing action of bacterial polysaccharides (Foster and Nicolson, 1981a,b). Mycorrhizal infection can help plants withstand root diseases either by protecting the root system against the pathogen attack or by compensating for root damage (Schenck and Kellan, 1978; Mam and Krupa, 1978; Schonbeck, 1979). These effects are more clearly understood for ectomycorrhizas in which the fungal sheath provides a mechanical barrier to infection by soil-borne root pathogens, and in which the fungi produce antibiotics. The role of VAM in deterring pathogens is much less defined. Mycorrhiza-induced changes in root exudation, especially increase of the arginine content and in thickening cortical root cell walls, can account for the deterence of bacteria, fungi, and nematodes in some


15

cases. However, the resistance to diseases can be largely attributed to improved host-plant nutrition (Graham and Menge, 1982). There are also reports indicating that disease symptoms were worsened in mycorrhizal plants. Thus, because of all of these facts the biological control of plant diseases by mycorrhizas is a problem deserving further research. 5. Carbon Nutrition in Mycorrhizas Under normal conditions, heterotrophic mycorrhizal fungi obtain carbon compounds from their autotrophic hosts by means of biotrophic transfer across the living membranes of both partners (Lewis, 1975; Smith, 1980). Direct evidence for such a transfer was obtained by detecting 14C-labeledcompounds in fungal structures associated with plants that photosynthesized with 14C02 (Ho and Trappe, 1973; Bevege et al., 1975; Cox et al., 1975). In ECM, as well as in ericoid, the photosynthate, mainly sucrose, is rapidly converted to the specifically fungal metabolites mannitol and trehalose, and eventually to glycogen. This is the storage sink which can represent a significant photosynthate diversion (Bevege et al., 1975; Harley, 1975). In contrast, the fungus in VAM does not appear to form trehalose or mannitol (Hayman, 1974), and lipids seem to be the alternative sink in these mycorrhizas (Cox et al., 1975; Cooper and Liisel, 1978; Liisel and Cooper, 1979). A large proportion (43.8%) of fatty material was found in the VA mycelium (Cooper and Liisel, 1978) either as deposits (storage sink) or involved in the extensive formation of membranes (growth sink) especially at the arbuscular phase of the infection (reviewed by Smith, 1980). This could be a circumstantial drain of photosynthate, but in general the relative size of the fungal biomass seems small enough to make such a diversion .unlikely (see Tinker, 1978). C. FACTORS AFFECTING MYCORRHUAESTABLISHMENT, DEVELOPMENT, AND FUNCTION

The establishment of successful entry points on host roots, the internal and external development of the fungus, and the resulting plant responses are all dependent on interactions between prevailing fungal, plant, and environmental factors. Vesicular-arbuscular endophytes are present in virtually all soils, but mycorrhizal population levels may differ greatly under various ecological conditions. Indigenous mycorrhizal populations can be diminished by agricultural practices such as heavy fertilization and pesticide treatments. This section reviews these factors and the ways in which they influence mycorrhiza formation and efficiency.

16


1 . Plant Species: Mycorrhizal Dependency Certain plant species require mycorrhizas to a much greater extent than do others, and this is usually referred to as mycorrhizal dependency, which is “the degree to which a plant is dependent on the mycorrhizal condition to produce its maximum growth or yield, at a given level of soil fertility” (Gerdemann, 1975). In general, plants having rootlet diameters of more than 0.5 mm and lacking root hairs are highly dependent on VAM. Conversely, plants with a dense cover of long root hairs and root systems that have rootlet diameters less than 0.1 mm respond to mycorrhiza only in P-deficient soil (Baylis, 1970; St. John, 1980). However, differences in the relative mycorrhizal dependency between crop species, or even cultivars ( A z c h and Ocarnpo, 1981), are also related to other inherent plant factors (Warner and Mosse, 1982), such as root structure and metabolism and plant growth rates, which could affect the demand for P (Hall, 1975). Legumes, for example, are more mycotrophic than grasses and thus probably will benefit more from mycorrhizas. Consequently, there is a critical effect of VAM on interspecies (legumes-grasses) competition for P (Crush, 1974; Hall, 1978; Haynes, 1980). Ectomycorrhizal hosts usually require their fungal associates in order to thrive (Trappe and Fogel, 1977).

2. Endophyte Species: Specificity Some ectomycorrhizal fungi are relatively restricted in their hosts, although most of them have a broad host range (Marx and Krupa, 1978). Vesiculararbuscular mycorrhizal fungi have very little host specificity and any of them can infect virtually any potential host plant (Mosse, 1978; Hayman, 1982);however, they differ in their effectiveness which appears to be more dependent on the specific soil-plant system they colonize than on the host plant itself. A factor determining its effectiveness in enhancing plant growth is the ability of the fungus to develop a great amount of external mycelium, a characteristic inherent with the fungal endophyte (Mosse, 1972) that is affected by the plant-soil system in question. This parameter, which appears to be independent of the rate of fungal colonization of the root cortex, is positively correlated with the enhancement of plant growth by the VAM fungus (Graham et al., 1982). Another critical factor for the effectiveness of VAM endophytes may be the speed at which they infect the root and start to function (Hayman, 1982). In this context, Abbott and Robson (1981b) found a close correlation between the effectiveness of fungi species and the percentage of root length infected at early stages of plant growth.


17

3 . Rate of Photosynthesis and Contributing Factors

Because the heterotrophic partner in mycorrhizal symbioses is supplied with carbohydrates from the photosynthate of the autotrophic host (Harley, 1975), any factor that modifies the photosynthetic products available for distribution would affect mycorrhizal development. Obviously, light is one such factor. High light intensities appear to enhance arbuscule formation (Hayman, 1974) and spore production (Furlan and Fortin, 1977). It seems that the largest plant-growth stimulation by VAM occurred under conditions of light and temperature optimal for the development of the host plant (Hayman, 1974; Daft and El-Giahmi, 1978; Johnson et al., 1982a,b). Light and temperature also influence ECM formation and function (Shemakhanova, 1972; Slankis, 1974; Picht and Fortin, 1982). Changes in soluble sugar levels in the root is the logical consequence of the effect of these factors on the rate of photosynthesis (Bjorkman, 1970; Hayman, 1974; Johnson et al., 1982a,b), and the concentration of these products in root extracts and exudates is closely related to mycorrhizal development (Ratnayake et al., 1978).

4. Soil Conditions In general, ectomycorrhizal fungi are acidophilic (Mam and Krupa, 1978), but there is no correlation between VAM and soil pH (Read et al., 1976). In spite of this it has been shown that soil pH can influence the predominance of a given type of spore. In fact, some species are better adapted for acid soils and others are better adopted for neutral and alkaline soiis (Mosse, 1973a). Mycorrhizal fungi are sensitive to soil moisture status; Redhead (1975) demonstrated that the optimal water supply for plant growth is also suitable for mycorrhizal infection. With obligate aerobes (see Saif, 1981), flooding is detrimental to mycorrhyzal activity. The mycorrhizal effect in saline soils is receiving current attention, but it requires further research (Allen and Cunningham, 1983; Ojale et al., 1983). Soil temperature affects the preinfection stages of mycorrhizal development in that the number of “entry points” increases as the temperature rises from 12 to 25°C (Smith and Bowen, 1979). The level and type of nutrients affect the formation of and the response to mycorrhizas (Smith, 1980; Hepper, 1983). A general principle is that a low-tomoderate soil fertility enhances the degree of mycorrhizal development and plant response. Hence mycorrhizal infection could be excluded in fertile agricultural soil (Mosse, 1978). In particular, the level of plant-available phosphate appears to be a suitable index for predicting a growth response to VAM in a given soil.

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5 . Fertilizers Heavy fertilizer applications, whether of .nitrogen or of phosphate, are often detrimental to mycorrhizal development and function, but field responses to these treatments are unpredictable because their effects are dependent on initial soil fertility (Slankis, 1974; Hayman, 1975b; Kruckelmann, 1975). As some VA species are more tolerant to added fertilizers than others, it follows that these soil amendments can change the species composition of VAM fungi in a given field soil. The degree of tolerance and compatibility of a mycorrhizal species to added phosphate is a relevant criterion for selecting suitable endophyte strains for field inoculation (Powell, 1977b; Hayman and Mosse, 1979). Special attention has been paid to the effect of soluble phosphate on mycorrhizal development (Mosse, 1973b). The general conclusion obtained is that P levels in the plant, rather than those in the soil, control the establishment and functioning of mycorrhizas (Sanders, 1975; Azcbn et al., 1978b; Menge et al., 1978; Allen et al., 1981). It has become increasingly apparent that P inhibition of VAM formation is associated with a membrane-mediated decrease in root exudation (Ratnayake et al., 1978). In summary, it appears that a cause-effect relationship exists; the higher the P content in the plant, the lower the soluble carbohydrate content in the roots and exudates and the lower the frequency of entry points (Jasper et al., 1979).

6. Soil Microorganisms

As soil inhabitants, mycorrhizal fungi are probably immersed in the framework of microbial interactions that take place in soil microhabitats (Stotzky, 1972),but our understanding of these events is still incomplete.When the mycelia reach the rhizosphere the interactions between mycorrhizal fungi and other microorganisms are increasingly apparent. The experimental evidence to support the existence of these interactions in the epidemiology of root colonization and mycorrhiza formation and on the mycorrhizal effect on plant growth has been reviewed by Barea and Azcbn-Aguilar (1982a). A range of situations including depression, neutrality, and stimulation of ectomycorrhizal fungi by soil bacteria in the rhizosphere of Pinus radiafa was demonstrated by Bowen and Theodorou (1979). Hence these authors proposed that compatibility with a wide range of soil microorganisms must be taken into account in selecting mycorrhizal fungi. Current literature also indicates that soil microorganisms can stimulate the mycelial growth of VAM fungi and the infection process. The suggested mechanisms include the production of compounds increasing root-cell permeability and the synthesis of plant hormones or vitamins by the soil microbiota, but changes in the physicochemical properties of the microenvironments might be also involved.


19

Interactions related to nutrient cycling have been described for nitrogen-fixing bacteria (Rhizobium spp., root-nodulating actinomycetes, Azospirillum spp., Azotobacter spp.) , nitrifying bacteria, phosphate-solubilizing microorganisms, and others. These beneficial microorganisms can be, in turn, stimulated in the ‘‘mycorrhizosphere.” Interactions with root pathogens have already been discussed. On the other hand, the existence of parasites of mycorrhizal fungi has been also described (Daniels and Menge, 1980). These may reduce the inoculum level in a soil or even destroy the external hyphae of the mycorrhiza, hence disrupting nutrient translocation (Rhodes, 1980).

7 . Pesticides Relatively few studies have been concerned with effects of agricultural pesticides on the establishment and efficiency of mycorrhizal associations. Stunting of seedlings of the genus Citrus following methylbromide treatment of the soil was associated with an inhibition of VAM fungi by the fumigant (KleinSchmidt and Gerdemann, 1972). It is now accepted that pesticides, especially fungicides, are detrimental to mycorrhizal development. They reduce the infection and, in some cases, completely eliminate the plant growth stimulation by mycorrhizas (Safir, 1980). The effects of herbicides, insecticides, nematicides, and others on mycorrhizal symbioses is, however, a topic deserving further study because contradictory results have been found in some instances (Hayman, 1982).

8. Other Factors Afecting Mycorrhizas Mycorrhizal establishment and/or function could also be affected by other factors [e.g., the removal of surface soil horizons, as in excavations and mining practices (Rhodes, 1980; Zak and Parkinson, 1982), crop rotation involving nonhost plants (Ocampo and Hayman, 1981; Ocampo et al., 1980)l. D. APPLICATIONS OF MYCORRHIZAL POTENTIAL IN AGRICULTURE, PASTURE,HORTICULTURE, AND FORESTRY

After the recognition that mycorrhizas improve plant growth and nutrition, the next logical step in mycorrhizal research to investigate the possibilities of their practical exploitation by harnessing the advantages of the symbioses. The first point of interest is to attempt to preserve the natural mycorrhizal potential as it exists in the field; the second is to try to improve this potential. Naturally occurring strains of mycorrhizal fungi need, in some cases, to be replaced with more effective mycosymbionts by means of inoculation. The principles that

20


govern this are derived from the fact that natural selection has usually not led to the dominance of the most effective strains of mycorrhizal fungi in a given area, since “the trend of evolution has been for survival, not high productivity [Bowen, 1980al.” Hence the indigenous symbionts have probably evolved to be compatible and adapted to their environments but, in most cases, they have solved their survival problems at the price of their effectiveness. If indigenous endophytes are efficient but sparse, they must be multiplied on stock plant cultures to obtain a suitable inoculum to maintain an adequate mycorrhizal level. Artificial inoculation with suitable mycorrhizal fungi is receiving increasing attention; it is a common practice in the case of ECM (Mikola, 1970; Trappe, 1977; Molina, 1977; Mam, 1980; Mexal, 1980). It is widely accepted today that the inoculation of tree seedlings with ECM fungi is a crucial step in reforestation programs in regions where these fungi are sparse or inappropriate. The incorporation of V A M inoculation into crop production systems is less well developed but there is increasing interest in this application (Hayman, 1982). Only when the scientific basis of mycorrhizal ecology and biology is completely established will it be possible to exploit mycorrhiza inoculation on a practical scale (Abbott and Robson, 1982; Gianinazzi et al., 1982; Powell, 1982; St. John and Coleman, 1983; Plenchette et al., 1983a,b).

1 . Factors Determining Plant Benefits from Mycorrhizal Inoculation Four major factors can determine the success of the inoculation (Hayman, 1982): the crop species involved, the size and effectiveness of the native mycorrhizal populations, the fertility of the test soil, and the cultivation practices. a. Crop Species. It is important to know which plant species derive more benefit from mycorrhizas (Pope et al., 1983). Obviously, for each plant there will be a level of available phosphate in the soil to which this plant will respond similarly whether or not it is mycorrhizal. Above this level, inoculation is not necessary. b. Test Soil Fertility. Because positive responses are to be expected mainly in low-phosphate soils and where the native endophytes are sparse or inefficient, the number and effectiveness of propagules of the family Endogonaceae and the phosphate status are usually determined in the soil where inoculation is to be tried. Olsen’s method for evaluating the available phosphate is usually used, but other methods are also applied, mainly for acid soils (Mattingly, 1980). Other factors affecting P availability, such as pH, organic matter, texture, and buffering capacity of the soil, should be also considered. c. The Size and Effectiveness of the Native Mycorrhizal Populations. The assessment of the number of VA spores and of the amount of mycorrhizal infection in roots of preceding hosts has been used as a measure of the natural


21

infectivity of a soil, but these data, because they do not give accurate information about the viability of the propagules, have not always correlated with the actual infectivity. For this reason, soil-dilution techniques have been developed to assess the most probable number of mycorrhizal propagules (i.e., spores, hyphae, or root fragments present in viable conditions) (Powell, 1980c; Wilson and Trinick, 1983). Nevertheless, it is widely assumed that is necessary not only to determine the natural infectivity of the test soil but also to estimate the effectiveness of native fungi and the advantages of the inoculation (Dodd et al., 1983; Jensen, 1983). The effectiveness of inoculated VA endophytes could be merely a consequence of a more rapid spread of the infection by increasing the inoculum level (Tinker, 1978). This is important because the demand for phosphate is higher at the early stages of plant growth. d. CulrivarionPractices. Finally, if cultivation practices (heavy fertilizer and pesticide applications, topsoil removal, long fallow periods, inadequate crop rotation, etc.), which are known to reduce the natural mycorrhizal potential of a soil have been practiced, the introduction of fresh mycorrhizal inoculum will be essential to restore the biological potential.

2 . Selection of Fungi for Inoculation Purposes As recommended by Hayman (1981), a research approach to select the best possible strains of VA endophytes should include a screening of local isolated endophytes concurrently with some strains requested from international collections. The behavior of these fungi under several phosphate regimens is relevant for selection purposes, but the interactions of the introduced endophytes with the natural soil microbiota, including other mycorrhizal propagules, must be taken into consideration. As these tests are carried out in pots, they cannot always predict the real growth effect in the field. Hence only small-scale field trials can evaluate the significance of mycorrhizal inoculation with selected endophytes (Mosse and Hayman, 1980). The selection of the best ectomycorrhizal fungus for a particular host or habitat is obviously critical also. The major selection criteria are (1) easy isolation; (2) growth rate in pure culture; (3) infectiveness; (4) effects on host growth; ( 5 ) ecological adaptation; (6) interactions with other microorganisms; and (7) host specificity (Molina, 1977). 3 . Production of Inocula

The production of a high-quality pathogen-free inoculum is actually a limiting factor for large-scale inoculation with VA mycorrhizal fungi. As these depend on association with living plant roots, the inocula used now consist of the following:

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J. M. BAREA AND C. AZCON-AGUILAR

1. Rhizosphere samples taken from stock plant cultures containing mycorrhizal root fragments, hyphae, and spores, either attached to the root or free in the rhizosphere soil. A practical scheme to produce inoculum of this type was proposed by Menge et al. (1977) and has proved to be successful for citrus inoculation. 2. Infected roots. Because clean material is preferable, stock plants must be grown on sand or liquid cultures. The infectivity of this kind of inoculum is suitable (Mosse and Hayman, 1980), and the advantages of this material are that it is cleaner and less bulky than whole-rhizosphere inoculum. The nutrient film culture technique (Elmes and Mosse, 1980; Howeler et al., 1982b) should prove to be the most useful way to obtain inoculum of this type, and the storage, viability, and longevity of the heavy infected clean roots are topics under investigation. 3. Fungal structures. Because pure fungal cultures cannot be obtained axenically, isolated resting structures produced on stock plant cultures have been tried. Resting spores are difficult to detach from the mycelia; thus they cannot be obtained in quantity and, in spite of the fact that they survive better than hyphae from infected root fragments, the spores have a lower infectivity power. These facts limit their efficiency as commercial inocula. Such problems, however, are obviated in the case of Glomus epigaeus, a VA endophyte that forms sporocarps on the soil surface. These sporocarps, which can be easily scraped off, contain many spores (Daniels and Trappe, 1979). Their commercial potential as an inoculum has been evaluated with promising results (Daniels and Menge, 1981). Four sources of ectomycorrhizal inoculum have been used (Molina, 1977): soil inoculum, mycorrhizal nurse seedlings, spores and sporocarps, and pure cultures. The last seem to be the best. In fact, pure cultures of many ECM fungi can be easily obtained for small-scale experiments, nursery beds, containerized seedlings, and other uses (Molina, 1977, 1979; Trappe, 1977). The production of massive quantities that would allow commercialization is receiving increased support, most of it devoted to the production of Pisolothus tinctorius because this fungus possesses many advantages as an inoculum (Rhodes, 1980). 4 . Inoculation Techniques

An important distinction should be made between annual and perennial crops. Annuals must be sown directly and the inocula must be introduced with the seeds or seedlings. Obviously, from the point of view of agronomy, it is crucial to ensure a subsantial early infection, and a strategic placement of the inoculum is essential. Hayman (1982) gives an exhaustive presentation on this subject. The techniques he enumerates and discusses are (a) preinoculated transplants; (b)


23

direct incorporation into seed furrows; (c) fluid drilling; (d) seed pelleting; (e) multiseeded pellets; and (f) highly infective soil. Perennial plants are usually preinoculated in containers, nursery beds, or small field plots; when the root systems are heavily mycorrhizal the seedlings can be transplanted. 5. Field Experiments

It is clear that the problems associated with VA inocula limit field experimentation on a practical scale; this would be facilitated by availability of pure fungal cultures. Field experiments have been developed in the meantime with the inocula now available. These studies are usually carried out in microplots which must be carefully selected in order to be representative of larger areas. These microplot assays are valuable because they allow us to establish some basis for a correct and successful inoculation and to predict future results when suitable inocula will be available. The completed field trials of VAM inoculation have been reviewed by Mosse and Hayman (1980), Plenchette (1982), Hayman (1982), Nemec (1983), and Menge (1983). Experiments in unsterile soil, some of them in arable fields under normal cultivation, have produced encouraging results. Among the assays performed in unsterile soil, some were conducted with cereals (Khan, 1975; Saif and Khan, 1977; Owusu-Bennoah and Mosse, 1979; Powell et al., 1980; Clarke and Mosse, 1981), potatoes (Black and Tinker, 1977), apple trees (Plenchette et al., 1981), and onions (Owusu-Bennoah and Mosse, 1979), cassava (Howeler et al., 1982a), chilli (Bagyaraj and Sreeramulu, 1982). Studies using legumes as the test crop will be treated in detail in the next part of this article when we discuss the role of mycorrhizas in the growth, nodulation, and N 'fixation of legumes.

111. MYCORRHIZAS IN LEGUMES A. INTRODUCTION

The largest contribution of biological N fixation to world agriculture is derived from the symbiosis between legumes and species of Rhizobium (Evans and Barber, 1977). The significance of this symbiosis is strengthened by the interest in legumes for food production, forage, green manure, and horticulture. The relevance of the legume-Rhizobium sp. symbiosis in agricultural and marginal environments has focused the attention of scientists from diverse disciplines

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J. M. BAREA AND C. AZCON-AGUILAR

(plant and microbial physiologists and genetists, plant breeders, and others) on a common objective, namely, the enhancement of the efficiency of symbiotic N faation [see Phillips (1980) for review]. The processes of infection and nodulation of legumes by their specific species of Rhizobium, and the biochemistry, physiology, and genetics of the N fixation, have been well documented and are clearly beyond the scope of this article (Dazzo and Hubbell, 1975; Nutman, 1977; Bergersen, 1978; Broughton, 1978; Casadesds and Olivares, 1978; DCnari6 and Truchet, 1979; Gibson and Newton, 1981; Bauer, 1981). Nevertheless, it is noteworthy that the infection mechanism of a legume root by its appropriate species of Rhizobium, particularly the process of N fixation, has a high energy requirement. The nitrogenase activity is dependent on ATP for the reduction of atmospheric dinitrogen to ammonia: approximately 21 mol of ATP are converted to ADP per mol N, reduced (Shanmugan et al., 1978). This explains why the scarcity of soluble P in soil is a critical limiting factor in the case of legumes, because it not only affects plant growth but also nodulation and N fixation (van Schreven, 1958; Andrew and Robins, 1969; Gates, 1974; Gates and Wilson, 1974). Mineral nutrients other than P, such as Zn, Cu, and Mo, may limit rhizobial growth, nodulation, or symbiotic N fixation (Demeterio et al., 1972; Robson, 1978; Munns and Mosse, 1980; Shukla and Yadav, 1982). Phosphorus and some of these minor elements, of course, may be supplied by mycorrhizal infection (Rhodes and Gerdemann, 1980). Thus, all of these circumstances account for the key role of mycorrhizas in legume production systems. B. OCCURRENCE

The coexistence of a bacterium and a fungus as root endophytes of legumes, establishing a tripartite symbiotic association, was first described by Janse (1896). Jones (1924) and Samuel (1926) found that most of the nodulated legumes they examined were also infected by mycorrhizal fungi of the VA type, but it was Asai (1944) who first reported that the nodulation of several legumes depended on the formation of mycorrhizas. This effect, however, was not attributed to the improvement of P nutrition by mycorrhizas because it was as yet unknown. Actually we know that most of the nodulated legumes so far examined are also mycorrhizal. The majority of these are of the VA type, which develops on both herbaceous and woody legumes, but in the latter the presence of ECM has also been reported (Ross and Harper, 1973; Trappe, 1979; Malloch et al., 1980; Thomazini-Casagrande, 1980; Moiraud et al., 1981; Malajczuk et al., 1981).

Two of the three subfamilies included in the family Leguminosae (i.e., Papilionoideae and Mimosoideae) are known to have VAM and rhizobial nodules.


25

The third subfamily, Cesalpinoideae, rarely forms nodules and, in spite of the fact that some members of this subfamily form endomycorrhizas, the two large groups, Amherstieae and Detariae, commonly have ECM. These two groups, with nonnodulated and ectomycorrhizal species, constitute those legumes that diversified first in the evolution of this family. As ECM have the ability to obtain nitrogen from organic sources, nodules and ECM seem to act as alternative means to supply plants with N (Malloch et al., 1980). Indeed, the nodulation of plant roots is an evolutionarily advanced character (Trappe, 1979). Vesicular-arbuscular mycorrhizas have been found in legumes growing in a wide range of habitats (Possingham et al., 1971; Ross and Harper, 1973; Khan, 1974, 1978; Crush, 1975, 1976; El-Giahmi et al., 1976; Sanni, 1976; Thomazini-Casagrande, 1980; Janos, 1980a; Pfeiffer and Bloss, 1980; Schenck and Smith, 1981; Rose, 1981; Diem e t a l . , 1981). The widespread occurrence of VAM in cultivated legumes is clearly demonstrated in the study by Strzemska (1975). The pioneering paper by Asai (1944) already showed the different degrees of mycorrhizal dependency in legume species and pointed out that species of Vicia and especially Lupinus have little dependence on mycotrophy. The case with lupines is noteworthy because these plants can get P from very low phosphate soils, although no more than 10% of their root system becomes mycorrhizal (Trinick, 1977). Lupines are able to induce abnormal VAM infections in clover (Morley and Mosse, 1976), suggesting that a certain type of antagonism toward mycorrhizal fungi may be involved. This relationship, however, remains unclear and mycorrhizal independence can be explained without the implication of fungus toxicants. An illustrative example relating mycorrhizal dependency in legumes to root morphology has been described by Crush (1974). He compared the development of mycorrhizal infection in four legume species and found that Lotus pedunculatus, which has well-developed root hairs, was able to grow well without mycorrhizal inoculation; the tropical legumes Centrosema pubescens and Stylosanthes guyanensis, which form few root hairs, exibit a strong dependence on mycorrhizas; finally, Trijolium repens holds an intermediate position with regard to both root hair production and response to mycorrhizas. C. INTERACTIONSBETWEEN SPECIESOF RHIZOBILIM, MYCORRHIZAL FUNGI, AND LEGUMES

The experiments by Asai (1944), which suggested that mycorrhizal status was a “precondition” for effective growth and nodulation of legumes, were not fully appreciated for a long time. The role of mycorrhizas in the growth, nodulation, and N fixation of legumes has been a subject of increasing interest. The published information on these topics, almost entirely concerned with VAM, is

26


summarized in this section with emphasis on some aspects of the physiology and biochemistry of these tripartite associations. The implications of these facts of legume ecology and production will be considered later. In the context of these interactions, it should be stated that the formation of VA fungal entry points and nodules on a legume root occurs simultaneously, usually within a few days after seed or seedling inoculation, and it appears that the two endophytes do not compete for infection sites (Smith and Bowen, 1979). Legume nodules are usually not invaded by the VAM fungus (Lanowska, 1966; Crush, 1974; Mosse, 1975; Smith et af., 1979). I . Signijkance of Dual Symbioses in Legume Growth and Nutrition

From the point of view of plant ecology and crop production, legumes are undoubtly a special case because they can be supplied with the two major nutrients, P and N, by naturally existing biological systems. In fact, as a consequence of the simultaneous infection with Rhizobium spp. and mycorrhizal fungi, legumes can receive growth benefits because of improved P and N supplies and also those resulting from the N-P interactions (Munns and Mosse, 1980). The double symbioses in legumes not only reduce the inputs of synthetic fertilizers, thereby saving energy, but they also appear to reduce the cost of the system itself in terms of photosynthate drain (Bevege et al., 1975; Pang and Paul, 1980; Kucey and Paul, 1982). Studying the distribution of I4CO, carbon fixed by Vicia fuba, Kucey and Paul (1982) showed that mycorrhizal fungi utilized -4% of the carbon, whereas nodules used 6% of the carbon fixed by the nonmycorrhizal beans and 12% of that fixed by the mycorrhizal plants. In spite of this, as the rates of CO, fixation for the symbiotic host were higher than for the nonsymbiotic plants, it appears that the host legume compensates for the carbon drained to the endophytes. At early stages of mycorrhizal and rhizobial infection the carbon drain, not yet compensated for, may induce a transitory negative growth response (see Smith, 1980). Mycorrhizal and nodulate plants usually have a lower root/shoot ratio than plants inoculated with either symbiont alone (for examples see Daft and El-Giahmi, 1974; Asimi et af., 1980; Redente and Reeves, 1981). This is a typical response to improved mineral nutrition, as discussed by Smith (1980, 1982) after the experimental testing of the inflow of phosphate into mycorrhizal clover. Moreover, it appears that dual inoculation with a suitable species of Rhizobium and mycorrhizal fungi not only enhances the nutrient content in the aboveground plant material but also seems to provide a nutrient supply that is well balanced. Consequently, the biosynthetic processes taking place in these adequately established legume-Rhizobium sp.-mycorrhiza associations can lead


27

to an improvement in the production of seed and/or foliar proteins. This is the beginning of a trophic chain of great relevance in animal alimentation. The role of mycorrhizal infection in legume production was established in studies carried out in the 1970s and early 1980s. These studies using nodulated soybeans were of particular importance for our understanding of this subject. The experimental assays by Ross and Harper (1970) and Ross (1971) demonstrated that mycorrhizal inoculation increased yield, and that mycorrhizal plants contained a higher N concentration and content in their tops than nonmycorrhizal controls. Further, mycorrhizal infection induced an improvement (53%) in the N content in shoots and seeds of nodulated plants, but the effect of mycorrhizal inoculation on a nonnodulating isoline of soybean was not significant Schenck and Hinson (1973). These results reinforced the connection of mycorrhizal effects with an enhancement of nodule function. 2. Mycorrhizal Effect on Nodulation and Nitrogen Fixation The effect of mycorrhizas in stimulating nodulation was reaffirmed in a study (crush, 1974) that also produced more consistent evidence of the mycorrhizal stimulation of N fixation by legume-Rhizobium sp. systems as measured by the acetylene reduction technique. More advanced information on this subject is available (Daft and El-Giahmi, 1974, 1975, 1976). These authors found that several parameters directly related to N-fixation processes in species of Phaseolus, Medicago, and Arachis were affected by mycorrhizal infection. In fact, the amount of nodular tissue, the concentration of legume hemoglobin, and the rates of acetylene reduction were greater in mycorrhizal and nodulated plants than in the nonmycorrhizal but nodulated controls. Using the 15N, tracer technique, Kucey and Paul (1982) confirmed that mycorrhizal and nodulated plants (faba beans) fix more N than those nodulated but nonmycorrhizal. This was attributed to an increase in nodule biomass as induced by mycorrhizal inoculation. As a consequence of these mycorrhizal effects on nodulated legumes, increases in fruit yield, plant growth, and nutrient content of shoots, roots, and seeds were recorded. Early observations on this subject suggested several approaches to the elucidation of physiological aspects of the legume-Rhizobium sp.-mycorrhiza interactions. One of these tried to ascertain whether mycorrhizas enhance symbiotic N fixation only through the stimulation of host-plant nutrition, or whether they also exert a more direct effect on nodulation and nitrogenase activity. The existence of such a direct availability of P to the nodules by mycorrhizal hyphae does not preclude, however, the importance of a suitable P nutrition, as achieved by mycorrhizal inoculation, of the host as a condition for effective symbiotic N fixation. This is not only because of the role of the host as a partner in the association as concerns the expression of the activity (N fixation), but also

28

J. M.BAREA AND

c. AZCON-AGUILAR

because the nodules are actually part of the plant. If the plant is well nourished, the nodules also will receive a suitable P supply for their function. The logical result, therefore, is the existence of a close relationship between host nutritional status and nodule formation. For example, Mosse et ul. (1976) found that plants did not nodulate unless their P concentrations were at least 0.15%; mycorrhizal infection helped the plants to reach this required level, and nodulation then occurred. The conclusions of Abbott and Robson (1977) and the further elaborations of Robson et ul. (1981) also support the idea that the effect of VAM on nodulation and N fixation in subterranean clover closely parallels responses in growth and nutrition. These authors assume that mycorrhizal effects on nodulation take place through host nutrition and that these occur at the same time as the growth responses. In contrast, Smith and Daft (1977) reported that mycorrhiza-induced increases in N fixation rates in Medicago sufivu preceded any effect on plant growth. This suggested the idea that nodules demand phosphate first. At all phosphate additions tested, Smith and Daft (1977) found that mycorrhizal plants had higher values of %N than the nonmycorrhizal controls and mycorrhizal plants of the same size (dry weight and root-to-shoot ratio) higher than that of the nonmycorrhizal controls. Studies comparing matched plants confirmed that VAM increases the rates of N fixation (Smith and Daft, 1978). Moreover, nitrogenase activity in Pueruriu sp. still increased when the growth phosphate response curve became asymptotic (Waidyanatha et ul., 1979). The latter papers therefore support the suggestion that nodule function may be preferentially stimulated by mycorrhizal infection, which makes phosphate directly available to the nodules. Smith et al. (1979) then used time-course experiments to elucidate the development of interactions between the components of these tripartite symbioses, and c o n f i i e d that the mycorrhizal effect on nodulation, nitrogenase activity, and nodule efficiency occurrs before any positive growth response to VAM in a lownutrient soil but not in soil more fertile. This indicates that the “special” demand for P of nodular tissues apparently results when the P supply is a limiting factor, but this is a common situation in most unamended soils. Smith et al. (1979) also reported the mycorrhizal clover roots had a higher P concentration than did the nonmycorrhizal control plants. Because they did not find mycorrhiza-induced increases of P concentration in the nodules, the enhancement of nodule efficiency could be explained by the increases in root P, which accumulated mainly as polyphosphate. These facts seem of great relevance; as stated by these authors “a steady supply of P to root cells, and to adjacent nodules, as such as would be available from continuous polyphosphate conversion at the fungus-root interface, would be stimulating to the development of effective nodules symbiosis and associated N-fixation’’. The stimulation of nitrogenase activity by VAM in the system soybean-


29

Rhizobiumjaponicum was also evident before the plant growth responses (Asimi el al., 1980). This also suggests “a particular sensitivity” of Rhizobium to mycorrhizal effect and confirms the role of VAM to satisfy the high P demand for the processes of nodulation and N-fixation. Moreover, increased phosphate additions first eliminated mycorrhizal effects on growth and then, progressively, those on nodulation and nitrogenase activity (Asimi et al., 1980). This gives additional indirect evidence that plant growth and nodule functioning show differential responses to and demand for P, as derived from their different P dependencies. Because there is no contact between the mycorrhizal fungi and Rhizobium bacteroids, any P supply must pass through the host cells. The role played by the VAM-specific phosphatases in the arbuscles developed inside root cells adjacent to nodules was suggested as being of great significance in phosphate transfer to bacteroids (Asimi et al., 1978; 1980). Time-course experiments using 32P and autoradigraphic techniques would be of interest to clearly define the statement that the phosphate apparently is made directly available in some way, to the bacteroids in legume nodules. Another question on the mycorrhizal effects on symbiotic N fixation is whether these are exclusively phosphate-mediated or, conversely, if mycorrhizas have any implications in addition to those resulting from the improved P supply. Carling et al. (1978) found that soluble phosphate can replace VAM in increasing nitrogenase and nitrate reductase activities in nodulated soybeans. Thus they suggest that P is almost exclusively responsible for the mycorrhizal effects and that neither soybean endophyte interacted directly, at least in their experimental conditions. In spite of this, some responses to mycorrhizal inoculation in legumes reported in other situations are difficult to explain on the basis of improved P supply alone (Munns and Mosse, 1980). The increased mycorrhizal uptake of water and elements other than P, which is related to the infectivity and/or effectiveness of legume-Rhizobium sp. systems, may be also involved (Safir et al., 1972; Gray and Gerdemann, 1973; Mojallah and Weed, 1978; Munns and Mosse, 1980). 3. Nonnutritional Interactions

Although the main reasons for the cooperation between VAM fungi and Rhizobium have a nutritional basis, other secondary, nonnutritional effects of the symbiosis have also been suggested (Mosse, 1977b). Further experimental studies seem to support this idea; both types of microorganisms appear to interact, whether at inhabitation of the rhizosphere, at the formation of the symbioses, in the development of these, or even in their effects on plant physiology. As is commonly accepted, soil microorganisms can stimulate the mycelial growth of mycorrhizal fungi and their penetration into susceptible roots of the

30


infective hyphae. The basis of such interactions and the suggested mechanisms of the microbial activity are the following: a. Production of CompoundsIncreasing Cell Permeability (CIPC). Common rhizosphere microorganisms are able to increase losses of microbial substrates from living roots, and the local production of CIPC by these microorganisms could result from these effects (Bowen, 1980a). These compounds, by relaxing root cell walls, would increase the rates of root exudation, and this could stimulate mycorrhizal fungi in the rhizosphere and facilitate root penetration. The increased leakage of substrates from pine roots caused by species of Bacillus, has been suggested as a cause of the enhancing effect that these bacteria exert on ECM formation (Bowen and Theodorou, 1979). Because extracellular p l ysaccharides (EPS) from Rhizobium spp. behave as CIPC, their role in VAM infection in legumes has been studied, indicating that EPS from Rhizobium meliloti enhance VAM formation on Medicago sativa (Azc6n-Aguilar et al., 1980). The EPS could act in the same way proposed for Rhizobium spp. according to the polygalacturonase hypothesis (Ljunggren and Fahraeus, 1959), hence improving the formation of VAM symbiosis through the establishment of entry points. Alternatively, EPS could act merely by increasing root exudation (Olivares et al., 1977), thereby favoring the development of the preinfection phase of the VAM infection. These results agree with an early observation by Mosse (1 962) indicating that in axenic conditions the fungus failed to penetrate clover roots unless a soil microorganism, a species of Pseudomonas, was also present. This bacterium possesses pectolytic activity, and the author suggested that the most probable explanation is that the bacterial compounds act on the structure of the cell wall, thereby affecting its plasticity. This also can affect the susceptibility of the plant root to fungal infection. b. Production of Plant-Growth-Regulating Substances. Many microorganisms isolated from the rhizosphere, and particularly species of Rhizobium, are able to produce substances with phytohormonal activity. Because this ability seems of interest in rhizosphere biology (Brown, 1975), considerable attention has been paid to its possible influence on mycorrhizal infection. The role of plant hormones (PH), mainly auxins, in the formation of sheathing mycorrhizas is already well established and, to some extent, the stimulation exerted by certain soil microorganisms on ECM infection appears to be through this mechanism (Slankis, 1974). The effect of plant hormones on the formation of VAM in M . sativa has been the subject of some studies. Infection levels in mycorrhizal plants were compared after treatment with pure substances and preparations from cultures of R. meliloti, which are known to contain auxins, giberellins, and cytokinins. Cell-free supernatants from the R. meliloti cultures tested increased VAM infection in M . sativa to an extent similar to that of the pure plant hormones applied in doses similar to those of the supernatants (Azc6n et al., 1978a; Azc6n-Aguilar and Barea, 1978).

MYCOFSHIZAS IN NODULATING N-FIXING PLANTS

31

The morphological and physiological changes that plant hormones can induce in the host plant may favor the establishment of VA symbiosis and its activity, thus leading to a greater rate of nutrient absorption by the plant (Azc6n et al., 1978a). In fact, it is known that gibberellins increase leaf area and the development of lateral roots, that cytokinins are involved in many basic processes of plant growth, including improvement of photosynthetic rate, and that auxins control root formation and increase the elasticity of the cell wall (Torrey, 1976; Thimann, 1977; Tien et al., 1979). All of these activities can affect the formation or effectiveness of VAM. The hormonal interactions in the rhizospheres of legumes seem more complicated as VAM fungi (Barea and Azc6n-Aguilar, 1980, 1982b) are also able to produce PH. These substances can be involved in the mycorrhizal effects. For example, Mosse (1962) found that VAM stimulated branching of infected roots (a typical hormonal effect), and Allen et al. (1980, 1982) demonstrated that VAM infection increases the hormonal level in the host plants. These facts could be important because PH play a role in the infection mechanism of legume roots by Rhizobium spp. (Nutman, 1977). Because mutualistic symbioses involving plants and microorganisms depend, both for their formation and for their function, on a series of interactions between the constituent partners, PH synthesized either by the host or by the endophytes appears to be involved in the establishment and development of these biotrophic associations. Another point of interest for future research is the fact that the exudates of mycorrhizal roots are probably different both quantitatively and qualitatively from those of nonmycorrhizal roots. This will induce changes in the rhizosphere that might affect the development of Rhizobium spp. (Mosse, 1977b).

D. INTERACTIONS BETWEEN ADDEDFERTILJZERS AND MYCORRHIZAS IN LEGUME-Rhizobium Sp. Systems

Although the nonnutritional interactions just discussed may act in some way on the formation and development of the tripartite symbioses, the protagonism of phosphate-mediated mycorrhizal effects on N fixation in legumes is obvious. As can be deduced from the universally accepted role and mode of action of external hyphae in taking up phosphate ions from solution in the soil, mycorrhizas not only enlarge the zones around roots that are depleted of phosphate, but also cause these zones to be more greatly depleted of this plant nutrient. Consequently this produces an impoverishment of the soil after several harvests. The phosphate stock must then be restored, which can be accomplished either by applying soluble phosphate fertilizers or, in some circumstances, by using less expensive, sparingly soluble forms of P. The interactions of these compounds with mycor-

32


rhizal fungi in the development of legumes have been the subject of several studies of great interest because of their basic and applied implications. 1. Effect of Soluble P Additions

It has been pointed out that mycorrhizal inoculation experiments should include testing of the interactions of VAM fungi at a series of phosphate levels (Abbott and Robson, 1977; Hall, 1978; Powell, 1980a) in order to select the P doses optimal for the mycorrhizal effects. Several greenhouse experiments on the phosphate response curves of mycorrhizal and nonmycorrhizal Rhizobiwn-inoculated legumes were carried out, mostly using clovers (Trifolium sp.) as the test plant (Crush, 1976; Abott and Robson, 1977; Hall et al., 1977; Sparling and Tinker, 1978; Powell, 1980a; Pairunan et al., 1980). These authors applied ranges of soluble phosphate fertilizers at rates equivalent to about 0-300 kg P/ha and have reached a the general conclusion that mycorrhizas markedly increase P uptake, growth, and nodulation in clover at low and intermediate rates of applied P, although plant growth depressions may occur at high levels of available P (Crush, 1976). Large P additions to the soil are known to decrease mycorrhizal infection in several legumes (Abbott and Robson, 1977; Powell and Daniels, 1978; Barea et al., 1980; Pairunan et al., 1980; Asimi et al., 1980; Powell, 1980b; Nielsen and Jensen, 1983; Bethlenfalvay, 1983). This could lead to host immunity to infection. However, certain phosphate additions which reduce the percentage of root lengh infected by VAM do not affect the length of mycorrhizal root per plant. This situation has been observed by Asimi et al. (1980) (soybean) and by Smith (1982) (clover), and it can be explained on the basis of a simultaneous rapid growth of roots at increasing levels of soluble phosphate. From the practical point of view, however, the results of the interrelationships between phosphate additions and VAM on legume-Rhizobium sp. systems are not always predictable and generalizable, because the responses are modulated by the incidence of several factors. These include the characteristics of the soil, the plant species cultivars or lines, the endophyte species involved, and, finally, the interactions between these factors. a. Characteristics ofthe Soil. It is to be expected that the effect of increasing additions of phosphate will depend not only on the doses applied but also on the ability of the test soil to retain a greater or lesser amount of the added phosphate. The period of contact between the applied fertilizer and the soil before planting also influences the P available in a given soil and the resultant mycorrhizal response (see Barrow et al., 1977). Obviously these factors will affect the size of the labile phosphate pool which is known to limit mycorrhizal activity. For legumes, Powell (1980a) demonstrated that the interactions of increasing phosphate additions with mycorrhizal inoculation of white clover depended on the P


33

retention capacity of the soil tested. The magnitude of the mycorrhizal effects in stimulating the growth and nodulation of Leucaema leucocephala was also found to vary with the test soil (Munns and Mosse, 1980). Plant response to mycorrhizas in different soils is not always related to soil P content (see Stribley et al., 1980). This could explain why the mycorrhizal infection level in Phaseolus sp. was independent of the level of N and P in the field soils tested (Hayman et a l . , 1976), indicating the importance of the previous fertilizer history of the soil in affecting the development of adaptative strategies of the extant native endophytes against the soil amendments. b. Legume Species. It is well known that nonmycorrhizal plant species, cultivars in species, and even clones in cultivars can differ in their P uptake capacity. This differential behavior is related to the degree of their mycorrhizal dependency and could condition changes in the patterns of the phosphate response curves of these plants, whether or not mycorrhizal. Some studies with legumes have been devoted to this subject. Differences between species have been clearly shown; for example, lotus plants become nodulated independently of mycorrhizal inoculation in soil where clover plants show low nodulation unless mycorrhizal (Crush, 1974). Because lotus needs less P than clover to nodulate (Gibson et al., 1975), these plant species differ in their phosphate response curves (Hart et al., 1981). The combined effects of increasing soil P levels and VAM on plant growth and nodulation were compared in two systems, M . sativa-R. meliloti and Hedysarum coronarium-Rhizobium sp., developing in the same soil. In the case of M . sativa, mycorrhizal plants grew and nodulated significantly better than nonmycorrhizal ones in unamended soil and at intermediate P additions. Higher doses eliminated the mycorrhizal effect on plant growth and nodulation and became supraoptimal for these processes. In the case of H . coronarium, mycorrhizal inoculation also significantly improved plant growth and nodulation at low phosphate additions. However, the response differed from that of M . sativa in that high phosphate levels did not become supraoptimal for growth and nodulation and the response curve to added phosphate became asymptotic. An explanation for the behavior of H . coronarium could be that this legume forms a sort of modified root that accumulates large amounts of Ca (J. Yaiiez, personal communication). These “roots” can then retain the phosphate surplus so that the shoots do not reach P concentrations supraoptimal for growth. Variations in response to VAM have been reported for cultivars of clover (Hall et al., 1977), soybean (Skipper and Smith, 1979), and alfalfa (Lambert et al., 1980a). However, Hall et al. (1977) only found VAM-clover cultivar interactions at the lowest levels of phosphate. The lack of interaction in clovers at higher P additions was also reported by Crush and Caradus (1980). Conversely, Lambert et al. (1980a) even found significant interactions between lines in the alfalfa cultivars, VAM, and the P level.

34

J. M. BAREA AND

c. AZCON-AGUILAR

c. Endophyte Species. Mycorrhizal endophytes are known to differ in their relative efficiences in stimulating growth nodulation and N fixation in legumes. The variation in the efficiency among mycorrhizal fungi has been studied in an attempt to find correlations among several factors including the rate of establishing the mycorrhiza, the number of entry points per unit of root length, the extent of mycorrhizal infection, the degree of enhancement of phosphate recovery from the soil by the plant, and their condition of native or introduced (Abbott and Robson, 1977, 1978, 1981a,b; Mosse, 1977a; Powell, 1980a; O’Bannon et al., 1980; Smith and Smith, 1981b). Although all of these factors are important, there is no close generalizable correlation between any of these parameters and the relative symbiotic efficiency of the endophyte species (Munns and Mosse, 1980). d. Interactions between the Factors Enumerated Previously. Interactions between the prevailing ecological factors affect mycorrhizal activity more than individual factors alone. For instance, the relative efficiency of several VAM fungal species on legume development was demonstrated to depend, in turn, on interactions with the soil and the amount of available P (Powell, 1977a; Mosse, 1977a; Sparling and Tinker, 1978; Powell and Daniels, 1978; Carling and Brown, 1980; Lambert et al., 1980a). Because of the several types of interactions actually operating, it is difficult to extrapolate conclusions from one study to another, and so Carling and Brown (1980) stated that “each system appears to be unique, and each must be evaluated experimentally before the question pertaining to effectivity of a specific VA mycorrhizal fungus in that system can be answered.”

2. EfSect of Rock Phosphate There is some evidence that legumes benefit from VAM in the presence of insoluble phosphates (Ross and Gilliam, 1973). Accordingly, it was suggested that mycorrhizal soybean plants might be able to utilize nonlabile forms of soil phosphate that nonmycorrhizal plants cannot use. However, assays in 32P-labeled soils indicate that mycorrhizal (five different endophytes, which caused a high, but differing, degree of infection in clover roots) and nonmycorrhizal clover plants take up P from the same source, as expected (Powell, 1975). As is known, VAM achieves a better exploitation of the sparingly soluble phosphate because hyphae make a closer contact than roots can with phosphate particles where the soluble ions are being chemically (or biochemically) dissociated. Therefore, the utilization of a nonlabile phosphate source by a mycorrhiza occurs on the condition that, at least slowly, a liberation of some phosphate ions takes place. Undoubtedly, rock phosphate (RP) has been the most utilized source of sparingly soluble fertilizer for the study of restoring soil phosphate. General conclu-

35

MYCORRHIZAS IN NODULATINC N-FIXING PLANTS

sions indicate that in acid soils RP can improve growth, nodulation, and N fixation in nonmycorrhizal legumes, and that inoculation with appropriate VA endophytes greatly enhances its utilization. On the other hand, in near-neutral and alkaline soils RP remains unavailable for both mycorrhizal and nonmycorrhizal legumes (Mosse et al., 1976; Mosse, 1977a,b; Powell and Daniels, 1978; Sparling and Tinker, 1978; Waidyanatha et al., 1979; Delorenzini et al., 1979; Barea et al., 1980; Islam et al., 1980; Munns and Mosse, 1980). These ideas are summarized in Table 11. VAM infection can increase plant growth and nodulation of legumes even when these are growing on neutral and alkaline soils with added RP. This indicates that this kind of phosphate fertilizer might be suitable to maintain the stock of phosphate in a soil; in addition, it does not reduce the level of mycorrhizal infections as does soluble P (Barea et al., 1980). Because VAM plants also take up their P from the plant-available phosphate fraction, the pool of labile P that is restored by chemical dissociation of phosphate ions from RP, it might be speculated that this could be a useful substrate for P uptake even in high pH soils. Furthermore, using a range of nonacidic soils, a situation was found in which clover plants, either Glomus sp.-inoculated plants or the uninoculated controls, were able to use RP. In this case the soil differed in a biological property; the number of phosphate-solubilizing bacteria (PSB) able to dissolve the RP present in it was significantly higher than in the other soils. It is also noteworthy that the number of PSB was stimulated in the root zone of the Glomus sp.-inoculated plants (Barea et al., 1981). A great number of microorganisms can release phosphate ions from sparingly insoluble inorganic and organic phosphate, as deduced from assays carried out in vitro (Greaves and Webley, 1965; Tardieux-Roche, 1966; Barea et al., 1970). However, the effectiveness of these microorganisms either in the untreated soil or when massively inoculated into the soil is doubtful. There are, in fact, some

Table II Interactions between Rock Phosphate (W) and Mycorrhizal Inoculation (VAM)in the Growth and Nodulation of Legumes as Affected by Soil pH Soil reaction0 Treatment

Acid

RP versus no treatment VAM X RF' versus VAM VAM x RP versus RP

+ + +

"+, Significant positive effects on growth and nodulation; -, nonsignificant effects.

Neutral or alkaline

36

J. M. BAREA AND

c. AZC~N-AGUILAR

problems that make an efficient solubilization of phosphates in soil difficult. These difficultiesare inherent in the scarcity of available energy sources and with problems in the translocation to the root surface of any available solubilized phosphate ions (Hayman, 1975a; Tinker and Sanders, 1975). Nevertheless, it was hyphothesized (Barea et al., 1975) that if inoculated PSB could solubilize some phosphate ions, these would be taken up by the mycorrhizal hyphae, thus avoiding refixation problems in the translocation of phosphate to the absorption places at the root surface. Thus, a synergistic interaction between both types of microorganisms was suggested. A series of experiments using clover and other test plants (Azc6n et al., 1976; Delorenzini et al., 1979; Barea et al., 1981) indicated the feasibility of this hypothesis. As mycorrhizal plants can explore microhabitats in nonrhizosphere soil, these plants would gain more benefit from the presumed activity of PSB on RP, as shown in Fig. 1. This possibility is supported by experiments using 32P(Raj et al., 1981). A synergistic cooperation between these symbiotic and asymbiotic microorganisms was also evidenced by the utilization of organic phosphates in volcanic ash-derived soils (Borie and Barea 1981).

Slow diffusion

.:;IS

FIG. 1. Interaction of Va mycorrhiza and phosphate-solubilizing bacteria (PSB). RP, rock phosphate.


37

3. Effect of N Fertilizer Because they are not dependent on combined N, legumes do not usually need N fertilizers when they are adequately nodulated. Moreover, these compounds are deleterious for nodulation and N fixation (Gibson and Newton, 1981; Dazzo and Brill, 1978). There is an obvious scarcity of information about interactions between these fertilizers and other treatments, such as mycorrhizal inoculation, on legume-Rhizobium sp. associations. VAM infection (in addition to nodulation) is suppressed by N fertilizers in Pisum sutivum (Lanoswska, 1966) and Trifoliurn substerruneum (Chambers et ul., 1980a,b). Additions of NH4+ were more deleterious than additions of NO,- (Chambers et al., 1980a), which can be explained on the basis of their different pathways of initial assimilation, cation carboxylate storage, and pH regulation (Raven and Smith, 1976; Smith, 1980). It follows that N fertilizer application must be rather limited for the correct development of the tripartite legume-Rhizobium sp. -VAM symbiosis. 4. Effect of Other Fertilizers

Information about interactions between VAM and additions of other nutrients for which nodulated legumes have special demands (Munns and Mosse, 1980) is still scant. Boron (B), one of these nutrients, was investigated for its interactions with VAM in T. prutense and M. sutivu (Lambert et al., 1980b). This study showed that an adequate B supply increases mycorrhizal activity. Boron deficiency in particular delayed the onset of VAM infection. This can affect the mycorrhizal effect on nodulation; however, the experimental design of these assays did not allow assessment of the direct effect of B supply on rhizobia.

E. ECOLOGICAL SIGNIFICANCE OF VESICULAR-ARBUSCULAR MYCORRHIZAS IN LEGUMES

It has been snown repeatedly that plants bearing dual mutualistic symbioses, such as nodulated and mycorrhizal legumes, may be well adapted to habitats with low availability of both N and P (Harley, 1970, 1973). Consequently these symbioses enable the plants to play an extremely important role as pioneer colonizers of nutrient-deficient soils (Janos, 1980b). The ecological and evolutionary significance of such symbioses in the history of terrestrial plants has been already discussed (Malloch et ul., 1980). Today mycorrhizal legumes have similar advantages for colonizing and thriving in adverse situations of different types.

38

J. M. BAREA AND

c. AZC~N-AGUILAR

I . Establishment and Improvement of Pastures in Marginal Soils As the production of pastures, especially when including legumes, is often limited by the low level of available P in the soil, the potential for managing VAM as a strategy to improve productivity in tropical dryland was demonstrated by Jehne (1980). Similarly, a positive effect of VAM in stimulating clover development related to hill-country soil improvement has been already demonstrated (Powell, 1976b; Hayman, 1977). Mycorrhizal fungi help the introduction of forage legumes in new habitats (see Redente and Reeves, 1981; Azc6nAguilar et al., 1982).

2 . Revegetation of Strip Mines and Other Industrial Wastelands Some mine spoils do not contain topsoil, and because they are not vegetated, they usually lack VAM fungal propagules. The establishment and survival of forage legumes (Lathyrus silvestris, Coronilla varia, and Lotus corniculatus) was aided significantly by mycorrhizal inoculation (Lambert and Cole, 1980), indicating the importance of VAM for revegetation purposes in these soils. Actually, plants colonizing coal wastes are mycorrhizal, and the legume species are also nodulated (Schramm, 1966; Daft et al., 1975; Daft and Hacskaylo, 1976; Khan, 1978). Thus these authors support the significance of mycorrhizal and nodulated plants in the rehabilitation of these wastelands into stable plant communities. In fact, the presence of both symbioses in Acacia hofosericeaand other woody and herbaceous legumes growing in restored areas after surface mining has been demonstrated (Langkamp and Dalling, 1982). These authors suggested that such symbiotic associations are a prerequisite for the successful establishment of long-term vegetation on these sites.

3 . Eroded Soil Reclamation The loss of topsoil and vegetation is an obvious consequence of erosion; therefore, eroded soils tend to be depleted of VAM propagules. The reintroduction of these fungi helped the growth and survival of white clover (Powell, 198Oc) and lotus (Hall and Armstrong, 1979) in such soils. The extensive network of fungal hyphae associated with white clover has been reported to be involved in the improvement of soil structure, thereby reducing soil erosion (Tisdall and Oades, 1979). 4 . Sand Dunes Stabilization Programs

Legumes are usually included in the lists of dominant plant species growing in sand dunes (Koske and Halvorson, 1981; Koske, 1981). Because mycorrhizal

MYCORRHIZAS IN NODULA'MNG N-FIXING PLANTS

39

beans proved to be suitable for sand aggregation by binding sand grains to the extensive VAM mycelium (Koske et al., 1975), the possibility of using legumes for dune stabilization seems attractive.

5 . Legumes in Arid and Semiarid Soils The mycorrhizal condition appears especially relevant for legumes growing in those habitats where pasture productivity is limited to a large extent by water availability (Jehne, 1980; Diem et a f . , 1981; Rose, 1981; Trappe, 1981). Because soil water content also affects P availability, the effect of mycorrhizas as related to water-use efficiency seems to be a key factor in pasture productivity in arid and semiarid regions (Safir, 1981). As prompt revegetation of such sites is needed, the use of mycorrhizal legumes that make such processes independent of N and P inputs is an appropriate choice.

6. Tolerance of Legumes to Other Stress Situations Published reports indicate that mycorrhizal plants have a greater tolerance of salinity, low pH, and high soil temperature than their nonmycorrhizal counterparts (Jehne, 1980; Bowen, 1980b). The yield of white clover in a low-pH (4.5) soil was significantly improved by mycorrhizal inoculation (Lambert and Cole, 1980), whereas the growth of noninoculated controls was poor.

7 . Role of VAM in Mixed Cropping Including Legumes The practice of growing legume and nonlegume (grasses) mixtures for pasture production is important economically. The agronomic events associated with grass-legume interactions have been reviewed (Haynes, 1980). Correct balances between species are important for maintaining pasture productivity, because grasses commonly have a competitive advantage over legumes, and many factors have been studied to obtain equilibria. One of these is mycorrhizal infection. Certainly VAM markedly increase the ability of clover to compete against ryegrass (Crush, 1974; Hall, 1978; Buwalda, 1980), because clover is more mycotrophic than the grass. Mycorrhizas also have other implications for plants growing together in the sward. These derive from the fact that the network of VA mycelium is able to link one plant to another. These mycorrhizal conections play an important role in the transport of P between plants (Heap and Newman, 1980; Whittingham and Read, 1982). As legumes also bring N to companion plants in the sward, the role of dually symbiotic legumes in the recycling of N and P in these ecosystems is clearly relevant.

Table III Greenhouse Experiment to Assess the Feasibility of the Introduction of a V A M Fungus (Gbmus mosseae) into a Legume (Hcdysarum coromuium) Rhizosphere in Two Test S o i a Soil l b Parameter % VA infection NodulationC Shoot weight (g/plant) Root/shoot ratio

C 0 2 2.1 1.3

I 57 4

3.8 0.9

Soil

I

N

N+I

C

20 2 2.5 1 .o

74 4 4.1

0 3 4.1

4.2

0.8

0.5

0.5

“After Azcbn-Aguilar er al. (1982). bVesicular-arbuscular endophytes were either not present (C), inoculated G. msseae (I), or native (N). ‘On a scale of 0 to 4.

3 3

26

N

N+I

52 3 5.0 0.5

60 4 4.9 0.5


41

F. PRACTICAL FIELDAPPLICATION OF MYCORRHIZAL EFFECTS ON LEGUME PRODUCTION

It is already clear that the degree of efficiency with which a VAM fungus can improve plant growth and fertilizer use is affected by its ability to establish, spread, and survive when inoculated into a plant rhizosphere in a living soil. Hence, preliminary evaluation of the interactions between indigenous and introduced VA fungi, Rhizobium sp., and phosphate fertilizers will be required initially. These effects must be assessed under conditions that are as similar as possible to those prevailing in natural habitats where the field study will be performed. Tests to determine the feasibility of a field inoculation have been proposed, studies to estimate the rate of spread and persistence of mycorrhizal effects are being developed, and a series of small-scale field trials have been carried out. These utilize a range of VA endophytes and legume-Rhizobium sp. systems.

1, Tests for the Assessment of Field Situations in Which VAM inoculation of Legume Crops May Be Feasible A rapid method for assessing the effectiveness of native endophytes under conditions most similar to those governing their natural environments was conceived by Mosse (1977a) to predict whether field inoculation would be rewarding. Powell (1977a) studied a technique using Trifoliurn repens as the test plant and soil cores taken from field plots and kept in pots in the greenhouse. The use of such soil cores has many advantages as they represent undisturbed soil profiles. Another technique with a similar purpose is being developed and seems to be useful for defining the type of interaction (additive, synergistic, or antagonistic) occurring between native and introduced endophytes (Barea et al., 1980) and to predict the feasibility of field inoculation (Azc6n-Aguilar et al., 1982). This technique compares the effect of inoculation in unsterile and steamed aliquots of test soil. The microbiota (except mycorrhizal propagules) was reinoculated into the treated soil in order to get a suitable control (see Smith and Smith, 1981a). Data summarized in Table I11 suggest that VAM inoculation may be rewarding in Soil 1 (5 ppm Olsen P) but not in Soil 2 (33 ppm Olsen P). In the former, it appears that an additive cooperation between native and introduced VA endophytes occurs. The subsequent field experiment on Soil 1 proved to be successful (Azc6n-Aguilar et al., 1982).

2 . Studies to Determine the Spread and Persistence of Mycorrhizal Effects Because the production of sufficient inoculum limits large-scale inoculation, it is of interest to ensure a more efficient use of the available inocula and prevent

42


the application of excessive amounts. Thus it is important to follow a suitable strategy for an adequate placement of inocula and to know their rate of spread and the residual growth effects in the following years (Mosse and Hayman, 1980). Studies by Powell (1979) and Mosse et ul. (1982) indicate that VA fungi are able to spread to about 4 m from the inoculation points in unsterile field soils. Mosse et u1. (1982) further pointed out a residual growth effect on M . sutivu used as the following year’s crop. Persistence of the mycorrhizal effect on alfalfa was also found in serial cutting of this crop under field conditions (Azc6n-Aguilar and Barea, 1981). 3 . Field Inoculation Experiments

The reports available on field trials of VAM inoculation of legumes for agricultural and revegetation purposes are recorded in Table IV. The published experiments show that efficient mycorrhizal fungi can be introduced into the rhizosphere of legumes growing in the field. This results in an improvement in Table IV Field Experiments on Inoculation of Legumes with VAM Fungi under Natural Conditions in Nollrumigated Soils Crop White clover

Red clover Faba beans Lucerne (alfalfa)

Inoculation technique“ Preinoculation transplants and direct incorporation into seed furrows Preinoculation transplants Seed pelleting Preinoculation transplants Preinoculation transplants Preinoculation transplants Direct incorporation into seed furrows Several Highly infective soil Preinoculation transplants Direct incorporation into seed furrows

Direct incorporation into seed furrows Direct incorporation into seed furrows Peas Direct incorporation into seed furrows soybean Direct incorporation into seed furrows Cowpea Preinoculation transplants Hedysarum coronarium Direct incorporation into seed furrows Lorn sp. Seed pelleting Highly infective soil

Reference Powell (1977b) Powell and Daniels (1978) Powell (1979) Powell (1982) Hayman and Mosse (1979) Hayman er al. (1979) Rangeley er al. (1982) Hayman ef al. (1981) Kucey and Paul (1983) Azc6n-Aguilar et al. (1979) Owusu-Bennoah and Mosse ( 1979) Azc6n-Aguilar and Barea (1981) Mosse et al. (1982) Jakobsen and Nielsen (1983) Bagyaraj et al. (1979) Islam et al. (1980) Azcbn-Aguilar et al. (1982) Hall (1980) Lambert and Cole ( 1980p

“See Section II,D,4 for details on inoculation techniques. forage legumes (of the genera Latyrus and Coronilla) were also inoculated in this study.


43

the growth and nodulation of these plants. Grain on forage legumes growing under conditions of tropical and temperate regions that have been assayed indicate that they benefit from mycorrhizal inoculation. Most of the tests recorded in Table IV gave positive responses to VAM. There are indications that the introduced VAM fungi swiftly become established, as deduced from the plant response, but sometimes the establishment was actually ascertained because of differences in the anatomical characteristics of the infections produced by introduced and indigenous endophytes. Inoculation experiments with M . sativa and its symbiotic partners R. melitoti and the VA fungus G. mosseae, carried out under agricultural conditions in untreated arable soil, illustrate the interactions in the tripartite symbiosis (Azc6nAguilar et al., 1979; Azc6n-Aguilar and Barea, 1981). These experiments were designed for the same soil but in plots that had supported different agronomic practices and which differed from one another in some characteristics; plot B possessed twice as much available phosphate and three times as much VA propagules as plot A. In the latter, inoculation with G. mosseae was always effective in promoting plant growth, but R. meliloti was only able to enhance the growth of G. mosseae-inoculated plants. Probably P was the limiting factor for R. meliloti activity, because of the scarcity of available nutrient and the low number of spores of the family Endogonaceae in this test soil (A). Hence, plants did not repond to the R. meliloti inoculation unless they were also inoculated with VA fungi. In contrast, R. meliloti was effective when inoculated alone in plants growing in plot B. The efficiency of indigenous VA fungi, together with the higher concentration of available P in the soil, could cause plants to respond to R. melitoti inoculated alone. In both cases, however, inoculation of R. meliloti plus G . mosseae more than doubled the yield compared to the uninoculated controls. In general, published field trials demonstrate that mycorrhizal inoculations improve the N content in leguminous plants, but they do not indicate whether the extra N results from increased N, fixation or increased N uptake from the soil. The use of 15N to label soil-assimilable N is a suitable way to quantitatively estimate the amount of N in the plant coming from symbiotic dinitrogen fixation by legumes growing under field conditions. This should be a subject of future research. Although positive inoculation responses are to be expected mainly in lowphosphate soils, it seems that the best responses have been obtained in soils of moderate fertility. For example, a twofold increase in growth after inoculation was obtained in plots of white clover given 90 kg P/ha (Hayman and Mosse, 1979), agreeing with other observations (Powell, 1977b; Owusu-Bennoah and Mosse, 1979). The compatibility of certain phosphate levels and mycorrhizal effects is, obviously, an interesting principle in legume production systems, as is the exploitation of interactions between rock phosphate and VAM. The use of

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this sparingly soluble form of phosphate seems of interest, especially in phosphate-retentive soils. The incorporation of Rp into pellets together with the seeds and the rhizobial inocula is a tantalizing possibility among several areas needing further research concerning the field inoculation of VAM fungi plus Rhizobium sp. for increasing legume productivity.

IV. MYCORRHIZAS IN NODULATING NITROGENFIXING NONLEGUME PLANTS The nonlegume plant species that are able to form N-fixing root nodules, as induced by soil bacteria, are also known to form mycorrhizal associations. The formation, role, and function of the root nodules and the significance of mycorrhizas in these plants are similar to those just discussed for legume-Rhizobium sp. symbioses. Consequently the subject will be only briefly reviewed, condensing information from the few available reports. A. OCCURRENCE AND DISTRIBUTION

The main group of these plants consists of the actinomycete-nodulated, socalled actinorrhizal plants. These are shrubs or small trees distributed t b u g h many ecosystems of temperate regions. Torrey (1978) reported actinorrhizas in 160 species of 15 genera belonging to 8 families. The genus Datisca must now also be listed (Rose, 1980). According to Trappe (1979) and Rose (1980) the actinorrhizal genera bearing VAM are Casuarina, Eleagnus, Hyppophae, Ceanothus, Colletia, Discaria, Purshia, Robus, and Datisca; VAM, ECM, and actinorrhizas coexist in Alnus, Myrica, Comptonia, Dryas, and Coriaria. The mycosymbionts involved were recorded by Rose (1980), Rose and Trappe (1980), and Molina (1981). Members of the order Cycadales (gymnosperms) also form typical N fixing root nodules. The endosymbionts are the blue-green algae (cyanobacteria) Nostoc or Anabaena (see Akkermans, 1978). Vesicular-arbuscular mycorrhizas have been reported to coexist with the nodules in these plants (Trappe, 1979). Species of Parasponia (formerly Trema) in the family Ulmaceae are nodulated by a species of Rhizobium (Akkermans et al., 1978) and appears to possess VAM (Trappe, 1979). B. S T R U ~ R AAND L . PHYSIOLOGICAL FEATURES

An unusual characteristic of mycorrhizal development in actinorrhizas is that VA hyphae have often been found in the nodular tissues (Rose, 1980; Rose and


45

Youngberg, 1981). The VAM structures can be surrounded by ECM in species of Ahus and Ceanothus. Thus it seems that the typical exclusion by nodular and mycorrhizal endophytes does not occur in actinorrhizal plants (Rose, 1980). As in legumes, these mycorrhizas act mainly by their well-known P-mediated mechanism (Mejstrik and Benecke, 1969; Le Tacon and Diagne, 1982), thereby increasing the number of nodules, the weight of nodular tissue, nitrogenase activity, and N, Ca, and P shoot content (Rose and Youngberg, 1981). However, hormone-mediated interactions between endosymbionts can be also involved as these substances seem important in the formation of actinorrhizal symbioses (Miguel et al., 1978). The possible interactions between mycorrhizas and actinorrhizas derived from the former produce calcium oxalate, which is needed by the latter (Trappe, 1979). C . ECOLOGICAL ASPECTS

Actinorrhizal plants are usually involved in the early successional stages of plant communities at low nutrient sites (Harley, 1973). These plants are therefore found colonizing disturbed and marginal habitats such as sand dunes, volcanic ash-derived soils, coal wastes, peat and sphagnum bogs, (Daft and Hacskaylo, 1976; Khan, 1978; Rose, 1980). Rose (1980) reported that 23 of the 25 actinorrhizal plants he tested were VA mycorrhizal. This is, therefore, similar to the situation found with legumes, suggesting the suitability of applying the mycorrhizal effects to actinorrhizal plants (typically woody and perennial) for the successful reforestation of stressed ecosystems.

V. CONCLUSIONS AND PERSPECTIVES Mycorrhizal associations play an important role in the growth and nutrition of higher plants. This results primarily from their more efficient use of soil P. Mycorrhizas appear to have an ecological and evolutionary relevance in the history of terrestrial plants. It is increasingly recognized that this symbiosis can be harnessed in order to improve nutrient cycling and crop productivity by reducing industrial fertilizer inputs, thereby conserving and reducing environmental costs. In addition, mycorrhizal infection can help plants to become reestablished in eroded or degraded habitats, to thrive in arid conditions, to deter pathogens, and to cope with various stress situations. The commonest mycorrhizal types, the VAM, are nearly omnipresent and are now being studied intensively throughout the world. Their contribution to the more efficient use of added P fertilizers, whether soluble (as applied at suboptimal rates) or sparingly soluble (rock phosphate), is being widely appreciated.

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The infectivity and the operativity of VAbi aic self regulated by a mechanism that assures that the P supply to the host will be optimal over a wide range of soil P levels. Vesicular-arbuscular mycorrhizas enhace growth, nodulation, and N fixation in grain and forage legumes, crops of the greatest interest for food production in the biosphere. They have a similar function in N-fixing nonlegumes, largely actinorrhizal plants, mostly of interest in forestry. Nodulating, N-fixing plants are usually mycorrhizal in the ecosystem, but the responsible VA fungi are not always the most suitable and can be replaced with more effective strains by means of inoculation. In general, the inoculation of VAM fungi has problems that limit its extensive use on a field scale. Field experiments with VAM must be supported by ecological and physiological studies including (1) the evaluation of the dependency of the test plant on the mycorrhiza; (2) the assessment of field sites where mycorrhizal inoculation with preselected (efficient and ecologically adapted) endophytes may be worth trying; (3) the production of high-quality inocula and the development of suitable inoculation techniques. Further research on mycorrhizas is needed with regard to several topics pointed out in this article. Current research mentioned has included (1) studies of the population ecology and epidemiology of mycorrhizal fungi; (2) studies investigating the causes of the host dependency of VAM fungi for carrying out their life cycle axenically; (3) physiological studies of mycorrhizal symbioses to discover new mechanisms of action; (4) the application of isotope and radiation techniques to make possible the effective management of mycorrhizas in increasing food-crop production; and (5) field inoculation experiments in small plots to establish bases for future mycorrhizal programs. Mycorrhizas therefore can be regarded as an alternative strategy for a more rational agricultural program. However, because the mycorrhizal condition is nearly universal, the natural mycorrhizal potential of a soil needs first to be preserved (avoiding detrimental practices), second to be optimized (manipulating soil conditions to be conducive to the symbiosis), and third, finally, to be considered when inoculation is required.

REFERENCES Abbott, L. K . , and Robson, A. D. 1977. Aust. J . Agric. Res. 28, 639-640. Abbott, L. K., and Robson, A. D. 1978. New Phytol. 81, 575-585. Abbott, L. K . , and Robson, A. D. 1981a. Aust. J . Agric. Res. 32, 621-630. Abbott, L. K . , and Robson, A. D. 1981b. Aust. J . Agric. Res. 32, 631-639. Abbott, L. K . , and Robson, A. D. 1982. Aust. J . Agric. Rex 33, 389-408. Allen, M. F., and Boosalis, M. G . 1983. New Phytol. 93, 67-76. Allen, E. B., and Cunningham, G . L. 1983. New Phytol. 93, 227-236. Akkermans, A. D. 1978. I n “Interactions between Non-Pathogenic Soil Microorganisms and Plants” (Y. R. Dommergues and S. V. Krupa, eds.), pp. 335-372. Elsevier, Amsterdam.


41

Akkermans, A. D., Abdulkadir, S., and Tnnick, M. J. 1978. Nature (London) 274, 190. Allen, M. F., Moore, T. S . , and Christensen, M. 1980. Can. J . Bot. 58, 371-374. Allen, M. F., Sexton, J. C., Moore, T. S . , and Christensen, M. 1981. New Phytol. 87, 687-694. Allen, M. F., Moore, T. S., and Christensen, M. 1982. Can. J . Bot. 60, 468-471. Andrew, C. S., and Robins, M. F. 1969. Aust. J . Agric. Res. 20,675-685. Asai, T. 1944. Jpn. J . Bot. 13, 463-485. Asimi, S., Gianinazzi-Pearson, V., and Gianinazzi, S. 1980. Can. J. Bot. 58, 2200-2205. Azcbn, R., and Ocampo, J. A. 1981. New Phyrol. 87, 677-685. Azcbn, R., Barea, J. M., and Hayman, D. S. 1976. Soil Biol. Biochem. 8, 135-138. Azcbn, R., Azcbn-G. de Aguilar, C., and Barea, 3. M. 1978a. New Phytol. 80, 359-364. Azcbn, R., Mm’n A. D., and Barea, J. M. 1978b. Plant Soil 49, 561-567. Azcbn-Aguilar, C., and Barea, J. M. 1978. Can J . Microbiol. 40, 520-524. Azcbn-Aguilar, C., and Barea, J. M. 1980. Invest. Ciencia 47, 8-16. Azcbn-Aguilar, C., and Barea, J. M. 1981. Soil Biol. Biochem. 13, 19-22. Azcbn-Aguilar, C., A z c h , R., and Barea, J. M. 1979. Narure (London) 279, 325-327. Azcbn-Aguilar, C., Barea, J. M.,and Olivares, J. 1980. Trans. Int. Symp. Microbial Ecol. 2nd, p. 129. Azcbn-Aguilar, C., Barea, J. M., Azcbn, R., and Olivares, J. 1982. Agric. Environ. 7, 199-206. Bagyaraj, D. J., and Sreeramulu, K. N. 1982. Plant Soil 69, 375-381. Bagyaraj, D. J . , Manjunath, A., and Patil, R. B. 1979. New Phytol. 82, 141-145. Barea, J. M., and Azcbn-Aguilar, C. 1980. Trans. Int. Symp. Microbial Ecology 2nd. p. 130. Barea, J. M., and Azcbn-Aguilar. C. 1982a. Trans. Int. Col. Mycorrhizes Biol. Perspectives Urilization, Dijon. Barea, J. M., and Azcbn-Aguilar, C. 1982b. Appl. Environ. Microbiol. 43, 810-813. Barea, J. M., Ramos, A., and Callao, V. 1970. Microbiol. ESP.23, 257-270. Barea, J. M., Azcbn, R., and Hayman, D. S. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), p. 417. Academic Press, New York and London. Barea, J. M., Escudero, J. L., and Azcbn-Aguilar, C. 1980. Plant Soil 54, 283-2%. Barea, J. M., Azcbn, R., and Azc6n-Aguilar, C. 1981. FAOIIAEA Consult, Meet. Use Isotopes Stud. Nutrient Availability Food Crops Endomycowhizal Vienna. Barrow, N. J., Malajczuk, N., and Shaw, T. C. 1977. New Phytol. 78, 269-276. Bauer, W. D. 1981. Annu. Rev. Plantfhysiol. 32, 407-449. Baylis, G. T. S. 1970. Plant Soil 33, 713-716. Bergersen, F. J . 1978. In “Interactions between NonpathogenicMicroorganismsand Plants” (Y. R. Dommergues and S . V. Krupa, eds.), pp. 305-337. Elsevier, Amsterdam. Bethlenfalvay, G. J. 1983. Physiol. Plant. 57, 543-548. Bevege, D. I., Bowen, G. D., and Skinner, M. F. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 149-174. Academic Press, New York and London. Bieleski, R. L. 1973. Annu. Rev. Plant Physiol. 24, 225-252. Bjorkman, E. 1970. Plant Soil 32, 589-610. Black, R. L., and Tinker, P. B. 1977. Nature (London) 267, 510-51 1 . Bolin, B., and Anhenius, E. 1977. Ambio 6, 96-105. Bone, F., and Barea, J. M. 1981. Anal Edafol. Agrobiol. 40, 2351-2381. Bowen, G. D. 1973. In “Ectomyconhizae” (G. C. Marks and T. T. Kozlowski, eds.), pp. 151-205. Academic Press, New York. Bowen, G. D. 1980a. In “Contemporary Microbial Ecology” (D. C. Ellwood, J. N. Hedger, M. J. Latham, J. H. Slater, and J. M. Lynch, eds.), pp. 283-304. Academic F’ress, New Yo& and London. Bowen, G. D. 1980b. In “Tropical Mycorrhiza Research” (P. Mikola, ed.), pp. 165-190. Oxford Univ. Press, London. Bowen, G. D., and Bevege, D. I. 1976. Rev. Rural Sci. 3, 103-112.

48


Bowen, G . D., and Theodorou,C. 1979. Soil Biol. Biochem. 10, 119-126. Brill, W. J. 1979. Am. Sci. 67,458-466. Bmughton, W. J. 1978. J . Appl. Bucreriol. 45, 165-194. Brown. M. E. 1975. In “Soil Microbiology” (N. Walker, ed.), pp. 21-38. Butterworth, London. Buwalda, J. G. 1980. N.Z. J . Agric. Res. 23, 379-383. Buwdda, J. G., and Goh, K. M. 1982. Soil Biol. Biochem. 14, 103-106. Callow, J. A., Capaccio, L. C. M., Parish, G., and Tinker, P. B. 1978. New Phytol. 80, 125-134. Capaccio, L. C. M., andcallow, J. A. 1982. NewPhyrol. 91, 81-91. Carling, D. E., and Brown, M. F. 1980. Soil Sci. SOC. Am. J . 44, 528-532. Carling, D. E., Riehle, W. G., Brown, M. F., and Johnson, D. R. 1978. Phyroparhology 68, 1590- 1596. Casadestis, J., and Olivares, J. 1978. Anal. Edafol. Agrobiol. 37, 915-948. Chambers, C. A., Smith, S. E., and Smith, F. A. 1980a. New Phyrol. 85,47-62. Chambers,C. A., Smith, S. E., Smith, F. A., Ramsey,M. D., and Nicholas, D. J. 1980b. Soil Biol. Biochem. 12, 93-100. Chapin, F. S., III 1980. Annu. Rev. Ecol. Sysr. 11, 233-260. Chilvers, G. A., and Harley, J. L. 1980. New Phyrol. 84, 319-326. Clarke, C., and Mosse, B. 1981. New Phyrol. 87, 695-703. Cooper, K. M., and Liisel, D. 1978. New Phyrol. 80, 143-151. Cooper, K. M., and Tinker, P. B. 1981. New Phyrol. 88, 327-339. Cox, G., and Sanders, F. E. 1974. New Phyrol. 73, 901-912. Cox, G., and Tinker,P. B. 1976. New Phyrol. 77, 371-378. Cox, G., Sanders, F. E., Tinker, P. B., and Wild, J. A. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 297-312. Academic Press, New York and London. Cox, G., Moran, K. J., Sanders, F., Nockolds, C., and Tinker, P. B. 1980. New Phyrol. 84, 649-659. Cress, W. A., Thronebemy, G. O., and Lindsey, D. L. 1979. Planr Physiol. 64, 484-487. Crush, J. R. 1974. New Phyrol. 73, 743-752. crush, I . R. 1975. N.Z. J . Agric. Res. 18, 361-364. crush, J. R. 1976. N.Z. J . Agric Res. 19, 473-476. Crush, J. R . , and Caradus, J. R. 1980. N.Z.J . Agric. Res. 23, 233-237. Daft, M. J., and El-Giahmi, A. A. 1974. New Phytol. 73, 1139-1147. Daft, M. J., and El-Giahmi, A. A. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 581-592. Academic Press, New York and London. Daft, M. J., and El-Giahmi, A. A. 1976. Ann. Appl. Biol. 83, 273-276. Daft, M. J., and EI-Giahmi, A. A. 1978. New Phyrol. 80, 365-372. Daft, M. J., and Hacskaylo, E. 1976. J . Appl. Ecol. 13, 523-531. Daft, M. J., Hacskaylo, E., and Nicolson, T. H. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 561-580. Academic Press, New York and London. Daniels, B. A., and Menge, J. A. 1980. Phytopurhology 70, 584-588. Daniels, B. A., and Menge, J. A. 1981. New Phytol. 87, 345-354. Daniels, B. A., and Trappe, J. M. 1979. Can. J . Bot. 57, 539-542. Dazzo, F. B., and Brill, W. J. 1978. Plant Physiol. 62, 18-21. Dazzo, F. B . , and Hubbell, D. H. 1975. Proc. Soil Crop Sci. SOC. Flu. 34, 71-74. Delorenzini, C., Barea, J. M., and Olivares, J. 1979. Rev. Larinoarn. Microbiol. 21, 129-134. Demeterio, J. L., Ellis, R., and Paulsen, G. M. 1972. Agron. J . 64, 566-570. Mnari6, J . , and Truchet, G. 1979. Physiol. Veg. 17, 643-667. Dexheimer, J., Gianinazzi, S.,and Gianinazzi-Pearson. V. 1979.2. Pjlunzenphysiol. 92, 191-206. Diem, H. G . , Gueye, I., Gianinazzi-Pearson, V., Fortin, J. A., and Dommergues, Y. R. 1981. Acta Ecol. Ecol. Plant. 2, 53-62.


49

Dodd, J., Krikun, J., and Haas, J. 1983. Isr. J. Bot. 32, 10-21. Dommeregues, Y. R. 1978. In “Interactions between Nonpathogenic Soil Microorganisms and Plants’’ (Y. R. Dommergues and S. V. Krupa, eds.), pp. 1-38. Elsevier, Amsterdam. Elmes, R., and Mosse, B. 1980. “Rothamsted Report 1979.” El-Giahmi, A. A., Nicolson, T. H., and Daft, M. J. 1976. Trans. Br. Mycol. SOC. 67, 273-274. Evans, H. J., and Barber, L. E. 1977. Science (Washington. D . C . ) 197, 332-339. Fogel, R. 1980. New Phytol. 86, 199-212. Foster, S. M., and Nicolson, T. H. 1981a. Soil Biol. Biochem. 13, 199-203. Foster, S. M., and Nicolson, T. H. 1981b. Soil Biol. Biochem. 13, 205-208. Furlan, V., and Fortin, J. A. 1977. New Phytol. 79, 335-340. Gates, C. T. 1974. Aust. J. Bot. 22, 45-55. Gates, C. T., and Wilson, J. R. 1974. Plant Soil 41, 325-333. Gerdemann, J. W. 1975. In “The Development and Function of Roots” (J. G. Torrey and D. T. Clarkson, eds.), pp. 575-591. Academic Press, New York. Gerdemann, J. W., and Trappe, J. M. 1974. Mycol. Mem. 5, 1-76. Gianinazzi-Pearson, V., and Gianinazzi, S. 1981. In “The Fungal Community; Its organization and Role in the Ecosystem” (D. T.Wicklow and G. C. Carrol, eds.), pp. 637-652. Dekker, New York. Gianinazzi, S., Gianinazzi-Pearson, V., and Trouvelot, A. (eds.) 1982. “Les Mycorrhizes partie int6grante de la plante: biologie et perspectives d’utilization”. Colloq. INRA No. 13. Gibson, A. H., and Newton, W. E. (eds.) 1981. “Current Perspectives in Nitrogen Fixation.” Aust. Acad. of Sciences, Canberra. Gibson, D. I., Hayes, P., and Laidlaw, A. S. 1975. J. Br. Grassl. SOC. 30, 295-301. Graham, J. H., and Menge, J. A. 1982. Phytopathology 72, 95-98. Graham, J. H., Linderman, R. G., and Menge, 3. A. 1982. New Phytol. 91, 183-189. Gray, L. E., and Gerdemann, J. W. 1973. Plant Soil 39, 687-689. Greaves, M. P., and Webley, D. M. 1965. J . Appl. Bacteroil. 28, 454-465. Hall, I. R. 1975. N.Z. J. Bot. 13, 463-472. Hall, I. R. 1978. N.Z. J. Agric. Res. 21, 509-515. Hall, I. R. 1980. N.Z. J. Agric. Res. 23, 103-105. Hall, I. R., and Armstrong, P. 1979. N.Z. J . Agric. Res. 22, 479-484. Hall, I. R., Scott, R. S., and Johnstone, P. D. 1977. N.Z. J. Agric. Res. 20, 349-355. Hardy, R. W. F., and Haveka, U. D. 1976. In “Symbiotic Nitrogen Fixation in Plants” (P. S. Nutman, ed.), pp. 421-439. Cambridge Univ. Press, Cambridge. Harley, J. L. 1970. Trans. Bor. SOC.Edinburgh 41, 65-70. Harley, J. L. 1973. J . Nut. Sci. Coun. Sri L a n k 1, 31-48. Harley, J. L. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 1-24. Academic Press, New York and London. Harley, J. L. 1978. Proc. R . SOC. London Ser. B 203, 1-21. Hart, A. L., Jessop, D. J., and Galpin, J. 1981. N.Z. J . Agric. Res. 24, 27-32. Hayman, D. S . 1974. New Phytol. 73, 71-80. Hayman, D. S. 1975a. In ‘‘Soil Microbiology” (N. Walker, ed.), pp. 67-92. Buttersorth, London. Hayman, D. S. 1975b. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 495-509. Academic Press, New York and London. Hayman, D. S. 1977. ARC Res. Rev. (U.K.) 3, 82-85. Hayman, D. S. 1978. In “Interactions between Nonpathogenic Soil Microorganismsand Plants” (Y. R. Dommergues and S. V. Krupa, eds.), pp. 401-442. Elsevier, Amsterdam. Hayman, D. S. 1981. FAOIIAEA Consult. Meet. Use Isotopes Stud. Nutrient Availability Food Crops Edomycorrhizae, Vienna. Hayman, D. S . 1982. In “Advances in Agricultural Microbiology” (N. S. Subba Rao ed.). INH New Delhi.

50

J. M. BAREA AND C. AZCdN-AGUILAR

Hayman, D. S. 1983. Can. J . Bor. 61, 944-963. Hayman, D. S., and Mosse, B. 1979. Ann. Appl. Biol. 93, 141-148. Hayman, D. S., Barea, J. M., and Azc6n. R. 1976. Phyropathol. Medirerr. 15, 1-6. Hayman, D. S., Moms, E. J., and Page, R. J. 1981. Ann. Appl. Biol. 99, 247-253. Haynes, R. J. 1980. Adv. Agron. 33, 227-261. Heap, A., and Newman, E. I. 1980. New Phyrol. 85, 173-179. Hepper, C. 1979. Soil Biol. Biochem. 11, 269-277. Hepper, C. M. 1983. New Phyfol. 93, 389-399. Ho, I., and Trappe, J. M. 1973. Nature New Biol. 244, 30-31. Howeler, R. H., Cadavid, L. F., and Burckhard, E. 1982a. Plant Soil 69, 327-339. Howeler, R. H., Asher, C. J., and Edwards, D. G. 1982b. New Phytol. 90, 229-238. Islam, R., Ayanaba, A., and Sanders, F. E. 1980. Planr Soil 54, 107-117. Jakobsen, I., and Nielsen, N. E. 1983. New Phyrol. 93, 401-413. Janos, D. P. 1980a. Ecology 61, 151-162. Janos, D. P. 1980b. “Tropical Succession 1980.” Janse, J. M. 1896. An. Jard. Bor. Buirenz. 14, 53-212. Jasper, D. A., Robson, A. D., and Abbott, L. K. 1979. Soil Biol. Biochem. 11, 501-505. Jehne, W. 1980. Trop. Grassl. 14, 202-209. Jensen, A. 1983. Plant Soil 70, 155-163. Johnson, C. R., Menge, J. A., Schwab, S . , and Ting, I. P. 1982a. New Phyrol. 90, 665-669. Johnson, C. R.,Graham,J. H., Leonard, R. T.,and Menge, J. A. 1982b. New Phytol. 90,671-675. Jones, F. R. 1924. J . Agric. Res. 29, 459-470. Khan, A. G. 1974. J. GenMicrobiol. 81, 7-14. Khan, A. G. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 419-435. Academic Press, New York and London. Khan, G . G. 1978. New Phytol. 81, 53-63. Kleinschmidt, G . D., and Gerdemann, J. W. 1972. Phyropafhology 62, 1447-1453. Koske, R. E. 1981. Trans. Br. Mycol. SOC. 76, 411-416. Koske, R. E., and Halvorson, W. L. 1981. Can. J . Bor. 59, 1413-1422. Koske, R. E., Sutton, J. C., and Sheppard, B. R. 1975. Can. J . Bor. 53, 87-93. Kruckelmann, H. W. 1975. I n “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 511-525. Academic Press, New York and London. Kucey, R. M. N., and Paul, E. A. 1982. Soil Biol. Biochem. 14, 407-412. Kucey, R. M. N., and Paul, E. A. 1983. Can. J. Soil Sci. 63, 87-95. Lambert, D. H., and Cole, H. 1980. Agron. J. 72, 257-260. Lambert, D. H . , Cole, H., and Baker, D. E. 1980a. Crop. Sci. 20, 615-618. Lambert, D. H . , Cole, H., and Baker, D. E. 1980b. Planr Soil 57, 431-438. Langkamp, P. J., and Dalling, M. J. 1982. Ausr. J. Bor. 30, 107-119. Lanowska, J. 1966. Pam. Pulowski. 21, 365-386. Le Tacon, F., and Diagne, 0. 1982. Trans. Int. Col. Mycorrhizes Biol. Perspecrives Urilisarion, Dijon. Levy, Y., Sylvertse, J. P., and Nemec, S. 1983. New Phyrol. 93, 61-66. Lewis, D. H. 1973. Biol. Rev. 48, 261-278. Lewis, D. H. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 119-148. Academic Press, New York and London. Ling-Lee, M., Chilvers, G. A., and Ashford, A. E. 1975. New Phyrol. 75, 551-554. Ljunggren, H., and Fahraeus, G . 1959. Nature (London) 184, 1578-1579. Lijsel, D. M., and Cooper, K. M. 1979. New Phyrol. 83, 415-426. MacDonald, R. M., and Lewis, M. 1978. New Phyrol. 80, 135-141. Malajczuk, N., Linderman, R. G . , Kough, J., and Trappe, J. M. 1981. New Phyrol. 87,567-572.


51

Malloch, D. W., Pirozynski, K. A., and Raven, P. H. 1980. Proc. Narl. Acud. Sci. USA 77, 21 13-21 18. Marx, C., Dexheimer, J., Gianinazzi-Pearson, V.,and Gianinazzi, S. 1982. New Phyrol. 90,37-43. Marx, D. H. 1980. In “Tropical Mycorrhiza Research” (P. Mikola, ed.). Oxford Univ. Press, London. Marx, D. H., and Krupa, S. U. 1978. In “Interactions between Non-pathogenic Soil Microorganisms and Plants” (Y. R. Dommergues and S. V. Krupa, eds.), pp. 373-400. Elsevier, Amsterdam. Mattingly, G. E. G. 1980. Chem. Ind. (London) 1980, 690-693. Mejstrik, V., and Benecke, U. 1969. New Phyrol. 68, 141-149. Menge, J. A. 1983. Can. J. Bor. 61, 1015-1024. Menge, J. A., Lembright, H., and Johnson, E. L. V. 1977. Proc. Int. SOC. Citric. 1, 129-132. Menge, J. A., Steirle, D., Bagyaraj, D. J., Johnson, E. L. V., and Leonard, R. T. 1978. New Phytol. 80, 575-578. Mexal, J. G. 1980. N.Z. J . For. Sci. 10, 208-217. Meyer, F. H. 1973. In “Ectomycorrhizae” (G. C. Marks and T. T. Kozlowski, eds.), pp. 79-106. Academic Press, New York. Miguel, C., Caiiizo, A., Costa, A., and Rodriguez-Barmeco, C. 1978. In “Limitations and Potentials for Biological Nitrogen Fixation in the Tropics” (J. Nbereiner ezal., eds.), pp. 121-134. Plenum, New York. Mikola, P. 1970. Inr. Rev. For. Res. 3, 123-196. Mojallali, H., and Weed, S. B. 1978. Soil Sci. Soc. Am. J . 42, 367-372. Moiraud, A., Capellano, A., and Birtschi, H. 1981. Can. J . Bot. 59,481-490. Molina, R. 1977. In “Mushrooms and Man: An Interdisciplinary Approach to Mycology” (T. Walters, ed.), pp. 147-161. Molina, R. 1979. For. Sci. 25, 585-590. Molina, R. 1981. Can. J. Bor. 59, 325-334. Morley, C. D., and Mosse, B. 1976. Trans. Br. Mycol. SOC. 67, 510-513. Mosse, B. 1962. J. Gen. Microbiol. 27, 509-520. Mosse, B. 1972. Rev. Ecol. B i d . Sol 9, 529-537. Mosse, B. 1973a. Annu. Rev. Phyropazhol. 11, 171-1%. Mosse, B. 1973b. New Phyrol. 72, 127-136. Mosse, B. 1975. In “Soil Microbiology” (N. Walker, ed.), pp. 39-66. Butterworth, London. Mosse, B. 1977a. New Phyzol. 78, 277-288. Mosse, B. 1977b. Coll. Trop. Agric. Univ.Hawaii Misc. Publ. 145, 275-292. Mosse, B. 1978. In “Structure and Functioning of Plant Populations” (A. H. T. Freysen and B. W. Woldendorp, eds.), pp. 269-298. Mosse, B., and Hayman, D. S. 1971. New Phytol. 70, 29-34. Mosse, B., and Hayman, D. S. 1980. In “Tropical Mycorrhiza Research” (P. Mikola, ed.), pp. 213-230. Oxford Univ. Press, London. Mosse, B., and Hepper, C. 1975. Physiol. P h Purhol. 5 , 215-223. Mosse, B., Hayman, D. S., and Arnold, D. J. 1973. New Phytol. 72, 809-815. Mosse, B., Powell, C. L., and Hayman, D. S. 1976. New Phyrol. 76, 331-342. Mosse, B., Warner, A., and Clarke, C. A. 1982. New Phyzol. 90, 521-528. Munns, D. N . , and Mosse, B. 1980. In “Advances& Legume Science” (R.J. Summerfild and A. H. Bunting, eds.), pp. 115-125. HM Stationery Office,London. Nemec, S. 1983. Trop. Agric. 60, 97-101. Newman, E. 1979. Biol. Rev. 53, 511-554. Ng, P. P., Cole, A. L. J., Jameson, P. E., and Mcwha, J. A. 1982. New Phyrol. 91, 57-62. Nicolson, T. H. 1960. Trans. Br. Mycol. SOC. 43, 132-145.

52


Nicolson, T. H. 1975. In “Endomycomhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 25-34. Academic Press, New York and London. Nielsen, J. D., and Jensen, A. 1983. Plum Soil 70, 165-172. Nutman, P. S. 1977. In “Recent Developments in Nitrogen Fixation” (W. Newton, J. R., Postgate, and C. Rodriguez-Barmeco, eds.), pp. 443-447. Academic Press, New York and London. Nylund, J. E., and Unestam, T. 1982. New fhyrol. 91, 63-79. O’Bannon,J. H., Evans, D. W., and Peaden, R. N. 1980. Can. J. flanr Sci. 60, 859-863. Ocampo, J. A., and Hayman, D. S . 1981. New fhyrol. 87, 333-343. Ocampo, J. A., Martin, J., and Hayman, D. S . 1980. New fhytol. 84, 27-35. Ojala, J. C., Jarrel, W. M., Menge, J. A., and Johnson, E. L. V. 1983. Agron. J. 75, 255-259. Olivares, J., Montoya, E., and Palomares, A. 1977. In “Recent Developments in Nitrogen Fixation” (W. Newton, J. R. Postgate and C. Rodriguez-Barmeco, eds.), pp. 375-386. Academic Press, New York and London. Owusu-Bennoah, E., and Mosse, B. 1979. New Phyrol. 83, 671-679. Owusu-Bennoah, E., and Wild, A. 1979. New fhyrol. 82, 133-140. PairUnan, A. K., Robson, A. D., and Abbott, L. K. 1980. New fhytol. 84, 327-338. Pang, P. C., and Paul, E. A. 1980. Can. J. Soil Sci. 60, 241-250. Pfeiffer, C. M., and Bloss, H. E. 1980. Mycologia 72, 1038-1041. Phillips, D. A. 1980. Annu. Rev. Plant fhysiol. 31, 29-49. Pichk, Y., and Fortin, J. A. 1982. New fhyrol. 91, 211-220. Pirozynski, K. A,, and Malloch, D. W. 1975. BioSystems 6, 153-164. Plenchette, C., Furlan, V., and Fortin, J. A. 1981. Can. J . Bot. 59, 2003-2008. Plenchette, C. 1982. fytoprocr. 63, 86-108. Plenchette, C., Fortin, 1. A., and Furlan, V. 1983a. Plant Soil 70, 199-209. Plenchette, C., Fortin, J. A., and Furlan, V. 1983b. Plant Soil 70, 211-217. Pope, P. E., Chaney, W. R., Rhodes, J. D., and Woodhead, S . H. 1983. Can. J. Bot. 61,412-417. Possingham, J. V.,Groot Obbink, J., and Jones, R. K. 1971. J. Aust. Insf. Agric. Sci. 37, 160-161. Postgate, J. R., and Hill, S. 1979. In “Microbial Ecology” (J. M. Lynch and N. J. Poole, eds.), pp. 191-2 13. Blackwell, Oxford. Powell, C. L. 1975. Newfhyrol. 75, 563-566. Powell, C. L. 1976a. Trans. Br. Mycol. Soc. 66, 439-445. Powell, C. L. 1976b. Nature (London)264,436-438. Powell, C. L. 1977a. N.Z. J. Agric. Res. 20, 59-62. Powell, C. L. 1977b. N.Z. J. Agric. Res. 20, 343-348. Powell, C. L. 1979. N e w f h y t o l . 83, 81-85. Powell, C. L. 1980a. N . Z . J. Agric. Res. 23, 225-231. Powell, C. L. 1980b. N.Z. J. Agric. Res. 23, 219-223. Powell, C. L. 1980~.Soil Biol. Biochem. 12, 247-250. Powell, C. L. 1982. flanr Soil 68, 3-9. Powell, C. L., and Daniels, J. 1978. New fhytol. 80, 351-358. Powell, C. L., Groters, M.,and Metcalfe, D. 1980. N.Z. J . Agric. Res. 23, 107-109. Raj, J., Bagyaraj, D. J., and Manjunath, A. 1981. Soil Biol. Biochem. 13, 105-108. Rangeley, A., Daft, M. J., and Newbould, P. 1982. New Phyrol. 92, 89-102. Ratnayake, M., Leonard, R. T., and Menge, J. A. 1978. New fhyrol. 81, 543-552. Raven, J. A., and Smith, F. A. 1976. New Phyrol. 76, 415-431. Raven, J. A,, Smith, S. E., and Smith, F. A. 1978. Trans. Bor. SOC.Edinburgh 43, 27-35. Read, D. J. 1983. Can. J. Bor. 61, 985-1004. Read, D. J., and Stribley, D. P. 1975. In “Endomycomhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 105-117. Academic Press, New York and London. Read, D. J., Koucheki, H. K., and Hcdgson, J. 1976. New Phyrol. 77, 641-653.


53

Redente. E. F., and Reeves, F. B. 1981. Soil Sci. 132, 410-415. Redhead, J. F. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 469-484. Academic Press, New York and London. Rhodes, L. H. 1980. Outlook Agric. 10, 275-281. Rhodes, L. H., and Gerdemann, J. W. 1975. New Phytol. 75, 555-561. Rhodes, L.H., and Gerdemann, J. W. 1980. In “Cellular Interactions in Symbiosis and Parasitism” (C. B. Cook, P. W. Pappas, and E. D. Rudolph, eds.), pp. 173-198. Ohio State Univ. Press, Columbus. Robson, A. D. 1978. In “The Mineral Nutrition of Legumes on Tropical and Subtropical Soils” (C. S. Andrew and E. J. Kamprath, eds.), pp. 277-293. CSIRO, Melbourne. Robson, A. D., O’Hara, G. W., and Abbott, L. K. 1981. Aust. J. Plant Physiol. 8, 427-436. Rose, S. L. 1980. Can. J . Bot. 58, 1449-1454. Rose, S. L. 1981. Can. J. Bor. 59, 1056-1060. Rose, S. L., and Trappe, J. M. 1980. Mycotaxon. 10, 413-420. Rose, S. L., and Youngberg, C. T. 1981. Can. J. Bot. 59, 34-39. ROSS,J. P. 1971. P h y t o ~ a t h o l o61, ~ ~ 1400-1403. Ross, J. P., and Gilliam, J. W. 1973. Soil Sci. SOC. Am. Proc. 37, 237-239. Ross, J. P., and Harper, J. A. 1970. Phytopathology 60, 1552-1556. Ross, J. P., and Harper, J. A. 1973. J. Elishu Michell Sci. SOC. 89, 1-3. Safir, G. R. 1980. In “The Biology of Crop Productivity” (P. S. Carlson, ed.), pp. 231-252. Academic Press, New York. Safu, G. R. 1981. FAOIIAEA Consult. Meet. Use Isotopes Stud. Nutrient Availability Food Crops Edomycorrhizae, Vienna. Safir, G. R., Boyer, J. S., and Gerdemann, J. W. 1972. Plant Physiol. 49, 700-703. Saif, S. R. 1981. New Phytol. 88, 649-659. Saif, S. R., and Khan, A. G. 1977. Plant Soil 47, 17-26. Samuel, G. 1926. Trans. Proc. R. SOC.South Aust. 50, 245-246. Sanders, F. E. 1975. In “Endomycomzas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 261-276. Academic Press, New York and London. Sanders, F. E., and Tinker, P. B. 1971. Nature (London) 233, 278-279. Sanders, F. E., Tinker, P. B., Black, R. L. B., and Palmerley, S. M. 1977. New Phyrol. 78, 257-268. Sanni, S. 0. 1976. New Phytol. 77, 667-671. Scannerini, S., and Bonfante, P. 1983. Can. J. Bot. 61, 917-943. Scannerini, S., Bonfante, P. F., and Fontana, A. 1975. In “Endomycorrhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 213-324. Academic Press, New York and London. Schenck, N. C., and Hinson, K. 1973. Agron. J. 65, 849-850. Schenck, N. C., and Kellan, M. K. 1978. Agric. Exp. Stn. Tech. Bull. No. 798. Schenck, N. C., and Smith, G. S. 1981. Proc. Soil Crop Sci. SOC. Flu. 40, 171-175. Schonbeck, F. 1979. In “Soil-Borne Plant Pathogens” (B. Schippers and W. Gams, eds.), pp. 271-280. Academic Press, New York and London. Schramm, J. R. 1966. Trans. Am. Phil. SOC. 56, 1-194. Shanmugam, K. T., O’Gara, F., Anderson, K., and Valentine, R. C. 1978. Annu. Rev. Plant Physiol. 29, 263-276. Shemakhanova, N. M. 1972. “Mycotrophy of Woody Plants.” US Dep. Agric. Natl. Sci. Found., Washington, D.C. Shukla, U. C., and Yadav, 0. P. 1982. Plant Soil 65, 239-248. Skipper, H. D., and Smith, G. W. 1979. Plant Soil 53, 559-563. Slankis, V. 1974. Annu. Rev. Phytopathol. 12, 437-457. Smith, F. A., and Smith, S. E. 1981a. New Phytol. 88, 311-325.

54


Smith, F. A., and Smith, S. E. 1981b. New Phytol. 88, 299-309. Smith, S. S. E. 1980. Biol. Rev. 55,475-510. Smith, S . E. 1982. New Phytol. 90, 293-302. Smith, S. E., and Bowen, G. D. 1979. Soil Eiol. Biochem. 11, 469-473. Smith, S. E., and Daft,M. J. 1977. Aust. J . PInnr Physiol. 4,403-413. Smith, S . E., and Daft,M. J. 1978. In “Microbial Ecology” (M. W. Loutit and J. A. R. Miles, eds.), pp. 312-319. Springer-Verlag, New Yo&. Smith, S. E., Nicholas, D. J. D., and Smith, F. A. 1979. Aust. J . Plant Physiol. 6, 305-311. Sondergaard, M., and Laegaard, S. 1977. Nature (London) 268, 232-233. Sparling, G. P., and Tinker. P. B. 1978. J. Appl. Ecol. 15, 959-964. St. John, T. V. 1980. N ~ wPhytol. 84, 483-487. St. John, T. V., andColeman, D. C. 1983. C u n J . Eor. 61, 1005-1014. Stotzky, G. 1972. CRC Crit. Rev. Microbiol. 2, 59-137. Stribley, D. P., Tinker, P. B., and Rayner, J. H. 1980. New Phytol. 86, 261-266. Strullu, D. G. 1982. Trans. Int. Col. Mycorrhizes Eiol. Perspectives Utilisation, Dijon. Stnemska, J. 1975. In “Endomyconbizas” (F. E. Sanders, B. Mosse,and P. B. Tinker, eds.), pp. 537-543. Academic Press, New York and London. Sutton, J. C., and Sheppard, B. R. 1976. Cun. J. Eor. 54, 326-333. Tardieux-Roche, A. 1966. Ann. Agron. 17,403-479. Thimann, K. V. 1977. “Hormone Action in the Whole Life of Plants.” Univ. of Massachusetts Press, Amherst. Thomazini-Casagrande, L. I. 1980. Fyton. 39, 113-120. Tien, T. M., Gaskins, M. H.,and Hubbell, D. H. 1979. Appl. Environ. Microbiol. 37, 1016-1024. Tinker, P. B. 1975. In “Endomycomhizas” (F. E. Sanders, B. Mosse, and P. B. Tinker, eds.), pp. 353-371. Academic Press, New York and London. Tinker, P. B. 1978. Physiol. Veg. 16, 743-751. Tinker, P. B. 1980. In “The Role of Phosphorus in Agriculture,” pp. 617-653. Amer. Soc. Agron., Madison, Wisconsin. Tinker, P. B., and Sanders, F. E. 1975. Soil Sci. 119, 363-368. Tisdall, J. M., and Oades, J. M. 1979. Aust. J. Soil Res. 17, 429-441. Torrey, J. G. 1976. Annu. Rev. Plant Physiol. 27,435-459. Torrey, J. G. 1978. EioScience 28, 586-592. Trappe, J. M. 1977. Annu. Rev. Phytoputhol. 15, 205-222. Trappe, J. M. 1979. In “Symbiotic Nitrogen Fixation in the Management of Temperate Forests” (J. C. Gordon., C. T. Weeler, and D. A. Perry, eds.), pp. 276-286. Oregon State Univ. For. Res. Lab., Corvallis. Trappe, J. M. 1981. In “Advances in Food Producing Systems for Arid and Semi-Arid Lands” (J. T. Manassah and E. J. Briskey, eds.), pp. 581-599. Academic Press, New York. Trappe, J. M., and Fogel, R. C. 1977. Range Sci. Dep. Sci. Ser. (Color. State Univ.) 26,205-214. Trinick, M. J. 1977. New Phytol. 78, 297-304. van Schreven, D. A. 1958. In “Nutrition of the Legumes” (E. Hallsworth, ed.), pp. 137-163. Buttenvorth, London. Waidyanatha, U. P., Yogaramam, N., and Ariyaratne, W. A. 1979. New Phytol. 82, 147-152. Warner, A., and Mosse, B. 1980. Trans. Er. Mycol SOC. 74,407-410. Warner, A., andMosse, B. 1982. New Phytol. 90, 529-536. Whittingham, J., and Read, D. J. 1982. New Phytol. 90, 277-284. Wilson, J. M., and Trinick, M.J. 1983. Aust. J . Soil. 21, 73-81. Zak, J. C., and Parkinson, D. 1983. Can. J . Eot. 61, 798-803.

ADVANCES IN AGRONOMY, VOL. 36

SUBMICROSCOPIC EXAMINATION OF SOILS E. B. A. Bisdom Netherlands Soil Survey Institute Wageningen, The Netherlands

I. Introduction .......................................................... 11. Submicroscopic Techniques ............................................. A. Electron Microscopy.. ................. ........ B . Ion Microscopy.. ..................... ..... C. Additional Submicroscopy. ......................................... 111. Applications of Electron Microscopy ..................... A. Unhardened Samples .............................................. B. Thin Sections .................................................... IV. Applications of Ion Microscopy ................ V. Applications of Other Forms of Submicroscopy ...................... VI. Conclusions .......................................................... References .................. ....................................

55 57 57 61

62 65 66 77 88 89

90 91

1. INTRODUCTION Submicroscopy of soils usually begins after light microscopy has been done, and light microscopy itself often supports field studies. This article primarily describes soil materials studied in thin sections, soil peds, and 1arger.mineral grains. Submicroscopy may be regarded as a young field in soil science, although instruments such as the transmission electron microscope (TEM) and the electron microprobe analyzer (EMA) have been in use for more than a decade. In situ light-microscopic studies of soils are usually performed by micromorphologists representing the field of soil micromorphology. For technical reasons, this is itself a relatively young branch of soil science; soil micromorphology could only develop after thin sections had been prepared (i.e., after development of the technique of the impregnation of samples with plastics necessary for their hardening). This technical difficulty did not exist in geological studies of hardrock and, as a consequence, thin-section studies of cohesive rocks were already being made many years before light-microscopic studies of soils were possible. However, as soon as thin sections of soils were feasible, these 55

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ISBN 0-12-000736-3

56

E. B. A. BISDOM

techniques were also applied to weathered rocks and uncohesive geological deposits. Thin-section studies of soils with the light microscope indicated that available magmfkations were frequently insufficient to study the finer soil particles. The TEM was initially used to investigate especially clayey material that had been pretreated and was disturbed. In sifu studies using the TEM were first made on replicas, and it has recently become possible to examine ultrathin sections of soft soil constituents. Such in sifu studies became popular with the introduction of the scanning electron microscope (SEM) (i.e., with the potential to study soil constituents in soil peds and to examine mineral grains with or without coatings). “Three-dimensional” pictures were obtained and the morphologies of the soil constituents were studied at various magnifications. The investigation of very small particles in soils requires TEM and scanning transmission electron microscope (STEM) investigation of ultrathin sections that are transparent to the electron beam. Such ultrathin sections of organic matter and clays have been prepared, but those from harder soil materials are still difficult to obtain. Ultrathin sections also offer the possibility of electron diffraction of individual soil particles instead of X-ray diffraction of bulk samples that have been pretreated and disturbed. Submicroscopy with the SEM, especially after the introduction of new detector systems, can be used to study the form of pores and minerals in a thin section if used in combination with equipment for image analysis. So far, however, thin sections have been used more often to investigate chemical elements of soil particles. Such microchemical analyses started with the EMA [also called EPMA (electron probe microanalyzer)]. This instrument, as do most of the other instruments used in submicroscopy, requires a polished surface of a thin section or the surface of a thin polished block. More recently, the SEM has been equipped with an energy dispersive X-ray analyzer (EDXRA). This SEM-EDXRA allows in situ analysis of soil materials in unhardened soil components and in thin sections. A scanning electron microscope equipped with a wavelength dispersive X-ray analyzer (SEM-WDXRA) can only analyze chemical elements in polished surfaces. Two problems remained with EMA and SEM-EDXRA-WDXRA analyses of soil components; the lightest elements of the periodic system of chemical elements and trace elements could not be studied using electron microscopy. These problems were solved with the introduction of ion microscopy. The instruments involved use either primary ions for the excitation of secondary ions from materials in thin sections of soils [e.g., ion microprobe mass analyzer (IMMA) and Cameca IMS 3F (ion microanalyzer)] or a laser for the excitation of primary ions [e.g., LAMMA 500 and LAMMA lo00 (laser microprobe mass analyzers)]. Quantification of all chemical elements in soil constituents of a thin section became possible with the Cameca IMS 3F.


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The technical possibilities of in situ submicroscopic research are significant. This young branch of soil science, together with light microscopy, allows us to obtain knowledge on soil components in their natural environment and at a variety of magnifications. Several examples of submicroscopic studies will be given after discussing the capabilities of a number of machines.'

11. SUBMICROSCOPIC TECHNIQUES A. ELECTRON MICROSCOPY

I . Transmission and Scanning Transmission Electron Microscopy The transmission electron microscope is commonly used for the study of loose and very small particles in soils using a static electron beam. A lateral resolution of 0.2 nm can be achieved (Boekestein et al., 1981). If ultrathin sections can be made with a thickness of about 1 pm, the TEM can be used because the electrons can pass through the specimen. Ultrathin sections of clayey and of organic material were prepared and studied with the TEM by Bresson (198 1) and Foster (1981), respectively. The high-voltage electron microscope (HVEM) uses a much higher kinetic energy than the TEM, about lo00 keV compared to 20-200 keV. The HVEM can examine a specimen somewhat thicker than can the TEM but so far no results have been published in soil micromorphology submicroscopy. The polyester resin used for the embedding of soil material is expected to become brittle with high voltage electron microscopy. Samples somewhat thicker than 1 pm can also be studied with the STEM. The STEM (Fig. 1) can be used as a TEM, SEM, or STEM. The instrument makes it possible to study an ultrathin section by a scanning beam. Microchemical analysis is also possible with a TEM or an STEM that has a 'Abbreviations used in this paper: A E S , Auger electron spectroscopy; BESI, backscattered electron scanning image; EDXRA, energy dispersive X-ray analyzer (analysis); EMA, electron microprobe analyzer (analysis); EPMA, electron probe microanalyzer (analysis); ESCA, electron spectroscopy for chemical analysis; HREM, high resolution electron microscope (microscopy); HVEM high voltage electron microscope (microscopy); IMMA, ion microprobe mass analyzer (analysis);LAMMA, laser microprobe mass analyzer (analysis); LMA, laser microspectral analyzer (analysis); RS, Raman spectroscopy; SEM, scanning electron microscope (microscopy);SIMS, secondary ion mass spectrometer (spectrometry); STEM, scanning transmission electron microscope (microscopy); TEM, transmission electron microscope (microscopy); WDXRA, wavelength dispersive X-ray analyzer (analysis); XRD, X-ray diffraction.

58

E. B. A. BISDOM

FIG. 1. Scanning transmission electron microscope (Philips EM 400T/ST).

lateral resolution of about 5 nm. The instruments can be equipped with an EDXRA. Analysis of the heavier chemical elements in ultrathin or somewhat thicker thin sections thus becomes possible. Because of beam spot diameters that are smaller than those present in conventional SEM instruments, it is possible to analyze at magnifications which are larger than those possible with an SEMEDXRA (i.e., larger than X l0,OOO). Very small particles can be analyzed and identified with the TEM and STEM if equipment for electron diffraction is available. This technique is usually applied to study loose clay minerals but can also be used to study small particles in ultrathin sections of soils. Work has been done with the STEM-EDXRA on thin sections (5 pm thick) and at a maximum magnification of X50,OOO. Work with the TEM-EDXRA and STEM-EDXRA on ultrathin sections, and diffraction


59

studies of these specimens, must yet be done. The principal difficulty is that we still must learn how to prepare ultrathin sections of harder soil materials. Ionthinning techniques seem to give the best results at present. If ultrathin sections have been prepared, TEM and STEM instrumenis are available for various types of studies on an ultramicro scale.

2 . Electron Microprobe Analysis and Scanning Electron Microscopy The electron microprobe analyzer is the oldest machine used for microchemical analysis of soil materials in polished thin sections. It is used for microanalysis only (i.e., for semiquantitative and quantitative measurements using a set of standards with which to compare the results of the analyses). Older EMA instruments often caused localization problems for materials in a thin section of soil. Modem machines, however, can be equipped in such a way that one can find soil components in thin sections with relative ease, which is necessary for heterogeneous soils with fine particles and complicated fabrics. The EMA is equipped with a WDXRA system. Wavelength dispersive analysis is done with a WD detector which contains a crystal that is used for Bragg reflection and a gas-filled proportional counter (Boekestein et al., 1981). Only one element can be measured at a time, unless more detectors are used. The WD detector has a high efficiency, because of the thin entrance window of the proportional counter, and a high peak-to-background ratio. This ratio is important because element-characteristic radiation is represented by the peaks and noncharacteristic radiation is represented by the background. The detectable elements are B-U (22 5 ) . The maximum magnification of the EMA is about X500, which can be a problem (Bisdom et al., 1975, 1976); the lateral resolution is about 1 pm. The scanning electron microscope (Figs. 2 and 3) can also be equipped with a WDXRA system, which allows magnifications up to X10,OOO. The SEMWDXRA and the SEM-EDXRA have lateral resolutions of about 1 pm, as does the EMA. This means that the minimum diameter of a spot that is analyzed in a thin section is 1 pm. The SEM-WDXRA, like the EMA, can only work with polished surfaces, whereas both polished and rough surfaces can be examined with the SEM-EDXRA. Consequently, materials in soil peds are now investigated microchemically, not only on the basis of morphology, by using the SEM. An additional advantage of the EDXRA is that the current of the primary electron beam on the specimen is lo-" A, whereas it is lo-' A for the EMA. Consequently, in EMA the polyester resin of the thin section is easier to damage than in SEM. Energy dispersive X-ray analysis utilizes an ED detector which consists of a lithium-drifted silicon crystal. If the detector has a beryllium window, very soft

E. B. A. BISDOM

FIG. 2. Scanning electron microscope (Jeol-JSM-35C).

FIG. 3. Scanning electron microscope (Philips SEM 505).


61

X rays are absorbed and the characteristic radiation of elements with low atomic numbers is not detected. Elements Na-U (22 11) are detected in this way. If an ECON detector is used without a beryllium window, the radiation of C, N, 0, and F can also be measured. The ED detector has very small processing times and can give information on a range of elements simultaneously. However, the energy resolution is rather poor, which affects the peak-to-background ratio and the minimal detectable concentration. Ideally, SEM-EDXRA is used for reconnaissance and semiquantitativework and EMA and SEM-WDXRA for quantitative and semiquantitative work. Trace elements are usually not measurable with electron microscopes. However, under ideal conditions, lo-'* g of an element [approximately 0.1%can be measured (Boekestein et al., 1981)]. B. ION MICROSCOPY

Various instruments for the analysis of secondary ions, excited from the sample by primary ions, have been tested on soil samples, including the IMMA of ARL, Cameca IMS 300 (ion microscope), Cameca IMS 3F (Fig. 4), and LAS of Riber [an apparatus in which SIMS, ESCA, and Auger (see later discussion) analysis can be done]. Only polished thin-section material which has been removed from the support glass can be used in these instruments. The IMMA uses a scanning primary ion beam and a mass spectrometer for mass analysis of sputtered ions (Bisdom et al., 1977). The primary electron beam of the electron

FIG. 4, Ion microanalyzer (Cameca IMS 3F).

62

E. B. A. BISDOM

microscope has been replaced in the IMMA by a primary ion beam and secondary ions are produced instead of secondary electrohs. In the IMMA the sample can be viewed during analysis through a binocular microscope (Henstra et af., 1981a), whereas this is not possible with the Cameca instruments. Localization is done in the latter machines with a low-power optical microscope. A viewport is present in the LAS series of instruments. All four instruments are used for secondary ion mass spectrometry (SIMS). Such spectra of secondary ions give information on all chemical elements that are present in a sample including hydrogen. Both trace and major elements can be measured. Background problems, such as are present in electron microscopy, are virtually absent. Background readings are usually below 5 counts/sec, with total count rates on the order of lo8 counts/sec (Liebl, 1975). Trace concentrations can therefore be analyzed, usually down to the range and in many cases even down to the range. Trace amounts (10-I8 g) of sample material are measurable. The sentitivity of SIMS is much better than that of X-ray analytical techniques (i.e., 1OOO-10,OOO times) (Henstra er al., 1981a). All elements can be detected with SIMS but the secondary ion yield differs for various elements. Also, the same element in a different matrix may give a different secondary ion yield. The secondary ion yield of the sample can be strongly enhanced by bombarding with a reactive species such as oxygen or nitrogen. The primary ions used for bombarding the sample can be charged either positively or negatively. The sample is continuously eroded under ion bombardment; consequently, the determination of concentration as a function of depth is important. Depth-concentration profiling with a resolution of about 5 nm is possible. Probe diameters on the surface of the sample range from 500 to 1-2 FmIsotopic analysis is possible in SIMS, and isotopic abundance ratios can be measured with high accuracy. This should allow in siru age dating of materials in thin sections of soils and in horizons of soil profiles. Another possibility is to label chemical elements with stable isotopes, which should allow the study of transport phenomena in soils by measuring the position of these labeled isotopes in thin sections. Secondary ion mass spectrometry offers a large variety of measurement possibilities which can be added or compared to those of electron microscopy. As ion microscopy is a younger field than is electron microscopy, various measurement techniques are still being developed. Experiments have indicated, however, that quantitative and semiquantitative measurements of soil materials in thin sections of soils are now possible (see Section IV). C. A D D ~ O N ASUBMICROSCOPY L

Various additional submicroscopic techniques are available, but only a few (those which seem to be the most promising for research of soil materials) have


63

been tested so far, namely, Raman spectroscopy (RS), laser microprobe mass analysis (LAMMA), electron spectroscopy for chemical analysis (ESCA), and Auger electron spectroscopy (AES). Raman spectroscopy has been tested for soil materials by Jeanson (1981). Illumination of the soil sample is done with a laser beam. The Raman spectrum is obtained when monochromatic light is passed through a transparent substance. The light is scattered by the transparent substance and undergoes energy transformations. The frequency of some of the light is therefore changed, resulting in the addition of certain lines to the Raman spectrum. The lines of the spectrum are thus characteristic of the molecular structure of the examined area in a thin section or soil ped. An important aspect of RS is that it is nondestructive. Laser microprobe mass analysis, which is destructive, has been done with the LAMMA 500 of Leybold-Heraeus (Henstra et al., 1980a; Bisdom et al., 1981). An optical microscope is used to focus a high-power pulsed laser onto an area of the thin section or soil ped less than 1 pm in diameter. A microvolume of about 10- '*-lo- l 4 cm3 is evaporated and ionized. These ions are detected by a mass spectrometer, also used in ion microscopy. The difference between the two methods is that excitation of the ions takes place with a laser beam in LAMMA and with primary ions in ion microscopes. The LAMMA 500 was made for the investigation of ultrathin specimens of 0.1- 1 pm, and the light optical and ion detection systems were therefore placed on opposite sides of the piece of ultrathin section. Thicker thin sections of about 15 pm could therefore only be analyzed by applying laser milling, in which the laser shots evaporate soil material from the edges of the piece of thin section inward. So far, in the LAMMA lo00 (Fig. 5), the optical and ion systems are placed on the same side of the sample. Consequently, our common thin sections can now remain on their support glass during analysis. Electron spectroscopy for chemical analysis uses X rays or uv photons to irradiate materials in thin sections (Henstra et al., 1981b). Ultraviolet and X-ray electromagneticradiation can be used to excite outer- or inner-shell electrons and this causes the ejection of electrons (McCrone and Delly, 1973). Ultraviolet radiation, with its long wavelength and lower energy, can eject the outermost electrons, whereas X-ray radiation, which has a short wavelength and higher energy, can eject inner-shell electrons. The energies of the ejected electrons can be used to distinguish pure elements and elements in a bonded state; the difference can be observed as different peaks in an energy spectrum. Electron spectroscopy for chemical analysis is mainly used for chemical bonding studies in situ. Tables by Wagner el al. (1979) are available for ESCA studies; all elements except hydrogen can be studied. The depth of analysis is 2-10 nm, and the minimal detectable concentration is about 0.1%. Electron spectroscopy for chemical analysis will usually succeed for soil materials that are homogeneous over fairly great distances; the lateral resolution of analysis is about 3 mm, which

64

E. B. A. BISWM

FIG.5. Laser microprobe mass analyzer (LAMMA 10o0, bybold-Heraeus).

FIG. 6. AES (Auger electron spectroscopy), ESCA (electron spectroscopy for chemical analysis), and SIMS (secondary ion mass spectrometry) are possible with the LAS 3000 (Riber).


65

is too large for heterogeneous samples with small particles. Bemer and Holdren (1977) have used this technique for the study of weathered feldspar. Auger electron spectroscopy is a typical surface analytical technique. Primary electrons are used to obtain Auger electrons from the sample. The energy of the Auger electrons is recorded in an energy spectrum which can be used for quantitative and qualitative chemical analysis of soil materials up to a depth of 1-2 nm below the surface of the thin section. All elements except H and He can be analyzed, and the lateral resolution of analysis is about 0.1 pm. The minimal detectable concentration is about 0.1%(Henstra et al., 1981b). Experiments with LAS (see section II,B) gave results on iron-coated organic material from the Netherlands, but no information was obtained on nonconductive clayey material. The LAS instrument (Fig. 6) was able to perform SIMS and ESCA analyses.

Ill. APPLICATIONS OF ELECTRON MICROSCOPY Applied electron microscopy has been subdivided into studies of unhardened samples (Section II1,A) and thin sections (Section II1,B) because most of the in situ submicroscopic literature describes unhardened materials in soil peds. These studies were usually performed with an SEM that had no equipment for microchemical analysis. The main goal was the study of the morphology of soil particles in the peds and the arrangement of the soil into certain fabric patterns. Thin sections were used for microchemical analysis with the EMA, an instrument which was specifically built for this purpose. Analysis by the SEM-EDXRA and the SEM-WDXRA followed later. Currently, microchemical analyses can also be done in soil peds with the SEM-EDXRA, and the morphology of soil particles and certain types of fabrics can also be studied in thin sections. It remains true, however, that quantitative analysis of the chemical elements in soil constituents requires a polished surface and must be done with an EMA or an SEM-WDXRA. The most impressive three-dimensional morphology of soil components is found in unhardened soil peds using the SEM. Several review articles have been written on electron microscopy as applied to soils. The use of TEM, SEM, and EMA in pedology was discussed by Bocquier and Nalovic (1972). The use of light microscopy, TEM, and SEM in micropedology was treated by Stoops (1974). A number of submicroscopic techniques which can be applied to soil micromorphology were given by Smart (1974), and details of TEM and SEM techniques as applied to soils and sediments are discussed by Smart and Tovey (1981, 1982). Published studies using SEM and EMA were indicated by Bisdom et al. (1976). Two review papers have been published in which TEM, SEM, SEM-EDXRA-WDXRA, EMA, and nonelectron microscopic work on thin sections of soils (i.e., ion microscopy, laser

66

E. B. A. BISWM

analysis, and electron spectroscopy for chemical analysis) are discussed (Bisdom, 1981a,b) .

1. General

The submicroscopic study of unhardened soil samples can be done by TEM,

STEM,and SEM. The TEM and STEM can give magnificationsover X 1,OOO,OOO

depending on the type of soil particles that are studied. The TEM and STEM are usually used for very small soil particles that are present in ultrathin sections or in pretreated and disturbed samples. Ultrathin sections are discussed in Section III,B, but pretreated and disturbed samples form no significant part of this article. The SEM can reach magnifications of more than X100,OOO. The maximum magnification is again dependent on the type of soil particle that is investigated. The SEM is an ideal instrument for three-dimensional studies of soil constituents and therefore it is frequently used for morphological examination. Much attention has also been paid by specialists in soil mechanics and soil microscopy to the spatial relationships between individual constituents in soil peds.

2 . Clay Minerals Individual clay minerals in soil peds or aggregates are usually difficult to recognize with the SEM because they commonly form stacks that can be partly or wholly coated with other fine soil constituents. X-raypowder diffractograms of bulk and disturbed samples and TEM studies of pretreated and disturbed individual clay minerals are usually performed simultaneously with SEM studies of materials in soil peds. Keller (1976a,b,c, 1977a,b, 1978a) and Keller and Haenni (1978) studied kaolinite in various deposits around the world and were able to classify these deposits into transported and residual types on the basis of texture differences found in scanning electron micrographs. Gillott (1974) and Tessier and Berrier (1978) recognized that the in situ investigation of clay minerals in soil peds required ‘special preparation techniques such as freeze-drying or critical-point drying if air-drying does not give the required results. Smart and Tovey (1982) discuss these and other techniques for electron-microscopic work. Spherulitic halloysite in volcanic deposits was examined with the SEM and TEM by Sudo and Yotsumoto (1977) and Violante and Violante (1977). Differences in shape and mineralogical properties were found to exist between the spherulitic halloysite bodies. Sudo and Yotsumoto (1977) called the bodies “chestnut-shell-like” on a morphological basis and “allophane-halloysite-


67

spherules” on a genetic basis (i.e., halloysite formed from allophane which had originated from volcanic glasses). Violante and Violante (1977)explained that the spherulitic halloysite possibly may have formed inside vitreous bubbles as a result of processes exerted by surrounding minerals. Palygorskite in acid-etched carbonate nodules was studied with the SEM by Yaalon and Wieder (1976)and in calcareous crusts by Nahon et al. (1975).The morphology of allophane, immogolite, and halloysite in volcanic ash soils was studied with the SEM by Eswaran (1972). Various scanning electron micrographs of clays were presented by Smart and Tovey (1981).Scanning electron microscopy-energy dispersive X-ray analysis has obtained information on chemical elements present in clay of soil peds. The maximum magnification at which this is possible is XlO,OoO, and the analysis can be performed on a l-Fmdiameter spot. 3 . Weathered Minerals

Much SEM work has been done on various types of weathering minerals. Although clay minerals also weather, most such studies concern the larger primary minerals such as feldspar, olivine, mica, and quartz. Minerals like feldspar, mica, and quartz can also form a part of the clay fraction of a soil, but it is the larger particles which are studied because they can be compared with the results obtained from light-microscopic investigation of the same or similar samples. a. Feldspars. Scanning electron micrographs of feldspars weathering to halloysite and kaolinite have been published by Eswaran and de Coninck (1971). The feldspars weathered to halloysite in an Entisol and to kaolinite in an Ultisol. Feldspar altered to kaolinite and gibbsite in a granite profile from Malaysia. No intermediary crystalline or amorphous phase was found during such weathering, whereas the amorphous phase was present during the transformation of feldspar into halloysite (Eswaran and Wong, 1978). Weathered feldspar in decomposing basalt was photographed with the SEM by Benayas and Alonso (1978).In Israel, weathering of plagioclase gave halloysite pseudomorphs in the vesicularly weathered basalt and smectite pseudomorphs in the saprolite profiles (Singer,

1973). Feldspars in Scottish soils showed holes and pits due to continuous dissolution and etching (Wilson, 1975). Such holes and pits were thought to have originated where crystal dislocations met the surface of the feldspars. No residual layer was observed at the boundary between the unweathered feldspar and a void or crack. The existence of such a residual layer was found to be unlikely. Experimental etching of a microcline perthite (Wilson and McHardy, 1980) confirmed that etch marks developed along crystal dislocations emerging on the surface (i.e., dislocations associated with perthitic lamellae). Analyses with ESCA (see Section I1,C) by Berner and Holdren (1977)of surface layers of feldspars c o n f i e d

68

E. B. A. BISDOM

that weathering occurs along dislocations, cleavages, and fractures (i.e., a residual layer, which requires an equal rate of attack on all parts of a feldspar grain, is not necessary and probably does not exist). Pitted feldspar in an altered rock fragment was thought to be the result of dissolution (Taupinard, 1976), whereas Keller (1978b) explained this pitting as the result of uneven dissolution and nonuniformity in composition. Millot et al. (1977) indicated that such pits in feldspar could originate when secondary calcite replaced feldspar, a process which was called epigenesis. b. Quartzes. Many SEM studies concern the morphology of weathered quartz grains. If the degree of weathering of individual quartz grains can be assessed, it is possible to use such information to deduce the developmental history of individual horizons in a soil profile. Legigan and Le Ribault (1974) studied the evolution of quartz in a humic and fermginous podzol in France that was developed in aeolian sands. Surface features of the quartz grains were related to the sedimentary and pedological history of the profile. Well-polished surfaces indicated transport in streams, whereas polished surfaces found with shock imprints indicated a fluviatile or wind-transported origin. Striae with a certain density on the surfaces of quartz grains were interpreted as being formed by the rubbing of quartz grains against each other during glacial activity. If the quartz grain had dissolution figures on its surface or an iron crust with or without organic matter, it was thought to be caused by pedogenesis. This type of approach permitted the indication of various environments in the studied profile and also helped to unravel the history of the sands. Eswaran and Stoops (1979) worked with a zero phase in a weathering sequence of quartz, established in a Xerochrept formed on Keuper marls in Spain. The quartz crystals were idiomorphic to hypidiomorphic. The surface textures of quartzes were studied in a 19-m-deep profile developed on granite in a tropical environment. Weathering of the quartz started a few centimeters above the fresh rock with fragmentation of the quartz grains and the presence of hairline cracks in the weathered quartz. Etching of the quartz grains occurred at a depth of 18.5 m and the quartz showed large dissolution pits that were interconnected by grooves and hairline cracks. Some idiomorphic secondary quartz was precipitated between the depths of 9.5 and 16 m on the surface of heavily etched primary quartz grains. Triangular dissolution pits were developed in primary quartz grains at a depth of about 9.5 m and heavily etched quartzes with linear striations were found at a depth of 1 m. These linear striations differed from the etch grooves found on the surfaces of quartzes at greater depths in the profile. The surfaces of quartz grains from Neogene sands in the Ivory Coast were examined by Leneuf (1972). The quartzes came from depths of 3,30, and 90 m. Two classes of weathering figures were distinguished, one related to the crystal lattice of quartz and the other apparently not related to it. The first class comprised cavities with the same alignments; cavities with tetrahedral, rectangular,


69

irregular polyhedral, and wedge-shaped forms; fissures with a concentric outline; cubic figures in a regular network; and lines with a relief and at 30, 60, and 120 degrees. The second class contained irregular cavities, fissures related to desquarnation, vermiform fissures, fine particles on the surface of quartz grains, and newly formed secondary quartz from silicon which had passed through the profile. These surface features on the quartz grains indicated that silicon had been mobilized in the upper part of the strata and that only part of the silicon participated in the formation of kaolinite. Secondary quartz could form from the transported silicon at deeper levels in the profile. Scanning electron micrographs of quartz particles in surface soils of the Hawaiian Islands were studied by Jackson et ul. (1971). These soils developed over quartz-free mafic (basic) rocks. Wind deposition of the quartz was inferred by comparison of the sharp angular, chip- or shard-like morphology of the grains with that of quartzes in aerosolic dust and pelagic sediment. The percentage of quartz varied with the elevation of the soil, the age of the soil, and the amount and source of annual rainfall. Scanning electron microscope and X-ray diffraction (XRD) analyses of airborne particles indicated that the coarser ones, with radii of 10-100 pm, consisted predominantly of quartz, whereas the finer particles, with radii of 1-10 pm, were mainly clay minerals. The clay minerals were found in the air as constituents of aggregates, as coatings on quartz grains, and as individual platelets, and were derived from the soil by sandblasting. Riezebos (1974) studied weakly cemented Miocene sands of deposits from South Limburg, the Netherlands, with the SEM. Secondary quartz was found not only at grain contacts but also around detrital quartz grains. Overgrowth of secondary quartz on the larger grain surfaces formed steps and striations. Such steps and striations were therefore not the result of glacial environments. Douglas and Platt (1977) investigated the surface morphology of quartz and the age of soils in glacial material from Wisconsin. Quartz in late Pleistocene (Wisconsin) deposits was only slightly weathered with a mainly broad, flat or conchoidal breakage surface, and sharp or slightly rounded upturned plates. Quartzes in sands of Illinoian age showed both sharp and rounded upturned plates. Precipitation of secondary silica had occurred on the quartz grains and a modification of the surface morphology was the result. Some solution pits were also present. Corroded surfaces with solution Vs and highly rounded upturned plates were found to be associated with quartz grains of Kansan age. The rounded forms were caused by dissolution and precipitation of silica. Flaking was also found, representing intense chemical weathering. Moss and Green (1975) pointed out that the concept of deformation sheeting (i.e., forming plates, steps, etc. on the surface of quartz grains) probably is more realistic than explanations based on existing cleavages in quartzes. Attention was also paid to microfractures and the laminae of quartzes between them called “sheets.” Such a sheet of quartz, usually 2-20 Fm thick, was considered to be

E. B. A. BISDOM

70

the smallest weathering entity. Microfractures can subdivide the sheet into small-

er particles that are clay sized. In nature, however, quartz is frequently common in the 2- to 20-pm silt fraction and does not occur in a dominant form in the clay fraction. It was also pointed out by Moss and Green (1975) that quartz grains can already be well-rounded when they leave the source rock and that it is therefore unrealistic to always assume angular particles that gradually become more rounded with increasing maturity. Conversely, angular quartz can often be found in soils and sediments. Magaldi (1978) indicated that two contradictory interpretations exist, one which cites the more rounded and another that cites the more angular quartz grains during weathering. A cathodoluminescent (CL) study of quartz sand grains was made by Tovey and Krinsley (1980), who pointed out that the common secondary electron (SE) micrographs (emissive mode micrographs) do reveal surface information on the quartz grains but no subsurface information as seen in cathodoluminescent micrographs. The surfaces of quartz grains, cross sections of quartzes, etched grains, and heated quartzes were studied with the SE and CL modes. Cathodoluminescence is significantly affected by slight changes in the chemical composition of the quartz grain, and cracks that are not visible in the SE mode can often be recognized in the CL mode. Study of the spatial distribution of narrow and broader dark bands, of dark patches, and of other characteristics in the CL micrographs, together with information obtained from SE micrographs, allowed some insight into the various processes which affected the quartz grains. c. Micas. Scanning electron microscope studies of weathered micas are often done in combination with nonsubmicroscopic techniques. Jackson and Sridhar (1974) studied Li exfoliated and freeze-dried phlogopite flakes. Scanning electron micrographs indicated that the osmotic force and swelling created by Li+ resulted in the gliding out of interstratified saponite layers which became twisted and curled during this process. Saponite was formed from phlogopite with vermiculite as an intermediate. Gliding out of layers only occurred when salt was removed from the solution and electric double-layer swelling took place in distilled water during the experiments. Scanning electron microscopy allowed the study and portrayal of tracks and holes in micas (Lee et al., 1974). The tracks were produced by spontaneous fission of 238U under natural conditions (235U must be activated to give thermal neutron bombardment and induced fission particle tracks). Upon splitting of the uranium nucleus, large fragments can move through the micas with considerable energy and leave behind trails of damage called “tracks” which are about 20 pm long and have diameters of about 0.015 pm. These fission tracks play a role during the weathering of micas and also influence cation exchange capacity. Tarzi and Protz (1978) studied the weathering of micas obtained from rocks. Upon the start of weathering, the micas split at their edges and this process proceeds inward along planes. The exfoliated stage is reached when the layers +


71

become separated. During the exfoliation process bending may affect the individual layers which may then take various forms. Holes in the micas were thought to have been occupied by quartz and other minerals, rather than to have resulted from spontaneous fission processes as advocated by Lee et al. (1974). Secondary material could accumulate in the spaces provided by the weathering micas. Crusts could form in them and roots could penetrate the mica. Secondary micas are frequently observed in weathering micas. Verheye and Stoops (1975) made a scanning electron micrograph of kaolinite between biotite lamellae in a soil from the Ivory Coast. Illite was distinguished by Taupinard (1976) on weathering biotite flakes of an altering granite in France together with dissolution, new formation, and disaggregation features. Sousa and Eswaran (1975) found that large biotite flakes in a saprolite from Angola were pseudomorphically altered to goethite. Scanning electron microscope observations indicated that microdroplets of goethite covered the surfaces of weathered biotite. d. Other Minerals. Dissolution of olivine to deeply etched and pitted weathered olivine probably occurred at particular sites where structural dislocations emerged in the olivine, similar to weathering feldspars (Wilson, 1975). This weathering mechanism was confirmed during experimental studies by Grandstaff (1978). The initial dissolution of freshly crushed olivine was where lattice imperfections occurred (e.g., dislocations and cleavage planes). Pits and rounded edges were found in altered forsterite. Dissolution was more rapid along surface discontinuities than along the general surface in the initial phases of weathering, whereas surface dissolution could dominate the overall rate of reaction in subsequent phases. Berner et al. (1980) studied the weathering features of augite, hypersthene, diopside, and hornblende. In the initial phase, only part of the surface of the altering pyroxenes and amphiboles was affected, as was the case with olivine and feldspar. Lens-shaped etch pits formed parallel to the long and short axes of the minerals, according to SEM observations, and this gave different alteration patterns of deeply striated surfaces with end-to-end alignment along the long axes and rough-walled cracks with side-by-side alignment along the short axes. Secondary clay could be found in cracks of the weathered minerals. Tooth- or needle-shaped walls were present in the cracks because primary mineral fragments were maintained between expanding lens-shaped pits during weathering. Scanning electron micrographs of weathered amphiboles from Israel also showed tooth- and needle-shaped walls of cracks and pores (Williams and Yaalon, 1977). Detrital garnets from fluviatile, littoral, and aeolian desert sands were studied with the SEM by Magaldi (1977). Furrows, V-shaped pits, triangular pits, quadrangular pits, clusters of polygon-shaped pits, and coalescent etch figures were found. Flicoteaux et al. (1977) studied the alteration of phosphate minerals in phosphate-containing Cretaceous-Tertiary sediments of the Senegalese-

72

E. B. A. BISDOM

Mauritanian basin. Pseudomorphous transformation of wavellite to crandallite was found. Crandallite crystallites could take different orientations with respect fo wavellite. Scanning electron microscopy also demonstrated an increase in porosity during the transformation of wavellite to crandallite. M o m (1978) studied isotropic phosphatic nodules, probably weathered guano fragments, in the A1 horizon of a soil developed on basaltic colluvium on Santa Fe Island of the Galapagos archipelago. Small craters and globules were present in the nodules.

4 . Newly Formed Minerals

A considerable number of newly formed minerals in unhardened samples of soils have been studied by SEM. Submicroscopy has mainly been used to obtain information on the surface morphology of the minerals. Nonsubmicroscopic techniques were used primarily for identification purposes. a. Carbonate, Gypsum, Anhydrite, and Celestite. Needle-shaped calcite from Turkey, called lublinite, was studied with the SEM by Stoops (1976).The individual lublinite crystals were stacked in an echelon with their c-axes in a parallel position. This explained certain optical characteristics as determined in thin sections with the light microscope. Various scanning electron micrographs of lenticular gypsum, weathered lenticular gypsum with a comb structure, gypsum microlites, and a rosette-like aggregate of prismatic gypsum crystals were published by Stoops et al. (1978). The authors also studied anhydrite fibers, which were parallel to each other, on gypsum in soils from Peru. Celestite was found as long square prisms elongated according to (100)and had a well-developed (011)form. Stoops et al. (1978)found celestite in gypsiferous soils from Algeria, Iran, and Iraq. Upon weathering of celestite, grooves could develop normally to the prism faces. b. Halite, Thenardite, Bloedite, Hexahydrite, and Barite. The morphologies of halite, thenardite, and bloedite were studied with the SEM by Driessen (1970) and Driessen and Schoorl (1973).These salts came from the Konya basin in Turkey and were present in salt crusts. Mirabilite was recognized in the field but could not be transported to the lab because of its high water content. The porosity of the salt crusts could also be investigated, and it was found that the needle-shaped thenardite gave more porosity to the crust than the platy bloedite. Halite could seal the surface of the soil. Vergouwen (1981)studied salts from the same basin in Turkey with the SEM-EDXRA. Crystallographic properties and morphologies of individual salt crystals were examined. The relations between different salt crystals in salt assemblages were also studied. It was found that identification on the basis of morphology alone is not always possible; microchemical in situ analysis with the EDXRA is then necessary. Thenardite occurred in two crystal forms, as needles and in another crystal form when


73

associated with other salt minerals. Trona, bloedite, and hexahydrite made the salt crust very fluffy. Halite formed a smooth crust and sealed the soil. Several scanning electron micrographs of various morphologies of halite in soils were published by Eswaran et al. (1980). Attention was also given to crust formation by halite. Tursina et al. (1980) published scanning electron micrographs of thenardite in hydromorphous Solonchaks from the Soviet Union. Scanning electron microscopy also permitted the effect of salt crystallization on soil fabric and structure to be studied. Stoops et al. (1978), using the SEM, found hexahydrite on ped surfaces of a salic Gypsiorthid from Iran. Hexahydrite was mixed with gypsum crystals. Barite (microlites consisting mainly of prism) was found by Stoops and Zavaleta (1978) in a typic Haplustalt of Peru. c. Pyrite, Jarosite, and Gypsum. Scanning electron micrographs of pyrite, jarosite, and gypsum in a paleosol of eastern Nigeria were published by Moormann and Eswaran (1978). Pyrite framboids were found associated with organic matter, and fine gypsum needles could protrude from these. van Breemen and Harmsen (1975) photographed jarosite by SEM before and after dialysis with distilled water over a period of 4 months. Miedema et al. (1974) studied pyrite, jarosite, and gypsum in four soils of inland polders of the Netherlands. Paramananthan et al. (1978) investigated the effects of drainage on pyrite-containing marine clays in the coastal area of Malaysia and presented SEM photographs of pyrite, jarosite, gypsum, ferriorganans, fungal mycelia, and diatoms. d. Iron- and Manganese-Containing Minerals. Iron-containing minerals in laterites have been the subject of a number of SEM studies. Schmidt-Lorenz (1974a,b, 1975) studied many laterites of tropical regions and remnants of laterites in paleosols of Europe. Several scanning electron micrographs of hematite and various types of goethite were presented. The process of lateritization was subdivided into primary and secondary ferrallization. Kuhnel et al. (1975) studied goethite in laterite profiles and found that the highest crystallinity of the mineral was found near the surface of the laterite and the lowest at the base of the profile between the soil and bedrock (i.e., at the start of weathering). Poorly crystalline goethite could also contain nickel, chromium, and aluminium. Hematite and goethite crystallites were studied with the SEM in plinthite by Moormann and Eswaran (1978) and Eswaran et al. (1978). Iron-containing minerals have also been studied in nonlateritic soils. Lepidocrocite was found in the upper part of the B horizon of Molkenpodzols in the Vosges of France (Guillet et al., 1976). Lepidocrocite was a weathering product of hematite and occurred as stacks of subparallel platelets, with local intermineral porosity, on scanning electron micrographs. Babanin et al. (1976) indicated that very fine goethite particles with diameters of less than 5-6 nm were dominant in Ortstein. The forms and compositions of iron compounds in various soil concretions in a number of soils from the Soviet Union were investigated.

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E. B. A. BISDOM

Scanning electron microscope studies of manganese-containing minerals such as lithiophorite, nsutite, birnessite, and feitknechtite were made by Eswaran et al. (1978). The minerals were present in nodules found in tropical soils. e. Other Minerals. Various forms of gibbsite were studied in tropical soils by Eswaran et al. (1977). Gibbsite can be present in very small amounts in tropical soils but may also form gravel-sized aggregates or sheets that are recognizable in the field. Dobrovolsky (1977) studied gibbsite crystals with a diameter of 1-20 pm in peaty soils of the Kilimanjaro area of Africa at an altitude of 2950 m above sea level; he favored a biological origin of the mineral. Biogenic opal has been studied with the SEM in soils of the United States (Wilding and Drees, 1971, 1973, 1974; Wilding and Geissinger, 1973). Opal isolated from trees differed considerably in amount and size and was dependent on the tree species. Only hackberry produced enough opal to be incorporated in the soil. Scanning electron micrographs demonstrated that there was a characteristic difference between tree-leaf opal and grass opal. Opaline isolates of wet soils often contained sponge specules and diatoms. Wilding et al. (1977) presented a review on silica present in soils and the conversion of silica hydrogel to silica polymorphs (opal, chalcedony, quartz, cristobalite, and tridymite). 5 . Organic Matter Humic and fulvic acids (HA and FA, respectively), inclusive of metal and clay complexes, were studied with the SEM by Chen and Schnitzer (1976). Fulvic acid morphologies were investigated at pH 2-10 and those of HA at pH 6-10. Metal-FA and clay-FA complexes were also studied at different pH. The SEM was used by Bruckert et al. (1974) to investigate organomineral complexes in aggregates from Andosols of the Canary Islands and France. The morphology of these aggregates was different from that of aggregates consisting of a clayhumus complex. Benayas et al. (1974) published a scanning electron micrograph of plant remains and small soil components in the upper part of an Andosol in the Canary Islands. Organomineral complexes in alkaline extracts of soil were investigated by Dormaar (1974). Fungal aggregates in sand-dune soil from Canada consisted of threads of branching mycelium from fungi to which sand grains adhered (Clough and Sutton, 1978). It was also found that amorphous material could form a sheet on the hyphae and act as an adherent between fungal hyphae and sand grains. The amorphous material consisted of polysaccharides and was possibly produced by fungi or bacteria. Aggregates formed when the fungal mycelium was in active symbiosis with the host plant.

6. Soil Structure and Fabric Numerous studies have been performed with the SEM to obtain information on various aspects of soil structure and fabric. Specialists in soil mechanics have


75

done considerable work to obtain information on the behavior of especially clayey soils under different experimental conditions, whereas soil micromorphologists frequently have had a closer look at soil constituents in soil peds and aggregates. a. Arrangements, Orientations, and Behavior of Soil Components under Various Conditions. Scanning electron micrographs and X-ray diffraction measurements were made of oriented clay samples obtained at different pF values by Tessier and Pedro (1976). Micrographs were made parallel or perpendicular to the orientation plane of the clay platelets of calcium kaolinite, calcium montmorillonite, and calcium illite. It was seen that considerable changes in the clay structures could occur with only minimal changes in the measured ranges of pF values; changes were greatest in the lower pF ranges. Structural changes in soil pore systems induced by Na/Ca exchange were studied with the SEM by Chen et al. (1976). At a low sodium adsorption ratio (SAR), fine material adhered to the sand grains or formed large aggregates. At a higher SAR, the fine material separated from the sand grains and filled pores. Another result was that calcium montmorillonite formed large irregular porous aggregates when a suspension was quickly frozen and dried, whereas sodium montmorillonite gave very thin sheets that were usually folded. An explanation for this phenomenon was presented. Sheeran and Yong (1974) indicated that rearrangement of soil particles in the soil environment is relatively simple as long as the soil is porous, but can only occur by way of individual minerals if only little porosity is left. Experiments indicated that virtually all changes in the orientation of clay particles occurred at lower pF levels, a result which was also obtained by Tessier and Pedro (1976). Much work on the quantification of individual clay particle alignments, including those in scanning electron micrographs, has been done by Tovey (1974, 1980) and Tovey and Wong (1974, 1980). Attention was given to photogrammetric and quantification techniques used in TEM, light microscopy, and XRD. A film measuring technique and digital computer techniques for the quantitative analysis of the orientation of clay particles in scanning electron micrographs of peds and aggregates were discussed. Such techniques can help in the quantification of soil fabric types in such micrographs. Attention was also given to particle alignments in scanning electron micrographs caused by mechanical stresses during experiments and various preparation techniques such as oven-drying, airdrying, substitution-drying, freeze-drying, and critical-point drying. The SEM has also been used to study the broken surfaces of soil fragments from a thin iron pan, the argillic horizon of an alfisol, and the cambic, argillic, and oxic horizons of tropical soils formed on basalt (Eswaran, 1971). Argillans have also been examined with the SEM (Osman and Eswaran, 1974; Callot, 1978; Koppi, 1981). Using the SEM, an impression of the degree of orientation of clay and silt in pores can be gained; whether microlayers are present or absent in the argillan can also be ascertained.

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b. Aggregates and Crusts. Scanning electron microscope studies have been done by Moreno et al. (1978) of aggregates from black earth in Southern Spain. Clay minerals exhibited platy intermineral pores when observed at higher magnifications with the SEM. The aggregates also contained a few cylindrical pores with diameters of 0.5-2 pm. The aggregates in the soil had a similar microstructure. Buol and Eswaran (1978) investigated aggregates in oxisols and found that inter- and intraaggregateporosity could be considerable. Moura and Buol(l975) studied a Eutrustox in Brazil. The soil originally had a porosity of between 15 and 34%; continuous cultivation over a period of 15 years had decreased the porosity to 10-22%. The type of porosity was studied with the morphology of the minerals on the surfaces of pores, fractures, and aggregates. Only a few fine pores were present in clay balls, which had a higher density than the surrounding soil materials. Aggregates and weathered complexes in Andosols of France were studied in detail, with the inclusion of SEM and TEM, by Hktier (1975). Organomineral complexes were extracted from the aggregates in a step-by-step method, and each of the residues was studied. The aggregates had diameters of 5-50 pm and contained minerals, organic matter, and embedding cement which were apparent at higher magnifications. The minerals were coated with organomineral complexes. Extractions removed virtually all of the coatings, but an insoluble humic debris consisting of humin remained on part of the mineral surfaces. It was also demonstrated that humic acids, which were the most condensed and stable, were situated in clay-humus spherules, the central part of which were often occupied by glomerated halloysite. Toogood (1978) performed studies on aggregate stability in Ap horizons of various soils in Alberta, Canada. Only very weak correlations were found between the stability of the aggregates and their organic-matter content, clay percentage, carbonate content, or specific surface. On a microscale, considerable differences between individual aggregates were indicated by SEM,and the suggestion was made that general rules should be developed to explain aggregation and cementation for each individual soil type in separate regions and under different management systems. This could form a basis to obtain techniques for improving aggregate stability for individual soils. Intergranularcontacts were examined in sands, loesses, and clays by Barden et al. (1973) to study collapse phenomena when wetted under load. The SEM allowed the investigation of the arrangement of individual and of combinations of clay platelets on and between larger soil components at various magnifications. Ducloux and Ranger (1978) examined aggregates in fragipan horizons of French soils with the SEM. Strands and bridges of clay minerals and iron oxides were found between the aggregates. Such a structure can be rigid, and if it is broken will break by brittle failure. Wang et al. (1974) studied a large number of fragipans in Nova Scotia, Canada. Clay bridges were indicated by SEM between


77

coarser particles, and these linkages were interpreted to give brittleness under dry conditions. If the bridges become wet they can be deformed and may collapse to give slaking. The clay bridges also exhibited a low permeability to the fragipans because they create a discontinuous pore system between the coarser soil constituents. The SEM was used by Ehlers (1977) to study the morphology and texture of a thin silt crust, usually less than 1 mm thick, in the Ap horizons of “Parabraunerden” from the Federal Republic of Germany. The aggregates in the loess can easily break down because only limited binding substances such as calcium carbonate, organic matter, and clay are present, in addition to considerable quantities of silt. Erosion of the Ap therefore occurs and silt sheets with a thickness of several centimeters may form on the slope of a hill. Miehlich (1978) studied “Tepetate” (a duripan) in central Mexico. The cementing agent was amorphous silica, probably mostly opal, which was derived from the weathering of volcanic glass in ash deposits. Porosity in soil aggregates can be measured with the SEM combined with an image analyzer (Sergeyev et af., 1980a; Sokolov et af., 1980). The soil aggregate or soil ped is broken in half and the conjugated surfaces are photographed by the SEM. Subsequently, the film of the two surfaces can be manipulated to obtain the real pores in the aggregate. These can then be measured using an image analyzer such as the Quantimet. Gillott (1980) used SEM and Fourier methods to obtain information on grain shape and texture of sediments. This information was used to study provenance, correlation, pedogenesis, and environmental processes. Sergeyev et af. (1980b) examined the microstructure of many clay soils with the SEM and were able to identify five main types; turbulent, laminar, honeycomb, skeletal, and matrix. Mathematical morphology methods were used to obtain quantitative structural characteristics.

B. THINSECTIONS I . General

At present most of the submicroscopic work with thin sections concerns microchemical analysis. All such studies offer information on chemical elements, usually the heavier ones, in thin sections of soils 15-30 pm thick. Many of these studies, done with the EMA and SEM-EDXRA, concentrate on measurements of transported materials in soils, on minerals, and on soil constituents which are difficult to determine with the light microscope. With the SEM-EDXRA one can also analyze soil materials in unhardened soil peds, whereas with the SEMWDXRA one can only analyze materials in thin sections. Microanalysis is also possible with the TEM and the STEM if these instru-

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E. B. A. BISDOM

ments are equipped with an EDXRA. The TEM-EDXRA analysis can be done on materials in ultrathin sections; STEM-EDXRA analysis can be done on both ultrathin (about 1 p,m thick) and thicker-to-common (15-30 pm thick) thin sections. Such analyses can be done at magnifications much larger than are possible with the EMA or the SEM-EDXRA-WDXRA (Le., more than X 10,OOO to more than X 1OO,OOO, depending on the thickness of the thin section). Electron diffraction of individual soil components can also be done on ultrathin sections if they can be prepared.

2 . Clay Minerals a. Thin Sections. The EMA was used by Brinkman et al. (1973) to study the chemical elements of cutans in a thin section of a pseudogley horizon in coversand soil from the Netherlands. The SiO,/Al,O, ratio was 3.5 in unaltered parts of cutans and 4-6.5 in weathered parts. This indicated a relative accumulation of silica in the weathering cutans. X-ray diffraction microcamera work indicated that less clay minerals and more extremely fine-grained quartz were present in the altered cutans. This suggested the new formation of microcrystalline quartz from silica derived from weathering smectite and illite. The elements Fe, Al, Mg, and K were partly removed from the cutans, whereas residual enrichment of mile and minerals of the kaolinite group occurred in the cutans during weathering. The plasma in a tropical groundwater podzol (Tropaquod) from Surinam was examined by Veen and Maaskant (1971) with the EMA. Cutans of a hardpan present at a depth of 120-150 cm exhibited wide extinction bands when viewed with the light microscope. Fine soil material of the matrix between coarser grains in the hardpan could be isotropic, and EMA indicated that SiO,/Al,O, ratios were lowest for this isotropic plasma; this was also the case for the SiO, percentage. Percentages of A1,0, could be lower or higher when compared to the birefringent soil matrix. The conclusion was that the birefringent plasma was broken down to form a gel-like isotropic substance which had less SiO, but was rich in A1,0,. Bajwa and Jenkins (1978) investigated the potential of selected techniques for the identification of clay minerals in thin sections of soils. If relevant standards were available and if relatively pure accumulations occurred in an area of several square micrometers, the identification of individual clay minerals was possible. Under such conditions, major element ratios of individual clay minerals and the differential adsorption of Sr were adequate to distinguish most clay minerals. Jenkins (1981) introduced the technique of low temperature ashing in which features observed in thin section under the light microscope can be exhumed by destruction of the impregnating resin. When the plastic is removed from a pore it becomes possible to study the minerals which are present in the wall of such a


79

pore by SEM or SEM-EDXRA. This makes the transition of light microscopy to SEM of three-dimensional entities easier. Thin sections of various soils in Buenos Aires, Argentina were studied with the light microscope by Scoppa (1978- 1979). Scanning electron micrographs were made of acicular calcite and of an argillan with montmorillonite. Bocquier and Nalovic (1972) studied, among others, ferriargillans around a pore in a sesquioxidic nodule from Cameroun with the EMA and the SEM. The argillan had two zones: an outer zone nearest to the pore in which Si, Al, and some Fe were found, and an inner zone with more Fe and less A1 in the X-ray images. It was interpreted that kaolinite was present in the outer zone whereas iron hydroxides were dominant in the inner zone away from the pore and against the matrix of the nodule. Primary and secondary illuviation of “lessivd” soils, formed in silty materials from France, were examined with the SEM and the EMA by Jamagne and Jeanson (1978). Primary illuviation took place in brown leached soils with a Bt horizon (sols bruns lessivks”). Organomineral complexes were formed and a partial desorption of the adsorption complex took place. Ferriargillans are characteristic of these brown and leached soils. Secondary illuviation occurs in leached soils with tongues (sols lessivds glossiques). Cutans, formed by secondary illuviation processes in the leached soils with tongues, were derived from ferriargillans by deferration. An EMA was used to measure the compositional differences in primary and secondary argillans in situ and to compare these microchemical data with those of transition zones and soil matrices. de Oliveira (1981) used an SEM-EDXRA to investigate femargillans in a hydromorphic profile in the state of Bahia, Brazil. The microchemical data obtained were used for pedogenetical interpretation. An SEM-EDXRA was used by Ledin (1975-1976) to test its possibilities in natural and artificial samples, including clays. The SEM-WDXRA was also used to study limed clay soil from Sweden in thin sections and the crystalline masses of calcium carbonate that formed after the addition of CaO (Ledin, 1981). Calcium carbonate can have a cementing effect when it grows between clay packets or when it partly or totally surrounds microaggregates. Argillans from Glossaqualfs in western France were studied with the SEM-EDXRA by Ducloux (1976). Argillans which were in the process of deferration had a highly variable iron content compared with the primary ferriargillans. Measurements by SEM-EDXRA and EMA indicated that the deferration process occurred sirnultaneously with the extraction of potassium (Ducloux, 1978). Rubefaction (or reddening) of soils was studied by Bresson (1974a,b) in fluvioglacial calcareous materials in the Jura Mountains of France; EMA, SEM-EDXRA, and TEM were used. The red color of the B horizons of the reddish soils was caused by accumulation of reddish materials, derived by leaching from the upper horizons, on the outer sides of structural elements and on the

80

E. B. A. BISDOM

walls of root channels. The leaching of reddish materials started in the upper horizons of the profile after decarbonization of the fluvioglacial calcareous material. Rubefaction can therefore also occur today under humid conditions and in a temperate climate. Ducloux (1978) explained that rubefaction in the silty materials of western France occurred in a rather well-drained leached soil, in neutral and weakly acid environments, after leaching by peptization of iron compounds that were present on and between illuviated clay particles. These iron compounds could color the cutans as well as the soil matrix when dehydrated to form cryptocrystalline, red-colored hematite. Cutans from the B horizons of a Haplohumod, a Humaquept, and a Haplaquod in the Netherlands appeared to be homogeneous under the light microscope (Bisdom and Jongerius, 1978). The SEM-EDXRA was used to test whether this homogeneity was also present if the heavier elements were measured in these cutans. The cutans of the Haplaquod showed a nonhomogeneous composition with EDXRA, but the other two soils were homogeneous in their heavier chemical elements. b. Replicas and Ultrathin Sections. Microchemical analyses of 15- to 30-pm-thick thin sections have been discussed in this section on thin sections. However, small soil particles such as clays can also be studied in ultrathin sections or replicas. The replica technique duplicates the surface morphology or topography of a sample and gives a very thin film that is transparent to electrons. Details of this technique have been discussed by Gillott (1974), Smart (1974), Stoops (1974), and Smart and Tovey (1982). A thin film of metal and carbon is usually evaporated onto the surface of the sample. The replica is obtained when the film and the soil are separated. Such a stripped film is usually subjected to shadowing and needs support by a fine-mesh metal grid (McKee and Brown, 1977). Individual particles in the replica can only be recognized if their morphology is characteristic. Replicas do not allow microchemical or roentgen identification. Consequently, the use of replica techniques is limited but can give information on the microfabric of a soil sample at very high magnifications using a TEM (Benayas et al., 1974; McKyes et al., 1974; Singer and Norrish, 1974; Benayas and Alonso, 1978). Biogenic opal and inorganic opal in gems were studied by Wilding et al. (1977) using the replica technique. The bonding or cementing agents of the Champlain Sea clays were investigated using a combination of replica, XRD, and selective dissolution techniques. It was found that amorphous materials played an important role by coating the minerals in the soil. This made the soils extremely sensitive to machines (McKyes et al., 1974). The preparation of ultrathin sections of the harder soil samples is difficult. SO far, only soft soil materials such as clays and organic matter have permitted the preparation of ultrathin sections. These are usually prepared in two ways, either by ultramicrotomy or by ion thinning. Ultramicrotomy makes use of either a


81

glass or a diamond knife, whereas ion thinning erodes the sample slowly until a hole occurs in the thin section; the soil material can be studied in the ultrathin wedge which surrounds the hole, allowing the transmission of electrons. Details on the preparation of ultrathin sections have been discussed by McKee and Brown (1977) and Smart and Tovey (1982). Ultrathin sections can be studied with TEM and STEM. High resolution electron microscopy (HREM) has not been used for the study of undisturbed small soil materials in ultrathin sections. Disturbed, laboratory-treated clay samples in ultrathin sections were examined with HREM by Brown and Jackson (1973) and Lee et al. (1975a,b). Transmission electron microscope studies of soil microstructure in ultrathin sections of clays were done by Smart (1975). On this ultramicro scale, various types of open and dense microstructures were found. It was advocated that ultrathin sections should be made of clays under natural and experimental conditions. In this manner one can compare microstructures and obtain an idea of conditions under which microstructures in natural clays were formed. Such information is important for studies in soil mechanics (i.e., consolidation, deformation, failure, and compaction effects in soils). Ultrathin sections and TEM have been used to examine the biodegradation and humification of roots by microorganisms (Kilbertus et al., 1972), the decomposition of leaves (Kilbertus et al., 1973), and the decomposition of plant material (Kilbertus and Reisinger, 1975). The root environment was studied in various soils by Foster (1978). It was found that bacteria could play a role in the formation of crumbs in the soil. Some roots are able to secrete large amounts of polymeric material into the soil; this substance can bind both organic and mineral materials. Foster (198 1) used cytochemical techniques to localize organic materials in ultrathin sections (i.e., polysaccharides derived from roots and soil microorganisms, neutral polysaccharides from cell-wall remnants, and polyphenolrich humic materials). Ion thinning and TEM were used by Bresson (198 1) to study microfabrics of clays (plasmic fabrics). Attention was given to the form, size, and distribution of ultramicropores and to the morphology of fine soil constituents. It was found that even at the high magnifications offered by TEM, microfabrics could be complex. An important objective of TEM studies on ultrathin sections is the investigation of the relation between iron compounds and clay minerals in soil microaggregates. Such studies can only be done in a satisfactory manner, however, if the TEM is able to perform electron diffraction for identification purposes. McHardy and Birnie (1975) found, that electron diffraction and identification of single particles in mixtures of fine-grained minerals can be difficult. Coarser minerals in ultrathin sections give less difficulty (Wenk, 1976), as is also true of individual clay minerals (Sudo and Yotsumoto, 1977). Fulvic acid aggregates at various pH values were studied by Schnitzer and Kodama (1975). Jepson and Rowse (1975) studied individual kaolinite particles with TEM-

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EDXRA. When ultrathin sections of all soil materials can be prepared, this type of analysis will be used regularly for microchemical analysis of soil particles at magnifications larger than x 10,OOO. Scanning transmission electron microscopy-EDXRA can do the same. However, in the SEM-EDXRA mode, the microchemistry of normal thin sections (with a thickness of 15-30 pm instead of about 1 pm for ultrathin sections) can also be studied at magnifications above X10,OOO. The best way, however, is to analyze ultrathin sections with TEM-EDXRA or STEM-EDXRA and to combine this with electron diffraction measurements. 3 . Weathered Minerals

Seddoh and Pedro (1975) used EMA to investigate the alteration of biotite, plagioclase, and quartz in weathering granite. X-ray images showed that Fe was concentrated along cleavage planes and microcracks in weathering biotite, whereas Si and Al seemed relatively stable. Magnesium and K were leached along microcracks and cleavages. Special attention was paid to the micmhemistry of soil materials present in microcracks of the minerals. Whalley et al. (1982) studied the propagation of such cracks in igneous rocks by SEM. Meunier (1977) used EMA for the study of the weathering of biotite, muscovite, and feldspar in French granite profiles. Evolutionary trends in mineral weathering could be established using submicroscopic, XRD, and wet-chemical analyses. Stoch and Sikora (1976) studied the weathering of a dark-green mica from a Tertiary weathering crust on granites and gneisses of lower Silesia, Poland, with EMA. Curmi (1979) and Curmi and Fayolle (1981) used EMA, SEM-EDXRA-WDXRA, XRD, and wet-chemical techniques to study mineral weathering in granites of Brittany, France. Detailed submicroscopic and quantitative micorchemical analyses were done which allowed part of the alteration processes from primary to secondary minerals to be monitored. It was found, for example, that the transformation of exfoliated micas to hydroxy-aluminum vermiculite involved a significant loss of K, Fe, Mg, Ti, Ca, Na, and Si. This loss occurred at the edges of the exfoliated lamellae, whereas the central part of these lamellae remained unweathered in these profiles. Biotite, however, was completely weathered close to the surface of the profiles. Submicroscopic measurements of chemical elements were also used to obtain some insight into the nature of newly formed minerals in the profiles. The SEM and the EMA were used by Bottino et al. (1976) to examine the weathering of feldspar in gneiss and micaschist from Italy. Feldspar changed into kaolinite with an intermediate gel phase. If this gel lost Si, gibbsite could form. The same combination of instruments gave information on mycorrhizal weathering of biotite flakes (Mojahli and Weed, 1978). Potassium was lost from flake edges and adjacent to cracks in the biotite; muscovite was not weathered.


83

Berthelin and Belgy (1979) used organic acids produced by microorganisms (i.e., oxalic acids and other complexing agents) to experimentally weather phyllosilicates. Chlorite of granite was entirely destroyed and vermiculite was only partly altered. Biotite could be weathered to vermiculite or to a white, almost amorphous substance containing predominantly Si and Al, or it was completely broken down. Weathering sequences during the experiments were similar to those found during podzolization. Schwaighofer (1976) used SEM-EDXRA and other techniques to study the weathering of pyroclastic rocks from Tenerife, Canary Islands. Measurements were made on weathering titanaugite, titanbiotite, and anorthoclase. Moinereau (1977) examined the weathering of basalt in an organic and humid environment of a temperate climate. Various primary minerals and glass were dissolved by organic acids forming complexes with Al, Fe, Ti, and, to a lesser extent, Mg and Ca from the minerals. Potassium and Na were completely leached; Ca and Si were partly leached. Amorphous gel-like material on the surface of a basalt fragment consisted of Si, Fe, Mg, Al, some C, and a little Ti and Ca. This composition was too complex for allophane. Transmission electron microscopy indicated that halloysite and beidellite were present in the amorphous material. Delvigne et al. (1979) studied the weathering of olivine. Measurements with EMA were done on fermginous pseudomorphs of olivine from the Ivory Coast, and SEM-EDXRA were done of olivine and different types of iddingsite from the Galapagos Islands. Composite grains, which can be used as provenance indicators for Maas sediments in the southeastern part of the Netherlands, were studied by Bisdom et al. (1978) and Riezebos el al. (1978). The nature of the ore and other minerals in the composite grains was assessed. Weathered dune sands from Fraser Island, Australia were examined with SEM-EDXRA by Little et al. (1978). The original microstructure of the quartz grains was a very important factor during the different stages of alteration and influenced the rate of weathering. 4. Newly Formed Minerals

Hutton et al. (1972) used EMA to examine the chemical elements in silcretes and silcrete skins from soils in South Australia. Massive silcrete was formed when silicon from sources outside the studied profile was deposited between coarser quartz grains. These massive silcretes occurred in the lower part of the landscape. Silcrete skins, present at higher levels in the landscape, formed when quartzes in the profile were subjected to weathering and extensive leaching occurred. Only elements like Ti, Zr, Ce, and P remained because they formed parts of resistant minerals such as rutile, zircon, and cerium phosphates. Brewer et al. (1973) used EMA to study iron-manganese pans from Newfoundland, Canada. The thin pans were present in peaty soils. Most data could be

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correlated with light-microscopic observations. Some material, however, had the same chemical composition but was different with the light microscope. Manganese and Fe often occurred side by side but without contamination by the other. Childs (1975) studied Fe-Mn concretions in soils from New Zealand. Initial formation of the concretions occurred by precipitation of Fe and Mn oxides in pores between soil constituents. Scanning electron microscopy and EMA were used, with other techniques, to study femginous pans (crusts) and calcareous crusts in Senegal and Mauritania (Nahon, 1976). Two different and opposing geochemical weathering mechanisms were recognized in a toposequence of fenuginous pans. In one, iron oxides and hydroxides remained stable and quartz and kaolinite were dissolved (formation of an iron pan); in a second, iron was dissolved and quartz and kaolinite were stable. Various stages in the development of an iron pan were described. Calcareous crusts were best developed in the upper meter of profiles formed from Eocene marls. The explanation was that once a calcareous crust or an iron pan has formed, it develops downward into the landscape by recrystallization and reconstruction processes which affect the original profile components. This could explain why iron pans and calcareous crusts are often situated directly on rock. Wieder and Yaalon (1974) used SEM and EMA to investigate the formation of carbonate nodules in soils from Israel. Chemical elements of the nodules and of the surrounding soil material were compared. Microcalcite increased in the internal part of the carbonate nodules, whereas clay increased toward their fringes. This process could lead to expulsion of part of the clayey material from the nodule into the surrounding soil matrix. An SEM-EDXRA analysis of newly formed carbonate in a Humaquept and of secondary silica in a red-yellow padzolic Tertiary paleosol of the Netherlands was made by Bisdom et al. (1975). McKee and Brown (1977) analyzed loose, undisturbed soil materials (i.e., a fractured gross soil sample and a fractured barite nodule). B6rdossy et al. (1978) studied gibbsite and other minerals in bauxite samples of different ages and origins; minerals in a laterite paleosol of the Katmandu basin in Nepal were investigated by Miiller (1976). Newly formed pyrite, carbonate, and various forms of iron- and manganesecontaining cutans were examined by Kooistra (1978) in recent marine sediments of the intertidal zone of the southwest Netherlands. Various forms of pyrite were distinguished with the light microscope. When pyrite oxidized in polder soils, black forms changed to dark, red-brown, adhesive nodules. Iron remained in the dark, red-brown rings at the oxidized periphery of pyrite spheres, whereas S disappeared. Measurements by EDXRA were also used to study Fe and Mn accumulations in a neomangan-neoferran compound and in peat and root pseudomorphs of the intertidal zone. It was found that P was also present, and this probably indicated the presence of phosphate in neoferrans. These studies


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were continued by Kooistra (1981) and gave various new data on new formations in dark-to-blackish materials, with or without organic matter, that are difficult or impossible to analyze by light-microscopic techniques alone. 5 . Organic Matter Measurements by EMA of a decaying root indicated that P, Ca, Fe, and A1 were present (Qureshi et al., 1969).The highest concentration of P was present near the root margin together with some Fe. Qureshi et al. (1978)found that P was concentrated in the outer tissues of old and fresh roots together with Ca. Jeanson (1972)used SEM and EMA to investigate the results of earthworm and mycelial activities in silty B-horizon material to which glucose and peptone were added. Humidity was kept sufficiently high that a gley environment could be maintained during the experiment. Evaporation caused a 1-mm crust to form which consisted of three layers containing Fe, Mn, Ca, K, Si, and Al. Concentric rings with different compositions were formed in worm burrows and cracks. Nodules of the B horizon could be weathered, in part by microorganisms. Elements set free during weathering of the nodules (i.e., mainly Fe, Mn, and Ca) could be found again in the crust and in the cutans in the worm burrows. Righi (1975)used SEM and EMA to study the micromorphologicalfeatures of organic matter in spodic B horizons of Podzols. Organometallic complexes were found in the cemented spodic B of the B2h horizons. The complexes consisted of amorphous organic matter, A1 and Fe which illuviated into the B2h horizon, and coated sand grains. A loose spodic B was found in the Bh horizons of the podzols. Organic matter found as aggregates and pellets between sand grains was much less transformed than in the cemented spodic B and contained less fulvic acids according to wet-chemical analysis. The organic matter in the aggregates was mainly derived from decaying roots in the Bh horizon itself. Aluminum and Si were present in the aggregates, which also contained numerous fine quartz grains. Scanning electron microscopy-EDXRA can be used to distinguish heavier chemical elements (Z 2 11). If an ECON(EDAX Carbon Oxygen Nitrogen) detector is available one can also obtain information on these three elements. High A1 concentrations in a Haplaquod of the Netherlands were found primarily in strongly humified root mats in the B horizon (Bisdom and Jongerius, 1978) and were virtually absent in the strongly humified root remnants that occurred in the A2 horizon. Dupont and Jeanson (1978)studied the soil material in earthworm burrows in highly organic fine sandy sediments of the Somme Bay estuary in France, finding that the microchemical composition of the soil material at different distances from the central hole of the burrows varied significantly. Frozen specimens of plant roots were studied by Chino and Hidaka (1977) with SEM-EDXM (i.e., epidermis, cortex, endodermis, and the pericycle

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layer). Chlorine, Ca, and Fe occurred primarily in the endodennis, whereas K was situated in the endodermis and pericycle layer. Aluminum, Si, P, S, and Ti were also found in the root. Different treatment procedures gave different accumulations at different places in the same type of root tissue. Organans and argillans were studied with SEM and EMA by van Ranst et al. (1980). It was found that organans are not usually layered and are composed of discrete, very fine bodies that are stacked at random. Organans usually contained Al with or without Fe and without Si. All three elements were found in the argillans. McKeague (1981) studied compound cutans, cutans, and nodules of Canadian soils using SEM-EDXRA. Organic matter and Al were present in the isotropic parts of compound cutans in an Ortstein horizon. Pyrite and iron oxide were found in nodules with diameters of 20 pm, which initially were thought to be fecal pellets. Phosphorus was found in a black, Fe-rich nodule of the Aeg horizon of a Luvic Gleysol.

6. Soil Structure and Fabric McHardy and Birnie (1975) used SEM and EMA to study phenomena associated with gleying in a surface water gley developed on red-brown varved lacustrine clay from Scotland. Extremely thin gray coatings on ped surfaces or fractures of the mainly red-brown Bg and Cg horizons were examined. Electron microprobe and chemical analyses indicated that the Fe,O, content of the gray materials was lower than that of the red-brown parent material and that no significant differences in composition occurred between the two as far as the other elements were concerned. Features associated with gley and illuviation processes were studied in soils from the Soviet Union using SEM and EMA (Dobrovolsky et al., 1977). Ferruginous films were present on the surfaces of peds and minerals and on the walls of pores if gleyification was weak to moderate. If the gleyifcation process became stronger, ferrous and ferric films dissolved and EMA indicated that Si, Al, and Fe remained in amorphous floccules. Measurements by EMA were also done in cutans of Solonetzes of the west Caspian coastal lowland. More Si and Al and less Ca and Mg were found in these cutans than in the adjacent clay matrix. Phosphorus in calcite grains from a Haplaquept developed in marine Cretaceous (“Gault”) clay was studied by Qureshi and Jenkins (1978). Phosphorus was just detectable with EMA and was present in small quantities of about 0.5%. The phosphorus was dispersed at relatively constant levels across the calcite grains and accounted for 50-808 of the total soil phosphorus. This phosphorus was released in the upper soil horizons when the calcite dissolved. Desert varnish on pebble and rock surfaces in the Mojave Desert of California (United States) was examined by Potter and Rossman (1977). Over 70% of clays were found in the desert varnish. Iron and clay were found in the orange coat-


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ings, whereas iron and manganese were present in the black coatings. Ugolini et al. (1977) studied particle migration in a soil solution from a subalpine Podzol from the central Cascades, Washington (United States). Tension lysimeters were installed below various horizons of the Podzol and used to extract solutions. An SEM-EDXRA was used to study particles on a 0. I-pm Nuclepore filter through which the collected solutions had passed. Organic particles which were predominant in the migrant material of the A and B horizons were arrested in the B2hir horizon. Traces of Al, Si, P, S, and Fe were found in the organic particles. The B2hir horizon had a dual nature of organic particles illuviated and silicates eluviated. Cutans in cemented podzolic B horizons (Ortsteins) from podzolic soils in Nova Scotia and New Brunswick, Canada were examined with SEM-EDXRA. Organic matter itself could not be detected, because no ECON detector was available, but it was indicated indirectly if an S peak and a minor P peak were found (McKeague and Wang, 1980). An important role in the strengthening of the cement of the B horizons in the podzol was played by Al and Fe organic complexes. Brown and dark-reddish-brown cutans were also examined. A discussion of a possible step-by-step genesis of Ortstein was also included in the paper. McKeague and Pro& (1980) advocated a continuum between Ortstein and duric horizons in Canadian soils because of a similarity in cementing material and the knowledge that the levels of extractable A1 + Fe did not meet the requirement of a podzolic B. Duric horizons with discrete cutans that linked grains were selected for EDXRA. Variation in element distribution was found to be considerable. Aluminum, Fe, Si, and organic matter could act together or in various combinations as cementing agents of duric horizons. Pagliai et al. (1981) used SEM-EDXRA and image analysis to study structural improvements of soils in Italy after the application of sewage sludges, fertilizers, and soil conditioners. Porosities and the distribution of chemical elements were measured in the surface horizons. The paper also discussed the weathering of minerals and the formation of calcareous crusts. Image analysis of pores in thin sections of soils is now done by a number of laboratories with the Quantimet, an image analyzer, and other machines. The instrument can measure the area, perimeter, size, number, and other parameters of pores and makes possible the introduction of a classification graph in which the relationship between a variety of pore systems can be recognized (Jongerius et al., 1972; Jongerius, 1974,1975; Ismail, 1975; Murphy et al., 1977; Bullock and Murphy, 1980). Micromorphometric porosity data was integrated with saturated hydraulic conductivity data of undisturbed pedal clay soils by Bouma er al. (1977, 1979) for pores with diameters larger than 30 pm. Pores smaller than 30 pm in diameter were not measured for a technical reason, that is, transmitted light is used to obtain photographs from the pores and the soil constituents in the thin section; this thin section itself is about 30 pm

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thick and therefore pores with a diameter of less than 30 pm cannot be measured. The introduction of backscattered electron scanning images (BESI), obtained from a layer of a few micrometers in thickness just underneath the surface of the thii section, makes it possible to obtain micrographs in which the image analyzer can also measure capillary pores (i.e., pores with a diameter smaller than 30 pm) (Bisdom and Thiel, 1981; Jongerius and Bisdom, 1981). The BESI technique is very useful for material contrast studies in thin sections of soils.

IV. APPLICATIONS OF ION MICROSCOPY Ion microscopy is new to soil science and the literature is concerned mainly with the possibility of applying secondary ion mass spectrometry to soil constituents in thin sections (see Section II,B). Trace concentration, trace amount, depth concentration profile, isotope, and compound analyses can be done. Ion microprobe mass analysis of ARL has been used to examine an alder root fragment from a Humaquept in the Netherlands (Bisdom ef al., 1977; Bisdom and Jongerius, 1978). Brownish-colored, rather homogeneous fine material was present together with humified remnants of the root tissue. These remnants were identical in morphology and appearance to the clayified roots described by Parfenova et al. (1964). In siru formation of clayey material was found at some sites of the alder root with the light microscope, but such sites were too small for uncontaminated sampling and subsequent XRD analysis. Ion images and positive and negative ion spectra were made of the materials in the alder root. It was found that the chemical elements, including those found in organic compounds, were rather homogeneously distributed throughout the clayified root. The organic matter occurred together with heavier chemical elements which are also present in clay minerals. It could not be decided, however, whether biogenic clay (Parfenova ef al., 1964) had been formed (i.e., it was not possible to measure whether the organic components played an active or a passive role during the formation of clay minerals in the decaying alder root) (Bisdom ef al., 1977). The Cameca ion microscope type IMS 300 has been used to quantify both trace and major elements in thin sections of soils (Henstra et al., 1980b). The experiments involved bauxite from Surinam, and quantification was possible in an area of the thin section 300 pm in diameter. Frequently, however, such an area is too large for most micromorphological and submicroscopic studies; consequently the Cameca IMS 3F has been tested. This instrument has allowed quantitative analysis of trace and major elements in a spot with a diameter of 1.5 pm. Only the soil material of polished thin sections can be studied.


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V. APPLICATIONS OF OTHER FORMS OF SUBMICROSCOPY A few technical details on Raman spectroscopy, laser microprobe mass analysis, electron spectroscopy for chemical analysis, and Auger electron spectroscopy were discussed in Section I1,C. Jeanson (1981) has done RS on clay-iron coatings in earthworm burrows, aggregates constructed by soil animals, and decomposed straw fragments. It is not necessary to prepare thin sections and samples are not destroyed. Molecular analyses of an area or point in the sample were done and a distribution image of each component was obtained. Laser microprobe mass analysis was done in thin sections of a weathered granite from Spain (Bisdom, 1967a,b) with a LAMMA 500 (Bisdom et al., 1981). Laser microprobe mass analysis and SEM-EDXRA concerned titaniumcontaining clouds derived from weathering biotite [i.e., from Ti contained in the crystal lattice of the original biotite or the rutile (sagenite) inclusions of this mineral]. Light microscopy revealed minute and larger droplets in Ti-containing clouds together with turbid secondary anatase. Both instruments indicated that the Ti content of the clouds increased with the degree of crystallinity. The secondary anatase contained the most Ti. Scanning electron microscopy- EDXRA found Al, Si, Ti, and Fe in the clouds, whereas LAMMA measured these elements and Na, K, Mg, and Mn. Weaver (1976) studied the nature of TiO, in kaolinite from Georgia (United States) using SEM-EDXRA, EMA, and TEM. Titanium was released as Ti(OH), from primary minerals and formed an amorphous hydrous oxide gel upon precipitztion. Secondary anatase was formed upon dehydration of the gel as small crystals in a granular aggregate. The aggregates or pellets had various forms and diameters between 0.05 and 0.1 Fm. The LAMMA 500 usually works with ultrathin sections; the LAMMA lo00 can analyze thin sections and polished blocks and can also be used for the examination of materials in soil peds. Quantification is not yet possible with these machines. Electron spectroscopy for chemical analysis and SEM were used by Berner and Holdren (1977) to study the mechanism of feldspar weathering. Electron spectroscopy for chemical analysis can analyze very thin surface layers and was therefore used to study the surface reaction layer of weathering feldspar. No differences in composition were found between the immediate subsurface of the feldspar and the fresh feldspar itself, which indicates that no surface reaction layer existed. Auger electron spectroscopy with the combination instrument (AES,ESCA, and SIMS) of the LAS series of Riber has succeeded on ironcoated organic material from the Netherlands, possibly because of the conductivity conferred to the sample by the iron; nonconductive clayey material gave no results. Nonquantitative ESCA and SIMS analyses of the same samples as used in AES, however, gave no problems.

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Auger electron microscopy would be an ideal technique if no charging problems existed for soil materials in thin sections of soils. Electron spectroscopy for chemical analysis is a technique which will very often be applicable, but the lateral resolution (3 mm) is too large for most.micromorphological and submicroscopic studies. Secondary ion mass spectrometry is a good technique (see Section IV) for the analysis of thin sections of soils. However, LAS does not yet have the capability for quantitative analysis.

VI. CONCLUSIONS Submicroscopy makes possible the in situ investigation of soil materials in thin sections, soil peds, soil aggregates, minerals, and other entities. Electron microscopy, with or without equipment for in situ microchemical analysis, has been used for the study of clays, weathered and newly formed minerals, organic matter, and soil structure and fabric. More recently, ion microscopy and laser microprobe mass analysis were introduced. These techniques make it possible to study both trace and major chemical elements, which is not possible with electron microscopy. Raman spectroscopy is another field of submicroscopythat will be of major interest, especially because it is nondestructive. Electron spectroscopy for chemical analysis can only be used for samples which are homogeneous over larger areas of a thin section; auger electron spectroscopy is difficult to apply to materials in thin sections of soils when they are nonconductive. Consequently, electron microscopy, ion microscopy, laser microprobe analysis, and Raman spectroscopy at present seem to be the most promising techniques. However, many instruments that are regularly used in the metals industry have not been tested for possible uses in soil science, although the most promising ones have been. Submicroscopic studies can provide microscale in situ information, whereas X-ray diffraction and wet-chemical analyses are done on bulk and disturbed samples. Information from light microscopy, which usually precedes submicroscopy during the in situ study of soil materials, is a powerful determination technique when correlated with data from XRD and wet chemistry. The same is possible for soil physics. Porosity data obtained by physical measurements can be compared with data on porosity acquired with an image analyzer from photographs made with the transmitted light of a light microscope. Presently, such photographs can also be made with the scanning electron microscope using backscattered electrons. Thus, capillary pores can also be measured with the image analyzer, usually a Quantimet. Soil-physical, light-microscopic, and submicroscopic techniques can thus be combined to solve problems related to soil porosity.

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At present there exists the International Working-Group on Submicroscopy of Undisturbed Soil Materials (IWGSUSM), in which specialists in soil micromorphology, soil mechanics, soil physics, soil chemistry, soil biology, and other soil sciences have joined forces. The purpose and objectives of this group have been described by Bisdom and Wells (1981). The group has become part of the Subcommission on Soil Micromorphology of the International Society of Soil Science in 1983.

REFERENCES Babanin, V. F., Karpachevskiy, L. O., Opalenko, A. A., and Shoba, S. A. 1976. Sov. Soil Sci. (Engl. Transl.) 8, 314-320. Bajwa, I., and Jenkins, D. 1978. In ‘‘Soil Micromorphology” (M. Delgado, ed.),pp. 3-17. Univ. of Granada, Spain. Barden, L., McGown, A., and Collins, K. 1973. Eng. Geol. (Amsterdam)7 , 49-60. Biirdossy, Gy., Csanhdy, A., and CsordBs, A. 1978. Clays Clay Miner. 26, 245-262. Benayas, J., and Alonso, J. 1978. In “Soil Micromorphology” (M. Delgade, ed.), pp. 717-739. Univ. of Granada, Spain. Benayas, I., Alonso, J., and Fernandez Caldas, E. 1974. In “Soil Microscopy” (G. K. Rutherford, ed.),pp. 306-319. Limestone Press, Kingston. Berner, R. A,, and Holdren, G. R. 1977. Geology 5, 369-372. Bemer, R. A., Sjoberg, E. L., Velbel, M. A., and Krom, M. D. 1980. Science (Washington,D.C.) 207, 1205-1207.

Berthelin, J., and Belgy, G. 1979. Geoderma 21, 297-310. Bisdom, E. B. A. 1967a. Leidse Geol. Meded. 37, 33-67. Bisdom, E. B. A. 1967b. Geol. Mijnbouw46,333-346. Bisdom, E. B. A. 1981a. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 67- 116. Cent. Agric. Publ. Doc.,Pudoc, Wageningen. Bisdom, E. B. A. 1981b. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 117-162. Cent. Agric. Publ. Doc.,Pudoc, Wageningen. Bisdom, E. B. A., and Jongerius, A.’1978. In “Soil Micromorphology” (M. Delgado, ed.), pp. 741-756. Univ. of Granada, Spain. Bisdom, E. B. A., and Thiel, F. 1981. In “Submicroscopy of Soils and Weathered Rocks” @. B. A. Bisdom, ed.), pp. 191-206. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Bisdom, E. B. A., and Wells, C. B. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 17-27. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Bisdom, E. B. A., Henstra, S., Jongerius, A., andmiel, F. 1975. Neth. J . Agric. Sci. 23, 113-125. Bisdom, E. B. A., Henstra, S., Hornsveld, E. M., Jongerius, A,, and Letsch, A. C. 1976. Neth. J . Agric. Sci. 24, 209-222. Bisdom, E. B. A., Henstra, S., Jongerius, A., Brown, J. D., von Rosenstiel, A. P., and Gras, D. J. 1977. Neth. J . Agric. Sci. 25, 1-13. Bisdom, E. B. A., Gerlofsma, A., Poelman, J. N. B., and Riezebos, P. A. 1978. Geol. Mijnbouw 57, 407-416. Bisdom, E. B. A., Henstra, S., Jongerius, A., Heinen, H. J., and Meier, S. 1981. Neth. J . Agric. Sci. 29, 23-36. Bocquier, G . , and Nalovic, Lj. 1972. Cah. ORSTOM Ser. Pedol. 10, 411-434. Beekestein, A., Henstra, S., and Bisdom, E. B. A. 1981. In “Submicroscopy of Soils and Weath-

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ered Rocks” (E.B. A. Bisdom, ed.), pp. 29-44. Cent. A H c . Publ. Doc., Pudoc, Wageningen. Bottino, G., Rosa, M. A., Stafferi, L., and Veniale, F. 1976. Rend. Soc.Ital. Mineral. Petrol. 32, 521-537. Bouma, J., Jongerius, A., Boersma, O., Jager, A., and Schoonderbeek, D. 1977. Soil Sci. Soc.Am. J . 41,945-950. Bouma, J., Jongerius, A., and Schoonderbeek, D. 1979. Soil Sci. Soc.Am. J . 43,261-264. Bresson, L. M. 1974a. I n “Soil Microscopy” (G. K. Rutherford, ed.),pp. 526-541. Limestone Ress, Kingston. Bresson, L. M. 1974b. Thesis, Univ. of Paris, France. Bresson, L. M. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 173-189. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Brewer, R., Protz,R., and McKeague, J. A. 1973. Can. J. Soil Sci. 53, 349-361. Brinkman, R., Jongmans, A. G., Miedema, R., and Maaskant, P. 1973. Geoderma 10,259-270. Brown, 1. L., and Jackson, M. L. 1973. Clays Clay Miner. 21, 1-7. Bruckert, S., Hetier, J. M., and Gutienez, F. 1974. Sci. Sol 4, 225-245. Bullock, P., and Murphy, C. P. 1980. J. Microsc. (Oxford) 120, 317-328. Buol, S. W., and Eswaran, H. 1978. I n “Soil Micromorphology” (M. Delgado, ed.), pp. 325-347. Univ. of Granada, Spain. m o t , G. 1978. In “Sod Micromorphobgy” (M.Delgado, ed.), pp. 349-368. Univ. of Granada, Spain. Chen, Y.,and Schnitzer, M. 1976. Soil Sci. SOC.Am. J . 40, 682-686. Chen, Y., Banin, A., and Schnitzer, M. 1976. Scanning Electron Microsc., pp. 425-432. Childs, C. W. 1975. Ge&rma 13, 141-152. Chino. M., and Hidaka, H. 1977. Soil Sci. Planr Nu#r. (Tokyo)23, 195-200. Clough, K. S., and Sutton, J. C. 1978. Can.J . Microbiol. 24, 333-335. Curmi, P. 1979. Thesis, Univ. of Rennes, France. Curmi, P., and Fayolle, M. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 249-270. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Delvigne, J., Bisdom, E. B. A., Sleeman, J., and Stoops, G. 1979. Pedologie 29, 247-309. de Oliveira, J. J. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 271-276. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Dobmvolsky, G. V., Fedorov, K.N., Balabko, P. N., Stasyuk, N. V., and Shoba, S. A. 1977. In “Problems of Soil Science” (V.A. Kovda, ed.),pp. 446-455. Nauka, Moscow. Dobrovolsky, V. V. 1977. Sov. Soil Sci. (Engl. Transl.) 9, 42-48. Dormaar, J. F. 1974. SoilSci. SOC.Am. Proc. 38, 685-686. Douglas, L. A., and Platt, D. W. 1977. Soil Sci. Soc.Am. J . 41, 641-645. Driessen, P. M. 1970. Agric. Res. Rep. No. 743. Pudoc, Wageningen. Driessen, P. M., and Schoorl, R. 1973. J. Soil Sci. 24,436-442. Ducloux, J. 1976. Sci. Sol 1, 23-36. Ducloux, J. 1978. Thesis, Univ. of Poitiers, France. Ducloux, J., and Ranger, J. 1978. In “Soil Micromqhology” (M. Delgado, ed.), pp. 815-832. Univ. of Granada, Spain. Dupont, J. P.,and Jeanson, C. 1978. In “Soil Micromorphology” (M. Delgado, ed.), pp. 833-850. univ. of Granada, Spain. Ehlers, W. 1977. 2. Pfhzeneraehr. Bodenkd. 140, 79-90. Eswaran, H. 1971. SoilSci. SOC.Am. Proc. 35, 787-790. Eswaran, H. 1972. Clay Miner. 9, 281-285. Eswaran, H., and de Coninck, F. 1971. Pedologie 21, 181-210. Eswaran, H., and Stoops, G. 1979. Soil Sci. Soc.Am. J . 43,420-424.


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Eswaran, H., and Wong, C. B. 1978. Soil Sci. Soc.Am. J . 42, 154-158. Eswaran, H., Stoops, G., and Sys, C. 1977. J. Soil Sci. 28, 136-143. Eswaran, H., Lim, C. H., Sooryanarayana, V., and Daud, N. 1978. In “Soil Micromorphology” (M. Delgado, ed.), pp. 851-885. Univ. of Granada, Spain. Eswaran, H., Stoops, G., and Abtahi, A. 1980. J . Microsc. (Oxford)120, 343-352. Flicoteaux, R., Nahon, D., and Paquet, H. 1977. Sci. Geol. Bull. 30, 153-174. Foster, R. C. 1978. In “Modification of Soil Structure” (W. W. Emerson, R. D. Bond, and A. R. Dexter, eds.), pp. 103-109. Chichester, England. Foster, R. C. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 309-317. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Gillott, J. E. 1974. In “Soil Microscopy” (G. K. Rutherford, ed.), pp. 143-164. Limestone Press, Kingston. Gillott, I. E. 1980. J. Microsc. (Oxford)120, 261-277. Grandstaff, D. E. 1978. Geochim. Cosmochim. Acta 42, 1899-1901. Guillet, B., Rouiller, J., and Souchier, B. 1976. Bull. SOC. Geol. Fr. 18, 55-58. Henstra, S., Bisdom, E. B. A., Jongerius, A., Heinen, H. J., and Meier, S. 1980a. Beirr. Elektronenmikrosk. Direktabb. Oberfl. 13, 63-66. Henstra, S., Bisdom, E. B. A., Jongerius, A,, Morgan, A. E., Werner, H. W., and de Grefte, H. A. M. 1980b. In “Electron Microscopy” (P. Bredemo and V. E. Cosslett, ed.), Vol. 3, pp. 224-225. Eur. Congr. Electron Microsc. Found., Leiden. Henstra, S., Bisdom, E. B. A., and Boekestein, A. 1981a. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 55-65. Cent. Agric. Publ. Doc.,Pudoc, Wageningen. Henstra, S., Boekestein, A., and Bisdom, E. B. A. 1981b. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 45-53. Cent. Agric. Publ. Doc.,Pudoc, Wageningen. Hetier, J. M. 1975. Thesis, Univ. of Nancy, France. Hutton, J. T., Twidale, C. R., Milnes, A. R., andRosser, H. 1972. J. Geol. SOC. Aust. 19,31-39. Ismail, S . N. A. 1975. “Soil Survey Paper,” No. 9. Neth. Soil Surv. Inst., Wageningen. Jackson, M. L., and Sridhar, K. 1974. Soil Sci. Soc.Am. Proc. 38, 843-847. Jackson, M. L., Levelt, T. W. M., Syers, J. K., Rex, R. W., Clayton, R. N., Sherman, G. D., and Uehara, G. 1971. Soil Sci. SOC.Am. Proc. 35, 515-525. Jamagne, J., and Jeanson, C. 1978. In “Soil Micromorphology” (M. Delgado, ed.), pp. 935-965. Univ. of Granada, Spain. Jeanson, C. 1972. Rev. Ecol. Biol. Sol 9, 479-489. Jeanson, C. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), p. 307. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Jenkins, D. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 163-172. Cent. Agric. Publ. Doc.,Pudoc, Wageningen. Jepson, W. B., and Rowse, J. B. 1975. Clays Clay Miner. 23, 310-317. Jongerius, A. 1974. In “Soil Microscopy” (G. K. Rutherford, ed.), pp. 67-83. Limestone Press, Kingston. Jongerius, A. 1975. Vortr. IMANCO Symp., pp. 161-185. Jongerius, A., and Bisdom, E. B. A. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 207-216. Cent. Agric. Publ. Doc.,Pudoc, Wageningen. Jongerius, A., Schoonderbeek, D., Jager, A., and Kowalinski, S. 1972. Geoderma 7 , 177-198. Keller, W. D. 1976a. Clays Ckzy Miner. 24, 107-113. Keller, W. D. 1976b. Clays Clay Miner. 24, 114-1 17. Keller, W. D. 1976c. Clays Clay Miner. 24, 262-264. Keller, W. D. 1977a. Clays Ckzy Miner. 25, 311-345.

94

E. B. A. BISDOM

Keller, W. D. 1977b. Clays C h y Miner. 25, 347-364. Keller, W. D. 1978a. Clays Clay Miner. 26, 1-20. KeUer, W.D. 1978b. Geology 6 , 184-188. Keller, W. D., and Haenni, R. P. 1978. Clnys Clay Miner. 26,384-3%. Kilbertus, G . , and Reisinger, 0. 1975. Rev. Ecol. Biol. Sol 12, 363-374. Kilbertus, G., Reisinger, O., and Delon, R. 1972. Rev. Ecol. Biol. Sol 11, 27-46. Kilbertus, G., Mangenot, F., and Reisinger, 0. 1973. Rech. Coop. CNRS Ecol. Sol 3, 151-193. Kooistra, M. J. 1978. “Soil S w e y Paper,” No. 14, Neth. Soil Surv. Inst., Wageningen. Kooistra, M. J. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 217-236. Cent. Agric. Publ. Doc., Pudoc,Wageningen. Koppi, A. J. 1981. A m . J. Soil Res. 19, 251-254. Kiihnel, R. A., Roorda, H. J., and Steensma, J. J. 1975. Clays Clay Miner. 23, 349-354. Ledin, S . 1975-1976. Grunforbdttring 27, 145-150. Ledin, S. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 295-306. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Lee, S. Y.,Jackson, M. L., and Sachs, I. B. 1974. Soil Sci. Soc.Am. Proc. 38,423-428. Lee, S . Y.,Jackson, M. L., and Brown, J. L. 1975a. Clays Clay Miner. 23, 125-129. Lee, S. Y.,Jackson, M. L., and Brown, J. L. 1975b. Soil Sci. Soc. Am. Proc. 39,793-800. Legigan, P., and Le Ribault, L. 1974. C.R. Hebd. Seances Acad. Sci. Ser. D 279, 799-802. Leneuf, N. 1972. Cah. ORSTOM Ser. Geol. 4, 53-65. Liebl, H. 1975. J. Phys. 8, 797-808. Little, I. P., Annitage, T.M., and Gilkes, R. J. 1978. Geoderma 20, 225-237. Magaldi, D. 1977. Boll. Sew. Geol. Iral. 98, 117-128. Magaldi, D. 1978. In “Soil Micromorphology” (M. Delgado, ed.),pp. 967-988. Univ. of Granada, Spain. McCrone, W. C., and Delly, J. G. 1973. “The Particle Atlas,” 2nd ed. Ann Arbor Sci. Publ., AM Arbor, Michigan. McHardy, W. J., and Birnie. A. C. 1975. J. Soil Sci. 26, 426-431. McKeague, J. A. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 277-282. Cent. Agric. Publ. Doc., Pudoc, Wageningen. McKeague, J. A., and h a ,R. 1980. Can. J. Soil Sci. 60,45-52. McKeague, J. A., and Wang, C. 1980. Can. J. Soil Sci. 60, 9-21. McKee, T.R., and Brown, J. L. 1977. In “Minerals in Soil Environments” (R. C. Dinauer, ed.), pp. 809-846. Soil Sci. Soc. Am., Madison, Wisconsin. McKyes, E.,Sethi, A., and Yong, R. N. 1974. Chys Clay Miner. 22,427-433. Meunier, A. 1977. Thesis, Univ. of Poitiers, France. Miedema, R., Jongmans, A. G.,and Slager, S. 1974. In “Soil Microscopy” (G. K. Rutherford, ed.), pp. 772-794. Limestone Press, Kingston. Miehlich, G. 1978. Forsch. Geol. Paliionrol. 44-45, 27-41. Millot, G., Nahon, D., Paquet, H., Ruellan, A., and Tardy, Y. 1977. Maroc Sci. Geol. Bull. 30, 129-152. Moinereau, J. 1977. Cah. ORSTOM Pedol. 15, 157-173. Mojallali, H., and Weed, S. B. 1978. Soil Sci. Soc. Am. J. 42, 367-372. Moormann, F. R., and Eswaran, H. 1978. Pedologie 28, 251-270. Moreno, F., Comejo, J.. and Rives, V. 1978. In “Soil Mimmorphology” (M. Delgado, ed.),pp. 1335-1348. Univ. of Granada, Spain. M o m , H.J. M. 1978. In “Soil Micromorphology” (M. Delgada, ed.), pp. 1007-1018. UNv. of Granada. Spain. Moss, A. J., and Green, P. 1975. J. Geol. Soc. Aust. 22,485-495. Moura, W. F., and Buol, S . W. 1975. Experienria 21, 161-177.


95

Miiller, H. 1976. Abh. Math. Naturwiss. K1. A M . Wiss. Lit. Mainz, Abt. I 185, 43-53. Murphy, C. P., Bullock, P., and Turner, R. H. 1977. J . Soil Sci. 28, 498-508. Nahon, D. 1976. Thesis, Univ. of Aix (Marseilles III), France. Nahon, D., Paquet, A., Ruellan, A., and Millot, G. 1975. Sci. Geol. Bull. 28, 29-46. Osman, A., and Eswaran, H. 1974. In “Soil Microscopy” (G. K. Rutherford, ed.), pp. 846-857. Limestone Press, Kingston. Pagliai, M., Ferrari, G. A., and Magaldi, D. 1981. In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 283-294. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Paramananthan, S . , Sooryanarayana, V., Syed Sofi, S . O., and Eswaran, H. 1978. In “Soil Micromorphology” (M. Delgado, ed.), pp. 589-609. Univ. of Granada, Spain. Parfenova, E. I., Mochalova, E. F., and Titova, N. A. 1964. In “Soil Micromorphology” (A. Jongerius, ed.), pp. 201-212. Elsevier, Amsterdam. Potter, R. M., and Rossman, G. R. 1977. Science (Washington, D.C.) 196, 1446-1448. Qureshi, R. H., and Jenkins, D. A. 1978. SoilSci. SOC.Am. J . 42, 703-705. Qureshi, R. H., Jenkins, D. A., and Davies, R. I. 1969. Nature (London) 221, 1142-1143. Qureshi, R. H., Jenkins, D. A., and Davies, R. I. 1978. Soil Sci. SOC.Am. J . 42, 698-703. Riezebos, P. A. 1974. Geol. Mijnbouw 53, 109-122. Riezebos, P. A., Bisdom, E. B. A., and Boersma, 0. 1978. Geol. Munbouw 57, 417-431. Righi, D. 1975. Sci. Sol 4, 315-321. Schmidt-Lorenz, R. 1974a. Min. Dtsch. Bodenkd. Ges. 20, 68-79. Schmidt-Lorenz, R. 1974b. Min. Dtsch. Bodenkd. Ges. 20, 114-122. Schmidt-Lorenz, R. 1975. Joint Meet. ISSS G h a I975 ~ (unpublished data). Schnitzer, M., and Kodama, H. 1975. Geoderma 13, 279-287. Schwaighofer, B. 1976. Geoderma 6, 285-315. Scoppa, C. E. 1978-1979. Rev. Invest. Agropec. INTA 14, 37-69. Seddoh, F. K., and Pedro, G. 1975. Cah. ORSTOM Ser. Pedol. 13,7-25. Sergeyev, E. M., Spivak, G. V., Osipov, V. I., Rau, E. I., Sokolov, V. N., and Filippov, M. N. 1980a. Scanning 3, 262-272. Sergeyev, E. M., Grabowska-Olszewska, B., Osipov, V. I., Sokolov, V. I., and Kolomenski, Y.N. 1980b. J . Microsc. (Ogord) 120, 237-260. Sheeran, D. E., and Yong, R. N. 1974. In “Soil Microscopy” (G. K. Rutherford, ed.), pp. 179-189. Limestone Press, Kingston. Singer, A. 1973. Isr. J . Earth Sci. 22, 229-242. Singer, A., and Norrish, K. 1974. Am. Miner. 59, 508-517. Smart, P. 1974. In “Soil Microscopy” ( G . K. Rutherford, ed.), pp. 190-206. Limestone Press, Kingston. Smart, P. 1975. Soil Sci. 119, 385-393. Smart, P.,and Tovey, K. 1981, “Electron Microscopy of Soils and Sediments: Examples.” Oxford UNv. Press (Clarendon), Oxford. Smart, P., and Tovey, K. 1982. “Electron Microscopy of Soils and Sediments: Techniques.” Oxford Univ. Press (Clarendon), Oxford. Sokolov, V. N., Osipov, V. I., and Tolkachev, M. D., 1980. J. Microsc. (Ogord) 120,363-366. Sousa, E. C., and Eswaran, H. 1975. Pedologie 25, 71-79. Stoch, L., and Sikora, W. 1976. Clays Clay Miner. 24, 156-162. Stoops, G. 1974. In “Soil Microscopy” (G. K. Rutherford, ed.), pp. 101-108. Limestone Press, Kingston. Stoops, G. 1976. Am. Miner. 61, 172. Stoops, G., and Zavaleta, A. 1978. Geoderma 20, 63-70. Stoops, G., Eswaran, H., and Abtahi, A. 1978. In “Soil Micromorphology” (M. Delgado, ed.), pp. 1093-1113. Univ. of Granada, Spain.

96

E. B.

A. BISDOM

Sudo, T., and Yotsumoto, H. 1977. Clays Clay Miner. 25, 155-159. Tarzi, J. G.,and Rotz, R. 1978. Clays Clay Miner. 26, 352-360. Taupinard, J. 1976. Bull. Soc.Fr. Mineral. Cristallogr. 99, 246-250. Tessier. D.,and Berrier, J. 1978.In “Soil Micronmphology” (M. Delgado, ed.), pp. 717-739. univ. of Granada, Spain. Tessier, D.,and Pedro, G. 1976. Sci. Sol 2, 85-99. Toogood, J. A. 1978.In “Modification of Soid Structure” (W. W. Emerson, R. D. Bond, and A. R. Dexter, eds.), pp. 21 1-215. Chichester. England. Tovey, N. K. 1974.In “Soil Microscopy” (G. K. Rutherfoni, ed.), pp. 119-142. Limestone Press, Kingston. Tovey, N. K. 1980. J. Microsc. (Oxford)120, 303-315. Tovey, N. K., and Krinsley, D. H. 1980.J. Microsc. (Oxford)120, 279-289. Tovey, N. K., and Wong, K. Y. 1974. In “Soil Microscopy” (G. K. Rutherfoni, ed.), pp. 207-222. Limestone Press, Kingston. Tovey, N. K., and Wong, K. Y. 1980. J. Microsc. (Oxford)120,329-342. Tmina, T. A., Yamnova, I. A., and Shoba, S. A. 1980.Pochvovedenie 2, 30-43. Ugolini. F. C., Dawson, H.,and Zachara, J. 1977. Science (Washington, D.C.) 198, 603-605. van Breemen, N.,and Hannsen, K. 1975. Soil Sci. SOC. Am. Proc. 39, 1140-1148. van Ranst, E., Righi, D., de Coninck, F., Robin, A. M., and Jamagne, M. 1980. J. Microsc. (Oxford)120, 353-361. Veen, A. W. L., and Maaskant, P. 1971. G e o d e m a 6, 101-107. Vergouwen, L. 1981.In “Submicroscopy of Soils and Weathered Rocks” (E. B. A. Bisdom, ed.), pp. 237-248. Cent. Agric. Publ. Doc., Pudoc, Wageningen. Verheye, W., and Stoops, G. 1975.Pedologie 25, 40-55. Violante, P.,and Violante, A. 1977.Agrochirnica 21, 513-522. Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., and Muilenberg, G. E. 1979. “Handbook of X-Ray Photoelectron Spectroscopy.” Perkin-Elmer, Eden Prairie. Wang, C., Nowland, J. L., and Kodama, H. 1974. Can. J . Soil Sci. 54, 159-170. Weaver, C. E. 1976. Clays Clay Miner. 6, 101-107. We&, H. R. (ed.) 1976. “Electron Microscopy in Mineralogy.” Springer-Verlag. Berlin. Whalley, W. B., Douglas, G. R., and McGreevy, J. P. 1982.Z. Geomorphol. 26,33-54. Wieder, M., and Yaalon, D. H. 1974. Geodema 11, 95-121. Wilding, L. P., and Drees, L. R. 1971.Soil Sci. Soc.Am. Proc. 35, 1004-1010. Wilding, L. P., and Drees, L. R. 1913. Soil Sci. SOC. Am. Proc. 37, 647-650. Wilding, L. P., and Drees, L. R. 1974. Clays Clay Miner. 22, 295-306. Wilding, L. P., and Geissinger, H.D. 1973. J . Sediment. Petrol. 43,280-286. Wilding, L.P., Smeck, N. E., and Drees, L. R. 1977.In “Minerals in Soil Envrionments” (R. C. Dinauer, ed.), pp. 471-552. Soil Sci. Soc. Am.,Madison, Wisconsin. Williams, C., and Yaalon, D. H. 1977. G e o d e m a 17, 181-191. Wilson, M. J. 1975. Soil Sci. 119, 349-355. Wilson, M. J., and McHardy, W. J. 1980. J. Microsc. (Oxford) 120, 291-302. Yaalon, D. H., and Wieder, M. 1976. Clay Miner. 11, 73-79.


THE CONVERGENT EVOLUTION OF ANNUAL SEED CROPS IN AG R ICULTURE C. M. Donald1 and J. Hamblin2*3 ’Waite Agricultural Research Institute, The University of Adelaide, South Australia, and 2Department of Agriculture, Geraldton District Office, Marine Terrace, Geraldton, Western Australia

I. Introduction .......................................................... 11. Selection in Domesticated Crops ......................................... A. Charles Darwin’s Views .................. B. Selection within Annual rops .................................. 111. Ecotypic Parallelism in Crop Plants ....................................... IV. Selection, Evolution, and Crop Yield .................. Biological Yield, Harvest Index, and Grain Yield. ............. Progress and ent of Annual Seed Crops.. ............

B. V.

B. C. D. E. F.

Barley .......................... Rice.. .......................................................... Maize ................................... Sorghum ............................... ... Common or American Bean ........................................

........................... ............ ................................. ................................. .... ....................... .......................

References ..............

97 100 100 101 111 112 113 119 121 121 122 123 124 126 127 129 130 130 131 133 134 139

I. INTRODUCTION Crop evolution consists of three phases: the natural evolution of a species to the “roto-crop” stage, domestication, and further evolution within the domesticated species. In the proto-crop stage, the main requirement is for a species to possess some attribute desired by man. Domestication of a seed crop was often a 3All correspondence regarding this article must be addressed to Dr. J. Hamblin. 97

Copyright 0 by Academic Press, Inc. All rights of reproduction in any form reserved. ISBN 0-12-000736-3

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“once only” event, when certain major mutations ensured that a species was adapted into the agricultural environment and altered unfavorably for survival in the wild (Mangelsdorf, 1965), although this may be an oversimplification in some cases (Harlan, 1971). These mutations include such well-known features of crop plants as indehiscent fruiting bodies, which allow easier and more complete harvesting, and soft seededness, which ensures simultaneous germination of all seeds. The third phase of evolution within the domesticated species is a continuing phase. The crop changes under selection pressures, both natural and manimposed. It is the role of agronomists and plant breeders to optimize the environment and plant type to ensure maximum crop production within this environment. The world’s field crops are commonly grouped into cereals, pulses or grain legumes, fiber crops, oil crops, root crops, and rubber. This classification, based partly on botanical considerations and partly on crop product, has little ecological basis. However, many of these crops, such as the cereals, the grain legumes, and some of the oil crops, are annual seed crops with parallel ecological features. Increased understanding of the factors governing crop photosynthesis and respiration, distribution of assimilates, and seed growth permits us to compare and contrast the performance of annual seed crops. This may be in terms of their branching, leafiness, light profile, photosynthesis, biomass, flowering, seed setting, grain filling, harvest index, and yield, and/or in terms of agronomic factors such as soil fertility, plant density, and plant arrangement. At first sight, it may seem difficult to compare cotton with maize or sunflower with wheat, but such comparisons provide a major challenge to our thoughts. For instance, why are some annual seed crops so much more productive than others? And what can be done to remodel less efficient crops? Annual seed crops provide most of man’s food and, in some countries, a significant part of the animal feed as well as industrial products of great importance (fibers, oil, etc.). They are cultivated from the equator to near the Arctic circle and are adapted to diverse edaphic situations. Some are extremely tall (more than 4 m) whereas others are dwarf, some are climbers with tendrils. When these patterns of adaptation and the morphological and physiological characters are surveyed, Can we perceive common features, either plant or cultural, that may be exploited to increase seed yield per hectare in all the various annual seed crops and environments? We consider that the yield potential of annual crop species will increase at a faster rate than occurs with empirical selection for yield if suitable ideotypes are identified. A considerable list of common features and practices that influence yield in all annual seed crops can indeed be identified, and it may be possible to design a basic ideotype for all these crops, involving principles of crop physiology and associated agronomic practices equally applicable to any annual seed crop. Such a common model or ideotype is formulated here.

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A sharp distinction is drawn between the ecology of annual field crops grown for their seed and that of most horticultural crops. Horticultural plants are cultivated for their fruits, unripe seeds, roots, stems, or leaves. “We believe,” remarks Schwanitz (1966, p. 29), “that the transition from normal to giant growth is the most important step in the evolution of wild species into cultivated plants.” The modem apple, tomato, celery stalk, or lettuce heart are examples. However, it is important to recognize the large environmental as well as the genetic component in the improvement of horticultural crops. Letluce and many other species may be so widely spaced that the plants are almost noncompetitive. Alternatively, there may be deliberate reduction in the number of harvested parts, as in flower or fruit growing where huge blooms and fruits of some species are produced by deliberate thinning. Schwanitz’s view of gigantism through evolution under domestication is clearly valid with respect to horticultural plants; there has been remarkable progress toward gigantism of the harvested part within markedly more favorable environments. Under horticultural conditions, natural selection is heavily suppressed, but selection by man has been highly effective. The range of varieties within a single species (e.g., within Brussicu oleruceu, the cabbage, cauliflower, brussel sprouts, kohlrabi, kale, etc.) illustrates this point most vividly. For annual seed crops, however, Schwanitz’s views regarding gigantism of plant parts do not hold. The prime need for cultivators of these crops has always been the quantity of seed in the bag or basket, of the crop yield per unit of land, rather than the size of the individual seed or the seed yield per plant. In modem seed crops, a reduction in individual plant yield to as little as 5% of the yield of like plants growing in isolation is usual (Donald, 1963), yet for a long time agricultural scientists failed to recognize the significance of competition within these monocultures. With competition of such intensity, the extent of natural selection is limited only by the genetic variability between the plants and by any inequalities of the immediate environment of the individuals. If a drought occurs, there will be intense competition for water. If water is abundant, there may be equally intense competition for nitrogen. If water and all nutrients are freely available, there will be extreme competition for light. Nothing is farther from reality than the following analysis by Schwanitz, which is a common viewpoint (italics ours): “Cultivated plants are also exposed to the influence of their environment; they too are threatened by frost and drought, pest and disease. But man has been careful to protect the plants that are useful to him from excessive hazard. By tilling and fertilising the soil, by regulating the water supply and eliminating the struggle for life, and by protecting the plants from pests and disease, he has created an artificial environment that favours the plant more and above all exposes it to less rigorous requirements than those met in nature. Hence natural selection in cultivated forms is less harsh than among wildplants” (p. 116). Only in a few horticultural crops is that

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statement generally acceptable. For cultivated seed crops, the seedling environments have been substantially improved; nonetheless, intense competition among plants rapidly develops. This competition more than any other factor has governed their evolution. Many of the generalizations on crop evolution are largely based on horticultural crops and simply do not apply to annual seed crops, the principal component of man’s agriculture. Our views are essentially similar to those of Clements et al. (1929, p. 77), who state that “Competition is keenest when individuals are most similar and . . . make nearly the same demands on the habitat and adjust themselves less readily to their mutual interactions,” and also that “The closeness of competition between plants of different species varies directly with their likeness in vegetation or habitat form.”

II. SELECTION IN DOMESTICATED CROPS A. CHARLESDARWIN’S VIEWS

Darwin (1868) grouped the selection forces operating among domesticated animals and plants into three loosely defined categories: methodical or con-

scious selection by man “according to some pre-determined standard”; unconscious selection by man through retaining ‘better’ animals or plants (‘better’ in the eyes of the herdsman or the cultivator) and natural selection, occurring without any purposeful intervention by man. He drew no particular distinction between the operation of these three forms of selection among animals and plants. Darwin readily illustrated conscious selection with horticultural crops: the development of large gooseberries, double flowers, and early maturing peas, and the selection of high sugar content in beets;hence his tribute to man’s capacity to select methodically from within a varying population those features or attributes he values. However, he used the term unconscious selection when man selected for general superiority of animals or plants without attempting to define the specific factors for which selection occurred. Consequently, some confusion has arisen in the subsequent use of this term. Though the herdsman might choose a ‘better bull’ or the cultivator might choose a ‘better plant’ without any predetermined standard or without evaluating any array of desirable features, he neverthelessdoes make a perfectly conscious and deliberate choice. The term unconscious selection seems scarcely appropriate. Darlington (1%9) uses Darwin’s term unconscious selection but gives it a different meaning. He speaks of “unconscious selection by the cu1tivator”as the transformation of the crop by selection during cultivation, tilling, sowing, reap-


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ing, and threshing. But this is not selection by man. It is clearly the operation of natural selection within the environment of man’s cultural practices, involving no active selection by man himself, conscious or otherwise. It is proposed here, at least for annual seed crops, that Darwin’s “unconscious selection by man” be termed “nonspecific selection by man,” and that Darlington’s “unconscious selection by the cultivator” be regarded as “natural selection for adaptation to agriculture.’’ Darwin recognized the close adaptation by natural selection of numerous varieties of wheat to various soils and climates even within the same country; “that the whole body of any one sub-variety ever becomes changed into another and distinct sub-variety, there is no reason to believe. What apparently does take place, is that some one sub-variety . . . which may always be detected in the same field, is more prolific than the others and gradually supplants the variety that was first sown” (p. 389). B. SELECTION WITKIN ANNUAL SEED CROPS

We now recognize several categories within Darwin’s general processes of selection by man and natural selection. These are discussed here in relation to annual seed crops.

I . Selection by Man in Annual Seed Crops Conscious selection by man relates especially to fruiting organs and seed; to larger ears of wheat, larger cobs of maize, or heads of sunflowers, all undoubtedly contributing to an improved harvest index and grain yield in the early years of domestication. The choice of seed size, color, and flavor was also a basis for selection, notably in rice and beans. An early and important case is the selection of the dwarf habit in the naturally climbing common bean (Phaseolus vulgaris) by the American Indians (A. M. Evans, 1980). The dwarf mutant, in nature or mixed cultivation, would have been effectively lethal because of suppression by taller plants (Smartt, 1969; Hamblin, 1975); it could not have emerged by natural selection, but man has preserved and propagated it as a key mutant. A similar situation has occurred in rice with the development of short, high-yielding types which are rapidly eliminated in mixtures with tall, low-yielding types (Jennings and Aquino, 1968; Jennings and Herrera, 1968; Jennings and de Jesus, 1968). However, in many instances it is difficult to distinguish selection by man from natural selection within the changed environment that man has provided. How effectively did early cultivators select for better yield and to what extent did better yields arise through the natural selection of genotypes producing more seed? In present-day annual seed crops, it is certainly very difficult to select by

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subjective judgement those plants capable of higher seed yield in pure culture (Bell, 1963; McGinnis and Shebeski, 1968; Walker, 1969; Hamblin, 1971). The genetic worth of any plant is confounded by the influences of its immediate environment, including its neighbors (Hamblin et al., 1978). Percival (1921) quotes Virgil and other classical writers as advocating the selection of the best ears for use as seed, but we do not know to what extent this influenced yield per unit area. Nevertheless, many effective selections have occurred in the past; we have records for a century or more of successful cereal varieties being developed from single ear or plant selections by farmers, such as wheat cultivars; Chidman, Hunter’s White, and Squarehead (United Kingdom); Canadian Fife (Canada); Fultz (United States), and Purple Straw and Prior barley (Australia). Each was an arbitrary choice of a plant which, in the eye of the observer, looked more productive and indeed proved to have features which were sustained in pure cultures over a range of environments. But the unsuccessful and unrecorded conscious selections for yield in cereal crops doubtless occurred in great numbers. Wherever conscious or methodical selection has been practiced, correlated secondary selection for other characters has been almost inevitable. Thus the selection of the dwarf habit in beans was probably accompanied by unplanned and unrecognized selection for determinate growth, a feature of plant architecture no less important than reduced height itself. Increased cob size in corn initially increased yields over its progenitors (Galinat, 1965; Wilkes, 1977). However, once the modem types had become established, secondary selection probably was adverse to productivity in pure culture, reducing any gain resulting from the conscious component of selection. Large maize cobs, selected in the field, were almost inevitably from tall, broad-leaved, highly competitive plants that had exploited the habitat of their neighbors (see the work of Gardner and coworkers at Nebraska, considered in more detail on p. 115-1 16). A striking example of such secondary selection was reported by Wilcox and Schapaugh (1980), who selected “phenotypically superior single (soybean) plants, based on a visual estimate of seed yield and lodging resistance.” No change in seed yield or lodging resistance occurred, although the selected plants were significantly taller and later than the unselected control plants. It seems the selected plants were chosen only because of competitive advantage resulting from their tallness and longer growth period. When competing against like neighbors, these taller and later plants showed no advantage. However, it must be remembered that only recently has man become obsessed with yieldhnit area. In many situations, these competitive features would provide benefits in terns of animal feed, building materials, and so on (Hamblin and Rosielle, 1983). It is conceivable that nonspecific selection by man may have occurred among horticultural plants, for example the propagation of a fruit tree because of its general form, cropping capacity, fruit quality, and so on, without any precise


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definition or evaluation in the mind of the observer. It is difficult, however, to identify nonspecific selection of plants within annual seed crops. If the selection was made in the field, some particular feature, whether number or size of fruits or seeds, growth form, vigor, or some other specific factor, probably would have been the basis of selection. Today’s growers of annual seed crops are most interested in yield per unit area. This is a form of conscious selection for yield, not within the crop but between cultivars within the region. This was also the case when hexaploid wheats replaced tetraploids over northern Europe and when tetraploid cottons from the Americas replaced most of the diploid cottons of the Old World. In each instance further selection for adaptation to new environments occurred. 2 . Natural Selection in Annual Seed Crops

Within man’s crops, natural selection is always potentially operative. Darwin emphasized that “natural selection . . . a power incessantly ready for action, is as immeasurably superior to man’s feeble efforts as the works of Nature are to those of Art” (p. 77). Natural selection results from the relationship of each plant to its physical environment and to its neighbors. If the plants within a crop are genetically variable in even the slightest degree some biotypes will increase and others will decrease in ensuing generations. The relative advantage of a particular biotype may change with the environment (e.g., for barley, Harlan and Martini, 1938; for beans, Hamblin, 1975; for rice, Adair and Jones, 1946). Two mechanisms of natural selection may be recognized (Nicholson, 1962): environmental selection and selection through competition. Nicholson considered that the views of Darwin and Wallace toward natural selection were based primarily on one or other of these mecharismeDarwin’s on selection through competition and Wallace’s on environmental selection. a. Environment Selection. Nicholson wrote that environmental selection “removes all individuals which are not sufficiently potent to withstand the severe conditions to which the species is exposed from time to time, and so leads towards the production of a population in which all individuals can survive, even under these severe conditions” (p. 65). Examples of environmental selection abound in man’s crops. It occurs when early flowering individuals in a crop are prevented from setting seed by a late frost, when late flowering plants produce no seed because of hot, dry conditions, when some plants grow poorly on an acid soil or under saline conditions, or in any one of numerous circumstances within the physical environment. Cotton differs from most annual seed crops in that it entered cultivation as a perennial shrub. It thus continued, confined to the tropics, for several millennia. When taken to temperate areas, in less than 600 years natural hybridization and selection for earliness and avoidance of frosts gradually led to a branched shrub

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of strictly annual habit (Hutchinson, 1965). Similarly, maize, originally confined to the subtropics of the Americas, progressively extended northward into areas with shorter seasons and severe winters because of interspecific crossing and environmental selection (for earlier flowering forms having different photoperiodic responses (Galinat, 1965). At the experimental level, a barley composite involving crosses between 31 cultivars (Allardand Jain, 1962) rapidly adapted to the climate at Davis, California. The population shifted strongly to earliness in heading date for 5 generations and then more slowly for the next 15, a directional selection. At the same time there was a steady elimination of the “tails” (either the earliest or latest) and variance decreased, stabilizing selection around the optimum heading time for the locality. Similar environmental selection has been recorded for other climatic features, especially length of day, and to soil features, such as texture, pH, fertility, and salinity. “All those who have closely attended the subject insist on the close adaptation of numerous varieties (of wheat) to various soils and climates even within the same country” (Darwin, 1868, p. 388). Environmental selection within cultivated crops also occurs from the influence of man’s cultural practices (Harlan er al., 1973). Although it may be claimed that these practices are “artificial,” the plant responses and the selection of the “fit” are truly natural selection in the Darwinian sense. Successful plants must be more suited to, or less harmed than other plants by man’s treatment of the crop. In primitive agriculture, emergence from varying depths of planting was critical; in many earlier crops, and still today in some species, resistance to damage during temporary grazing gave selective advantage. There was advantage also for those plants that responded better to man’s cultivation practices, to his soil-water storage, or to his irrigation practices. More recently, those plants that respond to artificial fertilizers or are less harmed by pesticides are at an advantage. Faced by these practices, some plants will show a relative gain and others a reduction in prolificity (number of seeds produced). However, seed number per plant by itself will not ensure that any individual is represented in successive crops. A high proportion of each plant’s seed must go into the bulk seed used for sowing the next crop. This attribute is distinct from prolificity but just as significant in determining the pattern of natural selection. Initially seed must be retained on the plant until the crop is cut, but then must be threshable yet not so light as to be lost during winnowing. The seed must retain viability until the following sowing time, but it then must germinate without problems of hard-seededness, physiological dormancy, or undue delay. It must emerge and establish from varying depths of sowing. This list of selection filters, doubtless far from complete for many seed crops, indicates the powerful selection pressures that inevitably occurred from the earliest years of domestication, and future natural selection will follow with the adoption of new cultural practices. Natural selection in response to many of these features of the crop environ-


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ment must have occurred within a single, or at most a few, generations; the most ill-fitted plants, such as those that shed all their seed before the harvest, would have no descendants in the next generation of the crop. b. Selection through Competition. If any plant within a crop takes up more water, nutrients, or light than another at the expense of that other plant, it will have the potential to be advantageously represented by its progeny in the ensuing generations; it will be selected through competition. Successful competitors within seed crops have been selected for the following: i . For the Annual Habit. In perennial plants, part of the assimilates are distributed to storage organs, that is, to underground organs or enlarged stem bases (which may themselves be harvested, but are not relevant to this article). Storage competes directly with seed production. Where annual seed crops are descendants of perennial species, selection toward the annual habit has occurred simply because of the capacity of annuals to produce more seeds. However, remnants of perenniality remain in some annual crops, so that “ratooning,” the growth of a second crop from the bases of the first, is possible. Many sorghum cultivars and also some rice varieties, especially those that tiller freely, can be ratooned. But within seed crops evolution toward a strictly annual habit has been continuous, so that seed ripeness and plant death are coincident or nearly so. The avoidance of diversion of resources to vegetative organs combined with the adaptive value of an annual growth pattern in regions of limited season has led to the development of annual seed crops in many families and genera (Chang, 1976). ii. For Tallness. The most universal factor for which natural selection has occurred in crops has been plant height. Even slight superiority, through advantageous competition for light, can give a plant sufficient yield increment to ensure its dominance in a few generations. Fischer (1978) found that each centimeter of superiority in height among spaced wheat genotypes (40 X 40 cm) gave a yield advantage of 0.58%. At normal densities this advantage is considerably enhanced (Jensen and Federer, 1964; Khalifa and Qualset, 1974). Similar results have been obtained in a segregating population of wheat. During bulk breeding, Khalifa and Qualset (1975) found that the mean height of the population increased from the F, to the F, and that the shortest types were eliminated. There was also a negative relationship between F, height and pure culture yield. Data from other cereals have been similar. In a segregating barley population, there was a positive relationship between single plant height and yield in the F, population; this was reversed in the F, plots (Hamblin and Donald, 1974). Perhaps the most striking data are those available for rice. When two tall, leafy varieties of rice were mixed in equal proportions with three semidwarf, erect cultivars (each providing 20% of the seed sown), the low-yielding tall varieties suppressed the high-yielding semidwarf varieties within four generations to less than 0.5% of the population (Jennings and de Jesus, 1968). Similar results occurred with segregating rice populations (Jennings and Herrera, 1968). In

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maize, dwarf types yielded less when grown in competition with tall types than they did in pure culture (Pendelton and Seif, 1962). Thus over the millennia, and probably quite early in the history of cropping, annual seed crops became tall. Percival (1921) instances Triticum uestivurn to 150cm, T. turgidurn to 180 cm, and even T . compacturn (Club or Dwarf Wheats) to 140 cm. Yet under marginal soil and climatic conditions, even these tall wheats were superseded and replaced by the still taller rye (Secale cereale). Under fertile conditions in the United States, maize attained heights of more than 3 m by the mid-twentieth century; Goldsworthy (1970) records a local sorghum of more than 4 m in height in Nigeria. The advantages of height in competition are not confined, however, to cereals. We have estimated (from the data in Table I and Fig. 1 of Schutz and Brim, 1967) that each centimeter of height advantage in soybeans gave Jackson (a tall variety) a yield increase of 2.5%, and 4% of mean yield, when competing in hills 46 cm apart with the short varieties Hill and Lee. The advantage was 0.7-1% when competing in rows 107 cm apart. In pure culture, Hill and Jackson had similar yields whereas Lee was 13% higher yielding than Jackson, the tall variety. These results are partly confounded by maturity effects, but in all cases the taller lines were more competitive than the shorter lines, and the late lines were more competitive than the early lines. Tallness in seed crops had certain advantages to early cultivators, which continues in the village agriculture of many regions. It gives stem material of value for fuel, bedding, building, and thatching purposes, so that tallness is esteemed. Also tallness is at an advantage when competition with weeds is severe (Pal et al., 1960). It tends also to be linked, in the minds of many growers, with greater yield; as will be discussed later, this is a fundamentally mistaken belief in weed-free situations. But there is little reason to believe that tallness developed through selection by man, because it dominates through natural selection within a few generations of its appearance within a crop. Finally, an equilibrium is reached when the tallest plants suffer grain loss or collapse because of wind damage. Further, these tallest plants tend to have reduced harvest indices (Rosielle and Frey, 1975; Donald, 1981; Hamblin and Rosielle, 1983), so that they yield less grain than their slightly shorter neighbors. iii. For a Leafy Canopy. The third powerful influence of natural selection within seed crops is for a canopy of wide, horizontal or floppy leaves. Such plants are able to intercept light preferentially. The large, subcircular, horizontally disposed leaves of cotton and the large leaves of many sunflower cultivars permit considerable light interception even by crops of low leaf area index (LAI). The influence of canopy structure on competitive ability was demonstrated by the comparative behavior of wheat varieties of different leaf habit (Tanner et al., 1966). In a weedy situation, the floppy-leaved varieties were able to suppress weeds and yield well whereas those of erect leaf habit were depressed in yield. In


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a weed-free situation, the yields were reversed. Similarly, in an F3 population of barley at strongly competitive spacing (Hamblin and Donald, 1974), there was a positive correlation between leaf length and yield. Reference was made earlier to the almost total suppression of short, erect-leaved rices by tall, floppy-leaved cultivars. The leafhess of successful competitors must also be subject to stabilizing influences. Maize or rice plants cannot continue to grow taller and leafier indefinitely. The disadvantage of tallness and leafhess, especially of tallness beyond that needed for successful competition, is the tendency of the crop to lodge, leading to a disorganized light profile, reduced seed production, and harvest problems. The trend to leafhess and strong competitive ability also tends to be associated with heavy water use, prolonged growth, lateness in maturity, and a decreased harvest index (Donald, 1981). An unexpected feature of the evolution of wheat under domestication has been the much lower photosynthetic rate per unit area of leaf of modem wheats compared to primitive species of Triticum and Aegilops (Evans and Dunstone, 1970; Khan and Tsunoda, 1970; Evans and Wardlaw, 1976). Evans and coworkers suggested that this falling rate is caused by the reduced surface/volume ratio of the mesophyll cells, which Kranz (1966) had shown to have become progressively larger during domestication. However, Evans and Dunstone (1970) found that the leaf size had increased more than the photosynthetic rate had fallen, so that the photosynthesis per leaf was much greater in modem wheats. They also noted a positive relationship between leaf size and grain size and reasoned that selection for yield would lead progressively to increased grain size, increased leaf size, larger cell size, and lower photosynthetic rate. Khan and Tsunoda (1970) take the view that the change in leaf size and photosynthetic rate is caused by an improvement in the environment of plants under agriculture which has selected for a mesophytic habit from a wild xerophytic habit. Similar changes with domestication have occurred in cotton and tomatoes (Stebbins, 1974).

An alternative explanation has been offered for this enigma of falling photosynthetic rates during the evolution of wheat under domestication (Donald, 1981), based on competitive relationships. Within the crop canopy, plants with large, usually wide, floppy, drooping leaves would have had strong competitive ability for light and a clear selective advantage throughout domestication. As long as the photosynthetic rate per leaf was sufficiently maintained, a progressive increase in leaf size ensured natural selection, with a consequent relaxation of selection pressure on photosynthetic rate per unit of leaf area. Less leafy plants were at great selective disadvantage because of shading by their leafy neighbors which, although they might have been “physiologically weaker,” were “ecologically powerful.” Thus there would again be stabilizing selection between directional selection for larger leaves in the competition for light and opposing

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directional selection for an adequate or advantageous rate of photosynthesis. It is proposed that modem wheats have leaf sizes and photosynthetic rates representing the outcome of this selection. Selection for yield, it is suggested, has not been involved as the initiating factor, and man has played a role in the falling rates of photosynthesis only through his crop production. iv. For Tillering or Brunching. The selective advantage of free tillering is illustrated by the response of wheat to poor establishment, which was doubtless common in early agriculture. Abundant tillering compensates for low plant uumbers. When the stand density of wheat at Adelaide was reduced from 184 to 35 plants/m, the number of tillers per plant increased from 5.5 to 13.7, and the number of grains per plant increased from 46 to 215, with no significant change in yield per square meter (Puckridge and Donald, 1967). Similar results were obtained by Bremner (1969). In both studies, a genetically uniform cultivar was used; it is clear that if a genetically diverse crop were depleted in plant numbers for any reason, free-tillering genotypes would have great selective advantage over less tillered kinds. Under domestication there seems to have been little reduction in the tillering capacity of wheat or barley. Modem cultivars are capable of producing 30 or more fertile tillers (ears) per plant at wide spacing, although only 2 or 3 are produced when competing at crop densities (Puckeridge and Donald, 1967). In contrast to wheat and barley, there has been a strong trend in maize and sorghum toward single-stemmed plants. One may ask why this should be so, because they are all graminaceous plants of basically similar vegetative structure. The probable explanation lies in the culture of these four species by man. Wheat and barley have always been harvested as a plant community, with sickle, scythe, mower, or header, and there is no recognition of the individual plant. In contrast, for many millennia maize and sorghum have been hand-harvested by pulling the cob or by cutting off the inflorescence. Here lay the opportunity to set aside the largest cobs or heads for seed the following season. This would lead indirectly to the preferential selection of sparsely tillered and ultimately of single stemmed plants, an instance of secondary selection by man. The phenomenon of branching in dicotyledonous crops is ecologically parallel to tillering in cereals. The competitive ability of soybean cultivars was assessed in several experiments by their growth in pure cultures and mixtures (Mumaw and Weber, 1957). The most consistent feature linked with the competitive success of a cultivar in mixtures was the branching growth habit, even exceeding the influence of a 12-cm height advantage of some cultivars. In all comparisons over 2 years, branching varieties contributed about 60% of the total yields of the mixtures, and nonbranching varieties contributed about 40%. v. For Seed Size. In most natural communities, small seeds have marked selective advantage over larger seeds. Small-seeded annual plants probably will yield a greater number of seeds than large-seeded plants: small seeds are more


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easily dispersed and buried; grazing animals can pick up small seeds less easily, are less likely to crush them while chewing, and are more likely to pass some of the seed through the digestive tract undamaged. Most of these selective advantages disappear in the crop situation; there are no grazing animals, and dispersal and resowing do not depend on natural forces. Large seeds have one powerful selective advantage, namely, they produce large seedlings (Evans and Bhatt, 1977) which have strong competitive ability over smaller seedlings. In the wild state this advantage probably is reduced or lost because of defoliation by animals, but within a crop larger seedlings give rise to larger plants with more seed. When wheat seeds of 45 mg were sown alternately with small (27 mg) seeds of the same variety, the large seeds produced plants with seed yields 57% greater than those from the small seeds (Christian and Grey, 1941). However, in mixtures the advantages of large seed size do not necessarily lead to survival. Hamblin (1975) found that the relative competitive advantage of bean varieties (Phaseolus vulgaris) changed with environment; large-seeded types yielded relatively more when competition was most severe (i.e., when yield/plant was small), but in nearly all situations the smallest seeded variety produced the most seeds per plant (was more prolific), although it was never the highest yielding variety and often yielded only moderately. He also showed that as the small-seeded type dominated the mixture so the average yield of the population fell. In another study (Hamblin and Morton, 1977) involving segregating populations, natural selection was always for small seeds, and in three out of four cases it was for increased seed numbers. The results were obtained at crop densities and contrasted markedly with the results obtained at low density, illustrating the importance of not extrapolating from one situation to another with changed competitive relationships (Donald and Hamblin, 1976; Donald, 1981). A similar result was found for a bulk cotton population that was grown without conscious selection for 10 generations; there was a linear increase in seed numbers and a linear decrease in seed size over the generations (Quisenberry et al., 1978). These stabilizing factors for seed size (i.e., the competitive advantage of large seeds and the greeter prolificacy of plants with small seeds) would, despite fluctuations in their significance from season to season, lead to a loose equilibrium for seed size. But early in the history of cropping, another factor was superimposed: the conscious selection by man of larger and plumper grain, features associated in the minds of growers with high yield. Several Greek and Roman writers emphasized the importance of retaining large grain from the harvest to be sown the following year (Percival, 1921). Large grains could readily be separated during winnowing or by shaking grain on a shallow tray. Thus an added and powerful selective advantage, unrelated to field performance, lay with plants producing larger seeds. Although continuing recombination and segregation also would have ensured stabilizing selection for small-seeded

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prolific genotypes, the equilibrium size presumably would have tended to increase. The doubling of seed size from the wild wheats (Triticum rhaoudar, to 20 mg) to the cultivated wheats (T. aestivum, 40-45 mg) was doubtless partly the result of active selection for seed size by man, but the increase in competitive advantage of large seeds and seedlings under wheat cultivation as compared to that in the wild state was probably notable. In some crops there was intense selection of individual seeds, within the harvested crop, for color, shape, and greater size; this was particularly so in the common bean (P. vulgaris) and also in many grain legumes. Not only were distinct and strikingly different races of beans developed in nearby villages, but there was a fivefold range in seed size under domestication (Evans, 1973), probably far beyond the limits of any equilibrium relationship resulting from natural selection. Perhaps man’s concern for selecting large seeds in cereals was reduced, eventually, because it added little to yield. For example, in Christian and Grey’s study (1944), there was no difference in yield between crops established wholly from large seed (45 mg) and those established wholly from small seed (27 mg) of the same genotype. If genetic selection has been made for large seed size, there is usually a compensating decrease in the number of grains per plant (Grafius et al., 1976; Hamblin and Morton, 1977). Indeed the data of Grafhs er al. indicated that the best means of increasing yield was to select for grains per head and to allow seed size to vary more or less randomly. Although man has taken a strong interest in larger wheat seed, the influence of natural selection has been so allpervading and continuous that his direct influence, at least until plant breeding began, may have been quite limited. vi. For Speed of Germination. In the wild, the irregular or protracted germination of wheat or of any other annual grass is a partial protection against uncertain climatic conditions, so-called false starts to the rainfall season. But cultivated wheats germinate more rapidly and evenly than do wild wheats (Evans and Dunstone, 1970). This evolution to speedy germination was wholly because of natural selection. Within a crop sown in a prepared seed bed, plants that emerged first had strong competitive advantage over their neighbors. In a drilled barley crop (cv. Clipper) at Adelaide, plants that emerged 1 day earlier gave rise to seedlings 15% heavier at day 17 (from sowing) and to plants 14% heavier at day 70. There was a reduction of 14% in the mean number of grains per plant for each delay of 1 day in emergence (Soetono and Donald, 1980). In that instance, the differences in day of emergence were caused principally by individual seed environments (depth, soil physical condition, and so on), but the same effect of delay would occur when prolonged germination was a genetic character. Thus there would be a progressive selection under crop conditions toward rapid and simultaneous germination, a feature, albeit imperfect, of most modem annual seed crops.


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vii. For Root Characters. Knowledge of the role of root systems in natural selection under domestication is seriously lacking. If it is reasonable to extend to the root system our understanding of the features of plant tops giving selective advantage through competition, one would suggest that just as a tall, leafy, tillered plant secures an undue share of the light, so plants with a widely ramifying root system would be at a selective advantage by absorbing water and nutrients more rapidly and more extensively than could neighboring plants with a restricted system. Passioura (1972) has shown that restricting root growth early in a plant’s development reduces preflowing water use, and he has suggested that this characteristic may be important in situations where a crop is maturing grain in dry situations. However, O’Brien (1979) has pointed out that reduced root development in early stages of growth may lead to problems of nutrient uptake. It is not possible, with our current knowledge, to make any generalizations about root growth and crop development (Fischer, 1981).

111. ECOTYPIC PARALLELISM IN CROP PLANTS It is evident that all crops have been subject to many similar selective and evolutionary processes during domestication. They have become adapted to both the natural environment of the region and the manmade environment of local cultural methods. Despite the diversity of environments in the cropped areas of the world and the specific responses by individual crop species, there have been many common trends in all crops. Various writers have pointed to features which they regard as typical of wild plants and which commonly disappear under cultivation. These especially include morphologically wild characters associated with seed dispersal and seed burial (awns, brittle rachis, shattering pods, pointed seeds, wings, spines, etc.). Positive responses have included the development of synchronous ripening, rapid and simultaneous germination, and larger seeds. Selection through competition has been recognized in its more generalized expression; those genotypes that are prolific and produce more seed, and in the next generation more seedlings, contribute an ever-increasing proportion of the population. However, there has not been adequate recognition of the influence of competition in ensuring the success of plants of common architecture in all seed crops, irrespective of species, soil, or climate. Their structure can be clearly designated: tall, free branching (or tillering), a dense canopy of large, horizontally disposed leaves, and indeterminate habit. Although these may be combined and expressed in many variant ways, they integrate to give a plant of strong competitive ability. Turesson’s (1922) concept of an ecotype, “a product arising as a genotypical

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response by the ecospecies to the particular habitat,” is applicable to this situation. Within each crop species a cultivation ecotype has developed that is comparable to Turesson’s climatic or soil ecotypes. Turessm further recognized that botanically and morphologicallydiverse species in a common environment may show similar responses. For example, he pointed out that many unrelated species display a stable prostrate form on the exposed west coast of southern Sweden. Philipschenko (1927) called this phenomenon ecotypicparallelism, whereby the identical reaction of related or unrelated organisms to a particular environment resulted in the appearance of a series of similar ecotypes that may be of quite dissimilar genetic structure. There has been ecotypic parallelism among all annual seed crops within the cropping environment through the development of plants of strong Competitive ability having an improved capacity for seed production compared to their wild counterparts and a lack of dependence on burial and dispersal mechanisms for their seeds. A widespread assumption among crop ecologists is that all these changes during domestication have increased productivity; that because an individual plant produces more seed than its neighbors its frequency will increase in the next generation, and as the whole community progressively is made up of more prolific plants it thus becomes every higher yielding. This is false. The cultivation ecotype certainly has advantages over the wild ecotype in its capacity for grain yield, such as prolificacy, strong and rapid establishment, larger seed, and freedom from excessive spines or wings on the seed. But many cultivation ecotypes have serious weaknesses associated with their strong competitive ability; each plant competes against its like neighbors. Only now are these being slowly overcome, usually by empirical means. Recognition of this phenomenon, and of the consequent pattern of progress needed in annual seed species, can contribute toward substantially increased grain yields.

IV. SELECTION, EVOLUTION, AND CROP YIELD

Crop yield is a manmade concept. It does not necessarily relate to natural selection or to crop evolution and it is expressed by the nonbiologicalcriterion of weight of product per unit area. The harvest in some crops is a vegetative part (sterns, leaves, or roots) whereas in others it is a reproductive organ (the fruit or seed). Yet whatever the plant part man has used, natural crop evolution on the one hand and trends in crop yields on the other must be recognized as separate, if interrelated, phenomena. Many believe that in the case of crops grown for their seed, plant evolution and increased seed yield must inevitably proceed hand in hand. This is not so. In


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some instances, as evolution continues seed yield advances; in other instances, it declines. “Natural selection, it should not be forgotten, can act solely through and for the advantage of each being,” wrote Darwin (1868, p. 184). Within seed crops it ensures the greater abundance of certain genotypes in ensuing crops, without implications for yield by the entire seed-producing community. And therein lies the dichotomy: on the one hand the performance of the individual competing in a mixed community, and on the other the performance of that same individual genotype growing as a pure crop stand, each plant competing against like neighbors. During selection by man, the bases for many of his choices were, by selfdefinition, advantageous. If he preferred and selected large mottled seeds, then any increases of these in the next generation were advantageous, by his standards. A secondary effect, such as a reduction in the number of seeds produced per plant, might reduce his yield per unit area, however. When this occurred, he may have accepted it; more probably he was unaware of it. When man has sought to select for yield deliberately, he has usually based his attempts on the performance of individual plants or individual shoots. Selection of larger wheat ears, corn cobs, sunflower heads, or plants with more inflorescences was considered a route toward higher yields, and no doubt was highly successful during the early years of domestication. But, as considered later, such selection had progressive limitations. Natural selection for adaptation to the farm and to the farmer’s practices also offered fum prospects that the progeny would contribute to increased seed yield. Again we emphasize the distinction between biological prolificacy (many seeddplant) and crop-to-crop survival that ensures representation in the bulk seed sown for the next crop (see Section III,B,2a). Plants that have both high biological prolificacy and strong crop-to-crop survival will dominate because of exacting natural selection for performance within the agricultural environment. It is especially in relation to natural selection by interplant competition that evolution and increased yield do not go hand-in-hand. There is growing experimental and circumstantial evidence that successful competitors within mixtures of biotypes may be poor producers in pure stands. Instances in which these features are either unrelated or negatively related are recorded in many seed crops, particularly in the cereals, and they are discussed in the following section. A. COMPETITIVE ABILITYIN MIXTURESVERSUS SEEDYIELDIN PURESTANDS

The successful plant within a genetically uniform crop growing in a uniform environment will be the plant suffering the least competition from its neighbors.

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It follows then that the crop should be comprised wholly of plants of low competitive ability, interfering with each other’s growth only minimally (Donald, 1968a,b). Such plants are, in Darwinian terms, unfit plants because of their weak capacity to compete or survive against other plant types in natural communities or in crops of mixed genotypes. A further postulation may be made: strong competitors, which in general are tall, leafy, and freely branched, not only suppress short, erect, unbranched plants but when grown in a pure community may so interfere with each other’s growth (as, for example, by strong mutual shading) as to perform relatively poorly. If this is so, then one not only may expect that performance in mixtures will be no guide to performance in a pure stand (a nil relationship), but further that there will be instances of a negative relationship between performance in the two situations. Both these situations have been reported frequently.

1 . Wheat The first recorded case of yield reversal for varieties grown in pure and mixed culture was described by Montgomery (19 12). He found that one variety rapidly dominated the mixture but that it was not necessarily the one that was highest yielding in pure culture. Similar results have often been reported (Engledow, 1925; Klages, 1936; Christian and Grey, 1941; Laude and Swanson, 1942; Khalifa and Qualset, 1974). The results of Khalifa and Qualset (1975) on a segregating wheat population, suggesting that competition was eliminating the short, high-yielding lines, have already been discussed (see Section III,B ,2,b,ii).

2 . Rice Perhaps the most striking case of a negative relationship between competitive ability and yield among crop varieties is that reported between tall and semidrawf rice cultivars in the Philippines (Jennings and de Jesus, 1968). The strong competitive ability of these tall leafy varieties was discussed earlier (Section II,B,2,B,ii and iii), but these successful cultivars in mixtures were poor producers in pure culture, When the dwarf erect types were almost eliminated, the lower yielding of the two tall cultivars reduced the other tall cultivars to a low frequency in the mixture. Similar results were obtained by Sakai (1955) and Akihama (1968). Jennings and Herrera (1968) also demonstrated a negative relationship between competitive ability in mixtures and yield in pure culture for segregating populations. 3. Barley

Harlan and Martini (1938) grew 11 barley varieties at 10 centers across the United States for 4-10 years; the seed harvested at each site was resown at that


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site the following year. One variety rapidly dominated the mixture, but the particular variety varied with the site. In several instances, the variety most successful in terms of farm use in the region was reduced to a very low frequency. The poor competitive ability of genotypes that are high yielding in pure culture has also been reported by Suneson and Wiebe (1942), Suneson (1949), Wiebe et al. (1963), and Allard and Adams (1969). On the basis of morphology (long leaves, tall plants), Hamblin and Donald (1974) suggested that the high yield of individual F, plants was the result of high competitive ability. This situation was reversed in pure culture F, plots, where high yield was associated with short, small-leaved plants. The negative relationship between competitive ability and pure culture yield for these lines was confirmed by direct measurement (Hamblin and Rowell, 1975). 4 . Oats

Smith et al. (1970) examined all pair combinations of five oat varieties. The tallest variety (Rodney) was the lowest yielding in a pure culture but the most competitive in mixed culture. In pure culture, however, a variety of intermediate height (Brave) had the highest yield; nonetheless, competitive ability was closely related to plant height.

5 . Maize The yield of brachytic maize genotypes grown in alternate rows with normal maize genotypes was less than when this maize was grown as a pure stand (Pendleton and Seif, 1962). The mass selection experiments for yield of Gardner and co-workers (Gardner, 1961, 1968, 1969; Lonnquist et al., 1966, personal communication) can be interpreted in terms of selection for increased competitive ability (our interpretation) rather than for high-yield potential (Gardner’s interpretation). These workers mass-selected single plants on the basis of plant yield, using a grid system to give local control of environmental variation (Gardner, 1961). The selected individuals provided the progeny for the next round of hybridization and selection. Eventually the different generations were tested for yield, and it was found that yield increased linearly with time for several cycles and then reached a plateau. The plots used were either single or double unbordered rows in which the within-row spacing was the same as the between-row spacing. The height of the populations increased in step with the yield. This would mean that the most advanced generations were surrounded by shorter, earlier generations whose yield potential would not have been fully expressed because of interrow and intergeneration competition. Ultimately, the plants became so tall that increased competitive advantage was offset by the disadvantages of increased lodging, and no further yield increase was observed. Increased yield from mass selection is a rare event; therefore it is important

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that further work be done to determine whether mass selection for yield was effective, or whether selection was in fact for competitive ability. This is particularly important in view of the contrasting CIMMYT results to be considered in Section V,D.

6. Sorghum Averaged over 16 hybrids and two environments, Kern and Atkins (1970) found significant yield depression for short hybrids bordered by tall hybrids and significantyield increases for tall hybrids bordered by short hybrids. On average, a k m difference in height between rows increased or decreased yield by 0.2% for the taller and shorter hybrids, respectively. Kern and Atkins (1970) considered that small yield differences between genotypes of different heights were of doubtful validity unless the data were obtained from bordered plots.

7. Soybeans The performance of soybean cultivars in pure culture was not necessarily related to survival in mixture (Mumaw and Weber, 1957). Survival was related to branching pattern and height. Branching in dicotyledons may be ecologically parallel to tillering in graminaceous crops. Similar results were obtained by Schutz and Brim (1967) and by Hinson and Hanson (1962) in which height and maturity were the dominant factors. Mumaw and Weber (1957) concluded that “a relatively high yield of a variety in pure stands was not necessarily an indication of its ability to survive in mixed populations.”

8. Beans (Phaseolus vulgaris) The experiments of Hamblin (1975) have already been considered (Section II,B,2,b,v). In summary, he found that yield in pure culture and survival in mixtures were not related.

9. Sunflowers Working with eight varieties of sunflower, Fick and Swallers (1975) found that in one-row plots the tallest variety had the highest yield, and that there was a close correlation between height and yield (Fig. 1A). When three-row plots were used the height/yield correlation disappeared (Fig. 1B) and the yield rankings changed markedly. In three-row plots, there is still a suggestion that the shortest genotype suffereda competitive disadvantage. Also, it was probably sown at a density too low for maximum yield.


117

.r

*k

Height (cm)

FIG. 1A. The effect of height on yield of single rows of eight varieties of sunflower. These. rows are not bordered. Note there is a strong correlation between height and yield.

120

150 lecl Height (cm)

FIG. 1B. The effect of height on yield of the same varieties in 3-row plots, where data was obtained from the center TOW only. These rows are bordered. Note there is no correlation between height and yield, although it appears that the center row of the shortest genotype is still suffering interplot competition. (Data from Fick and Swallers, 1975; Table I.)

10. Cotton

Significant genotype-neighbor interactions were found for yield of four cotton varieties grown as single rows (Moran-Val and Miller, 1975). Competitive ability was in part related to height and the authors cautioned that “If competitive effects of the magnitude observed in this study are generally present in yield trials, however, fully bordered plots would be indicated.” 1 1 . Comments on the Results Presented in This Section

The papers just discussed, relating yield in pure culture to competitive ability in mixtures, suggest that there is frequently zero or negative association between these two factors. Although this negative association has not always been found between yield and competitive ability (e.g., Johnston, 1972; Blijenburg and Sneep, 1975), it is so common that it cannot be ignored. In the cases where a positive association has been found between yield and competitive ability, the

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comparisons are often between adapted and unadapted types so that the observed result is to be expected. It may be argued also that the results obtained using mixtures of varieties are artifacts caused by the small sample of varieties used in many of the experiments discussed. We believe that the latter argument is not correct for several reasons. First, there is the sheer number of experiments involved; the result has been observed so often that it is unlikely to be an artifact. Also the argument that results from mixture experiments using varieties may be artifacts applies equally whether the relationship between yield and competitive ability is positive or negative. Second, and more convincing, there are studies involving segregating populations (Jennings and Herrera, 1968; Hamblin and Donald, 1974, Khalifa and Qualset, 1975; Hamblin and Rowell, 1975) in which yield in pure culture was not associated with competitive ability in mixtures for a whole range of species. In these experiments the lines used were related and chosen at random. Third, the results make evolutionary and biological sense (see the next section and Section II,B,2,b,i-vii). 12. Principles Governing Competitive Success and Yield It follows from the foregoing studies that successful plants for pure-stand grain yield are often poor competitors (however, we do not equate poor competitive ability with physiological deficiency). In the study of wheat by Christian and Grey (1941), seed size was the feature giving competitive success; in the study by Suneson (1949), it was a prolific root system; in those for rice (Jensen and de Jesus, 1968) and barley, (Hamblin and Donald, 1974) it was height, leafhess, and leaf length; and for soybeans (Mumaw and Weber, 1957), it was a branching habit. In each instance of yield reversal, it was the shorter, less leafy, less branched cultivars or segregates that gave higher yields in pure stands. Three propositions may now be stated: 1. That, under domestication, competitive plants gained dominance through the natural selection of “the fit”; in some instances, man’s purposeful selection of successful competitors has maintained the place of these plants. 2. That, in pure crop stands, highly competitive plants give lower seed yields than do less competitive individuals, especially at high plant densities. 3. That, because natural selection has favored competitive plants with reduced capacity for yield in pure culture, various natural selection processes must be reversed by plant breeding and selection.

In the Darwinian sense of fitness, it is the unfit plants that will succeed in the pure culture crop situation. Acceptance of this proposition imposes on the plant breeder the need to avoid using selection on individual plant performance in


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situations where there is significant competition among plants of differing genotypes. In this circumstance, poor competitors, whatever their potential merit in pure stands, will give depressed yields. The unfit plant must be allowed to express its potential in pure culture; such plants will perform poorly in mixtures with other genotypes, as in a segregating population in rows or hill plots of mixed genotypes. They will also perform poorly in spaced plant situations (Donald and Hamblin, 1976). Individual plant yields in those circumstances may be positively misleading for predicting crop yields (Wiebe et af. (1963). However, a crop comprised of a single genotype of weak competitive ability still depends on effective exploitation of the environment for high yield. If the annual plants to be used in crops are weak competitors, then the number of plants per square meter must be increased to ensure that they compete with one another sufficiently to exploit the environment fully. It may seem a paradox to propose that productive annual crops will be comprised of weakly competitive plants and to say these plants must be sown at a density sufficient to ensure that they will compete intensely with one another. But these are not incompatible thoughts or objectives. Each is aimed at increased crop efficiency, the first by reducing the pressure of each plant on its neighbors through plant form, and the second by increasing the pressure of the whole community on the available resources through an increased population density. Thus, testing should not merely avoid competition between different genotypes. The lines to be evaluated must be tested in pure stands at densities sufficiently high to ensure that there is interplant competition of considerable intensity.

B.

BIOLOGICAL YIELD, HARVEST INDEX,

AND

GRAINYIELD

The relationships between biological yield and seed yield in seed-producing annual crops display important differences, as is illustrated in Fig. 2. The upper graph (Fig. 2A) shows the general relationships found in cereals (Donald, 1963). Biological yield increases with density until it reaches a plateau. This is maintained up to very high densities unless crop failure occurs from lodging or the advent of disease among the attenuated plants. Grain yield increases to a maximum at a density approximating the minimum density giving full biological yield. To the extent that when maximum seed yield is attained there is maximum exploitation of the environment in terms of biological yield, cereals are efficient in ensuring their prolificacy. On the other hand, the maximum yield of seed cotton or lint by cotton crops is achieved at a density at which the biological yield is still rising steeply (Fig. 2B). The biological yield of the highest yielding crop falls far short of the capacity of the cultivar to produce dry matter. In this regard it is an inefficient crop, which will eventually be replaced to advantage by cultivars giving increasing lint yields up to full biological yield. It may be that this contrast between an annual such as

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M. DONALD AND J. HAMBLIN

CEREALS

-

Dm~ity COTTON

Density

-

FIG. 2. The general relationship between biological yield (BY) and grain yield (GY)for cereals (afterbnald, 1%3)aodforcotton(dataderivedfromKirkeral.. 1%9;Fig. 14,16,19,21 (BY);21 (GY).(GY is the fruit weightlha; seed yield data was not available).

wheat and an annual such as cotton relates to the period for which they have been cultivated (i.e., wheat for some millennia, annual forms of cotton for little more than 500 years) (Hutchinson, 1965). The future trend in the relationship between biological yield and grain yield of annual crops may follow the pattern shown in Fig. 3. If nonbranched plants are used, the density @lants/ha) required to give the full biological yield will be greater. Because of the improved canopy structure, the biological yield attainable will be greater, although perhaps not markedly so (10% in the example shown). The main contribution to potential yield will be an improved harvest index, perhaps 0.35-0.40 to 0.50 in cereals, representing an increase of about 25% in grain yield. The influence of an increased biological yield of 10% and an increased harvest index of 25% would be a 37% increase in grain yield. The extent to which the trend toward increased harvest index is displayed in annual seed crops of diverse growth form is discussed later in this article. The same trends are evident in each crop, and there is no instance in which newer cultivars demonstrate a reversal of the relationship. However, whether harvest index itself is a useful selection criterion, or a useful description of past selection trends, has yet to be critically determined (Hamblin and Rosielle, 1983).


I

BY

121

--

0-

II fi'

Density

*

FIG. 3. Present relationship (solid lines) and possible future relationship (broken lines) between biological yield (BY) and grain yield (GY).

V. PROGRESS AND PROSPECTS IN THE DEVELOPMENT OF ANNUAL SEED CROPS In this section we briefly review crop evolution, including recent plant breeding efforts for a series of annual seed crops. This will allow a basis for proposing a generalized ideotype for all annual seed crops. A. WHEAT

Wild wheats, both diploid and tetraploid, are annuals and have primitive features associated with effective seed dispersal and burial (Bell, 1965). Less easily defined but no less important characteristics of these wild wheats relate to growth form, permitting survival and seed production under close grazing by sheep or goats. Many are fine stemmed and prostrate, so that some ears are borne on a nearly horizontal stem only a few centimeters above the ground and can lie within a grazed sward. Under cultivation, these primitive characters of wheat were at a selective disadvantage and have been lost, except for the brittle rachis and enclosed grain of T. munucuccum (Bell, 1965). Taller, more erect, and more leafy plants had major competitive advantage and probably became dominant quickly. Bruegel's paintings (sixteenth century) depict shoulder-high wheat crops. Although having a tendency to lodge, these tall crops were feasible in premechanized agriculture because most of the grain would be recovered during hand harvesting. In early wheat breeding programs, improved disease resistance was a major

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contributor to increased yields (Athwal, 1971). Yield potential also increased (Austin et al., 1980; L. T. Evans, 1980; Perry and Reeves, 1980; Kulshrestha and Jain, 1982). However, this yield potential increased markedly with the incorporation of the Norin 10 gene in wheat. These short, fertilizer-responsive wheats had been grown in Japan long before scientific plant breeding and were successfully used in Italy 60 years ago (Athwal, 1971). However, it was only when Vogel at Pullman and then Borlang at CIMMYT used this material that the so-called semidwarf wheats made such an impact on world wheat yields in the 1960s and 1970s. These wheats are resistant to lodging and have many tillers and grains per spikelet. Reduced stature and resistance to lodging are characteristics sought in the “all-crops ideotype;” increased number of grains per spikelet in the semidwarf wheats defines the expression of increased yield. On the other hand, the freetillering and relatively broad, lax leaves of these highly successful semidwarf varieties are a challenge to the all-crops ideotype. Many workers believe that only a small number of tillers is needed to give both maximum yields and sufficient plasticity to permit adaptation to the environment (MacKey, 1966; Hurd, 1969; Bingham, 1972; Jones and Kirby, 1977). This view was extended by Donald (1968a,b), who described a wheat ideotype for high grain yields with a short, strong stem, few small, erect leaves, a large ear in relation to the total dry matter (i.e., a high harvest index), an erect ear, awns, and a single culm. (In view of the authorship of that article, this wheat ideotype conforms to the common ideotype for all annual seed crops described in this article). Atsmon and Jacobs (1977) have produced uniculm wheat lines of medium height, high harvest index, and resistance to lodging; they appreciably outyielded the standard cultivar of the region. Further evidence for the potential of controlled tillering was presented by Islam and Sedgley (1981), who examined the effects of manually detillering wheat plants in the field to give biculms. The performance of these was compared with normal-tillered control plots of the same variety. The detillered plants outyielded the controls by 14 and 22%, respectively, in 1978 and 1979. B. BARLEY

Barley (Hordeurn distichurn) is a cereal with a growth form and physiology similar to wheat, so that most considerations of canopy structure, tillering, and leafiness are applicable to both species. Cultivated barleys vary in height from brachytic forms (40 cm), widely grown in the Middle East, to tall forms (1.5 m) (Reid and Weibe, 1968). They tiller freely at low density, especially the tworowed types, but there is a mutant form with a single stem per plant (uniculm). There are considerable differences in leaf size, with extremes of leaf shortness in


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brachytic kinds and of leaf narrowness in the mutant form governed by a single gene. Two-rowed barleys generally have narrower leaves than six-rowed forms. During this century there have been two principal trends relating to plant form and productivity, as illustrated among varieties released, in the United Kingdom. There has been a progressive reduction in height from about 1 m (cv. Spratt, pre-1900) to semidwarfs of about 70 cm. This has been accompanied by an increase in harvest index from about 0.4 to 0.5 (Cannell, 1968; Hayes, 1970; Donald and Hamblin, 1976). This increase has not been consciously sought but is the result of a substantially constant biological yield concomitant with advances achieved by selection for grain yield, early flowering, and reduced plant height (Hamblin and Rosielle, 1983). Interest in plant form as a contributory feature to future yield relates to height (further reduction seems probable), reduced leafiness, and less tillering. Jones and Kirby (1977) believe that although tillering is invaluable for adaptation to the environment, it can serve this role adequately with only a small number of tillers. Donald (1977), using his wheat ideotype as his model, has produced semidwarf, uniculm barleys which, when sown at about double the standard seed rate, outyield the leading local cultivars by 15-20%. However, these lines were not evaluated for grain quality. The initial attempts to produce radically new cereal plants (Atsmon and Jacobs, 1977; Donald, 1979) are sufficiently promising to warrant further effort. As well as the increases in yield that are evidently attainable through dwarfing and the elimination of tillering, further substantial increases may be possible through the development of nonleafy lines with short, narrow, erect leaves (Hamblin and Donald, 1974). The retention of awns seems desirable (Frey, 1971). The use of biological yield and harvest index as a means of interpreting behavior during breeding programs for yield has been strongly advocated (Donald and Hamblin, 1976).

Until 1900, the typical cultivated rice was a tall, strongly competitive plant which had emerged by natural selection within man’s crops because of its capacity to suppress weeds and more dwarf kinds of rice (Jennings, 1964; Athwal, 1971). It had long, broad, drooping leaves and thick culms, was strongly photoperiodic, and was subject to serious lodging, particularly if fertilizer was applied. The first development of a more productive rice of communal habit was in Japan early this century, when cultivars of Oryzu juponicu were bred with erect habit, reduced height, short, stiff straw, and fewer tillers. These varieties did not lodge with heavy applications of nitrogen. This was followed by 0. indicu varieties of similar noncompetitive habit, first in Taiwan with the release

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of the cultivar Takhung Native 1 (TN1) in 1956, and then in 1966 at the International Rice Research Institute with the release of IR8, a variety that transformed rice yields over a great part of Southeast Asia. These two 0.indica cultivars, TNl and IR8, each derived their semidwarf habit and erect leaf growth from the variety Dee-geo-woo-gen, a mutant from an old Chinese variety, Woogen (Athwal, 1971). Additional features of IR8 in relation to the common seed crop ideotype were its nonphotoperiodicity,permitting use over a much greater geographic area, and an increased harvest index. Six older, tall, competitive varieties had a mean harvest index of 0.36, whereas the dwarf, erect, short-leaved varieties had an index of 0.53 (Chandler, 1969). Poor tiller survival because of intense mutual shading and the cessation of growth after flowering were believed to contribute to the low harvest index of the tall, leafy varieties. The other features of the common ideotype and its culture that may offer opportunity in rice production are nontillering (Japanese cultivars already show duction in tiller number) add the use of high plant densities. These features are of course Wed. As long as most of the world’s rice is transplanted by hand at enormous human effort from seed bed to paddy field at low plant densities (about 20 plants/m2), heavy tillering is essential. But in situations where rice is broadcast or aerially sown there may be potential gains in yield from less freely t i l l e d or even uniculm rices of higher harvest index sown at heavier seeding rates. D. MAIZE

The wild progenitor of maize was probably relatively dwarf, with several tillers each having a terminal inflorescence carrying both male and female flowers and several small ears in leaf a i l s (Mangelsdorf, 1965). The terminal inflorescence broke easily, assisting seed dispersal. With the exception of the United States corn belt dent types, the principal Commercial types of maize were fully developed by the American Indians; little genetic advance was made until the development of commercial hybrid corn in the 1930s (Mangelsdorf, 1965; Galiiat, 1965). During domestication strong artificial selection by man for large ears occurred, but natural selection of fecund plants probably occurred in fertile, man-made environments (Wilkes, 1977). The trend to a single stem and large ear was a consequenceof man’s preference for large, easily hand-harvested cobs and easy cultivation between rows and of natural selection for tall competitiveplants with many offspring. Tall competitive plants were regarded favorably,’ but a direct consequence was low optimal plant stands [e.g., 26,0001ha was considered a high density in ‘An Iowan would boast, “I’m from Iowa where the tall corn grows!”


125

Iowa in 1924 (Stringfield, 1964)l. However, during the 1950s a growing appreciation of the interaction between genotype, density, and fertility occurred (Stringfield, 1964), particularly when it was found that dwarf plants suffered much less sterility (5%) than normal plants (62%) at high densities [105,000 plantslha (Sowell, 1960)]. The importance of leaf distribution was also realized. With leaves more vertically disposed above the cob at high densities, light penetrates deeper into the canopy and yields are higher (Pendleton et al., 1968; Winter and Ohlrogge, 1973; Vidovic, 1974; Pepper et ul., 1977). Horizontal leaves were better at low densities, whereas at intermediate densities or in widely spaced rows leaf angle was not important. If no response to leaf angle is found, this probably results from sampling too narrow a density range, from leaves that are not stiff enough along their entire length to maintain a constant leaf angle or from the range of leaf angles that are too small to allow differentiation (Mock and Pearce, 1975). Nonetheless, responses to high leaf angles occur only at leaf area indices rarely obtained in commercial crops. Mock and Pearce (1975) presented features for a maize ideotype that included (1) stiff and vertical leaves above the ear and horizontal leaves below; (2) maximum photosynthetic efficiency; (3) efficient conversion of photosynthates to grain; (4) short interval between pollen shed and silk emergence; (5) ear shoot prolificity; (6) small tassel size; (7) photoperiod insensitivity; (8) cold tolerance for germinating seeds and seedlings (in areas where soils are cold and wet at planting); (9) a grain-filling period as long as is practical; and (10) slow leaf senescence. Features (4) and (6) relate specifically to maize and (8) relates to summer crops. All other features (assuming that ear shoot prolificity and small tassels are components improving harvest index) are common to every annual seed crop. Temperature maize production now uses many of these ideas, and similar objectives are currently being applied to tropical maize. Tropical cultivars are often tall (up to 3.5 m) and leafy, an competitive evolutionary response. They lodge easily and have low harvest indices (less than 0.35). Selection at CIMMYT for reduced height and leafhess within the cultivar Tuxpeno (CIMMYT, 1979) has reduced height by 8 cm/cycle so that plants now are only 60% of their original height; at the same time yield increased 190 kg/ha or 3% per cycle (cf. comments on Gardner’s mass selection program, Section IV,A,5). This increase is associated with reduced lodging, higher harvest index [0.35-0.48 during 15 cycles; cf. comments of Hamblin and Rosielle (1983) on the height-harvest index relationship], and increased crowding tolerance (optimum planting density of 45,000 plantslha at cycle 12 and of 60,000at cycle 15). Flowering was 13 days earlier and there were three leaves less below the ear. Thus similar plant type-density relationships occur in both temperate and tropical regions. Questions for future investigation are, What will be the equilibrium situation between these features and yield in maize? Should we examine the potential of

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maize crops sown at 160,OOO plants/ha (25-cm square planted) producing leaf area indices similar to other cereal crops but with improved canopy-light relationships, a low incidence of barrenness, and a high harvest index? E. SORGHUM

Only circumstantial evidence is available regarding the early history of sorghum (Sorghum bicolor). Doggett (1965) proposed that domestication first occurred in the Abyssinia-Sudan region about 5000 years ago. However, Harlan (1971) considers that sorghum had a more diffuse sub-Saharan origin. From there it spread to other parts of Africa, to India before lo00 B.c., and to China about A.D. 1300. The wild relatives of sorghum, widely distributed over the African continent, characteristically have large, pyramidal, loose inflorescences with spreading branches. Although mainly annual, some are perennial with short rhizomes; the racemes articulate at maturity, assisting natural spread; and they have small grains (de Wet and Huckabay, 1968; de Wet and Schechter, 1977). Cultivated grain sorghums have heads of varying degrees of compactness, from loose to extremely dense with tough rachises and persistent spikelets, features ascribable to selection by man and to natural selection, respectively. Competition for light within sown crops gave tall types a powerful advantage, so that village crops in Africa may be as tall as 3.5 m (Goldsworthy, 1970). Grain sorghum in the United States prior to 1928 was commonly 140-180 cm tall (Quinby and Martin, 1954). These cultivars were annual or weakly perennial, although without rhizomes. They were capable of regrowth from the crown to produce a second crop, permitting ratooning. Because of the wide geographic distribution of cultivated sorghums and the free hybridization between genotypes, many distinctive races can now be found (de Wet and Huckabay, 1968; de Wet and Harlan, 1971). Breeding programs with sorghum have had several clear-cut objectives. Through the use of dwarfing genes, striking reductions in height have been achieved with associated increases in grain yields. By 1953, 98% of the grain sorghum cultivars in the United States were of dwarf stature and harvested by combine (Quinby and Martin, 1954). Reductions from 1.5 to 1.2 m in singledwarf material and to 0.75 m in double-dwarf lines were typical of the reductions in height (Queensland Department of Agriculture, 1970). In Africa the use of dwarfing genes occurred later, partly because of the value of the tall stems for building and fodder in village life. Reductions in height are, however, now occurring in that continent (Goldsworthy, 1970). Another objective has been the incorporation of one or more genes for insensitivity to photoperiod, giving earlier flowering and permitting the progressive extension of sorghum to cooler areas of shorter season. At least three additive genes are involved (Quinby and Karper, 1945; Quinby and Martin, 1954). Since


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1954, the great advance has been the discovery of cytoplasmic sterility, permitting commercial use of hybrid sorghums with yields about one-third greater than those of pure lines. The heterotic manifestations are higher metabolic efficiency, increased height, earlier flowering and longer grain-filling period, greater vegetative yield, and increased grain size and yield (Quinby, 1963). With reduced height, increased vigor, and higher soil fertility, there has been a growing need to manage the crop so as to regulate panicle number per square meter, taking account of the seeding rate, estimated seedling establishment, and the probable tillering behavior (Ross and Eastin, 1972). The row spacing adopted is commonly as close as will permit cultivation (75 cm, or double rows 30 cm apart, at 100 cm); dry-land populations of 50,000-80,000 and of 250-300,OOO plants/ha under imgation have been adopted in the United States. The natural evolution, under cultivation, to very tall competitive plants has thus been followed by a controlled move toward communal plants, that is, toward dwarf stature and much-reduced tillering. Some sorghum cultivars are described as “single-stemmed,” although they tiller at low densities. The opportunities for further progress toward communal plants and higher grain yields seem to lie in further increases in plant density; the development of lines of strictly uniculm habit, shorter, narrower leaves, more erect leaf disposition, and markedly narrower rows without interrow cultivation (Clegg, 1972). F. COMMON OR AMERICAN BEAN

The earliest known cultivation of the common bean (Phaseolus vulgaris) was at least 7000 years ago (Kaplan and McNeish, 1960; Kaplan el a l . , 1973). Beans probably evolved over a wide area (Harlan, 1971; A. M. Evans, 1980); they were a valuable component of the American Indian diet, and their use extended over much of central and north America to about 42”N and over western South America. P . vulgaris has been used both for green beans and as dry beans; it is with the latter use, as a seed-bearing, annual field crop, that we are concerned here. The wild progenitor is P . vulgaris f. aborigineus, the climbing thicket bean, a perennial form with strong branching and a tuberous root. The cultivated species is very variable in growth habit, ranging from indeterminate climbing types to determinate bush types with 3-6 nodes on the primary stem (A. M. Evans, 1980). The climber, grown on a trellis or in association with maize, was the earlier cultivated form, with a branching, indetenninate habit of growth. The dwarf or bush type, used for seed production in presentday mechanized agriculture, is known (from vegetative remains) to have been grown as a crop in indigenous Mexican agriculture at least 800 years ago. It is of determinate growth habit, the main stem and each branch having a terminal flower after 3-6 nodes. The responses to domestication in American Phaseolus beans are summarized

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C. M.

DONALD AND J. HAMBLIN Table I

Rcspoase of Phase&

v u f g d is to -u

chuacteristic of plant as _____

Type of selectionb

Wild

DomaSticated

2 2 1

W d Y pmnnial Short-day plant Indeterminate Many nodes Scrambling habit Small seeds Hard-seeded Physiological dormancy Testa colors and patterns few Small leaves Pods dehiscent Stems thin

Annual Day neutral Determinate Few nodes More erect to erect habit Large seeds soft-seeded No dormancy Testa colors and patterns, many Large leaves Pads indehiscent Stems thick

?

2 1 and 2 2 2 1 2 2 2

“From A. M. Evpos (1980),Smartt (1969).aad Purseglove (1968).

’1, pmbably conscious selection by man; 2, natural selection in agricultural environment.

and classified in Table I. The mechanisms of some of the changes may be debated, and some changes may have multiple causes, but the grouping of most of them is self-evident. Only man himself could have selected the dwarf determinate form; as Smartt (1969) remarks, “In nature or mixed cultivation the dwarf determinate mutant would have been effectively lethal . . . man has preserved and propagated a key mutant.” We thus see that the American Indians deliberately selected and developed for cropping a shorter, less competitive plant; a selection of the unfit. This step has been repeated in wheat and rice by modem workers loo0 years later. The dramatic increase in seed size, although undoubtedly leading to a reduced number of propagules, must have also been attained through deliberate selection. Some seed colors may have had natural selective advantage (fungistatic properties of pigments, less predation by birds), but the choice of particular colors by man has been the all-powerful factor in the local evolution of color patterns. Various consequences result from man’s conscious selection, particularly effects on growth form related to selection for reduced height. However, many important characteristics of modem field beans result from natural selection within the climatic or cultural environment of man’s crops. The most notable of these is the annual habit, an ability to complete the life cycle before killing frosts prevent the production of viable seeds. The perennial habit suffices in subtropical areas, but did not permit survival as the cultivation of the bean extended northward.The responses in time of maturity, photoperiodism, and ready germination


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were all responses to the climatic or man-made environment, and large leaves gave competitive advantage over neighbors for light. The loss of pod dehiscence enabled survival of the harvested seed to be sown the following year (A. M. Evans, 1980). There is no doubt that the evolution of the bean under domestication in the Americas was in many ways more advanced, by several centuries, than the evolution of rice as a crop in Asia or of wheat as a crop in Europe. What developments offer further increases in seed yields in the common bean? It was suggested by Adams (1973) that major reduction in branching is desirable, so that each plant has a main stem and a few short lateral branches with many pods at each nude. Smaller leaves are also indicated as a means of securing deeper light penetration into the canopy. To be effective, these changes must be accompanied by increased density of the stand and strong pursuit of improved harvest index of communal plants growing in a strongly competitive crop situation. G. FIELDBEAN Viciufubu includes the field bean and the broad bean; the former, Viciufubu var. minor, is here considered. It is an erect annual with a main stem and, depending on plant density, one to several lateral stems. Each stem is indeterminate in growth, with 5-10 basal vegetative nodes and about 10 nodes with axillary inflorescences followed by a continuing production of vegetative nodes (Poulsen, 1977; Chapman and Peat, 1978). Most of the seed is produced by the main stem, with one or two pods per inflorescence and four to six seeds per pod. There is competition both among the developing pods and between the pods and the further vegetative growth (Chapman and Peat, 1978), a situation closely comparable to the tall sorghum genotypes discussed earlier. The weaknesses in this plant structure are evident, namely, unnecessary height and vegetative growth associated with the indeterminate production of sterile nodes above the pods. Various useful genes, principally simple recessives, are available, including those for dwarf stature and for a terminal inflorescence. Crossing has shown (Chapman and Peat, 1978) that there are excellent prospects for developing field beans of reduced, erect stature with upright pods borne terminally and leaf sizes reduced to no more than two leaflets; the problem of massive amounts of vegetative tissue passing through the harvesting machinery would thus be alleviated. The yields of these semidwarf determinate types so far are less than those of current tall cultivars because of the inadequate yield of seed per node; more seeds per pod are sought. Two points may be made: first, the reported tendency of determinate forms to produce more branches (Chapman and Peat, 1978), may cancel the gains achieved through reduction of the vegetative tissues of the main stem. Nonbranching determinate forms seem highly desirable. Second, any test-

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ing of such types should be undertaken within high density communities with continued emphasis on harvest index as a guide to efficiency within the biomass of the crop. The efficiency or yield of isolated plants is irrelevant. A disadvantage of reduced branching and increased density may be seed requirements, as the large seed weight of this species will mean that seed costs will be appreciable.

H. SOYBEAN The wild ancestor of the cultivated soybean (Glycine man) is believed to be Glycine soja, indigenous to China, the Soviet Union, Korea, Japan, and Taiwan. G . l l u u ~and G . soju have few barriers to hybridization and on this and other grounds are regarded as conspecific (Hymowitz and Newell, 1980). Both species are annuals, but although the wild G . soju is a slender twiner, characteristic of hedges and roadsides, the cultivated soybean is a bushy shrub. The domesticated plant differs also in having reduced dehiscence of the pods and larger seeds of higher oil content. The soybean was probably domesticated in the eastern half of north China in the eleventh century B.c., spreading to Southeast Asia in the early centuries A.D. It was not known to European agriculture until the early eighth century nor to North American agriculture until the 1850s (Hymovitz and Newell, 1977, 1980). . Three growth habits are present in soya: determinate, semideterminate, and indeterminate; genetic control is by two genes. There are marked differences in the source-sink relationships between these types (Shibles, 1980). Narrow rows and higher plant populations often produce yield increases (Costa et ul., 1980). This may result from improved light relationships (Shaw and Weber, 1967) or improved water-use efficiency (Peters and Johnson, 1960; Timmons et al., 1967). Narrow leaf types give better light penetration, but this was not associated with increased yields (Hicks et al., 1969), although they had high water-use efficiencies (Hiebsch et ul., 1976). The potential for manipulating the soybean plant to develop communal plants appears excellent. However, as they will be poor competitors in mixtures, and as competitive ability is related to branching, height, and late maturity (Mumaw and Weber, 1957; Hinson and Hanson, 1962; Schutz and Brim, 1967), care must be taken to ensure their retention in segregating populations. Also, they must be yield tested in pure culture at high densities if their full yield potential is to be realized. I.

PEAS

Davies (1977a,b) has reviewed the dramatic developments in the pea (Pisum surivum) crop. There has been a marked reduction in stature from a height of 1-2


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m for garden peas to 0.3-0.6 m for field peas. Yet even with this considerable dwarfing, two major physical problems remain: the great bulk of vegetative material to be handled during harvest and the frequency of severe lodging (amounting almost to certainty) with loss of the canopy structure and further deterioration of the light profile. These dwarf pea crops, despite their reduced height, have much in common with the old, tall rice varieties (large, horizontally disposed leaves and poor physical stability). Two mutant genes now offer the prospect of improved canopy structure. The first reduces the leaflets to tendrils and the second reduces the leafy stipules to small bracts. With only the f m t of these genes the plant is known as “semi-leafless”; with both, it is “leafless.” Here then is a dramatic reduction in leafhess. Leafless, and particularly semileafless, crops promise to outyield standard varieties (Davies, 1977a,b; Hedley and Ambrose, 1981). The advantages for seed crops may be several. First, leaflessness permits a much deeper penetration of light into the crop and thereby a more effective mean illumination of the photosynthetic surfaces; second, the interlocking tendrils give such effective mutual support that lodging cannot occur; third, the reduction of vegetative parts contributes to a higher harvest index; and forth, leafless peas may use water more efficiently than leafy types. Perhaps the radical structure of the canopy of these peas may offer, for the f m t time, prospects of yields from legumes more closely comparable to those of cereals. A feature of the pea crop that warrants fuller examination is the extent of vegetative branching, which has already been reduced in some dwarf genotypes. Increased sowing rates of leafless, nonbranching plants probably would improve the crop productivity by these most unusual plants. It is of interest at this point to note the features that existing leafless pea plants have in common with the semidwarf rice varieties: reduced stature, reduced leafhess, better light profile, improved physical stability, improved synchrony of flowering, and, almost certainly, better harvest indices.

In pre-Columbian times, all the cultivated cottons of Central and South America were perennial shrubs confined to tropical regions. These perennial American cottons founded the crops of southern Europe, Africa, and India, but since the mid-eighteenth century, three annual forms have evolved within these crops; upland cotton (Gossypium hirsutum), and sea island and Egyptian cottons (barbadense) (Phillips, 1976). There was a progressive change under cultivation from xeric, wild species to cultigens adapted to more fertile soils and more abundant water (Stebbins, 1974). A major selective force was the extension of cropping into temperate regions, where the frost-free season was progressively shorter. Differential seed production, in favor of plants adapted to the climatic,

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soil fertility, and water regimes of new environments, ensured the natural selection of mesic, early-flowering annuals. Cotton culture, as exemplified by that in the United States, has been of branched, annual shrubs, typically in rows about 1 m apart with about 5-15 cm between plants. Where irrigation is practiced, these rows run centrally along flattopped “hills,” between irrigation furrows spaced at 1 m. In the mid-l960s, however, a major change in cotton culture was foreshadowed through the study of “narrow-row cotton.” In the f m t paper on narrow-row cotton, Ray and Hudspeth (1966) stated that the primary objective was to determine whether yields could be increased substantially through high popylations with large amounts of fertilizer and irrigation water. They found (Brashears et al., 1%8)that population increase was effective in raising yield only if it was achieved by the closer spacing of the rows rather than by increasing plant number within the row. When the population was increased to about 250,000/ha and the mean row width decreased to 50 cm (i.e., with 2 rows 40 cm apart on each 1-m hill), the following changes ensued: reduced plant stature, nonbranching, frost avoidance through earlier maturity (8-10 days), more simultaneous ripening of the bolls (more uniform cotton quality), and higher yield. Similar results have been reported elsewhere (for a review see Low and McMahon, 1973). The study of narrow-row cotton culture led cotton workers to ask themselves two questions: Can a more suitable genotype be developed for use in narrow rows at high population density? and, Can machinery be developed to harvest narrow rows, pferably in a “once-over” operation? The outcome has been a trend toward dwarf, determinate cotton varieties, with bolls borne on short fruiting stalks so that they lie close to the stalk. These varieties are also described as “stom proof.” They can, as was hoped, be harvested at a single stroke. These cotton cultivars and the system under which they are grown are parallel to the common ideotype to be discussed in the next section in many aspects. Bhardwaj et af. (1971), working in India, reported a negative correlation between yield of seed cotton and both plant height and leaf area. They emphasized the need for more dwarf cultivars with fewer branches and less leaf area. Constable (1977) states that in Australia there is also a need for varieties bred specifically for narrow-row culture with reduced leafhess. There are clear opportunities to improve the light profde of the crop through the use of Okra types of cotton, which have deeply cleft leaves of much reduced area, but the field evidence in favor of such foliage is as yet inconclusive (Andries et al.. 1969, 1970; Constable, 1977; Pegelow et al., 1977). Certainly, Okra leaf is unlikely to offer advantages in 1-m rows, but it may well prove of significant value at high density in narrower rows. There has been no clear statement regarding the influence of narrow-row culture on the ratio of lint or


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fruit weight to biological yields (harvest index), but there are good prospects for progress. The trend in cotton toward short, less branched, less leafy plants has occurred by a different path than that in other crop species. In most annual seed crops, the trend toward such plants has arisen through breeding for increased yield or through theoretical considerations of improved plant form, with the consideration of the value of increased plant density or reduced row spacing for such communal plants then following. In cotton, the development of shorter, less branched, less leafy plants did not arise in that way. The first move was an agronomic step toward narrower rows and denser populations, as a means of improving exploitation of the environment; it resulted in phenotypic changes to smaller, earlier maturing, less leafy plants of the standard cultivars. The second phase was the consideration of the need for suitable genotypes for this new cultural environment, cultivars that were genetically smaller, less branched, and less leafy, rather than to allow phenotypic suppression at high densities. The outcome will be the same as in other crops; only the sequence has differed. A specific limitation in cotton to the plant type or the adoption of narrower rows and high densities lies in agronomic practice. Although tests have shown that cropping with 12-cm rows and plants spaced at 8 cm in the row (1 million plants/hectare) have given very high yields (Kirk et ul., 1969), such spacings probably will not prove practicable under irrigation. This suggests that future cultivars for natural rainfall conditions and for irrigation may be appreciably different in plant form, especially in height and degree of branching, depending on the row spacing and the density at which they are sown.

K. SUNFLOWER Wild sunflower (Heliunrhus unnuus) is distributed from southern Canada to northern Mexico, with its greatest abundance in the southwestern United States. It is a single-stemmed, tall annual, commonly much branched with many small heads (2-5 cm in diameter) and with ovate or cordate leaves (10-20 cm long) (Purseglove, 1968; Heiser, 1978). No perennial ancestor has been recognized though there are many perennial species of Heliunthus native to the region. Archeological evidence indicates that the sunflower may have been cultivated in Arizona and Mexico as early as 3000 B.c., possibly earlier than maize in that region. It was grown for its edible seed and used principally as flour, but possibly also for oil (Putt, 1978). The types cultivated by the Indians were freely branched with many small heads, but other types had a main stem and one principal head. There is some evidence that prior to European settlement, some Indian tribes may already have had monocephalic, unbranched, very tall sunflowers with massive heads (Heiser, 1978; Fick, 1978).

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The development of modem sunflowers began with its introduction to Russia; by 1900, nearly half a million hectares were planted there. The Russian cultivars of the nineteenth century became progressively taller (up to 4 m) (Purseglove, 1968), almost certainly as a consequence of natural selection, and many village cultivars developed. Subsequent breeding was aimed at reduced height, earliness, and high oil content. The crop returned to North America with direct commercial seed importation from Russia of very tall (2-4 m), late cultivars, notably Mammoth Russian, in the late nineteenth century. These tall cultivars were at first used mostly for silage, but in 1936 and 1950 Canada and the United States, respectively, began breeding for high seed yield and oil production, using lines carried in by Memmonite immigrants and some newer lines from the Soviet Union. Some of these were earlier, dwarf (1-1.5 m) cultivars with small seed (Putt,1978). Considerable increases in yield followed the development of hybrid sunflower in the late 1940s and especially the discovery and use of cytoplasmic male sterility and genetic fertility restoration in the 1970s. Modem cultivars are dwarf (0.8-1.2 m), unbranched plants with a large head, seemingly desirable attributes in an annual seed plant. What major further changes in plant form might lead to worthwhile increases in grain yield by sunflower? The most evident weakness is the extremely large, undivided leaves, up to 40 cm long and 35 cm wide, with an almost horizontal disposition (within 8-10"; Hiroi and Monsi, 1966), the leaf pattern of a shade plant rather than of a crop grown in open fields. A very marked reduction in leaf size (perhaps even to 20% of the leaf size in modem cultivars), together with the development of a semierect leaf disposition, would give greatly improved light penetration into the crop, increased crop growth, and, it may reasonably be expected, greater yield. Many consequences would follow. The heads would be smaller and the stems would be lighter and shorter. Yield per plant would be low and the plant population needed to exploit the environment and give full yields would be considerably greater than those presently in use; narrow rows would be indicated. Interrow cultivation might be neither practicable nor necessary. The crop would, sadly, be reduced in beauty but increased in harvest index.

VI. A BASIC IDEOTYPE FOR ALL ANNUAL SEED CROPS Within the generd field of agronomy several developments are occuning which, to maximize grain yield, require integration through plant breeding. The development and use of suitable herbicides in many crops, especially those traditionally sown in wide rows to allow cultivation for weed control, has permit-


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ted higher densities, better plant arrangements, and higher yields (e.g., cotton, maize, sorghum, soybeans; see earlier discussion). With increasing crop density, optimizing canopy structure to maximize light fixation will be a potential avenue to increased crop yields. The theory behind canopy optimization is based on physical principles (Monsi and Saeki, 1953; Davidson and Philip, 1956; Wilson, 1960; Donald, 1961, 1962; Monteith, 1965a,b; de Wit, 1965; and many others). In a simplified model, three aspects of crop canopy structure affect light penetration; the leaf angle to the incident light and the vertical, and horizontal, distributions of the leaves. The deeper that light penetrates into the canopy, the greater is the photosynthetic capacity of the crop (Wilson, 1960; Blackman, 1961); penetration is increased if leaves are evenly distributed both vertically and horizontally and have a high leaf angle (assuming a high angle for the sun) (Wilson, 1960). Differences in cereal canopy structure have been related to yield differences [see Tanaka et al., 1964, 1966; Jennings, 1964; Jennings and Beachell, 1965; Beachell and Jennings, 1965; and Matsushima et al., 1964 (for rice); Hamblin, 1971; Hamblin and Donald, 1974; and Tanner et al., 1966 (for wheat, barley, and oats); Pendleton el al., 1968; Pepper et al., 1977; and Williams et al., 1968 (for maize)]. However, in many cases the lines used were not isogenic and results may be related to other factors such as reduced disease incidence, improved water use (Trenbath and Angus, 1975), or even slightly improved carbohydrate supply at critical times of development (Fischer, 1981). Blackman (1961) suggested that narrow, dissected leaves would be better than round or cordate leaves in dicotyledonous crops. The use of the Okra leaf type in cotton would appear to confirm the potential of this approach (Andries et al., 1969; Constable, 1977). The most extreme case of altered canopy structure is that of peas, in which the leaves have been dispensed with entirely, markedly altering the pattern of light distribution down the profile; yields have been increased (Hedley and Ambrose, 1981). Despite the criticisms of Trenbath and Angus (1975) concerning the comparisons of nonisogenic lines, it is probable that improved canopy morphology will lead to improved yields in many grain crops. This should occur more rapidly in short C , crops that are grown at high density than in tall C, crops grown at lower densities (Evans and Wardlaw, 1976) and in environments where the solar incidence in the growing season is high (Trenbath and Angus, 1975). However, the rate of change will also depend on other factors of crop production affecting yield-density relationships. When crops are grown at high levels of fertility, yields (at least of dry matter if not of grain) are frequently increased but crops tend to lodge. Lodging resistance, primarily through reduced height, is a breeding objective of many programs directed at high yield potential. Lodging resistance may also be increased if a plant has thick strong stems. These may allow a greater accumulation

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of stored photosynthate during the vegetative phase, which if it can be retranslocated to the grain at a later date will increase the harvest index. If biological yield is constant d u d height also will have a direct benefit in terms of an improved harvest index (Hamblin and Rosielle, 1983). Within a given cross, short plants tend to have short leaves (Chowdhry and Allan, 1966; Hamblin, 1971). On the average, short leaves will be more upright thanlong leaves because they have less bending momenL Therefore, short plants may also have an advantage in terms of canopy structure at high densities. In many agricultural situations the length of the growing season is clearly defined. This may be because of drought, frost, the rotational needs of the following crop, or other factors. Within that defined season, there will be an optimum relationship between the vegetative and reproductive phases of crop growth, in terms of both phenology and dry matter production. This problem has recently been discussed by Fischer (1979, 1981) for dry-land Mediterranean situations. He concludes that, within a given environment, there is an optimum level of dry matter production at anthesis for maximum grain yield. If biological yield at anthesis is above that optimum, then there is insufficient water to maximize grain yield; if biological yield is below the optimum, then there is insufficient sink for maximum grain yield (Fischer, 1979, 1981). In many situations, early flowering allows a longer period of grain filling and higher yields (Thorne, 1966). Early flowering may also put grain development into a more favorable season (Fischer, 1981); however, it may reduce the biological yield at flowering to suboptimal levels. This can be countered by selecting types with vigorous early growth, by growing crops at high levels of nutrition, and by using higher seeding rates (Fischer, 1981). Uniculm cereals would have an advantage here; seeding at high rates would give rapid development of leaf area without the presence of tillers. In many situations the development of photoperiod insensitivity allows wide adaptation for a variety. A major problem for agronomists aiming at optimum levels of dry matter production at flowering is to manipulate the cropping strategy to maximize the probability of achieving that optimum. This is particularly difficult in branching and indeterminate crops in which there is little control over the amount of vegetative dry matter likely to be developed. The tendency to determin:ncy, which will allow some control of the relationship between vegetative and reproductive growth, is apparent in several species (beans, cotton, soya). There is a similar tendency toward reduced branching (tillering) in many species (cotton, cereals). The logical development is to nonbranching (uniculm cereals) so that it is possible to manipulate the relationship between vegetative and reproductive growth by adjusting density. This option is already available in the uniculm species, that is, in maize and sunflowers. Workers on these crops, in contrast to people working on other grain crops do not consider the uniculm habit unusual. In species such as sorghum and rice, selection for the strictly annual habit will probably increase yield potential slightly as no resources would be diverted to


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perennating organs. Many of the characteristics considered in this section will automatically lead to a high harvest index; these include reduced height, earlier flowering, and nontillering. To date, however, the critical experiments on the use of harvest index per se as a selection criterion either have not been carried out or have produced inconclusive results (Hamblin and Rosielle, 1983). As a long-term objective, however, selection for harvest index alone must eventually lead to diminishing returns if biological yield remains constant. There is widespread acceptance of the value of reduced stature, reduced tiller number, and early flowering in many circumstances, but there is little interest in the uniculm (or nonbranching) habit; shorter, narrower, and more erect leaves, higher harvest index, increased plant populations, and narrower rows than the present 18- to 20-cm spacing. Yet these features, it is proposed, provide notable opportunities to increase yields in all environments. It was proposed that there is much to gain in the breeding of crop plants by designing ideotypes, “biological model which is expected to perform or behave in a predictable manner within a defined environment and . . . to yield a greater quantity or quality of grain, oil or other useful product when developed as a cultivar” (Donald, 1968a, p. 389). It is here proposed that the ideotypes of all annual crops grown for their seed will have major features in common, even to the extent that a basic ideotype can be conceived for cereal crops, cotton, peas, beans, soybeans, linseed, sunflower, or any other annual seed crop. One may observe that breeding toward many of these features is already in progress in many annual seed species, and further that there are trends toward like, though often independently conceived, agronomic practices. There may be much to gain by recognizing and systematizing those trends in plant breeding and agronomy. Most of the features of this common ideotype arise directly from the proposed need for communal plants sown at high density. Various other useful features and practices for annual seed crops which can be postulated are set out in Table 11. It will be seen that despite some conflict in the plant features needed to meet particular criteria of crop performance, a clear picture emerges. The principal characteristics of the ideotype proposed for all annual seed crops and their culture are 1. Strictly annual habit 2. Erect growth form 3. Dwarf stature 4. Strong stems 5. Unbranches or nontillered habit 6. Reduced foliage (smaller, shorter, narrower, or fewer leaves) 7. Erect leaf disposition 8. Determinate habit 9. High harvest index


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10. Nonphotoperiodic for most but not all situations 11. Early flowering for most but not all situations 12. High population density 13. Narrow rows or square planted Table II The Features of a Common Ideotype for All Seed-Producing Annual Crops, Together with Associated Cultural Practices Feature of crop Pure culture sown at high density

Features of ideotype

Good plant performance among like neighbors sown at high density, hence communal plants needed; plant yield in isolation or in competition with other genotypes of no relevance Strictly annual habit Determinate growth; plant death at seed ripeness; loss of residual features of perenniality (i.e., of vegetative branching, tillering, or vegetative storage organs) Crop must not lodge or collapse Plants of sound physical structure; short stature, strong or flexible stems, nonbranching, nontillering, nonleafy Effective form and disposition of foli- Deep light penetration within the leafy canopy; small, age for light utilization narrow or divided, erect leaves High seed yield sought High biological yield, attainable through high sowing rate, rapid emergence, rapid attainment of optimum LAI, high net assimilation rate High harvest index, involving annual habit, no excessive use of resources on plant framework, short stature, light stems, nonbranching, nonleafy Large sink for photosynthates, many seeds per unit of biological yield, long interval flowering to maturity, no sterility at high plant density Absence of those features associated with strong comMinimal competition between plants petitive ability (i.e., absence of tallness, large or horizontally disposed Leaves, branching, or widely ramifying root system) Plant density and plant arrangement to High plant density to compensate for lack of branching and lack of leafiness; close approach to uniform spacbe appropriate to the communal plant form ing through use of narrow rows Effective response to high nutrient Limited increase in competition between plants as fertillevels ity is raised; absence of plant responses giving increased competitive ability, especially minimal increase in height, leafhess, or branching As appropriate to the climatic region but commonly Wide climatic adaptation including nonphotoperiodicity; earliness of flowering to avoid early or late frosts, cold soil or cold irrigation water early in the season, drought, or wet or wintry conditions at harvest; wide temperature tolerance


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ACKNOWLEDGMENTS We would like to thank Drs. W. J. Collins, R. A. Fischer, R. Knight, A. J. Rathjen, A. A. Rosielle, and W. R. Stem, and Mr. N. J. Halse, for comments and criticisms of the manuscript. Dr. Hamblin was supported by a Wheat Industry Research Council Grant and this is gratefully acknowledged.

REFERENCES Adair, C. R., and Jones, J. W. 1946. J. Am. SOC. Agron.38, 708-716. Adams, M. W. 1973. Semin. Ser. No. 2E, pp. 266-278. CIAT, Cali, Columbia. Akihama, T. 1968. Ikushugaku Zusshi (Jpn. J . Breed) 18, 213-216. Allard, R. W., and Adams, J. 1969. Am. Nut. 103, 621-645. Allard, R. W., and Jain, S . K. 1962. Evolurion 16, 90-191. Andries, J. A,, Jones, J. E., Sloane, L. W., and Marshall, J. G. 1969. Crop Sci. 9, 705-710. Andries, J. A., Jones, J. E., Sloane, L. W., and Marshall, J. G. 1970. Crop Sci. 10, 403-407. Athwal, D. S. 1971. Q. Rev. Biol. 46, 1-34. Atsmon, D., and Jacobs, E. 1977. Crop Sci. 17, 31-35. Austin, R. B., Bingham, J., Blackwell, R. D., Evans, L. T., Ford, M. A., Morgan, C. L., and Taylor, M. 1980. J . Agric. Sci. 94, 675-689. Beachell, H. M., and Jennings, P. E. 1965. In “The Mineral Nutrition of the Rice Plant,” pp. 29-75. John Hopkins Press, Baltimore, Maryland. Bell, G. D. H. 1963. Barley Genet. 1, 285-306. Bell, G. D. H. 1965. In “Essays on Crop Plant Evolution” (J. B. Hutchinson, ed.), pp. 70-102. Cambridge, Univ. Press, London. Bhardwaj, S. W., Nath, V., and Mehra, R. B. (1971). Indian J. Agric. Sci. 41, 519-523. Bingham, J. 1972. Eucarpia Congr. Eur. Assoc. Res. Plant Breed. 6, 15-29. Blackman, G. E. 1961. Amos Mem. Lecture E. Malling Res. Srn. 5th. Blijenburg, J. G., and Sneep, J. 1975. Euphyrica 24, 305-315. Brashears, A. D., Kirk, I. W., and Hudspeth, E. B. 1968. Texas Agric. Exp. Srn. Misc. Publ. No. 872, 1-7. Bremner, P. M. J. 1969. J. Agric. Sci. 72, 273-280. Cannell, R. Q. 1968. J. Univ. Newcasrle-upon-Tyne Agric. SOC. 22, 3. Chandler, R. F., Jr. 1969. I n “Physiological Aspects of Crop Yield” (I. D. Eastin, F. A. Hoskins, C. Y. Sullivan, and C. M. H. Van Bavel, eds.), pp. 265-285. Amer. Soc. Agron., Madison, Wisconsin. Chang, T. 1976. Euphyrica 25, 425-441. Chapman, G. P., and Peat, W. E. 1978. Outlook Agric. 9, 267-272. Chowdhry, A. R., and Allan, R. E. 1966. Crop Sci. 6, 28-31. Christian, C. S., and Grey, S. G. 1941. CSIR J. 14, 59-68. CIMMYT 1979. CIMMYTRev. Maize Sect. Spec. Res. Proj. Mexico, pp. 35-37. Clegg, M. D. 1972. In “Sorghum in the Seventies” (G. P. Rao and L. R. House, eds.), pp. 279-301. IBH Publ. Co., New DeIhi. Clements, F. E., Weaver, J. E., and Hanson, H. C. 1929. Carnegie Inst. Washington Publ. No. 398. Constable, G. A. 1977. Aust. J. Exp. Agric. Anim. H u b . 17, 135-142. Costa, J. A., Oplinger, E. S., and Pendleton, J. W. 1980. Agron. J. 72, 153-156. Darlington, C. D. 1969. In “The Domestication of Plants and Animals’’ (P. J. Ucko and G. W. Dimbleby, eds.), pp. 67-72. Duckworth, London.


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Darwin, C. 1868. “The Variation of Animals and Plants under Domestication.” Murray, London. Davidson, J. L., and Philip, J. R. 1956. I n “Climatology and Mimlimatology.” UNESCO, Paris. Davies, D. R. 1977a. Appl. Biol. 2, 87-127. Davies, D. R. 1977b. Sci. Prog. (Oxjord)64, 201-214. de Wet, J. M. J., and Harlan, J. R. 1971. Ewlwion 25, 128-135. de Wet, J. M. J., and Huckabay, J. P. 1%7. Evolution 21, 787-802. de Wet, J. M. J., and Schechter, Y. 1977. I n “Crop Resources” (D. J. Seigler), pp. 179-191. Academic Press, New Yo&. de Wit, C. T. I. 1%5. Centre for Agricultural Publications and Documentation. Agric. Res. Rep. 663.

Doggett, H. 1%5. In “Essays on Crop Plant Evaluation” (J. B. Hutchinson, ed.), pp. 50-69. Cambridge Univ. Press, London. . EX^. BWl. 15,283-313. Donald, C. M. 1961. S y m ~ SOC. Donald, C. M. 1962. J. Aust. Inst. Agric. Sci. 28, 171-178. Donald, C. M. 1%3. Adv. Agron. 15, 1-118. Donald, C. M. 1%8a. Euphyrica 17, 385-403. Donald, C. M. 1968b. Proc. Int. Wheat Genet. Symp. 3rd, pp. 359-369. Donald, C. M. 1979. J. Agric. Sci. 93, 261-269. Donald, C. M. 1981. I n “Wheat Science-Today and Tomomw” (L.T. Evans and W. J. Peacock, eds.), pp. 361-405. Cambridge Univ. Press, London. Donald, C. M., and Hamblin, J. 1976. Adv. Agron. 28, 361-405. Engledow, F. L. 1925. J. Agric. Sci. 15, 125-146. Evans, A. M. 1973. Semin. Ser. No. 2E, pp. 279-286. CIAT, Cali Columbia. Evans, A. M. 1980.In“‘Advances in Legume Science” (R. J. Summefield and A. H. Bunting), pp. 337-347. Stationery Office, London. Evans, L. E., and Bhatt,G. M. 1977: Can.J . Planr Sci. 57,45-47. Evans, L. T. 1980. J. Aust. Inst. Agric. Sci. 46, 3-13. Evans, L. T., and Dunstone, R. L. 1970. AW. J . Biol. Sci. 23,725-741. Evans, L.T., and Wardlaw, I. F. 1976. Adv. Agron. 28, 301-359. Fick, G. N. 1978. I n “Sunflower Science andTechnology” (J. F. Carter, ed.), pp. 279-315. Amer. Soc. Agron., Madison. Wisconsin. Fick, G. N., and Swders, C. M. 1975. Crop Sci. 67, 743-745. Fischer, R. A. 1978. Proc. Int. Wheat Genet. Symp. 5th, pp. 767-777. Fischer, R. A. 1979. J. Aust. Inst. Agric. Sci. 45, 83-94. Fiiher, R. A. 1981. P h Soil 58, 249-278. FRY. K. J. 1971. ASA Spec. Publ. No. 20. 15-58. Galinat, W. C. 1%5. Econ. Bot. 19, 350-357. Gardner, C. 0. 1961. CropSci. 1, 241-215. Gardner,C. 0. 1%8. Proc. Int. Congr. Genet. 12th, pp. 57-77. Gardner, C. 0. 1%9. I n “Induced Mutations in Plants,” pp. 469-477. Atomic Energy Agency, Vienna. Goldsworthy, P. R. 1970. J. Agric. Sci. 75, 109-122. W u s , J. E., Thomas, R. L.,and B a m d , J. 1976. Crop Sci. 16,673-677. Hamblii, J. 1971. Ph.D. Thesis, Univ. ofA&kzide, Australia. Hamblin, J. 1975. Euphytica 24,435-445. Hamblin, J., and Donald, C. M. 1974. Euphytica 23, 535-542. Hamblii, J., and Morton, J. R. 1977. Euphyrica 26, 75-83. Hamblii, J., and Rosielle, A. A. 1983. The contribution of genetics to plant breeding. Raeger (in press). Hamblin, J., and Rowell, J. G. 1975. Euphyrica 24, 221-228.


141

Hamblin, J., Knight, R., and Atkinson, M. J. 1978. Euphyrica 27, 497-505. Harlan, J. R. 1971. Science (Washington, D.C.) 174,46-474. Harlan, J. R., deWet, J. M. J., and Price, E. G. 1973. Evolution 27, 311-325. Harlan, M.V., and Martini, M.L. 1938. J . Agric. Res. (Washington, D.C.) 57, 189-199. Hayes, J. D. 1970. Rep. Welsh P h r Breed. Srn. (Aberysrwyrh, Wales). Hedley, C. L., and Ambrose, M. J. 1981. Adv. Agron. 34, 225-277. Heiser, C. B. 1978. I n “Sunflower Science and Technology” (J. F. Carter, ed.),pp. 31-53. h e r . Soc. Agron., Madison, Wisconsin. Hicks, D. R., Pendleton, J. W., Bernard, R. L., and Johnson, T. J. 1969. Agron. J . 61, 290-293. Hiebsch, C. K., Kanemasu, E. T.,and Nickell, C. D. 1976. Can. J. Plant Sci. 56, 455-458. Hinson, K.,and Hanson, D. 1962. Crop Sci. 2, 117-123. Hiroi, T., and M o d , M. 1966. J. Fac. Sci. Univ. Tokyo 9 Sect. 3, 241-285. Hwd, E. A. 1969. Euphytica 18, 217-226. Hutchinson, J. B. 1965. I n “Essays on Crop Plant Evolution” (J. B. Hutchinson, ed.), pp. 166-181. Cambridge Univ. Press, London. Hymowitz, T., and Newell, C. A. 1977. I n “Crop Resources”(D. S.Siegler, ed.), p. 209. Academic Press, New York. Hymowitz, T., and Newell, C. A. 1980. I n “Advances in Legume Science” (R. J. Summerfieldand A. H. Bunting, eds.), pp. 251-264. HM Stationery Office, London. Islam, T.M. T., and Sedgley, R. H. 1981. Euphyrica 30, 277-282. Jennings, P. R. 1964. Crop Sci. 4, 13-15. Jennings, P. R., and Aquino, R. C. 1968. Evolution 22, 529-542. Jennings, P. R., and Beachell, H. M. 1965. I n “Mineral Nutrition of the Rice Plant,” pp. 449-457. John Hopkins Press, Baltimore, Maryland. Jennings, P. R., and de Jesus,+J., Jr. 1968. Evolution 22, 119-124. Jennings, P. R., and Herrera, R . M. 1968. Evolurion 22, 332-336. Jensen, N. F., and Federer, W. T. 1964. Crop Sci. 4, 641-645. Johnston, P. R. 1972. W.D. Thesis, Univ. of Adelaide, Australia. Jones, H. G., and Kirby, E. J. M. 1977. J . Agric. Sci. 88, 391-397. Kaplan, L., and McNeish, R. S. 1960. Bor. Mus. Led. Harv. Univ. 19, 33. Kaplan, L.,Lynch, T. F., and Smith, C. E. 1973. Science (Washington,D.C.) 179, 76. Kern, J. J., and Atkins, R. E. 1970. Agron. J . 62, 183-186. Khalifa, M. A., and Qualset, C. 0. 1974. Crop Sci. 14, 795-799. Khalifa, M. A., and Qualset, C. 0. 1975. Crop Sci. 15, 640-644. Khan, M. A., and Tsunoda, S. 1970. Ikushuguku Zusshi (Jpn. J . Breed.) 20, 133-140. Kirk, I. W., Brasheam, A. D., and Hudspeth, E. B. 1969. Texas Agric. Exp. Srn. Misc. Publ. No. 937.

Klages, K. H. W. 1936. J . Am. Soc. Agron. 28, 935-940. Kranz, A. R. 1966. Biol. Zentralbl. 85, 681-734. Kulshrestha, V. P., and Jain, H. K. 1982. Z. Pflanzenzuecht. 89, 19-30. Laude, H. H., and Swanson, A. F. 1942. J. Am. SOC.Agron. 3, 270-274. Lonnquist, J. H., Cota, A., and Gardner, C. 0. 1966. Crop Sci. 6, 330-332. Low, A., and McMahon, J. P. 1973. Corron Grow. Rev. 50, 130-149. MacKey; J. 1966. Yugoslav Symp. Res. Whear Srh, pp. 37-48. Mangelsdorf, P. C. 1965. I n “Essays on Crop Plant Evolution” (J. B. Hutchinson, ed.), pp. 23-49, Cambridge Univ. Press, London. Matsushima, S., Tanaka, T.,and Hoshino, H. 1964. Nippon Sakumorsu Gakkui Kiji (Proc. Crop Sci. SOC. Jpn.) 33, 44-48. McGinnis, R. G . , and Shebeski, L. H. 1968. Proc. Inr. Whear Genet. Symp. 3rd, pp. 109-114. Mock, J. J., and Pearce, R. B. 1975. Euphyrica 24, 613-623.

142


Monsi, M., and Saeki, T. 1953. Jpn. J. Bor. 14, 22-52. Monteith, J. L. 1965a. Field Crop Abstr. 18, 213-219. Monteith, J. L. 1%5b. Ann. Bot. (London) 29, 17-37. Montgomery, E. G. 1912. Bull. Nebraska Agric. Exp. Stn. 127, 3-22. Mom-Val, C. A., and Miller, P. A. 1975. Crop Sci. 15, 479-482. Mumaw, C. R., and Weber, C. R. 1957. Agron. J. 49, 154-160. Nicholson, A. J. 1962. I n “The Evolution of Living Organisms” (G. W. Leeper, ed.), pp. 62-73. Melbourne Univ. Press, Australia. OBrien, L. 1979. Ausr. J . Agric. Res. 30, 587-595. Pal, B. P., Upadhyaya, Y. M., Khan, H. R., and Ramanujam, S. 1960. Indian J . Gener. Plant Breed. 20, 102-112. Passioura, J. B. 1972. Aust. J. Agric. Res. 23, 745-752. Pegelow, E. J., Buxton, D. R., Briggs, R. E., Murramoto, H., and Gensler, W. G. 1977. Crop Sci. 17, 1-4. Pendleton, J. W., and Seif, R. D. 1962. Crop Sci. 2, 154-156. Pendleton, J. W., Smith, G. E., Winter, S. R., and Johnston, R. J. 1968. Agron. J. 60,422-424. Pepper, G. E., Pearce, R. B., and Mock, J. J. 1977. Crop Sci. 17, 883-886. Percival, J. 1921. “The Wheat Plant.” Duckworth, London. Perry, M. W., and Reeves, J. T. 1980. “Farmnote.” Western Australian Dept. Agric., Avondale Project 1979. Peters, D. B., and Johnson, L. C. 1960. Agron. J. 52, 687-699. Fhillips, L. L. 1976. In “The Evolution of Crop Plants” (N. W. Simmonds, ed.), pp. 198-200. Longmans, London. Philipschenko, J. 1972. “Variabilitat und Variation.” Berlin. Poulson, M. H. 1977. J . Agric. Sci. 89, 643-654. Puckridge, D. W., and Donald, C. M. 1%7. Aust. J. Agric. Res. 18, 193-211. Purseglove, J. W. 1%8. “Tropical Crops,” Vol. 1. Longmans, London. Putt, E. D. 1978. In “Sunflower Science and Technology” (J. F. Carter, ed.),pp. 3-29. Amer. Soc. Agron., Madison, Wisconsin. Queensland Department of Agriculture 1970. Advisory Leaflet No. 1082. Quinby, J. R. 1963. CropSci. 3, 283-291. Quinby, J. R., and Karper, R. E. 1945. J. Am. SOC. Agron. 37, 916-936. Quinby, J. R., and Martin, J. H. 1954. A h . Agron. 6, 305-359. Quisenberry, J. E., Roark, B., Bilbro, I. D., and Ray, L. L. 1978. Crop Sci. 18,799-801. Ray, L. L., and Hudspeth, E. B. 1966. S w t h Plains Res. Exr. Cenr. Curr. Res. Rep. No. 66-5, 1-13. Reid, D. A,, and Wiebe, G. A. 1968. In “Barley,” pp. 61-84. USDA, Agric. Handbook No. 338. Rosielle, A. A., and Frey, K. J. 1975. Crop Sci. 15, 544-547. Ross, W. W., and Eastin, J. D. 1972. Field Crop Absrr. 25, 169-174. Sakai, K. 1955. Cold Spring Harbor Symp. Quanr. Biol. 20, 137-157. Schutz, W. M., and Brim, C. A. 1967. Crop Sci. 7, 371-376. Schwanitz, F. 1966. “The Origin of Cultivated Plants.” Harvard Univ. Press, Cambridge, Massachusetts. Shaw, R. H., and Weber, C. R . , 1967. Agron. J . 59, 155-159. Shibles, R. 1980. In “Advances in Legume Science” (R. J. Summerfield and A. H. Bunting), pp. 279-286. HM Stationery Office. London. Smartt, J. 1%9. In “The Domestication and Exploitation of Plants and Animals” (P. J. Ucko and G. W. Dimbleby, eds.), pp. 451-462. Duckworth, London. Smith, 0. D., Kleese, R. A., and Stuthman, D. D. 1970. Crop Sci. 10, 381-384. Soetono, and Donald, C. M. 1980. Aust. J. Agric. Res. 31, 455-470.


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Sowell, W. F. 1960. Diss. Abstr. 20, 2476. Stebbins, G. L. 1974. “Flowering Plants: Evolution above the Species Level.” Harvard Univ. Press, Cambridge, Massachusetts. Stringfield, G. H. 1964. Adv. Agron. 16, 101. Suneson, C. A. 1949. Agron. J. 41,459-461. Suneson, C. A., and Wiebe, G. A. 1942. J . Am. Soc. Agron. 34, 1052-1056. Tanaka, A., Kawana, S . A., Garcia, C. V., Parao, F. T., and Ramirz, E. 1964. IRRI Tech. Bull. No. 7.

Tanaka, A., Kawana, S. A., and Yamaguchi, J. 1966. IRRI Res. Bull. No. 3. Tanner, J. W.,Gardener, C. J., Stoskopf, N. C., and Reinbergs, E. 1966. Cun. J . Plant Sci. 46, 690.

Thome, G . N. 1966. In “The Growth of Cereals and Grasses” (F. L. Mlthorpe and J. D. Ivins, eds.), pp. 88-120. Butterworth, London. Timmons, D. R., Holt, R. F., and Thompson, R. L. 1%7. Agron. J . 59, 262-265. Trenbath, B. R., and Angus, J. F. 1975. FieM Crop Absrr. 28, 231-244. Turesson, G. 1922. Hereditus, 3, 100-113. Vidovic, J. 1974. B i d . Plant. (Prague) 16, 174-183. Walker, J. T. 1969. Biol. Rev. 44, 207-242. Wiebe, G. A., Petr, F. C., and Stevens, W. 1963. In “Statistical Genetics and Plant Breeding,” pp. 546-557. NAS, NRC Publ. 982. Wilcox, J. R., and Schapaugh, W. T. 1980. Crop Sci. 20, 809-811. Wilkes, G. 1977. In “Crop Resources“ ( 0 . S . Seigler, ed.), p p . 211-223. Academic Press, New York. Williams, W. A., Loomis, R. S.,Duncan, W. G . , Dovrat, A,, and F. Nunez, A. 1968. Crop Sci. 8, 303-308.

Wilson, J. W. 1960. Proc. IN. Grussl. Congr. 8th, pp. 275-279. Winter, S . R., and Ohlrogge, A. J. 1973. Agron. J . 65, 395-397.



CURRENT STATUS AND FUTURE PROSPECTS FOR BREEDING HYBRID RICE AND WHEAT S. S. Virmanil and Ian B. Edwards2 1 International Rice

Research Institute, Manila, Philippines ‘Pioneer Hi-Bred International, Inc., Glyndon, Minnesota

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B. Heterosis for Other Plant Characters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............... C. Combining Ability . . . . . . . . . . . . . . . . . . . . . Advantages of Hybrids over Conventionally Bred Cytoplasmic-Genetic Male Sterility Systems in Rice and Wheat . . . . A. Early Research ........................................ B. Major Sources of Cytoplasmic Male Sterility. . . . . . . . . . . . . . . . . . . . . . . . . . . C. Additional Sources of Cytoplasmic Male Sterilit D. Techniques for Cytoplasmic Differentiation. . . . E. Cytoplasmic Effects on Other Plant Characters . . . . . . . . . . . . . . . . . . . . . . . . . F. Cytoplasmic Effects on Disease Resistance .................. Fertility Restoration. . . . . . . . . . A. Sources of Restorer Genes ................................. B. Inheritance of Restoration . . . . . . . . . . . . . . . . . . . . . . . . . C. Environmental Effects on Male Fertility Restoration. . . D. Influence of Female Genetic Background on Fertility R Use of Chemical Pollen Suppressants in Hybrid Production.. Factors Affecting Cross-Fertilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................. A. Flowering Behavior . . . . . . . . . . . . . B. Floral Structure.. . . . . . . . . . . . . . . . . . . . . . . . . C. Effect of Pollinator Distance . . . ......................... D. Effect of Plant Height and Other Morphological Traits. . . . .............. .............. Seed Production ........................................ A. Multiplication of Cytoplasmic Male-Sterile and Maintainer ines . . . . . . . . . . ...... B. Hybrid Seed Production.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Disease Problems Associated with Seed Production . . . . . . . ..... D. Seed Quality in Hybrids and Their Inbred Lines . . . . . . . . . . .............. QualityofHybrids ....................................... Economic Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............. Problems ............................................................. A. Rice ............................................................ B. Wheat ..........................................................

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Conclusion.. ......................................................... A. Current Outlook.. ................................................ B. Future Strategies.................................................. References ...........................................................

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1. INTRODUCTION Rice and whe& together constitute the world’s two most important food crops. Estimates (U.S.D.A., 1982) of the total land area devoted to production of the two cereals indicate that approximately 380 million ha are evenly divided between wheat and rice cultivation. Wheat is the staple food of many developed countries and constitutes the leading cereal in terms of total world production. Because of the importance of rice in many developing countries, especially in Asia, considerable effort has been expended during the past 2 decades to develop varieties capable of responding to improved management and to reduce production hazards through the incorporation of genes resistant to major biological, chemical, and physical stresses. The successful development of hybrid maize in the 1930s provided an important impetus for breeders of other crops, including self-pollinatingcereals such as wheat, rice, barley, and often cross-pollinating sorghum, to utilize the principles of hybrid production. The basis for such genetic manipulation is the phenomenon of hybrid vigor, the tendency for the offspring of crossed varieties to have greater productivity than the parental varieties. Unlike maize, the floral biology of rice and wheat ensures that both crops are almost 100% self-pollinating. Consequently, selection for a more open flowering habit, with improved anther extrusion in the male parent and stigma receptivity in the female parent, was crucial to successful development of inbreds. Cytoplasmic male sterility (CMS) was visualized as an essential genetic tool to develop F, hybrids in self-pollinating crops. Kihara (1951) was the first to report the Occurrence of cytoplasmic male sterility in wheat. Later, Wilson and Ross (1962) established the existence of usable male sterility in wheat from the interaction of the common wheat nucleus with Triticum timopheevi cytoplasm. Schmidt et al. (1962) and Wilson and Ross (1962) found that they could restore fertility in the “Bison” cytoplasmic male-sterile line by crossing it with a T . timopheevi bread-wheat derivative. This prompted extensive research into hybrid wheat production in the United States and in other countries. Consequently, some commercial hybrids were developed and marketed in 1975, but production has been limited. In rice, the role of the cytoplasm in causing male sterility was fist reported by Sampath and Mohanty (1954) and Weeraratne (1954). The first cytoplasmic

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male-sterile line in cultivated rice was developed by Shinjyo and Omura (1966a). Additional cytoplasmic male-sterile lines were developed in the early 1970s at the International Rice Research Institute (WU) (Athwal and Virmani, 1972), in the United States (Carnahan et al., 1972), and in China (Yuan, 1972). The Chinese were the first to use cytoplasmic male sterility to develop commercial F, rice hybrids in 1973 (Hunan Agricultural Bureau and Revolutionary Committee, 1977; Lin and Yuan, 1980). About 6 million ha are currently planted to hybrid rices in China, and hybrids have 20-30% higher yields than the best semidwarf commercial varieties. Developments in China have encouraged rice and wheat breeders elsewhere to expore the value of hybrid breeding for further increases in the yield potentials of these crops. This article reviews the current status and future prospects for breeding hybrid rice and wheat. An attempt has also been made to analyze existing breeding methodologies and to suggest some future strategies for hybrid improvement.

II. HETEROSIS IN RICE AND WHEAT Heterosis was f i s t reported in wheat when Freeman (1919) found that F, plants were generally taller than the tall parent. In rice, Jones (1926) observed that some F, hybrids had more culms and higher yields than their parents. Subsequently, other workers have reported the Occurrence of this phenomenon in various agronomic traits of rice and wheat such as yield, grain weight, number of grains per panicle or spike, number of panicles or spikes per plant, plant height, number of days to flowering, and general plant vigor. A critical prerequisite for the successful production of hybrid varieties is that sufficient hybrid vigor (heterosis) be available through specific parental combinations, so that yields of hybrids would significantly exceed those obtained from the best conventionally bred varities available; this difference is known as standard heterosis. The literature on heterosis in wheat has been reviewed by Briggle (1963), Johnson and Schmidt (1968), and Zeven (1972), and that on rice has been reviewed by Chang et al. (1973), Davis and Rutger (1976), and Virmani et al. (1981). A. HETEROSIS FOR YIELDAND YIELDCOMPONENTS

Reports in the literature have provided ample evidence of significant positive mid- and high-parent heterosis for yield, ranging from 1.9 to 368.9% in rice (Virmani et al., 1981) and from 0 to 100% in wheat (Briggle, 1963). However, a common problem in many of the reports in rice and in earlier reports in wheat was the limited scope and application of many of the studies. Generally, only a

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small number of crosses were evaluaw, parental selection was not necessarily designed to maximhe heterosis;small populations were space planted either in the field or in greenhouses; noncommercial and unproductive varieties were frequently used (thereby eliminating the opportunity to evaluate standard heterosis); and the effects on height, maturity, and yield components were measured more often than heterosis for grain yield. Despite these limitations, the levels of heterosis have been high in certain cross combinations. However, Murayama et al. (1974)provided evidence that heterosis in rice is not influenced by plant spacing and soil fertility. In wheat, Jost and Glatki-Jost (1976)noted that excep tional hybrids can produce more tillers per unit area than inbreds, regardless of the seeding rate. Suggestions of exploiting heterosis commercially by developing F, rice hybrids have been made from time to time (Stansel and Craigmiles, 1966;Shinjyo and Omura, 1966a,b; Yuan, 1966, 1972;Craigmiles er al., 1968;Huang, 1970; Watanabe, 1971; Athwal and Virmani, 1972; Carnahan et al., 1972; Swaminathan et al., 1972;Baldi, 1976). However, difficulties in hybrid seed production discouraged most of the researchers from continuing their efforts, the notable exceptions being Chinese scientists (Yuan, 1966, 1972). In wheat, siflicant hybrid advantages have been measured in some instances, while other studies have reported no hybrid advantage (Kronstad and Foote, 1964;Larrea,1966;Brown etal.. 1966;Briggle ef al., 1967;Fonseca and Patterson, 1968; Livers and Heyne, 1968; Wells and Lay, 1970; Singh and Singh, 1971;Bitzer and Fu, 1972;Allan, 1973;Widner and Lebsock, 1973;Jost and Glatki-Jost, 1976; Yadav and Murty, 1976;Jost ef al., 1976b;Hughes and Bodden, 1978;Cregan and Busch, 1978;Mihaljev, 1980;Jost and Jost, 1980; Bailey et al., 1980;Wilson et al., 1980). Livers and Heyne (1968)reported on a comprehensive 4-year study to determine hybrid vigor by intercrossing 9 varieties of well-adapted winter wheats at Hayes, Kansas. The 36 hybrids collectively exceeded all varieties by 20,37,37,and 35% in the 4 years (1964-1967), respectively, with an average hybrid superiority of 32%. The best hybrid was consistently better than the best variety for the area. Similar results were obtained when 10 hybrids were compared with leading varieties at three locations (Livers and Heyne, 1966). When wheat hybrids were made using cytoplasmic male sterility-fertility restoration systems and field tested at optimum population rates, the results were less favorable than those obtained from hand-produced hybrids (Allan, 1973; Hayward, 1975; Johnson, 1977, 1978; Edwards er al., 1980). Allan (1973) found high-parent hetemis ranging from 23 to 113% among soft white winter wheat hybrids grown in the state of Washington, but the results were highly sitespecific; when averaged over 4 locations, they indicated that no hybrid had outyielded the high parent. Jost and Milohnic (1975)tested 5 hybrids in Yugoslavia and, in this small sample, only 1 hybrid showed high-parent heterosis.


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Similarly, Hayward (1975) reported on a 1970-1971 study of 39 hard red winter wheat hybrids and 6 high-yielding check cultivars in Kansas; he found only 1 hybrid that equalled the yield of the highest check. However, Hayward did show significant improvements among later hybrids tested in 1973-1974; the mean yield of 4 hybrids over three locations was 19%greater than the mean of 8 check cultivars, and the best hybrid yielded 13.7%more than the top cultivar. Some possible reasons for the relatively poor performance of CMS-produced hybrids in the early studies are inadequate fertility restoration, adverse effects of T. rimopheevi cytoplasm, heavy selection pressure for restoration ability with less emphasis on agronomic performance during restorer development, and limited testing of different hybrid combinations. Most of the hard red winter wheat hybrids evaluated have shown more specific adaptability to certain regions and winter-hardiness zones than common check cultivars. Johnson (1977, 1978) reported on tests conducted at 11 sites in five states (Texas, Oklahoma, Kansas, Colorado, and Nebraska) during 1975-1979 and 1976-1977. Of the 15 hybrids tested in 1976, the leading hybrid produced 70 kg/ha less than the check cultivar ‘Centurk.’ In 1977,16 hybrids averaged 70 kg/ha less than Centurk, although the best hybrid exceeded Centurk by 130 kg/ha. Johnson concluded that the hybrids were not sufficiently superior to justify their use over the best available varieties. Published reports of spring wheat hybrid evaluation have been more limited compared with winter wheats. Edwards et al. (1980) reported on 1978 and 1979 tests with a series of spring wheat hybrids. Although the levels of high-parent heterosis ranged up to 35%, the top-yielding hybrids exhibited standard heterosis of only 10-14% above the leading check cultivar ‘Era.’In summary, hybrid wheats based on the cytoplasmic-genetic system have not expressed as much heterosis as the early handproduced hybrids, and the yield advantages to date have not been sufficiently great to justify widespread commercial production. Experiments on heterosis in rice conducted at Davis, California (Rutger and Shinjyo, 1980) indicated significant yield superiority of 11 of 153 rice hybrids over the best check variety. Standard heterosis ranged from 16 to 63% and averaged 41%. Hybrid corn seed producers in the U.S. maintained that this frequency (11 of 153 combinations) and degree of standard heterosis (41%) would make the prospects of hybrid rice exciting if sufficient hybrid seed could be produced (Rutger and Shinjyo, 1980). Studies conducted at the International Rice Research Institute in the Philippines during 1980-1981 have shown levels of as much as 73, 59, and 34% for mid-parent, high-parent, and standard heterosis, respectively (Virmani et al., 1982). Hand-crossed F, hybrids produced from elite breeding lines yielded up to 6.2 ton/ha compared with 5.0 ton/ha from the best check variety (‘IR42’)in the wet season, and 10.4 ton/ha compared with 7.9 ton/ha (IR54) under irrigation during the dry season. Of a total of 202 F, hybrids evaluated for yield during

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1980-1982, 63% showed positive high-parent heterosis (4-64.3%) and 50% showed positive standard heterosis (0.1-46.4%). Yields of the F, were found to be positively correlated with the parental mean. (r = 0.45**) and high-parent values (r = 0.33**). The parents had been selected for high per se yield performance, diverse genetic background, and resistance to diseases and insects incorporated though conventional breeding, Saini et al. (1974) also observed positive standard heterosis when the F, hybrids were derived from selected parents with improved plant type. However, the association between parental yield per se and F, hybrid performance may vary with the genetic background of the inbreds, and Khaleque et al. (1977) found no association between parental and hybrid yield performance. The most comprehensive commercial utilization of heterosis in rice has been that reported from the People’s Republic of China (Li, 1977; Lin, 1977; Lin and Yuan, 1980); more than 12 hybrids were officially released prior to 1980 (Shen, 1980). Yields under large-scale production have exceeded the best conventionally bred varieties by 20-30%. Results from replicated yield trials are given in Table I, and the data indicate that although the hybrids had fewer effective panicles per square meter, they had significantly more filled grains per panicle and larger seeds. The highest individual yield obtained from the F, hybrids was 12.8 tonslha, compared with 10.4 tons/ha from a conventionallybred variety (L. P. Yuan, personal communication). The major yield components in rice and wheat are number of panicles (or spikes) per square meter, spikelet number per panicle (or spike), spikelet fertility percentage, and 1000-grain weight. Significant positive mid-parent, high-parent, and/or standard heterosis have been observed for one or more of these components in a number of rice crosses (Pillai, 1961; Namboodiri, 1963; Rao, 1965; Dhulappanavar and Mensikai, 1967; Karunakaran, 1968; Carnahan et al., 1972; Chang et al., 1973; Mohanty and Mohapatra, 1973; Saini and Kumar, 1973; Sivasubramanian and Madhava Menon, 1973; Murayama et al., 1974; Saki er al., 1974; Parmar, 1974; Paramsivan, 1975; Davis and Rutger, 1976; Mallick ef al., 1978; Rutger and Shinjyo, 1980; Virmani et al., 1981, 1982). Virmani el al. (1981) observed negative heterosis for panicle number per square meter, but in combinations showing positive mid- and high-parent heterosis for yield this was overcompensated by positive heterosis in spikelets per panicle. Most crosses showing significant standard heterosis for yield have been found to show heterosis for more than one component (Saini et al., 1974; Mauya and Singh, 1978; Virmani et al., 1981, 1982). Results obtained in China and at the IRRI indicate that heterotic F, combinations usually show an increased sink size through increases in spikelets per panicle, spikelet fertility percentage, and 1000-grain weight. In wheat, Livers and Heyne (1968) pointed out that each yield component was important but that no single one was predominant in determining yield. Their


151

Table I Yield and Yield Components of Hybrid Rice Varieties in Regional Teats of 27 Sites in Hunan Province, china0 Hybrid combinations or check 1977 Wei You 6 Shan You 6 Nan You 6 Zhao You 6 Dong Ting Wan Xian (ck) 1979 Wei You 6 v20 x s Tan You 4 Dong Ting Wan Xian (ck)

Yield (tonsha)

Effective panicledm2

Filled grains per panicle

weight (g)

6.1 6.1 6.0 5.9 5.2

324 308 339 318 374

86.1 95.1 86.4 85.4 75.1

26.7 25.4 24.5 26.0 22.0

6.5 6.3 6.1 5.4

328 266 328 345

78.0 72.7 66.9 77.7

27.0 33.9 31.9 21.7

1OOO-grain

“Data from VimuCni ef d.(1981).

data showed that although top-yielding hybrids tend to have relatively high values in all three components, good performance is possible with a low value for any one component if the other two components have high values. Yieldcomponent compensation has been well documented in the literature (Donald, 1962; Bingham, 1967), and several workers have concluded that yield-component selection has limited value in breeding programs (Rasmusson and Camel, 1970; Fisher, 1975). However, kernel weight has been considered the most independent yield component because it is the last component developed, and its level of expression should not produce a compensating change in other components. In contrast, Sinha and Khanna (1975) hypothesized that heterosis in wheat will have commercial utility only when yield per spike increases, because tiller number per plant is strongly influenced by environment and can be manipulated by seeding rate. Briggle et al. (1967) also noted that heterosis for tiller number decreased as the population increased in b o a parents and hybrids. However, Jost and Glatki-Jost (1976) found that exceptional hybrids had the capacity to produce more tillers per unit area than inbreds irrespective of seeding rate, and Wilson et al. (1980) found the 17% high-parent heterosis in full dwarf X semidwarf hybrids to be primarily the result of an increase in spikes per unit area. B. HETEROSISFOR OFHER PLANT CHARACTERS

In both rice and wheat, a number of studies have shown that heterosis for plant height is highly cross specific (Pillai, 1961; Namboodiri, 1963; Briggle et al.,

152


B. EDWARDS

1964;Rao, 1965; Dhulappanavar and Mensikai, 1967; Livers and Heyne, 1968; Karunakaran, 1968; Amaya et al., 1972; Bitzer and Fu, 1972; Sivasubramanian and Menon, 1973; Ingold, 1974; Paramsivan, 1975; Khaleque et al.. 1977; Sreekumari et al., 1977; Mallick et al.. 1978; Wilson et al., 1980), and significant positive as well as negative heterosis has been reported. In rice, shorter height of F, hybrids may be attributed to the higher photoperiod sensitivity of some hybrids in comparison to their parents. In wheat, various studies have shown F, hybrids to exceed the tall-parent value (Ingold, 1974), exceed the midparent value (Amaya et al., 1972), approximate the mid-parent value (Wilson et al., 1980), and, in the case of “Olsen-dwarf” derivatives, approach the dwarfparent value (I. B. Edwards, personal observation). Because height is one expression of vigor that may lead to unfavorable grain/straw ratios and belowoptimum yields as a result of lodging, a number of hybrid programs are manipulating dwarfing genes in parents to obtain desirable height expression in the hybrids. A number of workers have observed the growth duration of rice hybrids to be shorter than the mid-parent value and, in some cases, shorter than that of the early parent (Dhulappanavar and Mensikai, 1967; Karunakaran, 1968; Chang et al., 1973; Bardhan Roy et al., 1975; Khaleque et al., 1977; Mallick et al., 1978). The dominance of Efgenes for the short basic vegetative phase (BVP) has been pointed out by Chang et al. (1969). However, in subtemperate-to-temperate China, most of the heterotic rice hybrids are later maturing than their parents (Lin and Yuan, 1980). This apparent discrepancy requires further investigation. Heading dates of wheat hybrids made on both normal and alien cytoplasms have tended to be earlier than the mid-parent value (Livers and Heyne, 1968; Amaya et al.. 1972; Bitzer and Fu, 1972; Wilson et al., 1980) and, in some combinations, to exceed the early parent value (Bitzer and Fu, 1972; Jost et al., 1976a; Jost and Hayward, 1980). I. B. Edwards (Table 11) found different sets of spring wheat hybrids, evaluated over a 5-year period, to consistently show dominance or overdominance for earliness. The latter is a desirable trait in the northern spring wheat region of the United States, because heat stress is frequently encountered during the early grain-filling period. A number of researchers have emphasized the need to maintain a complementary balance between “source” (photosynthate supply) and “sink” (potential grains) in cereals. Sinha and Khanna (1975) proposed that both source and sink capacity should increase in order to improve yield, and Virmani et al. (1981, 1982) observed this phenomenon in F, rice hybrids derived from semidwarf parents. Jennings (1967) found significant heterosis for vegetative growth to be negatively associated with yield in hybrids derived from tall parents. In contrast, Virmani et al. (1981, 1982) observed significant standard heterosis for both vegetative growth and grain yield in certain hybrid combinations. Increases in


153

Table II Average Hybrid Performance of Spring Wheat during a 5-Year Period“

Measurement Days to 50% heading Low parent Hybrid High parent Height (in.) Low parent Hybrid High parent Yield (kgha) Low parent Hybrid High parent

1978 (9)b

1979 (12)

1980 (13)

1981 (9)

1982 (32)

55.5 54.7 59.2

56.4 56.I 59.3

57.5 56.6 61.1

64.9 65.2 68.8

49.7 48.0 51.5

30.6 33.3 34.3

30.9 34.0 35.I

28.9 31.6 32.6

28.7 32.2 32.9

29.2 32.5 31.8

38.4 52.1 47.8

29.6 43.2 36.9

53.4 55.6 58.7

34.9 30.2 42.6

51.7 62.7 66.3

“Source: I. B. Edwards, Pioneer Hi-Bred International, Inc. bNumber of hybrids evaluated with their parents.

grain yield in certain hybrid combinations have been attributed to a more efficient distribution of dry matter in the plant, and the harvest index (ratio of grain weightkotal plant weight) has been examined by several workers (Sinha and Khanna, 1975). Benson (1978) found the high yields and heterosis in four spring wheat hybrids to result from both increased plant weight and harvest index. He attempted to use harvest index as a screening technique for yield in spring wheat hybrids. However, although a high correlation was shown between harvest index and yield in conventional-sized plots (2.44 X 1.22 m), the harvest index of small, single-row plots showed only a weak correlation with yield in conventional plots. In rice research, heterosis has been observed for such traits as cold tolerance (Sawada and Takahashi, 1977), salt tolerance (Akbar and Yabuno, 1975), photoperiod sensitivity, and rooting habit (Lin and Yuan, 1980). Chinese F, hybrids showed heterosis for root penetration rate, depth and width of the rhizosphere, number of adventitious roots per plant, and number of root fibrils (Anonymous, 1977; Lin and Yuan, 1980). Preliminary observations made at IRRI have also indicated that some hybrids are superior to their parents at comparable growth stages with regard to total root dry weight and root number, length, diameter, and pulling force (O’Toole and Soemartono, 1981). Superiority of F, rice hybrids in such physiological traits as photosynthetic area, chlorophyll content per unit area, photosynthetic efficiency, and mitochondrial activity has been reported in China (Hunan Agricultural College Depart-

154


ment of Chemistry, 1977; Lin and Yuan, 1980) and elsewhere (McDonald et al., 1971, 1974). In wheat, Sage and Hobson (1973) observed increased mitochondrial activity above the high-parent value in several mixtures, and found these to be significantly correlated with the percentage yield heterosis of fully restored hybrids grown at lower seed densities. Improvements in both plant type and physiological efficiency would appear to be a logical consequence of hybrid research in both rice and wheat. C . COMBINING ABILITY

The diallel analysis has been the major mating design used to estimate heterosis and the relative amounts of general combining ability (GCA) and specific combining ability (SCA) in rice and wheat. Most wheat studies have revealed that GCA is usually of greater relative importance for grain yield than is SCA (Kronstad and Foote, 1964; Brown et al., 1966; Gyawali et al., 1968; Walton, 1971; Bitzer and Fu, 1972; Widner and Lebsock, 1973). All workers reported significant GCA effects for grain yield, but significant SCA effects occurred only when the experiments were space planted (Kronstad and Foote, 1964; Gyawali er al., 1968; Yadav and Murty, 1976). The absence of SCA effects in competitive growth conditions suggests that nonadditive genetic variance may not be well expressed in wheat under these circumstances (Cregan and Busch, 1978). Widner and Lebsock (1973) evaluated a 10-parent diallel of genetically diverse durum wheat lines and found highly significant GCA effects for grain yield, tillers per unit area, kernels per spike, kernel weight, seedling vigor, maturity, height, and lodging. Specific combining activity effects among F, values were significant for kernel weight, seedling weight, and seedling vigor, suggesting that maximum grain production may be attainable under a system that can exploit both additive and nonadditive genetic effects. The largest levels of heterosis and the highest yielding hybrids involved genetically diverse parents, and other workers have concluded that in hybrid wheat the genetic diversity of the parents is as important as their mean performance (Nettevich, 1968; Yadav and Murty, 1976). The results of combining ability studies in rice have tended to be more variable than those in wheat. The predominant role of additive effects was established for all yield components except panicle number, which was affected by a certain level of nonallelic interaction (Chang et al., 1973; Li, 1975). Several workers have found high GCA effects in the parents to be associated with maximum SCA effects and heterosis for yield in the resulting hybrids (Ranganathan et al., 1973; Parmar, 1974; Maurya and Singh, 1977; Singh, 1977; Khaleque et al., 1977; Rahman et af.,,1981). In contrast, a number of inheritance studies have suggested dominant gene action for yield and/or yield components (Wu, 1968a,b; Chang, 1971; Sivasubramanian and Madava Menon, 1973; Shaalan el al.. 1975;


155

Singh and Nanda, 1976; Singh et af., 1979, 1980; Rahman et af., 1981). Chang (1980) found 1-2 pairs of dominant genes to affect the expression of heterosis for panicle and grain characteristics. Finally, some workers have suggested that their results showed little relationship between combining ability effects and the manifestation of heterosis in the corresponding hybrids (Mohanty and Mohapatra, 1973; Parmar, 1974; Singh and Nanda, 1976; Maurya and Singh, 1977; Rao et al., 1980; Haque et al., 1981; Rahman et al., 1981). Clearly there is a need for hybrid rice programs to investigate this subject further. The question of inbreeding depression has received comparatively little attention in rice and wheat although this is of major significance in assessing the merits of hybrid versus conventional breeding. Cregan and Busch (1978) studied the F, ,F,-F, bulks, and F5 lines from an eight-parent spring wheat diallel at two locations. The F, yields showed significant GCA and SCA mean squares. The latter was attributed to additive X additive epistasis and, although it was present in the F, progeny, it was less apparent in later generations. The significant F, heterosis and SCA for yield, coupled with significant inbreeding depression (0.23% yield reduction per 1% decrease in heterozygosity), indicated the possible desirability of F, hybrids to maximize yields. However, no F, hybrid significantly outyielded the best F5 line tested, and it was unclear whether yields would be maximized by pure line or F,-hybrid development. The inbreeding depression in these crosses between genetically related parents, although significant, was substantially smaller (one-half to one-third) than that reported in maize. This was attributed to the significantly less dominant genetic variance. Yadav and Murty (1976) were able to show varying levels of inbreeding depression in their eight-parent spring wheat diallel study. A high level of inbreeding depression was associated with high heterotic effects in diverse crosses. It is evident that hybrid programs should establish separate heterotic pools for male and female inbred development, that the genetic relationships between these pools should be minimized, and that further studies of the relationship between heterosis and inbreeding depression should be conducted.

Ill. ADVANTAGES OF HYBRIDS OVER CONVENTIONALLY BRED VARIETIES Hybrid advantages are not simply a function of heterosis. Three factors affect the end result: (1) breeding-method efficiency (a rate-of-progress factor), (2) the negative or positive effects of the cytoplasmic male sterility-fertility restoration system used to produce the hybrid, and (3) the inherent heterosis. Although the levels of heterosis in rice and wheat are comparable to those obtained in maize and sorghum, comparatively few studies have reported eco-

156


B. EDWARDS

nomically significant yield advantages over the best conventional varieties. The most promising results on hybrid rice have come from China (Lin and Yuan, 1980), where F, hybrids outyielded conventionally bred varieties by 20-30% under varying levels of on-farm management. Preliminary results from IRRI indicated that leading hybrids had high-parent and standard heterosis of 0-64 and 0-4796, respectively. In wheat, few studies have reported economically significant yield advantages of F, hybrids over the best conventional varieties. However, much of the research on hybrid wheat has been directed at perfecting the genetic system. Only since the early 1970s have the leading hybrid programs devoted more research effort toward the agronomic improvement of male inbreds (restorer lies). The identification of varieties potentially useful as female inbreds, their rapid incorporation into a male-sterile conversion program, and the maintenance of pure seed have provided a management challenge to hybrid breeders. It is against this background that one must assess the advantages of hybrids over conventionally bred varieties. A strong variety breeding program is fundamental for the production of female inbreds; to this extent hybrids and varieties are both complementary and competitive. When separate, large, and genetically diverse “pools” of male and female inbreds are available to a hybrid breeding program, it is reasonable to assume that consistently higher levels of heterosis will be obtained. The hybrid program may have considerable advantage over conventional programs that frequently suffer from inbreeding situations in which several of the top parents used in crosses have varying degrees of genetic relationship. At this point, the rate of progress factor in hybrid production becomes significant [i.e., the generations of selections (usually F,-F,) in conventional breeding are bypassed and the testing phase is immediate]. The hybrid,breeding approach can expedite the incorporation of dominant genes for resistance to major diseases and insects. For example, IR26 is a rice restorer line containing dominant genes for resistance to the brown planthopper and bacterial leaf blight; it has conferred resistance to a number of hybrids (Shen, 1980). If resistance is conditioned by recessive genes, these would have to be incorporated into both parents. In wheat, hybrids offer an advantage for trait complementation of certain quality factors. For example, a mixing time in dough development that is either too long or too short is considered an undesirable trait. Results indicate that mixing time in the hybrid is intermediate between that of the two parents, and this would produce a desirable result in such a cross. More vigorous vegetative growth, taller height, stronger root systems, and higher photoperiod sensitivity are some of the traits already observed in F, rice hybrids compared with their parents. These traits may aid in suppressing weed competition and enable hybrids to adjust to varying water and nutrient regimes. Yap and Chang (1976) repoM that hybrids performed better under dryland than under wetland conditions.


157

No yield-enhancing effects have been attributed to T. timopheevi or other cytoplasms used in hybrid wheat production. However, the significant nuclear-cytoplasmic interactions that have been obtained in certain instances (Gomaa, 1973) should not preclude such a possibility. So far, the use of T. timopheevi has produced no apparent adverse effects on agronomic traits, although unpublished data obtained by Hayward (1975) did suggest that heterosis was slightly lower in T. timopheevi compared with normal cytoplasm hybrids. In summary, environmental adaptation (i.e., the response of hybrids to heat and moisture stress, various insects, and diseases) combined with such factors as photoperiod sensitivity (rice), winter survival (winter wheats), and maturity will all contribute to relative hybrid advantage. These factors should be considered in conjunction with yield data. More extensive hybrid evaluation is now being conducted by a number of rice and wheat programs, and the results are encouraging.

IV. CYTOPLASMIC-GENETIC MALE STERILITY SYSTEMS IN RICE AND WHEAT The development of F, hybrid varieties of rice and wheat, both self-pollinating crops, must involve the use of an effective male sterility system. Among the available male sterility systems (genetic, cytoplasmic, cytoplasmic-genetic, and chemically induced), the cytoplasmic-genetic sterility system has been found the most effective and practical. Almost all of the 6 million ha of cultivated F, rice hybrids in China are developed from cytosterile and restorer lines. In wheat, all experimental as well as commercial hybrids that have been developed involve use of the cytoplasmic-genetic male sterility system. A. EARLYRFSEARCH

1 . Rice

The role of cytoplasm in causing male sterility in rice was first reported in 1954 (Weeraratne, 1954; Sampath and Mohanty, 1954). Katsuo and Mizushima (1958) observed completely male-sterile plants in the progeny of the first backcross Oryza sativa f. spontanealo. sativa cv. Fujisaka 5*. In the reciprocal cross, however, no male-sterile plant was observed, indicating the role of cytoplasmic factor@)and nuclear gene(s) interactions in inducing male sterility. Kitamura (1962a,b) observed slightly lower seed fertility in an indica/japonica hybrid, Tadukan/Norin 8, than in the parental varieties. Backcrossing with male parent Norin 8 increased spikelet sterility in the test progenies. Spikelet sterility

158

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AND IAN B. EDWARDS

was attributed to nondehiscence of anthers, because both male and female gametes were nomal. Shinjyo and Omura (1966a)developed the first cytoplasmic male-sterile line in cultivated rice by substituting nuclear genes of a japonica variety, Taichung 65, into the cytoplasm of indica variety Chinsurah Boro 11 (Shinjyo, 1970). Watanabe et’uf. (1968) observed male sterility in the progeny of the indica-japonica cross (Lead/Fujisaka 5), but those of the reciprocal cross were fertile. However, no male-sterile line was developed. Eiickson (1969)and Carnahan er uf. (1972)developed cytoplasmic male-sterile lines from crosses of an indica variety, Birco (PI279120),with Californian japonica rice varieties Calrose, Caloro, and Colusa; the F, plants were almost completely sterile whereas the reciprocal crosses produced about 50% seed set. The three Californian varieties, when used as recurrent paternal parents, always gave higher sterility in the Birco cytoplasm than in their own. The sterility increased with succeeding backcrossing of Californian japonica varieties into Birco cytoplasm, and the third backcross generation plants became completely male sterile (Camahan et ul., 1972). Watanabe (1971) also reported development of cytoplasmic-genetic male-sterile lines by means of indica-japonica crosses. A cytosterile line possessing. 0. gluberrima cytoplasma in the genetic background of variety Colusa was also developed in California (Camahan et u f . , 1972). Athwal and Virmani (1972)developed a cytoplasmic male-sterile line at the IRRI by substituting nuclear genes of indica rice variety Pankhari 203 into the cytoplasm of a semidwarf indica variety, Taichung Native 1. The first cytoplasmic male-sterile line used to develop commercial F, rice hybrids was developed in China in 1973 from a sterile plant (wild-aborted) occurring naturally in a wild rice population (Oryzu sutivu f. spontunea or 0. perennis) on Hainan Island in 1970 (Hunan Provincial Rice Research Institute, 1977;Yuan, 1977). Subsequently, cytoplasmic male-sterile lines have been developed from various accessions of 0. sutivu f. spontunea, indica variety Gambiaca (from Africa), and the Chinese variety 0-Shan-Tao-Bai (Lin and Yuan, 1980). Rutger and Shinjyo (1980)studied the distribution of male-sterile cytoplasms in various geographical forms of 0. perennis. In Asian and American strains, the frequencies of male-sterile cytoplasm were about 64 and 48,respectively. No malesterile cytoplasm was found in the African and Oceanean strains. 2 . Wheat

The first report of cytoplasmically induced male sterility (CMS) in wheat was that of Kihara (1951),who obtained cytosterile plants by substitution backcrossing of the common wheat genome into Aegilops cuudutu cytoplasm. Plants with A. cuudufu cytoplasm also showed partial female sterility and pistilloidy. Fukasawa (1953)obtained CMS plants from successive backcrosses of Aegifo-


159

tricum X Triticum durum". The Aegilotricum species had been synthesized from a cross of Aegilops ovutu with T. durum. Male-sterile T. durum plants with A. o v m cytoplasm had reduced plant height and delayed maturity compared with normal durum plants, and these effects were consistent through subsequent generations of backcrossing. In the reciprocal cross, A. ovutu plants with T. durum cytoplasm showed male and female fertility comparable to that of normal A. ovutu plants, but the T . durum cytoplasm delayed maturity and reduced plant height. Other studies by Japanese researchers established the presence of CMS plants in the intergeneric crosses, and the sterility persisted through backcross generations (Fukasawa, 1957, 1958, 1959; Kihara, 1958; Kihara and Tsunewaki, 1961).

B. MAJORSOURCES OF CYTOPLASMIC MALESTERILITY

I . Rice From the foregoing review of the literature, 19 sources of cytoplasmic male sterility in rice can be identified (Table III). Five of these [i.e., wild rice (designated as wild aborted or WA type), 0. sutivu f. spontuneu, Chinsurah Boro II (BT type), Gambiaca (Gam type), and 0-Shan-Tao-Bail are being used. More than 100 cytoplasmic male-sterile lines in indica and japonica backgrounds derived from these sources are currently available in China. The male-sterile lines from China are classified into three basic groups according to genetic properties and relation between restorer and maintainer lines (Lin and Yuan, 1980). Group I . The WA cytosteriles are typical of this group, but Gam type and some male-sterile lines derived from 0.sutivu f. spontuneu also belong here. The function of the male sterility gene is sporophytic; pollen grains abort at the uninucleate stage. Maintainer lines are found in both indica and japonica rices. Group ZZ. This group consists of BT-type male-sterile lines developed by Shinjyo and Omura (1966a). The function of the male sterility gene is gametophytic; pollen grains abort between the binucleate and the trinucleate stages. The restoration spectrum of Group I1 is wider than that of Group I; it is easier to sterilize japonica varieties than indica varieties to this type of male sterility. Group ZZZ. The cytoplasmic-genetic male sterility mechanism of this group is derived from some 0. sativa f. spontuneu lines. The Hong-Lien is typical of this group. Pollen grains abort at the binucleate stage and the relation between restorers and maintainers in this group is in contrast to that of Group I. For example, the maintainers of Group I, such as Zhen Shan 97 and Er-Jiu-Ai 4, become restorers of this group, and the restorers of Group I, such as Tai-Yin 1, are good maintainers of this group. Among the various sources of cytosterility, cytoplasmic male-sterile lines


160

Table IU Cytoplnsmie sourceS Identifkd to Induce Male Sterility in Rice

Cytoplasm source

PTB16

Nuclear source ?

Male-sterile lines developed (number)

-

Oryza sativa f. spontanea

Fujisaka 5

-

Oryzaf-

Fujisaka 5

-

Fujisaka 5 Several indica and japonica rim Norin 8 Taichung 65

Lead B h (PI279120)

w u 10 Several japonica rice varieties in China Fujisaka 5 calrose, calm

Orya glaberrima

Colusa IR36

Taichung (Native) 1

Pankhari 203

Akebono Wild rice with aborted pollen (0.sativa f. spontanea or 0.perennis) or WA

0.glaberrima

0.@pogon (KR 7) Gambiaca

0-Shan-Tao-Bai

Several

1

Several 2 or 3

1 1 -

Reference Weeraratne (1954);Sampath and Mohanty (1954) Katsuo and Mizushima (1958) Katsuo and Mizushima (1958) Heu and Chae (1 970) Lin and Yuan (1980) Kitamura (1962a) Shinjyo and Omura (1966a,b) Lin and Yuan (1980) L. P. Yuan (personal communication) Watanabe et al. (1%8) Erickson (1%9); Carnahan er al. (1972) Carnahan er al. (1972) s. s. Virmalli (unpublished) Athwal and V i (1972) Yabuno (1977) Yuan (1972);Lin and Yuan (1980)

Er-Jiu-Nan 1,

Several

Zhen Shan 97, V20, V41, and several other indica and j a p onica rices Taichung 65 Gang-Yi-Ya Ai Zhao, Yat-AiZhao Toride 1 and several other indica and japonica rices

Several

Cheng and Huang (1979) Lin and Yuan (1980)

Several

Lm and Yuan (1980)


161

Table III Continued

Cytoplasm source IARI 10061 IARI 10560 Jeerege Samba 0. perennis (Wl080) 0 . perennis (W1092)

Nuclear source

Male-sterile lines developed (number)

IARI 11445 IARI 11445

Reference

-

Parmar et al. (1981)

-

Shinjyo et al. (1981) Shinjyo and Motomura

IR24 Taichung 65 Taichung 65

(1981)

derived from the WA cytosterility system have been found to be the most stable in China and at the IRRI for their complete or nearly complete pollen sterility (Lin and Yuan, 1980; Virmani et al., 1981). According to L. P. Yuan (personal communication), the probability of developing stable male-sterile lines is higher from relatively wider crosses where the female parent is a primitive line and the male parent is an advanced line. The closer the relation between the two parents, the harder it is to obtain a stable male-sterile line, and vice versa. In the IRFU hybrid rice breeding program, 11 cytosterile lines representing 5 cytoplasmic sources (i.e., Gambiaca, Birco, 0 . sativa f. spontanea of Group I, Taichung Native 1, and BT) are available. Only 7 of these lines [Zhen Shan 97A, V20A, Er-Jiu Nan lA, and V41A (all WA type), Yar Ai Zhao A (Gam type), Pankhari 203A (TN type), and Wu 10A (BT type)] are relatively stable for pollen sterility. The lines MS519A and MS577A possess stainable pollen as do fertile plants, but these pollen grains do not germinate or affect fertilization. All these lines are highly susceptible to major diseases and insects in the tropics, and they cannot be used to develop commercial F, hybrids. Pankhari 203A is tall and photoperiod sensitive, and Wu 10A is a japonica type. At the IRRI, the cytosterility system(s) of some of these lines is being transferred into the genetic background of improved breeding lines and varieties that possess disease and insect resistance.

2 . Wheat Early Japanese research on cytoplasms, coupled with the discovery by Wilson and Ross (1962) that T. timopheevi cytoplasm induces male sterility, led to the

162

S. S. VIRMANI

AND IAN B. EDWARDS

establishment of major research programs in Japan, the United States, and Bulgaria to determine the cytoplasmic variation in species of the genera Triticum and Aegilops. Cytoplasmic male sterility in wheat was reviewed by Maan (1973a) and by Sage (1976). Over 15 different cytoplasms have been recognized, several of which induce male sterility in common wheat and could form a basis for alternative systems of hybrid production (Maan,1975; Mukai and Tsunewaki, 1980). Nearly all hybrid wheat breeding research continues to be based on the T. timopheevi system, and its widespread use has been largely the result of its apparently neutral effect on agronomic and quality characters. Most other cytoplasms from Triticum and Aegilops have deleterious effects on various traits (Maan, 1973a; Sage, 1976). Of the altemative cytoplasms available, most show no advantage over that of T. timopheevi, and time and resources limit change. However, the potential for genetic vulnerability to a major disease is always present when a single cytoplasm is used; the southern corn leaf blight epidemic in the United States in 1970 (caused by Helminthosporium maydis, race T) is a good example. Ghiasi and Lucken (1982a) compared the reactions of A. speltoides and T. timopheevi cytoplasms to various restorer gene combinations and examined a number of agronomic and quality traits. They concluded that A. speltoides cytoplasm can be used interchangeably with that of T. timopheevi in hybrid wheat breeding, providing an alternative that can broaden genetic variability. On the basis of nucleocytoplasmicinteractions (Maanand Lucken, 1971, 1972) and cytogenetic evidence (Kimber and Athwal, 1972; Kimber, 1973; Shands and Kimber, 1973), it has been suggested that A. speltoides may have contributed the G genome and cytoplasm to T. timopheevi. However, the restorer line R5 (T. zhukovskyi/3* ‘Justin’) is an effective restorer for T. timpheevi cytoplasm but not for A. speltoides cytoplasm, and this interaction provides the genetic basis for differentiation of the two cytoplasms. Gomaa and Lucken (1973) compared the breeding behavior of the restorers R5 and BR4704 in T. timopheevi and T . boeoticum cytoplasms. Fertility restoration (RB genes effective in T. boeoticum cytoplasm were also effective in T. timopheevi cytoplasm, but not necessarily vice versa. The reduction in vigor noted when the genomes of common wheat are substituted into T. boeoticum cytoplasm (Hori and Tsunewaki, 1967; Maan and Lucken, 1967, 1972; Gomaa, 1973) has curtailed the use of T. boeoticum as an alternative cytoplasmic source for hybrid breeding. It should be recognized from the previous statements that nucleocytoplasmic interactions are often significant; in T. timopheevi cytoplasm, small differences between male-sterile lines and their maintainers have also been noted for a number of traits (Jost et al., 1976b). Several researchers have also observed reductions in germination and seedling vigor with progressive backcrossing in certain wheat genomes into T. timopheevi cytoplasm.


163

C. ADDITIONAL SOURCES OF CYTOPLASMIC MALESTERILITY

1 . Rice

Although a number of sources for cytoplasmic male sterility in rice have been identified, more than 90% of the area planted to hybrid rice in China is occupied by hybrids derived from WA cytosterile lines. This situation makes hybrid rice in China potentially vulnerable to disease or insect epidemics. Work is in progress in China (Lin and Yuan, 1980)and at the IRRI (Virmani etal., 1981)to diversify usable sources of cytoplasmic male sterility for hybrid rice development. Rice species (i.e., Oryza gluberrimu, 0. fatuu, Asian forms of 0. perennis, and 0 . rufipogon) and varieties (i.e., PTB16, Tadukan, Lead 35, Akeboro, IARI 10061, IARI 10560,and Jeerege Samba) that are known to induce cytoplasmic male sterility may result in cytosterile lines that possess different cytosterility systems. The use of protoplast fusion techniques should expedite the development of new cytosterile lines (E. C. Cocking, personal communication).

2 . Wheat Additional cytosterility systems that supply male-sterile plants with good vigor and female fertility have been produced with the cytoplasms of Zhukovskyi (2n = 42; AAA’A’GG), Triticum araraticum (2n = 28; AAGG), and T. dicoccoides var. nudiglumis (2n = 28;AAGG) (Maan, 1975;Maan and Lucken, 1971). The fertility restoration systems of the previously mentioned cytosterility systems are also under complex genetic control and cause difficulties in breeding agronomically suitable restorer lines. Franekowiak et al. (1976) presented a proposal for hybrid wheat using Aegilops squarrosa cytoplasm that seeks to avoid the breeding of restorer lines. The D genome of common wheat contains genetic factor(s) €or restoration of fertility of T. aestivum with A. squarrosa cytoplasm. The nucleus of T. aestivum was substituted into A. squurrosa cytoplasm and the seed was treated with a mutagenic agent (ethyl methanesulfonate, EMS)to inactivate the critical gene(s) that causes fertility. Ten male-sterility mutants from an M, population of 45,000 plants expressed sterility in F, or F, generation, which indicates control by a single recessive gene. Crosses with four spring wheats produced completely fertile F, progeny. However, the major weakness of the system was that no malesterile genes that function specifically in the A. squarrosa cytoplasm were found. The development of fertile T. aestivum B lines with homozygous recessive genes for the maintenance of the A lines was not completed. Mukai and Tsunewaki (1979)proposed a similar system using the cytoplasms of Aegilops kotschyi and A. variabilis. When 12 common wheat genotypes were

164

S. S. VlRMANI AND IAN B. EDWARDS

substituted into these cytoplasms, 3 were found to be male sterile. Crosses with Chinese Spring produced a fertile F, hybrid, and restoration was attributed primarily to a single dominant gene (Rfvl).Trificurnspelfa var. duhumelianum carries a gene on chromosome 1B which interacts with A. kofschyi and A . vuriabilis cytoplasms to give male-sterile plants. Cultivars that carry the 1B/lR rye translocation and thereby lack the short, satellited arm of chromosome 1B display the same male-sterile interaction. Normal wheat cultivars carry gene(@ that overcome this sterility and therefore constitute male or restorer parents for hybrids. Comparisons with T. fimpheevi cytoplasm for 12 agronomiccharacters showed that A. bfschyi cytoplasm influenced only dry matter (reduced to 12%). The A. vuriabilis cytoplasm reduced plant height 496, ear number 18%, and dry matter 26%. Several hybrid programs in the United States and elsewhere are currently evaluating this system.

D. TECHNIQUES FOR CYTOPLASMIC DIFPERENTIA~ON

The techniques available for detecting cytoplasmic variation in a crop species are

1. Substitution backcrossing 2. Use of cytoplasm-differentiating genes 3. Interaction of restorer (Rfigenes with the cytoplasm 4. Study of pollen abortion patterns 5 . Restriction endonuclease fragment analyses of organelle DNAs. Work on these lines in rice has been limited. Chinese scientists have used techniques 2, 3, and 4 and reported that the WA cytosterility system is different from the BT system because the restorer gene(s) for the former have sporophytic action and those for the latter have gametophytic action (Y. Y. Dong, unpublished). Shinjyo (1969, 1975) and Kinoshita et al. (1980) have also reported gametophytic action of the restorer gene for BT cytosterilelines. By studying the pollen abortion pattern of different CMS lines, Xu (1982) found that pollen of WA cytosterile lines abort at the uninucleate stage and those in BT cytosteriles abort at the binucleate and trinucleate stages (Q. L. Jiang, unpublished). Chaudhary ef al. (1981) also established differences between WA, TNl, and BT male-sterile lines maintained at IRRIon the basis of their pollen abortion pattern. Chaudhary ef al. further suggested that the pollen abortion stage in a CMS line depended on the distance of relation of its cytoplasmic and nuclear donor parents. It appears that cytosterile lines with pollen abortion at the uninucleate stage are more stable for complete pollen sterility than lines with pollen abortion at the binucleate or trinucleate stage. Cytoplasmic variation in the Triticinae was reviewed by Maan (1973b, 1975)


165

and Sage (1976). The literature reveals that all genomically distinct species of Triticwn and related genera examined differ cytoplasmically. The sterility- fertility interactions between genomes from Triticum spp. and cytoplasms from Aegilops spp. indicate that male fertility-restoring genes derived from one cytoplasm donor species may restore fertility to male-sterile wheats that have cytoplasms of one or more of the other related species. Maan (1973b) drew attention to the fact that a number of factors will influence the expression and detection of cytoplasmic effects: (1) stability of the cytoplasm, (2) stability of the nuclear genome, (3) choice of cytoplasm and genome donor, (4) persistence of nuclear genes from the cytoplasm donor, and ( 5 ) genotype-environment interactions. In substitution backcrossing of the genome of one species into the alien cytoplasm of another, sufficient backcrosses are required to eliminate all nuclear genes derived from the cytoplasm donor species and produce a new nucleocytoplasmic combination. If this results in relatively stable male sterility, the cytoplasms of the two species involved in the cross may be considered distinct. Maan (1973b) reported that genomes of common wheat and durum wheat generally have similar interactions resulting in male sterility and abnormalities of plant growth with the cytoplasms of most of the related species. When they differed, the durum genomes were more sensitive to certain alien cytoplasms than the common wheat genomes. Sasakuma and Maan (1978) introduced T. durum genomes into the cytoplasms of 6 species of Triticum, 14 species of Aegilops, and 1 species each of Secale and Haynuldia. O f the 22 alloplasmic lines, 14 were completely male sterile, 4 were partially fertile, and the rest, which had the cytoplasms of T. dicoccoides, A . kotschyi, A. variabilis, or H . villosa, had normal fertility. When new nucleocytoplasmic combinations are made using T. aestivwn or T. durum genomes, differences between cytoplasms in traits other than male sterility often occur. These differences indicate that distinctions between cytoplasms cannot be based on male sterility alone. This subject is dealt with jn Section IV,E. Cytoplasm-differentiating nuclear genes include male fertility-restoring genes, male fertility-inhibiting genes, and genes affecting plant vigor in various ways. The relationships among these different genes are not clearly understood. However, research during the past decade has led several hybrid breeding programs to apply the terms “hard-to-restore” and “easy-to-restore” to genotypes in which male fertility-inhibiting genes are present or absent, respectively. The cytoplasms of T. timopheevi and A. speltoides cannot be differentiated by the presence of T. aestivum genomes, because male sterility is the only deleterious effect in both. However, substitution of the genome of T. timopheevi into A . speltoides cytoplasm produces male-sterile plants with reduced vigor (Maan and Lucken, 1972). Therefore, nuclear gene differences between the genomes must control the behavior of T. timopheevi and A. speltoides cyto-

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S . S . VIRMANI AND IAN B. EDWARDS

plasms with either T. aestivum or T. tinwpheevi genomes. Such genes were termed cytoplasm-differentiating genes. In addition, T . tinwpheevi and A. speltoides cytoplasms differ in their reaction with restorer R5 (Maan, 1973a). The latter restores T. rimopkevi but not A. speltoides cytoplasm. This type of information on the components of interacting male sterility-male fertility restoration systems is important in hybrid wheat breeding. Restriction endonuclease fragment analysis of organelle DNAs, which is effective in demonstrating the heterogeneity of mitochondrial (MT) DNA among normal, fertile (Levings and Pring, 1977) and male-sterile cytoplasms in corn (Pring and Levings, 1978; Conde et al., 1979; Pring el al., 1980) and sorghum (Pring et al., 1982). Li and Liu (1983) found the heterogeneity of chloroplast (CT) DNA in CMS and maintaining lines in wheat, corn, and rape and suggested that changes in CT DNA may be involved in CMS.These techniques have not yet been used in rice and wheat for differentiating and identifying cytoplasm sources. E. CYTOPLASMIC Emcrs ON OTHER PLANTCHARACTERS

The effects of sterility-inducing cytoplasm on morphological traits have been reported for tobacco (Clayton, 1950; Chaplin and Ford, 1965), maize (Grogan and Sarvella, 1964; Grogan et al., 1971), and sorghum (Lenz and Atkins, 1981). Such effects in rice hybrids developed from WA cytosteriles V41A and Zhen Shan 97A and four fertility restorer lines (IR24, IR30, Xin-ni-ai-he, and Shiu Lian gu) have been reported in China (Lu et al., 1981). A comparison of F, hybrids A X R and B X R indicated that ‘Sterile’ cytoplasm had negative effects on number of spikelets per panicle, number of filled grains per panicle, 1OOOgrain weight, and yield per plant, although it had a positive effect on number of tillers per hill. Observations at the IRRI c o n f m that such effects are present but are cross specific; therefore it should be possible to eliminate the negative effects of cytoplasmic male sterility through the selection of appropriate restorer lines. Gomaa (1973) measured quantitatively and qualitatively the effects of T. boeoticum and T . timopheevi cytoplasms on five spring wheat cultivars. The male steriles in T. tinwpheevi cytoplasm were generally similar to their normal counterparts for the characters measured. With T . boeoticum cytoplasm a combined analysis revealed significant cultivar cytoplasm interactions for seedling vigor, vigor at heading stage, and spike length. C. F. Hayward (1975, unpublished) compared crosses of three restorer (male) lines with two male-sterile (T. tinwpheevi) lines and their normal cytoplasmic (B line) counterparts. Hybrids made on normal cytoplasm had yields 7.1% higher than their T . timopheevi counterparts, although the magnitude differed considerably between crosses (-3.5-22.0%). Mean heterosis levels averaged 19.9% in the normal cytoplasm and 12.8% in the T. tinwpheevi cytoplasm.

Table IV

Interactions between the Genomes of T. dunun, T. aesh'vum, and T. tinropheevi and the cytoplasms of Species of Aegilops, Tritieum, Secale, and Haynoldioa

Cytoplasm donor

Aegilops species A. speltoides A. bicornis A. longissimae A. sharonensisc A. mutica A. comosa A. heldreichii A. uniaristata A. caudatac A. umbellulata A. squarrosa A. cyIindrica A. ventricosa A. crassa A. ovata A. triaristata A. biuncialis A. columnaris A. juvanalis A. varzhbilis A. kotschyii A. triuncialisc Triticum species T. monoemcum T. boeoticum T. dicoccoides T. dicmcum T. dunun T. aestivum T. macho T. dicoccoides var. nudigIumis T. timopheevi T. araraticum T. zhukovskyi Other species Secale cereale Haynaldia villosa

Chromosome number (2n) and genome symbol

Nucleocytoplasmic interactionsb

T. durum

T. aestivum

T. timopheevi

14, SS 14, SbSb 14, SISI 14, SISI 14, M'Mt 14, MM 14, MM 14, MUMU 14, CC 14, C U C U 14, DD 28, CCDD 28, DDM'JM'J 28, DIDLMM 28, CUCuMM 28, CUCUMM 28, W M M 28, CuCuMM 42, DDMMCUCU 28, CUCUSISL 28, CuC'JSIS1 28, CUCUCC 14, 14, 28, 28, 28, 42, 42, 28,

AA AA AABB AABB AABB AABBDD AABBDD AAGG

28, AAGG 28, AAGG 42, AAAAGG 14, RR 14, HH

"After Maan (1975) and Sasakuma and Maan (1978). bF,male fertile; PF, partially fertile; S,male sterile; FS,female sterile; N, normal vigor; NN, near n o d vigor; BN, below normal vigor; w.weak (markedly reduced vigor); Z, zygote elimination (nonviable seed); B, bushy (stunted); E, early maturity; L, delayed maturity. =Some evidence of intraspecific cytoplasmic variability has been obtained by using two 01 more accessions.

168


The effects of various nucleocytoplasmiccombinations involving the genomes of common and durum wheat on a range of phenotypic characters were reviewed by Maan (1973a,b, 1975) and Sage (1976). Sasakuma and Maan (1978) reported on an extensive study in which the genomes of T. durum (selection 56-1) were substituted into 22 cytoplasms, and the effects on pollen fertility, seed set, heading date, and plant vigor were measured. Table IV represents the combined data of Maan (1975), Sasakuma and Maan (1978), and some data supplied by S. S. Maan (unpublished). It should be pointed out that the effects recorded are those that were most easily observable. In the future, more precise measurements may reveal different degrees of increased or reduced fertility and plant vigor. As has already been reported for rice, different accessions within certain species show nucleocytoplasmic interactions that differ sufficiently to suggest that cytoplasmic differences are present; these are indicated in Table IV (S.S. Maan, personal communication). The principal cytoplasmic effects resulting from various nucleocytoplasmic combinations included reduced vigor, delayed maturity, pistilloidy, and nongerminating grain. Mukai and Tsunewaki (1979) also showed that the degree of phenotypic deviation for a number of traits varied depending on the cultivar or genome used. The literature on gene interaction indicates that most genes do not act in isolation from other genes. Therefore, the alien cytoplasms and nuclear genes controlling cytoplasmic effects from alien sources may alter the phenotypic expression of various agronomic triats. Although no yield-enhancing effects from nucleocytoplasmicinteractions have been reported, the possibility of such an Occurrence should not be disregarded in the future as increasing numbers of substitutions are made by hybrid breeders.

F. CYTOPLASMIC E m s ON DISEASE RESISTANCE Although work to date has not revealed any relationship between CMS and disease susceptibility in rice (Y. Y.Dong, unpublished data), the desirability of hybrid rice and wheat breeders using more than one source of cytoplasmic male sterility as a safeguard against potential disease epidemics is widely recognized. Washington and Maan (1974) tested alloplasmic lines of T. aestivum (cultivars Chris and Selkirk) and T. durum with three physiologic races of wheat leaf rust (Pucciniu recondiru) at the seedling and adult plant stages. Although euplasmic and alloplasmic Selkirk lines were resistant to all races at both stages, differences were found in the cultivar Chris. Both forms were seedling susceptible,but adult plants of euplasmic Chris were resistant, whereas certain alloplasmic lines were susceptible or moderately susceptible to all races used. Other alloplasmic Chris lines were susceptible to one race but not to the other two. These results indicate that certain alien cytoplasms may alter the expansion of host nuclear genes for resistance to certain physiologic races of leaf rust. Furthermore, the host parasite


169

interaction was influenced by host cytoplasmic factors, host nuclear genes, and the rust fungus.

V. FERTILITY RESTORATION A. SOURCES OF RESTORERGENES

1. Rice

The practical use of cytoplasmic-genetic male sterility in developing hybrid varieties in grain crops is possible only when the effective restorer lines are identified and/or developed. In rice, effective restorer lines for the WA, Gam, and BT cytosterility systems have been identified among cultivated rice varieties and elite breeding lines (Shinjyo, 1969, 1972a,b, 1975; Lin and Yuan, 1980). Effective restorer lines for cytosteriles Pankhari 203A and MS577A have not been identified. Shinjyo (1975) found that 35% of 153 rice varieties that originated from outside Japan were effective restorers for the BT cytosterile line. Restoration ability was related to grain type (Matsuo, 1952). Effective restorer varieties were mainly distributed in the tropics where indica rice was exclusively grown. The Fist set of effective restorer lines used in China in commercial F, hybrids involving the WA cytosterility system was identified in 1973 (Hunan Provincial Rice Research Institute, 1977; Lin and Yuan, 1980). Since then, a number of these lines have been selected and/or developed for various cytosterility systems. The frequency of restorer lines, higher among rice varieties originating in lower latitudes, was about 20% of 75 varieties from southwestern and southern China but only 7.5% of 438 varieties from the Yangtze Valley (Hunan Academy of Agricultural Sciences, unpublished). The frequency of restorer lines was even less among varieties from nothern China, eastern Europe, Japan, and Korea; the highest frequency (35%) was found among 197 varieties originating in the lower latitudes of south and southeast Asia. The restoring ability of rice varieties has been found to be somewhat related to their origin. The frequency of restorer lines is higher and restoration ability is stronger among varieties closely related to the WA cytosterility source; it is higher among indicas because indica varieties originalted earlier than japonica varieties and are closely related to wild rice. The frequency of restorer lines among japonica varieties is negligible (Shinjyo, 1975; Lin and Yuan, 1980); consequently, japonica F, rice hybrids in China have been bred by transferring restorer gene(s) into the male parents from indica rice. Among indica rices, restorer lines have been more frequent among late-matur-

170


Table V Restorer Lines Used Widely in Commercial F1Rice Hybrids in China Line

Restorer of CMS system

Origin

IR24 IR26 IR30 IR661 IR665 Gu 154 Gu 223 Gui 630 Tai Yin 1 Yinni Ai He Ke Zhen 145 Zhao Hui 3533 c55 c57 Beijing 300 5350 5154 Hong Zhao Nou

WA, Gambiaca WA, Gambiaca WA, Gambiaca WA, Gambiaca WA, Gambiaca WA, Gambiaca WA, Gambiaca WA, Gambiaca WA, Gambiaca WA, Gambiaca WA, Gambiaca WA, Gambiaca Chinsurah Born I1 (BT) Chinsurah Boro I1 (BT) Chinsurah Boro I1 (BT) Chinsurah Boro I1 (BT) Chinsurah Born I1 (BT) Chinsurah Boro I1 (BT) Nan guang zhan

IRRI IRRI IRRI IRRI IRRI Cuba/IRRI CubdRRI CubamRRI ThailanMRRI Indonesia China China China China China China China China China

ing than among early-maturing varieties, perhaps because late-maturing indicas are primitive, relatively closer to wild rice. The restorer lines used widely in China are listed in Table V. The IR24, IR26, IR661, and IR665 restorers of the most promising, most widely cultivated indica hybrids in China originated at the IRRI. Additional restorers are being selected from IRRI elite breeding lines that possess multiple disease and insect resistances to be used in developing hybrid rices for the tropics and subtropics. 2 . Wheat The development of fertility restorer lines (R lines) which restore complete male fertility to F, hybrids presented a major challenge to hybrid wheat breeders and has constituted a key determinant of the pace of hybrid wheat development. The male fertility-restoring genes or gene combinations and cytoplasm used in hybrid wheat represents a substantial introgression of germ plasm from related species. Krupnov (1971) pointed out that all species capable of donating a cytoplasm that causes male sterility in common wheat are also sources of the corresponding Rf genes. This is to be expected if it is the absence of specific


171

genes, caused by their elimination during backcrossing, that causes pollen development to break down in male-sterile nucleocytoplasmic combinations. Wilson and Ross (1962) made the necessary breakthrough for hybrid wheat breeding when they successfully transferred Rf genes from T. timopheevi to common wheat by substitution backcrossing. Schmidt et ul. (1962) found malesterile and male-fertile plants in ‘Nebraska 542437,’ a T. timopheevi derivative (1279A9-111-4) crossed to ‘Nebred.’ Crosses between the male-sterile and male-fertile plants were fertile, demonstrating that a suitable restorer for T. timopheevi cytoplasm had been found. Most Rf genes are derived from T. timopheevi, but a number also occur in common wheat (Zeven, 1968; Lucken, 1973). Johnson and Schmidt (1968) pointed out that the discovery of Rf genes in a number of hexaploid and tetraploid varieties that provide at least partial restoration of CMS (T. timopheevi) lines suggests the possibility that at least one of the genes in the Nebraska restorer may have come from common wheat. Evidence from both the Kansas and Nebraska program suggested that a gene in ‘Cheyenne’ winter wheat is the same as one from T. timopheevi. When certain wheat cultivars are crossed into a CMS line, partial fertility varying from only a few seeds at the base of the spike to nearly full seed set may be observed in F, or BC, (F,) plants. In the latter case this would indicate that fertility is expressed only when the genes are homozygous. Zeven (1967) found that only ‘Redman’ spring wheat and a line from T. mcha (No. 2) caused partial fertility in A. cuudutu cytoplasm; in contrast, he was able to identify 30 common wheats and the T. macha (No. 2) line as having Rf genes for T. timopheevi cytoplasm. Joppa and McNeal (1969) identified 3 common wheats (PI 167841, PI 277013, and PI 277016) with genes for pollen fertility restoration to CMS (T. timopheevi) Selkirk. Oehler and Ingold (1966) reported that the hexaploid European cultivar ‘Primepi’ restored fertility to CMS (T. timopheevi) lines, and Milohnic and Jost (1974) reported Primepi as showing the highest level of restoration among the group of common wheats they tested. Lists of common wheats that carry Rf genes have been provided by several researchers (Zeven, 1967; Porter and Merkle, 1967; Johnson and Schmidt, 1968; Jost and Milohnic, 1976a; Ghiasi and Lucken, 1982b), and it is now a comparatively common Occurrence for hybrid wheat breeders to encounter Rf genes in common wheat varieties during substitution backcrossing into T. timopheevi cytoplasm. The reasons for this occurrence of Rf genes in conventional cultivars is not known. It can be hypothesized that partial Rf genes are retained because of pleiotropic effects that positively influence the phenotype, because they are linked with other desirable genes, or because their retention is a chance occurrence. Hughes and Bodden (1977) suggested that the high frequency of Rfgenes in wheats developed at the Plant Breeding Institute (Cambridge, England) may have been the result of linkage with genes for resistance to powdery mildew

Table VI

Origin of Some Sources of Male-Fertility-RestoringGenes Used in Hybrid Wheat Breeding Program

c 4

E3

Cytoplasm restored

Designation

-

Aegilops ovata P168

ABD-1

-

Aegilops caudata ABD- 13 R8 R9

Triticum timopheevi

R10 R1-Lee R2-Sonora 64 (= CIMMYT restorer)

Reference and/or other information

Derivation T. dicoccoides var. kotschyanum T. aestivum var. erythrospermum, where chromosome 1D is replaced by A. caudata chromosome C-sat-2 (1C) T. dicoccoides var. spontaneo-nigramlA.squarrosa typica No. 2 T. compactum T. dicoccum VemaYA. squarrosa strangulata A. speltoideslChmese Spring//4*Chris A. speltoideslChinese Spring/l2*SelkirkChinese Spring A. caudata/9*Chris//R8, F8 T. timopheevil2*Hussar-Hard. Fed//Comet Hussar-HardRed/Nebr (= Nebr 542437) T. iimopheevil2*Marquid/Sonora64

Fukasawa (1955) Kihara (1963a) Rf gene; Tahir and Tsunewaki (1971) Tahir (1969) Kihara (1963b)

Kihara and Tsunewaki (1964) Maan (1973a) Maan (1973a)

Maan (1973a) Schmidt et al. (1962)

Rf genes; Talaat (1969) CIMMYT (unpublished); Rf genes; Bahl and Maan (1973)

R3 (Wilson or Kansas restorer) R4 or RD RC RK

R5

R6 or BR4704 R7

TX73C9610-1

-

W 4

VS73-786 149-2-3 R11 R12, RE1 R13, RE3

T. timopheevi/3*Marquis T. T. T. T. T.

timopheevi-A. squarrosa/3*Dirk timopheevi-A. squarrosa/3*Canthatch rimopheevi-A. squarrosa/3*Kam zhukovskyi/3*Justin boeoticum-A. squarrosalT. durumNChinese spring T. boeoticum/2*T. durum/l2*Selkirk T. spelra var. duhumelianum T. aestivwn cv. Rimepi T. timopheevi/6/n*msBison/4*Selkirk/5/~s Bison/2*Selkirk/4/T. timopheevi/Justin//Marquis/3/Justin Complex cross-Rf source: Primepi, Maris Beacon, and R3 T. arararicum/3*Selkirk, F7 T. dicoccoides var. nudiglumis No. 1/6* T. durum T. dicoccoides var. nudiglumis No. 4/T. durum

Wilson and Ross (1962); Rf genes; Livers (1964) University of Manitoba, Canada

Rf genes; Yen et al. (1969)

Maan and Lucken (1967, 1970, 1972) Maan and Lucken (1970, 1972) Maan and Lucken (1970) Kihara and Tsunewaki (1966) Oehler and Ingold (1966) Gilmore et al. (1978)

Jost (1980) Maan (1973a) Maan (1973a); Sasakuma and Maan (1978) Maan (1973a); Sasakuma and Maan (1978)

174

S . S . VIRMAM AND IAN

B. EDWARDS

(Erysiphe graminis f. sp.) or other genes affecting productivity. In contrast, Ghiasi and Lucken (1982b) evaluated 40 F, lines derived from a single F, plant which was heterozygous for a gene(s) for partial fertility restoration. Testcrossing revealed that 27 lines had the Rf gene but 13 lacked it. Two years of evaluation for agronomic and disease traits failed to explain the retention of the partial Rf gene in the conventional variety breeding program for reasons other than chance. Sources of Rf genes widely used in hybrid breeding programs have been cited by several researchers (Maan and Lucken, 1972; Zeven, 1972; Maan, 1973a; Sage, 1976; Sasakuma and Maan, 1978; Jost, 1980), and a number of these sources are listed in Table VI. Two factors should be mentioned: first, the genome of the cytoplasm donor is clearly not the only source of Rf genes; and second, a number of the restoration sources are capable of restoring more than the cytoplasm indicated. For example, Maan and Lucken (1970) obtained highly fertile F, hybrids of normal vigor by crossing R5 and R6 into A lines with the cytoplasm of either T. timopheevi or the amphidiploid T. boeoticum-A. squarrow. Maan (1973a) transferred Rf genes to T . aestivum and T. durum genomes from several accessions of T. uraruticum (R1 1) and T. dicoccoides var. nudiglomis (R12, R13). These T. durum restorer lines restored fertility to CMS lines of durum with cytoplasms from several species of Triticum and Aegilops.

B. INHEIUTANCE OF RESTORATION Results of experiments designed to study the inheritance of fertility restorer genes using conventional genetic analysis are often difficult to interpret. The variable penetrance and expressivity of certain Rf genes, the presence or absence of fertility inhibitor genes, genetic background, and environment all affect genetic ratios in segregating populations. The information available on the two crops is reviewed in the following sections. I . Rice

Shinjyo (1969) identified a fertility-restoring single dominant gene, Rf, in the variety Chinsurah Boro I1 for the BT cytosterile line; its effect was gametophytic in the male sterility-inducing cytoplasm (MS-Boro). Kitamura (1962a) reported that high fertility in the F, hybrids of cytosterile Ta 820 (Kitamura, 1962b) was conditioned by a recessive gene combined with modifiers or polygenes. Shinjyo (1973, however, contended that the fertility-restoring gene of the variety Tadukan used by Kitamura (1962a) behaved similarly to the restorer gene from Chinsurah Boro II. Kitamura’s work had indicated that cytosterile Ta 820 pos-


175

sessed functional male sterility, because there was no abortion of male and female gametes but the anthers were nondehiscent. These two authors may have been working with two different systems of cytosterility. Shinjyo (1975) also studied the genetics of fertility restoration in the male sterility-inducing cytoplasm of the rice variety Lead identified by Watanabe et al. (1968). Two restorers, Fukuyama (a japonica variety) and a strain of MS-Boro Rf, Taichung 65 (Shinjyo et al., 1974), possessed the Rfx and the Rf gene, respectively, and had gametophytic effects in the Lead rice cytoplasm. The Rfx gene gave weaker restoration than did the Rfgene in the cytoplasm of Chinsurah Boro 11. Neither the Rf nor the Rfx genes restored the fertility of 0. sativa f. spontanea cytoplasm identified by Katsuo and Mizushima (1958). Shinjyo (1975) could not find any restorer source for Chinsurah Boro I1 cytoplasm, which had both effective and weak restoring genes. It was therefore concluded that the weak (Rfx) and effective (RB genes were probably allelic. The genetics of restoration in cytosterile lines developed by Katsuo and Mizushima (1958), Erickson (1%9), and Athwal and Virmani (1972) has not been reported. Therefore, the fertility restoration genes identified by Shinjyo (1969, 1975) cannot be related to fertility restoration in the cytosterile lines developed by the other workers. Inheritance of fertility restoration for cytosterile lines developed and used in China, although determined, is not well documented in the literature available outside China. Information gathered by S. S. Virmani during his various visits to China, and the results obtained by Gao (1981), indicate that restoration in the WA cytosterile line is controlled by one or two pairs of dominant genes, and the effect of the restoration genes is sporophytic. In some crosses, however, fertility restoration in WA cytosterile lines has also been found to be quantitative in inheritance (Anonymous, 1976).

2 . Wheat Sage (1976) reviewed the literature on fertility restoration and pointed out that in many studies the initial genetic explanation was proved by subsequent work to be an oversimplification. For example, restoration of T. tirnopheevicytoplasm by Primepi has been reported to be caused by a single dominant gene (Oehler and Ingold, 1966; Goujon and Ingold, 1967), two incompletely dominant genes with a major and a minor effect, respectively (Miller, 1970; Schmidt et al., 1971; Miller et al., 1974), two incompletely dominant genes with the epistatic action of a single recessive gene (Nettevich and Naumov, 1970), and one major and one minor dominant gene which act in a complementary manner together with one modifier gene and one inhibitor gene (Shebeski, 1971). Using monosomic analysis, it was shown that two Rf genes on chromosomes 1B and 5D controlled male fertility restoration in Primepi (Bahl, 1971; Bahl and Maan, 1973).

S. S. VIRMANI AND

176

IAN B. EDWARDS

Table W Chromoeomes Shown by Mom#lomie Analysis to InniKnee Male Sterility Restoration

Restorer line

Chromosome location of Rf genes

Reference

R1-Lee W-Sonora 64 R3

lA, 5A, 7D lA, 6B, 7D lA, 7D

Talaat (1969); Bahl and Maan (1973) Bahl (1971); Bahl and Maan (1973) Talaat (1969); Bahl and Maan (1973); Robertson and Curtis

R4-RD (Dirk)

lA, 7D

R4-RC (Canthatch) R4-RK (Karn)

6B, 6D lA, 6B lA, 7B, 7D 1B lB, 5D 1C (Sat-2) lB, 4B, 7D 6B

Yen et al. (1969); Talaat (1969); Bahl and Maan (1973) Yen et al. (1969) Yen et al. (1969) Bahl (1971); Bahl and Maan (1973) Tahir and Tsunewaki (1%9) Bahl (1971); Bahl and Maan (1973) Tahir and Tsunewaki (1971) Zeven (1970) Gilmore et al. (1977)

lA, 6B

Bravo (1982)

R5 T. spelta var. duhamelianwn Primepi P168 Minister Tam W-1oQ (rye translocation) R113

Monosomic analysis has been the principal technique used to reinforce the results obtained by conventional genetic analysis of restoration, and the results of analyses on several restorer lines are summarized in Table VII.Most monosomic analyses have been conducted on restorer lines that were the initial introgression lines or were early sources of restorer genes for T. timpheevi cytoplasm. However, Bravo (1982) used R113, a restorer line from the North Dakota State University hybrid program that had proved superior to other R lines in its ability to confer complete male fertility to F, hybrids. Monosomic analysis indicated that R113 had restorer genes on chromosomes 1A and 6B. Restorer genes of lower penetrance or modifier genes were located on chromosomes lB, 4B, and 2D, and inhibitors of restoration were located on chromosomes 5A, 6A, and 5B. The penetrance of these genes appeared to be influenced by environment, (i.e., they were detected in only 1 of 2 years). Because no major new restorer gene was detected, it was suggested that the high restoration capacity of R113 may result from the balanced, cumulative effects of genes on several chromosomes. The hybrid breeder is therefore selecting restorer lines from populations which are segregating for several genes, both restorer genes per se and modifying genes that may control the level of restoration needed for productive F, hybrids.

HYBRID RICE AND

177

WHEAT

c. ENVIRONMENTAL EFFECTS ON MALE FERTILITY RESTORATION The effects of environmental factors on male fertility restoration in hybrid rice would appear to be genotype-specific. Several of the commercial hybrid combinations involving restorer lines IR24, IR26, IR661, Gui 154, and others are widely adapted in China, implying that their restoration ability is stable in various enviornments. Lin and Yuan (1980), however, reported that the rate of seed set in hybrid rice, particularly under unfavorable climatic conditions, was only about 80%, less than that of the best conventional varieties in China. Combinations with better seed set than leading conventional varieties, even under unfavorable conditions, have been developed, confirming that the stability of fertility restoration in hybrid rice is genotype specific. IR54, another restorer line identified at the IRRI, has given normal seed set in the F, combination Zhen Shan 97A/IR54, both at the IRRI and in Fukien Province, China (Ren Cui Yang, personal communication). Results of a joint study between IRRI and China, aimed at identifying effective restorer lines among elite breeding materials developed at IRRI and other national programs, indicated that about 15 and 24% of these lines were effective restorers in China and at the IRRI, respectively, but only 6% were effective restorers at both sites (Table VIII). The restoration ability of the remaining restorer lines was site specific; the frequency of restorer genotypes was higher in a tropical than in a subtropical environment. In wheat, the effects of various environmental parameters such as temperature, photoperiod, and moisture stress on general growth and development are well

Table VIII Restoration Ability of 218 Elite Breeding Lines Tested in China and at the IRRI (1981)a Test in China ~

Test at the IRRI

Restorer

Partial restorer

Restorer Partial restorer Partial restorer; maintainer Partial maintainer Maintainer

13 17

37 66

-

-

-3 33

No data

Partial maintainer

Maintainer

Total

-

2 4 4 -

3 25 7 8 15 -5

2

53 111 7 10 23 14 -

113

63

9

218

3 -

4

“Figures given ar? number of lines. About 90% of the breeding lines were developed at the IRRI; the rest were developed in other national programs.

178

S. S. VIRMAM

AND IAN B. EDWARDS

documented. These factors may also influence the penetrance and expressivity of

Rf genes, and in some cases the interactions of cytoplasmic male-sterile lines with restoring genes in different environments have been highly unstable. This has been verified in the International Wheat Restorer Germplasm Screening Nursery (IWRGSN), where common sets of hybrids grown in several countries showed an unusually high level of instability and environmental sensitivity associated with fertility restoration (Jost, 1979, 1980, 1981). The genetic background of both male-sterile and restorer parents influenced seed set, but it was not possible to identify the environmental components that caused the wide variation between sites. Although several R lines in the IWRGSN have produced hybrids with relatively stable restoration and seed set, Lucken (1982) expressed doubt as to whether complete and normal fertility in F, hybrids has been achieved under all environmental conditions. In some instances, seed set may be complete, but the anthers may be slightly malformed at the tip of the spike. Based on field observations of restoration from Mexico to the northern United States, Wilson (1968a) classified environments as shallow sterile (Mexico), sterile (KansadOklahoma), and deep sterile (northern United StatesICanada). He suggested that restorers adequate in the shallow-sterile environment may be inadequate in the deep-sterile environment. Environmental factors that influence seed set and/or affect the stability of fertility restoration are listed in Table IX.

Table IX Environmental Factors That Af€ect the Stability of Fertility Restoration Environmental effects that may depress seed set Indirect effects Pot experiments Greenhouse environment Planting date Location

Short growing season Limited plant development Climatic effects Long photoperiod Long day and Low temperatures High temperatures Dry conditions High temperature and dry conditions

Reference

Miri et al. (1970) Rajki and Rajki (1966); McCuistion (1968); Schmidt er al. (1971) Kihara (1970) Monteagudo et al. (1967); Lucken and Maan (1967); Wilson (1968b); Schmidt er al. (1971); Jost (1979, 1980, 1981) Wilson (1968a) Wilson (1968a) Nanda and Chinoy (1945); Welsh and Klatt (1971); Johnson and Patterson (1973) Fukasawa (1953); Meletti (1961) Welsh and Klatt (1971); Johnson and Patterson (1973) Bingham (1966) Mihaljev (1972, 1976a,b)


179

It is apparent that temperature effects are complex and several interactions occur. Although higher temperatures prevail in the southern United States, flowering occurs earlier in the hard red winter wheats (April) as compared with the hard red spring wheats (July) in the northern United States. Heat stress can occur in the spring wheat region in July; consequently, spring wheats may encounter environmental stresses such as long photoperiods, cold temperatures (in some seasons), heat stress during pollen formation and anthesis (in some seasons), and dry conditions (where they prevail), which limit plant development. This has provided a challenge in restorer-line breeding and has necessitated the use of long-term R-line testers to monitor environmental effects each year when new testcross hybrids are evaluated in the field. D. INFLUENCEOF FEMALE GENETICBACKGROUND ON FERTILITY RESTORATION

Although no published data on this subject are available for rice, the widespread occurrence of intervarietal hybrid sterility attributable to gametic development (GD) nuclear genes randomly distributed among rice varieties (Oka, 1953, 1954, 1963) suggests that the genetic background of the female parent could influence the pollen and spikelet fertility of F, hybrids. In wheat, variation in the ease of restoration (EOR) of a number of genotypes in T. tirnopheevi cytoplasm has been reported by several workers (Lucken and Maan, 1967; Wilson, 1968a; Mihaljev, 1976a,b; Trupp, 1976; Jost, 1979, 1980, 1981). Wilson (1968a) interpreted this phenomenon as being the result of variations in sterility gene number and/or effectiveness among CMS lines, or of the presence of fertility genes that act in a complementary or additive fashion with restorer genes. Excess sterility genes could act as inhibitors of pollen fertility restoration in the F, generation. Trupp (1976) evaluated a range of soft red winter wheats for ease of restoration and found wide variability in the latter, a small portion of which could be accounted for by the presence of minor genes for restoration in the maintenance line. By using partially effective R-line testers as female parents, Trupp established that EOR is a genetically controlled trait for which prediction could be accomplished with a satisfactory level of accuracy. Jost (1980) reported that in the Second IWRGSN (1979), MS-Butte had the highest EOR (107.5% seed set) and MS-Tobari sib had the lowest (64.0% seed set). In the 1980 nurseries, the highest and lowest EOR levels were recorded in MS-Maris Hobbit (139.3%) and MS-Abe (47.1%) (Jost, 1981). It may be concluded that hybrid programs should place high emphasis on evaluating EOR among potential female inbreds in addition to searching for more effective Rf gene combinations. There is increasing evidence that stable restoration and consistent seed set under different environmental conditions results from

180

S. S. VIRMAM AND IAN B. EDWARDS

a balance between major Rf genes and modifer (fertility enhancing) genes that improve EOR.

VI. USE OF CHEMICAL POLLEN SUPPRESSANTS IN HYBRID PRODUCTION An alternative system for producing hybrids in rice and wheat is the use of chemical pollen suppressants (CPS). Male sterility induced by CPS is relatively convenient to use because there is no need to maintain it. Any variety can be sterilized and used as the female parent of a hybrid, and there is no need to identify a restorer line for making a commercial hybrid. Because there is no genetic segregation for male sterility in the F2 population, it is possible for farmers to plant heterotic F2 populations. An ideal chemical pollen suppressant should

1. Selectively induce only pollen sterility without affecting female fertility 2. Be systemic or sufficiently persistent to sterilize both early and late tillers 3. Have a reasonably broad “window” or target period of application to overcome the effects of adverse weather conditions and varible crop growth and permit treatment of large hectarages 4. Have minimum side effects on plant growth 5. Not induce only functional male sterility with viable pollen and nondehiscent anthers.

The proximity of the stamens and pistil within rice and wheat florets and the coordinated sequence of morphological development limit the ways to affect male and female fertility differentially. However, there exists an inherent differential sensitivity between male and female flowers, as is evidenced when environmental stresses such as high temperatures, low temperatures (frost, in wheat), and drought cause male but not female sterility. These observations and others, that several different nuclear and cytoplasmic genes can cause male sterility, and that there is both gametophytic and sporophytic control of male sterility in cereals, suggest several possible mechanisms to suppress pollen development without affecting the female (Lucken, 1982). A number of researchers have evaluated the use of CPS in wheat (Chopra et al., 1960; Porter and Weise, 1961; Rowell and Miller, 1971, 1974; Bennett and Hughes, 1972; St. Pierre and Trudel, 1972; Trupp, 1972; Borghi et al., 1973; Hughes et al., 1974; Jan et al., 1974; Johnson and Brown, 1976; Mihaljev, 1976a; Miller and Lucken, 1977; Dotlacil and Apltaverova, 1978; Jan and Rowell, 1981). The first studies (Chopra et al., 1960; Porter and Weise, 1961)


181

reported on the use of maleic hydrazide (250-1000 ppm); both of these groups obtained complete pollen sterility, but considerable female sterility and plant damage was also encountered. Foliar application of 2-chloroethyl phosphoric acid (ethaphon or ethrel) induced male sterility in wheat without significantly affecting female fertility (Law and Stoskopf, 1973; Hughes et al., 1974; Rowell and Miller, 1974). However, restricted spike emergence (phytotoxicity) and the need for precision in the time of application (narrow target period) have limited the commercial utilization of this chemical. Fairey and Stoskopf (1975) have since reported that soil application of granular ethephone overcame the phytotoxic effects associated with folial application, although high rates were required and average sterility was less than 100%. Ethephon has also been evaluated in rice (Perez et al., 1973;Cheng and Huang, 1978; Parmar et al., 1979a). Perez et al. (1973) applied ethephon (lo00 ppm) in three applications at 2-day intervals at early boot stage. Pollen sterility was 67% effective, but female sterility and phytotoxic effects were also encountered. Parmar et al. (1979a) used considerably higher dosage rates (a single application of 6000-8000 ppm at the boot stage or split applications of 4000-6000 ppm 1 week before and at boot stage). Pollen sterility was 94% effective but extremely low seed set resulted, and it was not clear whether insufficient pollen load or female sterility was the major cause of this low seed set. Cheng and Huang (1978) also reprted female sterility, and current research on rice suggests that ethephon is largely ineffective because application rates sufficiently high to induce pollen sterility also cause female sterility. The compound RH53 1 [sodium 1-(p-chloropheny1)-1,Zdihydro4,6-dimethyl-Zoxonicotinate; Rohm and Haas Co.] has been evaluated as a CPS in wheat and rice (Perez et al., 1973; Jan et al., 1974). RH531 gave a mean pollen sterility of 99% when applied at 100 ppm at the prebooting stage in rice (Perez et al., 1973), but it also caused complete ovule sterility. Jan et al. (1974) found that treatment with RH531 several days prior to meiosis at a rate of 2.0 kg/ha active ingredient (ai) gave maximum reduction in fertility with the spring wheats ‘Anza’ and ‘Yecora 70.’ Anza showed increased spike density and thickened floral tissues and was more sensitive to RH53 1 than was Yecora. Floret opening was not conducive to cross-pollination, and very low seed set resulted. Clearly, the effective use and versatility of CPS will depend on genotype-chemical, environment-chemical, and genotype-environment-chemical interactions. Where desirable heterotic combinations are identified, maximum seed production may be expected to result from the specific tailoring of application rates and timing for the cultivar and environment. Jan and Rowell (1981) compared high and low application rates of ethephon, RH532, and RH2956 on wheat tillers at various stages of development in Anza and Yecora 70. Treatment with RH2956

182


at the high application rate induced uniform and maximum male sterility in early and late tillers of both Anza and Yecora 70. Treatment with RH522 (high rate) was effective for Anza but not for Yecora 70. Ethephon proved the least effective, affecting only the late tillers treated at or before meiosis. Miller and Lucken (1977) evaluated four compounds (RH531, RH532, RH2956, and RH4667) for northern spring wheats. Several chemical companies in the United States are currently evaluating CPS compounds in wheat, and one company has commenced limited CPS production on several soft red winter and hard red winter hybrids. Chemical pollen suppressant rice hybrids have been released in China. Two chemicals, zinc methyl arsenate (CH,As,03Zn) and sodium methyl arsenate (CH,As,Na), have proved effective when sprayed 5-9 days before flowering at a concentration of 0.02% (Anonymous, 1978a); the sodium salt is superior. Three hybrids (Gang-hue-da-tian, Gua hua 2, and Bei-hua gu 67) were released for commercial production and have performed well. The costs of preparing these CPS compounds are also low (Shen, 1980). Other Chinese institutionshave evaluated CPS compounds. The Hunan Academy of Agricultural Sciences (unpublished) has reported finding zinc and sodium methly arsenate (200 ppm), calcium sulfamate, and fluoroacetamide (15OO-2OOO ppm) to be effective male sterilants. However, the general consensus apparently indicates a preference for CMS systems over CPS systems in hybrid production. The advantages of using a CPS method for hybrid seed production may be summarized. (1) Breeding procedures are simplified by eliminating the need for cytoplasms and Rf genes; (2) costly and difficult operations of male-sterile increase are avoided; (3) genotypes with poor anther extrusion can still be used as female parents; (4) the time lag in converting promising new genotypes into CMS (female) inbreds is avoided; and (5) evaluation of lines for general and specific combining ability is simpler. The principal disadvantages of the CPS method are; (1) chemical dosage levels sufficient to ensure male sterility often induce some female sterility and reduced seed set as compared with CMS hybrid production; (2) extended rainfall and prolonged winds can prevent field application of the chemical at the optimum time; (3) genotype-environment-chemical interactions must be evaluated; and (4) male fertility or selfing in hybrid fields can result in seed that does not conform to seed law specifications for a hybrid. Clearly, both systems have their advantages and disadvantages, but they can complement one another in advancing the development of hybrid rice and wheat. Chemical pollen suppressants can aid CMS programs in evaluating the combining ability of new lines prior to male-sterile conversion. Conversely, CMS lines can be used to evaluate the performance of a CPS. By comparing seed sets on equivalent (unsprayed) CMS lines and normal lines treated with a CPS, a measure of the effect of the chemical on female fertility can be determined.


183

VII. FACTORS AFFECTING CROSS-FERTILIZATION Floral structure, anthesis, and anther dehiscence patterns in rice (Van Breda de Haan, 1913; Hector, 1913; Rodrigo, 1925) and wheat (Percival, 1921; Leighty and Sando, 1924) make these crops strictly self pollinating. The extent of natural outcrossing in cultivated varieties varies from 0 to 6.8%in rice (see Sahadevan and Namboodiri, 1963) and from 0 to 4%in wheat (Heyne and Smith, 1967). In wild rice forms of 0. perennis Moench, 16.5-100% outcrossing has been observed (Sakai and Narise, 1959; Oka and Morishima, 1967). Male-sterile plants of cultivated rice have shown outcrossing of 0-44% (Stansel and Craigmiles, 1966; Athwal and Virmani, 1972; Carnahan et al., 1972) and 20-92% (Trees, 1975). Seed set as much as 75%on male-sterile wheats is a common occurrence. Variability in extent of natural outcrossing in these crops can be attributed to variations in flowering behavior, floral characteristics of varieties or species, and variations in environmental factors. An analysis of the factors affecting cross fertilization in the two groups should be useful. A. FLOWERING BEHAVIOR

Early descriptions of the flowering process were provided by Rodrigo (1925) in rice, and by Percival (1921) and Leighty and Sando (1924) in wheat. Rice researchers have addressed the subject of flowering behavior on the basis of bloom initiation, blooming duration of the panicle, and the angle and duration of floret opening, and wheat researchers have examined factors affecting flower opening, the duration of stigma receptivity, and pollen viability. Flowers of rice and wheat reach their peak opening between 9:00 and 11:30 AM on a sunny day. In contrast to rice, a second flush of flower opening is common around 3:OO to 5:OO PM in wheat, although Parmar et al. (1979~) identified two rice cultivars in India (IAFU 6193-B and IARI 7216) which also showed a second flowering flush between 5:30 and 6:OO PM. Work at the IRRI has also shown species differences in the time at which peak flowering takes place; approximately 60% of the spikelets of 0. glaberrimu flowered at 9:OOAM, whereas less than 5% of those in 0. sutivu reached anthesis at this time (WU, 1978). Flowers of both crops have two lodicules located at the base of the ovary, and their primary function is to open the floret at anthesis. As pollination approaches, the lodicules increase in turgidity, pushing apart the palea and lemma (Kadam, 1933), and consequently they acquire a significance in hybrid breeding because of their role in aiding anther extrusion and pollen interception. Studies on rice by Parmar et al. (1979~)suggested that the inner floral organs, such as the filaments and stigma, exert slight pressure on the linear joints of the palea and lemma as they become turgid whereas the lodicules exert leverage pressure at the

184


basal joint of the lemma and palea. Simultaneously the anthers protrude and thereby help increase the angle of opening, which varies among varieties from 25" in long, slender spikelets to 35" in short, coarse spikelets. In wheat, the lodicules are dependent on adequate moisture and cool temperatures for proper functioning. McNeal and Ziegler (1975) measured lodicules taken from spring wheat florets at daily intervals after heading and found an increase in size until the sixth day after complete emergence from the boot. Lodicules from both emasculated and male-sterile spikes were significantlyheavier, wider, and thicker than those from the spikes of fertile cultivars. Significant varietal differences in lodicule size were also noted. Varietal differences in the duration of flower opening from 28 to 93 min have been reported in rice (Virmani and Athwal, 1973; Parmar et al., 1979~);floret opening in CMS rice plants ranged from 105 to 280 min (IRRI,1983). Parmar et al. (1979~)also noted a longer duration of flower opening in late-maturing varieties (50-70 min) compared with early-maturing varieties (28-35 min). Delay or failure in pollination was found to prolong flowering (Grist, 1953) and, consequently CMS plants have a longer duration of floret opening than fertile plants (Saran et al.. 1971). A mean temperature of 2530°C and a relative humidity of 7 5 4 0 % was found to provide optimum conditions for anthesis in male-sterile, maintainer, and restorer lines, with the flowering period lasting 6-7 days (Hunan Academy of Agricultural Sciences, personal communication). Higher or lower temperatures affected synchronization;both low (18-26°C) and high (27-35°C) temperatures affected a number of agronomic and floral characters that influence outcrossing. High temperatures delayed anthesis and slightly increased the period of floret opening (IRRI, 1983). Genetic studies of traits affecting flowering behavior in rice have been limited, Estimates of heritability and the genetic coefficient of variation for floweropening duration were found to be low (Virmani and Athwal, 1973). Nagao (1951) reported nonclosure of glumes after anthesis to be a simple, recessive trait, whereas others have shown that temperature, light, and humidity markedly influence blooming behavior (Chu et al., 1970; IRRI, 1983). In wheat, flowering behavior has been examined from the perspective of the duration of stigma receptivity rather than from that of floret opening per se. Prolonged stigma receptivity is considered essential, because exact synchronization of anthesis in the male with flowering in the female is often difficult to accomplish in seed production blocks. Stigma receptivity has generally been measured by seed set on the male sterile, and most studies suggest that environmental effects are of greater significance than genetic effects (Bardier, 1960; Rajki and Rajki, 1966; Johnson and Schmidt, 1968; Zeven, 1968; Khan et al., 1973). Under field conditions, fluctuations in temperature and relative humidity each day, and from day to day, generally encompass both optimum and adverse conditions. The period of stigma receptivity is reduced as a direct result of these


185

fluctuations, and studies have shown a range from as few as 2 days (Khan et af., 1973) to as many as 13 days (Rajki and Rajki, 1966) in wheat, and from 2 to 6 days in rice (Virmani and Tan, 1982; Hunan Academy of Agricultural Sciences, personal communication). Stigma receptivity has also been shown to decrease as the elapsed period flowering and actual pollination increases (Imrie, 1966). Johnson et af. (1967) suggested that parental lines should flower on the same day to maximize seed set and reduce floral disease, but others have suggested that the pollinator should flower 1-4 days later than the male sterile. A 2-year study of seed set at two North Dakota locations by Miller and Lucken (1976) suggested that seed set is maximized by planting the maintainer a projected 44°C growingdegree days after the male sterile. This difference indicated that maximum anthesis in the maintainer occurred approximately 4 days ahead of optimum floret opening in the male sterile. Because seed set constitutes an indirect measure of stigma receptivity, the question of pollen viability cannot be excluded. In growth-chamber studies, Watkins and Curtis (1967) found that wheat pollen viability decreased as the temperature increased, for all relative humidities tested (20, 50, and 80% RH), and also as the relative humidity decreased, for all temperatures tested (65, 75, 85, and 95°F). Field experiments in Colorado c o n f i i e d the growth-chamber findings, and similar results were reported by Zeven (1968) in Holland and by Mihaljev (1976a,b) in Yugoslavia.

B. FLORALSTRUCTURE

Stigma size, style length, stigma exsertion, stigma receptivity, anther size, filament size, and pollen number are important floral characters that influence outcrossing in rice and wheat. Significant varietal differences in these traits have been observed in cultivated and wild rices (Copeland, 1924; Sampath, 1962; Oka and Morishima, 1967; Virmani and Athwal, 1973; Lyakhovkin and Singil’Din, 1975; Parmar et al., 1979b; Virmani ef al., 1980a; IRRI, 1983) and in wheat (Cahn, 1925; Kherde et al., 1967; Atashi-Rang, 1970; De Vries, 1974a;Jost and Milohnic, 1976a,b). The range of variation for these traits and the source of desirable floral traits as reported in the literature are given in Tables X and XI. Jost and Milohnic (1976b) tested 20 Rf sources in wheat and found the highest pollen count in the cultivar Primepi. There was a highly significant positive correlation between restoration ability and number of pollen grains per anther in the 3 years of testing and for the three female testers used. They suggested that anther length and numberofpollen grains per anther might be used to identify plants in segregating progenies that possess Rf genes, thereby reducing the number of testcrosses made each year. However, Jost and Glatki-Jost examined parents and progeny from the cross Primepi X R1 (Nebraska) at two sites. The F7


186

Table X Range of Variation and Varietal Sources P d p Maximum Value of Floral Traits Muenring Outcrossing in Rice as Reported in the Literature Trait Stigma length (mm) Stigma breadth (mm) Style length (mm)

Extent of stigma exsertion (%) Anther length (mm) Anther breadth (mm) Filament length (mm) Pollen nurnberhther

Varietal source possessing maximum value

Range of variation Cultivated, 0.2-2.6 Wild, 0.3-5.0 Cultivated, 0.2-1.0 Wild, 0.4-1.3 Cultivated, 0.6-3.2 Wild, I .2-2.3 Cultivated, 0.2-87.8 Wild, 0-100 Cultivated, 0.9-3.7 Wild, 1.6-5.4 Cultivated, 0.2-0.9 Wild, 0.2-1.0 Cultivated, 0-23 Wild, 0-14 Cultivated, 463-3833

IARI6637, IAR17332, IARI10979A" Genetic stock 6209-36 Not reporteda Genetic stock 6209-3 IARI7332, IARI10754, IARI10871, IARI10979A, IARIl 1205" Oryza sativa f. spontaneaa BPI76-1 (n.s.)= Genetic stock 6209-36 IARI5819, lAR15823a Oryza australiensis" IARI5819, IARI5823a Oryza sufivu f. spontanea" Not known Not known IR13526-41- 1-2'

"Parmar et al. (1979b). bIRRl (1963). c V h a n i and Athwal(I973). "Hunan Academy of Agricultural Sciences (personal communication). (unpublished).

lines and parents had significantly shorter anthers at Szeged than at Zagreb, but the mean number of pollen grains per anther for all lines was essentially the same. Substantial line-site interactions were obtained, and the authors questioned the validity of using anther dimensions as a basis for selecting genotypes containing Rf genes from segregating progenies. Inheritance studies (Virmani and Athwal, 1974) for floral traits such as anther length, stigma length, and stigma exsertion in rice indicated that these traits were governed by polygenes. Huang and Huang (1978) reported that stigma exsertion was dominant, partially dominant, or recessive depending on the cross. They also found a negative correlation between stigma exsertion and pollen fertility. Both additive and nonadditive effects were important in the inheritance of floral traits (Virmani and Athwal, 1974). The prevalence of duplicate type epistasis was considered a barrier to fixing these traits at higher levels of manifestation through conventional breeding. Therefore, recurrent selection in biparental progenies was proposed by Virmani and Athwal(l974) to improve these characters,

187

HYBRID RICE AND WHEAT Table XI

Range of Variation in Anther Size and Number of Pollen Grains in Wheat as Reported in the Literature Number of cultivars tested

Range of variation

Reference

Anther length (mm) 30 5 9 20 45 26

3.45-5.09 2.99-3.84 3.22-4.29 3.29-5.07 3.19-4.41 3.67-5.25

Atashi-Rang (1970) De Vries (1974a,b) Fisher (1977) Jost and Milohnic (1976b) Kherde er al. (1967) Milohnic and Jost (1970)

Anther width (mm) 20 45

0.29-0.97 0.57-1.07

Jost and Milohnic (1976b) Kherde er al. (1967)

Number of pollen grainslanther 22 4 5 4 8 20 1

581-2153 856-1380 1746-2856 2236-3022 2867-3867 2188-3983 8100

Beri and Anand (1971) Cahn (1925)

De Vries (1974a) D’Souza (1 970) Joppa et al. (1968) Jost and Milohnic (1976b) Yeung and Larter (1972)

because they may have to be transferred from wild rice in which an undesirable linkage relationship exists among the floral traits and agronomic characteristics. Attempts to transfer the long stigma (5 mm) trait of the genetic stock designated as 6209-3 (IRRI,1983) into the genetic background of selected maintainer lines, from which it would be transferred to cytoplasmic male-sterile lines in rice, are being made at the IFUU and the Sichuan Academy of Agricultural Sciences, China (IRRI, 1983). Atashi-Rang and Lucken (1978) found significant differences in both GCA and SCA effects of parents for anther length, anther extrusion, and glume tenacity in wheat. The GCA-SCA variance component ratio was approximately 5:l for anther length, 1:1 for anther extrusion, and 8:l for glume tenacity. Narrow-sense heritabilities on a per spike basis were 0.61 for anther length, 0.19 for anther extrusion, and 0.54 for glume tenacity. The authors suggested that selection for altered anther length and glume tenacity should be effective, and that selection in stress environments could provide a means of improving and stabilizing anther extrusion data.

188

S. S. VIRMAIW AND IAN B. EDWARDS

Ghiasi and Lucken (1982b) reported that visual mass selection for anther extrusion in F3 and F5 bulks of spring wheat R/B, R/R, and A/R crosses effectively increased the mean anther extrusion of successive generations. This increase was 19% per cycle in the F3 bulk mean. Selection for anther extrusion did not adversely affect other agronomic and quality traits. Komaki and Tsunewaki (1981) found that anther length was correlated with flowering data among parent lines as well as the F, progeny of 13 cross combinations. Anther length and flowering date fitted a curvilinear regression wherein reduction in anther length in both early- and late-flowering cultivars was attributed to poorer environmental conditions for floral development. This phenomenon was not found in the F2, and it was concluded that the genes controlling flowering date differ from those controlling anther length. The heritability estimate for anther length was 0.65. The pollen load in the air at a given time is a function of the amount of pollen produced per anther, the amount of anther extrusion, and the number of anthers per unit area. Joppa ef al. (1968) investigatedthe relative pollen-shedding ability of 11 hard red spring wheats and 3 durum varieties. Percentage of anther extrusion had the largest direct effect on pollen shedding. The expected pollen load in the air per 10 mm2 was compared with the actual number obtained on slides situated within each plot. The calculated and observed numbers of pollen grains were of the same order of magnitude, with a correlation coefficient (r) of 0.91. Their results indicated that the relative pollen-shedding capacity of a variety can be predicted from a knowledge of the number of pollen grains per anther, percentage of anther extrusion, and fertile florets per plot. These studies indicate that anther length and extrusion score are the simplest measurements of pollen-shedding capacity for R-line breeding. After the confirmation of restoration ability based on testcross fertility, the yield evaluation of new R lines will have dual significance. The economics of hybrid production will be improved, and high-yielding lines will have more fertile florets per unit area and an increased pollen load. C. E

m

OF

POLLINATOR DISTANCE

Provided that an adequate pollen load is present in the field, and that there is correct synchronization of flowering between male and female parents, seed set on male-sterile plants is also influenced by the distances that pollen can travel and remain viable. De Vries (1971) reviewed the environmental factors influencing pollen viability in wheat. The factors investigated by rice and wheat researchers have included both pollinator distance and wind direction. Rodrigo (1925) observed that in cultivated rice varieties pollen grains are carried by the wind, in the same plane as the flower itself, as far as 1.5-2.0 m


189

from the source depending on wind velocity. Virmani er al. (1980b) found that seed set from cross-pollination was higher on plants located 30 cm from the pollen source than on those located 30-110 cm from the source. However, differences were less pronounced when the CMS line was located downwind from the pollinator, and it was concluded that satisfactory seed set could be obtained in hybrid seed production plots by growing five rows of the CMS line (1-1.25 m wide) alternately with one row of the restorer (pollinator) line. Similar malelfemale ratios are used in China (Lin and Yuan, 1980). Pollen dispersal trials conducted in China (Hunan Academy of Agricultural Sciences, personal communication) have shown that in hybrid seed production plots with isolation distances of 10, 20, 30, and 40 m, contamination resulting from foreign pollen was 5.2, 1.0, 0.2, and O I , respectively. Consequently, a minimum isolation distance of 40 m is set for hybrid seed production in China. In wheat, a number of studies on the effects of pollinator distance and wind direction on seed set have been conducted (Kihara and Tsunewaki, 1964; Holland and Roberts, 1966; Porter er d.,1966; Bitzer and Patterson, 1967; Popov and Gotsov, 1968; Rajki and Rajki, 1968; Keydel, 1969; Tsunewaki, 1969; Ezrokhin and Nettevich, 1970; Anand and Beri, 1971; Stoskopf and Rai, 1972; De Vries, 1974b; Miller and Lucken, 1976). However, a major limitation of several of these studies is that they were conducted in very small plots where the basic pollen load was so small, and the seed set was so low, that they provide little guidance for commercial field production of hybrid seed. In Kansas, Holland and Roberts (1966) showed that percentage seed set decreased from 57.6 to 15.3% as distance of the female increased from 0.3 to 29.0 m from the pollen source. The importance of wind direction on percentage seed set was demonstrated by De Vries (1974b) in Holland and by Bitzer and Patterson (1967) in Indiana, where wind direction affected seed set in their experiments as much as range in pollinator distance (1.5-7.6 m). Evidence to date indicates that the isolation requirements for hybrid wheat seed production (and particularly malesterile increase) are greater than those indicated for rice. Jensen (1968) showed that although 90% of the wheat pollen shed remained within 6 m of its source, pollen could travel as far as 60 m. Researchers at Pioneer Hi-Bred International, Inc. (Kansas) conducted pollen slide studies and found that viable pollen could be obtained as far as lo00 m from a very large pollen source (C. F. Hayward, personal communication). In another Kansas study, Arp (1967) measured onethird more pollen at 12.2 m from the pollen source than at 1.5 m, which again confirms the potential buoyancy of air-borne wheat pollen. It appears that the size of the pollinator strip and its direction relative to the female will have a considerable effect on seed set, effects of pollinator distance, and applicability of the data to commercial seed production situations. The buoyancy of the pollen grain and the distances that viable pollen can travel will


190

influence the isolation requirements for male-sterile increase and hybrid production blocks.

D. EFFEC~ OF

PLANT HEIGHTAND OTHERMORPHOLOGICAL TRAITS

Because wind velocity will affect the rate at which pollen descends following dispersal from an extruded anther, a number of wheat researchers have suggested that there would be a positive effect on seed set if the male-sterile parent were shorter than the pollen donor (Lelley, 1966; Lein, 1967; Zeven, 1969; De Vries, 1972; Fisher, 1977). De Vries (1972) showed that the pollen concentration 20 cm above the spike level was considerably less than at the spike level, which in turn was less than that measured 20 cm below the spike level. Lelley (1966) showed that wheat pollen under calm conditions falls from a height of 1 m at a rate of 60 cm/sec. Fisher (1977) found a significant decrease in seed set when a tall CMS line was matched with a semidwarf pollinator. These studies suggest the need for a taller pollinator. However, it is possible that insufficient attention was paid to the pollen-shedding ability of short-statured wheats in some of these studies. Several workers have reported lower anther extrusion among semidwarf lines compared with conventional lines, and this factor could be of greater significance than height effects per se. Semidwarf restorer lines with good anther extrusion are-known to produce excellent seed set (more than 75%) on taller females (I. B. Edwards, personal observation). Chinese researchers have used rice restorer lines that are 10-20 cm taller than the CMS line. Use of a “recessive tall” gene in rice (Rutger and Camahan, 1981) should also enable the use of even taller R lines with semidwarf CMS lines to develop semidwarf hybrids. Other traits that might hinder cross-pollination in rice include long and broad flag leaves and incomplete panicle exsertion. Consequently, selection of CMS and restorer lines with short, narrow flag leaves and good panicle exsertion would be advantageous. Parental lines possessing high tillering capacity and/or larger numbers of spikelets per panicle would also enhance cross-pollination potential. Although Sections VI1,A-D suggest that much has been learned about the external and internal floral features that control access to wind-borne pollen, we have yet to establish a floral trait in the female that is as important for crossfertilization as is anther extrusion in the pollinator. That some females set more seed through cross-pollination than others becomes evident during male-sterile increases. The reasons for these differences are not clearly understood at present, but there is significant economic significance to improving our understanding of those floral traits in the female that enhance cross-fertilization and in learning how to manipulate them in breeding programs.


191

VIII. SEED PRODUCTION Hybrid seed production using the cytoplasmic-genetic male sterility system involves three steps: (1) multiplication of CMS (A) lines; (2) multiplication of maintainer (B) and restorer (R) lines; and (3) production of hybrid seed (A X R). Multiplication of B and R lines is done in the same manner as with conventional varieties; however, multiplication of A line and production of hybrid seed require different methods. A. MULTIPLICATION OF CYTOPLASMIC MALESTWLE AND MAINTAINER LINES

Procedures for CMS and maintainer line multiplication in rice have been described by Chinese researchers, and the methods are highly labor intensive (Ye, 1980). Four rows of A line are alternated with two rows of B line (2:l ratio), and to improve synchronization of flowering a split seeding of the maintainer is carried out at 4 and 8 days after seeding the male-sterile line. Both the A and the B lines are transplanted on the same day, and plants from the early and late B-line planting are alternated in the rows. Cross-pollination is improved by planting the rows perpendicular to the prevailing wind direction, and flag leaves of both the A and the B lines are clipped at the boot stage to aid pollen movement. This procedure requires 25 man-days/ha and has been found to increase seed yields by 42.9% compared with unclipped checks (Lin and Yuan, 1980). One or two applications of 20 ppm gibberellic acid are applied to the A and B lines after clipping to improve panicle exsertion. Pollination is improved on calm days by manual techniques designed to agitate the pollen parents. Rope pulling (Fig. 1) and shaking of the pollen parent with a pole (Fig. 2) require 5 and 15 man-days/ha, respectively. Isolation in time (21 days) or distance (100 m) from other rice crops prevents contamination by foreign pollen. Using these techniques, the average seed yields per production hectare in China are 400-900 kg of A line and 1000-1500 kg of B line. The two techniques commonly used to evaluate the most economical production scheme in wheat have involved varying the ratio of male sterile to pollinator and varying the basic drill strip width. Seed-increase methods were reviewed by Zeven (1974) and by Miller and Lucken (1976). MS/pollinator ratios (Wilson, 1968a; Schmidt et al., 1971; Rogers and Lucken, 1973; De Vries, 1974b; Miller et al., 1974; Miller and Lucken, 1976) evaluated have been 1:2, 1:1, 2:1, and 3:l. Several conclusions can be drawn from these studies. (I) The 2:l MS/pollinator ratio has usually proved the most effective when yields are expressed on a production unit basis; (2) increasing drill strip widths has resulted in decreased

192


FIG. 1. Supplementary pollination using rope-pulling method as practiced in China.

FIG. 2. Supplementarypollination using pole as practiced in China.

HYBRID RICE A N D WHEAT

193

seed set toward the center of the strip, the level of reduction being affected by genotype and environment; and (3) any environmental conditions or production practices that enhance normal wheat yields will positively affect male-sterile production yields. The effect of the alien cytoplasm on CMS wheat lines usually has been delay of seedling emergence and flowering compared with the maintainer line. Miller and Lucken (1976) evaluated the growing-degree day technique for timing planting dates between the male-sterile and maintainer lines and found seed set to be maximized by planting the maintainer a projected 44°C growing-degree days after the male-sterile line. B. HYBRIDSEEDPRODUCTION

Similar techniques are used in hybrid seed production. The selected R line serves as the pollen donor parent for the male-sterile A line, and seed harvested from the A line constitutes the commercial hybrid seed. Seed harvested from the R line is the result of self-fertilization and may be used again in hybrid production, or the surplus can be sold as commercial grain. The effects of strip width and parent ratio on pollen load and distribution are basic considerations in hybrid production. Increasing the ratio of female to pollinator decreases pollen production per unit area and also increases the distance over which pollen must travel. A change to a narrower drill strip width without altering the ratio alters only the distance pollen must travel and may result in a more homogeneous pollen distribution. Hybrid rice seed production in China involves six to eight rows of A line and one to two rows of R line. These parental lines are seeded and transplanted on different dates, depending on their growth duration, to synchronize their flowering. The R line is seeded on three or four different dates, but seedlings of different ages are transplanted the same day in the field to increase the duration of pollen availability at flowering. The other techniques of seed production, such as direction of row planting, flag leaf cutting, application of gibberellin, supplementary pollination, and isolation distance, are the same as described in the preceding section. Using these techniques, seed yields of 0.45-1.5 tonslha have been obtained in China (Lin and Yuan, 1980). In one instance, seed set on a CMS line in a hybrid seed production plot was 74% (Hunan Provincial Rice Research Institute, 1977). Average seed yield in hybrid seed production plots has increased steadily up to 2.5 tons/ha as seed producers have become more experienced. In hybrid seed production plots at the IRRI, 18-34% natural outcrossing on three cytosterile lines has been observed, resulting in a hybrid seed yield of 0.66-1.68 tons/ha (S. S. Virmani, unpublished).

194


Miller and Lucken (1976)used the restorer line R5 and four females in hybrid wheat production blocks at 1 :1, 2:1, and 3:1 MS/pollinator ratios in nine environments. Mean hybrid seed yields were 1.37,1.00,and 0.84 tons/ha, respectively, but the yields were 0.69,0.67,and 0.63 tons/ha when expressed on a production hectare basis. Narrowing the basic drill strip width from 3.1 to 1.5 m at the 1:l and 2:l ratios did not increase seed yields. Miller and Lucken concluded that adequate hybrid seed production could be accomplished with the existing drill sections (2.1-3.7 m) used on North Dakota farms. Experience gained from hybrid seed production in a number of environments has demonstrated the importance of using several sites to spread the risks and ensure a more consistent seed supply. It has now been established that high seed set can be obtained, given the right parental combinations and favorable conditions. Lucken (1982)cited data from hard red winter wheat hybrid production (Dwight Glenn, DeKalb Agresearch, Inc.) in which female (hybrid) seed yields ranged from 90 to 101% in three irrigation environments. A 3:l ratio was used, with the female strip width ranging from 21.8 to 18.9 m. As an alternative to the drill strip method of hybrid production, consideration has been given to blend production (Caroline, 1977;Mann, 1977). This method involves seeding a mixture of male-sterile and restorer seed, and Carolina (1 977) tested the effects of varying the blend proportions (70,80,and 90%) of two CMS lines (Minn 11-54-30and DeKalb 3047) in hybrid production blocks with three restorer lines (R101,R106,and R5). The percentage of hybrid seed was 81, 74, and 63% when entire blocks of 90,80, and 70% female blend were harvested. Hybrid seeds yields were significantly reduced at the 90% blend compared with the 70 and 80% blends because of inadequate pollen load. Mann (1977)investigated the possibility of using different seed sizes in the pollinator and female parents and mechanically separating the female (hybrid) seed from the R-line component in a mixture or blend. Separation was achieved with a Carter Disk Separator (Carter-Day Co., Minneapolis), provided the kernel size ranges within each component did not overlap. In practice, such a seed size differential would severely restrict the number of parental combinations that could be used. The extent of natural outcrossing on cytosterile lines in rice and wheat can be further increased by selection and/or breeding of CMS, maintainer, and restorer lines that possess desirable floral as well as agronomic characters influencing outcrossing. A recurrent selection program involving genetic male sterility (Athwal and Borlaug, 1967) should help reconstitute parental lines that possess an improved level of outcrossing. C. DISEASE PROBLEMS ASSOCIATED WITH SEED PRODUCTION

Ergot (Claviceps purpureu) is a potential problem in the two phases of hybrid wheat production that utilize a male-sterile parent, and infection can be particu-


195

larly severe in the northern spring wheat region of the United States. Most cultivars have morphological resistance, an exclusion mechanism resulting from the short time the florets are open during flowering. This form of resistance is undesirable in hybrid production where open flowering is essential in female parents, and a physiological or biochemical resistance is necessary. Schmidt et al. (197 1) attributed the increase in ergot infection to increasing divergence of blooming of the male-sterile and pollinator lines. Although others have observed this phenomenon, genotype differences are present, and Schmidt (1976) found partial resistance to ergot in the spring wheats ‘Chris’ and ‘Cajeme 71,’ intermediate susceptibility in ‘Waldron,’ ‘Bonanza,’ and ‘Kenya Farmer,’ and high susceptibility in ‘Nadadores 63. ’ Another disease, more common in hybrid wheat, is loose smut (Ustilago tritici), which is also partly the consequence of a more open flowering habit in the parents. Systemic chemicals (oxathiins) are available for loose smut control, and seed treatment is essential in some regions. Observations on early-generation breeding lines by I. B. Edwards (unpublished) would suggest that restorer lines are significantly more susceptible to loose smut than are conventional B lines. Whether this is a function of genetic background, nucleocytoplasmicinteraction with T. timopheevi, open-flowering habit, or a combination of these factors has not been clearly established. No specific disease problems have been reported in hybrid rice seed production plots in China. However, under tropical conditions where high levels of disease inoculum are present, the potential for disease problems remains high. D. SEEDQUALITY IN HYBRIDSAND THEIR INBRED LINES

A number of lines of common wheat, when converted into CMS (T. timupheevi) lines, have been found to produce shriveled kernels (Johnson el al., 1967; Rai et al., 1970; Schmidt et al., 1971). These shriveled kernels were extremely prone to preharvest sprouting (Rai and Stoskopf, 1974; Jonsson, 1976; Rai, 1979) and resulted in a serious loss of seed viability under unfavorable environmental conditions. The shriveling appears to result from a nucleocytoplasmic interaction and is associated with higher a-amylase activities (Rai, 1979). The degree of kernel shriveling was influenced by the genetic background of the B lines, the level of seed set, and environmental conditions. Unpublished data by I. B. Edwards would support the suggestion that a nucleocytoplasmic interaction causes shriveling. Certain spring wheat R lines in T. timopheevi cytoplasm have shown significant kernel shriveling and test-weight reduction in the absence of environmental stress, but other R lines have produced high test weights in the same experiment. In those affected R lines where reciprocal crosses were made into normal (T. aestivum) cytoplasm, a significant im-

196


provement in kernel pluinpness resulted. Comparativeinformation on seed quality of hybrid rice is not presently available.

IX. QUALITY OF HYBRIDS Because most rice is consumed as whole grain, there is conwm that the codring quality of commercial F, hybrids (F, grains) would be impaired because of genetic segregation for chemical characteristics of the grain. Results from China, however, indicate that there is no apparent adverse effect on the cooking quality of the F, hybrids. The table quality and swelling ratio of hybrids were intermediate between those of their parents (Lin and Yuan, 1980). The protein content in hybrid rice grains was reported as 9-1 1% (Lin and Yuan, 1980) and 10-12% (Anonymous, 1978b), which is 1-4% higher than that of conventionally bred variaties. Mohamed Sayed (1975) found that protein bodies in an intervarietal rice hybrid (‘Cherumodan’/ ‘ADT 27’) were uniformly distributed from the periphery to the interior of the grain. Protein was more concentrated in the peripheral layers of the parents, and milling and polishing caused a greater protein reduction in the parents than in the hybrid. Dzuba and Kolesnikov (1976) found additional protein complexes in F, hybrids that were not present in the parents. Heterotic effects for amylose content and alkali spreading value (Singh et al., 1977) and for protein content (Chao, 1972; Singh et al., 1977) have been reported. In general, these traits showed a nonsignificant association among themselves as well as with yield, indicating that simultaneous improvement of yield, protein, and cooking quality should be possible in F, hybrids (Singh et al., 1977). Five high-yielding rice hybrids and their parents were evaluated for physicochemical and cooked-rice characteristics at the IRRI in the Philippines. Hybrids were generally intermediate in characteristics and occasionally approached or exceeded the high parent in certain quality traits. Therefore, by complementation of quality traits in the parents, it should be possible to produce F, hybrids of desirable grain quality. The basic definition of quality in wheat will vary with the market class. Discussion of quality in this article will be confined to those traits that affect the milling and bread-making properties of hybrids in the hard red winter and spring wheat classes. Two factors of importance in maintaining quality in hybrid combination are a knowledge of the heritabilities and mode of gene action among quality traits and a knowledge of the effects of T. timophemi and other cytoplasmic sources on bread-making quality. Larrea (1966) evaluated parents and their F, and F, progeny from an eightparent diallel for nine quality traits in spring wheat. Parental performance pre-


197

dicted F, performance for milling and most baking characteristics, and only in a few cases did the hybrid exceed the best parent. The high narrow-sense heritabilities (close to 1) for test weight, loaf volume, mixogram pattern, and protein revealed the close relationship between the F, and mid-parent values for these traits. The heritability for flour yield was also high (0.72). Most GCA variances were significant, indicating the importance of additive gene action. Only crumb color had a significant SCA variance, and this trait had a low heritability. Larrea suggested that the advantage of additive gene action for quality traits was the ability to select parents and predict hybrid quality. However, the parents should be of good or at least of fair quality to be useful, and this could restrict the use of certain diverse germplasms. In contrast, Shebeski (1966) pointed out that parents of high quality may not necessarily confer these traits to their hybrids. ‘Pembina,’ a good quality spring wheat, had excellent GCA for all quality traits measured, while ‘Canthatch’ (another high quality wheat) proved a very poor combiner among the genetically diverse group of wheats examined. Many of the initial reports on the effects of T. timopheevi cytoplasm on the bread-making quality of hybrids were published in 1966 (Gilles and Sibbitt, 1966; Larrea, 1966; Rooney et al., 1966; Shebeski, 1966; Wilson and Villegas, 1966). Wilson and Villegas (1966) reported that T. timopheevi had little or no adverse effect on dough-mixing and sedimentation properties, and the quality of F, hybrids was intermediate between the quality of the parents. Rooney et al. (1969) crossed the R line ‘BA 130’ to the male-sterile (T. timopheevi) and maintainer (T. aestivurn) lines of the three winter wheats, and found that cytoplasm had no effect on several quality traits. Only in the A lines does T. timopheevi appear to have an adverse effect on quality. Schmidt et al. (1971) found dough-mixing time, mixing tolerance, and loaf volume of CMS ‘Gage’ to be impaired compared with normal Gage. However, hybrids of CMS Gage and Bison, pollinated by NBR 3547, had high protein, intermediate mixing properties, and good loaf volume. Thus, NBR 3457 may have contained genes (possibly linked with the Rf genes) that overcame the adverse effects of T. timopheevi cytoplasm on the quality of Gage. Aside from T. timopheevi studies, comparatively little information is available on the quality of fertile alloplasmic wheats with alien cytoplasms. Busch and Maan (1978) substituted the genomes of the hard red spring ( H R S ) wheats Chris and Selkirk into the cytoplasms of T. macha, T. dicoccoides, and A . squarrosa. Agronomic traits were not adversely affected, but the limited quality data indicated that loaf volume was the trait most adversely affected, followed by slightly reduced grain protein, compared with normal (T. aestivurn) cytoplasm. In contrast, Kofoid and Maan (1982) reported on a study of 14 alloplasmic lines, each with the nuclear genome of Selkirk and cytoplasm from a species of Triticum, Aegilops, or Haynuldia grown at three locations. Lines with a cytoplasm from a species of Aegilops had more protein and better loaf volume, and in general the

198


alloplasmic lines exceeded the euplasmic control for these traits. Sasaki et ul. (1979) found that four cytoplasms (T. fimopheevi, A . cuudutu, Aegilops umbellukztu, and Secule cereule) enhanced grain protein compared with the euplasmic control. However, they attributed these differences to the effects of the cytoplasm on yield-component characters. In summary, the need for using alternative cytoplasms in hybrid production has already been emphasized, and these preliminary quality studies may assume a greater significance in the future.

X. ECONOMIC CONSIDERATIONS The adoption of hybrid breeding technology in a sexually propagated crop plant implied that the farmer would be buying the F, seed every crop season, because he would not be able to use his harvest as seed to raise the next crop. Acceptance of hybrid seeds by farmers usually depends on the relative cost of hybrid seed compared with the economic gain obtained by the cultivation of the hybrid over the nonhybrid variety. The price of commercially produced hybrid seed will reflect research and development expenditures as well as the costs of production, processing, and marketing. Of these, the actual cost of seed production will be the single most important factor (Johnson and Schmidt, 1968). Key factors that will influence the cost of seed production are the percentage of seed set in the male-sterile increase and hybrid production fields, the ratio of female parent to pollinator, the seed multiplication ratio, and the proximity of the seed production operation to the areas of commercial seed use. Currently, China is the only country growing.hybrid rice. The following information was made available to S. S. Virmani during several visits: (1) the relative seed costs, in United States dollars, are $1.20 and $0.12/kg for hybrid and varietal seed, respectively. At a seeding rate of 20-25 kg/ha, the added cost of growing hybrid seed is $25.00 per ha; (2) hybrid yields average 1 ton/ha above varietal yields, and provide an added income of $120/ha (or $95 net); (3) the extra production costs for hybrid seed are $290/ha, and seed producers harvest about 0.75-1 ton/ha of hybrid seed and 1 ton/ha of restorer seed, compared with variety seed yields of 5 tons/ha. Thus, with the seed price differential of 1O:l (hybrid/variety), the hybrid seed producers have an extra income of approximately $130-430/ha. This advantage has prompted communes to specialize in hybrid seed production (L. P. Yuan, personal communication). The 20-33% yield advantage from hybrids has made hybrid rice production a profitable venture for both farmer and seedsman. The levels of standard heterosis needed to cover the added costs of hybrid seed at commercial yield levels ranging from 1 to 10 tons/ha are shown in Fig. 3. Under irrigated conditions, where yields of 5-6 tons/ha are attainable, standard

HYBRID RICE AND WHEAT 24

-0. 5

199

p I ton/ho

15

25

I

1

35

45

Increased cost hybrid seed/ha [over cost nonhybrid (US$)]

FIG.3. Percentage of standard heterosis necessary to pay the additional cost of hybrid seed at different commercial yield levels in rice. The price of paddy is assumed to be $2OO/ton (Virmani et al., 1981).

heterosis of 3.7-4.5% would cover added seed costs up to $45/ha. In rainfed situations, where yields vary from 1 to 3 tons/ha, standard heterosis of 8-22.5% would cover seed costs. Compared with rice, wheat has the problems of requiring higher seeding rates; also, its seed multiplication ratios are lower, and most of it is grown under dryland conditions where farm yields average slightly less than 1 ton/ha (35 bu/ac). Sage (1967) drew attention to the constraints of plant population and seed multiplication ratio in wheat compared with maize and sorghum. Lucken (1982) suggested that seed multiplication ratios for wheat, maize, and sunflower were 25,215, and 450, respectively, under comparable growing conditions in the Red River Valley of North Dakota and Minnesota. An important consequence of these constraints is the amount of lead time needed for initial increase of the parents, and the additional land area that would have to be set aside for hybrid seed production (approximately double) compared with conventional variety increases. Male-sterile increase, in particular, is a difficult and costly operation, necessitating intensive management and adequate isolation. Reitz (1967) suggested that a seed set below 50% would result in prohibitively high hybrid seed costs. Hayward (1975) suggested that seed set in the 50-70% range, with an MS/pollinator ratio of 2: 1-3: 1, would make hybrid wheat economically feasible. Mounting evidence suggests that with producible parents a

200


seed set in excess of 70% is attainable, and several seed companies in the United

States are using irrigation to improve the consistency of hybrid seed production. Johnson and Schmidt (1968) presented data showing the amounts of standard heterosis necessary to pay the additional costs of hybrid seed at different commercial yield levels and at different additional seed costs per hectare (comparable to the variables used in Fig. 3). In 1982, two seed companies in the United States marketed winter wheat hybrids at $l.lO/kg, compared with approximately %0.40/kg for variety seed. At a seeding rate of 65-70 kglha, an average yield of 0.95 tons/ha in the winter wheat region, and a grain market price of $150/ton, standard heterosis levels of 11-16% would cover the additional seed costs. This is closely comparable to the figures provided for dry-land rice. Important commercial production experience has been gained in the United States during the past decade, and some marked improvements in efficiency have been accomplished as a result of using more producible inbreds. Any production practice that will increase yields and reduce seed rates will improve the profitability of hybrid seed production. Several workers have suggested that seed rates of adapted hybrids could be reduced by as much as 50% without reducing yield, because of the heterotic expression of emergence, vigor, and tillering capacity in hybrids. Conflicting results have been obtained in a number of studies, which do not provide adequate justification for seed-rate reductions in hybrid wheat environments.

XI. PROBLEMS

Although hybrid rice has been developed and cultivated in China, several problems remain. First, the commercial hybrids have a longer growth duration (125-140 days) than do varieties and thus cannot be grown during the first cropping season (March-April to June-July) which is short because of the low temperature prevailing before March-April. Second, inadequate heterosis and lack of suitable restorers in japonica rice has prevented extensive use of hybrids in the japonica rice belt north of the Yellow River. Third, difficulties remain in hybrid seed set, and production is highly labor intensive. Last, the lack of multiple disease and insect resistance in the current hybrids has prevented their spread to the subtropical areas. However, researchers in China and at the IRRI report progress in shortening the maturation period, transferring Rf genes from indica to japonica varieties, improving pest and disease resistance, and selecting for traits that will improve hybrid seed set. Outside China, there are a number of factors that will affect the spread of


20 1

hybrid rice. Many scientists are not fully convinced that heterosis sufficient to pay for the extra seed cost exists, and the development of economically feasible seed production methods has yet to be resolved. The extent to which the laborintensive and time-consuming techniques of hybrid seed production developed in China can be adopted elsewhere is uncertain. In the absence of comprehensive analytical data on hybrid rice seed production and cultivation in China, many countries are skeptical about the economics of hybrid rice. Finally, many riceproducing countries do not have an efficient organizational setup to produce, certify, and market hybrid seed.

B. WHEAT

A major constraint in hybrid wheat development relates to the high seeding rate and low multiplication ratio discussed in the preceding section. These increase the labor and costs of hybrid testing and limit the number of hybrids evaluated in a single season. In maize, sorghum, and sunflower a single pollination produces enough seed to conduct yield trials at several locations, whereas a corresponding hand pollination in wheat may produce only 10-30 seeds. In consequence, inadequate (in general) combining ability studies have been conducted in hybrid wheat programs to realize the potential of the material on hand. Considerable research effort has been spent on developing an adequate and stable pollen fertility restoration system, and less selection pressure has been imposed for the agronomic performance of male parents. This factor, coupled with delays in converting desirable B lines into male steriles, has resulted in hybrids with comparatively low levels of standard heterosis because yield gains have been made through conventional breeding. A further problem is the producibility of the female parent. Most hybrid programs to date have tended to incorporate agronomically desirable varieties or lines into male steriles. Comparatively few crosses have been made with the objective of improving female producibility, and there is a growing realization that the objective of a true B-line program can differ considerably from those of a conventional variety breeding program for such traits as plant height, anther extrusion, and combining ability. Finally, a problem encountered especially in the spring wheat region is obtaining consistent seed production yields. However, considerable progress has been made in the hard red winter wheat areas. Hybrids have been marketed on a limited scale in the United States since 1974, but cropped hectarage has remained small and performance has not lived up to expectations. A number of institutions, both private and public, embarked upon hybrid research in the mid 1960s. The complexity of the system and the magnitude of the task were not adequately appreciated by some administrators, and continued funding became a problem. The small yield advantages of hybrids

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were attributed by some to limited heterosis in wheat rather than to a lack of combining ability studies on an adequate number of parents.

XII. CONCLUSION A. CURRENT OUTLOOK

A review of rice breeding during the past decade would indicate that although conventional breeding procedures have succeeded in maintaining adequate levels of pest and disease resistance in cultivars, they have not shown significant advances beyond the yield plateau established by IRS. Yield results from numerous wheat research programs have been somewhat varied; certain countries and regional programs have reported genetic gains and others a more static situation. Both crops would undoubtedly benefit from the adoption of more efficient breeding procedures, and the exploitation of heterosis through hybrid breeding offers an important option. Significant heterosis for yield and other agronomic and physiological characteristics have been reported in the literature on rice and wheat. Rice hybrids are grown on approximately 6 million ha in China, and hybrid wheats are now being routinely produced, tested, and marketed by several hybrid wheat programs in the United States. Small quantities of hybrid wheat seed are also being marketed in several other countries, and the potential for hybrid rice production is being evaluated in India, Indonesia, the Philippines, and South Korea, and by two United States-based seed companies. Aside from the considerable breeding progress made during the past 20 years, rice and wheat researchers have played a major role in increasing our understanding of nucleocytoplasmic interactions. Several usabIe cytoplasmic male-sterile systems are available in diverse genetic backgrounds and may be used to sterilize maintainer varieties (B lines) that possess desirable agronomic characteristics, insect and disease resistance, and adaptability to local conditions. The frequency of rice maintainer lines is adequate among elite breeding lines developed by national and international programs, and both these and restorer lines have been found to possess multiple disease and insect resistance. Although a number of wheat B lines contain genes for partial fertility restoration, most are maintainers that can be easily converted into totally male-sterile lines through nuclear substitution into an alien cytoplasm. Because of the complexity of the restoration system, the frequency of R lines that provide complete fertility restoration with desirable agronomic and disease resistance traits is proportionately lower. Standard heterosis of as much as 30% for rice hybrids and 20% for wheat hybrids, has been obtained.


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Hybrid seed production techniques have been developed for both crops, and rice seed yields of 0.45-1.5 tons/ha have been obtained in China. Hybrid wheat seed yields of 0.6-2.0 tons/ha have been obtained in the United States. Genetic variability in floral characters influencing outcrossing have been obtained in both crops, and breeders should be able to further improve the outcrossing potential of maintainer and restorer lines. Results obtained in China and at the IRRIindicate that the problems found in breeding and producing hybrid rice in China, and those foreseen elsewhere, are technically surmountable. Progress made in the United States since 1969 in developing a stable male sterility-fertility restoration system, in increasing the levels of standard heterosis, and in stabilizing and improving hybrid yields provides a sound basis for expanding the use of this technology. The adoption of F, hybrids in rice and wheat will ultimately depend on (1) the magnitude of the yield advantage obtained; (2) the cost/benefit ratio of using hybrid versus pureline seed; and (3) the efficiency of seed production, certification, and distribution agencies available in the country.

B. FUTURESTRATEGIES

Hybrid rice and wheat breeding should provide farmers with an opportunity to improve productivity, particularly in potential high-yield areas and where conventional breeding has apparently reached a yield plateau. The early vegetative vigor and stronger root system of heterotic rice hybrids should make them adapted to rainfed areas, and there is limited evidence to suggest that certain wheat hybrids may also have improved dry-land adaptability. Sorghum hybrids in India have shown greater yield advantages under drought conditions than under adequate moisture conditions. Some spring wheat hybrids appear to show a similar response, but both rice and wheat hybrids require further evaluation under stress conditions to confirm whether these advantages exist. Heterosis for protein content has not been clearly established in rice and wheat, but the identification of high-protein hybrids could further enhance the prospects for both crops. Aside from China, countries that may have prospects for hybrid rice in the near f u m e include India, Indonesia, the Philippines, and South Korea. In the United States, the prospects for hybrid rice are limited at present by the high costs of seed production and the unacceptable quality of heterotic hybrids introduced from China. There are 32 million ha of wheat grown annually in the United States, and Reitz (1967) estimated that hybrid wheat could find a place on 12 million ha. This represents a formidable challenge to the seed industry to manage a large hectarage of hybrid production fields and provides a strong incentive for increasing the scope of hybrid development. Seed companies and producers will have to face the realities of environmental effects upon seed set

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B. EDWARDS

US. wheat crop is produced in low rainfall areas, irrigation may prove a prequisite for increasing seed set and stabilizing seed production. The approach of the breeder to stabilizing production will be to continue identifying floral features that improve pollen load and female receptivity, to establish variation for these traits, and to incorporate favorable genes into new inbred lines. The stability and adaptability of available A, B, and R rice lines should be studied in different environments for which hybrids are to be developed. To protect F, hybrids against potential genetic vulnerability to disease and insect epidemics which may be linked to the use of a certain CMS system, studies of the effects of existing cytoplasms on resistance or susceptibility are essential, and alternative sources of CMS should be sought. In rice, a suitable alternative to the W A cytosterility system needs to be discovered; in wheat, Aegilops speltoides appears to offer the best alternative to T. timophemi, and adequate Rf genes are available. A sterility-inducing factor(s) in the cytoplasms of available CMS lines should be identified by restriction endonuclease fragment analyses of organelle DNAs. This will not only help in differentiating the available CMS systems but could provide a means of screening for prospective donors for CMS and fertility restoration. Interspecific and intraspecific hybridization and/or tissue culture, protoplast culture, and fusion techniques should be examined to identify new potential sources of cytoplasmic male sterility. As an alternative to the cytoplasmic-genetic system, the search for effective chemical pollen suppressants for rice and wheat should be continued. Whether the CPS system of hybrid production will be as effective as the CMS system in the long run remains to be seen. However, the use of a CPS system in facilitating combining ability studies will be an important new ingredient in hybrid testing. There is now improved understanding of the importance of a balance between genes for pollen fertility restoration in the male parent and fertility-enhancing genes that affect the ease of restoration in the female. It may be possible to identify CMS females that exhibit partial fertility in a favorable (shallow-sterile) environment. Such lines in hybrid combination may interact with restorer genes to provide a highly fertile hybrid that is stable over a range of climatic conditions. In wheat, monosomic analyses provide a definitive means of identifying restorer genes but are time consuming and expensive. The development of a key for the identification of restorer genes (or gene groups), possibly based on a combination of differential lines and several cytoplasms, would be worthy of investigation; it would provide hybrid breeders with a more simple means of managing and deploying Rf genes in their programs. Another breeding strategy available to the hybrid wheat breeder is using the semidwarfing genes Rh? I and Rht 2 in different parents to ensure short stature. and, because much of the


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Wilson et al. (1980) reported on the successful development of full dwarf x semidwarf hybrid wheats. They suggested that in developing semidwarf lines, some of the best genetic combinations for vigor will probably result in too much height. The use of Olsen dwarf derivatives with partial dominance for dwarfism offers an additional means of improving lodging resistance in hybrids targeted for high-potential or irrigation environments. If a dominant gene for semidwarfing is discovered in rice, tall varieties possessing wider genetic diversity can be used as parents to develop hybrids with high-yield potential (Athwal and Virmani, 1972). Improved techniques of hybrid seed production should be developed to economize seed production cost. Critical economic analyses of seed production and rice and wheat hybrid cultivation are required to determine under what situations and with what cost/benefit ratio hybrid breeding technology can be adopted in different countries. Because of the considerable differences that exist between wheat and rice, a comparative assessment of the prospects for developing hybrids is neither simple nor entirely valid. However, a few factors merit consideration: 1. Because rice is first sown in seedbeds and subsequently transplanted into production fields, it has been possible to match parents with widely different maturities. In contrast, because of environmental constraints and seed production methods, hybrid wheat breeders have had to work within clearly defined maturity groups. This can limit genetic variability and may be partially responsible for the lower (15-20% versus 20-30%) levels of standard heterosis reported in wheat versus rice. 2. Stable CMS systems in rice are derived from both intraspecific (BT and Gam type) and interspecific (WA) crosses. In wheat, CMS systems are developed by substituting the genomes of desirable genotypes into an alien cytoplasm and backcrossing to the nuclear donor parent. Although the effects of the alien cytoplasm on heterosis in wheat have not been widely studied, it seems that the effects (positive or negative) will be cross specific. 3. Fertility restoration appears to be less complexly inherited in rice than in wheat. Therefore, although Rf genes (and their associated linkage blocks) have been obtained from a number of common wheats, related genera, and species, their transfer into desirable agronomic types has been more time consuming. 4. The extent of natural cross fertilization in hybrid rice production blocks is lower at the present time than in hybrid wheat production blocks (35-45% versus 50-80%), and a considerable amount of hand labor has been used to produce hybrid rice seed. This could limit hybrid rice production to those countries where labor is abundant and inexpensive. 5 . Seeding rates of rice hybrids are lower (20-25 kg/ha) than those of wheat (60-70 kg/ha), and seed multiplication rates in hybrid production blocks are higher (approximately twice). This could offer certain advantages to the rice farmer and seed producer.

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6. Comparative costs of hybrid versus pure-line seed suggest a 6-10 1 ratio for rice in China versus a 3 . 5 1 ratio for wheat in the United States. These price ratios will undoubtedly fluctuate but could influence farmer preference.

In addition to the factors influencing farmer preference for hybrid versus pureline seed, the capability of countries to organize production, certification, and distribution of hybrid seed will have an important bearing on industrial expansion. Progress during the next decade will determine how much hybrids in these two major cereal crops can help to increase world food production.

REFERENCES Akbar, M., and Yabuno, T. 1975. Ikushugaku Zusshi (Jpn. J . Breed.) 25, 215-220. Allan, R. E. 1973. Froc. Int. Wheat Genet. Symp. 4th. pp. 311-317. Amaya, A. A., Busch, R. H.,and Lebsock, K. L. 1972. Crop Sci. 12, 478-481. Anand, S. C., and Ben, S. M. 1971. Indian J . Gener. 31, 132-135. Anonymous. 1976. Zhon Guo Nongye Kexue (Chinese Agric. Sci.) 1, 51-65. Anonymous. 1977. Int. Rice Res. Newsl. 2, 27-28. Anonymous. 1978a. Chih Wu Hsuch Fa0 (Acta Bor. Sin.) 20, 305-313. Anonymous. 1978b. Agric. Ind. Life 40, 10. Arp, A. L. 1967. M.S. Thesis, Kansas State Univ., Manhattan. Atashi-Rang, G. 1970. Ph.D. Thesis, North Dakota State Univ., Fargo. Atashi-Rang, G., and Lucken, K. A. 1978. Crop Sci. 18, 267-272. Athwal, D. S . , and Borlaug, N. E. 1967. Indian J . Genet. 27, 136-142. Athwal, D. S., and Virmani, S. S. 1972. In “Rice Breeding,” pp. 615-620. Int. Rice Res. Inst., Los B&m, Philippines. Bahl, P. N. 1971. Ph.D. Thesis, North Dakota State Univ., Fargo. Bahl, P. N., and Maan, S. S. 1973. Crop Sci. 13, 317-320. Bailey, T. B., Jr., Qualset, C. O., and Cox, D. F. 1980. Crop Sci. 20, 339-342. Baldi, G. 1976. Ann. Accaa‘. Agric. Torino 119, 15-23. Badhan Roy, S. K., Mitra, A. K., and Mukherji, D. K. 1975. Sci. Culr. 41, 118-119. Bardier, N. G. 1960. Plant Breed. Abstr. 32, No. 4599. Bennett, M. D., and Hughes, W. G. 1972. Nature (London) 240,566-568. Benson, R. M. 1978. M.S. Thesis, North Dakota State Univ., Fargo. Ben, S. M., and Anand, S. C. 1971. Euphytica 20, 327-332. Bingham, J. 1966. Ann. Appl. B i d . 57, 365-377. Bingham, J. 1967. J . Agric. Sci. 68, 411-422. Bitzer, M. I., and Fu, S. H. 1972. Crop Sci. 12, 35-37. Bitzer, M. J., and Patterson, F. L. 1967. Crop Sci. 7, 482-484. Borghi, B., Bonali, F., and Boggini, G. 1973. Proc. Int. Wheat Genet. Symp. 4th. pp. 337-343. Bravo, J. M. 1982. M.S. Thesis, North Dakota State Univ., Fargo. Briggle, L. W.. 1963. Crop Sci. 3, 407-412. Briggle, L. W., Dawn, R. J., and Stevens, H. 1964. Crop Sci. 4, 220-223. Briggle, L. W., Cox, E. L., and Hayes, R. M. 1967. Crop Sci. 7,465-470. Brown, C. M., Weibel, R. D., and Seif, R. D. 1966. Crop Sci. 6, 382-383. Busch, R. H., and Maan, S. S. 1978. Crop Sci. 18, 864-866. Cahn, E. 1925. J . Am. SOC.Agron. 17, 591-595.


207

Carnahan, H. L., Erickson, J. R., Tseng, S. T., and Rutger, J. N. 1972. In “Rice Breeding,” pp. 603-607. Int. Rice Res. Inst., Los Baiios, Philippines. Caroline, J. J. 1977. M.S. Thesis, North Dakota State Univ., Fargo. Chang, T. M. 1971. Nung Yeh Yen Chih (J. Tuiwun Agric. Res.) 20, 9-26. Chang, T. M. 1980. Chung-huu Nung Yeh Yen (J. Agric. Res. of China) 29, 107-114. Chang, T. T., Li, C. C., and Vergara, B. S. 1%9. Euphyticu 18, 79-91. Chang, T. T., Li, C. C., and Tagumpay, 0. 1973. Bor. Bull. Acud. Sin. 14, 83-93. Chao, C. N. 1972. T’ui-wan Nung Yeh Shuung Yueh K u n (Tuiwun Agric.) 8,60-65. Chaplin, J. F., and Ford, Z. T. 1%5. Crop Sci. 5, 436-438. Chaudhary, R. C., Virmani, S. S., and Khush, G. S. 1981. Oryzu 18, 149-142. Cheng, Y. G., and Huang, Z. S. 1978. Chung-huu Nung Yeh Yen (J. Agric. Res. China) 27, 267-290. Cheng, Y. K., and Huang, C. S. 1979. Chung-huu Nung Hsueh Hui Puo (J. Agric. Assoc. China) 106, 11-22. Chopra, V. L., Jain, S. K.,and Swaminathan, M. S. 1960. Indian J . Genet. 20, 188-199. Chu, Y. E., Tsai, K. S., and Li, H. W. 1970. Bof. Bull. Acud. Sin. 11, 105-112. Clayton, E. E. 1950. J . Hered. 41, 171-175. Conde, M. F., Pring, D. R., and Levings, C. S., III 1979. J. Hered. 70, 2-4. Copeland, E. B. 1924. “Rice.” MacMillan, London. Craigmiles, J. P., Stansel, J. W., and Flinchum, W. T. 1968. Crop Sci. 8, 720-722. Cregan, P. B., and Busch, R. H. 1978. Crop Sci. 18, 247-251. Davis, M. D., and Rutger, J. N. 1976. Euphyricu 25, 587-595. De Vries, A. P. 1971. Euphyricu 20, 152-170. De Vries, A. P. 1972. Euphyricu 21, 185-203. De Vries, A. P. 1974a. Euphyricu 23, 11-l9. De Vries, A. P. 1974b. Euphyticu 23,601-622. Dhulappanavar, C. V., and Mensikai, S. W. 1967. Mysore J . Agric. Sci. 1, 117-122. Donald, C. M. 1962. J. Ausr. Inst. Agric. Sci. 28, 171-178. Dotlacil, L., and Apltaverova, M. 1978. Euphyricu 27, 353-360. D’Souza, L. 1970. 2. Pflun zen Zuechr. 63, 246-269. Dzuba, V. A., and Kolesnikov, G. P. 1976. Risu 19,24-26 (Byul. Nauch. Tekh. Inf. No. VIVII). Edwards, I. B., Thompson, W. G., and Pingree, D. W. 1980. Annu. Wheat Newsl. 26, 36-37. Erickson, J. R. 1969. Agron. Absir., p. 6. Ezrokhin, L. M., and Nettevich, E. D. 1970. Plunr Breed. Absrr. 42, 7320. Fairey, D. T., and Stoskopf, N. C. 1975. Crop Sci. 15, 29-32. Fisher, J. J. 1977. M.S. Thesis, North Dakota State Univ., Fargo. Fisher, R. A. 1975. Proc. Int. Winrer Wheur Conf. 2nd. pp. 178-1%. Fonseca, S., and Patterson, F. L. 1968. Crop Sci. 8, 85-88. Franckowiak, J. D., Maan, S. S., and Williams, N. D. 1976. Crop Sci. 16, 725-728. Freeman, G.F. 1919. Generics 4, 1-9. Fukasawa, H. 1953. Cyrologiu 18, 167-175. Fukasawa, H. 1955. Cytologiu 20, 211-217. Fukasawa, H. 1957. Cyrologiu 22, 30-39. Fukasawa, H. 1958. Wheurlnf. S e n . 7, 21. Fukasawa, H. 1959. Jpn. J . Bor. 17, 55-91. Gao, M. W. 1981. Acra Genet. Sin. 8, 66-74. Ghiasi, H . , and Lucken, K. A. 1982a. Crop Sci. 22, 527-531. Ghiasi, H., and Lucken, K. A. 1982b. Euphyricu 31, 253-259. Gilles, K. A., and Sibbitt, L. D. 1966. Cereal Sci. Today 11, 510-516. Gilmore, E. C., Porter, K. B., and Tuleen, N. A. 1977. Annu. Wheat Newsl. 23, 112-113.

208

S. S. VIRMANI AND

IAN B. EDWARDS

G i ,E. C., Porter, K. B., and Tuleen. N. A. 1978. Annu. Wheut Newsl. 24, 130-131. GMnaa, A. S. 1973. W.D.Thesis, North Dakota State Univ.. Fargo. Gomaa, A. S., and Lucken, K. A. 1973. Proc. Int. Wheut Genet. Symp. 4th, pp. 345-350. Goujon, C., and Ingold, M. 1%7. C.R. Hebd. Seunces Acud. Sci. Ser. D 264, 334-336. Grist, D. H. 1953. “Rice.” Longmans Green, London. Grogan, C. O., and Sarvella, P. 1964. Crop Sci. 4,567-570. Grogan, C. O., Francis, C. A., and Sarvella, P. A. 1971. Crop Sci. 11, 295-297. Gyawali, K. K., Qualset, C. O., and Yamazaki,W. T. lW8.Crop Sci. 8,322-324. Haque, M. M., Faridi, M.N.I., Razzaque, C. A., and Newaz, M. A. 1981 .Indim J. Agric. Sci. 51, 711-714. Hayward, C. F. 1975. Proc. Int. Winter Wheut Conj. 2nd. pp. 84-104. Hector, S. P. 1913. Mem. Dep. Agric. Indiu Bot. Ser. 6, 1-10. Heu, M. H., and Chae, Y. A. 1970. Res. Rep. Rurul Dev. (Crop) Koreu 13, 9-13. Heyne, E. G., and Smith,G.S. 1967. I n “Wheat and Wheat Improvement” (K. S. Quisenberry and L. P. Reitz, eds.), pp. 269-306. Amer. Soc. Agron., Madison, Wisconsin. Holland, R. F., and Roberts, T. H. 1966. Publ. DeKalb Agric. Assoc. Hori, T., and Tsunewaki, K. 1967. Jpn. J. Breed. 19, 19-24. Huang, C. H. 1970. JCRR Plant Ind. Ser. 29, 1-5. H u g , C. S., and Huang, H. H. 1978. Publ. Agric. Res. Inst. Taipei, pp. 21-255. Hughes, W. G., and Bodden, J. J. 1977. Theor. Appl. Genet. 50, 129-135. Hughes, W. G., and Bodden, J. J. 1978. Theor. Appl, Genet. 53, 219-228. Hughes, W. G., Bennett, M. D., Bodden, J. J., and Galanopoulou, S. 1974. Ann. Appl. Biol. 76, 243-252. Hunan Agricultural Bureau and Revolutionary Committee of Yan-Pai-du Commune. 1977. Actu Bot. Sin. 19, 47-52. Hunan Agricultural College, Department of Chemistry. 1977. Acta Bot. Sin. 19, 226-236. Hunan Provincial Rice Research Institue. 1977. utongguo Nongye Kexue (ChineseAgric. Sci.) 1, 21-26. Imrie, B. C. 1966. Aust. J. Exp. Agric. Anim. Husb. 6, 175-178. Ingold, M. 1974. Extruit Rech. Agron. Suisse 13, 235-243. International Rice Research Institute. 1978. “Annual Report 1977.” Int. Rice Res. Inst., Los Baf~os, Philippines. International Rice Research Institute. 1983. “Annual Report 1981.” Int. Rice Res. Inst., Los BafIos, wilippines. Jan, C. C., and Rowell, P. L. 1981. Euphytica 30, 501-504. Jan, C. C., Qualet, C. O., and Vogt, H. E. 1974. Euphytica 23, 78-85. Jennings, P. R. 1967. Int. Rice Comm. Newsl. 16, 24-26. Jensen, N. F. 1968. Annu. Wheat Newsl. 15, 26-28. Johnson, J. W., and Patterson, F. L. 1973. Crop Sci. 13, 92-95. Johnson, R. R., and Brown, C. M. 1976. Crop Sci. 16, 384-387. Johnson, V. A. 1977. Farm Seed Conf. Am. Seed Trade Assoc. 23rd. pp. 27-34. Johnson, V. A. 1978. Cereal Foods World 23, 84-86. Johnson, V. A., and Schmidt, I. W. 1968. Adv. Agron. 20, 199-223. Johnson, V. A., Schmidt, J. W., and Mattern, P. J. 1967. Q w l . Plant. Muter. Veg. 14, 193-21 1 . Jones, J. W. 1926. J. Am. SOC.Agron. 18,423-428. Jonsson, J. 0. 1976. Cereal Res. Commun. 4, 115-119. Joppa, L. R., and McNeal, F. H. 1969. Crop Sci. 9, 839-840. Joppa, L. R., McNeal, F. H., and Berg, M. A. 1968. Crop Sci. 8, 487-490. Jost, M. 1979. Int. Wheut Restorer Germphm Screening Nur. 2nd. pp. 1-64.


209

Jost, M. 1980. Inr. Wheat Restorer Germplasm Screening Nur. 3rd, pp. 1-67. Jost, M. 1981. Inr. Wheat Restorer Germplasm Screening Nur. 4th, pp. 1-41. Jost, M., and Glatki-Jost, M. 1976. I n “Hybrid Wheat Research in Yugoslavia,” pp. 155-163. Univ. of Zagreb, Zagreb, Yugoslavia. Jost, M., and Hayward, C. F. 1980. Theor. Appl. Genet. 57, 177-180. Jost, M., and Jost, M. 1980. Proc. Inr. Wheat Conf. 3rd. pp. 122-136. Jost, M., and Milohnic, J. 1975. Proc. Int. Winter Wheat Conf. 2nd, pp. 153-162. Jost, M., and Milohnic, J. 1976a. I n “Hybrid Wheat Research in Yugoslavia,” pp. 73-77. Univ. of Zagreb, Zagreb, Yugoslavia. Jost, M., and Milohnic, J. 1976b. I n “Hybrid Wheat Research in Yugoslavia,” pp. 93-98. Univ. of Zagreb, Zagreb, Yugoslavia. Jost, M., Glatki-Jost, M., and Milohnic, J. 1976a. I n “Hybrid Wheat Research in Yugoslavia,” pp. 113-121. University of Zagreb, Zagreb, Yugoslavia. Jost, M., Glatki-Jost, M., Hrust, V., and Milohnic, J. 1976b. In “Hybrid Wheat Research in Yugoslavia,” pp. 39-57. Univ. of Zagreb, Zagreb, Yugoslavia. Kadam, B. S. 1933. IndiunJ. Agric. Sci. 3, 577-588. Karunakaran, K. 1968. Agric. Res. J . Kerufu 6 , 9-14. Katsuo, K., and Mizushima, U. 1958. Ikushugaku Zasshi (Jpn. J . Breed.) 8, 1-5. Keydel, F. 1969. Vorg. F . Pflonzenzuchter. 12, 123-142. Khaleque, M. A., Jorder, 0. T.,and Eunus, A. M. 1977. Bangladesh J. Agric. Sci. 4, 137-145. Khan, M. N. A., Heyne, E. G., and Arp, A. L. 1973. Crop Sci. 13, 223-226. Kherde, M. K., Atkins, I. M., Merkle, 0. G., and Porter, K. B. 1967. Crop Sci. 7, 389-394. Kihara, H. 1951. Cyrologiu 16, 117-193. Kihara, H. 1958. Proc. Inr. Congr. Genet. 106th 1, 142-171. Kihara, H. 1963a. Seiken Jiho 15, 13-23. Kihara, H. 1963b. Proc. Inr. Congr. Genet. 11th 1, 231. Kihara, H. 1970. Seiken Jiho 22, 107-1 11. Kihara, H., and Tsunewaki, K. 1961. Seiken Jiho 12, 1-10. Kihara, H., and Tsunewaki, K. 1964. Seiken Jiho 16, 1-14. Kihara, H., and Tsunewaki, K. 1966. Seiken Jiho 18, 55-63. Kimber, G. 1973. Proc. Inr. Wheat Gener. Symp. 4rh, pp. 81-85. Kimber, G., and Athwal, R. S. 1972. Proc. Narl. Acud. Sci. USA 69, 912-915. Kinoshita, T., Mori, K., and Takahashi, M. 1980. J . Fuc. Agric. Hokkuido Univ. 60, 23-41. Kitamura, E. 1962a. Ikushuguki Zusshi (Jpn. J . Breed.) 12, 81-84. Kitamura, E. 1962b. lkushugaku Zusshi (Jpn. J . Breed.) 12, 166-168. Kofoid, K. D., and Maan, S. S. 1982. Crop Sci. 22, 725-729. Komaki, M. K.,and Tsunewaki, K. 1981 . Euphyricu 30, 45-53. Kronstad, W. E., and Foote, W. H. 1964. Crop Sci. 4, 616-619. Krupnov, V. A. 1971. Genetika (Moscow)7, 159-174. Lama, P. E. 1966. M.S. Thesis, North Dakota State Univ., Fargo. Law, J., and Stoskopf, N. C. 1973. Can. J . Plant Sci. 53, 765-766. Leighty, C. E., and Sando, W.J. 1924. J . Agric. Res. (Wushingfon, D.C.) 27, 231-244. k i n , A. 1%7. Vorrr. Pfinzen. Zucht. 11, 97-105. Lelley, J. 1966. Zuechrer 36, 314-317. Lenz, M. C., and Atkins, R. E. 1981. Crop Sci. 21, 946-950. Levings, C. S., 111, and €‘ring, D. R. 1977. J. Hered. 68, 350-354. Li, C. C. 1975. Chung-huu Nung Hsueh Hui Puo (J. Agric. Assoc. China) 92, 41-56. Li, J., and Liu, Y-N. 1983. Theor. Appl. Gener. 64, 231-238. Li, P. H. 1977. Acru Bot. Sin. 19, 7-10.

210

S . S. VIRMANI AND IAN B. EDWARDS

Lin, S. C. 1977. Int. Rice Res. Newsl. 2, 27-28. Lin, S. C., and Yuan, L. P. 1980. In “Innovative Approaches to Rice Breeding,” pp. 35-51. Int. Rice Res. Inst., Los Bairos, Philippines. Livers, R. W., and Heyne, E. G. 1966. Agron. Absrr. 58, 11. Livers, R. W., and Heyne, E. G. 1968. Proc. Inr. Wheat Genet. Symp. 3rd. pp. 431-436. Lu, H. R., Yang, Rf. C., and Liu, K. M. 1981. Fujien Agric. Coll. Sci. Res. Bull. No. 79062. Lucken, K. A. 1973. Proc. Int. Wheat Genet. Symp. 4th. pp. 361-366. Lucken, K. A. 1982. Am. Soc. Agron. Publ. Wheat Yields (in press). Lucken, K. A., and Maan,S . S. 1967. Agron. Abstr. 14. Lyakhovkin, A. G., and Singil’Din, G. A. 1975. Selek. Semenovodmo 6, 28-29. Maan, S. S . 1973a. Proc. Symp. Gener. Breed. Durum Wheat Bari, Italy, pp. 117-138. Maan, S . S . 1973b. Proc. Inr. Wheat Gener. Symp. 4th. pp. 367-373. Maan, S. S. 1975. In “Prairie: A Multiple View” (M. K. Wali, ed.),pp. 255-281. UNv. of North Dakota Press, Grand Forks. Maan, S. S., and Lucken, K. A. 1967. Wheat Inf. Ser. Kyoto Univ. 23, 6-9. Maan, S. S., and Lucken, K. A. 1970. Euphyrica 19, 498-508. Maan, S. S . , and Lucken, K. A. 1971. J. Hered. 62, 149-152. Maan, S. S., and Lucken, K. A. 1972. Crop Sci. 12, 360-364. Mallick, E. H., Ghosh, H. N., and Bairagi, P. 1978. Indian J . Agric. Sci. 48, 384-386. Mann, H. A. 1977. M.S. Thesis, North Dakota State Univ., Fargo. Matsuo, T. 1952. Bull. Narl. Insr. Agric. Sci. (Japan), Ser. D . No. 3. 1-111. (In Japanese). Maurya, D. M., and Singh, D. P. 1977. Indian J . Agric. Sci. 47,65-70. Maurya, D. M., and Singh, D. P. 1978. Indian J . Genet. Plant Breed. 38, 71-76. McCuisition, W. L. 1968. Ph.D. Thesis, Oklahomn Stare Univ., Stillwater. McDonuld, D. J . , Gilmore, E. C.,and Sransel, J . W . 1971. Agron. Abstr., pp. 11-12. McDonald, D. J.. Stanse.1, J. W., and Gilmore, C. 1974. Proc. Gen. Congr. SABRA0 2nd. pp. 1067- 1373. McNeal, F. H., and Ziegler, D. M. 1975. Cereal Res. Commun. 3, 199-203. Meletti, P. 1961. Planr Breed. Absrr. 33, No. 2%1. Mihaljev, I. 1972. Conremp. Agric. 20 (Suppl.), 1-45. Mihaljev, I. 1976a. In “Hybrid Wheat Research in Yugoslavia,” pp. 59-66. Univ. of Zagreb, Zagreb, Yugoslavia. Mihaljev, I. 1976b. In “Hybrid Wheat Research in Yugoslavia,” pp. 67-72. Univ. of Zagreb, Zagreb, Yugoslavia. Mihaljev, I. 1980. Proc. Int. Wheat Con$ 3rd. pp. 102-109. Miller, J . F. 1970. M.S. Thesis, Univ. of Nebraska, Lincoln. Miller, J. F., and Lucken, K. A. 1976. Crop Sci. 16, 217-221. Miller, J. F., and Lucken, K. A. 1977. Euphyrica 26, 103-112. Miller, J. F., Rogers, K. J., and Lucken, K. A. 1974. Crop Sci. 14, 702-705. Milohnic, J., and Jost, M. 1970. Acra Agron. Acad. Sci. Hungaricae 19, 17-23. Milohnic, J., and Jost, M. 1974. Proc. Congr. Eucarpia 7th Miri, R. K., Amawate, J. S., and Jain, fi. K. 1970. Indian J . Genet. 30, 383-394. Mohamed Sayed, A. A. 1975. Agric. Res. J . Kerala 12, 120-123. Mohanty, H. K., and Mohapatra, K. C. 1973. Indian J . Genet. 33, 264-270. Monteagudo, A., Sanchez-Monge, E., and Lacadena, J. R. 1967. Bol. Inst. Nac. Invest. Agron. (Spain) 27, No. 56, 79-81. Mukai, Y., andTsunewaki, K. 1979. Theor. Appl. Genet. 54, 153-160. Mukai, Y., and Tsunewaki, K. 1980. In “Genetic Diversity of the Cytoplasm in Triricum and Aegilops” (K. Tsunewaki, ed.), pp. 237-250. Jpn. Soc. Remotion of Sci., Tokyo.


21 1

Murayama, S., Omura, T., and Miyazata, K. 1974. Ikurhugaku Zusshi (Jpn. J . Breed.) 24, 287-290. Nagao, S. 1951. Adv. Genet. 4, 181-212. Namboodiri, K. N. 1963. Rice News Teller 11, 92-96. Nanda, K. K., and Chinoy, J. J. 1945. Curr. Sci. 14, 241. Nettevich, E. D. 1968. Euphyrica 17, 54-62. Nettevich, E. D., and Naumov, A. A. 1970. Plant Breed. Abstr. 43, No. 4865. Oehler, E., and Ingold, M. 1966. Wheat Inf. Sew.Kyoto Univ. 22, 1-13. Oka, H. I. 1953. lkushugaku Zarshi (Jpn. J . Breed.) 3, 31-39. Oka, H. I. 1954. Ikushugaku Zusshi (Jpn. J . Breed.) 3, 1-6. Oka, H. I. 1963. In “Genetics and Cytogenetics,” pp. 158-174. Elsevier, New York. Oka, H. I., and Morishima, H. 1967. Evolution 21, 249-258. O’Toole, J. C., and Soemartono. 1981. Euphytica 30, 283-290. Paramsivan, K. S. 1975. Madras Agric. J . 62, 456-457. Parmar, K. S. 1974. Ph.D. Thesis, Sardar Patel Univ. Vallabh Uidyanayar, Gujarat, India. Parmar, K. S., Siddiq, E. A., and Swaminathan, M. S. 1979a. Indian J . Genet. 39, 529-541. Parmar, K. S., Siddiq, E. A,, and Swaminathan, M. S. 1979b. Indian J. Genet. 39, 551-559. Parmar, K. S., Siddiq, E. A., and Swaminathan, M. S. 1979c. Indian J . Genet. 39, 542-550. Parmar, K. S., Siddiq, E. A,, and Swaminathan, M. S. 1981. Z. PJlanzenzuechr. 86, 1-10. Percival, J. 1921. “The Wheat Plant.” Duckworth, London. Perez, A. T., Chang, T. T., Beachell, M. H., Vergara, B. S., and Marciano, A. 1973. SAERAO Newsl. 5, 133-139. Pillai, M. S. 1961. Rice News Teller 9, 15-17. Popov, P., and Gotsov, K. 1%8. Genet. Plant Breed. (Sofia) 1, 19-28. Porter, K. B., and Merkle, 0. G. 1967. Agron. Absrr, p. 16. Porter, K. B., and Weise, A. F. 1961. Crop Sci. 1, 381-382. Porter, K. B., Lahr, K. A., and Merkle, 0. G. 1966. Res. Texas 13,6. Pring, D. R., and Levings, C. S., III 1978. Generics 89, 121-136. Pring, D. R., Conde, M. F., and Levings, C. S., 111 1980. Crop Sci. 20, 159-162. Pring, D. R., Conde, M. F., and Schertz, J. F. 1982. Crop Sci. 22, 414-421. Rahman, M., Patwary, A. K.,and Miah, A. J. 1981. Indian J . Agric. Sci. 51, 543-546. Rai, R. K. 1979. Proc. Int. Wheat Genet. Symp. 5th. pp. 291-305. Rai, R. K.,and Stoskopf, N. C. 1974. Z. Pflanzenzuechr. 71, 307-318. Rai, R. K., Stoskopf, N. C., and Reinbergs, E. 1970. Can. J . Plant Sci. 50, 485-491. Rajki, E., and Rajki, S. 1966. Novenyrermeles (Crop Prod.) 15, 5-20. Rajki, E.,and Rajki, S. 1968. Euphytica 17 (Suppl. I),43-48. Ranganathan, T.B., Madhava Menon, P., and Sree Rangasamy, S. R. 1973. Madras Agri. J . 60, 1134- 1138. Rao, G. M. 1965. Andhra Agric. J . 12, 1-12. Rao, A. V., Krishna, T. S., and Prasad, A. S. R. 1980. Indian J . Agric. Sci. 50, 193-197. Rasmusson, D. C., and Camel, R. Q. 1970. Crop Sci. 10, 51-54. Reitz, L. P. 1967. In “Wheat and Wheat Improvement” (K.S. Quisenbemy and L. P. Reitz, eds.), pp. 1-18. Amer. Soc. Agron., Madison, Wisconsin. Robertson, L. D., and Curtis, B. C. 1967. Crop Sci. 7, 493-495. Rodrigo, P. A. 1925. Philipp. Agric. 14, 155-171. Rogers, K. J., and Lucken, K. A 1973. N.D. Farm Res. Rooney, L. W., Merkle, 0. G., Porter, K. B., and Lahr,K. A. 1966. In “Hybrid Wheat Research in Texas,” pp. 15-19. Texas A&M Univ., College Station. Rooney, L. W., Gustafson, C. B., and Porter, K. B. 1969. Crop Sci. 9,431-435.

212


Rowell, P. L., and Miller, D. G. 1971. Crop Sci. 11,629-631. Rowell, P. L., and Miller, D. G. 1974. Crop Sci. 14, 31-34. Rutger, J. N., and Carnahan, H.L. 1981. Crop Sci. 21, 373-376. Rutger, J. N., and Shinjyo, C. 1980. I n “Innovative Approaches to Rice Breeding,” pp. 53-66. Int. Rice Res. Inst., Los Bahos,.Philippines. Sage, G. C. M. 1967. Ann. Appl. Biol. 59, 319-321. Sage, G. C. M. 1976. Adv. Agron. 28, 265-298. Sage, G. C. M., and Hobson, G. E. 1973. Euphyrica 22, 61-69. Sahadevan, P. C., and N m W , K. M. N. 1963. Proc. Indiun Acud. Sci. Sect. B 58, 176-185. Saini, S. S., and Kumar, I. 1973. Indian J . Gener. 33, 197-200. Saini, S. S., Kumar, I., and Gagneja. M. R. 1974. Euphytica 23,219-224. Sakai, K. I., and Narise, T. 1959. Annu. Rep. Natl. Inst. Genet. 10, 65. Sampath, S. 1%2. Oryza 1, 1-29. Sampath, S., and Mohanty, H.K. 1954. Curr. Sci. 23, 182-183. Saran, S., Sahu, S. P., and h a m , M. A. 1971. Indian J. Agric. Sci. 41, 515-519. Sasaki, M., Yasummuru, Y., and Nekata, N. 1979. Proc. Int. Wheut Genet. Symp. 5rh, pp. 293-298. Sasakuma, T.,and Maan, S. S. 1978. Can. J . Genet. Cyrol. 20, 389-398. Sawada, S., and Takahashi, M. 1977. J . Fuc. Agric. Hokkaido Univ. 58,497-509. Schmidt, H.J. 1976. M.S. Thesis, North Dakota State Univ., Fargo. Schmidt, J. W., Johnson, V. A., and Maan, S. S. 1962. Nebr. Exp. Stn. Q. No. 9. Schmidt, J. W., Johnson, V. A., Moms, M. R., and Mattem, P. J. 1971. Seiken Jiho 22, 113-1 18. Shaalan, M. I., Gad, A. M., Ali, A. E., El Fawal, M. A., and Elkhishen, A. A. 1975. Egypr. J . Gener. Cytol. 4, 474. Shands, R., and Kimber, G. 1973. Proc. Inr. Wheat Genet. Symp. 4rh, pp. 101-108. Shebeski, L. H. 1966. Can. J . Genet. Cyrol. 8, 375-386. Shebeski, L. H. 1971. II$ Bull. Near Easr Cereal Improvement Prod. Proj. 8, pp. 1-7. Shen, J. H.1980. I n “Rice Improvement in China and Other Asian Countries,” pp. 9-36. Int. Rice Res. Inst., Los Bahos, Philippines. Shinjyo, C. 1969. Jpn. J. Genet. 44, 149-156. Shinjyo, C. 1970. Sci. Bull. Coll. Agric. Univ. Ryukus 17, 261-272. Shinjyo, C. 1972a. Jpn. J. Genet. 47, 237-243. Shijyo, C. 1972b. Ikushugaku Znsshi (Jpn. J . Breed.) 22, 329-333. Shinjyo, C. 1975. Sci. Bull. Coll. Agric. Univ. Ryukus 22, 1-57. Shinjyo, C., and Motomura, K. 1981. lkushugaku Zasshi (Jpn. J . Breed.) 31 (Suppl. 1). 240-241. Shinjyo, C . , and Omura,T. 1966a. Ikushuguku Zasshi (Jpn. J . Breed.) 16 (Suppl. l), 179-180. Shinjyo, C., and Omura,T. 1966b. Sci. Bull. Coll. Agric. Univ. Ryukus 22, 1-57. Shinjyo, C., Nishime, R., and Watanabe, Y. 1974. Ikushuguku Zusshi (Jpn. J . Breed.) 24 (Suppl. l), 130-131. Shinjyo, C., I s m e , Y.,and Tanaki, M. 1981. Ikushuguku Zusshi (Jpn. J . Breed.) 31 (Suppl. l), 238-239. Singh,D. P., and Nanda, J. S. 1976. Indian J . Genet. 36, 10-15. Singh, K. B., and Singh, J. K. 1971. Euphyticu 20, 586-590. Singh, N. B., Singh, H. G., and Singh, P. 1977. Indian J. Genet. 37, 347-352. Singh, R. P., Singh, R. R., Singh, S. P., and Singh, R. V. 1979. Oryza 16, 115-118. S i g h , R. P., Singh, R. R., Singh, S. P., and Singh, R. V. 1980. Orym 17, 24-27. Singh,R. S. 1977. Indian J. Genet. 37, 384-387. Sinha, S. K.,and Khanna, R. 1975. Agron. J. 27, 123-174. Sivasubramanian, S., and Madava Menon, P. 1973. Madras Agric. J . 60, 1097-1102. Sivasubramanian, S., and Menon, M. 1973. Mudrus Agric. J . 60, 1139-1144.


213

Sreekumari, A. J., Namboodki, K. M. N.,and George, M. K. 1977. Agric. Res. J . Kerala 15, 13-16. Stansel, J. W., and Craigmiles, J. P. 1966. Rice J. 69, 14-15, 46. Stoskopf, N. C., and Rai, R. K. 1972. Can. J . Plnnt Sci. 52, 387-393. St. Pierre, C. A., and Trudel, M. J. 1972. Agron. Abstr., p. 28. Swaminathan, M. S., Siddiq, E. A., and Sharma, S. D. 1972. In “Rice Breeding,” pp. 609-613. Int. Rice Res. Inst., Los Baiios, Philippines. Tahir, C. M. 1969. Whear I f . Sew. Kyoto Univ. 29, 15-17. Tahir, C. M., and Tsunewaki, K. 1969. Jpn. J. Genet. 44, 1-9. TAU, C. M., and Tsunewaki, K. 1971. Can. J . Genet. Cytol. 13, 14-19. Trees, S . C. 1975. Diss. Abstr. Inr. B 35, 3387-3388. T ~ p p C. , R. 1972. Agron. Abstr., p. 29. Trupp, C. R. 1976. Crop Sci. 16,453-456. Tsunewaki, K. 1969. Seiken J i b 21, 1-5. U.S. Department of Agriculture. 1982. Foreign Agriculture Circular No. FG 32-82. Van Breda &-Haan, JYI913. Bull. Econ. Indo-China 16, 16-28. Virmani, S. S., and Athwal, D. S . 1973. Crop Sci. 13, 66-67. Virmani, S . S., and Athwal, D. S. 1974. Crop Sci. 14, 350-353. V i a n i , S. S., and Tan, Z. H. 1982. Int. Rice Res. Newsl. 7 , 3. Vinnani, S. S., Khush, G. S., and Yang, R. C. 1980a. Inr. Rice Res. Newsl. 5, 4-5. Virmani, S. S., Khush, G. S., Bacalangco, E. H., and Yang, R. C. 1980b. Int. Rice Res. Newsl. 5, 5-6. Vinnani, S. S., Chaudhary, R. C., and Khush, G. S. 1981. Oryza 18, 67-84. Virmani, S. S., Aquino, R. C., and Khush, G. S. 1982. Theor. Appl. Genet. 63, 373-380. Walton, P.D. 1971. Crop Sci. 11, 422-424. Washington, W. J., and Maan, S. S. 1974. Crop Sci. 14, 903-905. Watanabe, Y. 1971. Oryza 8,9-16. Watanabe, Y.,Sakaguchi, S., and Kudo, M. 1968. Ikushugaku Zasshi (Jpn. J . Breed.) 18 (Suppl. 2), 77-78. Watkins, R. E., and Curtis, B. C. 1967. Crop Sci. Abstr., p. 17. Weeraratne, H. 1954. Trop. Agric. (Colombo) 110, 93-97. Wells, D. G., and Lay, C. L. 1970. Crop Sci. 10, 220-223. Welsh, J. R., and Klatt, A. R. 1971. Crop Sci. 11, 864-865. Widner, J. N., and Lebsock, K. L. 1973. Crop Sci. 13, 164-167. Wilson, J. A. 1968a. Euphytica (Suppl. l), 13-33. Wilson, J. A. 1968b. Proc. Inr. Wheat Genet. Symp. 3rd, pp. 423-430. Wilson, J. A., and Ross, W. M. 1%2. Whear lnf. S e n . 14, 29-30. Wilson, J. A., and Villegas, E. 1966. Cereal Sci. Today 11, 306-309. Wilson, J. A., Wilson, J. A., and Schmidt, H. J. 1980. Proc. In?. Whear Conf. 3rd, pp. 210-214. Wu, H.P. 1968a. Bor. Bull. Acad. Sin. 9, 1-9. Wu, H. P. 1968b. Bot. Bull. Acad. Sin. 9, 121-138. Xu, S. 1982. Sci. Agric. Sin. 2, 9-14. Yabuno, T. 1977. Euphytica 26, 451-463. Yadav, S. P.,and Murty, B. R. 1976. Indian J. Genet. 36, 184-196. Yap, T. C., and Chang, S. C. 1976. SABRA0 J . 8, 35-40. Ye, D. H. 1980. “Seed Production Techniques for Indica Type Three Line Hybrid Rice.” Agric. Publ. Soc., Beijing, China. Yen, F. S., Evans, L. E., and Larter, E. N. 1969. Can. J . Gener. Cyrol. 11, 531-546. Yeung, K. C., and Larter, E. N. 1972. Can. J . Plant Sci. 52, 569-574. Yuan, L. P. 1966. Sci. Bull. 4, 32-34.


214

Yuan,L. P. 1972. Proc. Workshop Genet. 2nd. Hainan Guangdong. Yuan,L. P. 1977. ulongguo Nongye Kexue (Chinese Agric. Sci.) 1, 27-31. Zeven, Zeven, Zeven, Zeven, Zeven,

A. C. A. C. A. C. A. C. A. C.

1967. Euphytica 16, 183-189. 1%8. Annu. WheatNewsl. 15,52. 1%9. Wheat Inf. Sew. 29, 8-9. 1970. Annu. Wheat Newsl. 17,56. 1972. Euphytica 21, 468-488.


THERMODYNAMICS AND POTASSIUM EXCHANGE IN SOILS AND CLAY MINERALS Keith W. T. Goulding Soils and Plant Nutrition Department, Rothamsted Experimental Station Harpenden, Hertfordshire, United Kingdom

I. Introduction ... ....................................................... 11. The Thermodynamics of Ion-Exchange Equilibria. . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Basic Equations . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. ... . . . . . . . . . . . . B. Standard States.. . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . .. . .. ... . . . . . . . . . . . C. Ionic Strength and Hysteresis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Standard Free Energies, Enthalpies, and Entropies . . . . . . . . . . . . . . . . . . . . . . E. Adsorbed-Ion Activity Coefficients.. . . . . . . . . . . . . . . . , . . . . . . . , . F. Excess Functions . . . ...................... G. Incomplete Exchange ge............................. H. Ternary Exchange. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. ... . . . . . . . . . . . . 111. Calorimetry in Ion-Exchange Studies.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .. ... . . . .. . . . . . A. History and Techniques . . . . . . . . . . . . .. . . . . . . . . B. Standard, Integral, and Differential Enthalpies of Exchange.. . . . . . . . . . . . . . IV. Thermodynamics Applied to Potassium Exchange in Soils and Clay Minerals. . . . . A. Background.. . .. . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . . .. . .. . . . . . . . . . .. B. Comparing Potassium with Other Cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Comparing Clay Minerals . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Comparing Soils .................................................. E. Potassium Selectivity and Fixation . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Potassium Potentials. . . ..................................... V. Exchange Equilibrium and the xchange . ... .. . . . . . . . . . . VI. Summary and Conclusions . . . . ....................... VII. Appendix: List of Symbols.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References ...........................................................

.

215 217 217 221 223 223

227 227 228 228 230 233 233 236 241 243 248 253 256 258 259 260

1. INTRODUCTION In 1972, reporting on the second working session of the ninth Colloquium of the International Potash Institute (IPI) entitled “Ion Exchange System of the Soil,” Walsh said, “The use of thermodynamic functions . . . is in many cases 215

Cojyight 0 by Academic Ress, Inc. All rights of repoduaion in m y form reserved. ISBN 0-126007363

216

KEITH W.T.GOULDING

remote, as they tend to integrate a variable quantity over a range of K saturations to give a kind of ‘average’ value which may be intellectually satisfying but not always useful . . . As a final comment, it might be advanced that perhaps at least some of the work now in progress becomes too theoretical and academic, beiig far removed from what actually happens to the soil as a specific entity in the field.” These comments will no doubt find sympathy with many readers. However, in his concluding remarks at the same colloquium, Schroeder (1972) said, “In the field of potassium in the exchange system it is my opinion that the thermodynamic approach has helped to overcome the stagnation of the past two decades.” More recently, Cooke and Gething (1978) said, at the eleventh Congress of the P I , “Walsh (1972) gave a pessimistic assessment of practical progress but we have in fact advanced, if only in that empirical methods have been largely abandoned.” This article attempts to show that, as well as stimulating research on potassium exchange as suggested by Schroeder (1972), the use of thermodynamics has greatly increased our understanding of the exchange complex and potassium furation and release and therefore has had positive practical results. Results of the last 10 years are emphasized; research before 1972 has been well reviewed by Schuffelen (1972), van Blade1 (1972), and Talibudeen (1972). However, earlier papers considered to be of particular importance are discussed where relevant. When referring to the thermodynamics of potassium exchange, workers invariably mean exchange equilibria and thus equilibrium thermodynamics. The attainment of equilibrium in the laboratory is entirely possible, but in the field the exchange of potassium between soil and solution is a “dynamic equilibrium,” if it is an equilibrium at all (see Hoagland and Martin, 1933; Cooke and Gething, 1978; Cooke, 1979). Some have tried to accommodate this probem by applying nonequilibrium thermodynamics to ion exchange in soils (Ravina and de Bock, 1974), but although results from the application of such methods agreed with those obtained from equilibrium thermodynamics, the method has never been put to use. The history of the development of our understanding of ion exchange, from its discovery in agriculture by Way (1850) to the present, has been recounted by Thomas (1977). The history of the development of the mathematical equations and theoretical concepts used has been covered by Sposito (1981a,b). Only the essential elements of these approaches that specifically relate to potassium will be considered. The basic elements of the thermodynamic treatment of ion exchange will be examined and then the application of these to potassium exchange will be discussed. Thermodynamics is about equations. These will be kept to a minimum and as simple as possible, but they cannot be avoided because an understanding of them is necessary for a full appreciation of the thermodynamics of potassium exchange.

THERMODYNAMICS AND POTASSIUM EXCHANGE

217

II. THE THERMODYNAMICS OF ION-EXCHANGE EQUILlBRlA A. BASIC~ U A T I O N S

The 2:1 layer aluminosilicate minerals in soils have a permanent negative charge caused by isomorphous substitution in the lattice of AP for Si4+ and of Mg2+ for A P + . They also have a pH-dependent charge, which is negative at high pH because of the dissociation of protons from surface hydroxyl groups and positive at low pH because of the adsorption of protons by such groups. The pHdependent charge is therefore positive below and negative above the isoelectric point of the solid, and it is superimposed on, and invariably less than, the permanent negative charge. The excess negative charge is neutralized by adsorbed cations which exchange with other cations in solution when spatially accessible. The 1:l layer silicate minerals and amorphous oxides of Fe, Al, and Si usually have a pH-dependent charge that contributes to the exchange capacity of the inorganic fraction of the soil. Soil organic matter also has a pH-dependent charge, arising from the association and dissociation of protons linked with amino, phenolic, and carboxylic groups. Although it is usually only a small percentage of the soil by weight, organic matter has a negative charge in the range 2000-4000 peq/g; thus, it can make an important contribution to the cation-exchange capacity (CEC) of the soil. More complete descriptions of the nature of ion exchange and the sources of charge were given by Schuffelen (1972) and Talibudeen (1981). The exchange reaction between adsorbed cation A of valency u and solution cation B of valency v is described by the general equation +

vA-(soil),

For the Ca2 + K +

+

+ uBY+

uB-(soil),

+ vAu+

(1)

exchange, the equation becomes Ca-(soil)z

+ 2 K+

2 K-(soil)

+ CaZ+

(2)

The double arrow (=)implies that the reaction is reversible (Section 11,G). For a reversible reaction such as Eq. (1) at equilibrium, the thermodynamic equilibrium constant K is given by

K =

+I" [B-(s~il),]~[A~ [A-(soil),]"[BV 1"

(3)

+

or again, for Ca2 + K +

+

exchange,

K=

[K-(soil)12[Ca2 ] [Ca-(soil),][K l2 +

+

where the square brackets refer to activities.

(4)

KEITH W.T. GOULDING

218

Many workers have suggested empirical relationships similar to Eq. (3) in an attempt to define an equilibrium constant, because such a constant would be valuable to soil science for predicting the state of the equilibrium at different concentrations. Some of the better known exaniples are those of Ken (1928), Vanselow (1932), and Gapon (1933); all were well reviewed by Bolt (1967) and by Sposito (1981a). In a series of papers, Sposito (1977), Sposito and Mattigod (1979), Oster and Sposito (1980), and El-Prince and Sposito (1981) have shown that these empirical “constants” can be derived from thermodynamic principles. However, in practical tests none of them have been found to be truly constant over the whole of the exchange process, although some, such as Gapon’s constant, KG, have proved to be very useful in practice. They are thus better described as equilibrium or selectiviry coeficients. The true thermodynamic equilibrium constant is exactly what is required. Unfortunately, it cannot be obtained directly because, although the activities of ions in solution can be measured, those of adsorbed cations cannot. Nevertheless, the latter activities can be approximated by relating them to experimentally measurable quantities, and as Sposito (1981a,b) shows, all, of the empirical constants can be derived from Eq. (3) by choosing suitable expressions for the activities. The two most important forms of the selectivity coefficient as regards the thermodynamics of K+ exchange are those of Vanselow (1932) and Gaines and Thomas (1953). Vanselow (1932) approximated adsorbed-ion activities by mole fractions, N, and wrote for the reaction in Eq. (1)

K, =

Nf; [Au+Iv NS [Bv+IU

The Vanselow selectivity coefficient, K,, equals K only if the mixture is ideal, that is, if activities = mole fractions (Guggenheim, 1967). Ca2+-Mg2+ exchange and K -Rb exchange most nearly approximate this. When the mixture is not ideal, activities must be related to mole fractions by activity coefficients, J and thus, in the Vanselow convention, +

+

K =

NvB[Au N r A [Bv

+

+

1” 1”

where f A

= -[A1 NA

(7)

Gaines and Thomas (1953) also defined an adsorbed-ion activity coefficient, g, but using equivalent fractions, E. Thus


219

(The question of the use of different conventions to define adsorbed-ion activity coefficients is discussed in full in Section II,E). Gaines and Thomas’ selectivity coefficient, K,, is thus

K, =

Eij[A” +Iv PA[BV+lU

(9)

and, by the Gaines and Thomas convention,

The Gaines and Thomas thermodynamic treatment of ion exchange, from which Eqs. (8)-(10) are taken, stimulated much research into ion exchange in both “pure” clays and soils and other exchange materials such as resins and zeolites (see Section IV,A). Therefore, when explaining the derivation of equations used, many workers have referred to the “Gaines and Thomas Method” or the “Gaines and Thomas Treatment” (e.g., Hutcheon, 1966; Deist and Talibudeen, 1967b; Laudelout et al., 1968a; Talibudeen, 1981). However, others have often referred to Argersinger et al. (1950) or to the “Argersinger Thermodynamic Approach” when making reference to the source of thermodynamicequations used (e.g., Jensen, 1973a). Argersinger et al. (1950) and Hogfeldt and co-workers (Ekedahl et al., 1950; Hogfeldt et al., 1950) first derived (independently) a set of general thermodynamic equations for ion exchange. They were based on the Vanselow (1932) convention of mole fractions. Gaines and Thomas (1953), although referring to Argersinger et al. (1950) and Ekedahl et al. (1950), made their own, thermodynamically more rigorous derivation of a set of equations based on equivalent fractions. For homovalent exchange (exchange between ions of the same valency), mole and equivalent fractions are equal and so the two approaches give the same results. For heterovalent exchange (exchange between ions of unequal valency), mole and equivalent fractions are not equal and thus neither are most of the thermodynamic parameters derived by the two methods. Any paper reporting thermodynamic data must therefore be carefully examined for the convention used, and of course direct comparison between data derived from the two conventions may not be possible (but see Section II,E). As stated previously, K, or K, do not equal K unless the mixture is ideal. However, K can be calculated from K, or K, by integrating over the whole exchange [i.e., EB = 0 to 1, as shown by Gaines and Thomas (1953)]. For an exchange as in &. (l), this gives a complex equation following Gaines and Thomas’ convention

220

KEITH W.T.GOULDING

The last term of the equation represents the change in water activity (in effect the change in water content of the soil) in going from an A-(soil) to a B-(soil). This term has been found in practice to be negligible (Gaines and Thomas, 1955; Laudelout and Thomas, 1965). The third term on the right-hand side of Eq. (11) is made zero by the choice of suitable standard states (Section II,B), or by assuming that g L = gff, which is not generally true. So there remains the simplified form of Eq. (1 1) most often used In K = (u-v)

+

L

In K, dE,

In the Vanselow convention, this becomes In K

=

I:

InK,dE,

We now have basic equations for obtaining a selectivity coefficient, Eq.( 5 ) or (9), and a thermodynamic equilibrium constant, Eq. (12) or (13), from experimental exchange equilibrium data. Other parameters are estimated as outlined in Section II,D,E, and F. Exchange isotherms are often presented in therodynamic analyses of exchange data. These are plots of the equivalent fraction of an adsorbed cation against that of the same cation in solution (Fig. 1). Their application is discussed in Section IV,A. Also, sometimes an ‘‘uncorrected selectivity coefficient” is used, called Khor KL (e.g., van Blade1 and Laudelout, 1967). This is given, again following the Gaines and Thomas convention, by

Equivaknt fraction of K+ in solution

.,

FIG.1. The exchange isotherm. A graph of the equivalent fraction of an adsorbed cation versus its equivalent fraction in solution. This example: K+-CaZ+ exchange on soil showing hysteresis. C a + K,0,K -+ Ca. After Deist and Talibudeen (1967a).


22 1

where mA and mg are molarities. It thus represents a selectivity coefficient uncorrected for activities in solution. B. STANDARD STATES

To understand what these are and why they are important one must look at the definition of ion activity and of equilibrium itself. The condition for chemical equilibrium in any system is that the chemical potentials (p)of each component of the system are equal throughout the system. Thus in a cation-exchange reaction such as that given in Eiq. (l), vp[A-(soil),]

+ up[B"+] = up[B-(soil),] + vp[AU+1

(15)

But p represents an intrinsic chemical property that cannot be identified with a universal scale (such as temperature), nor accorded a reference value of zero in the absence of the substance to which it refers. It is thus necessary to adopt a conventional reference or standard state for the substance at which p is zero (Sposito, 1981b; Talibudeen, 1981). The chemical potential in its standard state is written as po,and it can be shown (see Sposito, 1981b, Chapter 2) that p = po + R T l n a

(16)

where R is the gas constant, T is the absolute temperature, and a is the activity. Thus the activity of an ion is a measure of the deviation of the chemical potential of that ion from its value in the standard state, and the activity of an ion in its standard state is 1. Therefore, before thermodynamic quantities for exchange equilibria can be calculated, standard states must be defined for each phase; their choice affects greatly the value of such quantities and their physical interpretation. The various standard states adopted for exchanger and solution phases were discussed in full by Sposito (1981b). A list of the more important ones, and the practical results of their use, is given in Table I. The only ones commonly used are those suggested by Gaines and Thomas (1953), with a slight modification for practical reasons. In practice, the standard state for adsorbed cations is taken as being a homoionic exchanger in equilibrium with a solution of the saturating cation at constant ionic strength. The experimental results can be obtained at several ionic strengths and extrapolated back to zero, the standard state specified by Gaines and Thomas, as suggested by van Blade1 and Laudelout (1967) (but see Section 11,C). However, it appears that the values of activities in exchange reactions on soils and clays depend very little on concentration (Jensen, 1973a; Jensen and Babcock, 1973); this is a fortunate result, as such an extrapolation is rarely made in practice.

Table I

Some of the Standard States Used in Calculating the Thermodynamic Parameters of Cation-Exchange Equilibria ~~

~~

Standard states Adsorbed phase

Solution phase

Implications

Reference

Activity = mole fraction when the Activity = molarity as concentration Can calculatef,Kv, etc., but all depend on Argersinger er al. (1950) latter = 1 +o ionic strength Homoionic exchanger in equilibrium Activity = molarity as concentration AG' expresses relative affiiity of exchanger Gaines and Thomas (1953) with an infinitely dilute solution +0 for cations of the ion Activity = mole fraction when the Activity = molarity as concentration AGO expresses relative affiinity of exchanger Babcock (1963) latter = 0.5. Components nor in +0 for cations when mole fraction = 0.5 equilibrium


223

C. IONICSTRENGTH AND HYSTERESIS

van Blade1 and Laudelout (1967) found hysteresis of exchange isotherms during heterovalent exchange reactions involving the selectively adsorbed NH, ion (almost identical in size and hydration to K +). Hysteresis means that forward and reverse exchange isotherms are not the same, as in Fig. 1. They also found a large variation in the uncorrected selectivity coefficient, Kf, with ionic strength I and suggested that both were caused by clay aggregation at finite ionic strength. They reasoned that such aggregation would not occur at the standard state ionic strength of zero. Therefore, to avoid the problem of hysteresis and the need to calculate activity coefficients of ions in solution (y), they plotted log Kf against (2J)f (finding this empirically to be a linear relationship) and extrapolated to (2Z)l = 0 where, by definition, y = 1 and thus Kf = K,. This supported earlier theoretical work by Laudelout and Thomas (1965), who had derived an equation predicting a linear relationship between In K, and solution concentration at any one cation ratio. However, Laudelout et al. (1972) found a maximum change in In K , of only 9% in going from 0.01 M to 0.2 M ,showing that much of the variation in Kf is corrected for by calculating activity coefficients in solution. In addition, although isotherms for heterovalent exchange do often exhibit hysteresis, selectivity for the ion of higher valency, as shown by the exchange isotherm or Kf, increases continuously as ionic strength decreases. Thus, as the ionic strength approaches zero, isotherms become rectangular (i.e., become increasingly close to the x and y axes of the graph) and Kf tends to infinity (Barrer and Klinowski, 1974). Thus the log Kf versus ( U ) d relationship cannot have a finite linear slope over a large range of I, and any extrapblation to (2J)f = 0 which gives a finite value of log Kf is incorrect. It would seem much more sensible in experimental work, therefore, to calculate y values and use K, at a known ionic strength to determine ion selectivity. It is also worth noting that Barrer and Klinowski (1974) presented a method for calculating exchange isotherms (and therefore K, values) at any solution concentration when an isotherm has been experimentally measured. Thus with modem computing methods little effort is required to measure cation selectivity in a soil over a whole range of soil solution concentrations. +

D. STANDARD FREEENERGIES,ENTHALPIES,AND ENTROPIES

Many publications have examined cation selectivity during the exchange process by using selectivity coefficients and have drawn important conclusions from them (e.g., with respect to potassium, see Bolt et al., 1963; van Schouwenberg and Schuffelen, 1963; Marques, 1968). Often, however, the overall selectivity or preference of the soil for one of a pair of cations is required, perhaps for

KEITH W. T. GOLJLDING

224

comparison with other cation pairs (Section IV,A) or of soils (Section IV,B). This could be achieved through the thermodynamicequilibrium constant, which integrates selectivity over the whole exchange process, although it is usually expressed by the free energy function. The standard Gibbs free energy of exchange, AGO, is calculated from the experimentally determined thermodynamic equilibrium constant, K, using the relationship AGO = -RT In K

(17)

It is the difference in free energy between the two homoionic forms of the soil or clay at the chosen standard state. It has been stated that AGO defines the difference in the strength of binding between the soil and the two cations (Drake, 1964; Deist and Talibudeen, 1%7a), but this is incorrect. The free energy term is the sum of ion binding strength, expressed by the standard enthalpy of exchange, AH",and the degree of order of the system, expressed by the standard entropy of exchange, Af'. The relationship between these three standard functions is given by the familiar Gibbs equation

AG"

=

AH" - TAP

(18)

As well as being directly measurable by calorimetry (Section III), enthalpies can be calculated from measurementsof the thermodynamicequilibriumconstant at two temperatures, T, and T2,using the Van? Hoff equation ln(K2/K,) =

-AH(11T2 - l/Tl) R

The standard entropy of exchange is then calculated from AGO and AH'values using Eq. (18). The physical interpretation of the three parameters is discussed fully in Section IV. E. ADSORBED-ION ACTIVITY COEFFICIENTS

Absorbed-ion activity coefficients are central to the development of a set of thermodynamic equations describing cation exchange (Section 11,A). Although they cannot be measured experimentally, they can be calculated from the measured selectivity coefficient (for derivations, see Gaines and Thomas, 1953; Sposito, 1981b). For the general exchange reaction described in Eq.(l), and for Coefficients (g) defined by equivalent fractions according to the Gaines and Thomas conventions, vln g,

=

E,[ln K, - (u-v)] - I S l n K , dE,


225

and

For Ca2+ + K + exchange, these become In g,

= EK(ln K,- 1) -

In K, dEK

and 2 In g, = (l-EK) (1-ln K,)

+

I

1

In K , dE,

(23)

EK

These equations have been used in the majority of papers where adsorbed-ion activity coefficients have been calculated (e.g., Hutcheon, 1966; Deist and Talibudeen, 1967a,b; Goulding and Talibudeen, 1980). The equations forf values, derived according to Vanselow ’s convention using mole fractions instead of equivalent fractions, are of necessity slightly different (see Sposito, 198lb) and have been used only by Jensen (1973a). Activity coefficients, by definition, correct the equivalent or mole fraction terms for departure from ideality (Section 11,A). They thus reflect the change in the status, or fugacity, of the ion held at exchange sites, and thus the heterogeneity in the exchange process, as is shown experimentally in Section IV,B,C, and D. Adsorbed-ion activity coefficients used in soil and clay studies have almost always been calculated according to Gaines and Thomas’ (1953) procedure, but Sposito and Mattigod (1979) and Sposito (1981a,b) have questioned this. They state that the Gaines and Thomas-type adsorbed-ion activity coefficients are not true thermodynamic activity coefficients because they are defined by equivalent fractions [Eq.@))I rather than by mole fractions as in Vanselow’s convention [Eq.(7)]. This problem has been discussed in detail by Goulding (1983). Briefly, although the absolute values of the two types of coefficients are not the same (except at Ei= 1, where gi=& = 1 by definition), plots of g j versus Ei are very similar to those of f i versus Ei, as shown in Fig. 2, and result in similar conclusions as to cation behavior during an exchange reaction. Also, as will be shown later (Section IV,B,2), the girelate to heterogeneity as shown by calorimetrically measured enthalpies of exchange and thus have a sensible physical interpretation. Sposito and Mattigod (1979) gave expressions relating gi and fi, and also K , and K,. In this article, the symbolfwill be used for adsorbed-ion activity coefficients, and all values referred to have been calculated according to the Gaines and Thomas convention.

KEITH W . T . GOULDING

226

.4-

O u)

4.0-B

L)

c

0

0.2

0.4

0.6

0.8

1.0

Fractional K saturation

FIG.2. Adsorbed ion activity coefficients, calculated according to Vanselow’s v) and Gaines and Thomas’ (g) conventions, as a function of fractional K + saturation for (A) Ca2+ + K+ exchange on Hanvell series soil, U.K. (Deist and Talibudeen, 1967a); (B) A13+ + K+ exchange on Palm Garden Soil, Tea Research Institute, Sri Lanka (Talibudeen, 1972). From Goulding (1983).

F. EXCESSFIJNCITONS

Excess functions form the “ultimate” calculation from exchange equilibrium data and have rarely been used in soil and clay studies. They account for the properties of the exchange complex in terms of the activity coefficients of both adsorbed ions and were first introduced in studies of ion exchange in zeolites (Barrer et al., 1963). As was found for adsorbed-ion activity coefficients (Section II,E), they describe exchange heterogeneity qualitatively in soils and clays (Goulding, 1980). Excess free energies (AGE), enthalpies (AHE), and entropies (AYE) are calculated at chosen cation saturations (EB)from the following equations:

UVACE= vE,RT In fA

+ u(l -E,)RT

ASE = [AHE - ACE]/T

In fB

(24)

(26)

A H E values can also be calculated from the temperature coefficient off, and fB (Talibudeen, 1971). Excess functions were used to describe NH,+-Sr2+ exchange on mont-

227


morillonite by Laudelout et al. (1968b) and K -Ca2 on soils by Talibudeen (1971, 1972). +

+

and K -AP +

+

exchange

G . INCOMPLETE EXCHANGE AND MIXED EXCHANGERS

Incomplete exchange implies that in an exchange such as that described in Eq. (l), entering cations (B) cannot replace all the adsorbed cations (A). The reaction is thus not completely reversible. There are three cases in which this can occur: 1. A time-dependent hysteresis occurs between forward and reverse isotherms (i.e., a maximum in the equivalent fraction of the entering cation appears to have been reached, but it increases with time) 2. A definite maximum content of B is reached which is less than complete exchange and independent of temperature 3. A definite maximum is reached which varies with temperature. The first case cannot be analyzed by equilibrium thermodynamics, but Barrer et al. (1973) present a method for treating the second and third which will not be examined in detail because it has not been used in soil or clay exchange work. It involves separating exchange sites into those that can be occupied by A or B ions and those that can only be occupied by A ions. Selectivity coefficients and thermodynamic equilibrium constants are obtained for the two sets of sites separately. The ion-exchange complexes of soils are always mixed exchanger systems. As Sposito (1981b) says, thermodynamic systems in soil may often be treated as if they were homogeneous for the analysis of experimental data (and almost always have been in ion-exchange work). But soils are truly polyfunctional ion exchangers and really should be treated as such. Sposito shows how this can be achieved, based on work of Barrer and Klinowski (1979), again by splitting exchange sites into classes, considering each separately, and then obtaining a weighted geometric mean of the thermodynamic functions at the end. Such a treatment is very complex and has not yet been used in practice, although Munns (1976) separated K + adsorbed on volcanic ash soils into tightly and loosely bound fractions by a similar procedure. However, modem computing methods make the treatment of such mixed exchanger systems, and incomplete exchange, perfectly feasible. Thus, although not yet of great importance in relation to K + exchange, these methods may well prove more useful in the future. H. TERNARY EXCHANGE

Cation-exchange experiments in the laboratory can be restricted to binary (two-cation) exchange. In the field, however, ion exchange is rarely binary, although in many soils the real situation can be well approximated by considering

228

KEITH W.T.GOULDING

only the dominant cations (e.g., K+-Ca2+ in calcareous soils, K+-Na+ or Ca2+-Na + in saline soils, and K -A13 in acid soils). As a move toward a more realistic approximation of field conditions, attempts have been made to develop a thermodynamic treatment of ternary (three-cation) exchange. El-Prince and Babcock (1975) were the first to try this, basing their equations on a model developed by Wilson (1964) for calculating activity coefficients for three-component systems from mole fractions. It was thought then that all the constants in the model could be calculated from binary exchange data. El-Prince and Babcock (1975) calculated isotherms for Na+ -Rb+ -Cs exchange on Chambers montmorillonite and for Na -K -Cs exchange on attapulgite. These isotherms suggested that the qualitative selectivity rules that applied to binary exchange also applied to ternary exchange, in that selectivity followed the lyotropic series (Section IV,B,l). Wiedenfeld and Hossner (1978) used the same equations for Ca2+-MgZ+-Na+ exchange in saline soils, and plotted threedimensional exchange isotherms. They found that the results were “in agreement with recognized properties of the cations,” in that Ca2+ and Mg2+ were selectively adsorbed. In neither of these reports were experimental data provided to test the model, however. El-Prince et al. (1980) tested this “subregular model” of Wilson (1964) against data for NH, -Ba2 -La3 exchange on a Nevada montmorillonite and found calculated results in “reasonably good agreement with experimental data.” The model has been questioned by Chu and Sposito (1981). They calculated a set of general thermodynamic equations for ternary exchange and showed with them that the subregular model was not solely dependent on binary exchange data. One of the model constants required data from ternary exchange for its calculation, although its value was often insignificant by comparison with other terms. This perhaps explains the good agreement between calculated and experimental results found by El-Prince et al. (1980). Unfortunately, Chu and Sposito (1981) did not have enough experimental data for ternary exchange to test their set of equations. +

+

+

+

+

+

+

+

+

111. CALORIMETRY IN ION-EXCHANGE STUDIES A. HISTORY AND TECHNIQUES

The enthalpy change of a chemical reaction expresses the gain or loss of heat during the reaction. The reaction may be exothermic, in which case the change of enthalpy is negative and heat is lost to the surroundings. Alternatively it may be endothermic, in which case the enthalpy change is positive and heat is gained from the surroundings. Very few reactions have an enthalpy change of zero. The


229

enthalpy change is the result of the breakage and formation of chemical bonds, and the enthalpy of a reaction is the sum of all such events in the reactants and products, including solvent molecules (i.e., hydration enthalpies). A negative enthalpy change implies stronger bonds within the products and a positive enthalpy change implies stronger bonds in the reactants. The enthalpy of ion exchange is therefore a direct measurement of binding strength and is an important quantity. Coleman (1952) was the first to use calorimetry to measure standard enthalpies of ion exchange (W). He obtained enthalpies of H -Na+ and H -K exchange on a “Volclay” bentonite and an ion-exchange resin by direct measurement, and also from enthalpies of neutralization of the H+-clay or resin by subtracting the enthalpy of neutralization of the corresponding acid and alkali. Results for these two methods agreed to within 0.5 W/mol. Enthalpies of Na+-K+, Ba2+-Ca2+, and Ca2+-K+ exchange were also measured and compared with values calculated from exchange isotherm data at two temperatures using Fq. (19) in Section I1,D. Agreement was again good, always within *lo% or k0.2 kJ/mol. Coleman’s values were somewhat uncertain because he made no attempt to assess the extent of exchange after the reaction, assuming that adding a large excess of the replacing ion would give complete exchange. [That this error was small can be inferred from the results of Maes and Cremers (1977) who obtained exchange levels of 93-94% in Ca2 -Na+ exchange using this “excess” method.] Cruickshank and Meares (1957) overcame this problem by measuring the enthalpy of the forward and reverse reactions of A-B exchange to a common equilibrium point. By ensuring that the total molality at equilibrium was the same for both experiments, and taking the algebraic difference between the enthalpies with correction for dilution, etc. (see Section III,B), they obtained the standard enthalpy of A-B exchange. Meanwhile, Calvet and Pratt (1956) designed a microcalorimeter for measuring enthalpies of various physicochemical and biological reactions. Martin and Laudelout (1963) compared three methods for determining AZf” using this type of calorimeter: +

+

+

+

1. Coleman’s (1952) method of measuring the enthalpy of neutralization of an H+-clay with an alkali, X - O H , which, after subtracting the enthalpy of neutralization, gives A Z f O for H -X exchange 2. For exchange not involving H + , the enthalpy of complete exchange is measured at several ionic strengths, I , and extrapolated to I = 0 where the measured enthalpy = AZf” by definition 3. Cruickshank and Meares’ (1957) method of measuring enthalpies of A + B and B + A exchange to a common equilibrium point and subtracting the values. +

230

KEITH W.T.GOULDING

The results from the three methods were in reasonable agreement; those between the second and third methods differed by as much as 1.2 kJ/eq, and those between the f m t method and the other two differed by as much as 3 kJ/eq.Many workers have since used the third method. Laudelout et al. (1968a) measured enthalpies of exchange for various ion pairs on Camp Berteau montmorillonite and obtained results in good agreement (10% lower) with those calculated from exchange isotherm data, as did Gast et al. (1969). Calorimetry has also been used to obtain a more detailed picture of enthalpy changes during an exchange reaction. In fact, the f m t important application of calorimetry to ion exchange was made by Barrer et al. (1963), who showed how enthalpy changes during ion-exchange reactions on zeolites, involving Li , Na+ , Cs+ , K + , Rb+ , and Ca2+, reflected different types of exchange sites. The integral and dzyerential enthalpies obtained by Barrer et al. (explained in Section III,B) were laboriously obtained by making many individual experiments during several days. The third method listed was used and the enthalpies were measured at as many as a dozen different equilibrium points. Pipetting and injecting devices have been developed which enable enthalpies at different cation saturations to be measured in one continuous experimental run. Harter and Kilcullen (1976) designed a pipetting device for adding and mixing exact amounts of solution to a Calvet microcalorimeter which makes multiple reactions possible. They tested the device on reactions between clay and organic materials. Unfortunately, in making the additions and mixes, the equipment generated amounts of frictional heat comparable with heats measured in ion-exchange experiments. Talibudeen et al. (1977) and Minter and Talibudeen (1982) developed an automated injection system for an LKB microcalorimeter and used it to measure enthalpies of K+-Ca2+ exchange on soils and clays. The technique enables a complete exchange reaction, including as many as 20 points on the isotherm, to be measured in 2-3 days; it is sensitive enough to measure heat changes as small as 0.1 ml. The technique is outlined in Section III,B and its applications are described in Section IV,A-E. +

B. STANDARD, INTEGRAL,AND DIFFERENTIAL ENTHALPIES OF EXCHANGE

The calorimetrically measured enthalpy of an ion-exchange reaction cannot be equatedper se with the standard enthalpy of exchange, M ,as obtained directly from exchange isotherm measurements at two or more temperatures. The measured enthalpy change represents the sum of all the enthalpy changes from (1) the cation exchange reaction, involving the exchange of cations and the hydration of cations and surface; (2) the solvation of the solid; (3) the dilution of the salt solutions when mixed, and (4) the mechanical injection and mixing processes in the calorimeter.


23 1

The design of modem microcalorimeters, with two reaction cells connected electrically in opposition, enables these enthalpy changes (2)-(4) to be compensated experimentally (Maes et al., 1976; Talibudeen et al., 1977). We are thus left with component (l), the enthalpy of exchange, which corresponds to the experimental conditions used and not the standard state. However, the correction required to convert the experimental enthalpy to the standard enthalpy is merely the enthalpy of dilution of the two salts from their final state to infinite dilution (often referred to as the “difference in the apparent molar heat contents of the two salts”; see Laudelout et al., 1968a; Talibudeen et al., 1977). These values are available from tables, but at the concentrationsused are always within experimental error (Cruickshank and Meares, 1957; Barrer et al., 1963; Talibudeen et al., 1977). That this is a reasonable assumption is confirmed by the excellent agreement between enthalpies measured calorimetrically and those calculated from exchange isotherms (see Laudelout et al., 1968a; Maes et al., 1976). The standard enthalpy of exchange expresses the difference in binding strength between one homonionic form of an exchanger and another. Although this gross comparison is very useful, a more detailed picture of how binding strength varies at different cation ratios would be even more useful (see Section I). For K + exchange in particular, the first 5-1096 K + saturation is the part of the exchange process most important to crops, because the Kf saturation of a soil seldom exceeds 10% even after many years of treatment with K fertilizers. The measurement of enthalpies by calorimetry makes possible a detailed analysis of such regions as well as the determination of the enthalpy of the whole reaction, as shown by Talibudeen et al. (1977), Goulding and Talibudeen (1979, 1980), and Talibudeen and Goulding (1983a,b). The technique for Ca2+ --* K + exchange involves adding a small amount of KCl solution to a suspension of a known amount of the Ca2+ form of a soil or clay. The enthalpy of the resulting exchange of some of the Ca2+ ions for K + ions is measured and the procedure is repeated. The extent of exchange at each step is measured in a separate exchange isotherm experiment, and the cumulative, or integral, enthalpy change (AHx) can then be plotted against the cumulative K + saturation (x), as in Fig. 3. A detailed description of the method was given by Talibudeen et al. (1977). The reverse reaction, K + --* Ca2+, can also be followed to check variability and reversibility (as is also shown in Fig. 3). The value of AHx at x = 1 is equal to the standard enthalpy of exchange within experimental error as explained earlier. The mxversus x curve, in almost every case, takes the form of a series of linear segments separated by sharp changes in slope, a feature also noted for some ion-exchange reactions in zeolites by Barrer et al. (1963). This characteristic is very significant, suggesting sharply defined groups of homogeneous exchange sites within a heterogeneous structure. To ensure that this observation is correct, a model consisting of a series of linear segments is fitted by the least

KEITH W.T.GOULDWG

232

X

FIG. 3. The integral enthalpy of exchange (AH,)as a function of fractional K+ saturation (x) for K -Ca* + exchange on Upton montmofionite and Fithian ate.Some data points are omitted for clarity. D, Ca + K 0, K -P Ca. After Godding and Talibudeen (1980). +

squares approximation to the AHx versus x plot and compared with the best smooth curve that can be fitted (Goulding and Talibudeen, 1980). In every case the stepped straight lines give the best fit, and so there is a clear indication of heterogeneity in the exchange process. This heterogeneity is more clearly seen when the slope of the AHx versus x curve, the differential enthalpy of exchange [d(AH,)ldr], is plotted against x , as in Fig. 4.Such an analysis has been used to give new information on clay mineralogy (Section IV,C). To complete the set of differential functions, differential free energies [d(AGx)ldr]can be calculated from selectivity coefficients as shown by Clearfield and Kullberg (1974) with

Montmorillonite

0

0.2

0.6

0.4

0.8

1.0

X

FIG. 4. The differential enthalpy of exchange [d(M,)/dx)] as a function of fractional K+ saturation (x) for K+-Ca2+ exchange on Upton montmorilloniteand Fithian illite. After Goulding and Talibudeen (1980).


233

Differential entropies can then be calculated using a modified form of E q . (18):

and entropy changes during an exchange reaction can thus be analyzed.

IV. THERMODYNAMICS APPLIED TO POTASSIUM EXCHANGE IN SOILS AND CLAY MINERALS A. BACKGROUND

Ion exchange was first studied systematically by Thompson (1850) and Way (1850, 1852), who examined the adsorption of ammonia by soil and the cations subsequently released. Such an interest in the subject from the aspect of plant nutrition has continued, but in the early-to-middle twentieth century, naturally occurring and synthetic zeolites, and then synthetic (organic) resinous exchangers, became the main field of study. Interest in clay minerals as cation exchangers was renewed in the 1940s and 1950s, however, when large quantities of radioactive wastes began to be produced. The fixing of such nuclides as 137Cs ,Y j r 2 ,'Wo2 ,and 64Zn2 in clay mineral deposits underground was seen as a cheap and easy way of disposing of them. Therefore much research effort was put into understanding and predicting their ion-exchange properties, and the Gaines and Thomas (1953) method (Section II,A) was a direct result of this. Thomas and co-workers in the United States (Gaines and Thomas, 1953, 1955;Faucher and Thomas, 1954; Merriam and Thomas, 1956) were responsible for most of the early applications to clay studies. Laudelout set up a research group in Belgium, again concerned primarily with clays (e.g., Martin and Laudelout, 1963; Laudelout and Thomas, 1965; Cremers and Laudelout, 1966; van Blade1 and Laudelout, 1967; Laudelout e l al., 1968a,b), and Gast and coworkers in the United States (Gast, 1968, 1969, 1972; Gast et al., 1969) concentrated on alkali metal cation selectivity. None of these groups was primarily interested in potassium or even in soils. Hutcheon (1966) first applied the Gaines and Thomas method specifically to potassium exchange, using montmorillonite as a relatively simple exchanger. Tailbudeen and co-workers in Great Britain (Deist and Talibudeen, 1967a,b; Coulter and Talibudeen, 1968; Talibudeen, 1972; Goulding and Talibudeen, 1979, 1980) used Gaines and Thomas' equations to study potassium exchange in soils and clays, and Jensen in Denmark also +

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KEITH W.T.GOULDING

234

investigated K exchange using thermodynamic techniques (Jensen, 1972, 1973a,b, 1975; Jensen and Babcock, 1973) but based on equations developed by Argersinger et al. (1950) (see Section &A). The following parameters are derived from a thermodynamic analysis of cation exchange, presented with their physical interpretation. +

1. The exchange isotherm (Fig. 1) relates the equivalent fraction of the adsorbed cation with its equivalent fraction in solution. It can be used to indicate selectivity in an exchange process under certain conditions (see later) or to calculate selectivity coefficients. Exchange isotherms were classified by Sposito (1981b) into four common types, depending on their behavior at low values of the ordinate and abscissa (Fig. 5): (a) S type, indicative of an exchangeable ion whose relative affinity for the exchanger is not large; (b)L type, indicative of an ion with a high relative affinity for an exchanger; (c) H type, an extreme case of an L type; and (d)C type, a linear isotherm indicative of nonpreference. Isotherms for K+-Ca2+ exchange have been found to be S type (Hutcheon, 1966), L type (Jensen, 1973a), and H type (Deist and Talibudeen, 1967a), depending on temperature, concentration, and the exchanger. Isotherms can vary greatly with ionic strength (see Sposito, 1981b), hence the need for caution when interpreting them. However, an isotherm at one concentration can be used to calculate isotherms at any other concentration for the same cation pair, temperature, and exchanger (Section 11,C). 2. The (corrected) selectivity coefficient (K,) expresses the selectivity of an exchanger for a pair of cations at a certain cation ratio. It is less ambiguous than the exchange isotherm because it is virtually independent of ionic strength (Barrer and Klinowski, 1974; Sposito, 1981b). A plot of K,, or as is more commonly used, In K,, against fractional saturation gives a quantitative indication of selectivity changes during an exchange reaction.

FIG.5. The four classes of exchange isotherm. Sposito (1981b).


235

3. The Gibbs free energy of exchange (AGO) expresses the overall selectivity of an exchanger at constant temperature and pressure, and independently of ionic strength. It has been called the driving force of a reaction. For A 4 B exchange, a negative AGO implies that B is the preferred or selected cation, and vice versa. 4. The enthalpy of exchange (A@) indicates the relative binding strength of the two cations and forms one part of the driving force (AGO) of a chemical reaction according to AGO = AHO - T A P

(29)

5 . The entropy of exchange (AS”)expresses the difference in degree of order of all components of the exchange between the two homoionic forms of the exchanger. It is the second part of the driving force of a reaction according to Eq. (29). Entropies have been used to assess the relative importance of solid and solution phase changes in exchange reactions (e.g., Hutcheon, 1966; Deist and Talibudeen, 1967b). 6. Adsorbed-ion activity coefficients u> reflect the fugacity of an adsorbed ion. Fugacity is the degree of freedom an ion has to leave the adsorbed state, relative to a standard state of maximum freedom of unity. Plots off versus fractional saturation therefore show how this “freedom” alters during the exchange, thus indicating exchange heterogeneity. 7. Excess thermodynamic functions also indicate’departurefrom ideality and thus heterogeneity but allow for the behavior of both cations (Talibudeen, 1971). 8. Integral and differential thermodynamic functions show how free energy, enthalpy, and entropy vary during an exchange reaction. Differential free energies are calculated directly fi’om selectivity coefficients (Section II1,B) and so give no more information than do the latter. However, directly measured differential enthalpies give a clear picture of exchange heterogeneity and provide another means of investigating surface chemistry. Differential entropies, although complex, can interpreted in terms of the structural order of all the components of the exchange system. In presenting the physical interpretation of the latter three sets of parameters, “exchange heterogeneity” has been mentioned (see also Sections II,E,F and 111,B). Heterogeneity of exchange is caused by one or more of the following: a heterogeneous distribution of ions on the exchanger; a heterogeneous distribution of exchange sites in terms of their position and energy; differences in the properties of the two cations (e.g., size, polarizability, and hydration); in soils, a heterogeneous clay mineralogy and a complex mixture of organic and inorganic exchangers. The application of thermodynamic reasoning to cation exchange in soils and clays, and of the Gaines and Thomas method in particular, is not without problems. The method assumes a constant exchange capacity and a negligible adsorp-

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KEITH W.T. GOULDING

tion of anions from solution. The latter is usually true for cation exchange in soils and clays in dilute solutions. However, the CEC has been found to change during an exchange experiment, particularly for K+-Ca2+ exchange where some K+ may be fixed. Faucher and Thomas (1954) estimated that a change in CEC from 123 to 135 p q / g during Cs+-K+ exchange affected the isotherm by about 4% and was thus negligible. Hutcheon (1966) found a similar change in CEC during K+-Ca2+ exchange and therefore ignored it. However, Deist and Talibudeen (1967b) showed more satisfactorily that the decrease in CEC during Ca2+ --f K + exchange in their experiments on soils resulted from Ca2+ ions trapped inside the exchanger and not from an irreversible fixation of K + . They therefore suggested that if exchange sites were homogeneously distributed and some sites were physically restricted, the thermodynamic treatment was not invalidated. This view was supported experimentally when Goulding and Talibudeen (1980) obtained identical plots of In K, versus K + saturation (and thus identical AGO values) and identical AHO values for Ca2 --f K and K + Ca2 exchange on a montmorillonite, a kaolinite, and a vermiculite clay. Deist and Talibudeen (1967b) also discussed the problem of extrapolating exchange isotherms, and thus of calculating K, values, at very small saturations of the preferred ion where selectivity is high. They thought that a linear extrapolation was reasonable but could not estimate the errors involved. Sposito and Mattigod (1979) and Sposito (1981a,b) questioned the thermodynamic meaning, and thus the application, of adsorbed-ion activity coefficients and selectivity coefficients as calculated by the Gaines and Thomas method. This problem was discussed by Goulding (1983); the argument is summarized in Section II,E. Studies of the thermodynamics of potassium exchange have concentrated on either the comparison of the exchange properties of K + with those of other cations or the comparison of a series of soils or clays using potassium exchange with another cation as the reference point. The research will therefore be reviewed under these main headings, with special sections discussing potassium selectivity and furation, and potassium potentials. +

+

+

+

B. COMPARING POTASSIUM WITH OTHERCATIONS

The reviews by van Blade1 (1967, 1972) cover the early research in this area. The main aspects are the following: 1. Comparison of cation selectivities 2. Estimation of the relative effects of ionic polarizability and hydration 3. Comparison of the relative contributions of enthalpy and entropy to free energy 4. Use of exchange parameters for A-B exchange and A-C exchange to predict those for B-C exchange (Hess’s triangle rule)

237


5 . Use of selectivity coefficients, adsorbed-ion activity coefficients, and ex-

cess thermodynamic functions to elucidate the behavior of the exchange complex in detail at various compositions of the exchange sites. The topic is best covered under the headings homovalent and heterovalent exchange. 1. ffomovalent Exchange Martin and Laudelout (1963) comprehensively examined the exchange of NH, with Na , K , Li ,Rb , and Cs on Camp Berteau montmorillonite. Selectivity, as expressed by In KL and AGO, was a function of ion polarizability (z /r, where z is the ion charge and r is the radius of the anhydrous cation). The order of selectivity for the monovalent alkali metal cations was that of the Hofmeister or lyotropic series Cs > Rb > NH, = K > H,O > Na > Li+ [the ionic radii of these anhydrous cations, in nanometers, are as follows: Cs+, 0.167; R b + , 0.147; NH4+, 0.143; K + , 0.133; Na + , 0.097; and L i+ , 0.068 (Weast, 1971); those of the hydrated cations are Cs+, 0.228; Rb+ ,0.228; K + , 0.232; Na+ , 0.276; and Li+ , 0.340 (Cotton and Wilkinson, 1972)l. The K + + M2+ exchange on a montmorillonite gave similar results for divalent cations (van Bladel, 1967): Ba2+ > Sr2+ > Ca2+ > Mg2+ [where the anhydrous ionic radii are Ba*+, 0.134; Sr2+, 0.112; Ca2+, 0.099; and Mg2+, 0.066 (Weast, 1971)l. It is interesting to note that the reverse order was found for Na+ -+ M2+ exchange on vermiculite by Wild and Keay (1964) because of the different characteristics of the Na+ ion and vermiculite. Deist and Talibudeen (1967a) obtained AGO and adsorbed-ion activity coefficient values for K -Na+ and K -Rb exchange on soils, which again gave the order of preference Rb+ > K+ > Na+ . The widely contrasting soils differed little in their selectivities. The mean AGO value for Na+ 3 K + exchange was -4.08 & 0.29 kJ/eq; for Rb+ -+ K + exchange it was -2.11 2 0.36 kJ/eq. By contrast the same soils exhibited AGO values for Ca2+ -+ K+ exchange ranging from -4.40 to - 14.30 kJ/eq. Adsorbed-ion activity coefficients showed that for K + -+ Na+ exchange (two ions of the same valency but different in size and thus in selectivity), fugacity for each ion increased smoothly with increasing saturation (Fig. 6). For K+-Rb+ exchange [two monovalent ions of very similar size and selectivity and thus with almost ideal behavior (Section &A)], fugacity changed very little from the standard state value of 1. Therefore, for homovalent exchange potassium selectivity is a function of its polarizability (i.e., its size only). Gast and co-workers (Gast, 1968, 1969, 1972; Gast et al., 1969) found the same selectivity series again for alkali metal cation exchange on Wyoming bentonite and Chambers montmorillonite; it was not changed by pH. They found +

+

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KEITH W.T.GOULDING

238

1, or 0.4

-

0.2

3C

FIG. 6. Adsorbed ion activity coefficientsfK (0)and fNa (m) as a function of hctional K + saturation x for K+-Na+ exchange on soils. Deist and Talibudeen (1967a).

that selectivity was determined primarily by the enthalpy term, counterbalanced by a smaller entropy term, but that entropy changes became more important as the surface charge density of the clay increased. They concluded from these observations that selectivity was governed chiefly by electrostaticforces between hydrated cations and the surface, and assumed that all changes in entropy resulted from changes in ion hydration. Work on Na -Cs exchange on clays (Maes and Cremers, 1978) has supported the former conclusion, but not the latter. +

+

2 . Heterovalent Exchange More research effort has been expended on heterovalent exchange than on homovaknt exchange because the former is generally of greater agricultural and industrial importance. Results can be separated into mono-divalent, monotrivalent, and di-trivalent exchange. a. Mono-Divalent Exchange. From the early work, particularly of Hutcheon (1966), Deist and Talibudeen (1967a,b), and Laudelout et al. (1968a,b), and from the more recent work of Goulding and Talibudeen (1980, 1984a,b) and Talibudeen and Goulding (1983a,b), several conclusions can be drawn. 1. It is unwise to use the exchange isotherm to assign selectivity because it varies greatly with ionic strength and may show hysteresis. Deist and Talibudeen (1967b) suggested that the ionic strength effect resulted mainly from entropic forces. 2. Overall selectivity is best expressed by K or AGO, and changes in selectivity during the exchange process are best expressed by a graph of In K,, or the differential free energy, versus K+ saturation. In heterovalent exchange, AGO

239


depends on ion size and valency, that is, the coulombic or charge factor. Thus AGO for Ca2 ---* Na2 exchange in soils is usually positive (Poonia and Talibudeen, 1977) and for Ca2+ ---* K + exchange it is usually, although not always, negative (Deist and Talibudeen, 1967b). Thus Goulding and Talibudeen (1984a,b) found AGO for Ca2 + K exchange on 14 soils to vary greatly (from +2.2 to - 10.0 kJ/eq) depending on mineralogy, pH, fertilizer, cropping history, and organic matter content. 3. The relative binding strength between two cations and the surface is expressed by the enthalpy of exchange (Hutcheon, 1966). Laudelout et af. (1968a) for M2+ + NH4+, and Hutcheon (1966), Deist and Talibudeen (1967b), and Goulding and Talibudeen (1979, 1980, 1984a,b) for Ca2+ + K + exchange, found that AtP was negative, implying stronger binding for K + , NH, , and, by implication, other selectively adsorbed cations such as Rb and Cs+. In addition, Goulding and Talibudeen (1984a,b) found that although this greater binding strength for K + in soils may be slightly decreased by K + fertilizer treatment, it never reverses to make Ca2+ the more strongly bound, even after 140 years of heavy FWM or inorganic K + fertilizer applications. The reaction M2+ --* K + becomes increasingly exothermic (i.e., the binding strength for K + increases) as the polarizability of M2+ decreases (van Bladel, 1967). This is because the less easy a cation is to polarize, the less easily it displaces K , and the stronger, relatively, is the K -surface bond. 4. The entropy of exchange expresses the rearrangement of cations, surfaces, and solvent molecules during the exchange process. Hutcheon (1966) and Deist and Talibudeen (1967b) found AF for Ca2+ + K + exchange on soils to be negative. They attempted a qualitative discussion of entropy changes in the solid and solution phases, and concluded that those in the solution phase must dominate, as they are large and negative for the K (solution) to Ca2 (solution) exchange. However, Goulding and Talibudeen (1980) found that Ca2+ + K + exchange on clay minerals was accompanied by a negative AF value for the expanded 2:l minerals vermiculite, illite, and montmorillonite, but by a positive AF value for the collapsed 2:l mineral muscovite mica. They therefore suggested that rearrangements within the solid phase contribute most to entropy changes. (For a full discussion, see Section IV,C.) 5 . Adsorbed-ion activity coefficients and excess functions for Ca2 + K exchange showed a marked difference in the behavior of Ca2+ and K + ions. Although the fugacity of Ca2+ decreased smoothly as K + saturation increased, that of K + varied greatly, the graph of fK versus K saturation showing maxima, minima, and inflections (Talibudeen, 1972) (see Fig. 2A). This was taken as reflecting the different distributions of Ca2+ and K + ions in the Gouy and Stem layers (Deist and Talibudeen, 1967a). The interpretation with respect to the surface is given in Section IV,D,l. It is interesting to note that the behavior of the selectively adsorbed K ion, as +

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KEITH W.T.GOULDING

shown byf,, was influenced by both enthalpic and entropic forces, whereas that of the nonselectively adsorbed Caz+ ion depended only on entropic forces (Goulding and Talibudeen, 1980). This suggests the primacy of strength of binding in determining K+ selectivity and fixation (but see Section IV,E,l where the relative importances of enthalpy and entropy are discussed in full).

b. Mono-Trivalent Exchange. The only work on the thermodynamics of mono-trivalent exchange has been on K -AP exchange by Coulter (1969), Sin& and Talibudeen (1971), and Sivasubramaniam and Talibudeen (1972), which is summarized by Talibudeen (1972, 1981). Aluminium dominates the exchange complex in strongly leached acidic tropical soils. However, its ionexchange behavior is complicated by the existence of polymeric A l - O H forms at pH values above 4. For example, the precipitation of A l - O H polymers as “islands” in the interlayer space of 2:1 minerals prevents their collapse on K adsorption and thus prevents K+ fixation (Rich, 1972). Experiments at low pH values have clarified the K+-A13+ exchange process. Although exchange isotherms suggested A13 preference in all soils and clays (vermiculite, illite, and montmorillonite) examined, In K, and AGO values indicated K + preference in seven out of nine soils and all three clays (Talibudeen, 1972). It is important to note that if what Coulter (1969) called “difficultly exchangeable K ’ is ignored when calculating K, values, results sometimes suggest A13+ preference. However, this K + should be included in the calculations because it indicates strong K+ preference over the first 20-30% of A13 -K exchange because of specific (ion-size) effects. Illite, montmorillonite, and soils dominated by these minerals exhibited greatest preference for K + and less for vermiculite and chlorite. The presence of organic matter also decreased the preference of a soil for K over AP ,because organic matter chelates polyvalent, but not monovalent, cations (Talibudeen, 1981). The more strongly leached the soils (and therefore the less 2:l clay minerals present), the lower was the preference for K + . Indeed, when devoid of such minerals, soils preferred A13 , presumably because in other minerals there is no effect of ion size and valency alone controls selectivity. As in K -Ca2 exchange, fK versus K + saturation curves showed maxima and inflections, andf,, versus K + saturation curves changed smoothly (see Fig. 2B). This again shows the contrasting behavior of selectively adsorbed (K+) and nonselectively adsorbed (A13+) cations. +

+

+

+

+

+

+

+

+

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+

c. Di-Trivalent Exchange. Only Ca2 -A13 exchange has been investigated (Coulter and Talibudeen, 1968). For vermiculite, illite, montmorillonite, and two acidic (pH 4.8) soils, exchange isotherms and In K, values showed strong A13+ preference which decreased with surface charge density for the clays, illustrating the importance of coulombic (charge or valency) effects in the exchange of such strongly hydrated ions. +

+


24 1

C. COMPARING CLAYMINERALS

Potassium exchange with other cations has been used to examine and compare clay minerals (aluminosilicates), but only recently have thermodynamic methods been used for a systematic comparison of a suite of minerals. Homovalent exchange shows little difference between exchangers for any one cation pair, as was shown in Section IV,B, 1. Heterovalent exchange is therefore used, the main reference cation pair involved being K+-Ca2+ with, to a lesser extent, K+-A13+. Potassium selectivity and fixation in clays have also been of great interest, but these are considered separately in Section IV,E. Hutcheon (1966), examining K + -Ca2+ exchange on Chambers montmorillonite, related entropy changes to changes in lattice spacing and cation and surface hydration, and enthalpy changes to cation binding strength. He also related variation in the adsorbed-ion activity coefficients with changing K saturation to changes in lattice spacing. His aim was to segregate solid and solution effects, and, the behavior of ions in solution being fairly well defined, to learn more about those which occurred in the solid phase. He concluded that the overall exchange reaction was governed by a balance of interlayer cation hydration forces and attraction forces between the cations and the surface. A similar conclusion was reached by Gast (1969, 1972) and Gast et af. (1969). Although Hutcheon’s (1966) work was extremely thorough, it could be argued that he arrived at no new conclusions regarding clay properties, particularly for montmorillonite which had been extensively examined by other methods (e.g., Norrish, 1954). However, he was the first to apply thermodynamic methods specifically to K + exchange and to attempt a physical interpretation of the resulting data. He thus opened the way for a comparison of the more important clay minerals, which is of much greater interest than studies of a single clay or soil. Goulding and Talibudeen (1980) examined five aluminosilicate minerals chosen as representatives of groups of aluminosilicates commonly occurring in soils. They were a muscovite mica from Norway, Fithian illite, Montana vermiculite and Upton (Wyoming) montmorillonite from the United States, and a kaolinite from England. Free energies of exchange indicated that all of the minerals selectively adsorbed K + , selectivity decreasing in the order mica > vermiculite = illite > kaolinite > montmorillonite. Excepting kaolinite, this is approximately the same order of selectivity as that suggested by Talibudeen (1971) based on K -Ca2 exchange in soils and mineralogical data for those soils and identical to that found by Assa (1976a,b), who examined K+-Ca2+ exchange on a similar suite of minerals using Gapon’s constant. Because kaolinite is usually considered to have a very low preference for K if not actually preferring Ca2 ,the kaolinite examined by Goulding and Talibudeen (1980) had an anomalously high K + preference. A reason for this became apparent when +

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KEITH W. T. GOULDING

-

*

0

0.2

O

0.6

0.4 X

1

0.8

0

FIG. 7. Differential enthalpy of exchange, [d(AHx)/)ldx)],as a function of fractional K+ saturation, x, for a Montana vermiculite and an English kaolinite. After Gouldmg and Talibudeen (1980).

the enthalpy values, and particularly the differential enthalpy curves, were examined. From the AH” values, the order of decreasing binding strength for K + was mica > illite = kaolinite > vermiculite > montmorillonite. Again, the kaolinite occupies an anomalously high position. Its differential enthalpy values were the same, within experimental error, as those of the Montana vermiculite (Fig. 7). Only the disribution of enthalpy values was different; there were about 160 p q / g of strong K binding sites in the vermiculite and only 19 peq/g of these sites in the kaolinite. This suggests, therefore, that the presence of about 2% by weight of a vermiculitic impurity [i.e., weathered micaceous interleaves of the type shown by Lee et al. (1975) using scanning electron microscopy] is responsible for the exchange characteristics of the kaolinite (see also Lim et al., 1980). The same effect can be seen in some data of Bansal (1982) on K+-Ni2+ exchange in a kaolinite from Bath, South Carolina (United States). The Lw” and AGO values indicated a stronger binding and selectivity for Ni2+. However, plots of K,,fK, AGE, and M Eversus Ni2+ saturation all suggested the existence of a few sites (about 20 w q / g of a total CEC of 107 peq/g) which had high selectivity for K . The differential enthalpy curves of Goulding and Talibudeen (1980) also suggested the presence of a small amount of mica in the montmorillonite, modifying its exchange properties. Subsequent research on kaolinites and montmorillonitesfrom different sources (Talibudeen and Goulding, 1983a,b) has shown that virtually no so-called montmorillonite is completely free of micaceous impurities (the < 0.2 pm fraction of an Upton montmorillonite was the only “pure” sample found), and that all of the cation-exchange properties of kaolinites can be explained by 2:l mineral impurities. Such detailed quantitative mineralogical analyses, which have hitherto been impossible using X-ray diffraction techniques, thus open up new possibilities in analysis. +

+


243

Integral and differential entropy values of the five minerals studied by Goulding and Talibudeen (1980) supported to some extent Hutcheon’s (1966) view of the importance of lattice expansion and contraction, but not the dominance of solution forces over solid effects. X-ray diffraction evidence by Plancon et al. (1979) had suggested that montmorillonite surfaces rearrange and reorder before collapsing when the mineral is subjected to wetting and drylng cycles following K -Ca2 exchange. Based on this and their own data, Goulding and Talibudeen (1980) suggested three physical mechanisms that contribute to entropy changes during Ca2 +-K exchange in clays: (1) in the solid phase, replacing Ca2+ by K+ realigns the aluminosilicate layers in the 001 direction such that the hexagonal holes in adjacent sheets can accommodate K + ions [as suggested long ago by Jackson (1963)], resulting in a negative entropy change; (2) in the solid phase again, replacing Ca2+ by K + increases the randomness of distribution of exchangeable cations, resulting in a positive entropy change; (3) in the solution phase, replacing K + by Ca2+ increases the structural order of water molecules, decreasing the entropy of the system. The fact that all the expanded 2: 1 minerals examined exhibited negative entropy changes during Ca2+ + K + exchange, whereas muscovite mica, a collapsed 2: 1 mineral, exhibited a positive entropy change, suggested that the solid-phase effects (1) and (2) dominate. Adsorbed-ion activity coefficients gave little new evidence on mineral characteristics, but agreed quantitatively with differential enthalpy values as to the disposition of site groups (see Section IV,D,I). +

+

+

+

D. COMPARING Sons

1. Temperate Soils Deist and Talibudeen (1967a,b) first used a thermodynamic treatment of cation exchange to compare soils, examining K+-Na+, K+-Rb+, and K -Ca2 exchange on 10 important British arable soils. The clay content of the soils ranged from 13 to 4396, the pH ranged from 5.4 to 7.1, and the CEC (measured by M NH,Ac leaching at pH 7) ranged from 124 to 307 Feq/g. Little differentiation between soils was apparent in the homoionic exchange reactions, and the results can be summarized by saying that all soils preferred K to Na+ , and Rb and K + , in accordance with the expected order of selectivity (see Section IV,B, 1). Adsorbed-ion activity coefficients showed no heterogeneity for these cation pairs, preference for one ion over another being equally distributed over all the exchange sites. For Ca2+ + K + exchange, the characteristics were very different. Some isotherms showed hysteresis, and that of a Harwell series soil exhibited selectivity reversal. Free energy changes showed that K+ was preferred to Ca2+ on +

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KEITH W.T. GOULDING

244

all the soils, and a comparison of enthalpy and entropy changes demonstrated that this was always because of stronger binding of K+ (negative AH"),offset by an increase in order (negative AS"). The soils differed greatly in their AGO, W , and ASO values because of differences in their clay and silt mineralogies. Graphs of adsorbed-potassium activity coefficients (fK)versus K+ saturation exhibited maxima, minima, and inflections (Fig. 2A). Later (Talibudeen, 1971, 1972) these characteristics were qualitatively related to the clay mineralogy of the soils, the assumption being that each maximum or inflection reflected a changeover of Ca2 form to K form by one group of K -selective sites after another. It was thought that such graphs might improve the description of soil mineralogies. Excess functions (Section I1,F) were also used for this purpose, but being calculated from fK and fca values offered no additional information. In relation to practical agriculture, Talibudeen (1971) thought that the change in excess free energy with K + saturation expressed the reciprocal of the K + buffering capacity of a soil (seealso Section IV,F,l). Soils in which this function changed least as K + saturation approached zero were expected to release most K+ . This was not tested experimentally, but changes infK (from which A P is derived) as K+ saturation approached zero agreed qualitatively with K uptake by ryegrass in pots from the soils. This and later work (Talibudeen and Weir, 1972) also explained the unusual K+-Ca2+ exchange characteristics of the Harwell series soil mentioned earlier as the result of the presence of a zeolite, clinoptilolite, mainly in the coarse clay and fine silt (0.3-5 pm) fraction of the soil. Calorimetric measurements of enthalpies of K -Ca2 exchange have shown that the interpretation of changes infK versus K + saturation curves and excess functions in terms of mineralogical differences were correct (Goulding, 1980; Goulding and Talibudeen, 1980). The maxima in the former coincided with the major steps in differential enthalpy curves from K+-selective to nonselective sites, as shown in Fig. 8, However, differential enthalpies were a much better guide to heterogeneity and soil mineralogy than fK values because, being directly measured by calorimetry, they were much more precise. Similar enthalpy measurements on the separated particle size fractions of a soil (Goulding and Talibudeen, 1979) showed the dominance of the fine (

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