Horticultural Reviews Volume 2

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HORTICULTURAL REVIEWS VOLUME 2

HORTICULTURAL REVIEWS VOLUME 2

edited by

JulesJanick Purdue University

avi

AVI PUBLISHING COMPANY, INC. W estport, Connecticut

0 Copyright 1980 by THE AVI PUBLISHING COMPANY, INC. Westport, Connecticut

All rights reserved. No part of this work covered by the copyright hereon may be reproduced or used in any form or by any means-graphic, electronic, or mechanical, including photocopying, recording, taping, or information storage and retrieval systems-without written permission of the publisher.

ISSN-0163-7851 ISBN-0-87055-352-6

Printed in the United States of America by Eastern Graphics, Inc., Old Saybrook, Connecticut

Horticultural Reviews is co-sponsored by the American Society for Horticultural Science and The AVI Publishing Company

Editorial Board, Volume 2 Frank G. Dennis, Jr. Donald N. Maynard Marlin N. Rogers

John Robert Magness

Dedication

This book is dedicated to Dr. John Robert Magness, premier pomologist, whose outstanding contributions to fruit research covered nearly 50 years. Dr. Magness has been a leader in fruit research for most of this century. He talks about his many accomplishments in his characteristically shy manner. In preparing a recent review on the evolution of fruit nutrition during the lifetime of our professional society, I asked Dr. Magness to relate his first-hand experiences in fruit nutrition during this period. “I cannot do t h a t . . . , ” he replied, “ . . . the society started in 1903 and the earliest I can remember professionally is 1910.” Since 1910, Dr. Magness’ accomplishments have marked every phase of fruit research and production. In the early 1920’s he was the leader in postharvest fruit physiology. His desire to quantify changes during the maturation of fruit resulted in the development of the Magness-Taylor pressure tester that is still in wide use today. Dr. Magness was the leading innovator in nutritional research in the 1930’s and by introducing leaf analysis into commercial orchards, he revolutionized fruit nutrition. He was in the forefront in studying noninfectious physiological disorders-most notably, internal cork of apples. Under his guidance the first applications of growth regulators, stop drop sprays, and fruit thinners were made in the 1940’s. These chemical treatments not only had a great impact on fruit production, but started an important trend toward the use of growth regulators for agriculture in general. He recognized and encouraged breeding for disease resistance 30 years before this became a general concern. Dr. Magness was an outstanding research administrator. He had an unusual ability to stimulate research and to appreciate the accomplishments of others. His love of research and dedication to improvement of fruit production remained with him throughout his career as a research administrator for USDA’s Bureau of Plant Industry. vii

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HORTICULTURAL REVIEWS

H e was uncomfortable when he was not doing research himself. When . . . personnel thought th at administrators should not do research . . . ” he retired and dedicated his services to horticulture as the Editor of 15 volumes of the Proceedings of the American Society for Horticultural Science. Dr. Magness’ success may be attributed to his excellent knowledge of tree physiology and fruit production; his unparalleled ability to observe various phenomena in the orchards; his unusual ability to evaluate his own work and that of others in an unbiased manner; his willingness to cooperate with others and to encourage them to seek the new; and his optimism and enthusiasm th at has been an inspiration to all his peers. John Magness, a t the age of 86, is still active and interested in new developments. Th e dedication of this book to him is but a small tribute to his greatness. “

Miklos Faust Fruit Laboratory Horticultural Research Institute Science and Education Administration U S . Department of Agriculture Beltsville, Maryland

Contributors

ATKINSON, DAVID, Department of Pomology, East Malling Research Station, Maidstone, Kent, United Kingdom BARKER, ALLEN V., Department of Plant and Soil Sciences, University of Massachusetts, Amherst, Massachusetts BASS, L.N., United States Department of Agriculture, Science and Education Administration, Agricultural Research, National Seed Storage Laboratory, Fort Collins, Colorado CALDAS, L.S., Departmento de Botanica, Universidade de Brasilia, Brasilia, DF, Brazil CAMPBELL, LOWELL E., United States Department of Agriculture, Beltsville, Maryland CATHEY, HENRY M., United States Department of Agriculture, Beltsville, Maryland DOUD, S.L., Division of Agriculture, Fort Valley State College, Fort Valley, Georgia FAUST, M., Fruit Laboratory, Horticultural Research Institute, Science and Education Administration, United States Department of Agriculture, Beltsville, Maryland FERY, RICHARD L., United States Vegetable Laboratory, Agricultural Research, Science and Education Administration, United States Department of Agriculture, Charleston, South Carolina JACKSON, JOHN E., Department of Pomology, East Malling Research Station, Maidstone, Kent, United Kingdom MARAFFA, S.B., Department of Horticulture, Ohio State University, Columbus, Ohio MILLS, HARRY A., Department of Horticulture, University of Georgia, Athens, Georgia RYDER, EDWARD J . , United States Agricultural Research Station, ix

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1636 East Alisal Street, P.O. Box 5098, Salinas, California SHARP, W.R., Pioneer Research Laboratory, Campbell Institute for Agricultural Research, 261 1 Branch Pike, Cinnaminson, New Jersey SHEAR, C.B., Fruit Laboratory, Horticultural Research Institute, Science and Education Administration, United States Department of Agriculture, Beltsville, Maryland SONDAHL, M.R., Departmento de Genetica, Instituto Agronomica, Caxia Postal 28, 13.100 Campinas, S.P., Brazil WHITAKER, THOMAS W., United States Department of Agriculture, P.O. Box 150, La Jolla, California YADAVA, U.L., Division of Agriculture, Fort Valley State College, Fort Valley, Georgia

Contents

DEDICATION vii 1 The Short Life and Replant Problems of Deciduous Fruit Trees 1 U.L. Yadava and S.L. Doud 2 Seed Viability During Long-Term Storage 117 L.N. Bass 3 Nutritional Ranges in Deciduous Tree Fruits and Nuts 142 C.B. Shear and M. Faust 4 The Lettuce Industry in California: A Quarter 164 Century Edward J. Ryder and Thomas W. Whitaker 5 Light Interception and Utilization by Orchard 208 Systems John E. Jackson 6 The Physiology of Asexual Embryogenesis 268 M.R. Sondahl, L.S. Caldas, S.B. Maraffa and W.R. Sharp 7 Geneticsof Vigna 311 Richard L. Fery 8 Ammonium and Nitrate Nutrition of Horticultural Crops 395 Allen V. Barker and Harry A. Mills 9 The Distribution and Effectiveness of the Roots of 424 Tree Crops David Atkinson 10 Light and Lighting Systems for Horticultural Plants 491 Henry M. Cathey and Lowell E. Campbell 539 INDEX (VOLUME 2) CUMULATIVE INDEX (VOLUMES 1-2) 541 xi

Horticultural Reviews Edited by Jules Janick © Copyright 1980 The AVI Publishing Company, Inc.

1 The Short Life and Replant Problems of Deciduous Fruit Trees1 U.L. Yadava and S.L. Doud2

Fort Valley State College, Fort Valley, Georgia 31030 I. TheProblem 3 A. Introduction 3 B. Economic Impact. 4 C. Distribution 5 D. Symptomology 10 15 11. Methods to Study the Problem A. Electrophysiological 15 B. Chemical and Biochemical 17 18 C. Isolation, Culture, and Bioassay D. Inoculation 19 23 E. Discoloration and Tissue Integrity F. Regrowth 23 G. Other Methods 24 111. Causal Factors 26 A. Environmental Factors 26 1.Macroclimatic (Natural) 26 a. Cold (Winter, Freeze or Frost) Injury i. Dormancy 27 ii. Extent of Injury 30 iii. Type of Injury 30 iv. Mechanism of Injury 31 32 v. Effect on Tree Life vi. Cold Hardiness 33

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'The survey of literature pertaining to this review was completed in May 1978. The authors wish to thank Drs. C.N. Clayton, M. Faust, A. Jones, R.E.C. Layne, E.L. Proebsting, Jr. and M.N. Westwood for helpful suggestions regarding the outline of this manuscript, and Mrs. Cynthia J. Andrews and Miss Donna M. Bird for assistance in preparation. 2Research Scientists, SEA/CR Agricultural Research, Division of Agriculture.

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b. Other Stresses 34 i. Water (Drought, Desiccation) 34 ii. Oxygen Deficiency (Wet-feet) 36 iii. Temperature Extremes 36 37 iv. Combination of Above Factors 2. Microclimatic (Cultural) 37 a. Cultural Practices 37 i. Crop Rotation 37 ii. Pruning and Training 38 39 iii. Cover Crops, Mulch, and Weed Control 40 iv. Irrigation and Tillage Operations b. Nutritional 41 41 i. Soil pH and Liming 42 ii. Type and Amount of Fertilizers iii. Time of Fertilization 43 44 iv. Nutritional Deficiencies and Interactions B. Pathogenic Factors 46 1.Bacteria 46 2.Fungi 41 3. Nematodes 49 50 4. Viruses and Mycoplasma-Like Organisms (MLO) 5. Insects 52 6. Pathogenic Interaction 52 C. Physio-Biochemical Factors 53 1.Phytotoxins 53 a. Microbial Phytotoxins 53 b. Plant Residues 54 c. Spray Residues 55 d. Other Phytotoxins 56 2. Biochemicals 57 a. Carbohydrates (CHO) 57 58 b. Proteins and Other Nitrogenous Compounds 58 c. Fatty and Organic Acids d. Phenolics 59 e. Other Biochemicals 59 60 3. Phytohormones and Growth Regulators a. Promoters 61 i.Auxins 61 ii. Gibberellins (GA) 62 iii. Cytokinins (CYK) 62 b. Inhibitors 63 i. Abscisic Acid (ABA) 64 ii. Other Inhibitors 64 c. Phytohormonal Interaction 65 IV. Control Measures 66 66 A. Plant Improvement Through Breeding for Resistance 70 B. Rootstocks

SHORT LIFE, REPLANT PROBLEMS OF DECIDUOUS FRUIT TREES

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C. Cultural Practices 75 D. Control of Pathogens 79 E. Miscellaneous Controls 81 1.Fumigation 82 2. Steam Sterilization 83 3. Other Methods 84 V. Conclusion 84 VI. Literaturecited 85

I. THE PROBLEM A. Introduction There are numerous causal factors that can devitalize, weaken or even kill the trees in a fruit orchard. The “short life” or “replant” problem is only one of many such problems which face the growers. The situation is widespread throughout the world (Shannon and Christ 19541, but the same problem as such does not necessarily occur in different growing areas or in all orchards within a specific region. The so-called fruit tree short life or replant problem, as recognized in different forms in various parts of the world, has been reported for more than two centuries (Gilmore 1959; Savory 1966). Other problems causing premature death of young trees are more recent. Some of these problems, such as pear decline and stem pitting, are detrimental for all ages of trees, but they could be a major cause for failure to establish an orchard a t a given site. For this reason, these problems are included in this review. The terms “replant” and “replanting” indicate, by definition, the second or following plantings of the same or a closely related species a t a given site (Savory 1966). The problem was named specific apple replant disease (SARD) by preference over “specific problem” or “specific sickness.” Discussing specific replant diseases of apple and cherry, Savory (1967) gave the following characteristic features of these types of disorders: (a) they are specific to a certain degree-occurring when a species is planted following its own type; (b) they inhibit root growth-affected plants have weak, necrotic, and sparsely branched roots and the top/root ratio of such trees is reduced; (c) there are no leaf symptoms-characteristically, however, in the first year shoot growth on affected trees ceases earlier than that on healthy trees, while in the second year of growth retarded replants have considerably fewer growing shoots than healthy trees; (d) replant diseases directly affect trees only in the first year after planting whereafter replants and healthy trees have very similar relative growth rates, though affected replants do not catch up with healthy trees soon; (e) replant diseases persist in the soil for very

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long periods-just how long is not known. Donoho e t al. (1967) used the term “apple tree decline” (ATD) for the condition of loss of tree vigor and productivity th at occurred for no apparent reason with no improvement due to management practices. Proebsting (1950), Gilmore (1949,1959), and Clayton (1968) pointed out the basic characteristic of the replant problem, by indicating the high degree of specificity as evident from peach failure following peach but not when following apple, and the reverse also being true where other fruits do well after peach. According to Clayton (1975a), the problem might be considered to be of two overlapping types: (a) stunting or retardation of growth, an d (b) death of trees. T h e latter is now called peach tree short life (PTSL), a term coined a few years ago to denote the disease syndrome consisting of bacterial canker, blast, decline, sour-sap, die-back, sudden death, gummosis, apoplexy, winter injury, cold injury, etc., as applied to the death of peach trees in late winter or early spring which for all appearances were healthy in the preceding fall (Clayton 1977). From the conflicting observations it appears th a t the short life and replant problem is a complex one, for which no single factor is responsible. Since there are so many problems by name and nature, obviously there will be no total agreement on one single nomenclature or classification for each problem or complex thereof. However, we will attempt to limit our discussion to those well documented problems which contribute to shortened tree life expectancy or failure in the establishment of the replants, usually through soil or site factors in conjunction with environmental and/or other stresses. This generally would exclude the disorders which are purely pathological in nature.

B. Economic Impact Hickey (1962) found in New York state th a t Cytospora canker severely reduced the production life of peach trees which earlier were weakened by cold injury. In Poland, bacterial canker of sweet cherry, incited by Pseudomonas mors-prunorum Wormald, recently has become a devastating disorder resulting in an acute shortage of sweet cherry trees in nurseries (Lyskanowska 1976). Gardan (1975) reported 300,000 tree deaths in the principal peach-growing area of France as a result of the cold injury-bacterial canker complex. McGlohon and Ferree (1976) reported t ha t the peach tree population in Georgia dropped from 1 6 million trees in 1930 to less than 3 million trees in 1960, and th a t the primary causal factor for this decrease in tree number was peach tree decline, now called peach tree short life (PTSL). According to Sharpe (1974), a n estimated half million trees, mostly under six years of age, succumbed to


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PTSL in 1972-1973 in the peach areas of southeastern United States. In central Georgia alone, for example, the recent average life-span of peach trees was cited as eight years and in the whole Southeast, as only about ten years (Fogle 1975; Hendrix and Powell 1969). This contrasts with a nearly 20-year life expectancy only a few decades ago. In fact, trees begin dying before the orchard reaches full productivity and this tremendous tree loss soon leaves the orchard operations unprofitable, making another replanting absolutely necessary. Many replanted orchards never reach production stage, i.e., the replants fail to establish, especially when replanted immediately following peaches on old (short-life) sites (Hendrix and Powell 1969).

C. Distribution The purpose of this review is not to deal individually with each of the problems affecting the different deciduous fruit trees, nor even all of the problems of the same tree fruit. Instead, it will focus on the distribution of only those problems which are recognized a s contributing towards the shortening of the tree vitality, as well a s those having a significant economic impact on cultivation of pome and stone fruits in the major production areas. Some of the well documented problems of these pomaceous fruit trees are enumerated as follows. Parker et al. (1966) reported from New York state th a t in some sites where root damage was especially severe new trees of apple, cherry, peach, plum, and pear could not be established after old orchards of the same crops were removed. Referring to the apple replant problem in quartz sand in Germany, Bunemann and Jensen (1970) mentioned th a t thoroughly washed quartz sand used previously in culture experiments inhibited seed germination and growth of apple seedlings, a s well as grafts. Banta (1960), Beattie (1962), Beattie et al. (1963), and Donoho et al. (1967) described a serious disorder of old apple orchards (usually 20 to 25 years or older) in the eastern United States a s apple tree decline. Ross and Crowe (1973, 1976) have reported incidences of a n apple replant disease in Nova Scotia, Canada. Refatti (1970a,b) has reported from the Valtellina area of Italy, where apple decline, affecting only ‘Delicious’ trees of ages from 5 to 19 years, has reached epidemic levels. However, a sharp increase in incidence was observed in the first two years of growth after the initial outbreak. This specific decline phenomenon, therefore, appeared to be different from usual decline experienced in Italy and also from certain other syndromes specific to ‘Delicious’ trees in other parts of the world. Stouffer et al. (1977) reported a n apple union necrosis and decline disorder which also affects the bud union integrity and scion growth of young trees in the Pennsylvania area. Occurrence of stem

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pitting and necrosis in some bodystocks for apple trees has been observed in New Hampshire by Smith (1954). Welsh and Spangelo (1971) reported that necrotic stem pitting and decline, two bud-transmissible diseases of apple, affected M a l u s robusta No. 5 apple rootstock severely in British Columbia but not in eastern Canadian provinces. Kaminska (1973a,b), Kaminska e t al. (1971), Kaminska and Zawadzka (1973), and Kunze (1972) provided a detailed discussion of the apple proliferation disorder in Poland which was more acute on 1- to 5- than on 12-year-old trees. Paulechova and Rakus (1971) found apple proliferation problem severely damaging to ‘Golden Delicious’ and other native cultivars in Czechoslovakia. Benson (1974a,b) mentioned an apple replant problem on old apple sites (apple following apple on the same site) in Washington state, and associated it with soil arsenic toxicity, non-specific disease organisms, and SARD. The latter problem has been emphasized in recent years (Benson and Covey 1976; Benson e t al. 1978). Apple replant disease also has been reported from England (Jackson 1973; Pitcher e t al. 1966) and New Zealand (Ryan 1975a,b). This disorder specifically affects apples on old apple sites without an apparent involvement of nematodes. Hoestra (1967, 1968) reported an apple replant problem in the Netherlands, with two main distinguishable characteristics: damage by Pratylenchus penetrans (Cobb) nematode on light soils, and SARD, not caused by nematodes, on heavy soils with near-neutral soil pH. A similar decline problem has been reported from Australia by Sitepu and Wallace (19741, who correlated P y t h i u m species, nematodes, and soil p H with the inhibition of growth (in terms of trunk circumference) of apple trees. Colbran (1953), also from Australia, reported poor and unthrifty growth of apple trees when used as replants in old apple orchards. Researchers from Germany (Borner 1959; Otto 1972a,b,c,d; Winkler and Otto 1972) reported a similar problem, occurring mainly as a result of soil sickness, which the nurserymen encountered after a cultivation of apples for one to two years on old apple sites. Apple decline, as related to canker and die-back diseases caused by Stereum p u r p u r e u m (Pers.) Fr., has been reported recently in two separate studies in Wisconsin (Setlife and Wade 1973) and in Himachal Pradesh, India (Shandilya 1974). Phytophthora collar rot, which produces cankers below the ground line and mostly on clonal rootstocks, has been reported in Michigan (Jones 1971b) as well as in the Rio Negro and the Nequen Valleys in Argentina (Sarasola and de Bustamante 1970). A thorough review on decline, replant, and other short life problems of pear has been published by the Pear Research Task Group of the University of California (Anon. 1971). Researchers from various regions have reported “Pear Blast,” which affected all commercial cultivars of pear in Connecticut (Sands and Kollas 1974), is widespread in the USSR

SHORT LIFE, REPLANT PROBLEMS OF DECIDUOUS F R U I T T R E E S

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(Dorozhkin and Griogortsevich 1976), and had existed for a t least a de cade in Chile (Cancino et al. 1974). The problem typically occurs in wet weather near bloom time. In the Pacific Coast states of the United States, pear decline, which was first recognized in 1956 (Westwood and Lombard 1977), develops with a gradual loss of tree vigor and reaches the advanced stage upon the attainment of severe or complete suppression of terminal growth (Batzer and Schneider 1960; Woodbridge and Lasheen 1960). In recent years, increases in the occurrence of pear decline have been reported from other parts of the world also (Agrios 1972; Blattny and Vana 1974; Luisetti and Paulin 1972; Luisetti et al. 1973; Rallo 1973; Sarasola and de Bustamante 1970; Schmid 1974; Seemuller and Kunze 1972; Soma and Schneider 1971). Spivey and McGlohon (1973) noted that decline and eventual death of young peach trees has been a world-wide problem for more than a century. Of the peach replants which die, most are those planted in locations from which an old tree recently has been removed (Upshall and Ruhnke 1935). Chandler et al. (1962) used the term “sudden decline” for a disorder of peach trees in Georgia which, in the early spring of 1962, resulted in the sudden death of about 200,000 trees a t or shortly after bloom. Chitwood (1949) associated the decline and replant problems in Maryland peach orchards with ring nematodes of the genus Criconemoides (now called Macroposthonia (Raski) Loof de Grisse). PTSL in North Carolina has been linked with the cold injury-Cytospora cankerbacterial canker complex (Clayton 1968, 1972, 1975a,b, 1977). Often trees from four to seven years old are most affected, and tops are killed to the soil line while roots are still alive. Daniel1 and Crosby (1970, 1971) preferred the terminology “quick” and “slow” decline for the PTSL syndrome, depending on its rate of advance towards killing or devitalizing trees. In Pennsylvania, the peach replanting problem on old sites has been associated with the failure to give replants a rapid early start with readily available source of nitrogen to overcome inhibition caused by previous crops (Hewetson 1957). Hung and Jenkins (1969) have recognized the peach replant syndrome in New Jersey to be a complex of nematodes and one or more cultural or other factors. The disorder also has been well documented in the western United States; however, the “peach replant problem” is the terminology preferred (Gilmore 1949, 1959, 1963; Proebsting 1950; Proebsting and Gilmore 1941). T h e reduction of tree growth in old peach soil, both in containers and in the field, has been associated with various soil-borne factors (Proebsting and Gilmore 1941). Another report from California by De Vay et al. (1967) stated that the decline of peach trees is a chronic root problem affecting 2- to 16-year-old trees, particularly in light soils. In Ontario, serious difficulties are frequent in the establishment of replants on old

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orchard sites (Koch 1955; Mountain and Boyce 1957, 1958a,b; Mountain and Patrick 1959; Patrick 1955; Ward and Durkee 1956; Wensley 1956). Agrios (1971) reported from Greece that peach and apricot trees die within three to four years of first tree decline symptoms. Mizutani et al. (1977) reported that the peach replant problem in Japan associated with root cyanogenesis under anaerobic conditions. The microbial aspect of peach replant disease, as related to rhizosphere effects, has been reported from Italy by Lepidi et al. (1974). Scotto La Massese et al. (1973) and Vigouroux et al. (1972) have presented a complete analysisof peach decline in France. A bark gummosis of peach trees caused by Botryosphaeria dothidea (Moug. ex Fr.) Ces. and de Not. reportedly has seriously affected thousands of trees in central Georgia (Weaver 1974b). Chirilei et al. (1970) showed that gummosis was responsible for premature decline of apricot trees in Romania. Heimann (1968) described an apricot gummosis in Germany which caused incidences of die-back over a period of six years. Gummosis has been reported also to be a result of Pseudomonas syringae (van Hall) infection in Hungary (Babos et al. 1976) and of Vulsa canker in Czechoslovakia (Rosik et al. 1971). Peach rosette and decline have been reported from Australia (Smith and Neales 1977; Smith et al. 1977a,b; Stubbs and Smith 1971) a s an important problem but not as a short life or replant problem. A decline condition of plum, closely resembling that affecting prune trees in New York state, probably caused by a strain of Prunus necrotic ringspot virus, has been reported in England (Posnette and Cropley 1970). Preliminary reports on cherry decline (Fos 1976) and apricot decline (Gardan et al. 1973) in France have been published. Cherry decline also has been reported from East Germany (Kegler et al. 1973). The apoplexy disorder of apricots has been reported from Hungary (Babos et al. 1976; Klement et al. 1972, 1974; Rozsnyay and Barna 1974; Rozsnyay and Klement 1973), Greece (Kouyeas 1971), Poland (Paclt 1972), Bulgaria (Iliev 1968), and other countries (Frenyo and Buban 1976). Perennial canker, caused by two related fungi, Cytospora cincta Sacc. and C. leucostoma Sacc., has been associated with killing young trees of peach, apricot, prune, plum, and sweet cherry plantings in many parts of the world; C. cincta is more common on apricots while C. leucostoma is predominant on peach (Clayton 1971; Hampson and Sinclair 1973; Hickey 1962; Jones 1971b; Stanova 1977; Weaver 1963). Bacterial canker, incited by Pseudomonas syringae van Hall and Ps.mors-prunorum Wormald, is another serious cankerous disorder prevalent on many stone fruits around the world (Davis 1968; Davis and English 1965, 1969a,b; Dorozhkin and Griogortsevich 1976; English 1961;


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Jones 1971a,b; Klement et al. 1974; Lyskanowska 1976; Prunier, Gardan and Luisetti 1970; Prunier, Luisetti and Gardan l970,1973a,b; Psallidas and Panagopoulos 1975; Weaver et al. 1974). Pseudomonas syringae canker problem has been closely associated with PTSL or decline disease of peaches (Cameron 1971b; Clayton 1968; Dowler and Petersen 1966; French and Miller 1974; Gardan et al. 1975; Lyskanowska 1976; Savage and Cowart 1942a; Zehr et al. 1976; Petersen 1975; Petersen and Dowler 1965). X-disease, so named because of its mysterious nature, still causes large losses in peach (Cochran 1975), and cherries (Granett and Gilmer 1971; Jensen 1971) on the West Coast of the United States though reported to be under control in the eastern United States (Sands and Walton 1975). In the eastern United States and Ontario, X-disease attacks peach, nectarine, sweet cherry, and tart cherry (Dhanvantari and Kappell978; Jones 1971b; Lukens et al. 1971; McKee et al. 1972; Rosenberger and Jones 1977; Sands and Walton 1975). In mid-Atlantic states of the United States, a new disorder called “stem pitting” was found to be responsible for the girdling, decline, and death of peach trees of various ages in 1967 (Jones 1971b). This disorder causes a sizeable problem in almost all stone fruits. A survey of declining stone fruit orchards in California (Mircetich et al. 1977) revealed widespread incidences of P r u n u s stem pitting (PSP),which is graft-transmissible (Smith and Stouffer 1975). Hutchins (1933) related phony virus peach disorder as one of the several major factors reducing productive life of peach trees in Georgia. Savage and Cowart (1942a) and Rhoads (1954) reported similar findings about the impact of phony on PTSL. Armillaria root rot, caused by fungus Armillaria mellea (Vahl) Quel, is another disorder which often eliminates otherwise good orchard sites from production (Jones 197l b ; Wilbur et al. 1972), and appears as one of the important factors responsible for affecting peach tree longevity in Georgia (Savage and Cowart 1942a). Other fungal disorders associated with the short life and decline of fruit trees have been cited as Clitocybe tabescens (Scop. ex Fr.) Bres. (Chandler 1969; Cohen 1963; Petersen 1961; Savage and Cowart 1942a, 1954; Savage et al. 1953; Weaver 1974a), Cylindrocladium species (Sobers and Seymour 1967;Weaver 1971), Pythium species (Hendrix and Powell 1970b; Hine 1961b; Mircetich and Keil 1970; Powell et al. 1965; Taylor et al. 1970), Phytophthora species (Hendrix and Powell 1970b; Jones 1971b; Mircetich and Matheron 1976; Powell et al. 1965), Thielauiopsis basicola (Berk. and Br.) (Hoestra 1965; Pepin et al. 1975; Sewell and Wilson 1975), Physalospora persicae Abik. and Kit. (Abiko and Kitajima 1970), and species of Fusarium and Rhizoctonia (Hine 1961b).

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D. Symptomology The short life and replant problem has a wide variety of symptoms. The general problem may be the result of injury to the root system, the top (shoots and leaves), or both. However, it is not uncommon for affected plants to manifest as part of their physio-pathological syndrome some type of exaggerated growth disorder, leaf malformation, adventitious root formation, thickening or elongation of plant parts, lack of or excessive branching, bending of stems, stunting, and disorganized growth characteristic of tumors and galls (Viglierchio 1971).In this discussion, we largely emphasize those symptoms characteristic of devitalization of tree growth and vigor as well as production span. Banta (1960) and Beattie (1962) observed the reduction and cessation of apple tree growth in a relatively short time with ultimate tree death within three to five years.Donoho et al. (1967) described ATD symptoms as a general loss of tree vigor resulting in short terminals, small leaves, and reduced productivity. Benson and his co-workers (1974a,b, 1976, 1978) have reported impaired or poor growth of apple replants in Washington state, as did Bollard (1956) in New Zealand. T o characterize SARD, Hoestra (1968) stated that SARD attack is confined to the feeder roots, is specific to apple, and is non-lethal; the causal factor stays in soil for several years and trees recover quickly upon transplanting to fresh soil. Apple trees affected with proliferation disease were shorter with thinner trunks and smaller crowns than healthy trees and had more chlorophyll in the leaves (Kaminska 1973a,b; Kaminska et al. 1971; Kaminska and Zawadzka 1973). Savory (1966) gave the following account of replant disease of apple and cherry in England. The root systems of affected trees were weak and small with discolored and necrotic fine roots, and no presence of pathogens. Aboveground symptoms included reduction in tree vigor and size. The condition of poor growth became obvious in the first year of replanting, normal vigor being restored on transfer to fresh soil. The problem is persistent but does not spread through soil. However, the symptoms for both apple and cherry are similar. Phytoph thora collar rot disorder of apple produces cankers below the ground line where the roots are attached to the crown or lower trunk. These cankers girdle the roots and lower trunks, causing poor terminal growth, foliar discoloration, and eventual tree death in severe cases (Jones 1971b). Describing “stem pitting” and necrosis in bodystocks of apple, Smith (1954) noted typical symptoms of dwarfing, development of “wood pitting,” and breakdown of body-stocks themselves. The formation of “black-hearted wood,” together with death of parenchyma cells and occlusion of vessels and rays, has been included in the apparent symptoms of cold injury to the apple trees (Steinmetz and Hilborn 1938). Simons (1970) reported trunk splitting in scions of ‘Stark-


11

ing Delicious’ and ‘Golden Delicious’ apples on M 7 rootstock as a result of subfreezing temperatures. Apple union necrosis and decline are characterized by tree girdling, sparse, small, and pale green leaves, reduced terminal growth, bunchy-type twigs, fragile graft union with brown necrotic tissue imbedded in the union interface, and breaking-off of the scion portion of the affected trees (Stouffer et al. 1977). Sitepu and Wallace (1974) observed marked variability in apple tree growth, where the main visual symptom was extensive reduction in tree growth, particularly the trunk circumference. Colbran (1953) noted very poor and unthrifty growth of apple trees affected by “baffling slow decline.” This unsatisfactory growth of stunted trees was associated with the development of an abnormal root system with an abundance of discolored fibrous roots which later decayed. The symptoms of declining trees on “sick soil” as described by Borner (1959) include the manifestation of retarded growth and shortened internodes, which resulted in a rosette-like appearance, with varying degrees of root discoloration and reduced growth of the tap root. These symptoms were reversed when the plants were shifted from sick to normal soil. Pear decline has been characterized by phloem necrosis that occurs as a result of a mycoplasma vectored by Psylla pyricola Foerster, an insect of pear (Hibino and Schneider 1970; Westwood and Cameron 1978). Blattny and Vana (1974), Blodgett et al. (19621, Rallo (1973), and Seemuller and Kunze (1972) observed that sieve-tube necrosis is accompanied by various reactions leading to a girdling effect, deterioration of collar and feeder roots, callose development, early fall reddening of leaves, sparse foliage, decline in vigor, and wilting, with rapid or slow death of trees. Batzer and Schneider (1960) noted that a gradual loss of vigor in declining pear trees was associated with a series of anatomical changes initiated by sieve-tube necrosis immediately below the bud union. In advanced stages of pear decline, leaves became sparse, small, and pale green, and trees usually made no terminal growth and often suddenly wilted and died during the periods of high summer heat. Affected trees lacked the fibrous root systems of normal healthy trees (Woodbridge and Lasheen 1960). Sands and Kollas (1974) described the symptoms of “pear blast” condition in Connecticut as black leaf spots, sometimes with yellow halos, and frequent death of whole leaf and petiole, but no bacterial ooze or cankers on trees. Although frequently confused with fire blight, physiological disorders, or pesticide toxicity, severe outbreaks of “pear blast” in Chile have been characterized by blasting of flowers, leaf necrosis, and cankers of fruit spurs and small branches, but with no exudates on lesions (Cancino et al. 1974). In Greece, Agrios (1972) characterized stem pitting, graft incompatibility, and pockets of necrotic wood parenchyma from pear scions on quince rootstock as decline or “moria” symptoms.

12

HORTICULTIJRAL REVIEWS

Clayton (1968) observed that the replants of peaches often were stunted, grew poorly, and died from the bacterial canker-Cytospora cankercold injury complex where tops were typically killed to the soil line and roots remained alive. He mentioned th at PTSL is more common in light soils than in heavier soils, and more prevalent on old orchard sites (Clayton 1977). Gilmore (1959) noticed slower growth of peach on replant sites than on new peach sites. Th e difficulties frequently encountered in the establishment of peach replants on old sites in Ontario are characterized by Koch (1955) as variable from slight stunting to a complete absence of growth; in addition to stunting, Savory (1966) reported chlorosis of peach trees on replanted land. Hung and Jenkins (1969) described the death of peach trees as either a slow decline over a period of several years or a quick decline within a year or two of planting on an old orchard site. In California, the decline of peach trees has been reported to be a chronic root disease of uncertain etiology often characterized by poor growth of the trunk and branches, by underdeveloped and chlorotic leaves, and by the frequent death of young trees during winter (De Vay et al. 1967). Davidson and Blake (1936) described some macro-nutrient deficiency symptoms in declining young peach trees: die-back following leaf discoloration, leaf necrosis and mottling, defoliation, inhibition of linear as well as spatial growth of shoots, restricted root growth, and breakdown, necrosis, and weakening of feeder roots. Dekock and Wallace (1965) have reported phosphorus and iron chloroses in decline-affected peach trees as a result of excess nitrogen. Taylor et al. (1970) described the condition called peach tree decline as induced by cold injury with symptoms of tissue discoloration above ground level. Daniell and Crosby (1968) noted anatomical abnormalities in cold-injured trees, including occlusion of xylem elements with a “gum-like” substance and “slime-like” material, and damaged ray cells. Later, cambial browning, bud mortality, and retardation of leaf development were observed a s additional symptoms of cold injury (Daniell and Crosby 1971). Yadava and Doud (1977, 1978a,b) reported th at the severity of trunk cambial browning (TCB) in early spring was directly proportional to the cold injury th a t trees suffered. Cold injury resulted in death of those trees th a t showed visual T CB ratings of 8 or 9 on a scale of 1 to 9. According to the observations of Chandler et al. (1962), the trees affected by PTSL showed symptoms of wilting o f immature foliage, with cambial browning on trunks and limbs accompanied by yellowish exudate from bark and a characteristic sour-sap odor. Bacterial canker, which is also called blast, bacterial gummosis, and sour-sap, occurs in the southeastern and western United States, and causes death of trees and limbs by cankers th a t girdle the trunks, crotches, and limbs (Petersen 1975; Petersen and Dowler 1965).

SHORT LIFE, REPLANT PROBLEMS OF DECIDUOUS F R U I T TREES

13

These observations indicate a progression of disease from infected dormant buds to the development of cankers in the limbs. “Bacterial gummosis” has been found in phloem, xylem, tracheae, and fibers (Babos et al. 19761, with symptoms of bark cracking, leaf curl, persistence of wilted leaves, and death of trees within a year or so (Dorozhkin and Griogortsevich 1976). Prunier, Gardan and Luisetti (1970) and Prunier, Luisetti, and Gardan (1970, 1973a,b) reported additional symptoms of bacterial canker on peach and apricot in France to be cortical necrosis in winter followed by die-back of new growth in the spring, and twig necrosis on established trees followed by decline of all branches. This disease in France was first noticed in the Rhone Valley in 1968 (Prunier, Luisetti and Gardan 1970), and was characterized by small bud lesions appearing in fall, developing into a die-back in winter. Shortly after budburst in spring, trees partly dried out or died and numerous leaf spots and fruit gummosis were observed, while cankers caused the death of branches during summer. Root-knot nematode infestation is another weakening factor in the peach orchards suffering from PTSL (Clayton 1968; Hutchins 1936; Malo 1967). In Connecticut, Johanson (1950) presented evidence on the importance of nematodes, Pratylenchus and/or Macroposthonia, a s a factor in the short life of peach planted after peach in an orchard rotation where the nematodes attacked root tips and finally caused induction of ‘‘witches’-broom’’ root system. Johnson e t al. (1978) from Ontario attributed significant growth reduction of Siberian C rootstock seedlings to infection by Pratylenchus penetrans. Lownsbery (1959) and Lownsbery et al. (1968, 1973) attested th a t the growth reduction was the principal effect of the nematode infestation. Hung and Jenkins (1969) concluded from their greenhouse and laboratory experiments th a t the feeding by Macroposthonia curvatum Loof de Grisse on peach caused extensive lesions and pits on roots, which were later affected by other factors contributing to low vigor of the root system, finally resulting in the decline symptoms. Orion and Zutra (1971) demonstrated the role of Meloidogyne javanica (Treub) Chitwood as predisposing almond trees to infection by canker bacteria. Mountain and Patrick (1959) found th a t Pratylenchus caused peach root necrosis even in the absence of invading bacteria and fungi. Banko and Helton (1974) reported th a t tree wilting due to Cytospora canker was associated with gum-plugging in the xylem of affected trees. Studies of Hampson and Sinclair (1973) on Valsa (Cytospora) canker in New York peach orchards showed th a t wilting and resulting defoliation of infected branches not girdled by the cankers were characteristic of the condition. Additionally, the greenhouse tests gave evidence of xylem dysfunction as an inportant cause of canker symptoms. In Georgia, PTSL symptoms have been characterized a s

14


scanty growth, leaf chlorosis, and eventual tree death due to lack of feeder roots, probably caused by P y t h i u m infection (Hendrix and Powell 1970a; Hendrix et al. 1966; Owen et al. 1965; Spivey and McGlohon 1973; Taylor et al. 1970). In addition to P y t h i u m spp., other fungal organisms, such as species of Fusarium and Rhizoctonia, also have been found to cause poor growth of peach replants (Hine 1961b). Savage et al. (1953) reported that Clitocybe causes root and trunk rot of peach trees in Georgia with characteristic symptoms of whitish mycelial mats beneath the bark on the lower trunks of trees which are dying or already dead. Another fungal organism, Cylindrocladium floridanum sp.n., has been associated with PTSL and was found to cause wilting, root-rot, and death of seedlings of several peach cultivars in the greenhouse (Sobers and Seymour 1967; Weaver 1971). “Blister canker,” incited by Physalospora persicae, appears as a swelling of newly invaded bark tissue leading to rough and blistered appearance with gum exudation later on and eventual twig death (Abiko and Kitajima 1970). Weaver (197413) characterized a gummosis bark disease of peach trees caused by Botryosphaeria dothidia, with typical symptoms of sunken lesions around lenticels, circular- to oval-shaped necrotic areas in bark beneath infected lenticels, and blisters on the surface of shoots and twigs. Numerous gum deposits on trunks, limbs, and twigs of affected trees are commonly seen. Chirilei et al. (1970) distinguished two types of gummosis: xylem gummosis which typically produces water-insoluble gums (pectic acid), and cortical and cambial gummosis which results only in water-soluble exuding gums (pectins). The first gummosis is responsible for apricot apoplexy, whereas the second type causes the slow decline of apricots. X-disease, which affects peaches, nectarines, and cherries, shows symptoms of inward curling of leaves in early summer, followed by development of watersoaked spots on these curled leaves which become yellow to reddishpurple and fall prematurely starting a t the basal end of shoots (Jones 1971b). Affected trees often are winter-killed after a few years (Lukens et al. 1971). Klement et al. (1974) accounted for apoplexy symptoms on apricot in Yugoslavia as a consequence of infection by Pseudomonas syringae, which caused the phloem and cambium to die during the course of winter. If the phloem and cambial necroses are not of such an extent that they girdle the branch or the trunk, then in summer following the infection the surrounding healthy tissues will try to overgrow the necrotized areas resulting in cankerous wounds. When cambial necrosis completely engirdles the branch or the trunk, the healthy parts above the infected area suddenly die in spring or in the course of summer. If the cambial necrosis girdles only one or two branches, partial apoplexy occurs; but if the trunk is engirdled, the result is complete apoplexy or tree death. Apoplexy


15

of apricot in Bulgaria (Iliev 1968) has been described as a necrosis of bark on young trees on their own roots, the damage being severe a t the collar and lower parts of the trunk with xylem remaining unaffected. The characteristic symptoms of bacterial canker of sweet cherry, caused by Pseudomonas mors-prunorum, are shot-hole leaves, infected buds, shoot cracks, and die-back of branches and young trees (Lyskanowska 1976). Cherry replants following cherry on the same land either made very poor growth or died as a result of Pratylenchus penetrans infestation (Mai and Parker 1967). The symptoms of cherry decline, as associated with a mycoplasma, have been described by Fos (1976) as growth irregularities and leaf and flower anomalies-lack of production with tree death soon following when growth is rapid. Plum decline, incited by Prunus necrotic ring-spot virus, has been symptomized as a general growth reduction followed by progressive tree decline with necrotic “incompatibility” between rootstock and scion (Posnette and Cropley 1970). As reported by Cochran (1975), Prunus stem pitting induces serious derangement in the xylem with abnormal lignification. Trees infected early in their life are dwarfed, tend to break off a t the ground line, and usually die after one to five years of infection. The woody cylinder near the ground line of affected trees is variously but characteristically pitted and fluted. The symptoms of tree decline in some stone fruits also include leafing out four to eight weeks early, general growth suppression, and discoloration of wood with occasional stem pitting (Agrios 1971). 11. METHODS TO STUDY THE PROBLEM

A. Electrophysiological Electrolytic conductivity (EC) of tissue is recorded in pmhos on the conductivity bridge after emersing an electrode in leachate; whereas in the case of electrical resistance (ER) the points of electrodes are clamped into plant tissues and the resistance is determined directly in ohms (Wilner et al. 1960). A highly significant correlation exists between EC and ER methods (Wilner 1961). Greenham and Cole (1950) found electrical capacitance measurements on diseased plants to be the reciprocal of ER measurements. A method of exotherm analysis also has been used to determine the condition of plant tissue in relation to cold temperatures (Quamme et al. 1973,1975; Yelenosky 1975). This method gives a direct measurement of the temperature a t which the experimental tissue is injured. Oscilloscope technique, as described by Ferguson et al. (19751, uses oscilloscopic square wave form, the pattern of which changes in relation to periods of plant dormancy, activity, or death. “Index of injury” is a simple method for expressing freeze injury since the per-

16


centage of released electrolytes is converted to a scale where the unfrozen sample is given a value of 0 and the heat-killed sample a value of 100 (Flint et al. 1967). Stuart (1939) showed that the ratio between the conductivity produced by complete killing of tissue and that produced by injured tissue termed “percent electrolytes” (5% EC), is more reliable than EC of a sample alone. De Plater and Greenham (1959) proved that measuring ER a t both low (1 K Hz) and high (100 K Hz to 1 M Hz) frequencies, and using a ratio between these measurements resulted in more sensitive determinations of injury than with either alone. Measuring the flow of an electric current in the trunk of black cherry, P r u n u s serotina, Levengood (1973) determined that trees infected with crown gall had lower ER than did healthy trees. Gardner et al. (1974) found that the toxic chemical produced by Helminthosporium fungus caused a rapid hyperpolarization in membrane electropotentials of tissues from susceptible lines of corn, but not from resistant lines. Using an exosmotic method, Filinger and Cardwell (1941) successfully determined cold injury in bramble canes which, following death by freezing or by boiling, offered 72 to 82% less resistance to electric current than when alive. Dostalek (1973) reported that the roots of apple trees infected with the proliferation disease were much lower in electrical impedance than roots of healthy trees in late fall. No differences were found when root impedance measurements were taken in late summer; stem tissue showed no difference a t either time. De Plater and Greenham (1959) found that a low/high frequency ratio of ER in healthy tissue usually would be about 4, while in tissue that has been severely injured by cold the ratio would be less than 1.5. Swingle (1932) showed that higher EC readings as a result of speedy exosmosis from frozen apple tissue samples indicated a corresponding high degree of cold injury to the woody tissue. Osterhout (1922) mentioned in an earlier report that the increase in the membrane permeability, which usually accompanied death of tissue, was paralleled in a striking manner by a simultaneous increase in EC of tissue leachate. Changes in membrane permeability and tissue respiration have been found to be characteristic of most plant disorders (Wheeler and Luke 1963). Similarly, Stadelmann (1969) concluded that an increase in permeability often reflects pathological or premortal conditions. Golus (1935) reported that lower permeability of the membranes and, hence, low EC normally characterized more winter-hardy plants. Wilner et al. (1960) presented data on decreasing trends in cold resistance of outdoor roots and shoots of apple trees which agrees closely with the increasing trend in EC of tissue extracts. Diffusion of electrolytes from roots, however, did not follow the same pattern as from shoot tissue (Wilner 1959). Wilner (1960) also established quantitative values for the ultimate frost hardiness


17

of apple trees, viz., no sign of any appreciable injury when EC from hardened tissues was 200 to 250 pmhos or less. An EC of above 350 to 450 pmhos generally signified total killing; whereas, intermediate readings indicated partial injury to the twigs. In another study on apple rootstocks, Stuart (1941) showed that freezing injury increased the EC of the stems of all the rootstock types studied. Yadava et al. (1978), Ketchie and Beeman (1973), and Stergios and Howell (1973) compared EC with other methods to assess cold hardiness in fruit trees. Tree mortality has been correlated to greater EC readings from acclimated as well as dormant twigs of fruit trees (Ketchie et al. 1972; Nesmith and Dowler 1976; Yadava et al. 1978).

B. Chemical and Biochemical Chirilei et al. (1970) suggested that the decline disorder of fruit trees is a complex pathological phenomenon caused by major physiological and biochemical disturbances. Most plant disorders are characterized by changes in cell permeability and tissue respiration (Wheeler and Luke 1963). Allen (1953) studied in detail the respiration of roots in relation to soil toxins. He found that the toxins would act on some of the many steps in the respiratory process and that their effect would be reflected by a change in the respiratory rate of the actively respiring meristematic region of the root. Bergman (1959) stated that the ability of roots to survive an oxygen-deficient period in wet soils varies greatly between species; some plants die within a few days, others survive for weeks or months, or, in some particular trees, even for several years. But death always ensues. Rohrbach and Luepschen (1968) discussed the relationship among polyhydric alcohols in peach tree bark, winter injury, and the initiation of Cytospora canker infection. They stated that the least winter-hardy cultivar, ‘Earlyglo’, was found to have a slightly higher mannitol level. Siminovitch et al. (1967) found the augmentation of protoplasm to be a part of the mechanism of freezing resistance, since they noted in early fall an abrupt rise in RNA from the low summer value, closely followed by a similar rise in protein, protein synthetic capacity of tissue, and freezing resistance. Thus, they developed a quantitative method of estimating resistance to freezing injury based on ninhydrin-reactive compounds (NRC) which measures amino acids that are released from the injured cells. Yadava et al. (1978) used the NRC method of Siminovitch et al. (19671, modified by Wiest et al. (1976), and successfully correlated with peach tree survival and trunk cambial browning; however, due to greater variation in NRC values of fresh tissue, they were not satisfied with the modification made by Wiest et al. Yadava et al. (1978) found

18


that the triphenyl tetrazolium chloride (TTC) reduction method as refined by Steponkus and Lanphear (1967a), when used with artificially frozen tissue, was significantly correlated with peach tree survival, trunk cambial browning, as well as NRC method, but not with EC. However, the work on dormant twigs under natural freezing conditions showed no consistent correlation among any of the methods (Yadava et al. unpublished). Similarly, Stergios and Howell (1973) found that the T T C method was not as suitable as EC for cherry and raspberry. Gallaher et al. (1975) studied the levels of leaf calcium (Ca) from healthy and declining ‘Loring’ peach trees on limed as well as unlimed field plots. They found that most Ca was nonextractable in acetic acid and that the concentration of extractable leaf Ca was less than 100 ppm. They also reported that concentration of total leaf Ca was highest from declining trees but that declining trees had fewer and smaller leaves, resulting in less total Ca than in healthy trees. Higher concentrations of soluble Ca in the leachate from dormant peach twigs have been found to be closely correlated with tree survival on short-life and non-short-life sites (Yadava et al. unpublished). The contents of prunasin, the predominant cyanogenic glucoside in peach roots, have been correlated with the peach replant problem in Japan (Mizutani et al. 1977). Patrick (1955) carried out an extensive study to evaluate the importance of soil toxins (toxins from peach roots and pure amygdalin) in relation to their interaction with certain microorganisms in old peach sites. Such inhibitors were not produced when old soil was autoclaved before amygdalin was added, when other soils were used in which no breakdown of glycoside had occurred, or when roots from species other than peach were added. However, in California, no differences in the rate of amygdalin hydrolysis and resulting production of hydrogen cyanide (HCN) were found in peach replant problem soils, peach non-replant soils, or soils used for a crop other than peach (Hine 1961a). The rate of amygdalin breakdown varied significantly only with soil depth-deeper soils showed less activity than shallow and light soils. Thus, it was theorized that amygdalin hydrolysis probably does not account for the difficulty of establishing peach replants in some areas of California. Amygdalin added to autoclaved soil was toxic to growing peach seedlings only if they were planted immediately after its addition; whereas two weeks following this, no HCN was detected in the soil and, likewise, no signs of HCN injury on plants were noticed. C. Isolation, Culture, and Bioassay

The subject of phytotoxins produced by plant parasites has been covered in detail in a review by Strobe1 (1974). Sands and Kollas (1974)


19

isolated Pseudomonas syringae from diseased (pear blast) pear trees and the isolates produced characteristic blast symptoms when inoculated into pear. In Michigan, green fluorescent bacteria were isolated from cherry trees diseased with bacterial canker and, when bioassayed, the organisms were found to be pathogenic (Jones 1971a). Dowler and Weaver (1975) were able to readily isolate pathogenic and non-pathogenic fluorescent pseudomonads from twig and trunk tissues of apparently healthy peach trees on a monthly routine, except in summer. These pathogenic isolates were found to be closely related to Pseudomonas syringae. In a review on Pseudomonas pathogens of deciduous fruit trees, Crosse (1966) specified two distinct biochemical types of flourescent pseudomonads causing disease, corresponding to Pseudomonas syringae and Pseudomonas mors-prunorum, respectively. De Vay e t al. (1968) reported t ha t the isolates of Pseudomonas syringae th a t causes canker of peach trees produce a wide spectrum antibiotic, syringomycin, whose production is reduced with the loss of pathogenicity by Pseudomonas syringae. Weaver (1971) isolated Cylindrocladium floridanum from soil around roots of dying or dead peach trees on a short-life site. In bioassays on peach seedlings, the fungal isolates caused root rot of the seedlings. Water-soluble extracts from root, shoot, and seeds of peach were toxic to the seedling growth of peach, apple, and beans when applied to soil of potted plants (Oh and Carlson 1976). However, synthetic amygdalin, when bioassayed with potted peach seedlings, did not produce a phytotoxic effect, although it did alter the levels of certain nutrient elements. Patrick (1955) isolated from peach roots a highly physiologically active inhibitor which resembled amygdalin. Upon bioassaying, the inhibitory response was obtained only when amygdalin was hydrolysed by emulsin enzyme in the treatment consisting of amygdalin emulsin. Ross and Crowe (1973) studied the presence of apple replant disease in pot bioassays using different orchard soils and fumigation with chloropicrin. For the development of apoplexy of apricots, Rozsnyay and Barna (1974) bioassayed Cytospora toxin obtained from three different isolates of the fungus, which caused leaf collapse, gum production, and necrotic wounds when absorbed by young attached apricot shoots.

+

D. Inoculation Weaver (1974b) reported 28°C as the optimum temperature for mycelial growth of Botryosphaeria dothidia, the incitant of peach bark gummosis disease; however, good growth a t 36°C and slight growth a t 38°C were not uncommon. A greenhouse test from New Zealand (Dye 1957) indicated t hat the optimum temperature for stone fruit stem infection by Pseudomonas syringae was 18.3"C. According to Crosse and Garrett

20

HORTICIJLTIJRAL REVIEWS

(19661, little or no recovery of Ps. syringae could be expected (after inoculation) when daily maximum temperature averaged 30°C. Canker length and peach seedling mortality in containers, filled with soil from a short-life site, were positively related to temperature (Daniel1 and Chandler 1974). Seedlings held a t v iriable outdoor temperatures (-17” to 14°C) with mean minimum of 3 3 ° C and maximum of 8.5”C, respectively, developed longer cankers than a t a constant temperature of 8°C. Dormant season hypodermic inoculations of several isolates of Ps. syri n g a ~into the bark of ‘Bin$ and ‘Berryessa’ sweet cherries, ‘Blenheim’ apricot, ‘Eldorado’ and ‘President’ plums, and ‘French’ prune showed that these trees were highly susceptible to infection and pathogenesis from mid-December until early February under California conditions (English and Davis 1969).Additionally, they found th a t Ps. syringae was more pathogenic to ‘Lovell’ peach seedlings a t 12°C than a t 28°C’ but caused essentially no infection near 7°C. When 3-month-old ‘Lovell’ seedlings were exposed to 6°C for 25 to 30 days prior to inoculation with Ps. syringae, then held a t 16”C,they were more susceptible to canker development than those held constantly a t 16°C or those in a greenhouse. Relative population development of Macroposthonia xenoplax on a good host (Thompson Seedless grape) and on poor hosts (Lovell and S-37 peaches) is influenced greatly by soil temperature (Lownsbery 1961). A soil temperature of 26°C is more favorable for population increase of M. xenoplax than 13”,18”,21”’ or 28°C. Weaver and Wehunt (1975) showed the effect of various soil p H levels on the performance of ‘Elberta’ peach seedlings grown in pots of soil from a peach orchard with a bacterial canker history, and artificially inoculated with Ps. syringae after they had become dormant. Seven weeks later, sizeable mortality occurred in soils with a pH of from 5.6 to 6.1, but no plants died in soils with p H levels adjusted to 6.4 to 7.2. In addition, it was found th a t the number of propagules of Pythium spp. in the soil and recovery from roots were positively correlated with soil pH. In December, highest population of M. xenoplax was recorded in soil having a p H of 6.1, but differences during March or April were not significant. On the other hand, Lownsbery (1961) detected no differences in population levels of M. xenoplax on ‘Lovell’ peach grown in soil between p H 5.0 and 7.0. Further, in a lathhouse pot test, peach seedlings were not injured by the nematode a t populations as high as any yet found in California peach orchards. Luisetti and Paulin (1972) and Luisetti et al. (1973) studied the pathogenicity of Ps. mors-prunorum f. sp. persicae in relation to inoculum concentration, and concluded th at incubation time was inversely related to inoculum concentration. Prunier et al. (1973a) successfully reproduced decline symptoms on peach and apricot by inoculations with concentrated cultures of the same bacterium.


21

At Geneva, New York, apple breeders (Cummins 1977; Cummins and Aldwinkle 1974b) emphasize the importance of screening by utilizing inoculation for resistance to several important fungal organisms before beginning actual horticultural testing. Similarly, in Oregon Westwood (1976), while working on the inheritance of pear decline resistance, graftinoculated the progeny of resistant, susceptible, and resistant X susceptible crosses, using them as rootstocks for ‘Bartlett’ pear. By inoculating Virginia crab K 6, a virus indicator clone, with certain isolates of apple stem grooving virus (ASGV), Stouffer et al. (1977) observed apple decline and union necrosis symptoms comparable to those occurring on clones of ‘Delicious’ apple propagated on M M 106 rootstock. Since Malling and Malling-Merton apple rootstocks had not been reported to be affected by the ASGV-induced disorder, the authors declined to include ASGV as the causal factor for the union necrosis and decline syndrome of apple in Pennsylvania. Campbell (1971) ascertained the importance of the amount of virus inoculum in assessing its effect on the growth of apple trees. His observations of four apple cultivars, bud-grafted on virus-infected rootstocks in England, have revealed varying degrees of growth reduction which were proportional to inoculum strength in both the first and second years of growth. Kunze (1972) used a graft inoculation technique to study apple proliferation disease in a series of tests where 80% of the surviving grafts developed typical symptoms within 2 years. Mircetich et al. (1977) inoculated peach seedlings with root chips from peach orchard trees carrying Prunus stem pitting (PSP) or yellow bud mosaic (YBM). These later developed only PSP or YBM symptoms, depending on the inocula used. However, ‘Mazzard’ cherry seedlings that received inoculum through root chips from root chip-infected peach, or buds from naturally-infected trees of cherry, remained symptomless. The studies of Smith and Stouffer (1975) established that PSP was graft-transmissible, and that root bark patches were more efficient sources of inoculum than budwood. Posnette and Cropley (1970) used the inoculation method in a ten-year field trial to study the effect of five different viruses on the decline of three plum (Prunus dornestica L.) cultivars. They noticed the first appearance of symptoms after five years, whereafter the trees declined progressively with necrotic “incompatibility” between rootstock and scion. Rosenberger and Jones (1977) studied seasonal variation in virulence of peach X-disease inoculum, and found that infection rose from 8%for May to 100% for June inoculations, then declined to 82%, 32%, and 20% for August, September, and October, respectively. Various other inoculation studies to test the role of bacterial canker in PTSL have been reported by several workers (Chandler and Daniell 1974, 1976; Daniell and Chandler 1976; Davis

22


and English 1969b; Dowler and Petersen 1966; Dowler and Weaver 1975; Gardan et al. 1975; Yadava et al. 1978). Davis and English (196913) proved that the greatest expression of bacterial canker symptoms on peach results from inoculations made during the dormant period. They theorized that the susceptibility to Pseudomonas syringae inoculation varies directly with the degree of dormancy. Gardan et al. (1975) reproduced experimentally all the symptoms of bacterial canker on branches, stems, leaves, and fruits of peach by artificial inoculations with Ps. mors-prunorum f s p . persicae. Differences in susceptibility to Ps. syringae inoculations in presence of Phytophthora spp. on seven species of stone fruits in Greece were observed by Kouyeas (1971). Inoculations of injured and uninjured peach roots with several isolates of Clitocybe tabescens showed that most of the isolates had infected the injured roots, whereas only few isolates infected the uninjured roots (Weaver 1974a). In a three-year-study under Colorado conditions, Luepschen e t al. (1975) found seasonal differences in canker development on ten peach cultivars as a result of inoculations with Cytospora leucostoma. Benomyl sprays on peach trees before inoculating the limbs with C. leucostoma gave up to 98% and 80% control of inoculations made in May and June, respectively (Luepschen 1976). In New York, Hampson and Sinclair (1973, 1974) studied the pattern of Leucostoma canker development due to water stress on potted peach plants by inoculating the plants during the active growth period in greenhouse. They used eosin dye to learn about the infection’s movement in the xylem, since they suspected that the xylem dysfunction was the important cause of symptoms. Helton and Randall (1975) inoculated ‘Italian’ prune trees with a virulent strain of Cytospora cincta, and examined the infected branches to determine the longitudinal extent of visible gum in the cambial zone. Internal gummosis apparently was associated with the commonly observed wilting of terminals following infection. Rosik e t al. (1971) were able to artificially induce gummosis on apricots with C. cincta inoculations. Wilbur e t al. (1972) studied the seasonal development and pathogenicity of four clones of Armillaria mellea inoculations on peach trees and interclonal differences. Abiko and Kitajima (1970) established, on the basis of inoculation tests, that the blister canker fungus (Physalospora persicae) was specifically pathogenic only to peach. By using inoculation with Macroposthonia xenoplax as a routine procedure, Lownsbery et al. (1973) reported reduced growth of ‘Carolyn’ peach on Love11 rootstock as well as increased susceptibility to both Pseudomonas syringae and “wet-feet.” The full effect of the nematode was evident after only three growing seasons. Soil inoculations with P y t h i u m species a t planting time increased susceptibility to Ps. syringae less than the inoculation with M. xenoplax. P y t h i u m spp. did not reduce growth


23

of peach trees significantly and no correlation between M. xenoplax and Pythium sp. was noted. Hung and Jenkins’ (1969) greenhouse and laboratory experiments on peach inoculation with Macroposthonia curuatum and Pratylenchus penetrans produced typical symptoms of PTSL; however, under nonsterile conditions, the pits and lesions produced by feeding of these nematodes were invaded by other microorganisms, causing general discoloration and low vigor of the root system. Penetration, migration, establishment, and development of Meloidogyne javanica in the roots of two resistant (Okinawa and Nemaguard) and one susceptible (Lovell) peaches were histologically studied by Malo (1967), after inoculation with second-stage larvae of the nematode. It was concluded that the nature of resistance of ‘Okinawa’ and ‘Nemaguard’ to M. jauanica infection was based on a “walling-off’’ of the giant cells, followed by their breakdown. This was not the case with ‘Lovell’ roots where giant cells reached the reproductive stage 20 to 22 days after inoculation.

E. Discoloration and Tissue Integrity Daniell (1977) has reviewed the subject of PTSL, citing discoloration and tissue integrity as important criteria for determination of tree condition. Other workers also have used these methods for determining tissue disorders caused by different factors (Chandler et al. 1962; Daniell 1975; Daniell and Crosby 1971; Lapins 1961; Nesmith and Dowler 1976; Petersen 1961; Prince 1966; Prince and Horton 1972; Stergios and Howell 1973; Taylor et al. 1970; Yadava et al. 1978). Stergios and Howell (1973) compared five viability tests for cold-stressed plants and found that tissue browning was one of the most reliable, although it required considerable time and was only a qualitative measure. On the other hand, Yadava et al. (1978) compared trunk cambial browning (TCB) of orchard trees in early spring with laboratory tests for cold hardiness and bacterial canker development (BCD) and finally, peach tree survival (PTS) later in the season. TCB proved to be a quick, convenient, reliable, and quantitative method. A highly significant correlation was observed between TCB in mid-March and PTS through the end of June. Yadava and Doud (1978b) have found a close correlation between the ease of bark plug removal from the tree trunks and the TCB ratings. Bark removal was easiest (particularly in early spring) in the case of injured trees, thus indicating a direct relation with the degree of injury to the trunks.

F. Regrowth Lapins (1961, 1962) found that the hardiness differences between cultivars and seedling progenies were more reliable by tissue regrowth

24


following artificial freezing than by EC readings of the leachate from shoot sections. In a ten-year-study on peach budbreak as influenced by temperature, Weinberger (1967) found that the length of physiodormancy correlated with mean maximum November and December temperatures, respectively, but not with mean minimum for these months. On the basis of controlled freezing experiments and forcing, Meador and Blake (1943) showed that peach buds were most hardy near the coldest part of winter and that hardiness did not follow a smooth curve throughout, but fluctuated up and down during winter. Ormrod and Layne (1974) forced whole trees under controlled conditions following acclimation a t set regimes of day/night temperature and photoperiod. They found that temperature and scion cultivars had much greater effects on cold hardiness of buds and bark than did rootstock and photoperiod. Prince (1966) investigated the possible relationship between winter temperature patterns and tissue injury in the cambial zone of peach tree trunks and crotches during four winters in central Georgia. Injury was detected only on trees which had accumulated all or a substantial portion of their chilling, followed by exposure to abnormally warm weather, and then to a relatively sharp drop in temperature to below freezing. Hodgson (1923) provided guidelines for assessing regrowth. If the cuttings from dormant trees failed to start growth within two weeks of forcing, they were judged to be in deep dormant state. Yadava and Doud (1977) studied the effect of applied phytohormones and rootstocks on the budbreak and growth of scions by forcing two-year-old potted trees of ‘Babygold-5’ in the greenhouse. Differences due to rootstock and phytohormone treatments were significant for budbreak and shoot length but not for other characters studied.

G . Other Methods Based on a histological evaluation of cold injury to apple trees, Steinmetz and Hilborn (1938) concluded that if about 50%of the parenchyma cells were killed the branch might not recover; but if only 20% were killed, recovery was probable. Further, it was shown experimentally that the compression of cambial cells and the lateral displacement of wood rays were characteristic of low temperature injury in woody plants. Dorsey and Strausbaugh (1923) explained in a cytological study that the browning in the wood was due, a t least in part, to a condensation of storage materials apparently transformed into gums and tannins. An anatomical examination of Cytospora canker infection on one-year-old ‘Elberta’ peach seedlings confirmed that the wilting due to this cankerous disorder was associated with gum-plugging in the xylem (Banko and Helton 1974). Batzer and Schneider (1960) showed that positive


25

diagnosis of pear decline could be made only by anatomical examination of phloem a t the bud union, although macro-symptoms of a faulty bud union were helpful in determining the affected trees. Anatomical abnormalities in cold-injured peach trees also were observed by Daniel1 and Crosby (1968). Harvey (1923) measured winter cambial temperature of trees and correlated it with cold injury. He was most concerned with the cambial temperatures because injury is most serious within this layer. Tree trunks painted with white latex paint on sunlit sides were found to be 30°C cooler than non-painted trunks during mid-winter (Eggert 1944; Martsolf et al. 1975). According to Jensen et al. (1970), trunk cambial temperature appeared to be a sensitive indicator of peach tree vitality during growth; trees with low vitality had higher cambial temperature than trees in good health. Potter (1924) reported that apple roots, dried until 5% of their total moisture was removed, sustained less freezing injury than turgid roots, and high root moisture content contributed to winter root injury. Wildung et al. (1972a) determined root moisture levels and soil temperature in the root zone, concluding that roots with 3 to 4% less moisture in 1967 hardened to a greater extent than in 1968 when rainfall was above normal. Weekly root hardiness changes in 1967 also were highly correlated with the soil temperature during the week preceding hardiness testing. Tukey (1970) published an excellent review on the leaching of substances from plants, as applied to the removal of substances from plants by the action of rain, dew, mist, and fog. He mentioned that the young leaves from healthy and vigorous plants are much less susceptible to leaching than are leaves which are injured, whether by microorganisms, insect-pests, adverse climate, nutritional and physiological disorders, or by mechanical means. Leaching also may reduce physiological disorders in certain cases. Injections, but not foliar sprays, with several derivatives of tetracycline, under the bark or into the wood of peach trees infected with peach rosette, resulted in remission of rosette symptoms, and thus, confirmed the involvement of mycoplasma-like organism in peach rosette (Kirkpatrick et al. 1975a). Israel et al. (1973) reported that soil-applied potassium cyanide, mendelonitrile, benzaldehyde, peach root bark, and amygdalin reduced the total population of microorganisms, actinomycetes, Pythium, and pathogenic nematodes in an old peach soil. In search of the causative factors in PTSL, Gilmore (1963) designed experiments in soil pot culture using soils composted with peach root wood. This compost stimulated biological activity, thereby suppressing nitrogen content-hence, the stimulation of “replant” effect. Havis and Gilkeson (1947) studied toxicity of old peach roots in high-nutrient sand culture in 3-gal. earthenware crocks. Evidently, there was no “toxic” substance

26


in peach roots or leachate that adversely affected the growth of young ‘Elberta’ peach on ‘Lovell’ seedling rootstock that had failed under field conditions. However, Proebsting and Gilmore (1941) reported that root bark, but not wood, was phytotoxic in sand culture. Furthermore, the alcohol extract of bark was also toxic, while the residue from alcohol extract did not show any toxic effect in sand culture. Another study on soil treatments in the greenhouse (Prince et al. 1955) showed inconclusive results applicable only to the particular soil used. Van Gundy et al. (1962) measured the oxygen diffusion rate in the soil pore spaces and correlated it with nematode activity and their survival. The results revealed that the oxygen availability in soil was largely related to moisture content. Heuser (1972) obtained callus cultures from peach and two grafting understocks, ‘Nanking’ cherry (Prunus tomentosa Thumb.) and sand cherry (P. besseyi Bailey), to study their growth pattern, structure, and tolerance to cyanide toxicity. Peach callus was more tolerant to high cyanide concentration, whereas callus cultures from the two understocks were more sensitive to cyanide. This study suggested the existence of a detoxification system in peach. 111. CAUSAL FACTORS

A. Environmental Factors An understanding of the physiological responses of particular crops to changes in the environment must precede an evaluation of causal agents in the short life syndrome. It is generally agreed that environment exerts the most important controlling effect on the geographic distribution of organisms on earth. Of the various climatic factors, temperature often plays a leading role in its influence on plants. Thus, climatological data seem to furnish the most plausible explanation for a favorable tree condition (Cowart and Savage 1941). For this reason, those factors related to environment in both the broad sense (those natural factors affecting the entire region or growing areas) and the strict sense (those having influence on individual orchards or a group of plants or orchards on a limited area) are included in this section. 1. Macroclimatic (Natural).-In this section, we will emphasize such controlling factors as temperature, rainfall, and wind and their combinations. The impact of these prime forces as related to the prevailing weather in a specific fruit growing area is considered under the following headings.

a. Cold (Winter, Freeze or Frost) Injury.-Throughout the world, the most consistently and effectively hostile element of climate in a fruit tree’s environment is low temperature (Proebsting 1970). There is a


27

relatively small temperature interval between that causing slight injury and that which virtually eliminates the crop (Proebsting et al. 1966). Low temperature, either background, hardening, or damaging (Drozdor et al. 1976), usually limits the organic reactions that constitute the processes of plant life (Parker 1963). Langridge (1963) has reviewed a t length the biochemical aspects of temperature response. Sometimes both fall and spring damages occur, one kind of injury being additive to the other. Gerber et al. (1974) concluded that, aside from the winter injury, spring frost in temperate climates constitutes the most serious climatic hazard to the fruit trees. Once the tree is damaged, this injury is further complicated by secondary organisms that gain entry through injured tissue and become part of the complex that kills the trees (Blake 1938; Helton 1961; Hickey 1962; Lyons 1973; Panagopoulos and Crosse 1964; Savage 1970; Savage and Cowart 1942a). Blake (1928) stated t h a t cold injury may change the growth status of an entire or a sector of a tree within any one season with the effects continuing for an indefinite period of time. Furthermore, the extent of resulting injury often is affected by growth processes preceding a freezing spell (Dennis 1977; Olien 1967). Longevity of cold-injured trees is substantially shortened (Campbell 1948). I t is generally accepted that cold injury, which occurs to the bark and wood of tree trunks and limbs during sub-freezing winter weather following an unusually warm period, is alone more than enough to kill trees even in the absence of other agents. Cold or frost injury has been heavily implicated as the major cause in the short life syndrome of fruit trees by several research workers (Clayton 1968, 1972; Cowart and Savage 1941; Daniel1 and Crosby 1970; Edgerton and Harris 1950; Jensen et al. 1970; Paclt 1972; Prince 1966; Sarasola and de Bustamante 1970; Savage 1970, 1972; Savage and Cowart 1942a, 1954; Weinberger 1949). General aspects of freezing and chilling injuries to plants have been thoroughly reviewed (Burke et al. 1976; Lyskanowska 1976; Mazur 1969). In woody plants, consisting of tissues of more than one kind and age, frost killing occurs over a range of temperatures. The relative influence of such factors as length of growing season, tissue maturity, states of dormancy, nutrition, moisture, and temperature variations upon injury cannot be separated definitely from that of other factors, but all factors acting together determine the ability of a tree to withstand winter conditions (Dorsey and Bushnell 1925).

i. Dormancy. The subject of dormancy has been thoroughly reviewed by Lyons (1973), Olien (19671, Samish (19541, Samish et al. (1967), Taylorson and Hendricks (1976), Vegis (1964), and Wareing and Saunders (1971). A rest period (referred to as physiodormancy throughout this text) is characterized by an internal inhibition of growth resulting from physiological factors, and having certain distinct features such

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as onset, intensity, and duration; dormancy, however, is when most woody plants would not grow where even the physiodormancy is not the controlling factor. In other words, the physiodormancy refers to that portion of the state of dormancy when a given species cannot be induced to grow even if suitable conditions prevail. According to Vegis (1964), dormant plant organs have especially high resistance; thus, growth cessation and the onset of dormancy before the unfavorable season begins ensure the survival of the plants in question. He further stated that dormancy is often considered as a hereditary property with the beginning, duration, and end of the dormancy period. Frost in the fall season can be quite serious, especially on plants t h a t have not become dormant. Thus, suitable biochemical preparations for the development of resistance against cold weather are necessary for the survival of overwintering plants with aboveground parts (Parker 1963). Samish (1954) has pointed out that photoperiodism is not a factor universally involved in dormancy, but that cultivar differences in photoperiodic response with corresponding variance in chilling requirement indicate that photoperiodism is a factor in dormancy, a t least in some species and cultivars. Moreover, Piringer and Downs (1959) demonstrated that previous photoperiod(s) had no effect on the survival of apple and peach trees a t Beltsville, Maryland during the 1957-1958 winter. Similarly, Ormrod and Layne (1974) found temperature and scion cultivar to have greater effects on cold resistance of peach buds and bark than did photoperiod and rootstocks. Along with low temperature, light has been found to be the most notable of environmental factors to greatly enhance the rate and degree of cold acclimation as a photosynthetic rather than a photoperiodic stimulus (Steponkus and Lanphear 196713).Samish et al. (1967) reported that with peach buds light was of particular importance during the period of emergence from dormancy. However, this effect was preconditioned through preparation in the course of mid-dormancy darkness, since darkness during chilling increased the subsequent effect of light. T o produce normal growth under favorable conditions, it is necessary that physiodormancy be broken by certain periods of cold or low temperature during which growth may be interrupted. The length of this period, which differs with plant species, cultivars, and physiological conditions, is termed by Samish (1954) as the chilling requirement. T h e chilling requirement, in all its phases, is controlled by genetic factors (Vegis 1964). Lesley (1944,1957) emphasized that chilling requirement in peach depends on multiple genes, and transgressive segregation occurs in both directions. Generally, flower buds have a shorter chilling requirement than vegetative buds, and terminal vegetative buds have a lower chilling requirement than lateral buds. Symptoms of inadequate chilling include delayed and sporadic foliation, deformed and non-viable flower parts,


29

and flower bud abscission (Lammerts 1941). Continuous chilling breaks physiodormancy of more buds than alternating warm and cold periods even with similar total hours of chilling (Overcash and Campbell 1955). This is because the periods of warm temperatures interspersed with cold temperatures counteract some of the cumulative chilling effect. Cold injury is common on those trees which have accumulated all or partial chilling hours, followed by their exposure to abnormally warm and cold periods (Prince 1966). A heat requirement also is necessary for growth resumption after the chilling requirement has been satisfied (Lammerts 1941). However, high temperatures of from 15" to 23°C during winter have been shown to antagonize the dormancy-releasing effect of chilling (Bennett 1950). Chilling probably accomplishes more than a decrease in growth inhibitor content and synthesis of growth promoters; rather, a change in metabolism occurs, dependent in part on a higher oxygen supply permitted by low demands for respiration during winter (Taylorson and Hendricks 1976). Wareing and Saunders (1971) published an excellent account of dormancy in relation to phytohormones. Roots are not thought to have a true physiodormancy (Dorsey 1929; Samish 1954), although some roots may go through a partial one. Other parts, which are exposed to external environment, experience true dormancy to escape injury during varying and adverse conditions of winter weather. Dormant plant organs have maximum cold hardiness (Vegis 1964), but after spring dehardening the killing point begins to rise (Dorsey 1929). A rapid rate of cold hardiness development is better than slow rate to withstand cold. Moreover, loss or inability to regain cold resistance reduces the survival value of the plants (Brierley 1947). In peach buds, cold resistance is lost and regained repeatedly depending on the fluctuations in the winter weather. A peach cultivar in middle Georgia would suffer from prolonged dormancy if its chilling requirement were not satisfied and physiodormancy were broken by mid-February (Weinberger 1950a). Chilling early in the dormant period appears to be less effective than later chilling, and interruption of chilling with periods of high temperature reduces subsequent growth (Thompson et al. 1975). Prolonged dormancy of peaches, according to Weinberger (1950b), is a condition in which budburst is beyond the usual time for opening in the spring, even though favorable growing temperatures occur. Prolonged dormancy trouble with peaches has been experienced in all southern regions of the U.S.A., as well as in Italy and part of Latin America, where it is a serious problem. In China, races of peach that are resistant to prolonged dormancy have developed. This problem is associated with high mean temperature in winter. Continuous exposure to cold or periods of extreme cold is not necessary or even desirable to break physiodor-

30


mancy. However, high winter temperatures and sunlight have a delaying effect in breaking dormancy. The chilling requirement of peaches, while creating a hazard to production in southern regions, is very beneficial in other parts. I t keeps peach trees dormant during cold weather, while otherwise growth might begin very early and be susceptible to frost. Delayed, but not prolonged, dormancy provides an added advantage of escaping spring frosts.

ii. Extent of Injury. General weakening of tissue as a result of cold injury increases its susceptibility to decay or other disorders. The duration of exposure to low temperature, the maturity of plant tissue, the time of the year, and the dormancy status of the tissue all appear to have a bearing on the extent of injury. With the approach of maturity the killing point due to cold drops gradually, while during the break in the dormancy period it rises again (Dorsey 1929). Campbell and Hadle (1960) noted that between trees of different ages but of same cultivar considerable variation in amount of winter injury resulted from an extremely low temperature of -34°C experienced in Kansas in January, 1959. One-year-old trees of ‘Halehaven’ and ‘Redhaven’ peach suffered slight injury while five-year-olds sustained medium injury. Cullinan and Weinberger (1934) reported that buds on peach trees which were low in nitrogen and where shoot growth was short did not survive as well as those on more vigorous trees. Gerber et al. (1974) gave a comprehensive account of spring injury. Deciduous fruit trees undergo a transition a t spring blossoming and leafing that is intermediate between foliation and defoliation. This transition time is critical because the crop is setting and the threshold of lethal temperatures is rising rapidly. Aside from midwinter temperature injury, spring frosts in temperate climates constitute the most serious climatic problems in fruit production. The “black” frosts, which are the freezes occurring without white hoar due to lower dew point than ambient temperatures, are usually more damaging than the “white” frosts, which occur on nights with dew point near freezing and produce abundant white frost. Usually, black frosts feature very cold weather. iii. Type of Injury. Two types of injuries, viz., bud injury occurring on both flower and leaf buds, and bark and/or cambial injury which may occur on trunks and/or limbs, constitute the most common kinds of cold injuries. Probably both types of injuries result from insufficiently matured tissues’ inability to withstand low temperature (Brown 1943). Two types of bark injuries have been described by Blake (1938). One form of bark injury is the near- or complete-killing of a band of bark several centimeters in width on the main trunk just above ground level. This results in a girdling effect, and fungi soon cause rapid decay of


31

the wood. Another form of injury to peach trunks is bark splitting, sometimes confined to the lower 5 tolOcm of the trunk,but often extending to a height of a t least 30 to 50 cm. Although the bark on other areas of the trunk is uninjured, fungi enter from the injured side and cause weakening and decay of the inner wood. There is not always a correlation between resistance to bark and bud injuries (Blake 1935, 1938). Thousands of peach trees in middle Georgia were found severely damaged, principally on the windward side, due to cambial injury which occurred as a result of subfreezing temperature and high wind in the spring of 1949 (Weinberger 1949).

iu. Mechanism o f Injury. A number of mechanisms have been proposed to help explain the physiological and biochemical changes associated with cold injury. Perhaps the most spectacular effect of chilling temperatures on sensitive plant tissue is that on protoplasmic streaming (Lyons 1973). The external symptoms of injury and ultimate death of the tissue would reflect the inability of cells to withstand increasing concentrations of metabolites as a function of time. The mechanism of cell injury by chilling encompasses several factors operating simultaneously or independently-imbalance in metabolism, accumulation of toxic chemicals, and increased permeability. In nature, temperature decreases a few degrees per hour and the woody plant tissue freezes a t a slow rate, resulting in ice formation first occurring outside the protoplasm where the water is purest. As temperature continues to decrease, intercellular water increases a t the expense of intracellular water, resulting in cell dehydration. T h e review by Burke et al. (1976) centers around water in living tissues as related to freezing injury. They mention that water content or degree of dehydration in plants which acclimate (the hardy plant species that go through the seasonal transition from tender to hardy condition) almost always decreases with increasing hardiness and increases as plants deacclimate. Plants which are tolerant to freezing generally undergo extracellular freezing, while intracellular freezing is probably invariably lethal. The differences between hardy plants and those tender plants which withstand freezing to some extent can, therefore, be stated simply: hardy plants tend to survive when more of their water is frozen than do tender plants. Mazur (1969) theorized t h a t damage to cells from intracellular ice seems due to direct interaction of ice crystals with membrane systems rather than to indirect effects associated with the loss of liquid water. Cooling velocity and permeability of the cell to water are the primary factors determining the kind and extent of intracellular ice formation. Except when cooled at the highest of velocities, cells approach equilibrium during freezing. However, the fate of the cell is greatly influenced by whether it approaches equilibrium

32


by intracellular freezing or by dehydration and extracellular freezing. Cells that are cooled too slowly to freeze intracellularly, as is the case with most plant cells, equilibrate by dehydration which produces a t least six types of physical alterations: concentration of solutes, precipitation of solutes, reduction in cell water content, cell shrinkage (plasmolysis), changes in pH, and reduction in spatial separation of macromolecules. Several of these alterations, including changes in ionic strength, concentration of specific electrolytes, pH, and concentrations of protein, can lead to irreversible denaturation of proteins a t subzero as well as a t elevated temperatures. Four of them, viz., change in electrolyte concentration, removal of essential water, cell shrinkage, and reduction in the spatial separation of macromolecules, have been the bases of unitary theories of freezing damage. Another study showed rapid and more uniform ice spread in unhardened than in cold-hardened stems (Yelenosky 1975). Exothermic tests showed that a small fraction of water may remain unfrozen to as low as -42°C after freezing of the stems’ bulk water (Quamme et al. 1973). In view of the report by Scarth (19441, the protoplasm’s most important property in frost resistance is its ability to prevent coagulation or some other mechanical injury following freezing. Histologically the effects of low temperature injury become apparent first as death of the protoplasts in the parenchyma cells followed by an occlusion of the vessels by a gummy substance; thus, “blackhearted” wood is formed (Steinmetz and Hilborn 1938). I t has been shown experimentally that the compression of cambial cells and the lateral displacement of wood rays are characteristic of low temperature injury in woody plants. u. Effect on Tree Life. Campbell (1948) stated that those trees most severely injured by cold probably have reduced longevity. He further maintained that the mechanical strength of those branches with severe blackheart-type injury is likely to be reduced, resulting in splitting and breaking. When splitting and breaking do occur in the blackheart wood, it becomes more susceptible to wood-decaying organisms than uninjured sapwood or normal heartwood. Any such injured wood, when exposed to the air, rots rapidly; thus, the productivity and life span of the injured tree are decreased. Cold injury has been implicated as a prime predisposing factor for such disorders as Cytospora canker (Helton 1961; Hickey 1962), bacterial canker (Klement et al. 1972), blossom blight (Panagopoulos and Crosse 19641, other insects and diseases (Cowart and Savage 19411, and PTSL syndrome (Daniel1 and Crosby 1970). Brierley (1947) asserted that cold injury or easily lost resistance to it certainly would lessen tree survival.


33

ui. Cold Hardiness. Cold resistance refers to the ability of plant cells to survive ice formation in the tissue or the ability of the plant to withstand low winter temperature or frost in the spring (Chandler 1954). A complete knowledge of hardy plant adaptations to freezing stress may help us to reduce winter damage and resulting losses in fruit production, since cold hardiness has been directly associated with tree longevity (Nesmith and Dowler 1976). During the period of rapid growth in the spring, plants are exceptionally susceptible to cold injury (Dorsey 1918a). T he ability of plants to withstand cold depends on an inherent annual rhythm of complex metabolic functions t h a t have evolved through a n interaction between plant and environment. Levitt (1966) and Alden and Hermann (1971) agree th at development of cold hardiness in woody plants is inversely proportional to growth rate, and th a t the environmental factors th at influence growth will affect cold hardiness accordingly. Levitt (1951) theorized th at frost, drought or desiccation, and heat resistance are all basically similar, and any resistance to one of these factors carries with it a corresponding resistance to the others. Numerous factors affect cold hardiness of fruit trees: soil and air temperature, photoperiod and light intensity, soil and tissue moisture, rainfall, air movement, rate of cooling, tissue maturity, growth rate and factors controlling it, cultivar and tissue type, physiological stage, dormancy, stomata1 density, and defoliation (Alden and Hermann 1971; Brierley 1947; Chandler 1954; Dennis 1977; Edgerton 1954, 1960; Hildreth 1926; Howell and Weiser 1970; Irving and Lanphear 1967; Kennard 1949; Ketchie and Beeman 1973; Knecht and Orton 1970; Knowlton 1936; Levitt 1966; Meador and Blake 1943; Pellett 1971; Proebsting 1963; Proebsting amd Mills 1961; Wildung et al. 1972a, 1973). Temperature prior to freezes can strongly influence cold hardiness and survival of peach trees (Dennis 1977). Peach trees do lose hardiness following periods of warm weather during the dormant season (Edgerton 1960). Both soil and air temperatures during the dormant season affect hardiness, although the duration of cold appears to be more important than the degree of cold (Proebsting 1963). Loss of hardiness could occur before the end of physiodormancy if hardiness greater than the minimum level had been achieved previously. Peach scions have greater influence on cold hardiness of bark than do rootstocks (Ormrod and Layne 1974); however, apple rootstock hardiness is not influenced by scion (Wildung et al. 1972b). Comparing root and shoot differences in hardiness, Pellett (1971) showed th a t stem tissue achieved much greater hardiness than root tissue under similar conditions. This conforms with the details given in reports by Chandler (1954), and Dorsey and Bushnell(1920).Chandler said th a t roots of most orchard species are considerably less resistant to cold in winter than

34


parts aboveground. For example, roots of apple whose tops would withstand a temperature of -35" to -40°C are killed by a temperature of -10" to -15°C when frozen slowly in air. Pear, peach, P r u n u s cerasifera, and P. auium roots are about as tender as apple roots or slightly more so; however, P. mahaleb roots seem to be more resistant than the others'. Wildung et al. (1972a,b) correlated root hardiness with rainfall and soil and root moisture levels. Time and degree of defoliation have been reported to affect winter hardiness (Kennard 1949). There is evidence in plums, apples, and other such species that the maternal parent may transmit cold tolerance better than the paternal parent (Stushnoff 1973; Wilner 1965). Lantz and Pickett (1942) found that hardy apple cultivars transmit their hardiness to a relatively high percentage of progenies, even when crossed with a tender cultivar like 'Delicious'. They also found that a portion of the seedling trees produced by crossing two cold-tender cultivars may be hardier than either parent, and further indicated that apple progeny hardiness is predictable but comes from multiple factor inheritance.

b. Other Stresses.-i. Water (Drought, Desiccation). Brierley (1947) reported that when a large portion of tissue water is lost, some plants, or their younger wood, are severely injured by cold and cannot recover. Brown (1943) showed under Illinois conditions that both trunk and tree top injury in peach were the result of the inability of immature tissues to withstand cold. The immaturity of the tissues was associated with summer drought in 1941, followed by heavy rains in October and warm weather in November and December. Subsequent spring and early summer rains in 1942 to the verge of soil saturation may have contributed to cold injury during the following January. Under nearly adequate soil moisture conditions a t all times, even moderately injured trees live and maintain productivity. However, under the conditions of high temperature and frequently inadequate moisture supply, any injury to peach trees, no matter how slight, quickly weakens the trees and makes them even more susceptible to winter injury and other disorders (Cowart and Savage 1941). Howard (1924) reported that soil moisture seems important to root hardiness and survival in a variety of ways. Emerson (1903) and Howard (1924) found greater root injury under dry soil conditions than in moist soils. Hampson and Sinclair (1974) have detailed the development of Valsa canker of peach in relation to water supply. ii. Oxygen Deficiency (Wet-Feet). The ability of plants to survive a period of oxygen (02) deficiency varies greatly among species (Bergman 1959). An extensive review of literature by Rowe and Beardsell (1973) showed that waterlogging produces many adverse changes in the root environment which basically can be attributed to O2 deficiency. In ad-


35

dition to indirect effects on the soil, 0, deficiency has a direct effect on root metabolism itself. Literature also shows that the effects of O2 deficiency on plants may be compounded by the formation of hydrogen sulfide in some waterlogged soils and by the auto-toxic production of hydrogen cyanide (HCN) by the roots of species which contain cyanogenic glycosides. Waterlogging caused a rapid decrease in root prunasin (the predominant cyanogenic glycoside in peach roots) and in the chlorophyll content of the leaves (Mizutani et al. 1977). Excess carbon dioxide and ethylene formation in waterlogged soils can add to the direct effects of deficiency. Poor performance of plants under such conditions can therefore result from a complex of interacting factors which make it extremely difficult to isolate any one change as being more important than others. This is complicated by two additional factors. Firstly, the importance of each factor varies with the plant species, and secondly, not all soils undergo the same changes when they become 0, deficient due to waterlogging (Bergman 1959; Rowe and Beardsell 1973). Rowe and Catlin (1971) reported differential sensitivity to waterlogging and cyanogenesis by peach, apricot, and plum roots. Individual plants varied considerably, but peach and apricot were more sensitive to waterlogging than was plum. All three species became more sensitive with a temperature increase of between 17°C and 27”C, and a scion of a more tolerant species did not overcome the sensitivity of the roots. They also reported that both cyanogenic glycoside content and its proportion that was hydrolysed during waterlogging were higher in peach than in plum roots. An exposure of detached root systems of all three species to anaerobic conditions caused HCN to be released (cyanogenesis), and cyanogenetic rate was increased with both temperature and time. They found a close correlation between differential sensitivity, hydrolysis of glycoside, and cyanogenesis under anaerobic conditions; however, the latter may have been secondary, though contributory, to cellular disorganization as a cause of sensitivity. Anaerobic soil conditions cause cyanogenesis in peach roots but aeration reverses it (Mizutani et al. 1977). Chaplin et al. (1974) reported cultivar differences in peach root tolerance to waterlogging and “wet-feet” resistance, with ‘Rutgers Red Leaf’ being most tolerant and ‘Lovell’ least tolerant under Kentucky conditions. The cultivar differences for severe root damage in one-yearold peach seedlings with roots submerged for two to four days was noted also by Marth and Gardner (1939). In the case of apple roots, prolonged periods of submersion during tree dormancy caused little damage; however, if any leaf surface was present during root submersion there was likely to be more damage (Heinicke 1932). Adverse effects by O2 deficiency in terms of disease problems also have been cited. Biesbrock and Hendrix (1970) reported that the condition of

36


soil water and temperature had a marked and differential influence on peach root necrosis caused by P y t h i u m uexans d.By and P. irregulare Buis. Saunier (1966) evaluated a number of widely used rootstocks, which were subjected to controlled flooding, for their capability to recover (asphyxiation) and classified them in the following three groups in decreasing order of susceptibility: (1) apricot, cherry, and peach seedlings; ( 2 ) plum seedlings and M 9 and M 1 apples; (3) M 2 and M 13 apples, P r u n u s marianna, and quince. There was a close correlation between resistance to winter flooding and resistance to summer flooding. In addition, a strong influence of scions on the rootstock resistance to “wet-feet” was also observed. Influence of O2 supply on plant parasitic nematodes in soil was studied by Van Gundy et al. (1962), where they correlated nematode activity and survival with O2 diffusion rate in the soil pore spaces, which was largely related to the soil moisture content. The rate of O2 supply critical for all nematodes tested (Meloidogyne, Pratylenchus, Xiphinema, etc.) was around 30 X l o s gcm - 2 minute - l . However, some species were more sensitive to the length of exposure than to concentration. Highest incidence of gummosis in apricot was observed following a heavy rainfall causing waterlogging, and it was suggested that clogging of the vessels by gum was related to an inadequate supply of O2 to roots (Heimann 1968).

iii. Temperature Extremes. Literature dealing with the effects of temperature extremes on plant growth processes has been thoroughly reviewed by Langridge (1963). In many species, it has been shown that growth ceases a t a temperature only slightly above the optimum, but may be restored by addition of a single factor. If the growing temperature is raised a little higher, a further factor becomes necessary, and thus, these requirements become progressively more numerous with increased temperature. The process of chlorophyll formation appears to be very sensitive to cold. I t seems probable that the inactivation of enzymes a t low temperature can be attributed to an increase in intramolecular hydrogen bonding and is compatible with the concept of H-bond forming a t low temperature and H-bond breaking a t high temperature. Thus, a single enzyme reaction may become limiting to growth above or below a critical temperature. In another review, Levitt (1951) clearly showed that frost, drought, and heat resistance are all basically similar. In the peach-growing areas where high temperatures do not prevail over an extensive period, long-lived orchards are to be expected (Cowart and Savage 1941), whereas high temperature with inadequate water supply adversely affects survival. Also, root submersion accompanied by high temperature is likely to cause severe damage (Heinicke 1932). Bennett (1950) showed the antagonistic effect of high temperature on dormancy development in pears. T h e negative effect of direct solar ra-


37

diation during winter was emphasized by the benefits of winter shading to reduce bud injury. High temperatures of 15" to 23°C during winter on the West Coast were shown to antagonize the dormancy-releasing effect of cold. High cambium temperatures occurring in late winter as a result of solar radiation impinging on the bark of peach trunks caused a loss of hardiness following dormancy break. When these high daytime temperatures were followed by a rapid drop of temperature to near freezing a t night, severe injuries often resulted, contributing greatly to PTSL (Jensen et al. 1970). While working with the physiological dwarfing in peach seedlings, Pollock (1962) noted that germination a t 22°C produced almost normal plants whereas the plants produced a t 25°C were severely dwarfed. All tested peach cultivars responded in the same general way.

iu. Combination of Above Factors. The literature reviewed in preceding sections indicates that there are many instances where a combination of anaerobiosis, drought, and heat, when coupled with cold, can severely affect the plant processes, leading to complete plant destruction, devitalization or predisposition to other disorders. For example, Chandler et al. (1962) indicated that physical factors probably were responsible for PTSL in the Southeast. Excessive rainfall in late winter, resulting in waterlogged soils, and a freeze in February, following a period of unseasonably warm weather, definitely were contributing factors to the death of 200,000 peach trees in Georgia in the early spring of 1962. Similar informationon apple is provided by Wildung et al. (1972a, 1973). Thus, it is obvious that it is not easy to separate the effects of individual factors, especially when they act simultaneously or follow in succession. 2. Microclimatic (Cultural).-This portion of environmental factors will cover those items that affect plant microclimate in the immediate vicinity of trees or individual orchards in a limited area. The main consideration will be given to cultural practices and nutritional aspects.

a. Cultural Practices.-Such contributory practices as crop rotation, pruning and training, cover crops, irrigation, and tillage operations will be discussed in this section as related to their effects on short life and replant problems. Cultural practices play a vital role in hardiness (Hung and Jenkins 1969; Nesmith and Dowler 1976; Stuart 19391, and thus, bad cultural practices contribute significantly to the cause of PTSL (Savage 1970; Savage and Cowart 1942a). Any practice that stimulates a high level of vigor and/or retards normal hardening, increases the potential hazard of cold injury (Rollins e t al. 1962).

i. Crop Rotation. Most short life or replant problems may be avoided simply by switching to new land if it is available; however, new land suitable for orchards is unavailable in many fruit regions. Often the same

38


land is used repeatedly for the same crop because of specificity of certain crops to their respective areas of cultivation. Several reports indicate that successive plantings of peach trees on the same sites contribute to PTSL (Clayton 1977; Savage and Cowart 1942a; Taylor et al. 1970; [Jpshall and Ruhnke 1935). Most peach replants dying are those established on locations of previous peach trees. Mountain and Boyce (1958a) found three to four times greater soil populations of Pratylenchus penetrans in soils with previous peach history than in those with no such history of peach production. Similarly, repeated croppings with apple andcherry were found toaffect their respective specific replant diseases (Pitcher et al. 1966). On the other hand, Proebsting (1950) did not find any evidence of peach failure when peach succeeded apple; other fruits did well after peach. Smith and Stouffer (1975) have suggested that establishment of new orchards by interplanting young trees in mature orchards of the same crop should not be started until the ground has been thoroughly worked, the old roots removed, and the land planted to a grain in a rotation for a t least one to two years. As Savory (1966) noticed in greenhouse culture, crop rotations are seldom feasible since replant diseases are so persistent in the soil. In such cases, pre-plant fumigation following rotation should be beneficial (Miller and Dowler 1973). Good (1960) established th at nematode injury to peaches can be reduced substantially by use of crop rotations, in addition to fumigation a n d planting with resistant plant material. Rotation of deep-rooted cover crops between orchards has proved to be beneficial for tree longevity (Savage 1970).Day and Serr (1951) observed severe short life of peach in a repeated peach rotation in California.

ii. Pruning and Training. Both time and extent of pruning influence short life and replant problems. Hibbard (1948) found no great differences in size of the trees under various systems of pruning, viz., moderate, light, and corrective. The trees more heavily pruned made wood and shoot growth a t the expense of fruit production, a n d therefore corrective pruning was done accordingly. Different pruning methods can cause marked differences in the strength of peach wood (McCue 1915). Cain and Mehlenbacher (1956) did not find trunk growth to be a reliable guide to controlled pruning. The apoplexy problem of apricots has been directly connected, in relation to its severity, with the height and type of the trunk, damage being absent or very minor on high-budded trees with clean trunks (Iliev 1968). Increased pruning severity tends to decrease yield and red color on fruit and delay peach fruit maturity, but increases fruit size, terminal growth, and percentage of nitrogen in the foliage (Schneider and McClung 1957). Heavier pruning materially reduces horizontal root extension; however, the degree of pruning does not alter the nature of the root system other than its total growth and distribution


39

since there is generally a proportional reduction in weight of all sizes of roots in heavily pruned trees (Savage and Cowart 1942b).Cummings and Ballinger (1972) obtained highest yields with lightest pruning although fruit size was reduced. Daniell (1975) reported significant yield increases reflecting tree survival in peach due to time of pruning, where February, March, and May pruning proved superior to November- January pruning. Fall pruning of many leading apple cultivars is likely to result in severe injury if prolonged periods of sub-zero temperatures follow (Burkholder 1936). However, Way (1954) reported that fall pruning had no measurable effect on the hardiness of ‘Cortland’ apple twigs. Pruning time is very critical from the standpoints of tree short life and survival, vigor, trunk cambial browning, hardiness, bacterial canker, and cold injury (Chandler 1974; Chandler and Daniell 1976; Clayton 1968, 1971, 1977; Nesmith and Dowler 1975; Prince and Horton 1972). Daniell (1973) reported that the time of pruning, however, had little or no effect on longevity and tree growth, when trees were grown on new peach land. Prince and Horton (1972) noticed little injury and no tree death when peach trees were pruned on different dates on a site with no short life problem. They found, however, greater trunk cambial browning in trees on a short life site that were pruned in November and December than in trees that were pruned in January, and still less browning in trees pruned in February. Tree mortality followed the same trend as that for browning. Similar deleterious effects of fall and early winter pruning in peach have been cited (Chandler 1974; Chandler and Daniell 1976; Clayton 1971, 1972, 1975a,b; Correll et al. 1973; Dowler 1972; Nesmith and Dowler 1973, 1975,1976).Weaver et al. (1974) reported that peach tree mortality on a PTSL site wasdue to cold injury and bacterial canker, and was not influenced by time of pruning. Further, they reported that on new land adjacent to the PTSL site early pruning caused trees to be more susceptible to cold damage, but the trees recovered and none died. Stene (1937) observed variable effects of pruning severity on winter-injured peach trees. Relatively severe pruning after leafing out was most beneficial for tree health.

iii. Cover Crops, Mulch, and Weed Control. In a four-year study, Boynton and Anderson (1956) found that the effects of mulching on tree behavior were similar and additive to the effects of nitrogen fertilization, and caused a substantial increase in potassium uptake by tree roots. Bell and Childers (1956) studied the effect of three systems of soil culture (clean cultivation, sod, and sod plus mulch) on growth, yield, and manganese content of peach trees in New Jersey. Trees grown in sod culture made comparatively less growth than did trees in other systems. In the same area, no significant differences in tree growth or yield resulted from

40


sawdust mulch, straw mulch, or manuring, although corncob mulch supplemented with fumigation produced significantly more growth than other treatments (Shannon and Christ 1954). In peach orchards, the importance of cover crops in relation to nutrition and hardiness has been reported by Proebsting (1961) and Raw1 (1935). McBeth and Taylor (1944) suggested that in areas where root-knot nematode is a problem, peach tree performance can be significantly improved by growing only nematode-resistant and -immune cover crops. Yield was increased also by clean cultivation and trap cover crop treatments, but neither of these practices seems to be practical. Parker et al. (1966) found Sudan grass and both perennial and annual rye grass to be beneficial as cover crops to combat devastating effects of the lesion nematode. Deep-rooted cover crops like alfalfa, clover (Lespedeza sericea), and coastal bermuda grass used in a rotation between orchard plantings have increased peach tree longevity in Georgia (Savage 1970). In some cases, successful rouging programs for plant species which serve as alternate hosts for specific diseases have demonstrated the negative effects of poor sanitation and weeding practices. X-disease has been observed only in peach and nectarine orchards where infected chokecherry was found (Lukens et al. 1971). Where chokecherry was not weeded out, 72% of peach trees showed X-disease in 3 years but only 13% of the trees were affected where it was removed. Sarasola and de Bustamante (1970) obtained a sharp reduction in pear decline and apple collar rot in the western sectors of orchards in Argentina by planting Poplar windbreaks to protect apple and pear trees from the prevailing winds and to prevent abrupt changes of temperature a t sunset.

iu. Irrigation and Tillage Operations. Recent experiments in North Carolina showed that irrigation did not influence total yield, growth, or longevity of ‘Elberta’ or ‘Redhaven’ peach trees (Cummings and Ballinger 1972). On the other hand, Way (1954) reported that twigs from ‘Cortland’ apple trees that were irrigated in the fall suffered significantly greater freezing injury than did twigs from non-irrigated trees. Slater and Ruxton (1954) confirmed that on frosty nights the minimum air temperature immediately over uncultivated or compacted soil is higher than that over cultivated or loose soil. T h e studies of Hendrix and Powell (1969) as well as the ten-point program reported by Miller and Dowler (1973) stress modifications in some vital cultural practices to improve the chances of peach tree survival. According to these reports, tree root destruction by discing is harmful, though subsoiling following planting is beneficial, particularly where herbicides are used, to improve subsurface drainage and breaking of hardpan layers to allow better root colonization. Savage et al. (1968) presented the following view with regard to subsoiling in Georgia peach orchards. Many peach orchard soils in

SHORT LIFE, REPLANT PROBLEMS OF DECIDUOUS FRUIT T R E E S

41

the Georgia Piedmont have O2 levels of 15% or less during a great part of the growing season. Under these conditions, preplant subsoiling to a depth of about 50 cm has nearly doubled the growth and yield of peach trees and greatly increased tree longevity. T h e increased growth, production and longevity were accomplished even though soil moisture was decreased in the subsoiled plots.

b. Nutritional.-To avoid short life and replant problems, nutritional considerations focusing on type, amount, balance, and time of application have been stressed (Clayton 1975a; Harrison 1958; Otto 1972b; Parker et al. 1966; Savory 1966; Spivey and McGlohon 1973; Taylor 1972). It is considered t hat improper nutrition is devitalizing; thus, weakening of the trees indirectly affects survival. However, Gilmore (1963) showed th a t a less fertile soil did not promote the peach replant effect in soil pot culture. Beattie (1962) evaluated ATD in Ohio and found symptoms normally associated with nutritional troubles. Later, Donoho et al. (1967) concluded th at cultural and management practices or nutrition did not appear to be related to ATD, except when they directly contributed to the lowering of soil pH.

i. Soil p H and Liming. Acidity of the growing medium does not appear to be a universally crucial factor for short life problems, although its contribution is significant. Savory (1966) has clearly demonstrated th a t the apple replant problem seems to be absent or unimportant in some places such a s in sandy soils with moderate soil acidity. Similarly, Hoestra (1968) found th at low p H soils are less heavily infested with SARD than near-neutral soils, and acidification of the latter leads to a growthstimulating effect. However, ATD, in Ohio, has been found closely associated with a low soil pH, ranging from 4.2 to 5.0 (Banta 1960; Beattie 1962; Beattie et al. 1963; Donoho et al. 1967). Similarly, in Australia, Sitepu and Wallace (1974) found positive correlation between growth decline of apple trees and such factors as species of P y t h i u m and Phytophthora, nematodes, and texture, moisture, and p H of soil. Out of these factors p H was suspected to be the most important. Extensive studies under both controlled and field conditions were carried out by Weaver and Wehunt (1975) to correlate effects of low p H on bacterial canker development and population growth of Macroposthonia xenoplax to PTSL in middle Georgia. Sizeable mortality of peach seedlings was noticed in soil with unadjusted pH, whereas no plants died in soils adjusted to pH 6.4 to 7.2. In December, populations of M. xenoplax were greater in soils adjusted to above 6.1 pH, but differences were not significant in March and April. In addition, numbers of propagules of P y t h i u m species in soil and recovery from roots were positively correlated with soil pH. In greenhouse studies, addition of high-Ca or high-Mg lime benefited peach planted in old peach soil with low p H (Havis 1962). However,

42


soil treatment with lime and various fertilizers at two locations in Texas failed to show any effect on tree growth and survival (Havis et al. 1958). In addition to raising soil pH, lime also increased leaf Ca and Mg contents. In a similar study, Boynton and Anderson (1956) found that liming caused a decreased uptake of potassium by roots but a net gain in leaf magnesium. Parker et al. (1966) recommended lime application in New York orchards for combating replant problems, and a second application was suggested when soil pH appeared to be too low a t the start of planting. Another greenhouse study a t Beltsville, Maryland, showed that from treatments of high-Mg lime, CaS04, MgS04, and NaC03, the most striking growth response was obtained with the addition of a relatively low rate of high-Mg lime (Prince et al. 1955). It was recognized that much of the response obtained was not well understood and that the results of this study might be applicable only to the particular soil used. The present understanding in Georgia seems to be that yields can be maintained and tree survival extended by following good liming and balanced fertilization practices (Spivey and McGlohon 1973; Taylor 1972). Liming also has been included as an important practice in the 10-point program recommended by the PTSL Work Group (Miller and Dowler 1973). Based on well established field observations, the New Jersey Experiment Station has recommended a regular use of complete fertilization together with frequent liming in orchards planted on the light Coastal Plain soils (Davidson and Blake 1937). Havis (1962) reported that new peach trees had grown without any difficulty in old orchard locations a t Beltsville, Maryland, where about 1.5 tons of lime were thoroughly mixed into the loam soil. Supplemental nitrogen plus dolomitic lime produced a trend of further increases in tree survival and yield in Georgia (Giddens et al. 1972).

ii. Type and Amount of Fertilizers. In soil pot cultures designed to stimulate the peach replant problem, nitrogen suppression due to stimulation of biological activity of peachroot wood seemed to be the causative factor (Gilmore 1963). Soil as well as foliar applications of nitrogen on declining apple and peach trees have been found to help maintain yields and extend survival (Banta 1960; Spivey and McGlohon 1973; Taylor 1972). Hewetson (1953, 1957) and Higgins et al. (1943) were convinced that a readily available nitrogen supply gave replant trees a rapid early start, maintained vigorous growth, and increased their resistance to cold injury and hence, premature tree death. This treatment quickly establishes the trees, perhaps in turn helping to overcome any inhibiting effect of the trees previously grown on the same sites. It is further stated that if trees could be made to grow vigorously during their first year in the orchard, they might be expected to continue


43

satisfactory growth and top performance in subsequent years. Supplemental nitrogen significantly increased survival and yield of peach trees even in a severe decline area (Giddens et al. 1972); however, a similar treatment did not stimulate growth and fruiting of affected apple trees (Donoho et al. 1967). Additional reports show the beneficial effects of nitrogen applications on vigor, survival, resistance to some diseases, and nutrient levels in plant tissues (Boynton and Anderson 1956; Kennard 1949; Nesmith and Dowler 1976; Sarasola and de Bustamante 1970). In the opinion of Raw1 (1935), application of nitrogen alone is improper since other plant nutrients are essential as well. In addition, a given dose of one form of nitrogen does not bring about the same growth response in a peach tree growing on a poor soil as compared to a rich soil, or in an unpruned tree as compared to a pruned tree (Blake 1928). Different forms of different nutrients show mixed effects-some beneficial, others harmful (Batzer and Benson 1958; Kennard 1949; Sarasola and De Bustamante 1970; Way 1954). Sodium nitrate (NaNOJ applied a t the rate of 350 lblacre on August 1 did not increase cold injury of sour cherry trees 50% defoliated by August 10, but the same amount of NaN03 decreased cold hardiness of trees fully defoliated by August 10 (Kennard 1949). Woodbridge and Lasheen (1960) obtained significant differences in nitrogen content of pear leaves from normal and decline-affected trees, which they suspected were a result rather than a cause of decline. Dekock and Wallace (1965) noticed iron chlorosis induced by nitrogen application in peach trees growing on calcareous soil. Nitrogen fertilization at rates adequately high to maintain vigorous peach tree growth increased cold hardiness and hence, resistance to premature death (Higgins et al. 1943). Increased levels of applied nitrogen do not appreciably affect cold hardiness (Edgerton and Harris 1950) or further growth increase (Chandler and Tufts 1933); however, nitrogen significantly interacts with pruning (Schneider and McClung 1957). Cummings and Ballinger (1972) reported increased peach tree loss with low nitrogen rates, although some random tree deaths occurred with higher nitrogen rates. However, Correll et al. (1973) did not find peach tree survival to be influenced by nitrogen levels. Nutrients other than nitrogen have not been so extensively studied in relation to short life and replant problems.

iii. Time of Fertilization. Edgerton (1957) applied nitrogen to ‘Cortland’ apple trees in the fall and early winter for three successive years to evaluate the effect on cold hardiness of twigs and bark, and on nitrogen accumulation in plant tissue. October and early November applications appeared to increase susceptibility of both twigs and bark to freezing, the effect being more noticeable in early than in mid-winter. Evidently, October applications with urea sprays were safer than an equivalent

44

HORTIClJLTURAL REVIEWS

amount of ground-applied urea. However, ground application of NH4N0,r in December was not detrimental. In West Virginia Sudds and Marsh (1943) evaluated fall application of NaNOj to young apple trees in relation to winter injury. They concluded th a t under certain unknown conditions problems may occur when nitrogen is applied to orchards in the autumn, and that the nitrogen may help to predispose the trees to winter injury. In peach and some woody plants, time of nitrogen application does not seem to have much effect except for producing minor differences in foliar nitrogen levels (Schneider and McClung 1957) and cold acclimation (Pellett 1973). Late application of nitrogen significantly increases cold resistance of ‘Redhaven’ twigs (Chaplin and Schneider 1974) and wood hardiness in other peach cultivars (Higgins et al. 1943). Waltman (1937) reported that since root activity during the winter is minimal, peach trees fall-fertilized with calcium cyanamide may be less subject to winter injury because of the tissue’s lowered percentage of soluble nitrogen. Therefore, any nutritional practice increasing tissue nitrogen level during dormancy will seriously hamper cold hardiness and hence, tree survival. iu. Nutritional Deficiencies a n d Interactions. Davidson and Blake (1937) demonstrated th at an adequate amount of a nutrient may vary with the concentration of other nutrients present in the root medium. Based on extensive experiments on the response of young peach trees to nutrient deficiencies, Davidson and Blake (1936) produced an exclusive report on nitrogen, phosphorus, potassium, calcium, and magnesium deficiency symptoms in general, as well as on leaves, stems, and roots. Nutrient deficiencies may cause unbalanced metabolism, thereby initiating short life and finally tree death (Frenyo and Buban 1976; Parker et al. 1966). Nutrient deficiencies either in the plant tissues (Beattie 1962) or in the soil (Otto 1972b) have not been conclusively found to be associated with short life and replant problems. Lower calcium with higher manganese in the declining apple trees was thought to be associated with lower soil p H (Beattie et al. 1963); but soil potassium seemed to be related to decline, since decline soil contained lower potassium levels. Gallaher et al. (1975) reported higher concentrations of total calcium in peach leaves from declining trees, but since these trees had fewer and smaller leaves, less total calcium was found in decline than in healthy trees. Savage (1972) found th at there was a great increase in peach tree death (up to 83%)in field plots with low calcium level (soil and leaf analyses) as compared to plots with adequate calcium (up to only 10% mortality). Backman et al. (1969) found th a t calcium and magnesium ions suppress the toxicity of syringomycin, an antibiotic polypeptide t ha t is a toxin in bacterial canker (Pseudomonas syringa4 of peach. Stimulation of peach seedling growth by root-knot nematodes has


45

been associated with greater accumulation of calcium and magnesium (Chitwood et al. 1952). Batzer and Benson (1958) used Zn and Fe chelates to help overcome arsenic toxicity of peach trees in Washington. Zinc chelate was most economical in correcting arsenic toxicity, while iron chelate was phytotoxic and tended to intensify zinc deficiency. Bell and Childers (1956) found no relation between leaf Mn and peach tree growth which was positively correlated with Fe, P, Mg, or K in the leaves. Manganese deficiency was more pronounced a t soil p H of 6.5 or above, with intensity of deficiency symptoms directly related to the dryness of the season. Daniel1 and Chandler (1976) correlated Fe deficiency in liquid culture with bacterial canker development in peach, since plants receiving no Fe developed the longest cankers. No significant differences in canker length occurred between treatments containing Fe. An increase in bacterial canker severity also might be due to an Fe deficiency brought about by excess phosphate (Cameron 1962). Leaf chlorosis in peach has been attributed to deficiency of Fe or Mg or both (Chitwood et al. 1952). Hansen (1955) studied leaf and stem injury on almond and peach due to excess boron as influenced by the rootstock used. Almond and peach trees on almond rootstocks showed least injury. Bunemann and Jensen (1970) observed no improvement in the inhibition of apple seed germination and growth of seedlings and grafts by adding potassium to thoroughly washed quartz sand previously used as the medium. Beattie and Flint (1973) showed th a t optimum K level for frost hardiness seems to be within or below the optimum range for growth. With an increase in K supply, plant K levels increase with a corresponding decrease in plant nitrogen. Seedlings of several peach cultivars showed significant differences in the leaf levels of N, P, K, Ca, and Mg when grown in a sand medium supplied with nutrient solutions with high and low K (Thomas and White 1950). Leaves from seedlings grown in low K contained more P and Ca than those from high K solution. T he plant root system liberates into the surrounding medium a number of organic substances like amino and organic acids, the qualitative composition of which is markedly different with a change in the mineral composition of the medium (Tsitsilashvili 1977). Significantly discernible amounts of valine were found in the medium with abundant K, and oxalic acid was detected in K-deficient medium. Frenyo and Buban (1976) reported more distinct seasonal variations in the concentrations of ammoniacal N and phosphate P, but not K, in leaves from apoplexyaffected apricot trees than in leaves from healthy plum trees. Yablonskii (1975) identified several forms of P in buds and one-year-shoots of peach. The content of total P and its separate fractions in different cultivars did not correlate with their degree of winter hardiness. In comparison

46


with less winter-hardy cultivars, the winter-hardy ones were characterized by a closer correlation of P metabolism with environmental factors and other metabolic processes.

B. Pathogenic Factors Numerous pathogenic factors contribute to short life and replant problems. However, as discussed in the subsequent sections, it should be clearly understood th at they are not soleiy responsible for the difficulties in question; rather, they act sequentially or simultaneously following predisposition of tissue by environmental factors, or the environmental injury is more likely to be severe if the tree is weakened by pathogens. Otto (1972a,b,d) ruled out the possibility of soil physical and nutritional factors for apple replant disease in Germany, but supported the involvement of microorganisms like bacteria or actinomycetes in the weakening of roots. Benson and Covey (1976) and Rallo (1973) agree on minor effects of some specific pathogens, but only a t certain stages of plant growth. Still we lack total agreement on the decisive role of pathogens in SARD and replant diseases of apple (Bollard 1956; Ross and Crowe 1973; Savory 1966). In the early spring of 1962, no pathogenic organism was consistently associated with the diseased tissue, although yeasts and bacteria seemed a t least partially responsible for killing thousands of peach trees in Georgia (Chandler et al. 1962). However, in addition to cold injury and nutritional disturbances, peach replant failures have been reported to be caused by the activity of insects, bacteria, certain fungi, and nematodes (Clayton 1975a; Koch 1955). 1. Bacteria.-Short life problems related to bacteria generally lead to the sudden death of a susceptible plant in spring. Bacterial canker, apoplexy, blast, bacterial gummosis, dead-bud condition, bacteriosis, lilac blight, and sour-sap are common names given to a disease of several stone fruits around the world which is caused by two related species of bacteria-Pseudomonas syringae and Ps. mors-prunorum. T h e bacterium attacks the plant tissues only after defoliation, and tissue damage occurs only during the dormant stage of trees (Gardan e t al. 1975; Klement et al. 1974) where the infection results in a characteristic cellular degradation in the phloem and cambial region (Davis and English 196913). Several predisposing and/or enhancing factors including cold injury, pruning time, phytotoxins in the rhizosphere, other pathogens, and physiological activities of the tissue closely interact with the bacteria (Chandler and Daniel1 1974, 1976; Davis 1968; Dowler and King 1967; Dowler and Weaver 1975; Klement et al. 1972; Petersen 1975; Rozsnyay and Klement 1973).T h e pathogen, green fluorescent pseudomonads, can be isolated from infected tissue only up to a certain time of the year


47

(Cameron 1970; Jones 1971a; Kouyeas 1971). The distribution and frequency of infection varies depending on fluctuations in seasonal moisture and temperatures (Cameron 1970). A study in England (Anon. 1966) more or less ruled out the possibility of nematodes and viruses as causal organisms in the apple replant problem, and emphasized the bacterial involvement. Bacterial blast of pear in Chile (Cancino et al. 1974) and Connecticut (Sands and Kollas 1974) has been caused by Pseudomonas syringae infection, usually following either cold stress or wet weather. In Belorussia, Ps. syringae is widespread where it attacks cherry and pear, then apple and plum following low temperature exposure, with trees dying in the first year or after several years in chronic cases (Dorozhkin and Griogortsevich 1976). In Greece, Ps. amygdali sp. nov. causes bacteriosis only in almonds where infection persists throughout the year and induces perennial swollen cankers (Psallidas and Panagopoulos 1975). Bacterial decline or apoplexy of apricots in southern Europe has been a serious problem for some time, due to Ps. syringae infection compounded by cold, Cytospora, Phytophthora, and other Pseudomonas species (Babos et al. 1976; Gardan et al. 1973; Klement et al. 1974; Kouyeas 1971; Prunier, Gardan and Luisetti 1970). Bacterial canker of sweet cherry, caused by Ps. mors-prunorum, recently has become a destructive disorder causing a shortage in cherry tree supply in Poland (Lyskanowska 1976). Pseudomonas syringae also a t tacks cherries, causing bacterial gummosis or canker (also called lilac blight and dead-bud condition in the western United States), and a t times reaches disastrous proportions (Blodgett 1976; Jones 1971a). Bacterial canker caused by Ps. syringae has been very closely associated with the short life and death of peach trees in many growing areas (Clayton 1968, 1977; De Vay et al. 1968; Dowler and Petersen 1966; English 1961; English and De Vay 1964; Lepidi et al. 1974; Petersen 1975; Petersen and Dowler 1965; Weaver et al. 1974; Zehr et al. 1976). However, Dowler and Petersen (1966) could not assign Ps. syringae as the only cause of all the observed PTSL-related difficulties, since cold injury and pruning time played important roles. Clayton (1968) pointed out that cold-injured peach bark often is invaded by the cankers or wood decay organisms and, a t times, cold injury alone is sufficient to kill trees even without the involvement of the canker-causing organisms. Gardan et al. (1975) correlated peach decline with sensitivity to Ps. mors-prunorum f s p . persicae only during fall and winter months, with a climax during defoliation. 2. Fungi.-Sitepu and Wallace (1974) found P y t h i u m species, nematodes, and soil p H together inhibiting apple tree growth, which is the first step towards decline. Jones (1971b) described Phytophthora collar rot of apples in Michigan as causing poor terminal growth, foliar dis-

48

HORTICITLTITRAL REVIEWS

coloration, and eventual tree death in severe cases. In California, the Pear Research Task Group (Anon. 1971) has furnished a detailed account of decline and replant problems of pear and the general root rot and crown rot syndrome including Armillaria root rot. Several fungal organisms, such as Cstospora, Verticillium, and Phytophthora, have been noted to cause apoplexy and premature die-back of apricots (Babos et al. 1976; Kouyeas 1971; Paclt 1972; Rozysnyay and Klement 1973). In all these cases cold and/or pruning time and other predisposing factors have been recognized. Cambial gummosis in P r u n u s domestica infected with Cytospora cincta apparently is associated with cold injury as a prime contributing factor (Helton 1961; Helton and Randall 1975). In western Europe, Thielaviopsis basicola is involved in the cherry and plum replant problems (Hoestra 1965; Pepin et al. 1975; Sewell and Wilson 1975).Mircetich and Matheron (1976) implicated three species of Phytoph thora (P. cambivora (Petri) Buisman, P. megasperma Drechsler, and P. drechsleri (Tucker)) directly in the root and crown rots and death of cherry trees in poorly drained California commercial orchards. Further damage to nematode-injured cherry roots, and hence a loss in tree cold hardiness, may be caused by the invasion of certain fungi, disturbing the metabolism and water and nutrient uptake (Edgerton and Parker 195813). Blodgett (1976) explained recurring death of cherry trees in Washington state on the basis of Verticillium wilt, Phytophthora crown rot, Cytospora canker, and Armillaria root rot, all of which became severe under poor drainage and waterlogged conditions. T he most important fungal agent in PTSL is perennial canker complex of peach caused by the species of ValsalCytospora, which occurs most readily during the dormant season, with low temperature and Pseudomonas syringae canker implicated as important predisposing factors (Banko and Helton 1974; Cameron 1971b; Clayton 1971, 1975a, 1977; Hampson and Sinclair 1973; Hickey 1962; Hildebrand 1947). Cytospora fungi often finish killing the peach trees after cold and bacterial canker injuries (Clayton 1968, 1977). Another fungus closely linked with the PTSL problem is Clitocybe tabescens (Fr.) Brez. (Chandler 1969; Cohen 1963; Petersen 1961; Rhoads 1954; Savage and Cowart 1942a, 1954; Savage et al. 1953). However, this fungus has not been found to be the sole factor responsible for early mortality (Rhoads 1954; Savage and Cowart 1954), although it may cause very heavy losses on old sites. Phycomycetous fungi (water molds), especially Phytophthora and P y t h i u m species, appear to be the most damaging factor in PTSL under excessive soil moisture conditions (Biesbrock and Hendrix 1970; De Vay et al. 1967; Hendrix and Powell 1970a,b; Hendrix et al. 1966; Hine 1961b; Mircetich 1971; Mircetich and Keil 1970; Powell et al. 1965; Taylor e t al. 1970). However, Lownsbery e t al. (1973) did not find sig-


49

nificant growth reduction due to Pythium, and no interaction between the fungus and nematode (Macroposthonia xenoplax) was noted. Root necrosis and related effects of these organisms are secondary factors in PTSL, and the severity of tree-killing varies depending on temperature and soil moisture. Armillaria root rot is another cause of tree mortality, which often attacks already weakened tissues (Cameron 1971b; Savage and Cowart 1942a). Poor growth of peach replants, and severe decline and death of peach trees also have been associated with the disorders caused by Fusicoccum amygdale Del., Cylindrocladium floridanum, Botryosphaeria dothidea, Physalospora persicae, and some species of Fusarium and Rhizoctonia (Abiko and Kitajima 1970; Hine 1961b; Hung and fJenkins 1969; Sobers and Seymour 1967; Weaver 1971, 1974b). 3. Nematodes.-Nematodes play a significant role in the short life and replant problems of deciduous fruit trees. Hoestra (1961) reported that 65% of the apple orchards in Holland appeared to be infected with Pra tylenchus penetrans, and damages were severe. Further, nematodes form a part in the complex of‘ factors contributing to replant problems of apple, but they are not the cause of SARD (Hoestra 1967). Stylet-bearing parasitic nematodes were found to be partially responsible for stunting of apple trees in Australia (Sitepu and Wallace 1974). An earlier report by Colbran (1953) established that root-lesion nematode (Pratylenchus coffeae Zimm.) was widely distributed in the Stanthorpe district of Australia and was the most important contributor to the unthrifty growth of replants in many old orchards. Nevertheless, reports from some European countries and Canada show that nematodes are not involved in such apple problems as replant, decline, or SARD, despite the fact that soil steaming and fumigation alleviated growth reduction (Anon. 1966; Pitcher et al. 1966; Ross and Crowe 1976; Savory 1967; Winkler and Otto 1972). The longevity and productivity of ‘Montmorency’ cherry on both Mazzard and Mahaleb rootstocks in New York state were seriously hampered by the parasitic nematode (Pratylenchus penetrans Cobb.); pre-plant fumigation restored normal cold hardiness and survival (Edgerton and Parker 1958a,b; Mai and Parker 1967; Parker and Mai 1956). Although nematode involvement in specific cherry replant disease in England has been ruled out (Pitcher et al. 1966; Savory 1967), the disease is probably caused by a soil microbial organism(s), since soil sterilization often has eradicated it. Wehunt and Good (1975) have reviewed the literature regarding nematode involvement in PTSL. They said that the role of nematodes in the PTSL complex has not been determined; however, nematicide treatment of orchard soil usually brings improvement, indicating that nematodes

50


are somehow involved. Increasing evidence suggests that one or more species of nematodes contribute to the PTSL syndrome, and th a t nematode control would, a t least partially, eliminate PTSL (Davis 1968; English 1961; English and De Vay 1964; Hendrix and Powell 1969, 1970a; Savage and Cowart 1942a). Root injury is usually the result of the combined effect of root-knot galling by root-knot nematodes, root pruning by dagger nematodes, and necrosis and decay by root-lesion nematodes and associated microorganisms (Good 1960). Root-knot nematodes, including several species of Meloidogyne, have been found in the vicinity of certain declining peach trees, particularly in light soils, but have not been directly associated with PTSL (Burdett e t al. 1963; Chitwood et al. 1952; Dhanvantari et al. 1975; Foster 1960; Foster and Cohoon 1958). Clayton (1977) demonstrated th a t trees on root-knot resistant rootstocks, e.g., Yunnan, ,937, Nemaguard, or Okinawa, are more susceptible to PTSL than are those on root-knot susceptible Love11 rootstock. Ring nematodes (Macroposthonia xenoplax and M. curuatum) have been associated with PTSL, affecting tree growth and longevity during winter and/or spring when they are a t highest population density (Barker and Clayton 1973; Chitwood 1949; Chitwood et al. 1952; De Vay et al. 1967; Foster 1960; Hung and Jenkins 1969;Johanson 1950; Lownsbery 1959; Weaver et al. 1974; Zehr et al. 1976). Lesion nematodes (Pratylenchus penetrans and P. uulnus) are important primary parasites and true plant pathogens. Their main role in peach replant seems to be the ability to incite root degeneration by providing extensive infection sites for other pathogenic soil microorganisms (Foster 1960; Mountain and Boyce 1957; Mountain and Patrick 1959). These nematodes are associated with decline and PTSL, especially when peach follows peach in an orchard rotation (Barker and Clayton 1969; Bird 1968; Chitwood 1949; Johanson 1950). Mountain and Patrick (1959) reported that the main mechanism involved in the formation of lesions is the production of phytotoxic substances through hydrolysis of cyanophoric 0-glucoside (amygdalin). They reported th a t P. penetrans is capable of hydrolysing amygdalin in uitro. Other predominant nematode species t ha t received attention regarding their role in PTSL-related disorders are Belonolaimus, Trichodorus, Tylenchorhynchus, and X iphinema (Chitwood et al. 1952; Foster 1960).Smith and Stouffer (1975) reported t ha t the soil-borne nematode (Xiphinema americanum Cobb.) transmits the virus which causes P r u n u s stem pitting in peach. 4. Viruses an d Mycoplasma-Like Organisms (MLO).-Although viruses, along with nematodes, have not been thought to be so important in the replant problems in England (Anon. 19661, it is believed th a t the causal agent of apple decline in the Valtellina area of Italy is a virus or


51

mycoplasma (Refatti 1970a).Cameron (1971b) discussed the synergistic reaction between new cultivars and some viruses, which sometimes results in the death of apple trees. Apple stem grooving virus was found not to be the causal factor in the union necrosis and decline syndrome of apple on the East Coast; however, some scionlrootstock combinations were very severely affected (Stouffer et al. 1977). Pear decline probably is caused by an MLO carried by pear psylla (Anon. 1971; Blattny and Vana 1974; Cameron 1971b; Hibino et al. 1971; Hibino and Schneider 1970; Nyland and Moller 1973; Westwood and Cameron 1978). Blodgett (1976) used the term “induced incompatibility” when viruses or MLOs are involved in pear decline. Pear decline in Greece appears to be the result of union incompatibility, stem pitting, and wood necrosis, possibly due to some unidentified transmissible factor (Agrios 1972). Westwood and Cameron (1978) established th at remission of pear decline symptoms is dependent on the environment since it occurs during some dormant seasons, and th at reinfection by the psylla vector may be necessary for the disease to continue. Western X-little cherry disease and X-disease of cherry are caused by MLOs, where complete wilting and death of mature trees result in serious losses (Blodgett 1976; Granett and Gilmer 1971). Cherry decline in western Europe results from an MLO infection vectored by leafhoppers and other efficient virus vectors (Fos 1976; Kegler et al. 1973). A widespread incidence of Prunus stem pitting (PSP)in California’s cherry and other Prunus orchards has been revealed (Mircetich et al. 1977). PSP also has been reported from the East Coast (Smith et al. 1973; Mircetich and Fogle 1969). Posnette and Cropley (1970) associated decline disease of plum with Prunus necrotic ringspot virus (PNRSV) which is specific to only certain cultivars. Based on observations, PNRSV and related agents contribute to increased sunburn, perennial canker, chronic die-back, short life, rosetting and decline, and predisposition to other infections (Cameron 1971b; Scotto La Massese et al. 1973; Smith and Neales 1977; Stubbs and Smith 1971). PSP of peach is a recent disease which occurs sporadically all over the United States and affects almost all cultivars (Cochran 1975). According to Cameron (1971b), PSP appears to be a virus-induced rootstock-scion reaction responsible for severe problems in some parts of the United States. Ringspot viruses have been implicated as the causal factors of PSP (Smith and Stouffer 1975; Smith et al. 1973), though Agrios (1971) suspected MLOs a s the cause. MLOs also have been cited as being responsible for phony disease, Western X- and X-disease, and peach rosette, all of which, in one way or another, are involved in PTSL and related syndromes th a t cause heavy tree losses (Hutchins 1933; Jensen 1971; Kirkpatrick et al. 1975a; Sands and Walton 1975; Savage and Cowart 1942a).

52


5. Insects.-In most cases related to short life, insects act as vectors for disease agents. The prevalence of 17-year locust or cicada (Majicicada spp.), in addition to low pH, has been associated with ATD (Donoho et al. 1967). Heavy populations of cicada nymphs were found under most, if not all, declining apple trees (Banta 1960; Beattie 1962). Several research groups have confirmed pear psylla (Psylla pyricola) as the main vector for MLOs which cause pear decline (Cameron 1971b; Hibino et al. 1971; Hibino and Schneider 1970; Nyland and Moller 1973; Westwood and Cameron 1978). Psylla carries MLOs in the salivary glands and foregut (Hibino et al. 1971). However, attempts to transmit pear decline in Argentina by budding or psyllids failed (Sarasola and De Bustamante 1970). Cherry decline (caused by MLOs) in Tarn-et-Garonne, France is transmitted by leafhoppers, especially Fieberella florii (Fos 1976). Peachtree borer (Sanninoidea exitiosa Say) and lesser peachtree borer (Synanthedon pictipes Grote and Robinson) have been added by Savage and Cowart (1942a) to the list of factors in PTSL. However, these borers do not appear to play a major role as do cold injury, bacterial canker, and other similar factors. MLOs responsible for Western X-disease of peach are transmitted by Colladonus montanus (Van Duzee) leafhopper to the host plants (Jensen 1971).

6. Pathogenic Interaction.-Interaction and synergism frequently are noted among different pathogenic causal factors in producing severe short life and replant effects. Cameron (1971b) showed that PNRSV contributes to increased incidences of Cytospora canker, and that peach trees do not show symptoms of Armillaria root rot unless they have previously acquired PNRSV. In a glasshouse test a t Harrow, Ontario, Meloidogyne hapla injury on peach, with a 62-day incubation period, resulted in an increased infection by Agrobacterium tumefaciens, but no such interaction was noted when the incubation period was 157 days (Dhanvantari et al. 1975). Similar results in almonds were found by Orion and Zutra (1971). Lownsbery et al. (1973) reported that the addition of Pythium spp. a t planting time increased peach susceptibility to Pseudomonas syringae less than did the inoculation with Macroposthonia xenoplax. No interaction between M. xenoplax and Pythium spp. was noted. However, nematodes also are important in carrying, aiding entry of, or otherwise interacting with bacteria and fungi (Lownsbery and Thomason 1959). An interaction between M. xenoplax and Ps. syringae was responsible for the death of plum tree tops a s a result of cankers (Mojtahedi et al. 1975). The consistent association of PSPagents, Tom RSV and TbRSV, with the capability of Xiphinema americanum to transmit the agents has been established (Smith et al. 1973; Smith and Stouffer 1975). Rozsnyay and Klement (1973) reported that Valsa cincta and Ps. syringae produce similar symptoms of apricot


53

apoplexy, since simultaneous inoculations with both pathogens caused more extensive cankers than those produced by either alone.

C. Physio-Biochemical Factors I. Phytotoxins.-For a long time, “soil sickness” and phytotoxins in the soil have been suspected as factors responsible for the unsatisfactory growth of second and subsequent plantings of the same crop on the same sites (Gilmore 1949; Savory 1966; Otto 1972b). Phytotoxins may cause disease symptoms, by inhibiting or changing membrane permeability (Owens 1969), which are characteristic of most plant disorders and represent the initial response of plants to any pathogenic or non-pathogenic causal factor. Phytotoxins predispose the plant to infection as well as to stress and thus, are actively involved in the establishment of the disorders in the host plants (Wheeler and Luke 1963).

a. Microbial Phytotoxim-Literature on the phytotoxins from plant parasitic microorganisms has been thoroughly reviewed by Strobe1 (1974) and Wheeler and Luke (1963). Wheeler and Luke (1963) concluded that, in contrast to most phytotoxins, hydrocyanic acid or hydrogen cyanide (HCN), which is produced by an unidentified group of basidiomycetes and other organisms, may be directly involved in disease development. HCN is toxic to plants under most conditions (Hine 1961a). Patrick (1955) discussed the microbial phytotoxins in relation to the peach replant problem in Ontario. It was demonstrated that substances which inhibit the respiration of excised peach root tips are produced when peach root residues or chemically pure amydgalin is acted on by certain microbes occurring in old peach orchard soils. This reaction did not take place when other soils or root residues were used or when the soil was autoclaved before amygdalin addition. In tests on peach root tips aqueous extracts of phytotoxins showed 40 to 90% physiological activity within 30 minutes. In addition to inhibiting respiration, these substances also induced tissue darkening and finally necrosis of meristematic cells. All of these effects were irreversible after tips had been in the toxic leachates for five hours, whereafter the tips apparently were killed. On the basis of further studies with water solutions of pure chemicals and enzymes, it was concluded that the microbial action on the amygdalin fraction of peach roots is mainly responsible for the toxic factor frequently encountered in old peach sites. Rowe and Catlin (1971) reported that temperature and 0 2 deficiency played an important role in amygdalin hydrolysis and cyanogenesis. They also reported that differential sensitivity among peach, plum, and apricot roots existed for both cyanogenesis and HCN toxicity. Gardner et al. (1974) reported a rapid hyperpolarization in membrane electropotentials in the tissue susceptible to

54


the phytotoxin. A phytotoxin obtained from three isolates of Valsa cincta caused leaf collapse, gum production, and necrotic wounds when absorbed by young intact apricot shoots (Rozsnyay and Barna 1974). Furthermore, this toxin was an exotoxin of high molecular weight, where protein and lipid components of the toxin did not produce toxic effects; however, the presence of a carbohydrate component was believed to be an important part of the toxic agent. Syringomycin is another such phytotoxin which is produced by Pseudomonas syringae, the inciter of bacterial canker on peach and other stone fruits (Backman et al. 1969).

b. Plant Residues.-In some sites where root damage has been especially severe, new trees cannot be established when old orchards are removed. Toxic chemicals released from old roots occupy an important position in the list of causal factors of replant problems on old sites (Gilmore 1949; Havis and Gilkeson 1947; Israel et al. 1973; Oh and Carlson 1976; Parker et al. 1966). Borner (1959) presented the following account of the apple replant problem in Germany. Experiments were carried out to investigate the cause of the apple replant problem from the standpoint of a possible action of substances released from plant residues into the soil. Addition of as little as 2 g/liter dried apple root bark to water cultures produced a strong reduction in apple seedling growth. Paper chromatography revealed five phenolic substances from bark held in nutrient solutions for about a month. The same toxins were present in cold-water extracts from soils containing apple root residues. Of the five phenolic toxins present in the nutrient solution and water extracts of soil, only phloridzin could be identified as a natural constituent of the apple root bark and wood, although quercitrin also was detected. The other four phenolic toxins were detected in soils within two to ten days following addition of pure phloridzin to different soils. I t is quite obvious, therefore, that these other phenolics were the decomposition products of phloridzin. The detected compounds were identified, and were found to occur in soils as follows: Phloridzin +phloretin

f

phloroglucinol p-hydroxyhydrocinnamic acid +p-hydrobenzoic acid

Of these toxins, phloridzin and phloretin caused the strongest effects. T o what extent these phenolics participate in the apple replant problem is not clear a t this time. In discussing apple and cherry replant problems in England, Savory (1967) stated that although the cause(s) of replant disease is not yet known, it does not include phytotoxins, once thought to be released from the decomposing roots of the previous crops, since it was found that root residues were not harmful to the replants.


55

Peach trees growing on a severe decline site did not respond to peach root residue applications (Giddens et al. 1972). However, there are reports that the leachate from different tissues of peach shows a definitive role in reducing apple and peach seedling growth by directly affecting their root systems (Chandler and Daniell 1974; Israel et al. 1973; Oh and Carlson 1976; Patrick 1955; Proebsting and Gilmore 1941). Davis and English (1969a) and Chandler and Daniell (1974) correlated the effects of peach seed and root leachates with bacterial canker incidence in peach. The latter workers postulated that when trees are replanted on old peach sites, their uptake of some water-soluble toxic substance(s1 from dead peach roots may predispose them to bacterial canker and contribute to PTSL. Proebsting and Gilmore (1941) reported that addition of peach root residue inhibited growth of peach seedlings even in virgin soils. Further, in sand culture, the root bark-not the wood-was found to be toxic. The alcohol extract of bark also was toxic, while the residue from alcohol extraction was not. No specific compounds were identified or estimated. However, Havis and Gilkeson (1947) found no evidence of any toxic substance in peach roots or peach leachates which adversely affected the growth of young peach trees in high-nutrient sand culture. In fact, they found that new roots of trees, planted in crocks to which chopped old roots had been added, actually penetrated well into the bark and along the cambial zone of the old roots without apparent injury to themselves. Ward and Durkee (1956) studied seasonal and tissue variation in amygdalin content of peach trees. I t was shown that amounts varied from none in the woody tissue to more than 50 mglg of dry tissue in some roots, with highest concentrations found in the root bark. Factors such as cultivar and season significantly affected the content of this glycoside. Amygdalin breakdown probably does not account for the difficulty of establishing peach trees in certain old peach soils in some areas of California (Hine 1961a). This theory is based on the observations that amygdalin addition to non-autoclaved soils was toxic to growing peach seedlings, but if a 14-day period elapsed between these additions and planting, tests for HCN were negative and cyanide injury was not evident. Studies by Israel et al. (1973) in Georgia showed that peach root bark contained appreciable amounts of HCN, which was released into the medium from live roots following mechanical injury. Furthermore, extracts from peach soils caused greater inhibition to respiration of peach root tips than extracts from non-peach soils. They also observed that peach root bark and amygdalin reduced the total microorganisms, actinomycetes, Pythium, and pathogenic nematode population of an old peach soil.

c. Spray Residues.-Benson and his co-workers (1974a, 1976, 1978) studied arsenic (As) toxicity in Washington state apple orchard soils.

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Lead arsenate residues from insecticide sprays used for codling moth prior to DDT were still present in the soil in moderate to high concentrations in the 1970’s. Apple seedling growth has been negatively correlated with soil As concentration where 450 ppm total As completely inhibited growth. Total As is not a good measure of As toxicity, and is a minor factor in apple tree growth when the total soil As concentration is less than 150 ppm. Apple seedling growth decreases linearly with corresponding increments of freshly added As to the threshold above which there is no growth. Greenhouse studies of a soil without the As factor showed a 4.4fold seedling-growth increase with methyl bromide fumigation. Correlation coefficient values indicated that a large portion of As is not available to plants, and thus some other factor is a t least as important as the soil As in apple replant problem. Consequently, soil As concentrations less than the threshold of 150 ppm, which are often found in orchard soils, contribute less to the replant problem than their biological counterparts. However, in peaches, apricots, prunes, and cherries, As residues cause systemic As toxicity (Blodgett 1976). It was also reported that young apple trees grow poorly in As soils, whereas cherries are probably the least affected among the stone fruits. Batzer and Benson (1958) experimentally showed that zinc chelate economically corrected As toxicity in peach trees.

d. Other Phytotoxins.-Callus cultures derived from Prunus besseyi and P. tomentosa were more sensitive to sodium cyanide than were those from peach (Heuser 1972). Bernstein et al. (1956) studied the effect of

salt accumulation on growth of several stone fruits. They found that about half of the growth reduction was due to chloride toxicity, the other half to increased osmotic pressure of the saline solution. Aluminum (Al) toxicity also is considered a potent factor in PTSL (Kirkpatrick et al. 1975b). Data from this study suggested that an available concentration of soil A1 greater than 3 ppm which injured peach roots and inhibited plant growth, may do so by inducing an imbalance in such nutrients as Ca, Mg, Mn, and P in peach seedlings. Jones et al. (1957) found extracts from peach buds, twig bark, and leaves to cause growth inhibition in pea bioassay, which exceeded the inhibition caused by sodium cyanide solution containing an equivalent amount of cyanide. Any treatment which removed the cyanide from the extracts caused a loss of inhibitory activity. Later, Jones and Engie (1961) identified the toxic substance in peach extracts as mandelonitrile. Israel et al. (1973) reported that benzaldehyde and potassium cyanide were toxic to rooted peach trees in the greenhouse, and both chemicals inhibited respiration of peach root tips. They also reported a similar suppression of respiration by benzoic acid, mandelonitrile, and aqueous isolates of peach root bark incubated in peach and non-peach soils.


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2. Biochemicals.-Although major biochemicals, carbohydrates, proteins, lipids, nucleic and organic acids, and phenolics may not be the direct causes of short life and replant problems, they exert tremendous influence on plant condition. In fact, biochemical differences set the stage for action by other primary factors. In one way or another, almost all of these major cell biochemicals play critical roles in the replant problems (Bachelard and Wightman 1973; Borzakovska et al. 1975; Kaminska et al. 1971; Lasheen and Chaplin 1971; Lasheen et al. 1970; Lebedev and Komarnitskii 1971; Rozsnyay and Barna 1974).

a. Carbohydrates (CHO).-After growth ceases in late summer, maturation of wood and buds of deciduous plants begins with a corresponding accumulation of CHO, and then cold hardiness develops, due largely to conversion of insoluble starch to soluble sugars (Chandler 1954; Lasheen et al. 1970; Lebedev and Komarnitskii 1971; Levitt 1959; Raese et al. 1977). Dowler and King (1966) reported that in early fall peach twigs and branches contain large amounts of reserve CHO, which the tree may use during dormancy to maintain cold hardiness. Removal of these reserves through early pruning may render the plant more susceptible to PTSL. In addition, older trees are less sensitive than younger ones to such removal, possibly because greater CHO reserves are stored in large trunks and branches. Williams and Raese’s (1974) report indicated that sorbitol and sucrose are important reserves of storage CHO in apple trees during physiodormancy. Rozsnyay and Barna (1974) believed that a CHO component of the phytotoxin from Valsa cincta isolates was evidently responsible for toxicity to apricot trees. High and stable levels of soluble sugars usually have been correlated with the condition of frost resistance (EL-Mansy and Walker 1969; Lasheen et al. 1970; Lebedev and Komarnitskii 1971; Levitt 1959; Raese 1977; Raese et al. 1977; Williams and Raese 1974). However, Layne and Ward (1978) found that both bud and shoot hardiness of peach were closely correlated with total sugars, sucrose, and reducing sugars in the shoots from autumn to spring. They found no correlation between hardiness of buds and apical shoots and total CHO or starch. Lasheen and Chaplin (1971) have reported similar results between hardiness and endogenous levels of these CHO. Levels of starch usually are inversely related to soluble sugars (Raese et al. 1977). Stanova (1977) found that several isolates of Cytospora cincta, which caused typical canker symptoms on peach and apricot, effectively assimilated fructose and xylose but not glucose. Polyols (polyhydric or sugar alcohols), which make up another group of CHO amounting to about 40% of the total sugar content in some fruit species, are considered to play some part in the increase of frost resistance of plants (Sakai 1961). This is based on the observations t h a t seasonal variation (increase in polyol contents during cold period and decrease during warm weather)

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is often observed (Sakai 1966; Williams and Raese 1974). Raese (1977) reported that in most cases the combined content of soluble sugars and polyols, particularly sorbitol, in two-year-old apple trees was slightly higher in treatments that induced cold hardiness than in controls. In peach, however, Rohrbach and Luepschen (1968) found that sucrose, glucose, fructose, and mannitol increased significantly from August to January, while sorbitol decreased significantly. The least winter-hardy peach, ‘Earlyglo’, was found to contain a slightly higher mannitol concentration. Polyols in peach tree bark were correlated with winter injury and the initiation of Cytospora canker infection.

b. Proteins and Other Nitrogenous Compounds.-Proteins play a major role in plants’ development of freezing tolerance. Brown and Bixby (1975) reported that soluble protein concentrations remain relatively constant during early stages of freezing tolerance development, but increase significantly during later stages; whereas, insoluble proteins remain comparatively unchanged throughout the induction period. Qualitative changes in protein contents, evident by their appearance and disappearance corresponding to changes in hardiness levels, indicate some possible connections between hardiness and levels of soluble protein (Craker et al. 1969; Donoho and Walker 1960). Other workers also have associated proteins with cold hardiness in different species under varying conditions (Bachelard and Wightman 1973; Borzakovska et al. 1975; Holubowicz and Boe 1969, 1970; Kenis and Edelman 1976; Lasheen et al. 1970; Siminovitch e t al. 1967). In apple seedlings, a relationship exists between cold hardiness and certain amino acids (Holubowicz and Boe 1970). Kaminska (1973b) and Kaminska et al. (1971) showed that free amino acids were involved in the proliferation disease of apples. Alanine has been reported to play a positive role in defoliation (Larsen 1967; Rubinstein and Leopold 1962). According to Lasheen and Chaplin (1971), total free amino acids in peach leaves were high in the spring, but decreased quickly to a minimum in the fall. In shoots, the level was relatively high in spring, decreased in early summer, increased to a maximum in late summer, then gradually leveled off during fall and winter. c. Fatty and Organic Acids.-Phospholipid degradation in frozen cells of less hardy trees has been intimately associated with freezing injury (Yoshida and Sakai 1974). However, Siminovitch et al. (1975) found that quantitative augmentation of phospholipids per se, or of whole membranes in the cells, is the important component of the hardening process. The increased ratio of unsaturated to saturated fatty acids with increased hardening is the most striking change in the relation of fatty acids to hardiness in peach (Ketchie 1966). An increase in the unsatur-


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ated fatty acids in the various membranes of the plant cell would change the structure and some properties of the membranes, which might be suitable for frost protection. The increased levels of free unsaturated fatty acids also might alter the properties of the cytoplasm, thereby making it less viscous and more flexible, and hence the cytoplasm might be less vulnerable to freeze injury. St. John and Christiansen (1976) concluded that chilling resistance is related to the level of linolenic acid in the polar lipid fraction in the developing root tips. Lepidi et al. (1974) isolated and examined bacteria from the inner rhizosphere of young peach replants in normal and diseased sites in Italy. Bacteria from the diseased site were more numerous and more active in metabolizing compounds other than organic acids. The utilization of organic acids by rhizosphere bacteria was, however, greater in the control plants on the normal site. In another study on the peach replant problem in Japan (Mizutani et al. 1977), benzoic acid and other ultraviolet-absorbing substances were detected from roots sensitive to waterlogging and soil sickness.

d. Phenolics.-Borner (1959) investigated the cause of the apple replant problem in relation to a possible involvement of phenolics released from residues in the soil. All five phenolic substances revealed by paper chromatography strongly inhibited growth of apple seedlings. Cultivar differences in phenolics from apple trees with and without proliferation disease were noted by Kaminska et al. (1971). Masking of proliferation symptoms by other disorders reduced this variation. Decrease in phenolic inhibitors’ activity as a result of polyphenol oxidase activity was found to be an important event in the sequence of phenomena which lead to the dormancy release in peach (Kenis and Edelman 1976). e. Other Biochemicals.-Dorsey and Strausbaugh (1923) indicated that browning in the wood was due, a t least in part, to a condensation of storage materials, which apparently were thereby transformed into gums and tannins. Chirilei et al. (1970) distinguished two types of gummosis in apricot problems in Romania. The first, “xylem gummosis,” which produced water-insoluble gums (pectic acid), causes apricot apoplexy. The second, known as “cortico-cambial gummosis,” produces water-soluble and exuding gums (pectins), and is responsible for a slow decline of apricots. Siminovitch et al. (1967) ascertained that a rhythmic pattern of seasonal changes in water-soluble proteins, RNA, and protein synthetic capacity in living bark cells closely parallel similar rhythms in bark cells’ resistance to freezing injury. The cyclic variations begin with striking abruptions in the early fall with a rise in RNA from the low summer value, followed closely by similar rises in protein, protein synthetic capacity, and freezing resistance. Thus, augmentation of protoplasm becomes a part of the mechanisms of freezing resistance.

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HORTICIJLTURAL REVIEWS

Enzymes play a vital role in the biochemistry and physiology of replant problems through resistance and susceptibility reactions of plant tissues. Langridge (1963) reported that the probable inactivation of such enzymes a t low temperature may be due to an increase in intramolecular hydrogen bonding. Thus, below a critical temperature a single enzyme reaction may become limiting to growth. Conversely, Steponkus and Lanphear (1967a) showed that the high correlation between cold hardiness and T T C reduction might be due to a co-factor and substrate limitation rather than inactivation of dehydrogenases. McCown et a l . (1969) found in winter hardy plants a gradual synthesis of two to four new peroxidase isoenzymes during the hardening period, whereas only a relatively weak initiation of one isoenzyme was noted in tender plants. In addition, the formation of the new isoenzymes preceded the period of hardening by several weeks, depending on the specific isoenzymes and the plant type. Ladd et al. (1976) reported differentially-decreased soil enzyme activities, lowered viable bacterial population, and an increased ninhydrin reactivity as a result of soil fumigation. However, they found a non-consistent relationship between the release of ninhydrin-reactive compounds following fumigation and changes in bacterial population or changes in soil enzyme activity. 3. Phytohormones and Growth Regulators.-The evidence outlined in this section will strongly suggest that some degree of hormonal control is present over such physiological phenomena as dormancy, cold hardiness, disease condition, resistances to different problems, and overall plant growth and development. For details relative to these controls, the reader is referred to the articles by Taylorson and Hendricks (1976), Wareing and Saunders (1971), Samish et al. (1967), Jacobson (19771, and Viglierchio (1971). Current knowledge suggests that control of plant growth and development involves complex interactions of phytohormones in a system of checks and balances, and thus it would be expected that the above responses also would involve phytohormonal imbalances. Abscisic acid (ABA), gibberellins (GA), auxin (IAA), cytokinins (CYK), and perhaps ethylene may each be implicated in the variations of responses, plant species, and tissues or organs. Samish et al. (1967) explained the concept of dormancy regulation in peach with reference to phytohormones. An accumulation of inhibitors induces dormancy. This is followed by mid-dormancy, during which low temperatures and short photoperiods are connected with initial reduction in inhibitors and subsequent appearance of promoters. Finally, the release from dormancy is obtained through additional patterns of phytohormone balances. A full understanding of hormonal regulation can be achieved only by knowing the precise action of the phytohormones and the state of dormancy a t the molecular level (Wareing and Saunders 1971). In this respect, Jacobson


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(1977) believed that RNA synthesis and its regulation are required for all phytohormonal responses. Endogenous phytohormones controlling dormancy and growth can be employed to modify plant chemistry, thereby manipulating dormancy regulation to prevent untimely frost injury (Walker 1970). Viglierchio (1971) concluded that, along with mediating causes of growth disorders reflecting disease symptomology, one must, of necessity, consider synthetic and degradatory mechanisms that result in the establishment of pathological phytohormonal levels. a. Promoters-i. Auxins. Increased cell permeability often reflects pathological or premortal conditions, and IAA and GA have been reported to change permeability depending on concentration (Stadelmann 1969). According to Carter (1976), PTSL appears to be caused by coldinjury to the vascular cambium. He found an elevated IAA level in fallpruned trees and those growing in non-fumigated soil, as compared to control trees. I t was suggested that early breaking of the vascular cambium's dormancy, caused by an altered phytohormonal balance, was responsible for predisposing certain trees to death by cold injury. Blommaert (1955, 1959) reported high concentrations of several indole auxins in peach buds preceding bloom, which reached maximum a t the end of dormancy when the buds began to open. Kochba and Samish (1972a) reported significant differences in the activity of several basic-ether soluble auxin-like phytohormones between Meloidogyne jauanica-resistant (Nemaguard) and -susceptible (Baladi) peaches. Viglierchio (1971) concluded in his review that viruses and fungi have been shown to reduce endogenous IAA levels in hosts and that IAA-degrading enzymes have been implicated in phytopathological symptoms of some disorders caused by nematodes. Exogenous applications of auxins have effectively modified certain physiological processes. Raese (1977) reported a 5.0"C increase in apple tree shoots' cold hardiness in November when 100 ppm naphthaline acetic acid (NAA) was applied 11 days after a 500 ppm ethephon application. NAA treatments of 0.25 to 1.0% applied to pruning cuts on 'Sungold' peach induced gummosis around the cuts but controlled sprouting, both of which were generally proportional to NAA concentration (Couvillon et al. 1977). Kochba and Samish (1971, 1972a) discussed the role of NAA application in nematode susceptibility of peach rootstocks. They found that NAA supplied by wick-feeding increased root growth, but reduced top growth of the trees. NAA also caused the development of swelled and non-suberized branch roots which became susceptible to M. jauanica. In addition, NAA application in combination with kinetin produced a synergistic stimulating effect on the endogenous cytokinin-like activity in roots and hence, an increased nematode susceptibility of both resistant and susceptible cultivar roots. Auxin treatments on apple trees,

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when applied ten and four days before Phytophthora cactorum infection, suppressed collar rot disease development; however, the same application four days after infection showed no effect on collar rot development (Plich 1976).

ii. Gibberellins (GA). Only limited work has been done on endogenous GA as related to regulation of cold hardiness, dormancy, and disease resistance. Bottini et al. (1976) concluded that the trend in seasonal variation in endogenous GA points to a differential action of GA during physiodormancy, its break, and the subsequent resumption of growth. Walker and Donoho (1959) compared the effectiveness of exogenouslyapplied GA on dormancy break of detached shoots from young apple and peach trees, and found that GA broke dormancy of peach trees but not of apples. GA applications a t 100 ppm to apple seedlings showed no influence on the development of cold hardiness, killing point, or levels of amino acids (Holubowicz and Boe 1969, 1970). However, Holubowicz (1976) reported that GA, when applied to one-year-old apple trees a t weekly intervals in August and September, produced varying degrees of cold resistance a t different times during the winter season. GA alone, or in combination with ABA, did not induce or enhance cold acclimation of defoliated branches under short days (Fuchigami et al. 1971). Plich (1976) found no significant effect of GA application on the development of collar rot (Phytophthora cactorum) on apple trees. Dennis (1976) reported moderate to severe winter injury to cherry cambium and buds as a result of GA sprays. Furthermore, Proebsting and Mills (1974) showed that cherry trees treated with GA on August 22 and September 12 had much more severe winter injury than the trees sprayed earlier or later. Deleterious side effects caused by GA treatments include gummosis (Dennis 1976; Proebsting and Mills 19691,twig die-back prior to freezing injury (Proebsting and Mills 19691,bud abscission or failure to open, and reduced fruit set in cherries and peach (Dennis 1976; Yadava and Doud 1977). Bottini et al. (1976) reported that, with GA application, peach foliage persisted two weeks longer than on control trees. Davis and English (1969a) found that Pseudomonas syringae cankers were reduced in size and number when leaf senescence was inhibited by GA application, while the cankers were increased by the acceleration of senescence with peach seed leachate. GA nullified the effect of peach seed leachate. Unchilled ‘Lovell’ seedlings sprayed with 250 ppm GA prior to inoculation with 2 isolates of Ps. syringae were more resistant to canker development than untreated seedlings (English and Davis 1969).

iii. Cytokinins (CYW. Krupasagar and Barker (1966) reported that plant roots infected with Meloidogyne incognita contained substances exhibiting CYK-like activity. However, extracts from healthy roots


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showed only about 16% CYK activity compared to extracts from galled roots. They detected CYK in roots 75 days after inoculation with M. incognita, but attempts were unsuccessful after 35 days. The CYK activity in the roots of Nemaguard and Okinawa peach rootstocks, which are resistant to M. javanica, was significantly lower when compared with susceptible Baladi and L 198-12 peach rootstocks (Kochba and Samish 1972a). Discussing the peach replant problem in Japan, Mizutani et al. (1977) showed that waterlogging caused a rapid decrease in root CYK levels, resulting in reduced chlorophyll content of the leaves. Cytokinin application through wick-feeding increased endogenous CYK levels in roots of peach rootstocks resistant to M. javanica which had lower levels initially (Kochba and Samish 1972a). However, this CYK application had no visible effect on peach seedling growth (Kochba and Samish 1971). According to Weinberger (1969), the CYK, SD 8339 a t 100 to 200 ppm, most effectively stimulated normal peach bud development when only a little additional chilling was needed to break dormancy. A combination of CYK and abscisic acid was found by Yadava and Doud (1977) to delay budbreak and improve subsequent balanced growth (with no blind wood) of peach plants which had their physiodormancy broken before phytohormonal application. Benzyladenine application has been reported to induce mobilization of cold hardiness promoters (Steponkus and Lanphear 1967b). Plich (1976) studied modification of Phytophthora collar rot susceptibility of apple trees as influenced by exogenous phytohormones. He found that CYK application greatly increased the size of necroses. The effect of CYK and ABA depended on the cultivars used. He suggested that phytohormones act indirectly on susceptibility through their effect on plant metabolism. b. Inhibitors-Wareing and Saunders (1971) are convinced that dormancy regulation and, hence, susceptibility of plants to stresses involve an interaction between growth promoters and inhibitors. The induction of physiodormancy is determined by the accumulation of growth inhibitors (Samish e t al. 1967). It also was indicated that the mid and end phases of dormancy are characterized by an initial reduction of inhibitors with the subsequent appearance of, or increase in, certain growth promoters. This view has been confirmed by several workers who found inhibitors disappearing from dormant tissues as the dormancy progressed (Blommaert 1959; Bottini et al. 1976; Eagles and Wareing 1964; Henderschott and Bailey 1955; Henderschott and Walker 1959). Eagles and Wareing (1964) showed that reapplication of an inhibitor isolated from plants apparently could induce normal dormancy in actively growing seedlings of the same species. More growth inhibitors were excreted from the peach replant roots under anaerobic conditions than when the soil was aerated (Mizutani et al. 1977).

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i. Abscisic Acid (ABA). High endogenous ABA levels could not be correlated with the initiation of cold acclimation of apple tree bark (Hammond and Seeley 1977). However, decreasing temperatures after growth cessation and increasing endogenous ABA levels during the same period did correlate with further cold acclimation. Some anomalous stock/scion effects on cold hardiness have been suspected due to indirect effects of several factors including imbalances of IAA, GA, CYK, and ABA (Westwood 1970). Lipe and Crane (1966) were the first to isolate ABA from peach seeds, and to correlate the endogenous ABA levels with peach bud and seed dormancy. They also were able to induce conditions indicative of dormancy in peach seedlings with exogenously-applied ABA. Fuchigami et al. (1971) reported that ABA separately or in combination with GA did not influence cold acclimation under short-day conditions. Holubowicz and Boe (1970) reported no correlation between soluble protein and killing point when ABA was included in the treatments. Moreover, i t was reported that 20 ppm ABA effectively increased cold hardiness of apple seedlings, but this treatment also decreased the seedlings’ rate of photosynthesis (Holubowicz and Boe 1969). Initial budbreak and growth on greenhouse peach plants which had their physiodormancy broken by chilling were inhibited by ABA treatment (Yadava and Doud 1977). Depending on time between inoculation with collar rot disease and ABA application, ABA can modify apple’s susceptibility to Phytophthora cactorum (Plich 1976). ABA application, both before and after inoculation with the disease, caused larger necroses on the trees.

ii. Other Inhibitors. The basic and most plentiful endogenous apple inhibitor that belongs to this category is phloridzin or dihydrochalcone (Kolomiets et al. 1970). The high winter hardiness of apple tissue following physiodormancy break is determined by the presence of phloridzin. Further, dormancy regulation in peach, unlike apple, is determined not by a single inhibitor but by an inhibitory complex. Also, peach plants do not possess the ability to synthesize the inhibitors with the physiological properties of phloridzin of apple during the transition to autumn-winter dormancy. This is apparently one of the factors behind low frost resistance of peach. However, the increase in flavonols towards the end of dormancy indicates t h a t they possibly may play a protective role in peach shoots similar to that of phloridzin in apple shoots. Raese (1977) obtained a slight increase in apple shoot cold hardiness for three winters with two annual applications of SADH in June. When applied a t weekly intervals in August and September, SADH produced varying degrees of hardiness in apple and peach a t different times during the winter season (Holubowicz 1976).SADH sprays followed by benomyl fungicide did not substantially improve Cytospora canker control over that of benomyl alone (Luepschen 1976). The benomyl and oil combination provided the


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best control of canker. However, no post-inoculation sprays reduced canker extensions. Kochba and Samish (197213)examined the effect of seven growth inhibitors on Meloidogyne jauanica development in the roots of the nematode-susceptible Baladi peach rootstock. Maleic hydrazide, thiourea, TIBA, and actidione most effectively inhibited gall formation as well as nematode maturation in the galls. The other three inhibitors, 7-aza-indole, 2-hydroxy-5-nitrobenzyl bromide, and 2,6 diaminopurine, effectively prevented gall formation, but once galls had formed nematode development was affected less. c. Phytohormonal Interaction-Dormant plant tissues apparently contain many promotive and inhibitory phytohormones. Thus, Walker (1970) proposed a balance concept between promoters and inhibitors. If the plant contains more “inhibition units” than “promotion units,” then the plant remains dormant and does not grow. When “promotion units” outnumber the “inhibition units,” dormancy is broken and growth may occur, provided adequate and suitable environmental conditions become available. Martin and Corgan (cf Walker 1970) have theorized that if the ratio of GA and CYK to ABA is high, growth will occur. If the two promoters are low relative to the inhibitor, growth ceases. They further postulated that the ratio may be influenced by suppressors or inducers of specific sites on DNA. DNA transfers the message to RNA, which passes it on to the specific proteins that form the enzymes necessary for either growth promotion or senescence and dormancy, depending on the environmental circumstances. Interaction between promotive and inhibitory phytohormones based on their balances has been reported to be an important factor in such processes as dormancy regulation, hardiness, chilling requirement, stock/ scion effects, disease resistance, etc. (Bowen 1971; Davis and English 1969a; Fuchigami et al. 1971; Holubowicz and Boe 1969,1970; Kochba and Samish 1971; Plich 1976; Samish et al. 1967; Wareing and Saunders 1971; Westwood 1970; Yadava and Doud 1977). Plich (1976) demonstrated interactions among IAA, CYK, and ABA which modify apple’s susceptibility to Phytophthora canker. H e showed that the phytohormones which were active in uiuo had relatively weak or no fungitoxic effects in uitro, and, thus, suggested that they act on susceptibility indirectly through their influence on plant metabolism. Kochba and Samish (1971) studied the role of NAA and CYK in peach rootstocks’ susceptibility to M. jauanica. These substances produced a synergistic effect, stimulating nematode development in roots of resistant rootstocks and increasing the nematode population in the susceptible rootstocks. Thus, NAA and CYK played a significant role in altering the host-parasite relationship in peach-nematode problem. GA application reportedly

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antagonizes peach seed leachate and nullifies its stimulating effect on Pseudomonas syringae canker development on peach (Davis and English 1969a).

IV. CONTROL MEASURES Controls for all the different fruit crop problems discussed in this review are not available. Whatever measures and management practices have been offered for the solution or minimization of these short life and replant problems are discussed under the following categories. A. Plant Improvement Through Breeding for Resistance A resistant plant is resistant usually for several different reasons, and no one single mechanism can be designated as the most important (Rohde 1965). Any loss of or lack of retention of resistance leaves plants with less survival value (Brierley 1947). Plants’ ability to protect themselves against more than one harmful factor leads to adaptive flexibility which enhances plant survival potential. Thus, knowledge of plants’ resistance to freezing, pathogens, and other stresses may help to substantially reduce damages. I t is not difficult for fruit breeders to justify improving fruit trees’ resistance to frost and plant pathogens, and their survival. Past fruit breeding and improvement programs, designed to incorporate desirable characters in cultivars as well as in rootstocks, have made dramatic contributions to fruit production, especially in extending their range of adaptation (Dorsey 1921; Parker 1963; Rohde 1965; Stushnoff 1972, 1973). Thus, the development of hardy and resistant cultivars and rootstocks by breeding appears to be the most effective and economic way available to combat short life and replant problems. Samish and Lavee (1962) stated that while the plant’sgenetic make-up controls the dormancy and chilling requirement of the various organs, its physiological state may modify the expression of these characters. This, they reported, is true for the entire tree as well as for different parts of any one plant. The inheritance of nematode resistance does not appear to differ in any significant respect from the inheritance of any other type of resistance or other qualities in plants (Hare 1965). Genetic studies have shown that slight differences in plants, controlled by one major gene in many cases, can make a plant resistant. Thus, it becomes logical to expect the presence of nematode resistance in most crops (Rohde 1965). It must be recognized, however, that some nematodes may feed on plants without response other than injury, and resistance to these types will be rare. According to Lownsbery and Thomason (1959), no available fruit cultivar or rootstock is resistant to all the kinds of nematode parasites


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which may be present. For this reason, the control measures most certain of success involve a choice of material resistant to the most common nematode species in a particular area, in addition to plant protection practices. Based on tests for root-knot nematode resistance in apple, peach, cherry, pear, plum, almond, and apricot seedlings, Tufts and Day (1934) suggested that one must consider the possibility that, in the absence of susceptible host plants, nematodes may attack partly resistant hosts. Furthermore, in the presence of very susceptible cultivars, these parasites may be attacked from the partly resistant hosts. On the basis of progeny analyses from reciprocal crosses between the hardy, less hardy, and tender apple cultivars, Wilner (1965) showed that frost resistance in apple appeared to be regulated by hereditary factors. He also reported that the resistance of the progenies tended to reflect that of their parental types, and was more favorably influenced by the maternal than the paternal parent. Lantz and Pickett (1942) reported that hardiness in apple progeny is based on multiple factor inheritance. Those apple rootstock types which start earliest in spring (M 9) seem most injured by cold, while those types which start latest in spring (M 16) are least injured (Stuart 1941).Apple rootstock breeders a t Geneva, New York have been working for some time towards developing improved and satisfactory replacements for the rootstocks presently available (Cummins 1977; Cummins and Aldwinkle 1974a,b,c).They perceive that problems, objectives, and breeding strategies in developing improved rootstocks are similar for most tree fruits in most parts of the world. Stock-related factors which are of particular importance from the breeding standpoint have been identified and assigned relative levels of priority. Large numbers of seedlings from controlled crosses are being screened during their first year for susceptibility to such pathogens as Phytophthora coctorum, Eriosoma lanigerum, and Erwinia amylouora. Criteria for later selection include propagability, freedom from suckering, cold hardiness, dwarfing, and induction of early, efficient production. The entire selection period is expected to require a minimum of 15 years. Their experience with this apple rootstock breeding program suggests that there is excellent potential for substantial improvement over existing rootstocks. However, no such program for developing improved rootstocks exists for the Southeast, except that rapid screening procedures have been developed for some pathogens only. Genetic variations in susceptibility to some pathogens, pear decline, winter hardiness, and chilling requirement have been observed among pear cultivars, rootstock clones, and certain species of P y r u s (Brown and Kotob 1957; Rallo 1973; Seemuller and Kunze 1972; Westwood 1976; Westwood et al. 1971; Wilcox 1936). This variation, depending on the type of heredity, could be utilized beneficially for developing more de-

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sirable rootstocks and cultivars. Westwood (1976) reported on the inheritance of pear decline resistance. This research to study rootstocks from resistant and susceptible parent types and those from resistant and susceptible crosses was initiated in 1968. All trees were grafted to ‘Bartlett’, and in most cases 14 to 32 seedlings of each cross were used as rootstocks. All the progenies of resistant parents were resistant to pear decline-though not to the same degree. Pyrus betulaefolia, P. calleryana, and several crosses of resistant P. c u m m u n i s showed a relatively low percentage of severe decline. Crosses of resistant and susceptible types were intermediate in response, whether or not the resistant parent was P. c u m m u n i s . The lack of complete resistance to decline in resistant crosses and gradation from healthy to severe decline in most cases indicated a complex inheritance involving multiple genes. Of all tested Pyrus species, P. betulaefolia is most resistant to the decline problem. Recent serious peach tree losses in most southeastern peach areas have prompted greater interest in breeding suitable rootstocks as a possible solution to PTSL. However, there has been little effort towards obtaining dwarfing, size control, or specific disease resistance (Sharpe 1974). Dwarfing in peach trees is an objective in the search for rootstocks resistant to cold and nematodes, and generally long-lived (Fogle 1975). Thus, continued research on peach scion and rootstock improvement by breeding and selection is essential to the peach industry’s economic viability and future (Carlson 1975; Layne 1974). Peach has been described as the most heterogeneous of temperate fruits, and cultivar variation has been reported for such characters as chilling requirement (Bowen 1971; Lesley 1944), cold hardiness (Blake 1935, 1938; Layne 1975, 1976a; Mowry 1960; Oberle 19571, status of important nutrients and phytotoxic chemicals (Rowe and Catlin 1971; Thomas and White 1950; Ward and Durkee 1956)’nematode resistance (Barker and Clayton 1969; Burdett e t al. 1963; Day and Tufts 1939; Hansen e t al. 1956; Hutchins 1936; Minz and Cohn 1962; Sharpe et al. 19691, resistance to other diseases (Luepschen et al. 1975; Lukens et al. 1971; Rowe and Catlin 1971; Smith and Neales 1977; Smith et al. 1977b; Vigouroux et al. 1972; Wensley 1966, 1970), “wet feet” tolerance (Chaplin et al. 1974; Marth and Gardner 19391, and growth and vigor (Conners 1922; Hutchins 1936). The chilling requirement in peach appears to be genetically controlled by multiple genes (Bowen 1971; Lesley 1944), and physiologically controlled by a phytohormone balance (Bowen 1971). Blake (1938) established that lower trunk hardiness and resistance to bark injury are not always correlated with hardiness of fruit buds. Lukens et al. (1971) reported that peach cultivars differ in susceptibility to X-disease, but none of the tested cultivars showed a high degree of resistance. Similarly,


69

Luepschen e t al. (1975) found no significant resistance to Cytospora canker among ten peach cultivars; ‘July Elberta’ was the most susceptible of the group, while the rest were of intermediate susceptibility. Most of the breeding work for nematode resistance in peach has been on root-knot nematode species, with only a little on lesion nematodes. As surveyed in Israel, Shalil, Elberta, and Baladi peaches proved to be highly susceptible rootstocks to Meloidogyne jauanica, although Stribling’s 37 (S-37) peach showed a fairly high overall index of resistance to this nematode (Minz and Cohn 1962). On the other hand, Burdett et al. (1963) reported that, in addition to Elberta and Yunnan, S-37 supported a high population of M. jauanica, while M. incognita var. acrita reproduced only on Elberta. Under California conditions, Shalil, Bokhara, and Yunnan showed resistance to M. incognita var. acrita and were susceptible to M. jauanica, while seedlings of S-37 were reported to be resistant to the latter species. Hansen et al. (1956) reported that seedlings of Shalil and S-37 were immune or highly resistant to M. incognita var. acrita, whereas Lovell seedlings were very severely infected with this species as well as with M. jauanica. Five peach rootstock selections from a 1949 cross of P r u n u s dauidiana and a Chinese peach in Chico, California showed immunity to M . incognita and M. jauanica (Sharpe et al. 1969). I t was shown that resistance to both of these nematode species depends on different genes; to the former it was inherited as monofactorial dominant, while to M. jauanica it appeared to depend on two or more dominant genes. In 1966, a third type of root-knot nematode was discovered in Florida. I t reproduced readily on Okinawa, Nemaguard, and other lines which had been selected for resistance or immunity to both M. incognita and M. jauanica. Lesion nematodes (Pratylenchus u u l n u s and P. penetrans) are equally damaging to peach, and no resistant peach cultivars have been reported as yet (Barker and Clayton 1969). Lownsbery (1961) established that Lovell and S-37 peaches are poor hosts for Macroposthonia xenoplax. Since major breeding studies in the southeastern United States have concentrated on root-knot nematode resistance and general tree vigor, only a little attention has been paid towards obtaining tree size control or specific disease resistance. Sharpe (1974) suggested that wide crosses, involving various P r u n u s species, might offer dwarfing possibilities as well as resistance to nematodes and root rots. Clonal propagation of such material probably would be essential. Layne (1975, 1976b) has been involved for some time in breeding peach rootstocks a t Harrow, Ontario. His findings warrant that there is a need to find sources of resistance to nematodes and certain fungi, and tolerance to fine textured, imperfectly drained soils needs to be improved by testing peach X almond hybrids. More precise screening tests for evaluating rootstock hardiness and dis-

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ease and pest resistance, for testing resistance to waterlogging, and for assessing dwarfing capability and ease of propagation are also necessary. The breeding work on other P r u n u s species has not been so extensive. Pepin et al. (1975) found variation in cherry and other P r u n u s species, including their hybrids, for resistance to Thielaviopsis basicola root rot. Resistance to T. basicola in the P r u n u s pseudocerasus hybrids appears to be an important asset for these potential cherry rootstocks. Genetic variation among cultivars and species also has been reported for resistance to apoplexy of plums caused by Phytophthora spp. (Kouyeas 1971), and plum decline caused by P r u n u s necrotic ringspot virus (Posnette and Cropley 1970).

B. Rootstocks Sharpe (1974) emphasized that serious tree losses due to short life and replant problems have prompted greater interest in rootstocks as a possible solution. Rollins et al. (1962) suggested that consideration be given to cultivars, rootstocks, and interstocks prior to establishing orchards on problem sites. Foster et al. (1965) found that using resistant rootstocks along with more effective nematicides on a nematode-infected soil resulted in much more vigorous tree growth during the pre-fruitbearing years and much greater production in the early bearing years. Similarly, Ryan (1975b) stated that the use of resistant rootstocks contributes substantially to the control of specific replant diseases. Some rootstocks may enhance early winter hardiness simply by causing growth to cease earlier than on vigorous rootstocks (Westwood 1970; Layne et al. 1977). In addition to affecting tree vigor and growth, rootstocks are reported to considerably influence tree survival (Layne et al. 1976; Yadava and Doud 1978a). In some cases, tree mortality has been associated mainly with winter injury and canker infection but not with stionic incompatibility (Layne et al. 1976). Day and Serr (1951) noticed differential resistance to Pratylenchus vulnus among such crops as apricot, apple, pear, and quince rootstocks, as well as peach and plum cultivars used as rootstocks. Parker et al. (1966) reported that deeprooting rootstock types are less damaged by nematodes than are shallowrooting types, e.g., Mahaleb is a substantially better cherry rootstock than Mazzard for problem soils. Savory (1966) observed that rootstockscion combination had a significant effect on specific replant diseases of apple and cherry. Dorsey (1918b) showed that the cold hardiness of an apple scion was independent of the rootstock. However, a highly cold hardy scion does not give the rootstock measurably greater hardiness than does a less hardy scion (Chandler 1954). Filewicz and Modlibowska (1941) concluded that the freezing injury to the rootstock depends not


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only on its own resistance but also, to a large extent, on the scion cultivar growing upon it. They reported that root killing in Poland, during a cold snowless winter, was more influenced by the scion type. Stuart (1937) reported that scion hardiness was not measurably affected by rootstock hardiness, but that rootstock hardiness, on the other hand, was greatly influenced by the scion cultivar. However, Wildung et al. (1972b) recently reported that rootstock hardiness is not altered by scion cultivars. Saunier’s (1966) preliminary observations on rootstock resistance to root asphyxiation showed a strong influence of the scion on the rootstock resistance. Filewicz (1931) and Hilborn and Waring (1946) reported a reciprocal effect of rootstock and scion on cold hardiness. Certain apple scion/rootstock combinations seem to grow poorly and frequently show symptoms of tree decline, where affected trees often appear to be girdled (Stouffer et al. 1977). In this regard, clones of ‘Delicious’ propagated on MM 106 rootstock seem to be affected most severely; however, other scion/rootstock combinations exhibit similar symptoms which are typical of apple stem growing virus-induced disorders. Wildung et al. (1972b) investigated rootstock survival and root hardening pattern in apple. A comparison of M 9, M 7, M 26, MM 104, and M M 106 showed that M 26 was the most hardy and M 7 the least hardy of those tested. They concluded that scion hardiness seemed to be influenced more by maturity of the rootstock than by inherent hardiness in the rootstock. A sub-freezing temperature caused trunk splitting in scions on M 7 but these scions on seedling rootstocks were not injured, indicating that severity of cold injury was affected by the rootstock/scion combination (Simons 1970). Reporting on multiple-stock apple trees, Cummins and Forsline (1977) said that the effects of a rootstock on hardiness, anchorage, and suckering are somewhat altered by the interstem. The interstem tree is especially valuable for limiting certain disease problems and for permitting utilization of sites too wet or otherwise unsuitable for other apple rootstocks. Stuart (1937, 1941) indicated that the hardiness imparted to the rootstock by the scion bore no relation to the hardiness of the scion. Filewicz and Modlibowska (1941) emphasized that for rootstock hardiness, microclimatic conditions and, to a certain extent, the vigor of the rootstocks, are important. Rollins et al. (1962) pointed out that any practice tending to stimulate a high level of tree vigor and/or retarding normal hardening increases the potential hazard of cold injury. Campbell (1971) compared growth of young apple trees on virus-infected and healthy rootstocks. The amount of virus inoculum present was important in assessing the viruses’ impact on the growth. When four apple cultivars were bud-grafted on virus-infected rootstocks, substantial growth reduction resulted in the first two years depending on cul-

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tivars. The growth of ‘Jonathan’ and ‘Granny Smith’ apples on Sturmer seedling and MM 104 rootstocks, with and without apple mosaic strain of Prunus necrotic ringspot virus from mildly and severely diseased trees, showed significant interactions between cultivar and infection source, rootstock and infection source, and cultivar and rootstock (Johnstone and Boucher 1973). Ryan (1975a,b) strongly recommended the use of best rootstock (M 1 2 ) to control SARD in the Hawke’s Bay area of New Zealand. The recommendation of M 1 2 rootstock was made especially for those places where root diseases were a problem. In addition, the use of MM 115 and M 793 rootstocks in lighter soils and M 793 with fumigation on heavier soil was also recommended to ensure normal growth in orchards having SARD. Day and Serr (1951) reported that under California conditions, apple rootstocks were resistant to root-lesion nematode (Pratylenchus uulnus) attacks. Colbran (1953) reported from Stanthorpe district of Australia that the root-lesion nematode (P. coffeae Zimm.) was widely distributed in apple orchards, and that no apple rootstocks available in the district were immune to this species. Certain scion/rootstock combinations in Greece have been affected by pear decline with an increasing frequency in recent years (Agrios 1972). In these cases, graft union symptoms vary considerably with the scion/ rootstock combination, and often appear to be the result of graft incompatibility, particularly with pear on quince rootstock. Batzer and Schneider (1960) presented similar evidence indicating that pear decline in western United States is a bud union disorder closely associated with certain rootstocks. They also pointed out t h a t this disorder is not an inherent incompatibility of scion and rootstock, but appears to be an induced one. Furthermore, pear scions on oriental stocks like Pyrus serotina and P. ussuriensis were highly susceptible to pear decline, while those on imported French (P. communis) were intermediate, and trees growing on domestic Bartlett seedlings (P. communis) were highly resistant to decline. Blodgett et al. (1962) found no variation in the frequency and severity of decline symptoms due to scion cultivars or to origin of the scionwood. However, the variation was directly associated with the rootstock from light to severe as follows: P. communis ‘Bartlett’ followed by imported French, P. calleryana, P. ussuriensis, and P. serotina. Seemuller and Kunze (1972), who investigated pear decline in southwestern Germany, were able to experimentally transmit by grafting the decline symptoms which they suspected were responsible for the variation in rootstocks’ susceptibility. T h e most extensive pear rootstock research in relation to pear decline has been conducted in Oregon for a t least the past 50 years. Westwood et al. (1971) assessed pear plots established in 1923 and 1926 with trees composed of several rootstocks and trunk combinations for tree size and susceptibility to pear decline. In


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general, P. ussuriensis and P. pyrifolia rootstocks were dwarfing, P. communis and P. calleryana Decne. intermediate, and P. betulaefolia Bunge non-dwarfing. The latter was most resistant to decline, followed by P. calleryana and P. communis, with P. pyrifolia and P. ussuriensis being susceptible. The use of oriental hybrid cultivars ‘Variolosa’ and ‘Tolstoy’ as interstocks increased the severity of pear decline symptoms, whereas the use of P. communis cv. Old Home as scion-rooted trunkstocks decreased the degree of decline. Additional emphasis was placed on rootstock research for resistance to blight, decline, pear psylla, nematodes, etc., following severe occurrence of decline in 1956 (Westwood and Lombard 1977). In California, pear and quince rootstocks were reported to be resistant to attack by Pratylenchus vulnus (Day and Serr 1951). Fogle (1975) emphasized that dwarfing in peach is an important objective in the search for hardy, resistant, and long-lived rootstocks. For their various desirable characters, trees on Lovell rootstock have been heavily favored to control PTSL in the Southeast (Clayton 1975a,b; Correll et al. 1973; Miller and Dowler 1973; Yadava and Doud 1978a; Zehr et al. 1976). Siberian C rootstock gave best tree survival for northern conditions (Layne et al. 1976, 1977; Ormrod and Layne 1977), but tree mortality in the South, particularly on short-life sites, has been substantial on this rootstock (Yadava and Doud 1978a). Although Blake (1938) reported that lower trunk hardiness and resistance to bark injury are not always correlated with fruit bud hardiness, enhancement in bud hardiness as a result of rootstock effects has been found by several workers (Emerson et al. 1977; Layne et al. 1973,1977; Layne and Ward 1978; Winklepleck and McClintock 1939). In 1951, 14 of the 15 trees on Yunnan rootstock in Fort Valley, Georgia were injured by cold, while no trees on Lovell rootstock were affected (Weinberger 1952). The winter killing of trees on Yunnan was not confined to one cultivar alone, but all six cultivars lost some trees on this rootstock. Yadava and Doud (1978a) showed that Lovell, Halford, and NA8 rootstocks invariably imparted more cold hardiness to ‘Redhaven’ scions than other rootstocks tested, whereas maximum cold injury was sustained by trees on Siberian C and NRL4 rootstocks. Emerson et al. (1977) noted that graft incompatibility of scions on certain peach rootstocks resulted in excessive tree mortality. Nematode infestation is a weakening factor in peach orchards in most producing areas. The use of rootstocks resistant to causal nematode species offers a promising method to reduce tree losses. Zehr et al. (1976) feel strongly that tree losses are affected by rootstock as well as by nematode control. Several workers have studied peach rootstock resistance to root-knot nematode and found that S-37 and Okinawa were, in most cases, resistant to both Meloidogyne javanica and M. incognita

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(Burdett et al. 1963; Hutchins 1936; Kochba et al. 1972; Minz and Cohn 1962). Day and Serr (1951) presented evidence that Pratylenchus uuln u s was an important factor in peach decline on old sites. Peach rootstocks varied in susceptibility to this nematode, and Bokhara and Yunnan were, at least partially, resistant. The research group from Harrow, Ontario that reported Siberian C rootstock to be the most winter-hardy and otherwise desirable found that the seedlings of this rootstock were most severely affected by P. penetrans (Johnson et al. 1978). Lownsbery (1961) listed Lovell and S-37 peach rootstocks as very poor hosts for ring nematode (Macroposthonia xenoplax). Stem-pitting in peach has been reported to be a virus-induced rootstock-scion reaction which has been a severe problem in peach orchards in some parts of the United States (Cameron 1971b). Natural incidences of perennial canker (Leucostornu spp.) on ‘Dixiered’, ‘Babygold-5’, ‘Loring’, and ‘Redhaven’ peaches on Rutgers Red Leaf (RRL) and Harrow Blood rootstocks were significantly lower than on other rootstocks (Layne 1976a; Weaver 1963). Harrow Blood rootstock’s influence on promoting a lower incidence and severity of Leucostoma canker was postulated to be caused by its known effect on enhancement of stem hardiness of peach scions (Layne 1976a). Hutchinson and Bradt (1968) reported that on a clay loam soil in Vineland, Ontario ‘Golden Jubilee’, ‘Redhaven’, and ‘Veteran’ peach trees were of similar size on Elberta and Lovell seedling rootstocks, but were usually smaller on RRL seedlings. Tree losses which were greater on RRL than on the other two rootstocks were thought to be caused by incompatibility rather than cold injury or soil conditions. On the other hand, Chaplin et al. (1974) reported that, under Kentucky soil conditions, RRL seedling rootstock was most tolerant to waterlogging and Lovell was least tolerant. The rootstocks used for peach and almond have an influence on the amount of injury caused by an excess accumulation of boron in the leaves and stems (Hansen 1955). Unfortunately, almond, which is only partially satisfactory rootstock for peach, could not be recommended for use with peach even though it did reduce boron phytotoxicity. Compared to Lovell roots, the Shalil roots tended to produce somewhat higher chloride accumulation and less growth in both peach and almond scions (Bernstein et al. 1956). Westwood et al. (1973) tested 6 P r u n u s species, represented by 19 types, for their performance as rootstocks for prune ( P r u n u s domestica L.). Fewer trees on peach roots died from Pseudomonas syringae canker than did those on several clonal plum roots; however, some plum-rooted trees outgrew the canker and survived as well as trees on peach rootstock. Bacterial canker (Ps. syringae) ratings of ‘Napoleon’ and ‘Corum’ cherry and ‘Italian’ prune trees budded on various rootstocks were re-


75

corded by Cameron (1971a). Under Oregon conditions, about half as many ‘Italian’ prune trees had trunk cankers when on peach seedling rootstocks as trees on plum rootstock. Infection was particularly severe on trees with P. tomentosa rootstock. On both the East and West Coasts of the United States, Mahaleb cherry rootstock was found to be more winter hardy than Mazzard (Blodgett 1976; Carrick 1920; Edgerton and Parker 195813).However, according to Carrick (1920) there is no basis for assuming that stock hardiness in cherries would directly influence hardiness of the scion. Western X-little cherry disease often results in a complete wilt and death of mature trees, and trees on Mahaleb rootstock suffer comparatively serious losses (Blodgett 1976). Cherry trees on Mahaleb rootstock in Michigan are killed suddenly in mid-summer by Xdisease, whereas trees on Mazzard rootstock decline slowly (Jones 1971b). A deep-rooting rootstock, like Mahaleb, is substantially better than shallow-rooting Mazzard for problem soils infested with Pratylenchus penetrans (Parker et al. 1966). Furthermore, a progressive decrease in tree growth on fumigated soils occurred sooner in trees on Mazzard than on Mahaleb rootstock (Mai and Parker 1967). One-year-old Mahaleb seedlings had higher mortality due to Phytophthora root rot than Mazzard seedlings within a period of three months in a soil artificially infested with Phytophthora cambivora and P. megasperma (Mircetich and Matheron 1976). Trees on Mahaleb rootstock on poorly drained soils were more severely affected with the disease than trees on Mazzard rootstock, or on well drained soils. Rootstock resistance to Thielaviopsis basicola, which is responsible for cherry decline (Pepin et al. 19751, differed significantly among various cherry clones. It was suggested that resistance to T. basicola in the P r u n u s pseudocerasus hybrids was an important asset to the potential hybrid cherry rootstocks. Bernstein et al. (1956) reported t h a t Yunnan rootstock definitely increased chloride level and hence toxicity in the apricot while Marianna rootstock effectively reduced chloride accumulation and improved plum and prune tree growth. Since almond and peach trees on almond roots suffered the least phytotoxicity due to accumulated boron, almond rootstocks were suggested only for almond trees in locations where excess boron is a problem (Hansen 1955).

C. Cultural Practices A well grown plant in good health will be better able to survive adverse conditions than a plant in marginal health. In addition, tissue maturity is necessary before a plant can develop any resistance to cold (Brierley 1947). Early development of cold resistance results from decrease in protoplasm activity before leaf fall, and in such cases fertilizer appli-

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cations reduce plants’ cold resistance (Chandler 1954). The greater susceptibility of late growth in deciduous fruit crops to winter injury gives some indication of how necessary it is to modify the cultural practices in those regions where low temperatures are encountered. Though environmental factors cannot be modified to reduce cold injury in the orchards, potential hazards can certainly be reduced by certain precautions, such as proper care taken to avoid those practices that will result in late and excessive growth and delayed hardening of trees (Rollins et al. 1962). Beattie et al. (1963) found no clear cut relationship between cultural practices and ATD in Ohio. They suspected that ATD incidence was associated with lower soil pH (Beattie 1962; Beattie et al. 1963); consequently, liming to adjust pH between 5.5 and 6.5 and to help other biological activities in the soil was recommended (Banta 1960; Parker et al. 1966). In England, however, acidification of soil was found to be effective against apple replant problem (Anon. 1966). Apple trees became more resistant to collar rot when treated with chemical fertilizers and green manure, but more susceptible when treated with animal manures (Sarasola and De Bustamante 1970). In addition, nitrogen fertilization improved the vigor of pear trees, though without considerably affecting the decline symptoms. I t also was reported that the use of windbreaks to protect apple and pear orchards from winds and to prevent abrupt changes of temperature a t sunset sharply reduced pear decline and apple collar rot. Soil moisture appears to be important to root hardiness and survival in various ways. From Nebraska, Howard (1924) reported greater apple root injury under dry soil conditions than under moist conditions. Conversely, Way (1954) observed that ‘Cortland’ apple trees that were irrigated in the fall suffered significantly greater freezing injury than non-irrigated trees. Similar trees treated with 3 to 7 Ib of ammonium nitrate suffered significantly greater cold injury than non-treated trees. H e also showed that artificial drought and fall pruning had no measurable effect on the cold injury. Burkholder (1936) suggested that when heavy pruning is to be practiced, the work should not be done until late February, because some cultivars, if fall-pruned, may be severely injured when pruning is followed by prolonged periods of sub-zero temperatures. When it becomes necessary to prune in the fall, it would seem best to work first on such varieties as ‘Rome’, ‘Delicious’, and ‘Grimes Golden’. Still another possibility would be to confine the pruning to mature trees where the pruning cuts should be relatively small and mainly in the outer surface, well removed from the crotch and lower parts of the scaffold branches. Cultural practices strongly affect cold hardiness, which in turn is associated with peach tree longevity (Nesmith and Dowler 1976). Hen-


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drix and Powell (1969) concluded th at peach growers can improve the chances of tree survival by modifying cultural practices. Cultural practices that delay hardening in the fall tend to prolong dormancy period in spring (Proebsting 1970). Avoidance of those practices that leave peach trees susceptible to cold injury will also lessen the chances of severe damage by bacterial canker (Petersen 1975). Oh and Carlson (1976) suggested prompt disposal of plant parts as a practical sanitation practice to possibly reduce PTSL in old soils. Smith and Stouffer (1975) suggested prompt roguing of diseased plants, no replanting in old orchards, effective weed control, and thorough tillage before planting a s practical points to be considered for an effective measure against infection and spread of Prunus stem pitting. T h e spread of peach rosette and decline disease in Victoria, Australia was reduced by almost 33'k over a 2-year period by grubbing all affected trees before flowering (Smith et al. 1977a). Savage (1970) showed th a t insulation of trunk and scaffolds immediately above the crotch would prevent cold injury under most circumstances. He suggested that, if any easily applied economical insulation were available, the tree losses could be greatly reduced. Metal chelates have been found to be beneficial in overcoming arsenic toxicity of peach trees (Batzer and Benson 1958). I t was suggested th a t zinc chelate was most economic in correcting arsenic toxicity without any adverse effects. Various kinds of cover crops and mulches have been recommended to initially benefit replants and to reduce soil population of certain nematodes in established orchards (McBeth and Taylor 1944; Shannon and Christ 1954). Hendrix and Powell (1969,1970a) cautioned t ha t if PTSL is to be controlled, the first step in any such program should be to avoid injury and destruction of roots by discing. However, preplant subsoiling has been strongly recommended for the southeastern United States (Miller and Dowler 1973; Savage et al. 1968). T h e latter workers reported t ha t under Georgia peach soils, where oxygen levels were 15% or less during a great part of the growing season, preplant subsoiling to a depth of 50 cm had nearly doubled the growth and yield of peach trees and greatly increased tree longevity. The improved performance and increased longevity were accomplished even though soil moisture was decreased in the subsoiled plots. Under adequate soil moisture supply, even moderately injured trees lived and maintained productive lives (Cowart and Savage 1941). Aeration of anaerobic soils reverses cyanogenesis in the peach roots (Mizutani et al. 1977) and could be used to control peach replant problems. Georgia scientists have claimed th at on short-life sites peach yields can be maintained and tree survival extended by following good liming and nitrogen fertilization practices prior to planting (Giddens et al. 1972; Spivey and McGlohon 1973; Taylor 1972). T h e PTSL research efforts,

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conveyed through a 10 point program, have very strongly recommended preplant liming to aid in reducing tree losses (Miller and Dowler 1973).In addition to raising the soil pH, lime also increases leaf Ca and Mg contents (Havis 1962), and greatly reduces leaf abnormalities arising from Ca and Mg deficiencies (McClung 1953). Dolomitic and high-Mg limes have proved to be beneficial and caused even greater growth improvements than in the non-short-life sites in some cases (Giddens et al. 1972; Havis 1962; Prince et al. 1955). Nesmith and Dowler (1976) reported t ha t nitrogen applied alone or in combination with fumigation reduced cold hardiness of peach in early winter, but nevertheless increased vigor and survival. This would mean th a t any practice serving to increase the level of soluble nitrogen in peach tissues during the dormant season might lower the resistance of these tissues to freezing (Waltman 1937). McCue (1915) ascertained th at the fertilizer which was so balanced to produce the most healthy growth also would promote the strongest wood, with considerable hardiness in the peach tree. Higgins et al. (1943) reported th at fall nitrogen application increased the wood hardiness of peach trees. Waltman (1937) explained the usefulness of fall application of nitrogen. Since root activity during the winter is relatively slow, it would appear th at harmful effects from using calcium cyanamide (CaCNJ fertilizer are not likely to occur, and th a t trees fall fertilized with CaCNz possibly may be less subject to winter injury because of the lowered level of soluble nitrogen in tissues. Application of nitrogen has been reported to induce iron chlorosis in peach trees grown on calcareous soils (Dekock and Wallace 1965). Similarly, Davidson and Blake (1937) warned against those practices involving heavy applications of single fertilizer materials on,light sandy soils which are low in organic matter. They emphasized th at the significance of nutrient balance must be recognized, particularly when dealing with such soils. They recommended the regular use of complete balanced fertilizers, together with frequent liming in orchards on these soils. Studies reported by Raw1 (1935) also disapproved the use of nitrogen alone a s a fertilizer, and strongly suggested th at application of other plant nutrients is equally essential. However, nitrogen fertilization a t rates sufficiently high to maintain vigorous growth of peach trees was reported by Higgins et al. (1943) to increase their cold hardiness, and hence reduce tree injury. Similarly, supplemental nitrogen fertilization significantly improved tree survival and yield of ‘Elberta’ peach trees grown on a severe shortlife site (Giddens et al. 1972). Hewetson (1953) studied the feasibility of liquid fertilizer in replant peach orchards in Pennsylvania. This report showed t ha t the liquid fertilizer employed had the capacity to produce strong, vigorous peach trees superior to those produced with either ma-

SHORT LIFE, REPLANT PROBLEMS OF DECIDUOlJS FRUIT TREES

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nure or 1O:lO:lO fertilizer. The theory was th a t if replant trees could be made to grow vigorously during their first year in the orchard, they might be expected to continue satisfactorily in subsequent years. Four years later (Hewetson 19571, it was concluded th a t the readily available source of nitrogen gave the trees a rapid early start which quickly established the trees, perhaps in turn helping replants to overcome any inhibiting effect of the trees previously grown in these locations. McCue (1915) believed th at greater differences in peach wood strength could be obtained by different pruning methods than by fertilizer treatments. Stene (1937) reviewed the literature on pruning peach in relation to winter injury. His conclusion varied from no pruning until after defoliation to relatively severe pruning a t the usual time depending on the condition of late growth. Th e preponderance of opinion among present peach researchers favors very light to no pruning a t all until just before foliation time. Fall pruning is potentially a damaging practice which significantly contributes to PTSL (Clayton 1968; Correll et al. 1973; Nesmith and Dowler 1973,1975; Prince and Horton 1972; Weaver et al. 1974), and late winter pruning in February-March has been found to be beneficial to increase tree survival (Clayton 1975a, 1977; Correll et al. 1973; Miller and Dowler 1973).Luepschen and Rohrbach (1969) reported less likelihood of Cytospora canker infection on peach trees under Colorado conditions with pruning delayed until spring. However, Weaver et al. (1974) reported th at in middle Georgia peach trees grown on a n old site were killed by cold injury and bacterial canker, but the trees’ death was not influenced by time of pruning. On a n adjacent new site, early pruning caused susceptibility to cold, showing th a t early pruning is also a predisposing factor to cold and bacterial canker injuries. However, once these problems have set in, the pruning time has no bearing on death or survival of trees. Daniel1 (1973) reported th a t trees growing on old peach site and pruned in fall or early winter had greater mortality than nonpruned or those pruned in spring. Time of pruning, on the other hand, has little or practically no adverse effect on tree longevity when grown on new site. He found no consistent effect of pruning on tree growth.

D. Control of Pathogens Effective control of causal pathogens by chemical means has been very important in helping, a t least partially, alleviate some short-life-related problems. T he specific reasons for the success or failure of different soil treatments used to control SARD are not known, but appear to be related to their efficiency in destroying a wide range of microbial species, presumably the causal organisms (Savory 1966). Steaming soil a t 50°C for one hour reduced bacterial population to about 17% and benefited apple

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replants a t problem sites in Germany (Otto 1972c,d). Pseudomonas syringae infection on apple, pear, cherry, and plum has been satisfactorily controlled by wound treatments with “Santar M” paste and 3 subsequent treatments with 0.2‘,( benomyl or quinolin-fundazol a t budding, early petal fall, and after harvest (Dorozhkin and Griogortsevich 1976).Petersen (1975) reported th at no satisfactory chemical control was known for Ps. syringae canker of peach. The systemic existence of this microorganism explains the lack of effective control from protective bactericides applied to cherry tree surfaces (Cameron 1970). Such approaches as nematicide applications and fumigation with DD in the declining peach orchards in California reduced or completely controlled trees’ death due to bacterial canker (De Vay et al. 1968; English 1961; English et al. 1961). In field trials, copper compounds were only slightly effective against €?seudomonaa mors-prunorum f. sp. persicae, but autumn applications of oxytetracycline and kanamycin a t 1000 ppm each gave good control of the pathogen on peach trees and considerably reduced infection the following spring (Prunier et al. 197313). However, combined applications of copper compounds and antibiotic showed no further improvement over th at of either chemical alone. Very few fungi survived in soils which were fumigated with chloropicrin-methyl bromide (1:l mixture) applied a t 440 kg/ha a n d covered with polyethylene sheeting; but without polyethylene sheets some fungi did survive, especially a t or near the soil surface (Warcup 1976). Wensley (1956) reported similar results from peach replant studies in Ontario. Steaming a t 50°C for one hour almost eliminated microscopic fungi and reduced actinomycetes to 61% (Otto 1972c,d). At 60°C actinomycetes were reduced to 16%, and very few survived a t 70°C. Actinomycetes’ decreasing tolerance to increasing temperatures gave corresponding control of replant diseases (Otto 1972d). De Vay et al. (1967) and Hendrix and Powell (1970a) reported control of peach decline which was associated with reduced soil population of P y t h i u m spp. Of the several test chemicals which demonstrated systemic activity in one or more tests for control of Cytospora cincta invasion of peach trees, Na 2-pyridinethiol, 1-oxide and cycloheximide thiosemicarbazone were outstanding (Helton and Rohrbach 1967).Th e preventive effects were more pronounced than curative effects in all cases. Complete prevention of Cytospora cankers was achieved with cycloheximide thiosemicarbazone, whereas Na 2-pyridinethiol, 1-oxide promoted best healing of infection wounds. Both of these compounds demonstrated curative activity against established canker infections. Chandler (1969) reported th a t a t the Georgia Experiment Station preplant soil fumigation in peach orchards where the original planting was heavily infested with Clitocybe root rot resulted in significantly better tree growth for three years due to control of the fungus.


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Nematodes have not been associated often with short life and replant problems of pome fruits and, therefore, no specific literature is available concerning their control. Nematode control and elimination of reservoirs of infection are crucial to the effective control of PTSL, however (Scotto La Massese et al. 1973).Soil fumigation and nematicide application have been found to be effective against various nematode species in stone fruits (Barker and Clayton 1969, 1973; Chandler 1969; Foster 1960; Foster et al. 1965; Good 1960; Hendrix and Powell 1969; Hung and Jenkins 1969; Mountain and Boyce 1957, 1958b; Nesmith and Dowler 1975; Shannon and Christ 1954; Wehunt and Good 1975; Zehr et al. 1976). The growth and survival of peach trees have been substantially improved by effective nematode control. In New York cherry orchards, control of nematodes by soil fumigation caused increased cold hardiness and tree survival while all trees on non-fumigated soil were dead by the end of the third year (Edgerton and Parker 1958a; Mai and Parker 1967). Nyland and Moller (1973) chemically controlled pear decline using a tetracycline. Of the three basic effects of pear decline, viz., tree collapse, tree decline, and leaf curl, the latter two were prevented by transfusing a solution of oxytetracycline hydrochloride into affected trees. Six to eight quarts of a 100 ppm solution per tree given soon after harvest prevented leaf curl in autumn of the current season and greatly stimulated shoot and spur growth the following season. Two to three annual treatments in the autumn restored previously severely declined trees to a normal or near-normal condition. These tests, involving 75 growers and about 2,000 diseased pear trees, showed that such treatment is feasible. This chemical also inhibited peach rosette symptoms four to five times more effectively than other compounds (Kirkpatrick et al. 1975a). In this case, remission occurred when the chemicals were injected under the bark or into the wood, but not when sprayed on the foliage. The same chemical also has been used as injections in October via previously drilled holes beneath each scaffold limb to effectively control peach X-disease (Sands and Walton 1975). Following injection, the peach X-disease symptoms do not appear in early summer as they do on untreated trees. Treated trees, although still weak from X-disease of the previous year, produce more foliage and fruit, with continued improvement.

E. Miscellaneous Controls Soil sterilization by fumigation or steaming controls several soil-borne problems of fruit crops indirectly by eliminating a wide range of microbial species and nematodes (Bollard 1956; Hendrix and Powell 1969; Otto 1972c,d; Prince et al. 1955; Savory 1966; Warcup 1976; Winkler and Otto 1972; Youngson et al. 1967). Generally, pre-plant fumigation

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is more effective or desirable than post-plant treatments (Hendrix and Powell 1969; Lownsbery et al. 1968; McBeth 1954). McBeth (1954) suggested that the non-phytotoxicity and effectiveness of the fumigant should be given a high practical priority while choosing from various fumigants. Fumigation with methyl bromide (MB) has been reported to increase the effectiveness of liming (Havis 1962), but not to modify the depressing effect of soil toxins on the growth of peach replants which gradually diminishes with the passage of time (Wensley 1956). Ladd et al. (1976) and Beams and Butterfield (1944) have reported a differential decrease in soil enzyme activity and a corresponding increase in the amounts of ninhydrin-reactive chemicals extractable with acidified “Tris” buffer. Fumigation with M B has little effect, if any, on the rate of transpiration; however, since M B is heavier than air, it may cause some wilting by reducing O2 content of the soil by approximately 80% (Beams and Butterfield 1944). Dibromochloropropane (DBCP), a soil fumigant nematicide, is reported to be strongly absorbed by wet organic matter but not by wet clay. Therefore, when DBCP is applied with irrigation water, the depth of nematode control decreases with increasing soil organic matter (Youngson et al. 1967). Further, fumigation does not change the level of any nutrient from a deficiency to a sufficiency status (Aldrich and Martin 1952). 1. Fumigation.-For apple replant diseases, chloropicrin as a fumigant has been found to be a satisfactory control whether the problem is caused by nematodes (Colbran 1953), by microbial organisms (Savory 1967), or by causes still unknown (Hoestra 1967). Chloropicrin obviously eliminates the factor responsible for growth inhibition and tree loss, but it is dangerous, expensive, and unpleasant to handle (Anon. 1966). Several other workers found chloropicrin to be the best pre-plant fumigant for apples in different countries (Jackson 1973; Ross and Crowe 1973, 1976; Ryan 1975a,b; Savory 1966). However, Pitcher et al. (1966) found it more effective for cherries than for apple replants, but i t still proved to be better than other fumigants, including DD and MB. Soil fumigation with either M B or chloropicrin overcomes both non-specific apple replant disease and SARD in Washington state (Benson 1974b; Benson and Covey 1976; Benson et al. 1978). In greenhouse studies in a soil without the arsenic toxic factor, M B fumigation resulted in better growth of apple seedlings, however. For apple, fumigants which release methyl isothiocyanate are rarely of any benefit, nor usually are bromine compounds, although M B and DBCP are sometimes successful (Savory 1966). Pre- and post-plant fumigants play a crucial part in the control of PTSL (Clayton 1968, 1975a,b; English and De Vay 1964; Miller and Dowler 1973), by increasing tree survival (Correll et al. 1973; Nesmith


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and Dowler 1975) and alleviating severe tree losses on problem sites (English 1961; Johnson 1965; Nesmith and Dowler 1973). None of the nine soil fumigants, including M B and DD tested by Hayden et al. (1968), significantly increased peach tree survival, however. Soil fumigation also has been reported to substantially improve peach tree growth, vigor, and cold hardiness (Havis et al. 1958; Nesmith and Dowler 1973, 1975; Prince et al. 1955; Shannon and Christ 1954). Pre-plant soil fumigation reflected a lower index of root-knot nematode, and hence rapid tree growth in initial years (Foster 1960; Foster and Cohoon 1958; Foster et al. 1965,1972; Good 1960). Other nematode specieslike Pratylenchus (De Vay et al. 1967; Mountain and Boyce 1957), Macroposthonia (De Vay et al. 1967; Zehr et al. 1976), and Xiphinema and other nematode vectors of stem pitting virus (Smith and Stouffer 1975) also have been successfully controlled by soil fumigation. However, reports are conflicting concerning effects of fumigation on tree decline or death caused by bacterial canker and/or cold injury (De Vay et al. 1967; Zehr et al. 1976). Soil fumigation also inhibits several fungi in the rhizosphere of the peach replants (Wensley 1956), and improves tree stand on soils heavily infested with Clitocybe (Chandler 1969) and P y t h i u m (De Vay et al. 1967; Hendrix and Powell 1970a) by reducing the population of these fungal species in the soil. In the case of cherry, soil fumigation with chloropicrin proved to be outstanding on old cherry sites (Benson 1974b; Pitcher et al. 1966),but when cherry followed apple it was not so effective (Benson 1974b; Jackson 1973). In New York, sour cherry performed extremely well on old sites which were fumigated with DD (Mai and Parker 1967), while all trees on untreated sites were dead by the end of the third year. The fumigation controlled Pratylenchus penetrans nematode in these cherry plantings and resulted in increased cold hardiness and survival of ‘Montmorency’ cherry (Edgerton and Parker 1958a). For rate of survival, plum trees have been reported to respond better to fumigation with ethylene dibromide than other fumigants (Johnson 1965). 2. Steam Sterilization.-Steam sterilization of soil a t temperatures from 50” to 70°C considerably improved shoot growth of apple seedlings, leading to a complete soil recovery from pathogenic infestation (Otto 1972c, d). Steam sterilization of old soil in pot experiments in New Zealand permitted as much growth of apple on Northern Spy rootstock as in fresh soil, while normal growth on M 12 rootstock on decline soil was further improved following steaming (Bollard 1956). Winkler and Otto (1972) concluded that a higher temperature was necessary to eliminate apple replant problem than to inactivate most nematodes. Prince e t al. (1955) recognized the importance of steaming and fumigation to obtaining greater growth of peach trees in old peach soil; however, much of the

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response they obtained was not well understood. They suggested that results of this kind may not be applicable to all soil types. Hot-waterbath treatment of whole trees showed that thermal death time of peach rosette virus was less than one hour a t 4 5 T , about 18 minutes a t 50“C, and only 2 minutes a t 55°C (KenKnight 1958). 3. Other Methods.-Ketchie and Murren (1972,1976)used cryoprotectants (PVP, glycerol, ethylene glycol, and DMSO) as spray applications, individually or in combination with each other, on whole apple trees in the greenhouse and by terminal feeding to apple and pear trees in the field. In this study, excised apple bark was soaked in cryoprotectant. Freeze protection was tested by artificial as well as natural freezing. The cryoprotectant applications increased both cold hardiness and survival of test plants; however, cultivar variation was experienced. Effects of white latex paint on temperature differences between north and south sides of apple and peach tree trunks have been studied (Eggert 1944; Martsolf et al. 1975). The unpainted trunks showed a difference of 28” to 44°C between north and south sides, while the differences in painted trunks did not exceed 6°C. This kind of amelioration would certainly reduce trunk injury due to cold. Similarly, Bennett (1950) emphasized the beneficial effect of winter shade in lowering cold injury in pear.

V. CONCLUSION

A comprehensive review of the literature clearly reveals that short life and replant problems are seldom of a simple nature, but usually are established through the interaction of several environmental, pathogenic, and/or physiological factors. Control of any one factor does not necessarily overcome the problem, although i t may be an important prerequisite to subsequent effective treatments. In many cases, diagnosis, identification of causal factors, and agreement on nomenclature have proved to be as difficult as actual treatment of the specific problem. Successful alleviation of some problems provides a basis for continued progress. In general, it can be concluded that short life and replant problems reduce expected tree longevity, are not purely pathological in nature, and are usually related to repeated plantings of the same crop on a given site. Problems may be “specific,” involving the same species in succeeding plantings with no obvious causal organism, or “non-specific,” affecting related fruit tree species in the presence of certain causal organisms. Most deciduous fruit trees are affected on a worldwide basis, although the severity, form, symptoms, and time of occurrence may vary. Due to variation in problem severity from year to year and lack of precise data, it is difficult to place a monetary figure on the losses suffered by the fruit


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industry. It also is probable t h a t a number of short life problems remain undiagnosed or mistaken for other similar disorders. Past successes in methodology development, identification of causal factors, and implementation of control measures must be followed by continued progress in many areas. In most cases, it is no longer feasible t o plant new orchards on virgin sites or t o tolerate low tree populations and inefficient management. Therefore, plant improvement through breeding, selection a n d physiological amelioration, along with effective cultural practices a n d pathogen controls, will become increasingly essential to t h e future fruit industry.

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BARKER, K.R. and C.N. CLAYTON. 1973. Nematodes attacking cultivars of peach in North Carolina. J Nematol. 5:265-271. BATZER, L.P. and N.R. BENSON. 1958. Effect of metal chelates in overcoming arsenic toxicity in peach trees. Proc. Amer. SOC. Hort. Sci. 72:74-78. BATZER, L.P. and H. SCHNEIDER. 1960. Relation of pear decline to rootstock and sieve-tube necrosis. Proc. Amer. SOC. Hort. Sci. 76:85-97. BEAMS, G.H. and N.W. BUTTERFIELD. 1944. Some physiological effects of methyl bromide upon horticultural plants. Proc. Amer. SOC. Hort. Sci. 45:318-322. BEATTIE, D.J. and H.L. FLINT. 1973. Effect of K level on frost hardiness of stems of Forsythia. J Amer. SOC. Hort. Sci. 98:539-541. BEATTIE, J.M. 1962. An evaluation of the apple decline problem. Proc. Ohio State Hort. SOC.115:139-144. BEATTIE, J.M., C.W. DONOHO, JR., E.S. BANTA, and F. QUINN. 1963. Horticultural aspects of the apple decline problem. Proc. Ohio State Hort. SOC.116:83-91. BELL, H.K. and N.F. CHILDERS. 1956. Effect of manganese and soil culture on the growth and yield of the peach. Proc. Amer. SOC. Hort. Sci. 67:130-138. BENNETT, J.P. 1950. Temperature and bud rest period. Effect of temperature and exposure on the rest period of deciduous plant leaf buds investigated. Calif. Agr. 4:ll-16. BENSON, N.R. 1974a. Apple replant problem in Washington State-Effect of soil arsenate. HortScience 9290 (Abstr.). BENSON, N.R. 1974b. Apple replant problem in Washington State-Effect of soil fumigation. HortScience 9:290 (Abstr.). BENSON, N.R. and R.P. COVEY, JR. 1976. Specific apple replant disease (SARD) in Washington State. HortScience 11:331 (Abstr.). BENSON, N.R., R.P. COVEY, JR., and W. HAGLUND. 1978. The apple replant problem in Washington State. J. Amer. SOC. Hort. Sci. 103:156-158. BERGMAN, H.F. 1959. Oxygen deficiency as a cause of disease in plants. Bot. Rev. 25:417-485. BERNSTEIN, L., J.W. BROWN, and H.E. HAYWARD. 1956. The influence of rootstock on growth and salt accumulation in stonefruit trees and almonds. Proc. Amer. SOC. Hort. Sci. 68236-95. BIESBROCK, J.A. and F.F. HENDRIX, JR. 1970. Influence of soil water and temperature on root necrosis of peach caused by Pythium spp. Phytopathology 60:880-882. BIRD, G.W. 1968. Orchard replant problems. Can. Dept. Agr. Publ. 1375. BLAKE, M.A. 1928. Some serious weak points in field nutrition studies with Hort. Sci. 25:350-353. peaches. Proc. Amer. SOC. BLAKE, M.A. 1935. Types of varietal hardiness in the peach. Proc. Amer. SOC. Hort. Sci. 33:240-244.


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BLAKE, M.A. 1938. Hardy rootstocks for the peach should extend well above the surface of the soil. Proc. Amer. SOC. Hort. Sci. 36:138-140. BLATTNY, C. and V. VANA. 1974. Pear decline accompanied with mycoplasma-like organisms in Czechoslovakia. Biol. Plant. 16:474-475. BLODGETT, E.C. 1976. Why cherry trees die. Wash. State Uniu., Coop. Ext. Seru., Pullman. Ext. Bul. 668. BLODGETT, E.C., H. SCHNEIDER, and M.D. AICHELE. 1962. Behavior of pear decline on different stock-scion combinations. Phytopathology 52: 679-684. BLOMMAERT, K.L.J. 1955. The significance of auxins and growth-inhibiting substances in relation to winter dormancy of the peach tree. W. P. Fruit Res. Sta., Stellenbosch, Union of South Africa Sci. Bul. 368. BLOMMAERT, K.L.J. 1959. Winter temp in relation to dormancy and the auxin and growth-inhibitor content in peach buds. South African J. Agr. Sci. 2:507-514. BOLLARD, E.G. 1956. Effect of steam-sterilized soil on growth of replant apple trees. New Zealand J. Sci. Technol. 38:412-415. BORNER, H. 1959. The apple replant problem. I. The excretion of phlorizin from apple root residues. Contrib. Boyce Thompson Inst. P l a n t Res. 20:39-56. BORZAKOVSKA, I.V., I.M. SHAITAN, N.P. LEVCHENKO, and T.P. T E RESHCHENKO. 1975. Biochemical aspects of the rootstock effect on graft in relation to winter hardiness of the apple trees (English translation). Ukr. Bot. Zh. 32:708-716. BOTTINI, R., G.A. DE BOTTINI, and N.S. CORREA. 1976. Changes in the levels of growth inhibitors and GA-like substances during dormancy of peach flower buds. 11. Phyton 34:157-167. BOWEN, H.H. 1971. Breeding peaches for warm climates. HortScience 6: 153-157. BOYNTON, D. and L.C. ANDERSON. 1956. Some effects of mulching, N fertilization, and liming on McIntosh apple trees, and the soil under them. Proc. Amer. SOC. Hort. Sci. 67:26-36. BRIERLEY, W.G. 1947. Winter hardiness complex in deciduous woody plants. Proc. Amer. SOC. Hort. Sci. 5O:lO-16. BROWN, D.S. 1943. A report of injury by cold weather to peach trees in Illinois during the winter of 1941 and 1942. Proc. Amer. SOC. Hort. Sci. 42:298-300. BROWN, D.S. and F.A. KOTOB. 1957. Growth of flower buds of apricot, peach, and pear during the rest period. Proc. Amer. SOC. Hort. Sci. 69:158164. BROWN, G.N. and J.A. BIXBY. 1975. Soluble and insoluble protein patterns during induction of freezing tolerance in black locust seedlings. Physiol. Plant. 34:539-541.

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WEAVER, D.J. and E.J. WEHUNT. 1975. Effect of soil p H on susceptibility of peach to Pseudomonas syringae. Phytopathology 65:984-989. WEAVER, D.J., E.J. WEHUNT, and W.M. DOWLER. 1974. Association of tree site, Pseudomonas syringae, Criconemoides xenoplax, and pruning date with short life of peach trees in Georgia. P l a n t Dis. Rptr. 58:76-79. WEAVER, G.M. 1963. Influence of rootstocks on susceptibility of peach to peach canker. Fruit Var. & Hort. Dig 17:43-44. WEHUNT, E.J. and .J.M. GOOD. 1975. Nematodes on peaches. p. 377-387. In N.F. Childers (ed.) The peach. Horticultural Publications, Rutgers-The State University, New Brunswick, N.J. WEINBERGER, J.H. 1949. Field notes on cold injury taken a t the USDA Hort. Field Station, Fort Valley, Ga. (unpublished). WEINBERGER, J.H. 1950a. Chilling requirement of peach varieties. Proc. Amer. Soc. Hort. Sci. 56:122-128. WEINBERGER, J.H. 1950b. Prolonged dormancy of peaches. Proc. Amer. SOC. Hort. Sci. 56:129-133. WEINBERGER, J.H. 1952. Winter injury to peach trees on Yunnan stock. P l a n t Dis. Rptr. 361307-308. WEINBERGER, J.H. 1967. Some temperature relations in natural breaking of the rest of peach flower buds in the San Joaquin Valley, California. Proc. Hort. Sci. 91534-89. Amer. SOC. WEINBERGER, J.H. 1969. The stimulation of dormant peach buds by a cytokinin. HortScience 4:125-126. WELSH, M.F. and L.P.S. SPANGELO. 1971. Necrotic stem pitting and decline, two bud-transmissable diseases affecting Malus robusta No. 5 apple rootstocks in British Columbia but not in Eastern Canada. Phytoprotection 52:58-67. WENSLEY, R.N. 1956. The peach replant problem in Ontario. IV. Fungi associated with replant failure and their importance in fumigated and nonfumigated soils. Can. J. Bot. 34:967-981. WENSLEY, R.N. 1966. Rate of healing and its relation to canker of peach. Can. J . P l a n t Sci. 46:251-264. WENSLEY, R.N. 1970. Innate resistance of peach to perennial canker. Can. J. P l a n t Sci. 50:339-343. WESTWOOD, M.N. 1970. Rootstock-scion relationships in hardiness of deciduous fruit trees. HortScience 5:418-421. WESTWOOD, M.N. 1976. Inheritance of pear decline resistance. Fruit Var. J. 30:63-64. WESTWOOD, M.N. and H.R. CAMERON. 1978. Environment-induced remission of pear decline symptoms. P l a n t Dis. Rptr. 62:176-179. WESTWOOD, M.N., H.R. CAMERON, P.B. LOMBARD, and C.B. CORDY. 1971. Effect of trunk and rootstock on decline, growth, and performance of Hort. Sci. 96:147-150. pear. J. Amer. SOC.


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WESTWOOD, M.N., M.H. CHAPLIN, and A.N. ROBERTS. 1973. Effects of rootstock on growth, bloom, yield, maturity, and fruit quality of prune ( P r u n u s domestica L.). J Amer. Soc. Hort. Sci. 98:352-357. WESTWOOD, M.N. and P.B. LOMBARD. 1977. Pear rootstock and Pyrus research in Oregon. Acta Hort. 69:117-122. WHEELER, H. and H.H. LUKE. 1963. Microbial toxins in plant diseases. Annu. Reu. Microbiol. 17:223-242. WIEST, S.C., G.L. GOOD, and P.L. STEPONKUS. 1976. Evaluation of root viability following freezing by the release of ninhydrin-reactive compounds. HortScience 11:197-199. WILBUR, W., D.E. MUNNECKE, and E.F. DARLEY. 1972. Seasonal development of Armillaria root rot of peach as influenced by fungal isolates. Phytopa t hol ogy 62 :567-570. WILCOX, A.N. 1936. Material for the breeding of winter hardy pears. Proc. Amer. Soc. Hort. Sci. 34:13-15. WILDUNG, D.K., H.M. PELLETT, and C.J. WEISER. 1972a. The relationship of soil temperature and moisture to the hardening of apple roots. HortScience 7:335 (Abstr.). WILDUNG, D.K., H.M. PELLETT, and C.J. WEISER. 1972b. Factors influencing the cold hardiness of clonal apple rootstock. HortScience 7:335 (Abstr.). WILDUNG, D.K., C.J. WEISER, and H.M. PELLETT. 1973. Temperature and moisture effects on hardening of apple roots. HortScience 8:53-55. WILLIAMS, M.W. and J.T. RAESE. 1974. Sorbitol in tracheal sap of apple as related to temperature. Physiol. Plant. 30:49-52. WILNER, J. 1959. Note on an electrolytic procedure for differentiating between frost injury of roots and shoots in woody plants. Can. J. P l a n t Sci. 39:512-513. WILNER, J. 1960. Relative and absolute electrolytic conductance tests for frost hardiness of apple varieties. Can. J. P l a n t Sci. 40:630-637. WILNER, J. 1961. Relationship between certain methods and procedures of testing for winter injury of outdoor exposed shoots and roots of apple trees. Can. J P l a n t Sci. 41:309-315. WILNER, J. 1965. The influence of maternal parent on frost-hardiness of apple progenies. Can. J. P l a n t Sci. 45:67-71. WILNER, J., W. KALBFLEISH, and W.J. MASON. 1960. Note on two electrolytic methods and procedures of testing for winter injury of outdoor shoots and roots of apple trees. Can. J. P l a n t Sci. 40:563-565. WINKLEPLECK, R.L. and J.A. MCCLINTOCK. 1939. The relative cold resistance of some species of P r u n u s used as rootstocks. Proc. Amer. SOC. Hort. Sci. 3 7 :324-326. WINKLER, H. and G. OTTO. 1972. Investigations on the cause of replant disease in fruit trees. IV. Effect of various steaming temperatures on motile

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forms of nematodes in replant diseased soil. Zentralb. Bakteriol. Parasitenk., Infektionskrank. Hygiene 127:783-788. WOODBRIDGE, C.G. and A.M. LASHEEN. 1960. The nutrient status of normal and decline Bartlett pear trees in Yakima Valley in Washington. Proc. Amer. SOC.Hort. Sci. 75:93-99. YABLONSKII, E.A. 1975. Dynamics of phosphorus-containing substances and winter hardiness of amygdalaceous fruit cultures. Soviet Plant Physiol. 22: 88 1-886. YADAVA, U.L. and S.L. DOUD. 1977. Effect of exogenous phytohormones and rootstocks on budbreak and growth of peach ( P r u n u s persica (L.) Batsch). Proc. 4th Annu. Mtg. Plant Growth Reg. Work. Grp., Hot Springs, Ark., Aug. 9-11, 1977, p. 252-257. Plant Growth Regulator Working Group, Longmont, Colo. YADAVA, U.L. and S.L. DOUD. 1978a. Effect of peach seedling rootstocks and orchard sites on cold hardiness and survival of peach. J. Amer. SOC. Hort. Sci. 103:321-323. YADAVA, U.L. and S.L. DOUD. 1978b. Effect of rootstock on the bark thickness of peach scions. HortScience 13:538-539. YADAVA, U.L., S.L. DOUD, and D.J. WEAVER. 1978. Evaluation of different methods to assess cold hardiness of peach trees. J. Amer. SOC.Hort. Sci. 103 :3 18-3 2 1. YADAVA, U.L., S.L. DOUD, D.J. WEAVER, and J.H. EDWARDS. 1979. Evaluation of laboratory and field methods to assess cold hardiness and survival of peach trees under natural freezing conditions. J. Amer. SOC. Hort. Sci. (in press). YELENOSKY, G. 1975. Cold hardening in citrus stems. Plant Physiol. 56: 540-543. YOSHIDA, S . and A. SAKAI. 1974. Phospholipid degradation in frozen plant cells associated with freezing injury. Plant Physiol. 53:509-511. YOUNGSON, C.R., C.A.I. GORING, and R.L. NOVEROSKE. 1967. Laboratory and greenhouse studies on the application of fumazone in water to soil for control of nematodes. Down to Earth 23:27-32. ZEHR, E.I., R.W. MILLER, and F.H. SMITH. 1976. Soil fumigation and peach rootstocks for protection against peach tree short life. Phytopathology 6 6 :6 89 - 6 9 4.


2 Seed Viability During Long-Term Storage L. N. Bass USDA-SEA-AR National Seed Storage Laboratory, Fort Collins, Colorado 80523 I. 11. 111. IV. V. VI.

Introduction 117 118 Effects of Species and Cultivar Effects of Production Conditions 119 Effects of Seed Maturity 119 Effects of Harvesting and Processing 123 Effects of Storage Environment 124 A. Seed Moisture Content 125 B. Temperature 132 C. Controlled Atmosphere Storage 134 VII. Changes During Storage 135 A. Biochemical Changes 135 B. Cytological Changes 135 VIII. Summary 136 IX. Literature Cited 137

I. INTRODUCTION The need to store seeds for long periods of time arose with the development of improved agricultural practices and plant breeding programs. In recent years, much research has been devoted to the storage requirements of various kinds of seeds. Justice and Bass (1978) reviewed in depth the research that had been conducted prior to 1973. Other recent reviews on seed storage include those by Bass (1973a1, Harrington (1972), and Roberts (1972). Therefore, this paper deals principally with more recent literature. For commercial purposes, seeds usually are not stored for more than one or two years, but even for such a short period special conditions are required to maintain viability in some climatic 117

118


regions. For plant breeding purposes, small samples of many kinds of seeds must be preserved indefinitely, and maintaining viability requires a good knowledge of the storage requirements for each kind. 11. EFFECTS OF SPECIES AND CULTIVAR

Variations in seed longevity among species have been extensively documented. Harrington (1972) summarized the data available for species with (a) short-lived seeds and (b) seed longevity of ten years or more. Among the former are various aquatic, nut tree, and tropical species, and seed longevity varies widely. Some seeds may remain viable for only a few days or weeks, but others may remain viable for a few years. Many short-lived seeds cannot be dried to a low moisture content or be subjected to temperatures below freezing. Consequently, longevity of such seeds cannot be increased by conventional storage methods. However, seeds of some species which are short-lived under natural conditions can be stored for several years when carefully dried and stored under a low temperature and relative humidity or in a sealed moistureproof container . Long-lived seeds survive best under low temperature and low moisture storage conditions or in the soil under conditions unfavorable for germination. Seeds of some species have a long life under both conditions. Seeds of most temperate zone crops can be stored for more than ten years under conditions of low temperature and low relative humidity. Most seeds recorded as having survived for more than 100 years are of species which develop hard seeds. Because their seedcoats are impermeable to water, hard seeds do not fluctuate in moisture content with changes in atmospheric relative humidity as do seeds with permeable coats. Seeds of the following genera are known to have survived for 100 years or more (Harrington 1972): Albizia 147 years (Ramsbottom 1942), Cassia 158 years (Becquerel 1934), Goodia 105 years (Ewart 1908), and Trifoliurn 100 years (Youngman 1952). Seeds known to have survived for over 500 years are hard seeded. Examples include Canna (Anon. 19681, Lotus (Ohga 1923; Arnold and Libby 1951; Chaney 1951; Wester 1973), and Lupinus (Porsild et al. 1967). All have high percentages of hard seeds. Although barley does not have hard seeds, a sample survived 123 years of storage in a sealed glass tube at uncontrolled temperatures (Aufhammer and Simon 1957). Documentation of differences in seed longevity among cultivars of the same species is limited. ‘Oderbrucker’ barley seeds retained their viability during storage better than did seeds of other cultivars (Shands et al. 1967). Seeds of ‘Black Valentine’ bean stored better than did seeds of ‘Brittle Wax’ (Toole and Toole 1954).

SEED VIABILITY DURING LONG-TERM STORAGE

119

Seed longevity differed significantly among cultivars of bean, tomato, cucumber, pea, sweet corn, watermelon, and papaya (James et al. 1967; Bass 197313) (Table 2.1). Seeds of 27 muskmelon cultivars varied in their keeping quality within and among storage conditions over a 12-year period. Seeds of some genera retain their viability much longer than others during storage, and because of variations in keeping quality among cultivars (Table 2.1) germination of stored seeds must be monitored regularly. 111. EFFECTS OF PRODUCTION CONDITIONS Austin (1972) found no literature that related preharvest factors to the longevity of stored seeds. Weather probably is the most important production factor affecting seed quality. Rains just before harvest can cause wheat to sprout in the head (Moss et al. 1972) and cause delays in harvesting which create problems with seedborne fungi. Damp weather promotes the growth of fungi on seeds in the field. We have observed that seeds with a heavy mold population lose viability more rapidly during storage than seeds that are mold-free. Extremely dry conditions before harvest can cause seeds to be too dry, making them more susceptible to mechanical damage. Damaged seeds deteriorate more rapidly in storage than do undamaged seeds (Moore 1972). However, irrigation regime during production had little effect on bean seed longevity (Table 2.2). IV. EFFECTS OF SEED MATURITY

Seed maturity is the point in development a t which maximum dry weight is attained (Harrington 1972; Roberts 1972). Most plants flower and produce seeds over several days, weeks, or months; consequently, not all seeds on a plant mature a t the same time. Commercial seeds are harvested when the greatest yield of mature seeds can be obtained. Thus, each lot will contain both immature and mature seeds. Immature seeds usually do not store as well as mature seeds, and the viability of seeds left for some time on the plant before harvest may decline, depending upon weather conditions preceding harvest. Harvesting too soon results in excessive quantities of immature seeds, and harvesting too late results in decreased yield caused by shattering. Very few studies have addressed the question, “HOWwell do immature seeds store?” Most studies involving seed maturity have considered either yield or seedling vigor, not longevity in storage. Eguchi and Yamada (1958) harvested cabbage, carrot, Chinese cabbage, cucumber, edible burdock, eggplant, Japanese radish, pumpkin, tomato, watermel-

120


TABLE 2.1. PERCENTAGE OF GERMINATION OF BEANS, PEAS, SWEET CORN, CUCUMBER, WATERMELON, TOMATO, AND PAPAYA SEEDS WHEN STORED AND AFTER 8 OR 9 YEARS' STORAGE AT 10°C AND 50% RH

Germination ("lo) Cultivar Tomato Bonny Best Break O'Day Clarks Special Early Crack Proof Dwarf Champion Earliana Garden State Improved GrothensGlobeStrain 2 Homestead 2 Indiana Baltimore Livingston's Globe Marglobe Moscow Oxheart Pearson Improved Pinkshipper Purdue 1361 Rutgers San Marzano Sioux Small Fruited Red Cherry Stone Sunray Urbana Valiant Wisconsin 55 ~

Bean Alabama 1 Black Valentine Blue Lake Blue Ribbon Pak Brittle Wax Contender Corneli 14 Commodore Imp. Dwarf Horticultural Ex tender Genuine Cornfield Golden Wax Topnotch Kentucky Wonder Kinghorn Wax Lazy Wife McCaslin Seminole Slendergreen Sulfur Tendergreen Tendergreen Improved Tennessee Green Pod Top Crop White Half Runner White Kentucky Wonder 191

Initial

9 Year

93 95 94 95 84 92 94 90 88 76 92 89 96 88 90 84 87 96 95

90 97 89

81 89 96 94 90 78 95 95 94 89 .~ 90 82 87 90 96

88 90 94

84 85 92

91 ._

Meanchange Range of change 99 94 79 94 85 99 84 90 93 99 97 75 97 98 80 96 89 98 91 96 98 97 90 96 89

98 90 74 97 79 99 79 78 91 93 94 86 95 92 79 70 93 89 77 91 90 94 80 93 77

Meanchange Range of change

Change % -3 +2 -5 -4 -3 -3 +2 +4 +2 +2 +3 +6 -2 +1 0 -2 0 -6 0 -2 -4 +1 -1 -5 -5 -2 -0.9 +6 to -6

-1 -4 -5 +3 -6 0 -5 -12 -2 -6 -3 +11 -2 -6 -1 -26

+4

-9 -14 -5 -8 -3 -10 -3 -12

-5 t11 to-26


121

TABLE 2.1. (Continued)

Germination ("lo) Cultivar Peas Alaska Alderman Ameer American Wonder Bliss Everbearing Creole Dwarf Alderman Dwarf Grey Sugar Early Perfection First and Best Garden Alaska Glacier Gradus Improved Hundred Fold Laxton Progress Lincoln Little Marvel Laxtons Superb New Era Perfection Premier Pride ~ ~

.

~

Thomas Laxton Wando

.

Initial

9 Year

98 88 87 84 93 97 83 99 96 77 98 88 91 96 89 95 89 81 91 91 88 96 96 97

98 85 81 81 81 92 68 99 91 74 93 77 71 92 85 95 90 79 86 90 84 95 91 99

Mean change Range of change Sweet corn Calumet Country Gentleman Country Gentleman H brid Golden Bantam (8row7 Golden Cross Golden Early Market Golden Cross Bantam Goldrush Iochief Keystone Evergreen Hybrid Seneca Chief Spancross Superchief Sweetangold Tempo Barbecue

98 81 93 95 94 98 99 93 99 96 82 99 94 95 96 81

99 78 95 93 93 96 99 93 99 96 67 99 98 90 96 91

Mean change Range of change

Cucumber A&C Ashley Black Diamond Chinese Snake Crystal Apple Cubit Early Cluster Early Fortune

96 93 94

99 ..

95 93 99 94

95 91 92 95 83 95 97 91 ~~

Change % 0 -3 -6 -3 -12 -5 -15 0 -5 -3 -5 -11 -20 -4 -4 0 +1 -2 -5 -1 -4 -1 -5 +2 -4.6 +2 to -20 +1 -3 +2 -2 -1 -2 0 0

0 0 -15 0 +4 -5 0 +10 -0.7 +10 to-15 -1 -2 -2 -4 -12 +2 -2 -3

122


TABLE 2.1. (Confinued)

Germination ( ( k ) Cultivar Improved Long Green Improved Long Green Special Long Japanese Climbing Lemon Marketer Model Niagra Ohio M.R. 17 Palomar Staysgreen Stone Straight Eight Vaughan West Indian Gerkin White Wonder Wisconsin SMR 12

Initial

9 Year

95 99 99 99 98 95 98 96 99 89 99 97 90 97 97 98

96 95 93 95 98 95 96 93 98 77 97 99 89 94 95 96

Mean change Range of change Watermelon Black Diamond Blacklee WR Blackstone Calhoun Sweet Charleston Grey Congo Dixie Queen Florida Giant Garrisonian Golden Honey Harris’ Earliest Irish Grey Kleckley’s Sweet 6 Klondike Striped 11 New Hampshire Midget Peacock WR-50 Purdue Hawkesbury Striped Klondike Blue Ribbon Sugar Baby Summit Tendersweet Tom Watson White Hope Chilean Black Seeded

94 95 84 99 86 93 87 90 82 89 93 95 97 92 56 89 95 95 98 96 92 96 96 91

94 90 75 92 84 90 86 82 85 81 94 91 93 96 51 81 95 89 98 84 91 94 89 90

Mean change Range of change Papaya PR 6-65 PR 7-65 PR 8-65 P R 9-65 PR 10-65 S 64

78 71 89 48 55 66

44 46 36 50 26 70

Mean change Range of change

Chanae % +1 -4 -6 -4 0 0 -2 -3 -1 -12 -2 +2 -1 -3 -2 -2

-2.6 +2to-12 0 -5 -9 -7 -2 -3 -1 -8 +3 -8 +1 -4 -4 +4 -5 -8 0 -6 0 -12 -1 -2 -7 -1 -3.5 +4 to -12 -34 -25 -53 +2 -29 +4 -23.5 4-4 to -53


123

TABLE 2.2. PERCENTAGE OF GERMINATION OF WADE AND TOP CROP BEAN SEEDS PRODUCED UNDER FOUR IRRIGATION REGIMES, WHEN STORED AND AFTER 15 YEARS AT 5°C AND 40% RH

Germination (%I

Cultivar Wade

TopCrop

Irrigation Regime Wet Wet Wet Dry Dry Wet Dry Dry Wet Wet Dry Dry

Wet Dry Wet Dry

When Stored 93 93 93 93

After 15 Years 92 89 89 93 Mean change Range of change

94 94 93 93

92 92 92 90 Mean change Range of change

56 Change -1 -4

-4 0 -2 0 to -4 -2 -2

-1

-3 -2 - 1 to -3

on, and Welsh onion seeds a t three- to seven-day intervals, giving four to six stages of maturity over two or three years. For all kinds except Chinese cabbage, pumpkin, and tomato, mature seeds retained viability better than did immature seeds. The same was true for Kentucky bluegrass when both immature and mature seeds were stored under the same conditions (Bass 1965). After 93 months a t 2°C and 70% relative humidity (RH), mature seeds germinated 53%, immature seeds only 15%; a t 32°C and 15% RH, mature seeds germinated 81%and immature seeds 59%. When fruits of butternut squash were stored for several months, germination improved with time of storage of the fruits (Young 1949; Holmes 1953). However, the seeds were not stored after removal from the fruits. Van Staden (1978a) found that Protea neriifolia L. seeds were mature about seven months after fertilization. At that time, germination was 93%. Leaving the seeds in the influorescences longer resulted in decreased viability. Germination was 86% and 40% when harvested 1 and 5 months after maturity. For best quality, seeds generally should be harvested as soon as they reach full maturity. Because of the lack of uniform maturity within a field, seeds must be harvested when the best germination and yield can be obtained. Such seed should have the maximum storage potential under any given storage conditions. Austin (1972) discussed seed maturity and seed size jointly, as such seeds have the same problems in storage. V. EFFECTS OF HARVESTING AND PROCESSING

Harvesting practices can cause seeds to lose viability and vigor. Damage to seeds during harvest is influenced by factors such as seed size and

124


shape, firmness of the seedcoat, difficulty of extraction from the head or pod, and seed moisture content. Seeds of grasses which are harvested with the glumes still surrounding the caryopsis or grain usually are not as subject to damage as are seeds with the glumes removed. Damage to seeds like peas and beans increases or decreases as seed and pod moisture increases or decreases. Very dry seeds damage easily. Some kinds of seeds such as corn are harvested a t a moisture content too high for safe storage and are promptly and carefully dried to a safe moisture content. Drying too slowly can permit heat accumulation, resulting in decreased viability. Drying too rapidly or a t too high a temperature can damage seeds and reduce storage life. Most horticultural seeds should not be dried a t a temperature above 38°C. Adequate movement of air through the seeds is essential whether drying is done by heated or unheated air (Justice and Bass 1978). Small samples of seeds can be dried by storage with CaClz or CaCo. Seed lots directly from the field usually contain varying amounts of extraneous materials such as pieces of leaves and stems, broken seeds, immature seeds, insect parts, and other materials which may have a high moisture content or moisture holding capacity. For longest seed life, such extraneous materials must be removed before storage (Justice and Bass 1978). Dry seeds damage easily with rough handling (Dorrell and Adams 1969). Therefore, cleaning equipment and handling procedures must be properly adjusted for each kind of seed.

VI. EFFECTS OF STORAGE ENVIRONMENT The effects of environmental factors on seed longevity are interrelated, and therefore difficult to discuss separately. Seed moisture content and storage temperature are the major environmental factors affecting the preservation of stored seed, with seed moisture content usually more critical than temperature (Owen 1956; Barton 1961; James 1967; Bass 1973a; Justice and Bass 1978). For most seeds that can be dried to a low moisture content without damage, the lower the moisture content and storage temperature, the longer the viability. The third major factor in seed survival is oxygen partial pressure. Literature on the effect of partial pressure of oxygen is not extensive, and data from some of the earlier work (Owen 1956; Roberts 1961; Touzard 1961) were contradictory. But later work (Roberts et al. 1967; Roberts and Abdalla 1968) has shown that the higher the oxygen concentration, the shorter the life span of seeds of most species studied.


125

A. Seed Moisture Content Because earlier work has been thoroughly reviewed (Owen 1956; Barton 1961; James 1967; Kozlowski 1972; Roberts 1972; Heydecker 1973; Bass 1973a; Justice and Bass 1978), discussions here will be confined to more recent work. Some kinds of seeds cannot be dried to a low moisture content without loss of viability; others can be dried with a desiccant or by natural means but not with heated air; still others can be dried very rapidly without loss of viability. Most common crop seeds can be rapidly dried with heated air (Justice and Bass 1978). To illustrate the effect of drying method on seed viability, Japanese pear seeds dried to 3 to 5% moisture content in air or by silica gel showed no loss in germination as a result of drying; but vacuum dried seeds lost viability rapidly (Omura et al. 1978). Seed moisture content can be controlled by controlling the relative humidity of the storage area or by drying the seeds to the desired moisture content and storing them in sealed moistureproof containers (Harrington 1972; Bass 1973a; Justice and Bass 1978). Each kind of seed attains a moisture content in equilibrium with the relative humidity of the surrounding atmosphere. Seed equilibrium moisture content varies with both relative humidity and temperature (Table 2.3). Humidity control methods, drying methods, and protective packaging were reviewed by Justice and Bass (1978). Relative humidity in a seed storage facility is usually controlled by either a refrigeration-type or a desiccant-type dehumidifier. The refrigeration-type system takes the moisture out of the air as it passes over the cooling coil. With this type of system, the air is cooled to a temperature colder than that desired, then rewarmed, thereby reducing the RH. The desiccant-type humidity control system utilizes a liquid or solid desiccant to absorb moisture vapor from the air and later eject it from the room. Desiccant dehumidifiers usually use dry chemicals for small TABLE 2.3. EQUILIBRIUM MOISTURE CONTENT OF CRIMSON CLOVER SEEDS AT VARIOUS TEMPERATURES AND RELATIVE HUMIDITIES

Tem erature

RH

10

50

FC)

21

32

(%I ~~

70 90 50 I0 90 50 I0

Equilibrium Moisture (%)

9.4

12.5 14.0 8.7 14.1 17.6

1.7

9.4

126


systems and salt solutions for large systems. The dehumidifier incorporates one or two beds of granular silica gel or activated alumina. In a two-bed system, the air is circulated through one bed while the other is being dried out. In a rotary one-bed system, the air passes through part of the bed while the other part is being dried out (Justice and Bass 1978). Hermetically sealed metal cans and glass jars are impervious to moisture vapor and gases, as are containers made of some flexible materials. Studies carried out in our laboratory to determine the protective value of various heat-sealable materials indicated that only those materials which included a foil layer at least 0.35 mil thick provided moisture protection equivalent to that provided by a hermetically sealed metal can. For storage either in a moistureproof heat-sealable container (Bass and Clark 1974; Clark and Bass 1975) or a sealed metal can (Basset al. 1962, 1963a,b; Bass 1978; Bass and Stanwood 1978) seed moisture content must be low enough to be safe a t the highest temperature to which the seeds might be exposed. For example, 4% moisture content lettuce seeds which initially germinated 97%, germinated 85% after 19 years a t 21°C but declined to 0% at 32°C. In fact, seeds held a t 32°C germinated only 36% after just 8 years of storage. Seeds which contained 7% and 10% moisture lost all viability in less than one year a t 32°C but germinated 94% and 96%, respectively, after 19 years a t -12°C (Table 2.4). Not all seeds require the same moisture content a t a given temperature. Germination of 10% moisture content sorghum seeds was 91% when sealed. Germination was 43% after 5 years' and 0% after 8 years' storage a t 32"C, but was 59% after 16 years' storage a t 21°C (Bass and Stanwood 1978). Other kinds of seeds showed different responses. Therefore, one must know the maximum temperature to which the seeds will be exposed during storage and also the anticipated length of storage. Although there are lot and species differences in safe moisture content for sealed storage, most kinds of seeds which can be dried will retain their viability well for a t least 5 years a t temperatures as high as 21"C, provided their moisture content does not exceed 4 or 5%. Woodstock et al. (1976) found that freeze-dried onion, pepper, and parsley seeds retained their viability better than did seeds that were not freeze-dried when stored a t 40°C and 50°C. At 50°C, non freeze-dried seeds of onion, pepper, and parsley lost their viability in less than 3 to 6 months. Freeze-dried seeds of onion and parsley germinated 61% and 74%, respectively, after 12 months a t 50°C. Freeze-dried pepper seeds produced 77% abnormal seedlings and no normal seedlings. For seeds stored a t 21" to 25"C, only freeze-dried pepper seeds germinated significantly better than control seeds after 12 months' storage. Length of freeze-drying (one, two, or four days) had little effect on germination. However, moisture content when freezedried may have a significant effect on response. Seed moisture content


127

TABLE 2.4. PERCENTAGE OF GERMINATION OF LETTUCE SEEDS STORED AT APPROXIMATELY 4, 7, AND 10% MOISTURE CONTENT AT FIVE TEMPERATURES IN SEALED METAL CANS

Temperature ("C) - 12 - 1

10

21 32

Seed Moisture (96) 4 7 10 4 7 10 4 7 10 4 7 10 4 7 10

Initial Germination (96) 97 97 95 97 97 95 97 97 95 97 97 95 97 97 95

Germination (%I Years in Storage

1 96 94 92 97 92 92 95 ~~

9.1 ..

91 94 94 11 92 0

0

4 96 95 93 94 97 94 93 94 .. 21 89 4 0 90 0 0 ~~

8 96 98 ~. 94 90 76 91 90 9.1 ..

0 90 0 0 36 0 0

19 93 94 96 93 1 18 90 0 0 85 0 0 0 0 0 ~~

may need to be reduced to 8 or 9% by conventional drying before seeds can be freeze-dried safely (Woodstock et al. 1976). Seeds from the study reported by Woodstock et al. (1976) were stored in the National Seed Storage Laboratory a t 21", -12", and -70°C with only minor differences in germination of the freeze-dried and the non freeze-dried seeds after 6 years in storage (unpublished data). Seeds of Cheiranthus cheiri L. (wallflower), Vinca rosea L., and Salvia splendens F. Sellow ex Roem. and Schult. (scarlet sage) retained viability best a t a high relative humidity and low temperature and lost it most rapidly a t a low relative humidity (Nakamura 1975). Resistance to excessive drying varied among species. In vegetables, pea (Pisum sativum LJ, garden bean (Phaseolus uulgaris LJ, and broad bean (Vicia faba L.), and in flowers, pansy (Viola tricolor LJ, California poppy (Eschscholzia California Cham.), and Mexican firebush (Euphorbia heterophylla L.) showed least resistance to excess drying (Nakamura 1975). Perilla (Perilla acymaides L. var. crispa Benth) seeds, known to be short-lived, lost viability very rapidly when dried to 2.1% moisture content. Both controlled seed moisture content and a low temperature were required for maximum storage life of perilla seeds. Wallflower seeds stored open a t 5°C had 20% higher germination after 6 years' storage than seeds stored a t 5°C and room temperature (20" to 30°C summer and 3" to 8°C winter) sealed with CaClz and 60% higher than seeds sealed with CaO a t room temperature. Germination of Vinca rosea L. seeds stored 6 years open a t 5°C was 17% higher than for seeds sealed over CaClz and 13% and 81%

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higher than for seeds a t room temperature sealed over CaC12 and CaO, respectively (Nakamura 1975). Germination of scarlet sage seeds stored 6 years open a t 5°C was 19% higher than that of seeds sealed over CaClz.Seeds sealed over CaO germinated 4% after 2 years a t room temperature (Nakamura 1975). Lettuce seeds stored a t room temperature sealed over CaClz (6.3% moisture content) germinated 91% after 9 years, but only 2.5% after 15 years; but seeds sealed over CaO (2.5% moisture content) germinated over 40% after 20 years. The difference in longevity between lettuce seeds sealed over CaClz and CaO apparently resulted from the difference in seed moisture content (Nakamura 1975). At room temperature, edible burdock (Arctiurn lappa L.) seeds sealed over CaClz and CaO germinated about the same percentage after 9 years; but after 15 years, the seeds over CaO germinated 26% and those over CaC12 germinated 6%. Welsh onion seeds stored 15 years a t room temperature sealed over CaO had 2.6 to 3.1% moisture content and germinated 52 to 54%, whereas seeds sealed over CaClz had 5.9 to 6.9% moisture content and germinated 1%(Nakamura 1975). Carrot, Chinese cabbage, cucumber, spinach, and eggplant seeds stored a t room temperature retained their viability longer when sealed over CaC12 than when sealed over CaO. Carrot seeds sealed over CaO lost viability in less than 10 years, but seeds sealed over CaClz lost only about half their initial viability during 2 1 years' storage a t room temperature. When stored a t room temperature sealed over CaClz, Chinese cabbage seeds had declined from 92.5 to 39.5% after 17 years, but seeds sealed over CaO failed to germinate after 17 years (Nakamura 1975). At room temperature, cucumber seeds sealed over CaClz declined 6% in viability after 2 1 years, but seeds sealed over CaO declined 86%. Spinach seeds declined from 82.5 to 56% germination during 17 years' storage a t room temperature, but viability of seeds sealed over CaO declined from 82.5 to 4%. Eggplant seeds stored 21 years a t room temperature declined in viability only 11%when sealed over CaClz but declined 81% when sealed over CaO. Radish, pepper, and squash seeds retained their viability better when sealed over CaClz than when sealed over CaO and stored a t room temperature (Nakamura 1975). Powell and Mathews (1976) reported that pea seeds stored under both warm, humid (25"C/93% RH) and cool, extremely dry (lO"C/l% RH) conditions showed deteriorative changes after 6 weeks, although little or no loss of viability had occurred. At 25°C and 93% RH, viability began to decline after 6 weeks, but a t 45°C and 94% RH, viability began to decline after 2 days. Rate of loss of viability was closely associated with seed moisture content. Moisture content of seeds a t 25°C and 93% RH increased from 10.4 to 22.7% in 15 weeks, but that of seeds a t 10°C and 1%


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R H decreased from 10.4 to 5.7% during the same period. The high moisture content seeds lost viability rapidly, while the low moisture content seeds retained their viability . Arumugam and Shanmugavelu (1977) reported that the germination of papaya seeds declined during storage when sealed in glass bottles with either CaO or silica gel, but 9.5 to 10.1% moisture content seeds retained good viability for 9 months when sealed in glass bottles without a desiccant. The reductions in viability observed for seeds sealed with the desiccants apparently resulted from excessive drying. Bass (1975) found that papaya seeds retained their viability well when seed moisture content was in equilibrium with 10°C and 50% RH whether stored a t 10°C and 50% R H or sealed in containers made of a heat-sealable material with a good foil layer in the lamination and stored a t 5°C. After 6 years of storage, seeds at 5°C in the heat-sealable containers germinated 3% higher than the seeds stored a t 10°C and 50% RH. Van Staden (197813) reported that 98.7% of Protea neriifolia seeds harvested a t 8%moisture content germinated in 20 days. Germination of seeds stored 3 years in cloth bags a t 20°C and 26°C (7.54% and 6.89% moisture) was 54.3% and 38.7%, respectively. Seeds stored 3 years under nitrogen in sealed glass containers a t 20°C (6.1% moisture) germinated 96.3%. Seeds in plastic bags a t 5°C and -10°C contained 6% and 8% moisture and germinated 97.7% and 96.3%, respectively. Days from planting to start of germination increased with increased storage in cloth bags a t 20°C and 26°C. No delay in germination was observed for seeds stored a t 5°C and -10°C and a t 20°C in nitrogen. The delay in start of germination was accompanied by a decrease in germination percentage. Honjo and Nakagawa (1978) found that seeds of Citrus grandis (L.) Osbeck, C. otachibana Hort. ex Y Tanaka, C. sulcata Hort. ex Takahashi, C. natsudaidai Hayata, C. iyo Hort. ex Tanaka, and C. tamurana Hort. ex Tanaka remained fully viable a t seed moisture contents above 20%. Seeds of trifoliate orange (Poncirus trifoliata [L.] Rafin.) fluctuated in germination a t seed moisture contents above 20%, but no seeds with less than 20% moisture content germinated. To evaluate the tolerance of citrus seeds to cold temperatures, seeds of C. natsudaidai with 54%, 31%, 23%, and 17% moisture content were stored a t 4", l o ,- lo , -3", and -5°C. Seeds with 31% and 54% moisture content retained good viability for 3 years a t 4°C. At l o , -lo, -3", and -5"C, a t all seed moistures, and a t 4°C and 23% and 17% moisture content, the rate of viability decline was variable. In a second storage experiment, seeds a t 29%, 39%, and 54% moisture content retained good viability for 2 years a t 4°C. Seeds a t 1°C and -1°C and with 34% and 23% moisture content showed variable rates of deterioration. It was concluded that for long storage C. natsudaidai seeds should have a moisture content above 30% and be held a t 4°C.

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According to Kotobuki (1978), seeds of Japanese persimmon (Diospyros kaki L.f.) do not tolerate drying. Regardless of the rapidity or the method of drying, seeds with the same moisture content showed the same germination percentage. At seed moisture contents above 30% germination was above 90%, but as seed moisture was reduced to below 30%, germination declined, until a t about 10% seed moisture few seeds germinated. Japanese persimmon seeds with 42.7% and 51% moisture content retained their initial viability for 18 months when stored a t 0°C. Seeds with 31.9% moisture showed a sharp decline in germination during the first 6 months of storage a t 0°C with little additional decline in viability during the next 12 months. Storage of seeds a t relative humidities between 65% and 90% provides seed moisture contents favorable for the growth of storage fungimainly a few group species of Aspergillus and Penicillium (Justice and Bass 1978). Some authorities on seed storage and seed health question whether storage fungi are ever the primary cause of loss of viability of seeds. However, Christensen (1973) reported that seeds of pea, barley, corn, and wheat stored a t moisture contents and temperatures favorable for the growth of storage fungi germinated 95% or higher after several months of storage if kept free of fungi; but germination of seeds inoculated with storage fungi was reduced to near zero during the same period. For many kinds of seeds, seed moisture contents in equilibrium with various relative humidities have not been determined, and consequently moisture contents a t which seeds are likely to be invaded by storage fungi are not known. Christensen (1973) has reviewed the literature on the effects of fungi on stored seeds. Storage fungi can be controlled best by storing seeds under conditions unfavorable for the growth of such fungi. T h e optimum temperature for growth of most storage fungi is about 30" to 33"C, the maximum about 50" to 55"C, and the minimum 0" to 5°C. A few Penicillium species found on seeds can grow a t temperatures as low as -5°C. For best control of storage fungi, seeds should not be stored a t temperatures above 30°C and relative humidities above 65%. Respiration of insects may cause accumulations of moisture which could encourage the growth of fungi, transmit fungi from seed to seed, and generate enough heat to affect seed viability (Howe 1973). Insects can be controlled by chemicals or by storage in insect-proof containers. Insecticides do not damage dry seeds, but some damage moist seeds. For long-term storage of germplasm, insects are no problem when seeds are stored a t subfreezing temperatures. Seed lots of a given kind, variety, chronological age, and germination do not all maintain their viability equally well in storage under identical conditions. Delouche and Baskin (1973), in an attempt to predict dif-


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ferences in seed storage life, developed the accelerated aging test, which measures physiological differences among seed lots which are not evident from the results of standard germination tests. Seeds were stored for short periods (2 to 8 days) under conditions known to promote deterioration-40" to 45°C and 100% R H for most species, but for some species 30°C and 75% RH was better. Seed lots which declined in viability most rapidly under these conditions had the poorest storage potential. Use of the accelerated aging test is beneficial to the seedsman in determining which seed lots can be carried over to the next planting season and which seed lots should be sold first. Delouche et al. (1973) reported that high quality bean, lettuce, onion, radish, and watermelon seeds showed little or no deterioration during 2Y2 years' storage a t 45% R H and 7°C. Similar seeds stored a t 75% R H and 30°C lost their viability in less than one year. At ambient conditions (Mississippi State, Mississippi) radish, bean, and watermelon seeds germinated 9596, 90%, and 86%, respectively, after 2% years, lettuce 68% after 1%years, and onion 42% after one year of storage. Delouche et al. (1973) reviewed the literature on storage of seeds in tropical and subtropical regions. They noted that for short-term storage (1 to 9 months) seeds should be stored a t 30°C and 50% RH, 20°C and 60% RH, or comparable conditions. For storage up to 18 months, they recommended a 10% decrease in the relative humidities listed above and other comparable temperature/RH combinations, such as 10°C and 60% RH. For long-term storage, R H should be 45% a t 10°Cand 30 to 40% a t 0" to -5°C. Most research has been on the storage of dry seeds, but Villiers (1974) studied storage of fully imbibed seeds as well as dry seeds. Fully imbibed lettuce and Fraxinus americana (L.) seeds retained good viability a t 30°C and 22"C, respectively, while the rate of viability loss in dry stored seeds increased with increased seed moisture content and storage time. Villiers and Edgcumbe (1975) stored seeds of lettuce cultivars 'Big Boston', 'Arctic King', and 'Grand Rapids' a t a range of moisture contents and fully imbibed in contact with liquid water. As the moisture content of the seeds not in contact with the liquid water increased, storage life decreased. Fully imbibed seeds held a t 30°C lost no viability in 1 2 months as long as they remained in contact with liquid water. Increased numbers of seedling abnormalities, including stunted roots and shoots, distorted cotyledons with necrotic areas, subdivided first leaves, swollen roots, and necrotic radicle meristems, were observed with increased time in dry storage. Plants from seeds stored fully imbibed a t 30°C for over 18 months appeared normal, grew vigorously, and produced normal, high-germinating seeds which produced a second generation crop of normal, vigorous seeds.

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Seeds of ‘Oswego’, ‘Fulton’, ‘Imperial 44’, ‘Imperial 846’, and ‘Imperial 847’ lettuce retained viability well a t -12°C and 70% R H for 1 4 years, but a t 5°C and 40% RH produced almost no normal seedlings after the same period of storage (Bass, unpublished data).

B. Temperature Storage temperature is the second most important factor in determining seed longevity. At temperatures above freezing, seed storage life decreases as temperature increases. The response of seeds to temperatures below freezing has not been fully documented. Owen (1956), Barton (1961); Roberts (19721, Harrington (1972), Bass (1973a), and Justice and Bass (1978) reviewed the literature on effects of storage temperature on seed longevity. Temperatures between 5°C and -29°C were reported to be satisfactory for medium to long-term seed storage with temperatures below -5°C preferable. Kretschmer (1976) found that after 4 years’ storage a t 10” to 3OoC, 5°C or -18°C lettuce seeds germinated over 90%. He concluded that for best results lettuce seeds should be stored with a low water content in water vapor-tight polyethylene bags. Temperatures near -18°C and 5% moisture content are recommended for long-term storage of seeds for germplasm preservation. (IBPGR 1976). In recent years, considerable emphasis has been placed on storage of seeds a t subfreezing temperatures, especially -196°C. Sakai and Noshiro (1975) investigated factors contributing to the survival of seeds cooled to the temperature of liquid nitrogen. They found that as a safe, general practice, seeds should be dried to 8 to 15% moisture content, be enclosed in an aluminum foil or an aluminum or plastic vessel with a screw cap, and the container be immersed directly into LN2. After storage, the containers should be rewarmed at 0°C or room temperature. Onion seeds a t 4% and 16% moisture content in sealed glass vials showed no decline in germination or yield after 3.75 years of storage a t -196°C and -20°C (Harrison and Carpenter 1977). Seeds of Anemone coronaria L., Antirrhinum majus L., Asparagus officinalis L., Lactuca sativa L., Pastinaca sativa L., Phaseolus vulgaris L., Pisum sativum L., Primula sinensis Sab. ex Lindl., Raphanus sativus L., and Zea mays L. also showed no reduction in viability after a few months’ storage a t -196°C. Stanwood and Bass (1978) reported that a t moisture contents below 13% (wet-weight) seeds of 42 commonly cultivated species showed no decline in viability after cooling to -196°C. In studies with seeds below 8% moisture content, no damage was observed from cooling to -196°C and rewarming, except for soybean, for which optimum moisture content

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for slow rewarming was 14.6% and for fast rewarming was 15.8% (Sakai and Noshiro 1975). Lettuce seeds with 5 to 13% moisture content were not injured by cooling to -196"C, but seeds with 13 to 16% moisture content were injured when cooled below -40°C (Stanwood et al. 1978). Cassava seeds with 6.34% moisture showed no detrimental effects of freezing in the vapor above LN2, but freezing in LN2 resulted in decreased germination. Seeds with 2% moisture frozen in LN2 shattered upon thawing (Mumford and Grout 1978). Junttila and Stushnoff (1978), Stushnoff and Junttila (1978), and Mumford and Grout (1978) found that hydrated lettuce seeds avoid injury by super-cooling. Super-cooling as an avoidance mechanism depends upon the intact structure of the endosperm. Th; degree of resistance to low temperature was related to the degree of hydration in the 20 to 40% moisture range, and the killing point was detectable by differential thermal analysis. No exotherms were detected for seeds with less than 16% moisture, but 2 types of exotherms were found in seeds containing more than 20% moisture. The first exotherm, which appeared as a single peak a t -10" f 2"C, was not related to the injury of intact seeds, but the second exotherm, which was linearly correlated with seed moisture content within the limits of 20 to 40%, represented the killing point of individual seeds. For seeds between 40% and 50% moisture content, the exotherm was -16 f 2°C. When the endosperm was disrupted the secondary isotherm disappeared, and the first isotherm represented the killing temperature. Extremely rapid freezing rates of 80", 110", and 240°C per hour shifted the free water peak slightly but did not change the secondary peaks (Stushnoff and Junttila 1978). Lettuce seeds with seed moisture levels up to 28.3% survived a freezing rate of 4°C per hour but not 40°C per hour prior to storage at -20°C for 28 days. At 21.8% moisture, freezing rate was not a factor, and high survival also occurred a t 40°C per hour. Lettuce seeds with 47.3% moisture did not tolerate one day a t -20°C (Stushnoff and Junttila 1978). Stanwood et al. (1978) reported that no freeze damage was observed in seeds with 13.9% moisture or lower when cooled to 5", -lo,-18", -70", or -196°C. Damage was observed in 19.7% moisture seeds cooled to -70" or -196°C. Japanese pear seeds slowly dried to less than 10% moisture content showed no loss of germination when subjected to temperatures of 0", -lo", -25", and -196°C for 48 hours. Seeds a t 55% moisture content lost viability a t temperatures lower than 0°C. Seeds at 11.7% moisture lost all viability in less than one year a t room temperature, but seeds a t 4.3% moisture retained their full initial viability for 3 years. Both 11.7% and 4.3% moisture content seeds stored a t -20°C and -196°C showed no loss

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of viability during 3 years' storage. Several months of low temperature stratification were required for germination of dried stored seeds (Omura et al. 1978). Storage of cucumber seeds a t -196°C for 2 years did not decrease viability, but storage a t -20°C reduced viability somewhat (Fedosenko and Yuldasheva 1976).

C. Controlled Atmosphere Storage Bass (1973a) and Justice and Bass (1978) comprehensively reviewed the literature on controlled atmosphere storage of seeds. They found the reported results to be variable and in some instances contradictory, probably because of widely divergent test procedures. Some workers controlled either storage temperature or seed moisture content, and others controlled neither. Some workers used air-dry seeds and room temperature, which provided very little information about the conditions actually used. Air-dry moisture content and room temperature vary with the kind of seed, time of year, geographic location, and kind of building. To accurately assess the value of either a partial vacuum or a gas in prolonging seed storage life, all environmental conditions must be considered. Under certain circumstances, some kinds of seeds are benefited by storage under a partial vacuum or gas such as COz or N,. But what are those circumstances? In an effort to understand more fully the interrelationships involved, we initiated a study to evaluate the effects of seed moisture content, storage temperature, and surrounding atmosphere on seed longevity. Seeds of crimson clover, lettuce, safflower, sesame, and sorghum were conditioned to 4, 7, or 10% moisture content and sealed in metal cans with atmospheres of air, carbon dioxide, nitrogen, helium, argon, or a partial vacuum (51 f 1 mm mercury) and stored a t temperatures of -12", -lo, lo", 21", or 32°C (Bass et al. 1962, 1963a,b). After 16 years' storage there were no significant differences in germination among the atmospheres for sorghum seeds a t the respective moistures and temperatures (Bass and Stanwood 1978). Similar results were obtained for crimson clover (Bass 1978), lettuce, safflower, and sesame seeds (unpublished data) except that lettuce seeds with 7% moisture content sealed in air a t temperatures of -1°C and warmer lost viability more rapidly than did the seeds in the other atmospheres or a partial vacuum. After 1 9 years' storage, 7% moisture content lettuce seeds sealed in air germinated 1% compared to 96%, 94%, 89%, 92%, and 94% for seeds sealed in a partial vacuum, carbon dioxide, nitrogen, helium, and argon, respectively (Bass, unpublished data). Even with these comprehensive studies we still do not know the ultimate value of controlled atmosphere storage of seeds. The benefits of such storage, if any, may be for very long storage, and then only for

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certain kinds of seeds stored at specific seed moistures and storage temperatures. For most situations, the added expense of using an atmosphere other than air appears to be unnecessary. VII. CHANGES DURING STORAGE A. Biochemical Changes Seed storage begins when seeds reach full maturity, regardless of where or how they are held. Biochemical changes occur in all parts of the seed both during and after maturation. Although much has been written about biochemical changes associated with deterioration, our knowledge of these processes is still incomplete. Abdul-Baki and Anderson (1972) and Roberts (1972) have thoroughly reviewed the literature. Attempts have been made to correlate such factors as increased fat acidity, increased enzyme activity, depletion of food reserves, and changes in membrane permeability with deterioration. Villiers (1973, 1974) and Villiers and Edgcumbe (1975) proposed that deterioration in dry-stored seeds results from the lack of operable systems to repair and replace organelles. When lettuce seeds were stored fully imbibed in contact with liquid water a t temperatures which promote rapid deterioration of dry seeds, no deterioration occurred. The number of abnormal seedlings in dry-stored seeds increased with increased time in storage. When dry seeds which showed considerable deterioration were imbibed and stored in liquid water, no further deterioration occurred. This suggests that repair mechanisms are operable in fully imbibed seeds and not in dry seeds. Consequently, any repair in dry-stored seeds must occur after the seeds are imbibed during the germination process.

B. Cytological Changes Cytological changes in the form of chromosomal aberrations occur in seeds of numerous crop and native plants, such as Antirrhinum, Crepis, Datura, Nicotiana, Hordeum, Vicia, Zea, Allium, Pisum, Secale, Beta, and Triticum spp. Kolotenkov (1974) reported that the frequency of mutations in Pisum sativum seeds was doubled while the frequency of chlorophyll mutations was increased from 0.5 to 5.8% by soaking the seeds in water for 6 to 12 hours, followed by thorough drying before planting. Seeds soaked and planted immediately were not affected. Villiers (1974) and Villiers and Edgcumbe (1975) found that an increase in moisture content of seeds of Lactuca sativa L. and Fraxinus americana L. in dry storage caused an increase in the rate of accumula-

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tion of chromosome aberrations. Storage of fully imbibed seeds in water allowed a very low incidence of chromosome aberrations to be maintained. Seedlings grown from dry-stored seeds showed increased morphological abnormalities with increased time in storage, while those from imbibed stored seeds appeared normal. Villiers and Edgcumbe (1975) found that radiation damage in irradiated lettuce seeds decreased with time when stored fully imbibed, but increased when stored dry. Floris and Anguillesi (1974) found that when the embryos and endosperms of wheat seeds were aged separately, each produced mutagenic substances which induced nuclear damage in the radicle meristem. The nuclear damage observed in isolated embryos did not differ from that observed in intact seeds. Isolated aged endosperm induced some chromosome damage in young embryos but did not increase the aberrations in isolated aged embryos. Orlova and Nikitina (1972) studied the timing of the appearance of chromosome aberrations during storage of Welsh onion seeds. With the passage of time in storage under room conditions and a t 50°C and 75% RH the number of single bridges and fragments decreased as did the total number of all types of rearrangements. They concluded that chromosome changes in fresh seeds occur a t the moment of germination and can be partially removed by the action of a protective substance. In aged seeds, part of the chromosome damage occurred in dormant seeds and appeared to be irreversible. In later studies (Orlova et al. 1976) a decrease in the mitotic potential of Welsh onion seeds was observed during aging. Orlova and Ezhova (1976) reported that when a DNA inhibitor was applied to young and old Welsh onion seeds, an equivalent increase in chromosome aberrations occurred. They concluded that automutagens capable of inducing potential chromosome changes accumulated during aging of the seeds. In aged pea seeds, the occurrence of first mitosis was delayed with increased time in storage and occurred when root length was about 4 mm in seeds stored 4 days and 4.8 mm in seeds stored 14 days a t 38°C with 18% moisture content. Disturbance of cell divisions by seed aging appears to be a factor in the induction of chromosomal aberrations (Roos, unpublished data).

VIII. SUMMARY Within families and genera, differences in seed longevity have been demonstrated among species and among cultivars. Production conditions, maturity a t harvest, and harvesting and processing conditions affect seed longevity. The more critical factors affecting seed longevity are seed moisture content and storage temperature. For maximum longevity seeds must be

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stored dry a t a low temperature. Many kinds of seeds can be dried by heated air but some cannot. Storage with CaC12 or CaO also extends storage life; however, some kinds of seeds live longer with CaClz than with CaO, and others live longer stored with CaO than with CaClZ.Most seeds that can be dried live longer a t subfreezing temperatures than a t above-freezing temperatures. A temperature of 10°Cor lower will prolong seed life. Within limits, the lower the temperature and seed moisture content, the longer seeds will remain viable. Hermetic storage can increase seed storage life provided seed moisture content is sufficiently low when the seeds are sealed. Storage in an a t mosphere other than air offers no advantage except for very long storage of a few kinds of seeds. Results of recent investigations suggest that cryogenic (-196°C) storage may extend the longevity of seeds more than conventional storage does. Cryogenic storage, through stoppage of all metabolic activity and biochemical changes, may also prevent the development of chromosomal aberrations which have been shown to occur during storage by conventional methods. IX. LITERATURE CITED ABDUL-BAKI, A.A. and J.D. ANDERSON. 1972. Physiological and biochemical deterioration of seeds. p. 283-315. In T.T. Kozlowski (ed.) Seed biology, vol. 2. Academic Press, New York. ANON. 1968. 550-year old seed sprouts. Sci. News 94:367. ARNOLD, J.R. and W.F. LIBBY. 1951. Radiocarbon dates. Science 113:lll120. ARUMUGAM, S. and K.G. SHANMUGAVELU. 1977. Studies on the viability of papaya seeds under different environments. Seed Res. 5(1):23-31. AUFHAMMER, G. and U. SIMON. 1957. Die samen landwirtshaftlicher kulturpflanzen im grundstein des chamaligen nurnberger stadttheaters und ihre keimfahigkeit. Ztschr. f. Acker- u. Pflanzenbau. 103:454-472. AUSTIN, R.B. 1972. Effects of environment before harvesting on viability. p. 114-149. I n E. H. Roberts (ed.) Viability of seeds. Chapman and Hall, London. BARTON, L.V. 1961. Seed preservation and longevity. Leonard Hill, London. BASS, L.N. 1965. Effect of maturity, drying rate, and storage conditions on longevity of Kentucky bluegrass seed. Assoc. Off. Seed Anal. Proc. 55:43-46. BASS, L.N. 1973a. Controlled atmosphere and seed storage. Seed Sci. & Technol. 1:463-492. BASS, L.N. 1973b. Response of seeds of 27 Cucumis melo cultivars to three storage conditions. Assoc. Off. Seed Anal. Proc. 63:83-87. BASS, L.N. 1975. Seed storage of Carica papaya L. HortScience 10:232-233.

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BASS, L.N. 1978. Sealed storage of crimson clover seed. Seed Sci.& Technol. 6:1017-1024.

BASS, L.N. and D.C. CLARK. 1974. Effects of storage conditions, packaging materials, and seed moisture content on longevity of safflower seeds. Assoc. Off. Seed Anal. Proc. 64:120-128. BASS, L.N., D.C. CLARK, and E. JAMES. 1962. Vacuum and inert-gas storage of lettuce seed. Assoc. Off. Seed Anal. Proc. 52:116-122. BASS, L.N., D.C. CLARK, and E. JAMES. 1963a. Vacuum and inert-gas storage of crimson clover and sorghum seeds. Crop Sci. 3:425-428. BASS, L.N., D.C. CLARK, and E. JAMES. 1963b. Vacuum and inert-gas storage of safflower and sesame seeds. Crop Sci.3:237-240. BASS, L.N. and P.C. STANWOOD. 1978. Long-term preservation of sorghum seed as affected by seed moisture, temperature, and atmospheric environment. Crop Sci. 18:575-577. BECQUEREL, P. 1934. La longevite des graines macrobiotiques. Acad. des. Sci. Compt. Rend. 199:1662-1664. CHANEY, R.W. 1951. How old are the Manchurian lotus seeds? Gard. J. (N.Y. Bot. Gard.) 1:137-139. CHRISTENSEN, C.M. 1973. Loss of viability in storage: Microflora. Seed Sci. & Technol. 1:547-562. CLARK, D.C. and L.N. BASS. 1975. Effects of storage conditions, packaging materials, and moisture content on longevity of crimson clover seeds. Crop Sci. 15:577-580.

DELOUCHE, J.C. and C.C. BASKIN. 1973. Accelerated aging techniques for predicting the relative storability of seed lots. Seed Sci.& Technol. 1:427-452. DELOUCHE, J.C., R.K. MATHES, G.M. DOUGHERTY, and A.H. BOYD. 1973. Storage of seed in sub-tropical and tropical regions. Seed Sci.& Technol. 1:671-700. DORRELL, D.G. and M.W. ADAMS. 1969. Effect of some seed characteristics on mechanically induced seedcoat damage in navy beans (Phaseolus vulgaris L.). Agron. J. 61:672-673. EGUCHI, T. and H. YAMADA. 1958. Studies on the effect of maturity on longevity in vegetable seeds (in Japanese. English summary). Natl. Inst. Agr. Sci. Bul., Ser. E, Hort. 7:145-165. EWART, A. J. 1908. On the longevity of seeds. Roy. SOC. Victoria, Proc. 21: 2-210.

FEDOSENKO, V.A. and L.M. YULDASHEVA. 1976. The preservation of C u cumis sativus seeds at extremely low temperatures (in Russian). Vsesoyuznogo Ordena Lenina Instituta Rastenievodstva imeni N. I. Vavilova 64:60-62. VIR, Leningrad, USSR. FLORIS, C. and M.C. ANGUILLESI. 1974. Aging of isolated embryos and endosperms of durum wheat: An analysis of chromosome damage. Mutation Res. 22:133-138.

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HARRINGTON, J.F. 1972. Seed storage and longevity. p. 145-245. I n T. T. Kozlowski (ed.) Seed biology, vol. 3. Academic Press, New York. HARRISON, B.J. and R. CARPENTER. 1977. Storage of Allium cepa seed a t low temperatures. Seed Sci. & Technol. 5:699-702. HEYDECKER, W. 1973. Seed ecology. Pennsylvania State University Press, University Park, Pa. HOLMES, A.D. 1953. Germination of seeds removed from mature and immature butternut squashes after seven months of storage. Proc. Amer. SOC. Hort. Sci. 62:433-436. HONJO, H. and Y. NAKAGAWA. 1978. Suitable temperature and seed moisture content for maintaining the germinability of citrus seed for long term storage. p. 31-35. I n T. Akihama and K. Nakajima (eds.) Long term preservation of favorable germ plasm in arboreal crops. The Fruit Tree Research Station of the Ministry of Agriculture and Forestry, Japan [Kokusai Print Service, Tokyo]. HOWE, R.W. 1973. Loss of viability of seed in storage attributable to infestations of insects and mites. Seed Sci. & Technol. 1:563-586. IBPGR. 1976. Report of IBPGR working group on engineering, design, and cost aspects of long-term seed storage facilities. International Board for Plant Genetic Resources, Rome. JAMES, E. 1967. Preservation of seed stocks. Adu. Agron. 19237-106. JAMES, E., L.N. BASS, and D.C. CLARK. 1967. Varietal differences in longevity of vegetable seeds and their response to various storage conditions. Proc. Amer. SOC.Hort. Sci. 91:521-528. JUNTTILA, 0. and C. STUSHNOFF. 1978. Freezing avoidance by deep su-per-cooling in hydrated lettuce seeds. Nature 269:325-327. JUSTICE, O.L. and L.N. BASS. 1978. Principles and practices of seed storage. USDA Agr. Handb. 506. KOLOTENKOV, P.V. 1974. The mutagenic influence of drying on pea seeds. Soviet Genetics 10:924-925. KOTOBUKI, K. 1978. Seed storage of Japanese persimmon, Diospyros kaki. p. 36-42. I n T. Akihama and K. Nakajima (eds.) Long-term preservation of favorable plant germ plasm in arboreal crops. The Fruit Tree Research Station of the Ministry of Agriculture and Forestry, Japan [Kokusai Print Service, Tokyo]. KOZLOWSKI, T.T. (ed.). 1972. Seed biology, vol. 3. Academic Press, New York. KRETSCHMER, M. 1976. Mehrjahrige Lagerung von Lactuca sativa L. - bei unterschiedlichen Temperaturen 1. Veranderungen der Temperature-toleranz im Dunkeln und bei Licht. Gartenbauwissenschaft 41:S.229-235. MOORE, R.P. 1972. Effects of mechanical injuries on viability. p. 94-113. In E. H. Roberts (ed.) Viability of seeds. Chapman and Hall, London.

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MOSS, H.J., N.F. DERERA, and L.N. BALAAM. 1972. Effect of pre-harvest rain on germination in the ear and a-amylase activity of Australian wheat. Austral. J. Agr. 23:769-777. MUMFORD, P.M. and B.W.W. GROUT. 1978. Germination and liquid nitrogen storage of cassava seed. Ann. Bot. 42:255-257. NAKAMURA, S. 1975. The most appropriate moisture content of seeds for their long life span. Seed Sci. & Technol. 3:747-759. OHGA, I. 1923. On the longevity of seeds of Nelumbo nucife. Bot. Mag. (Tokyo) 37:87-95. OMURA, M., Y. SATO, and K. SEIKE. 1978. Long-term preservation of Japanese pear seeds under extra low temperatures. p. 26-30. In T. Akihama and K. Nakajima (eds.) Long term preservation of favorable germ plasm in arboreal crops. The Fruit Tree Research Station of the Ministry of Agriculture and Forestry, Japan [Kokusai Print Service, Tokyo]. ORLOVA, N.N. and T.A. EZHOVA. 1976. Effect of the DNA synthesis inhibitor 5-aminouracil on the appearance of chromosomal aberrations in Allium fistulosum seeds of various ages. Soviet Genetics 10:1476-1481. ORLOVA, N.N. and V.I. NIKITINA. 1972. The moment of appearance of chromosome aberrations during the aging of seeds. Soviet Genetics 4:11531158. ORLOVA, N.N., N.P. ROGATYKH, and G.A. KHARTINA. 1976. Decrease in the mitotic potential of cells in dormant seeds of Welsh onion during storage. Soviet Plant Physiol. 22:629-635. OWEN, E.G. 1956. The storage of seeds for maintenance of viability. Commonw. Agr. Bur. Pastures and Field Crops Bul. 43. POLLOCK, B.M. 1961. The effects of production practices on seed quality. Seed World 89(5):6, 8 , 10. PORSILD, A.E., C.R. HARINGTON, and G.A. MULLIGAN. 1967. Lupinus arcticus Wats. grown from seeds of Pleistocene Age. Science 158:113-114. POWELL, A.A. and S. MATHEWS. 1976. Deteriorative changes in pea seeds (Pisum sativum L.). J. Expt. Bot. 28:225-234. RAMSBOTTOM, J. 1942. Duration of viability in seeds. Gard. Chron. 111: 234. ROBBINS, M.L. and W.N. WHITWOOD. 1973. Deep-cold treatment of seeds: Effect on germination and on callus production from excised cotyledons. Hort. Res. 13:137- 141. ROBERTS, E.H. 1961. The viability of rice seed in relation to temperature, moisture content, and gaseous environment. Ann. Bot. 25:381-390. ROBERTS, E.H. (ed.). 1972. Viability of seeds. Chapman and Hall, London. ROBERTS, E.H. and F.H. ABDALLA. 1968. The influence of temperature, moisture, and oxygen on period of seed viability in barley, broad beans, and peas. Ann. Bot. (N.S.) 32:97-117. ROBERTS, E.H., F.H. ABDALLA, and R.J. OWEN, JR. 1967. Nuclear damage and the ageing of seeds with a model for seed survival curves. Symp. SOC. Expt. Biol. 21:65-100.

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SAKAI, A. and M. NOSHIRO. 1975. Some factors contributing to the survival of crop seeds cooled to the temperature of liquid nitrogen. p. 317-326. In 0. H. Frankel and J. G. Hawkes (eds.) Crop genetic resources for today and tomorrow. International Biological Programs, Vol. 2. Cambridge University Press, New York. SHANDS, H.L., D.C. JANISCH, and A.D. DICKSON. 1967. Germination response of barley following different harvesting conditions and storage treatments. Crop Sci. 7:444-446. STANWOOD, P.C. and L.N. BASS. 1978. Ultracold preservation of seed germplasm. p. 361-371. I n P.H. Li and A. Sakai (eds.) Plant cold hardiness and freezing stress. Academic Press, New York. STANWOOD, P.C., M.W. BROWN, and E.E. ROOS. 1978. Freezing damage of high moisture lettuce seeds. Newsletter Assoc. Off. Seed Anal. 52:38-39. STUSHNOFF, C. and 0. JUNTTILA. 1978. Resistance to low temperature injury in hydrated lettuce seed by supercooling. p. 241-247. I n P.H. Li and A. Sakai (eds.) Plant cold hardiness and freezing stress. Academic Press, New York. TOOLE, E.H. and V.K. TOOLE. 1954. Relation of storage conditions to germination and to abnormal seedlings of bean. Proc. Intern. Seed Testing Assoc. 18:123-129. TOUZARD, J. 1961. Influences de diverses conditions constantes de temperature et d’humidite sur la longevite des graines de quelques especes cultivees. Adv. hort. sci. and their applications. Proc. 15th Intern. Hort. Congr., Nice, I, 339-347, Pergamon, Oxford. VAN STADEN, J. 1978a. Seed viability in Protea neriifolia. I. The effects of time of harvesting and seed viability. Agroplantae 10:65-67. VAN STADEN, J. 1978b. Seed viability in Protea neriifolia. 11. The effects of different storage conditions on seed longevity. Agroplantae 10:69-72. VILLIERS, T.A. 1973. Aging and the longevity of seeds in field conditions. p. 265-288. I n W. Heydecker (ed.) Seed ecology. Pennsylvania State University Press, University Park, Pa. VILLIERS, T.A. 1974. Seed aging: Chromosome stability and extended viability of seeds stored fully imbibed. Plant Physiol. 53:875-878. VILLIERS, T.A. and D.J. EDGCUMBE. 1975. On the cause of seed deterioration in dry storage. Seed Sci. & Technol. 3:761-774. WESTER, H.V. 1973. Further evidence of age of ancient viable lotus seeds from Pulan tien deposit, Manchuria. HortScience 8:37 1-377. WOODSTOCK, L.W., J. SIMKIN, and E. SCHROEDER. 1976. Freeze drying to improve seed storability. Seed Sci. & Technol. 4:301-311. YOUNG, R.E. 1949. The effect of maturity and storage on germination of butternut squash seed. Proc. Amer. SOC. Hort. Sci. 53:345-346. YOUNGMAN, B.J. 1952. Germination of old seeds. Kew Bul. 6:423-426.


3 Nutritional Ranges in Deciduous Tree Fruits and Nuts C. B. Shear and M. Faust Fruit Laboratory, Horticultural Research Institute, Science and Education Administration, U. S. Department of Agriculture, Beltsville, Maryland 20705 I. Introduction 143 11. Deficiency and Toxicity Symptoms A. Nitrogen (N) 145 1.Deficiency 145 2. Toxicity 145 B. Phosphorus (PI 145 1.Deficiency 145 2.Toxicity 146 C. Potassium (K) 146 1.Deficiency 146 2. Toxicity 147 D. Magnesium (Mg) 147 1.Deficiency 147 2. Toxicity 147 E. Calcium (Ca) 147 1.Deficiency 147 F. Iron (Fe) 149 1.Deficiency 149 2. Toxicity 149 G. Manganese (Mn) 149 1.Deficiency 149 2. Toxicity 149 H. Zinc (Zn) 150 1.Deficiency 150 2. Toxicity 150 I. Boron (B) 150 1.Deficiency 150 2. Toxicity 151

142

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NlJTRITION RANGE I N DECIDUOUS F R U I T S & N U T S 143

J. Copper (Cu) 152 1. Deficiency 152 2. Toxicity 152 K. Chlorine (C1) 152 1.Deficiency 152 2. Toxicity 152 L. Sodium (Na) 152 1. Toxicity 152 M. Sulfur (S) 153 1. Deficiency 153 N. Arsenic (As) 153 1. Toxicity 153 0.Aluminum (Al) 153 1. Toxicity 153 111. Nutrient Concentration in Plant Tissues in Relation to Nutritional Disorders 153 IV. Literature Cited 161

I. INTRODUCTION Although all plants require the same minerals to complete their life cycles, the quantities and balances necessary for optimum growth and production of high yields of quality produce vary greatly among species. The nutrition of large woody trees including fruit trees differs in many ways from that of annual herbaceous plants. Fruit nutrition has been studied since the turn of the century and its history has been reviewed (Faust 1979). Several specific aspects of tree nutrition also have been reviewed (Ballinger et al. 1966; Benson and Linder 1966; Bould 1966, 1970; Boynton and Oberly 1966a,b; Cain and Shear 1964; Chapman 1966; Hansen and Proebsting 1949; Kenworthy and Martin 1966; Proebsting 1966; Shear 1966; Wallace 1961; Westwood and Wann 1966), and the nutritional ranges a t which fruit trees grow best or manifest nutritional deficiencies have been assembled (Kenworthy and Martin 1966). As time has passed and more data have become available, slight modifications have been made in absolute values a t which plant performance could be defined. All available lists giving ranges of nutrient requirements are out of print. Yet this information is needed because of newly developing orchards introducing scientific nutritional methods. Therefore, publishing an up-to-date list of ranges of nutrient requirements for deciduous trees is necessary. In fruit crops, perhaps more than in any other group of economic plants, nutrient imbalances may manifest themselves in quality characteristics of the fruit of otherwise normal appearing trees. Therefore, in considering the adequacy of nutrition for fruit and nut trees we must be aware

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not only of the level of nutrients sufficient to prevent abnormal growth, or leaf symptoms, but also of those necessary to prevent abnormal fruit color, texture, or keeping quality, or in nuts, to prevent reduced filling or oil content. Although each nutrient element usually enters into a number of metabolic processes in the plant, specific symptoms associated with failure of its dominant function are usually the first to appear when an element is in deficient supply. However, since nutrients are not taken up by plants entirely independently of one another, symptoms of deficiencies and excesses may not always be easily distinguished and a knowledge of nutrient interactions is essential for accurate diagnosis. Soil and climatic factors also greatly influence the uptake and function of nutrients. Therefore, the appearance of a symptom characteristic of the deficiency of a certain element may not indicate a lack of that element in the nutrient medium, or even in the plant. The uptake of the nutrient element may have been inhibited by a lack or excess of soil moisture or by an excess of some other element or, once within the plant, it may have been converted into a metabolically inactive form. For these reasons, identification of the deficiency characterized by specific symptoms may not provide the information necessary to correct the disorder. Symptoms are very useful in identifying nutritional disorders, but to diagnose the imbalance responsible for the occurrence of a specific symptom, a knowledge of the nutrient content of one or more portions of the plant is often necessary. The diagnostician must be aware of the many factors other than nutritional that may be responsible for reducing tree growth or inducing symptoms that can be confused with those of nutrient deficiency or excess (Woodbridge 1976). Drought, heat, cold, mechanical injury, insects, nematodes, pathogenic diseases, herbicides, and pesticides may produce symptoms almost indistinguishable from those of malnutrition. In fact, true symptoms of malnutrition may result from some of these agents, particularly from herbicides and virus diseases. Successful diagnosis of the cause of any nutritional disorder requires all available information on cultural and climatic conditions as well as a knowledge of symptomology and tissue composition. 11. DEFICIENCY AND TOXICITY SYMPTOMS Symptoms vary among species included in this section. Where possible, symptoms are described in such a way as to apply to as many of the species as possible. Characteristics unique to individual species are emphasized where necessary.

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A. Nitrogen (N) 1. Deficiency.-Nitrogen deficiency is common in fruit and nut trees. Reduced top growth with short, upright spindly shoots with pale, yellowish green leaves are the first symptoms. Symptoms are usually fairly uniform over the entire plant. The symptoms may develop a t any time during the growing season depending on weather conditions and level of available N. Unless additional N is made available, symptoms become more severe as the season advances. Thus, the best time to judge N deficiency is late in the season. In severely deficient trees, basal shoot leaves may develop extreme symptoms. These leaves may develop necrotic areas along their margins in tung, and in pecans and walnuts leaflets may drop from the rachis. In fruit trees, fall coloring may be brighter on N deficient trees. Leaf color differences are less marked on cherry, peach, and pear trees, whereas in apple and plum marked color reduction may appear by mid season. Fruits are usually smaller and earlier in maturing, especially on the stone fruits, and peach fruit is astringent and stringy. Nuts may be poorly filled. Fruit bud differentiation may be decreased and fruit set the following year may be greatly reduced.

2. Toxicity.-Though much less spectacular, the effects of excessive levels of N can be as economically disastrous as N deficiency. Symptoms of over-fertilization with N may manifest themselves in many ways depending on the species, the balance of N with other nutrients, the form of N (ammonium, nitrate, or urea) available to the roots, and the time of application. Nitrogen is a major factor in both growth and flowering, and effects of too much may be most readily recognized as excessive stimulation of shoot elongation and development of abnormally dark green leaves. Its most detrimental effects, however, are on fruit maturity and quality. As N supply increases above the optimum, fruit color is reduced in both stone and pome fruits; area and intensity of red color on red cultivars of apples is reduced, as is yellow on yellow cultivars. Maturity is delayed in both fruits and nuts. In pome fruits, flavor may be diminished, storage life shortened, and susceptibility to many physiological disorders increased, both on the tree and in storage. Though most of these disorders are associated primarily with Ca deficiency (see below) they are all aggravated by a high level of N. In the oil-producing nuts, high N, especially a high ratio of N to K not only delays maturity but lowers the oil content of fruit.

B. Phosphorus (P) 1. Deficiency.-Phosphorus deficiency severe enough to produce recognizable foliar symptoms is rare in fruit or nut trees in the orchard.

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Symptoms on all fruit and nut crops show up first as limited and slender growth. Young expanding leaves are abnormally dark green and the lower sides of the young leaves, especially along the margins and main veins, frequently show purplish discoloration. The leaves may have a leathery texture and form abnormally acute angles with the stem. Since the first symptom of P deficiency is an inhibition of active vegetative growth, the leaf symptoms frequently become less marked in late summer after active vegetative growth is complete and the P uptake by the roots is able to catch up with the reduced demands of the slowly growing plant. Lateral buds may remain dormant or die, and a s a result few lateral shoots appear. Blossoming and fruiting are reduced and bud opening in the spring may be delayed. Deficient stone fruits ripen early, have a greenish ground color, and may be highly flushed with a soft, puffy acid flesh of poor eating quality. 2. Toxicity.-Effects of excess P are expressed usually as a n antagonism of P and one or more of the essential heavy metals such a s Zn, Cu, Fe, or Mn. Since symptoms of deficiencies of these elements may be induced by excesses of other elements as well a s by a n insufficient supply of the metals themselves, the complex interactions responsible for their appearance cannot be interpreted from visual symptoms but require knowledge of plant composition and the soil factors responsible for th a t composition.

C. Potassium (K) 1. Deficiency.-"Scorching" of the margins of older leaves is the outstanding symptom of advanced K deficiency in all fruit and nut crops. In the stone fruits an upward lateral curling a n d chlorosis of the leaves is evident before the scorch appears. In most species of nut trees chlorosis may precede the scorching. Often, even before chlorosis develops, leaf petioles recurve and the leaves roll and droop, giving the tree the appearance of wilting even though the leaves are completely turgid. In apple, scorch may be preceded by a greyish green discoloration of the upper surface of the margins which later turns a reddish brown and works inward, mostly interveinally. Young leaves generally show less severe symptoms although they may be smaller than normal. A heavy fruit crop usually accentuates the symptoms. Nuts, especially those with high oil content, make a high demand on available K and even young leaves on K deficient trees may fall prematurely. In tung, fruit still clinging to leafless shoots is a typical symptom of severe potassium deficiency. Poor filling and low oil content of kernels also result from inadequate K.

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2. Toxicity.-Symptoms specific for excessive accumulation of K in fruit or nut trees are unknown. High concentrations of K in the soil, however, may affect the uptake of other cations and induce symptoms of deficiencies of these elements. The one most commonly observed is Mg deficiency, but the heavy metals Mn and Zn also may be reduced to deficiency levels by excess K.

D. Magnesium (Mg) 1. Deficiency.-Magnesium deficiency in its severest stages may cause marginal scorching similar to K deficiency, but its pattern of development is very distinctive. Its earliest appearance is a fading of the green color a t the terminals of older leaves or leaflet terminals in pinnateleaved species. The fading, followed by chlorosis, progresses interveinally towards the base and midrib giving the very typical “herringbone” appearance to the leaves. In pear, the symptoms can be very spectacular. Dark purplish islands surrounded by chlorotic bands may develop in the interveinal tissue on both sides of the midrib while the leaf margin maintains a more-or-less normal green color. Such leaves have a “Christmas tree” appearance. In some cases the whole leaf may turn yellow before falling. As the season advances, symptoms develop on progressively younger leaves and the older leaves are abscised. Heavily fruiting trees and branches are the most severely affected. Developing fruit has a high demand for Mg and will draw it from neighboring leaves, thus inducing severe symptoms and early defoliation. In extremely severe cases fruit may fail to mature and may drop early. In most fruit and nut crops certain stages of Mg and K deficiencies may be difficult to distinguish visually by even an experienced diagnostician. In such cases leaf analyses may be necessary for absolute identification.

2. Toxicity.-Symptoms of excessive concentrations of Mg are not specific but usually appear as a deficiency of either K or Ca, depending on the balance of cations available to the tree.

E. Calcium (Ca) 1. Deficiency.-Since Ca affects the supply of other nutrients in so many ways, indirect influences of insufficient Ca may confound the recognition of visible foliar symptoms of direct Ca deficiency. Specific foliar symptoms of Ca deficiency have been developed on many fruit and nut crops in artificial cultures, and these symptoms have sometimes, though rarely, been observed on orchard trees.

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The first indication of Ca deficiency on apple foliage is the upward cupping of the margins of the youngest leaves. On actively growing terminals the expanding leaves develop a uniform veinal and interveinal chlorosis. The very youngest leaves may become entirely chlorotic and terminal growth stops. Margins of older affected leaves may become necrotic and shatter. In severe cases, terminals die back. Various symptoms have been described as associated with low levels of soil Ca or low soil pH but these cannot definitely be ascribed to a deficiency of Caper se since, under these conditions, toxic levels of such elements as Na, Al, Mn, Zn, B, and perhaps H may become available. The fruit of many deciduous fruit trees often shows symptoms associated with a low level of Ca, even when the Ca level in the rest of the plant is sufficient for normal growth. Symptoms may be expressed in many different ways depending on species and cultivar. A common though not always distinctive (see boron deficiency) response to low Ca is cracking of the fruit, especially stone fruits and apples. Cracking usually occurs immediately after periods of heavy rainfall and/or high relative humidity. In extreme cases peaches are small, green, and hard. Pome fruits are especially sensitive to low levels of Ca and may exhibit one or more of the following disorders. Bitter pit appears as slight indentations in the skin, usually toward the calyx end of the fruit. These areas turn brown, and soft dessicated tissue develops in the flesh immediately beneath the spots. Bitter pit may develop while the fruit is still on the tree, but usually develops in storage or after fruit is taken from storage and kept for a few days a t room temperature. Cork spot may appear early in the development of the fruit, first as small blushed areas on the skin above hard brown spots in the flesh. These spots consist of hard compressed tissue caused by cell proliferation after normal cell division has ceased. Calcium-deficient fruit are also more susceptible to sunburn, which appears as large, depressed dehydrated areas on the exposed surface. Lenticel breakdown appears first as pale areas around the lenticels. White halos develop around the lenticels and later turn brown or black. These prominent lenticels may be the only symptom of Ca deficiency to develop on mildly deficient fruit. A number of other fruit disorders-watercore, internal breakdown, and low temperature breakdown-also are associated with inadequate Ca in the fruit. A bark symptom of apples called “measles” or internal bark necrosis results from a complex involving low Ca, high Mn, and sometimes low B. This symptom is described under B deficiency and Mn toxicity.

A. Nitrogen deficiency. Left to right-severe to none.

E. Manganese deficiency.

B. Potassium deficiency. Left to r severe to none.

F. Zinc deficiency. Le severe to none.

FIGURE 3.2. NUTRITIONAL DEFICIENCY SYMPTOMS II

C. Calcium deficiency.

right-

ACH

0. Iron deficiency. Left to rightsevere to none.

0. Manganese deficiency.

H. Copper deficiency. Left to rlght-severe to none.

1. Zinc deficiency on the tree.

M. Sulfur deficiency.

FIGURE 3.2 (Continued)

J. Zinc deficiency in sand culture.

N. Magnesium deficiency.

ciency on the tree.

0. Magnesium deficiency.

L. Boron deficiency on the tree.

P. Arsenic toxicity.

Photographs I.J.K.L. and M are courtesy 01 N. F. Childers. Rutgers University

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F. Iron (Fe) 1. Deficiency.-Iron deficiency symptoms are one of the most commonly observed on fruit and nut trees. This is true because of the many conditions other than an actual deficiency of Fe that can induce the symptoms. T h e initial symptom is the loss of green color in the very youngest leaves. While the interveinal tissue becomes pale green, yellow, or even white, the veins remain dark green. New leaves may unfold completely devoid of color but the veins usually turn green later. In acute cases, dieback of shoots and branches may occur.

2. Toxicity.-Though rare under field conditions, Fe toxicity usually results in Mn deficiency.

G . Manganese (Mn) I. Deficiency.-Chlorosis between the main veins starting near the margin of the leaf and extending toward the midrib is the typical symptom of Mn deficiency. Manganese deficiency symptoms are similar to those of both Fe and Mg. Unlike Fe deficiency, chlorosis does not appear on the very youngest newly expanding leaves nor do the finest veins remain green. Unlike Mg, Mn deficiency seldom develops so far as to produce interveinal necrosis. Whereas Mg deficiency is usually confined to the older leaves, Mn deficiency symptoms develop on the younger leaves shortly after they have fully expanded and persist with little change. In peach, terminal growth may become stunted. Symptoms observed on walnut are similar to those already described, but in severe cases some interveinal bronzing and necrosis may occur. Unlike the necrosis resulting from B excess, the necrotic areas are usually angular rather than rounded or blotchy. A disorder of pecans called “mouse-ear’’ has been attributed to Mn deficiency, though there is some question as to its cause. “Mouse-ear” is characterized by a shortening of the mid-vein of the leaflets which causes the leaflets to become rounded and wrinkled and to cup upward. T h e entire leaf may be smaller than normal. 2. Toxicity.-A disorder known as “measles” that occurs on apples, particularly on the cultivars ‘Delicious’ and ‘Jonathan’, is in part caused by an excess of Mn or, more specifically, an excess of Mn accompanied by low Ca in the bark. A part of the syndrome is an expression of B deficiency and will be described under that element. Manganese-related measles is characterized by the eruption of pimples on the bark of 2-year-old shoots. As the symptoms progress from year to year the pimples enlarge and erupt, producing sunken areas surrounded by callus.

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These areas coalesce and the bark becomes rough, cracked, and scaly. In severe cases, whole branches may die. H. Zinc (Zn) 1. Deficiency.-Zinc deficiency symptoms are variously described as “rosette,” “yellows,” “little leaf,” and “bronzing” in different fruit and nut trees. In all cases, however, the initial mild symptom is an interveinal leaf chlorosis difficult to distinguish from Mn deficiency. With Zn deficiency, however, more extensive symptoms usually develop. Newly developing leaves are smaller than normal, and reduced shoot elongation brings them close together giving rise to the popular names “little leaf” and “rosette.” In severe cases, older leaves may fall, leaving tufts or “rosettes” of leaves near the terminals of branches. In pecans, walnuts, and almonds, reddish brown areas or perforations may develop between the veins. In tung, the leaf margins become wavy, one side of the blade may fail to develop, and the leaf will curl towards the small side into a sickle shape. In very severe cases this uneven growth affects the whole tree, giving it a one-sided appearance. The lower surface of leaves may take on a purplish-bronze cast from which the name “bronzing” is taken. In severe cases, necrotic areas may develop a t random over the leaf surface and later disintegrate, giving the leaf a ragged appearance. In stone fruits, irregular chlorotic areas develop along the margins and later coalesce to form continuous yellow bands extending from midrib to margins. Red to purple blotches may develop within the chlorotic areas and later dry up and fall out, producing a shot-hole effect. Crinkling, cupping, and curving of the leaves are also common. Small crops of small misshapen fruit are produced. Deficient peach and plum fruits may be more flattened than normal while apricot fruits may be less flattened than usual. Nuts from deficient oil-producing nut species are usually poorly filled and low in oil content. 2. Toxicity.-As with most heavy metals, an excess of Zn usually appears as Fe chlorosis.

I. Boron (B) 1. Deficiency.-Boron deficiency shows up in many different ways, depending on the crop and the extent of the deficiency. In most fruit crops, symptoms usually appear on the fruit before vegetative parts are affected. Fruit symptoms in apples and pears are quite similar. Fruits do not develop normally and take on a gnarled misshapen appearance


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caused by depressions usually underlaid by hard corky tissue which rapidly browns when exposed to the air. The color of the surface of these areas is usually darker green than the surrounding tissue. If the deficiency does not become severe until late in their development, the main fruit symptom may be internal cork formation scattered throughout the entire apple from peel to core. This type of B deficiency often has been confused with cork spot (see Ca deficiency). Some apple cultivars’ fruit may crack, particularly if both B and Ca are low. In mild cases, the whole surface of the fruit may be covered with small cracks that have callused over, producing a russeted appearance. In plums, the symptom appears as brown sunken areas in the flesh of the fruit, ranging in size from small spots to almost the whole fruit. The firm flesh beneath these sunken areas may extend to the pit. Affected fruit usually colors earlier than normal, and falls. Gum pockets may be formed also in the flesh of the fruit, especially of almond. In peaches, the fruit develops brown, dry corky areas in the flesh adjacent to the pit, and some fruits may crack along the suture. The most typical vegetative symptom in all tree crops is the failure of apical meristems to develop and eventual death of shoot tips followed by forcing of new weak shoots below the dead tips. Boron-deficient leaves are darker green, thick, and brittle, and are abscised early, starting at the shoot tips. Boron deficiency may be part of the symptom complex of “apple measles”-the appearance of purplish pimples on young twigs progressing to rough cracked bark on older twigs. Lenticel proliferation, corky callus development, and bark splitting occurs on B deficient peach and tung also. Reproductive organs are particularly sensitive to lack of B and such incipient symptoms as failure to set fruit or even the wilting and dying of blossoms as in “blossom blast” of pears may be the only indication of B deficiency. 2. Toxicity.-Fruit, shoot, and bark symptoms are the most typical of B injury in the stone fruits, almond, apricot, cherry, peach, plum, and prune, and the pome fruits, apple, pear, and quince. Dieback of twigs, greatly enlarged nodes on one- and two-year-old twigs, gumming of the twigs and larger branches, splitting, early maturity, corking, and dropping of the fruit are typical responses of the stone fruits. Early maturity and shortened storage life are characteristic of toxicity in apples. Foliage symptoms on stone and pome fruits usually are yellowing along the midrib and large lateral veins, often followed by abscission. In contrast, most nut crops show a tip burning followed by marginal and interveinal necrosis. Older leaves are the first to be affected.

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J. Copper (Cu) 1. Deficiency.-Copper deficiency is occasionally seen on fruit and nut trees in the orchard. On almond, bark on the trunk and older branches becomes rough and in late winter there may be some gum exudation from these areas. On most species leaves become chlorotic and various degrees of mottling develop. In peach, new leaves may be narrow and elongated with wavy margins. Terminals are most severely affected, and wilting and defoliation occur in severe cases. In walnut and tung, marginal and interveinal necroses precede defoliation, and in all species death of the terminals is followed by growth of laterals, giving a bushy appearance to the trees. Copper and Zn deficiencies are often associated, producing a confusing symptom picture.

Toxicity.-Copper toxicity is practically unknown under orchard conditions. When induced under artificial culture conditions the symptom is usually that of Fe chlorosis, the severity being related to the level of c u . 2.

K. Chlorine (Cl) 1. Deficiency.-Although C1 is an essential nutrient element, specific symptoms attributable to a lack of C1 have not been observed on fruit or nut trees. 2. Toxicity.-Most stone fruits are known to be injured by excess chloride in the soil. Stunting without specific leaf symptoms may result from moderate salt concentrations. At higher levels apricot leaves cup upward and roll along margins and tip. Margins later scorch. In cherry, small pale leaves with marginal scorch develop and drop early. Almond, plum, and prune show tip and marginal leaf scorch. Symptoms on peach are similar to those on cherry.

L. Sodium (Na) Sodium has not been determined to be an essential nutrient for any deciduous fruit or nut tree species, nor has it been shown to be substitutable for any portion of the K requirement as is the case with some plants. 1. Toxicity.-In “alkali” soils where Na constitutes 15% or more of the exchangeable cations, or under saline conditions where Na is a major component of the soluble salts, Na may be toxic. Although some chlorosis may precede the appearance of burning, the most typical symptoms of excess Na on most fruits and nuts are tip


153

burning and marginal scorch. In apple, leaves may turn yellow and drop in a few days.

M. Sulfur (S) 1. Deficiency.-Sulfur deficiency occasionally occurs in apple, apricot, cherry, peach, plum, and pear orchards. Symptoms are a general yellowing of the younger leaves somewhat similar to N deficiency. In peach, the much smaller chlorotic leaves develop marginal necrosis. Rosettes of small laterals with small pale leaves may develop near terminals. In severe cases, large necrotic areas that result in leaf distortion occur in older leaves. In apple, some interveinal chlorosis occurs.

N. Arsenic (As) 1. Toxicity.-Arsenic toxicity may occur on stone fruit trees growing on old apple orchard sites where large amounts of arsenate spray residues have accumulated in the soil. Peaches, apricots, and almonds are very susceptible to injury. Prunes and cherry are moderately sensitive. Plums, pears, and apples are more resistant. Symptoms appear first on older leaves as a brown to red coloration along the leaf margins followed by a similar discoloration scattered between the veins. The tissue in many of these spots dies and drops out, giving a shot-hole appearance. In severe cases, complete defoliation may occur or young terminal leaves may remain normal. Yield is usually reduced and fruit is astringent.

0. Aluminum (Al) 1. Toxicity.-Aluminum is not considered to be an essential nutrient element, although in very low concentrations it may have stimulatory effects on plant growth. Aluminum, however, is very toxic in small concentrations, especially below pH 4.7. Root malformation, malfunction, and ultimate death are the usual responses to toxic levels of Al. Resulting leaf symptoms are not specific for A1 toxicity, but usually appear as those of Ca deficiency.

111. NUTRIENT CONCENTRATION IN PLANT TISSUES IN RELATION TO NUTRITIONAL DISORDERS The absence of deficiency or toxicity symptoms does not necessarily indicate an optimum plant nutrient status. Incipient nutrient imbalances that may seriously reduce crop yield and quality may exist even in the '

164

HORTICI'LTLTRAL REVIEWS

absence of noticeable growth reduction. Diagnosis of such conditions can be done accurately, rapidly, and economically by chemical analysis of appropriate plant tissues. Leaves are the most commonly used tissue for analysis, but because of the peculiarities of distribution of certain elements within the plant, feeding roots, petioles, fruit, or certain fruit parts may be used for some elements. Leaf analyses, to be of diagnostic value, must be based on standardized sampling methods and the results must be compared only with standard values obtained by those same procedures. Such values are given in Table 3.1. When available, these values refer to concentrations in leaves from the mid-portion of terminal shoots sampled a t about the cessation of terminal growth. This sampling would be during July or August in the North-Temperate Zone, the exact time depending on species and cultivar. For species having pinnate leaves, leaflets from the mid-section of the rachis of mid-shoot leaves should be sampled. If possible, leaves showing symptoms, especially those with necrotic tissue, should not be sampled. If it is impossible to collect leaf samples a t the proper time, or if other than mid-shoot leaves must be sampled, the direction of changes in concentration of the different elements with time and position on the shoot must be taken into consideration in comparing them with standard values. Any statement regarding the change in element concentration with time or position must be qualified. Calcium, Mn, Fe, and Na generally increase in concentration with time if they are continuously available for uptake by the roots. Nitrogen, P, K, Mg, and Zn generally decrease, while others are variable. Change in concentration of some elements from basal to terminal leaves on the shoot will vary, depending upon which side of the level of adequacy the concentrations fall. Nitrogen and P are usually higher in terminal than in basal leaves, whereas Ca is consistently higher in basal leaves. Potassium, Mg, Mn, Fe, Cu, and Zn shift their gradient on each side of the normal range. When these elements are in the deficiency range, retranslocation from the older to the younger leaves produces a lower concentration in the basal leaves. If a n excess of these elements is present, the concentration is usually higher in the older leaves. Boron, Al, and S are variable in this respect. It must be pointed out, however, that if changes in availability occur during the growing season a s a result of deficient or excess moisture, for example, these patterns may vary. Fruit analysis, particularly of pome fruits, has been found to be the most satisfactory means of diagnosing Ca and B deficiencies. As mentioned earlier, the fruit are the first to show symptoms of deficiencies and a number of physiological disorders occurring on the tree or in

Species

Element

NE

Leaf Leaf

July

Leaf Leaf Leaf Leaf Fruit flesh Fruit peel Leaf Leaf l-yearold bark Leaf Leaf

1.0 but 5 2.0 pm, and E = chromosome length < 1.0 pm. Das and Dana (1977) showed that the inheritance of base color mosaic spotting of seed coat can be explained with two independent, non-interacting genes of three alleles each. They proposed the following gene sap green color; tgb,garnet brown color; Md, symbols: T”’,straw color; tSR, dense mosaic spotting; m i ,light mosaic spotting; and m, no mosaic spotting. The orders of dominance are straw color > sap green color > garnet brown color and dense mosaic > light mosaic > nonmosaic. Chatterjee and Dana (1977) cited unpublished work of Das (1977) on the inheritance of a number of rice bean characteristics. Seedling stem color, flower bud pigmentation, petal color, hilum color, earliness, and seed coat color are all monogenically inherited and dry pod color is governed by two interacting genes. The genes conditioning stem color, flower bud pigmentation, seed coat color, and one of the pod color genes are linked. The size of primary leaves, seedling height, plant height, days to flowering, number of pods per plant, number of seeds per plant, and seed size were highly heritable (95.8 to 98.5%) and largely conditioned by additive gene action. Low heritabilities (8.0 to 55.8%) and significant dominance effects were found for green and dry forage yields, succulency of green forage, number of branches per plant, pod size, and seed yield. Considerable heterosis was noted for green forage yield. V. umbellata hybridizes with V. radiata (Ahn 1975; Baker et al. 1975; Dana 1966d, 1967; Evans 1976; Sawa 1974), V. mungo (Biswas and Dana 1975a), V. angularis (Ahn and Hartmann 1978b; Evans 1976), and a naturally-occurring allotetraploid Phaseolus species (Dana 1964, 1965a). Al-Yasiri and Coyne (1966) reported that the crosses between V. umbellata and the species V. mungo, V. angularis, and P. vulgaris resulted in pod set, but the pods collapsed in the early stages of develop-

376


ment. They concluded that these crosses were partially compatible because fertilization probably occurred. The amphidiploids between V. umbellata and the following species are fertile: V. r a d i a t a (Ahn 1975; Dana 1966d, 1967; Sawa 1974), V. mungo (Biswas andDana 1975a),and allotetraploid Phaseolus species (Dana 1964). X. LITERATURE CITED AGBLE, F. 1972. Seed size heterosis in cowpeas (Vigna unguiculata (L.) Walp.). Ghana J. Sci. 12:30-33. AHN, C.S. 1975. Interspecific hybridization between Phaseolus aureus Roxb. and P. calcaratus Roxb. Rural Dev. Rev. 9:63-70. AHN, C.S. and R.W. HARTMANN. 1978a. Interspecific hybridization between mung bean (Vigna radiata (L.) Wilczek) and adzuki bean ( V angularis (Willd.) Ohwi & Ohashi). J. Amer. SOC. Hort. Sci. 103:3-6. AHN, C.S. and R.W. HARTMANN. 197813. Interspecific hybridization between rice bean (Vigna umbellata (Thunb.) Ohwi & Ohashi) and adzuki bean (Vigna angularis (Willd.) Ohwi & Ohashi). J. Amer. SOC. Hort. Sci. 103:435438. AHUJA, M.R. and B.V. SINGH. 1977. Cross between P. aureus X P. mungo. G. B. Pant Univ. Agr. Tech. (India), Ann. Rpt. Res. 1975-1976. p. 71. AL-YASIRI, S.A. and D.P. COYNE. 1966. Interspecific hybridization in the genus Phaseolus. Crop Sci. 6:59-60. AMOSU, J.O. and J.D. FRANCKOWIAK. 1974. Inheritance of resistance to root-knot nematode in cowpea. Plant Dis. Rptr. 58:361-363. ANON. 1975. Mung bean culture and varieties. US. Dept. Agr., Agr. Res. Ser., Northeast Reg. CA-NE-11. ANON. 1976. Annual report, 1975. IITA, PMB 5320. International Institute of Tropical Agriculture, Ibadan, Nigeria. APPA RAO, S. and M.K. JANA. 1973. Inheritance of anthocyanin coloration in Phaseolus mutants. Indian Sci. Congr. Assoc. Proc. 60:302. APPA RAO, S. and M.K. JANA. 1974. Alteration of seed characters in blackgram. Indian J. Agr. Sci. 44:657-660. APPA RAO, S. and M.K. JANA. 1975. Characteristics and inheritance of chlorophyll mutations in Phaseolus mungo. Biol. Plant. 17:88-94. APPA RAO, S. and M.K. JANA. 1976. Leaf mutations induced in black gram by X-rays and EMS. Enuir. Expt. Bot. 16:151-154. APPA RAO, S., S. PADMAJA RAO, and M.K. JANA. 1975a. Induction of non-dormant mutants in black gram. J. Hered. 66:388-389. APPA RAO, S., S. PADMAJA RAO, and M.K. JANA. 1975b. New plant type in black gram. Curr. Sci. 44:679-680. APPA RAO, S. and B.M. REDDY. 1976. Crumpled petal mutants in black gram and cowpea. Indian J. Genetics Plant Breeding 35:391-394.

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ARYEETEY, A.N. and E. LAING. 1973. Inheritance of yield components and their correlation with yield in cowpea (Vigna unguiculata (L.) Walp.). Euphytica 22:386-392. BAKER, L.R., N.C. CHEN, and H.G. PARK. 1975. Effect of an immunosuppressant on an interspecific cross of the genus Vigna. HortScience 10:313. BALLON, F.B. and T.L. YORK. 1959. Crossing the common and scarlet bean (Phaseolus spp.) with Vigna species. Philip. Agr. 42:454-455. BAPNA, C.S. and S.N. JOSHI. 1973. A study on variability following hybridization in Vigna sinensis (L.) Savi. Madras Agr. J. 60:1369-1372. BAPNA, G.S., S.N. JOSHI, and M.M. KABARIA. 1972. Correlation studies on yield and agronomic characters in cowpeas. Indian J. Agron. 17:321-324. BARTON, D.W., L. BUTLER, J.A. JENKINS, C.M. RICK, and P.A. YOUNG. 1955. Rules for nomenclature in tomato genetics. J. Hered. 46:22-26. BHAPKAR, D.G. 1965. Uptake of P32and growth pattern in autopolyploids, and changes in nucleic acid content of germinating diploid and tetraploid Phaseolus seeds. Diss. Abstr. 26:44. BHARGAVA, P.D., N.N. JOHRI, S.K. SHARMA, and B.N. BHATT. 1966. Morphological and genetic variability in green gram. Indian J. Genetics Plant Breeding 26:3 70-3 7 3. BHATNAGAR, C.P., R.P. CHANDOLA, D.K. SAXENA, and S. SETHI. 1974. Cytotaxonomic studies on the genus Phaseolus. Indian J . Genetics Plant Breeding 34A:800-804. BHATNAGAR, P.S. and B. SINGH. 1964. Heterosis in mungbean. Indian J. Genetics Plant Breeding 2499-91. BHOWAL, J.G. 1976. Inheritance of pod length, pod breadth and seed size in a cross between cowpea and catjang bean. Libyan J. Sci. 6:17-21. BISWAS, M.R. and S. DANA. 1975a. Black gram X rice bean cross. Cytologia 40:787-795. BISWAS, M.R. and S. DANA. 1975b. Phaseolus aureus X P. lathyroides cross. Nucleus (India) 18:81-85. BLISS, F.A., L.N. BARKER, J.D. FRANCKOWIAK, and T.C. HALL. 1973. Genetic and environmental variation of seed yield, yield components, and seed protein quantity and quality of cowpea. Crop Sci. 13:656-660. BLISS, F.A. and D.G. ROBERTSON. 1971. Genetics of host reaction in cowpea to cowpea yellow mosaic virus and cowpea mottle virus. Crop Sci. 11:258262. BOLING, M., D.A. SANDER, and R.S. MATLOCK. 1961. Mung bean hybridization technique. Agron. J . 53:54-55. BORDIA, P.C., J.P. VADAVENDRA, and S. KUMAR. 1973. Genetic variability and correlation studies in cowpea (Vigna sinensis L. Savi ex Hassk.). Rajasthan J. Agr. Sci. 4:39-44. BOSE, R.D. 1932. Studies in Indian pulses: V. Urd or black gram (Phaseolus mungo Linn.) var. Roxburghii Prain. Indian J. Agr. Sci. 2:625-637.

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COOP.Rpt. 20:3-5. CLAYBERG, C.D., L. BUTLER, C.M. RICK, and P.A. YOUNG. 1960. Second list of known genes in the tomato including supplementary rules for nomenclature. J. Hered. 51:167-174. CUTHBERT, F.P. and R.L. FERY. 1975. Relationship between cowpea curculio injury and Choanephora pod rot of southern peas. J. Econ. Entomol. 68:105-106. CUTHBERT, F.P., R.L. FERY, and O.L. CHAMBLISS. 1974. Breeding for resistance to the cowpea curculio in southern peas. HortScience 9:69-70. DAHIYA, B.S., K. SINGH, and J.S. BRAR. 1977. Incorporation of resistance to mungbean yellow mosaic virus in black gram (Vigna mungo L.). Trop. Grain Legume Bul. 9:28-32. DANA, S. 1964. Interspecific cross between tetraploid Phaseolus species and P. ricciardianus Ten. Nucleus 7:l-10. DANA, S. 1965a. Hybrid between tetaploid Phaseolus sp. and P. calcaratus Roxb. Biol. Planta 7:7-12. DANA, S. 1965b. Phaseolus aureus Roxb. X tetraploid Phaseolus species cross. Rev. Biol. (Lisbon) 5:109-114. DANA, S. 1966a. Cross between Phaseolus aureus Roxb. and P. mungo L. Genetica 37:259-274. DANA, S. 1966b. Species cross between Phaseolus aureus Roxb. and P. trilobus Ait. Cytologia 31:176-187. DANA, S. 1966c. Spontaneous amphidiploidy in F, Phaseolus aureus Roxb. X P. trilobus Ait. Curr. Sci. 35:629-630. DANA, S. 1966d. Cross between Phaseolus aureus Roxb. and P. ricciardianus Ten. Genetics Iberica 18:141-156. DANA, S. 1966e. Interspecific hybrid between Phaseolus mungo L. and P. trilobus Ait. J. Cytol. Genetics (India) 1:61-66. DANA, S. 1967. Hybrid from the cross Phaseolus aureus Roxb. and P. calcaratus Roxb. J. Cytol. Genetics (India) 2:92-97. DANA, S. 1968. Hybrid between Phaseolus mungo L. and tetraploid Phaseolus species. Japan J. Genetics 43:153-155. DANGI, O.P. and R.S. PARODA. 1974. Correlation and path coefficient analysis in fodder cowpea (Vigna sinensis Endl.). Expt. Agr. 10:23-31. DARLINGTON, C.D. and A.P. WYLIE. 1955. Chromosome atlas of flowering plants. 2nd edition. George Allen & Unwin Ltd., London. DAS, N.D. 1977. Genetics of Phaseolus calcaratus. PhD Thesis, Bidham Chandra Krishi Vidyalaya, West Bengal. DAS, N.D. and S. DANA. 1977. Inheritance of seed coat color in rice bean. Seed Res. (New Delhi) 5:119-128. DE, D.N. and R. KRISHNAN. 1966a. Studies on pachytene and somatic chromosomes of Phaseolus mungo L. Genetica 37:581-587. DE, D.N. and R. KRISHNAN. 1966b. Cytological studies of the hybrid, Phaseolus aureus X P. mungo. Geneticu 37:588-600.

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DE ZEEUW, D.J. and J.C. BALLARD. 1959. Inheritance in cowpea of resistance to tobacco ringspot virus. Phytopathology 49:332-334. DE ZEEUW, D.J. and R.A. CRUM. 1963. Inheritance of resistance to tobacco ringspot and cucumber mosaic viruses in black cowpea crosses. Phytopathology 53:337-340. DHALIWAL, H.S. and K.B. SINGH. 1970. Combining ability and inheritance of pod and cluster number in Phaseolus mungo L. Theor. Appl. Genetics 40:117-120. DUNDAS, B. 1939. Inheritance of resistance to powdery mildew in runner beans (Phaseolus coccineus), tepary beans (P. acutifolius),. yard-long beans (Vigna sesquipedalis) and cowpeas (Vigna sinensis). Phytopathology 29:824. (Abstr.) EBONG, U.U. 1972. Optimum time for artificial pollination in cowpeas, Vigna sinensis Endl. Samaru Zaria Inst. Agr. Res. Samaru Agr. Newsl. 14:31-35. EMPIG, L.T., R.M. LANTICAN, and P.B. ESCURO. 1970. Heritability estimates of quantitative characters in mung bean (Phaseolus aureus Roxb.). Crop Sci. 10:240-241. ERSKINE, W. and T.N. KHAN. 1977. Genotype, genotype X environmental and environmental effects on grain yield and related characters of cowpea (Vigna unguiculata (L.) Walp.). Austral. J. Agr. Res. 28:609-617. ERSKINE, W. and T.N. KHAN. 1978. Inheritance of cowpea yields under different soil conditions in Papua, New Guinea. Expt. Agr. 14:23-28. EVANS, A.M. 1976. Species hybridization in the genus Vigna. Proceedings of IITA collaborators meeting on grain legume improvement, plant improvement. IITA, PMB 5320, June 9-13, 1975. International Institute for Tropical Agriculture, Ibadan, Nigeria. p. 31-34. FARIS, D.G. 1964. The chromosome number of Vigna sinensis (L.) Savi. Canadian J. Genetics Cytol. 6~255-258. FENNELL, J.L. 1948. New cowpeas resistant to mildew. J. Hered. 39:275279. FERY, R.L. and F.P. CUTHBERT, JR. 1975. Inheritance of pod resistance to cowpea curculio infestation in southern peas. J . Hered. 66:43-44. FERY, R.L. and F.P. CUTHBERT, JR. 1978. Inheritance and selection of nonpreference resistance to the cowpea curculio in the southernpea ( Vigna unguiculata (L.) Walp.). J. Amer. SOC.Hort. Sci. 103:370-372. FERY, R.L. and F.P. CUTHBERT, JR. 1979. Measurement of pod-wall resistance to the cowpea curculio in southernpea (Vigna unguiculata (L.) Walp.). HortScience 14:29-30. FERY, R.L. and P.D. DUKES. 1977. An assessment of two genes for Cercospora leaf spot resistance in the southernpea (Vigna unguiculata (L.) Walp.). HortScience 12:454-456. FERY, R.L. and P.D. DUKES. 1979. Genetics of root knot resistance in the southernpea (Vigna unguiculata (L.) Walp.). HortScience 14:406. (Abstr.)

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cowpea. Indian Phytopathol. 30:99-102. SINGH, D. and P.N. PATEL. 1977b. Studies on resistance in crops to bacterial diseases in India, Part VIII. Investigations on inheritance of reactions to bacterial leaf spot and yellow mosaic diseases and linkage, if any, with other characters in mung bean. Indian Phytopathol. 30:202-206. SINGH, D. and J.K. SAXENA. 1959. A semi-dominant lethal leaf mutation in Phaseolus aureus. Indian J. Genetics Plant Breeding 1933-89. SINGH, H.B., S.P. MITAL, B.S. DABAS, and T.A. THOMAS. 1974. Breeding for photoinsensitivity and earliness in grain cowpea. Indian J . Genetics Plant Breeding 34A:77 1- 77 6. SINGH, K.B., G.S. BHULLAR, R.S. MALHOTRA, and J.K. SINGH. 1972. Estimates of genetic variability, correlation and path coefficients in urd and their importance in selection. Punjab Agr. Uniu. Res. J. 9:410-416. SINGH, K.B. and H.S. DHALIWAL. 1971. Combining ability and genetics of days to 50 percent flowering in black gram (Phaseolus mungo Roxb.). Indian J. Agr. Sci. 41:719-723. SINGH, K.B. and H.S. DHALIWAL. 1972. Combining ability for seed yield in black gram. Indian J. Genetics 32:99-102. SINGH, K.B. and R.P. JAIN. 1970. Heterosis in mungbean. Indian J. Genetics Plant Breeding 30:251-260. SINGH, K.B. and R.P. JAIN. 1971. Analysis of diallele cross in Phaseolus aureus Roxb. Theor. Appl. Genetics 41:279-281. SINGH, K.B. and R.P. JAIN. 1972. Heterosis and combining ability in cowpea. Indian J. Genetics 32:62-66. SINGH, K.B. and L.N. JINDLA. 1971. Inheritance of bud and pod color, pod attachment, and growth habit in cowpeas. Crop Sci. 11:928-929. SINGH, K.B. and R.S. MALHOTRA. 1970a. Estimates of genetic and environmental variability in mung (Phaseolus aureus Roxb.). Madras Agr. J. 57: 155-159. SINGH, K.B. and R.S. MALHOTRA. 1970b. Interrelationships between yield and yield components in mungbean. Indian J. Genetics Plant Breeding 30: 244-250. SINGH, K.B. and P.D. MEHNDIRATTA. 1969. Genetic variability and correlation studies in cowpea. Indian J. Genetics Plant Breeding 29:104-109. SINGH, K.B. and P.D. MEHNDIRATTA. 1970. Path analysis and selection indices for cowpea. Indian J. Genetics Plant Breeding 30:471-475. SINGH, K.B. and J.K. SINGH. 1970. Inheritance of black spot on seed coat and shiny seed surface in mung bean (Phaseolus aureus Roxb.). Indian J. Hered. 2:61-62. SINGH, K.B. and J.K. SINGH. 1971a. Heterosis and combining ability in black gram. Indian J. Genetics Plant Breeding 31:491-498. SINGH, K.B. and J.K. SINGH. 1971b. Inheritance of leaf shape in black gram (Phaseolus mungo L.). Sci. Cult. 37:583.

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SINGH, K.K., W. HASAN, S.P. SINGH, and R. PRASAD. 1976. Correlation and regression in green gram (Phaseolus aureus Roxb.). Proc. Bihar Acad. Agr. Sci. 24:40-43. SINGH, M.K. 1973. Inheritance of seed coat color in Phaseolus radiatus L. (Syn. P. aureus Roxb.). Part 111, p. 321-322. Indian Sci. Congr. Assoc. Proc. 60th. Jan. 3-9, 1973, Panjab Univ., Chandigarh, India. Indian Sci. Congress Assoc., Calcutta. SINGH, P., I.B. SINGH, U. SINGH, and H.G. SINGH. 1975. Interspecific hybridization between mung (Phaseolus aureus Roxb.) and urd (Phaseolus mungo L.). Sci. Cult. 41:233-234. SINGH, S.P., H.B. SINGH, S.N. MISHRA, and A.B. SINGH. 1968. Genotypic and phenotypic correlations among some quantitative characters in mung bean. Madras Agr. J. 55:233-237. SINGH, T.P. 1974. Epistatic bias and gene action for protein content in green gram (Phaseolus aureus Roxb.). Euphytica 23:459-465. SINGH, T.P. and R.S. MALHOTRA. 1975. Crossing technique in mung bean (Phaseolus aureus Roxb.). Curr. Sci. 44:64-65. SINGH, T.P. and K.B. SINGH. 1970. Inheritance of clusters per node in mungbean (Phaseolus aureus Roxb.). Curr. Sci. 39:265. SINGH, T.P. and K.B. SINGH. 1972. Mode of inheritance and gene action for yield and its components in Phaseolus aureus. Canadian J. Genetics Cytol. 14:517-525. SINGH, T.P. and K.B. SINGH. 1973a. Association of grain yield and its components in segregating population of green gram. Indian J. Genetics 33:112117. SINGH, T.P. and K.B. SINGH. 1973b. Combining ability for protein content in mungbean. Indian J. Genetics Plant Breeding 33:430-435. SINGH, T.P. and K.B. SINGH. 1974a. Components of genetic variance and dominance pattern for some quantitative traits in mungbean (Phaseolus a u r e us Roxb.). 2. Pflanzenzucht. 71:233-242. SINGH, T.P. and K.B. SINGH. 197413. Heterosis and combining ability in Phaseolus aureus Roxb. Theor. Appl. Genetics 44:12-16. SINGH, U. and P. SINGH. 1975. Colchicine induced amphidiploid between mung (Phaseolus aureus Roxb.) and urd (Phaseolus mungo L.). Curr. Sci. 44:394-395. SINGH, U. and P. SINGH. 1976. Selection index as an effective aid for the improvement of grain yield in black gram (Phaseolus mungo L.). Indian J. Farm Sci. 4:26-28. SINGH, U., P. SINGH, and I.B. SINGH. 1976. Path coefficient analysis for yield attributes in black gram (Phaseolus mungo L.). Indian J. Farm Sci. 4:23-25. SINGH, U., P. SINGH, U.P. SINGH, and I.B. SINGH. 1976. Discriminant function technique for the improvement of grain yield in black gram (Vigna mungo (L.) Wilczek). Trop. Grain Legume Bul. 6:15-16.

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SINGH, U.P., U. SINGH, and P. SINGH. 1975. Estimates of variability, heritability and correlations for yield and its components in urd (Phaseolus mungo L.). Madras Agr. J. 62:71-72. SITTIYOS, P., J.M. POEHLMAN, and O.P. SEHGAL. 1979. Inheritance of resistance to cucumber mosaic virus infection in mungbean. Crop Sci. 19:5153. SOHOO, M.S., N.D. ARORA, G.P. LODHI, and S. CHANDRA. 1971. Genotypic and phenotypic variability in cowpeas (Vigna sinensis).under different environmental conditions. Punjab Agr. Uniu. J . Res. 8:159-164. SOUNDRAPANDIAN, G., R. NAGARAJAN, K. MAHUDESWARAN, and P.V. MARAPPAN. 1975. Genetic variation and scope of selection for yield attributes in black gram (Phaseolus mungo L.). Madras Agr. J. 62:318-320. SOUNDRAPANDIAN, G., R. NAGARAJAN, K. MAHUDESWARAN, and P.V. MARAPPAN. 1976. Genotypic and phenotypic correlations and path analysis in blackgram, Vigna mungo (L.). Madras Agr. J. 63:141-147. SPILLMAN, W.J. 1911. Inheritance of the “eye” in Vigna. Amer. Nut. 45: 513-523. SPILLMAN, W.J. 1912. The present status of the genetics problem. Science 35:751-767. SPILLMAN, W.J. 1913. Color correlation in cowpea. Science 38:302. SPILLMAN, W.J. and W.J. SANDO. 1930. Mendelian factors in the cowpea (Vigna species). Michigan Acad. Sci., Arts & Letters Papers 11:249-283. STRAND, A.B. 1943. Species crossing in the genus Phaseolus. Proc. Amer. SOC. Hort. Sci. 42:569-573. SWINDELL, R.E. and J.M. POEHLMAN. 1976. Heterosis in the mung bean (Vigna radiata (L.) Wilczek). Trop. Agr. 53:25-30. SWINDELL, R.E. and J.M. POEHLMAN. 1978. Inheritance of photoperiod response in mungbean (Vigna radiata (L.) Wilczek.). Euphytica 27:325-333. SWINDELL, R.E., E.E. WATT, and G.M. EVANS. 1973. A natural tetraploid mungbean of suspected amphidiploid origin. J. Hered. 64:107. TANAKA, Y., B. EPHRUSSI, E. HADARN, A. HAGBERG, T. KEMP, A. LOVE, H. NACHTSHEIM, G. PONTECORVO, and M.M. RHOADES. 1957. Report of the International Committee on Genetic Symbols and Nomenclature. Intern. Union Biol. Sci. Colloques. B30:l-6. THAKUR, R.P., P.N. PATEL, and J.P. VERMA. 1977a. Studies on resistance in crops to bacterial diseases in India. Part XI. Genetic make-up of mung bean differentials of the races of bacterial leaf spot pathogen, Xanthomonas phaseoli. Indian Phytopathol. 30:217-221. THAKUR, R.P., P.N. PATEL, and J.P. VERMA. 1977b. Independent assortment of pigmentation and resistance to Cercospora leaf spot in mung bean. Indian Phytopathol. 30:264-265. THAKUR, R.P., P.N. PATEL, and J.P. VERMA. 1977c. Genetical relationships between reactions to bacterial leaf spot, yellow mosaic and Cercospora leaf spot diseases in mungbean (Vigna radiata). Euphytica 26:765-774.

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TIKKA, S.B.S. and B.M. ASAWA. 1978. Note on selection indices in cowpea. Indian J. Agr. Sci. 48:767-769. TIKKA, S.B.S., B.M. ASAWA, and S. KUMAR. 1977. Correlated response to selection in moth bean (Vigna aconitifolia Jacq. Marechal.). Gujarat Agr. Uniu. Res. J. 3:l-4. TIKKA, S.B.S., B.M. ASAWA, R.K. SHARMA, and S. KUMAR. 1978. Path coefficient analysis of association among some biometric characters in cowpea under four environments. Genetica Polonica 19:33-38. TIKKA, S.B.S., S.N. JAIMINI, B.M. ASAWA, and J.R. MATHUR. 1977. Genetic variability interrelationships and discriminant function analysis in cowpea (Vigna unguiculata (L.) Walp.). Indian J. Hered. 9:l-9. TIKKA, S.B.S. and S. KUMAR. 1976. Association analysis Vigna aconitifolia (Jacq.) Marechal. Sci. Cult. 42:182-183. TIKKA, S.B.S., R.K. SHARMA, and J.R. MATHUR. 1976. Genetic analysis of flower initiation in cowpea (Vigna unguiculata (L.) Walp.). 2.Pflanzenzucht. 77:23-29. TIKKA, S.B.S., J.P. YADAVENDRA, P.C. BORDIA, and S. KUMAR. 1973. Variation in moth bean (Phaseolus aconitifolius Jacq.). Rajasthan J. Agr. Sci. 4:50-55. TIKKA, S.B.S., J.P. YADAVENDRA, P.C. BORDIA, and S. KUMAR. 1976. A correlation and path coefficient analysis of components of grain yield in Phaseolus aconitifolius Jacq. Genetics Agr. 30:241-248. TIWARI, AS. and S.RAMANUJAM. 1974. Partial diallel analysis of combining ability in mung bean. 2.Pflanzenzucht. 73:103-111. TIWARI, AS. and S. RAMANUJAM. 1976a. Combining ability and heterosis for protein and methionine contents in mung bean. Indian J. Genetics Plant Breeding 36:353-357. TIWARI, AS. and S. RAMANUJAM. 1976b. Genetics of flowering response in mung bean. Indian J. Genetics Plant Breeding 36:418-419. TOMAR, G.S., S. LAXMAN, and P.K. MISHRA. 1973. Correlation and path coefficient analysis of yield characters in mung bean. SABRAO Newsl. 5: 125-127. TOMAR, G.S., L. SINGH, and D. SHARMA. 1972. Effects of environment on character correlation and heritability in green gram. SABRAO Newsl. 4:49-52. TREHAN, K.B., L.R. BAGRECHA, and V.K. SRIVASTAVA. 1970. Genetic variability and correlations in cowpea, Vigna sinensis, under rainfed conditions. Indian J. Hered. 2:39-43. TRIPATHI, I.D. and M. SINGH. 1975. Association of yield components and their functions in black gram (Phaseolus mungo). Haryana Agr. Uniu. J. Res. 5:260-266. TYAGI, I.D., B.P.S. PARIHAR, R.K. DIXIT, and H.G. SINGH. 1978. Component analysis for green fodder yield in cowpea. Indian J.Agr. Sci. 48:646649. VAN RHEENEN, H.A. 1964. Preliminary study of natural cross-fertilization

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8 Ammonium and Nitrate Nutrition of Horticultural Crops Allen V. Barker

University of Massachusetts, Amherst, Massachusetts 01003

Harry A. Mills

University of Georgia, Athens, Georgia 30602

I. Introduction 396 11. Nitrogen Balances 397 A. Soil and Fertilizer Nitrogen 397 1. Forms of Nitrogen in Soils 397 2. Nitrogen Inputs 398 a. Nitrogen Mineralization 398 b. Fertilizer Nitrogen 399 401 3. Soil and Fertilizer Nitrogen Loss 401 a. Leaching Loss b. Loss of Nitrogen Through the Denitrification Process 402 c. Ammonia Volatilization 403 111. Soil/Plant Relations with Nitrogen Nutrition 404 A. Acquisition of Nitrogen by Plants 404 404 1. Environmental Factors Affecting Acquisition of Nitrates 404 a. Presence and Concentration of Nitrate b. Effects of Other Ions 405 406 c. Light d. Effectsof Carbon Dioxide 407 B. Factors Affecting Acquisition of Ammonium Nitrogen by Plants 407 1. Environmental Factors Affecting Acquisition 407 a. Ammonium Concentration 407 b.pH 408 c. Other Ions 409 409 d. Light and Carbohydrate Status 2. Genetic Factors Affecting Acquisition of Nitrate and Ammonium 410 Nitrogen 41 1 C. Crop Responses to Form of Nitrogen 1.Nitrate Versus Ammonium Form 411 41 1 a. Ammonium Toxicity 395

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b. Nitrate Toxicity 412 2. Physiology of Ammonium Toxicity IV. Literature Cited 414

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I. INTRODUCTION In most cropping systems, available nitrogen is often a more limiting factor influencing plant growth than is any other nutrient. Complicating this problem for agriculture is the fact th a t often less than 50% of nitrogen fertilizer applied to crops ultimately may be utilized by the crop (Allison 1966). Nitrate ions are highly mobile and are not adsorbed by soil colloids. Loss of NzO, N P ,and other oxides of nitrogen are recognized as major contributors to ineffective nitrogen utilization. T o offset these nitrogen losses, agriculturists often add nitrogen in large quantities to maintain adequate levels in the rhizosphere. This excessive use of nitrogen fertilizers can result in undesirable conditions such as the accumulation of nitrate in plant tissues and contamination of ground water supplies via nitrate leaching. More recently gaseous loss of nitrogen as N 2 0 has been recognized as a potential factor leading to deterioration of the ozone layer of the atmosphere (Bremner and Blackmer 1978). Application of ammoniacal fertilizer would seem to offer a potential means of increasing the utilization of applied nitrogen fertilizer because the ammonium ion is not as readily subject to leaching loss or volatilization losses as is the nitrate ion. Th e major disadvantage of ammonium nutrition is th at many plants are sensitive to continuous ammonium nutrition. However, nitrification, the soil process by which ammonium is converted to nitrate, occurs rapidly in most cropped soils, limiting the effectiveness of applying ammoniacal fertilizer as a means to increase the utilization of fertilizer N, but also alleviating the potential toxicity associated with continuous ammonium nutrition. Many sources of nitrogen contribute to the nitrogen balance under the diversified cropping conditions with horticultural crops, but the major inputs of nitrogen can be identified as mineralization of organic matter, nitrogen fixation, and fertilizers (Stanford et al. 1969). T h e contribution of nitrogen mineralization to the nutrition of most crops is limited, and supplemental nitrogen is required under most cropping conditions with mineral soils to achieve maximum yields. Though approximately 35,000 tons of elemental nitrogen cover each acre of the earth’s surface, this gaseous form of nitrogen can be utilized directly only by plants forming a symbiotic relationship with certain bacteria, or indirectly through plant decomposition and the decomposition or release from microorganisms capable of fixing nitrogen (Mulder et al. 1969). Thus, the inability of most horticultural crops to fix nitrogen limits the adaptation of sym-

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biotic fixation to the leguminous crops or to where a leguminous crop is incorporated into a crop rotation system. With horticultural crops it is generally recognized that nitrogen fertilizer is necessary to produce yields or growth rates considered to be economical under most soil and cropping conditions. Though several avenues of nitrogen loss from media have been identified (Parr 1973), the major avenues of nitrogen loss are leaching of the nitrate ion, denitrification, and ammonia volatilization (Keeney and Walsh 1972; Gardner 1965; Parr 1973; Allison 1973; Batholomew and Clark 1965). T h e rate and form of nitrogen utilized by plants are highly influenced by both internal and external factors. External factors such as the form of nitrogen (Maynard and Barker 1969), the concentration and ratio of nitrate and ammonium (McElhannon and Mills 1978), availability of molecular nitrogen (Pilot and Patrick 1972), pH, light, temperature and moisture (Bremner and Shaw 1958; Mahendrappa and Smith 1966), and the presence of a particular anion or cation (Kirby and Mengel 1967) influence the absorption and utilization of nitrogen by plants. Internal factors such as the dual or multiphasic patterns of ion uptake for a particular plant species (Nissen 1974), rate of absorption of other anions and cations (Barker et al. 1965), protein synthesis (Barker 1968), nitrate reductase capacity, and physiological age of the plant (McKee 1962) influence the rate of absorption and assimilation of nitrogen in horticultural crops. I t is our intention in this paper to review selected factors instrumental in the nitrogen nutrition of horticultural crops. 11. NITROGEN BALANCES A. Soil and Fertilizer Nitrogen 1. Forms of Nitrogen in Soils.-Analysis of the soil nitrogen composition shows that nitrogen assumes several valence states and exists in many ionic and molecular combinations. Elemental nitrogen (N2)is one of the more abundant forms of soil nitrogen and exists in the soil atmosphere, dissolved in the soil water, or adsorbed to the soil complex. In comparison to N2, some of the more highly oxidized forms of soil nitrogen are nitrous oxide (N20), nitrogen dioxide (NO,), nitric oxide (NO), and the nitrite (NOz -) and nitrate (NOa -1 ions. T h e primary reduced forms of nitrogen in comparison to Nz are the amino radical (-NH2) contained in amino acids, proteins, and other nitrogenous compounds, ammonia (NH,), and the ammonium ion (NH,'). Bartholomew and Clark (19651, Black (1968), Allison (19731, and Nielsen and MacDonald (1978a,b) have presented excellent reviews on nitrogen and may be consulted for addi-

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tional specific information on inorganic and organic forms of nitrogen in soils, their sources and fate. 2. Nitrogen Inputs.-a. Nitrogen Mineralization.-More than 90% of the total nitrogen in soils exists in organic combinations. Heterotrophic soil microorganisms are involved in the mineralization process termed ammonification in which the ammonium ion is released from organic combinations. Ammonium ions released from organic combinations are subjected to various fates: (1) leached with percolating water, (2) adsorbed to the negatively charged particles of clay minerals and soil organic matter, (3) immobilized by various soil microorganisms, (4) volatilized a s ammonia gas primarily under alkaline condition, (5) utilized by plants to satisfy their nitrogen requirements, or (6) nitrified to the highly mobile nitrate ion. Nitrification occurs rapidly in most soils; this is the primary fate of soil ammonium. Th e nitrification process is mediated by a number of genera of facultative aerobic bacteria, though with most cropped soils the autotrophs, Nitrosomonas and Nitrobacter, are the primary bacterial species concerned. T he contribution of the nitrogen mineralization process to the annual nitrogen balance on cropland was estimated by Stanford et al. (1969) to be in excess of 3 million tons annually. Though many factors such as environmental conditions and cultural practices influence the rate of nitrogen mineralization, the soil type and organic composition are primary factors influencing the quantity of nitrogen released for crop utilization. With mineral soils of the temperate region, nitrogen mineralization is generally 3% or less annually of the total organic N (Bremner 1965; Allison 1973), and as such is less than adequate to supply the nitrogen requirements of most crop plants. With organic soils or histosols, the normal rate of nitrogen mineralization is usually adequate in terms of supplying the quantity of nitrogen needed by most crops (Allison 1973). However, due to various factors such as leaching of the highly mobile nitrate ion and the gaseous loss of nitrogen through the denitrification process, adequate nitrogen may not be available during peak demand periods (Guthrie and Duxbury 1978). High-nitrogen-requiringcrops such as celery have responded to nitrogen fertilization in the peat soil of Everglades (Beckenbach 19391, and vegetable crops grown on the muck soils of New York (Guthrie and Duxbury 1978) and peat soils of Michigan (Allison 1973) have responded to nitrogen fertilization. In contrast, in well drained peats of the USSR (Skoropanou 1968) and in Israeli peats (Aunimelech 1971), higher nitrate levels are generally found, and crops in these soils show little response to nitrogen fertilizers. Th e differences between these organic soils probably can be attributed to the quantity of nitrogen lost through leaching and denitrification.


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The mineralization rate of nitrogen from nursery media composed of organic components, such as peat and bark, is generally considered to contribute little to maintaining the nitrogen balance (Goh and Haynes 1977). Though a high proportion of the nitrogen added as fertilizer may be immobilized in these types of media, the fate and subsequent availability of this nitrogen is presently not known.

b. Fertilizer Nitrogen.-Anhydrous ammonia, nitrogen solutions, and various formulations with urea are leading sources of nitrogen applied to various horticultural crops (Nelson 1968). The horticultural industry’s ever increasing dependence on ammoniacal fertilizers arises from the low efficiency of nitrogen fertilizers due primarily to nitrate leaching and denitrification losses (Allison 1966; Parr 1973). Ammoniacal fertilizers offer a means of increasing the efficiency of fertilizer nitrogen as the positively charged cation is not as readily subject to leaching loss or gaseous loss as is the nitrate ion (Parr 1973). However, rapid conversion of the ammonium ion to nitrate limits the effectiveness of applying ammoniacal nitrogen to cropped soils as a means of increasing the utilization of fertilizer nitrogen (Alexander 1965; Parr 1973). Goring (1962) reported that 66 to 92% of all added ammonium is converted to nitrate in most soils within 4 weeks after application. Lorenz et al. (1972) found that 90% of the nitrogen from (NH4I2SO4and urea had nitrified and leached from the fertilizer band within 40 days after application to a Hesperia fine sandy loam soil in California. For acidic soils of Florida, Polizotto et al. (1975) reported that (NH4)&304was toxic to potato plants. The adverse plant response to (NH4I2SO4was attributed to the low rates of nitrification in these acidic soils. The use of urea and ammoniacal fertilizers in the acidic soils of Florida has resulted in adverse effects on plant growth in the leather leaf fern and citrus industries (Mills, personal observations). Apparently during heavy rains or during periods of high temperatures when frequent irrigations are applied, the nitrate ion is leached from the rhizosphere, exposing these crops to a higher level of the ammonium ion. When these conditions exist, two adverse effects on plant growth occur. Competition between the ammonium ion and potassium reduces the potassium level in the plant tissue and in turn can cause plant wilting. A second effect is a reduction in plant growth. Several factors cause the reduction in plant growth, two of which are the acidifying effects of ammonium in the rhizosphere (see section on pH) and a reduction in the mobility of nitrogen compounds in plants cultured in 50% or more ammonium. Sparks (Sparks and Baker 1975) found that pecan seedlings grown in sand culture on NH4N03 developed symptoms of ammonium toxicity and that the mobility of nitrogen compounds from older to younger leaflets was very low. This reduced mobility of nitrogen compounds has been observed in

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southernpeas (Sasseville an d Mills 1979a,b) a n d lima beans (McElhannon and Mills 1978) when ammonium supplied a t least 50% of the nitrogen in solution culture studies. To increase the efficiency of fertilizer nitrogen, controlled or slowrelease nitrogen fertilizers are employed in all areas of the horticultural industry. Byrne and Lunt (1962) reported th a t 25% of the nitrogen in urea-formaldehyde is soluble in cold water and, in a greenhouse evaluation, found that the nitrogen mineralization rate was 6 to 7% per month. Lorenz et al. (1972) in a field study in California found th a t half of the nitrogen from urea-formaldehyde remained in the fertilizer band 120 days after application to a potato crop. Slightly higher release rates were reported with sulfur-coated urea. Results with these slow-release fertilizers indicate that excellent plant responses can be obtained with slowly maturing crops; however, for rapidly maturing crops, such as vegetables, the release rate of nitrogen may be too slow to be an effective nitrogen source (Lorenz et al. 1972). Even in media where the ammonium ions are rapidly adsorbed by the soil and organic fractions, loss of the nitrogen fertilizer may occur (Cribbs and Mills 1979). Mills and Pokorny (1978) found th a t the growth of tomato plants was reduced significantly as ammonium supplied more of the nitrogen form in a pine bark and sand media. However, incorporating the nitrification-denitrification inhibitor nitrapyrin into the media increased nitrate retention, total nitrogen in the plant tissue, and plant growth. Without the use of nitrapyrin these results would indicate th a t the restriction in growth was due to ammonium toxicity. However, the increase in growth, total nitrogen, and soil nitrates with nitrapyrin incorporated into the media suggests th at the ammonium was unavailable (possible tied up by the bark) and after being converted to nitrate was lost before its utilization by plants could occur. These same trends have been observed in field studies with corn in which soil nitrate levels, plant total nitrogen, and yield were increased when denitrification was inhibited with nitrapyrin (Mills, unpublished data). T he determination of whether to use a slow-release nitrogen fertilizer or single or split applications of nitrogen fertilizer must take into consideration t hat the nitrogen requirements of many crops may be greater a t certain times during the growth cycle. Sayer (1948), Terman and Noggle (19731, and Bar-Yosef and Kafkafi (1972) reported th a t the greatest accumulation of nitrogen in corn occurred between 30 days and 45 days after planting. With sweet corn a rapid increase in the nitrogen requirements was observed a t the whorl stage (25 days after planting), and the nitrogen requirements increased in an exponential fashion until tasseling (Mills, unpublished data). McColm and Miller (1971) reported t ha t maximum yields of cucumber fruits occurred 55 days after planting

AMMONIUM AND NITRATE NUTRITION

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with over 40% of the nitrogen being taken up during the period of rapid fruit development. Mills et al. (1976a) found that radishes ceased to absorb appreciable quantities of nitrogen when root was three-fourths of its marketable size. Roy and Wright (1974) found that nitrogen uptake in soybeans was greatest a t periods corresponding with peak vegetative growth and pod filling. Sasseville and Mills (1979a,b) observed a similar trend with southernpeas. With lima beans peak nitrogen uptake periods were identified with specific physiological stages, flower initiation, pod initiation, and pod filling (McElhannon and Mills 1978). In addition to the time of nitrogen application, the selective absorption of a particular nitrogen form during the growth cycle must be considered. McKee (1962) stated that seedlings of several species absorbed more ammonium than nitrate early in their growth cycle while later this trend was reversed. Ingestad (1972) found that nitrate was accumulated more rapidly by cucumber seedlings than was ammonium. With southernpeas, no preference for ammonium or nitrate was observed early in the growth cycle through flower initiation when these plants were cultured in a 50% nitrate-50% ammonium solution (Sasseville and Mills 1979a,b). However, a preference for nitrate was observed during reproductive development and was intensified under nitrogen-deficient conditions. Similar trends were observed with lima beans (McElhannon and Mills 1978). Kafkafi et al. (1971) reported an ammonium preference with tomatoes though physiological disorders occurred under continuous ammonium nutrition. Additional factors influencing the selectivity of plants for a particular nitrogen form are discussed in subsequent sections. Other problems associated with the extensive use of nitrogen fertilizers have been identified. The nitrification of ammonium to nitrate can result in a nitrate-rich growing medium, promoting conditions favoring excessive accumulation of nitrate in certain plant tissue and ground water resources (Wright and Davidson 1964). In addition, fertilizer applications and the evolution of nitrous oxides through denitrification have been implicated as having a detrimental effect on the ozone layer in our atmosphere (Bremner and Blackmer 1978). 3. Soil and Fertilizer Nitrogen Loss.-a. Leaching Loss.-Leaching of the negatively charged nitrate ion is a significant avenue of nitrogen loss from cropped soils and media. Allison (1966) identified several factors contributing to the leaching loss of nitrogen: (1)the quantity and form of soluble nitrogen, (2) the quantity and time of rainfall, (3) infiltration and percolation rates, (4)water-holding capacity and soil moisture content a t the time of rainfall, (5) evapotranspiration, (6) vegetative cover, (7) upward movement of nitrogen during droughts, and (8) nitrogen availability and uptake by the crop. In addition to these factors the

402


cations associated with the nitrate ion and temperature were found to significantly influence leaching loss (Burns and Dean 1964). Stanford et al. (1969) estimated th at over 2 million tons of N are lost annually from cropland through leaching, but Allison (1973) suggested th a t few d a ta show accurately the amount of nitrogen lost through leaching under field conditions. A summary of lysimeter experiments measuring the leaching loss of nitrogen was presented by Allison (1965, 1966). T he intensive use of irrigation in the horticultural industry increases the potential for nitrogen leaching loss. Studies by Bingham et al. (1971), Stewart et al. (1968), and Ward (1970) have shown a direct correlation between leaching of fertilizer nitrogen and irrigation practices. T he extensive use of irrigation with greenhouse and containerized crops and the subsequent effects on nitrogen loss have not been delineated. Preliminary studies with nursery media by Mills and Pokorny (unpublished da t a ) suggest th at up to 33% of the applied nitrogen can be lost by leaching.

b. Loss of Nitrogen Through the Denitrification Process-Denitrification is a dissimilatory reduction process in which molecular nitrogen or an oxide of nitrogen is formed from the reduction of nitrite and nitrate ions. In soils where atmospheric oxygen is available, Nz and NzO are the primary nitrogen gases evolved in appreciable quantities (Hauck and Melsted 1956; Nommik 1956). Several environmental factors have been identified as influencing biodenitrification. In general, biodenitrification in soils increases with increasing water content (Mahendrappa and Smith 1966),increasing temperature, pH and carbon substrate (Bremner and Shaw 1958), decreasing oxygen availability (Pilot and Patrick 1972), and the presence of a living plant (Cribbs and Mills 1979). A generalized pathway for biodenitrification is given by the following equation:

acid

However, biodenitrification losses from field soils are difficult to determine due to variations among bacterial species, the presence of molecular nitrogen, and changes in the soil environment resulting from the equipment necessary to measure the denitrification process (Bollag 1970).


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Most of the early investigators of the denitrification process indicated that this process was not a significant avenue of nitrogen loss in most soils, though more recent investigations have identified a significant loss of nitrogen through the biodenitrification process occurring as short bursts of NzO evolution (Bremner and Blackmer 1978; McGarity and Rajaratnum 1973). Woldendorp (1962) reported that biodenitrification losses are greater in the rhizosphere where there is a high demand for oxygen by both the roots and the rhizosphere microorganisms feeding on root exudates and cell debris. Mills and Pokorny (1978) and Cribbs and Mills (1979) were able to show that this nitrogen loss through the denitrification process from media containing a plant was great enough to reduce plant growth and total nitrogen content. This is significant in that most of the earlier studies determining denitrification losses were made without the presence of a plant and may indeed explain the earlier concepts regarding low losses of nitrogen through the denitrification process. In addition, the findings of Mills and Pokorny (1978) show that the quantity of nitrogen lost through the denitrification process increases with increments of organic matter, suggesting that denitrification from an organic nursery medium would be extensive. Though the total quantity of nitrogen lost is greater in organic media, loss of nitrogen from soils low in organic matter through the denitrification process is significant and reduces overall plant growth and yield (Mills and Pokorny 1978; Mills, unpublished data). Losses of gaseous nitrogen by chemical means from well drained acidic soils have been suggested (Nelson and Bremner 1969). The quantity of nitrogen lost by this means for cropped soils is generally not known. The complexity of the denitrification process is apparent in that gaseous nitrogen losses can occur partly by biological means and partly by chemical means (Steen and Stojanovic 1971).

c. Ammonia Volatilization.-Rapid losses of gaseous nitrogen have been reported after application of urea and ammonium fertilizers. Several environmental factors that enhance ammonia volatilization from soils have been identified. Ernst and Massey (1960) found that temperature, rate of soil drying, initial soil moisture content, depth of urea incorporation, method of nitrogen application, and soil reactions influenced nitrogen loss as ammonia. DuPlessis and Kroontje (1964) found that ammonia volatilization was directly related to soil pH. Mills et al. (1974) reported that loss of nitrogen as ammonia gas with the soil pH below 7.2 would not result in substantial volatilization loss. Also, the quantity of ammonium nitrogen applied to soils influences the quantity of ammonium volatilization (Parr and Papendick 1966; DuPlessis and Kroontje 1964). Ammonia volatilized from an organic medium is probably very low (Mills

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and Pokorny 1978) and would not be a primary avenue of nitrogen loss under these cultural conditions. 111. SOIL/PLANT RELATIONS WITH NITROGEN NUTRITION A. Acquisition of Nitrogen by Plants 1. Environmental Factors Affecting Acquisition of Nitrates.-The uptake mechanism for nitrate appears to be very complex and to be altered by a number of environmental factors which affect the external supply of nitrate and the physiological and biochemical processes operating within plants.

a. Presence and Concentration of Nitrate.-The nitrate uptake system of roots is apparently inducible, requiring the presence of nitrate for activation (see Jackson 1978). Plant tissues or cells cultured in the absence of nitrate exhibit a lag in uptake upon exposure to nitrate. The rate of uptake increases steadily to a relatively constant value (Minotti et al. 1968; Jackson et al. 1972,1973;Ezeta and Jackson 1975; Neyra and Hageman 1975; Rao and Rains 1976a,b). Nitrate has been shown to induce nitrate reductase (Heimer 1975), and a relationship between nitrate reductase and uptake is possible (Huffaker and Rains 1978; Jackson 1978; Schrader 1978). However, treatments with tungstate or vanadium which inhibits nitrate reductase have not been shown to prevent the development of the nitrate transport system (Heimer et al. 1969; Heimer and Filner 1971; Huffaker and Rains 1978). Transfer of bacteria to a nitrate-free medium resulted in the decline of activity for nitrate uptake (Goldsmith et al. 1973). This decline was more rapid than that of nitrate reductase activity. On the other hand, high concentrations of molybdenum, a constituent of nitrate reductase, appear to increase the activity of the nitrate transport system (Lycklama 1963). More than one nitrate transport system may operate in plants (Huffaker and Rains 1978). Nitrate uptake increases sharply with increases in the external supply of nitrate, and when the supply is high, nitrates will be absorbed in excess of the needs of plants and will accumulate internally. The external supply of nitrate is probably the most important environmental factor controlling the accumulation of nitrates in plants (see Wright and Davidson 1964; Maynard et al. 1976). Generous nitrogen fertilization of crops may elevate concentrations in edible plant parts sufficiently to be of concern in the health of humans or livestock consuming the vegetation (Wright and Davidson 1964; Maynard et al. 1976; Maynard 1978). Kinetic studies on the uptake of nitrate in relation to external concentrations are confounded by the metabolism of nitrate after its absorption. However, sufficient studies have shown that the mechanism is com-


405

plex. At low NOs - concentrations, less than 0.001 M , uptake fits a simple Michaelis-Menten equation (Huffaker and Rains 1978). At higher concentrations, a dual mechanism of absorption becomes apparent so that Michaelis-Menten kinetics are not followed. The second mechanism is probably the functional one which leads to nitrate accumulation in plants growing in a highly fertilized medium. Plants which have efficient mechanisms for nitrate absorption appear to have relatively low Michaelis-Menten (Km) values and, consequently, have a high affinity for nitrate in soils of low fertility. The ecological significance is that the ability of plants to survive and to compete in soils of varying fertility may be shown by differences in their kinetics of nitrate absorption (Huffaker and Rains 1978).

b. Effects of Other Ions.-Nitrate absorption may be affected by the presence of other ions in the environment of the root. Normally, one considers that ions with similar charge and chemical properties might compete in absorption by plants; but, on the other hand, ion absorption is very selective, and little interference is encountered by similar ions a t low concentrations (Elzam and Epstein 1965; Elzam and Hodges 1967; Epstein 1972). However, in the system of complex kinetics a t higher concentrations of ions, ion absorption is generally competitive. This phenomenon may occur in nutrient solutions or in highly fertilized fields. Nitrate absorption, nevertheless, appears to be influenced little by similar ions such as chloride, bromide, or sulfate (Rao and Rains 1976a), but cations, such as calcium, potassium, and ammonium, affect nitrate uptake significantly (Minotti et al. 1968, 1969a,b; Rao and Rains 1976a; Jackson 1978). Increasing the supply of calcium or potassium generally accelerates the rate of nitrate uptake, whereas ammonium ions have an inhibitory effect. The effect of cations, like calcium, on nitrate uptake may be to counter the negative charges on the roots’ cell walls so that nitrate ions may migrate more closely to the plasmalemma and its uptake sites than they could in the absence of these ions (Elzam and Epstein 1965). The mode of the inhibitory action of ammonium ions is also unclear. Jackson (1978) proposed and discussed a number of alternative mechanisms by which ammonium nutrition may alter nitrate accumulation in plants. In microorganisms and cell cultures, amino acids have been implicated as being the inhibitory factor (Goldsmith et al. 1973; Heimer and Filner 1971). However, in order for ammonium ions to exhibit their inhibitory effect on nitrate uptake by plants, they must be present continuously, for pretreatment of roots with ammonium ions does not inhibit subsequent nitrate uptake (Minotti et al. 1969a; Jackson et al. 1972). The inhibition of nitrate uptake in the presence of ammonium ions is apparently incomplete and may be nutritionally unimportant, for the toxicity of am-

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monium ions is diminished in the presence of nitrate, and the growth of plants in a medium with half of the nitrogen supplied as nitrate and half as ammonium is usually as good as that occurring when all of the nitrogen is supplied as nitrate (Mills et al. 1976a,b; McElhannon and Mills 1977). Uptake of nitrate is apparently sensitive to the external hydrogen ion concentration. Above pH 6, nitrate uptake decreases (Rao and Rains 1976a). High acidity does not affect nitrate uptake until the pH falls below 4.5 (Minotti et al. 1969a). Rao and Rains (1976a) observed no decline in nitrate uptake a t pH values as low as 4.0. The driving force for the influx of nitrate has been suggested as being a pH gradient across the plasmalemma (Hodges 1973) with an ATPase extruding hydrogen ion vectorially across the membrane. The reduction of nitrate in the cell could operate in maintaining a hydrogen or hydroxyl ion balance within the cell and a high pH relative to the ambient solution, thus again associating nitrate reductase activity with nitrate transport into the cell.

c. Light.-An effect of light on nitrate acquisition may be related to the supplying of materials to provide energy for nitrate uptake. The rate of nitrate uptake by decapitated wheat seedlings is considerably less than that attained by illuminated intact seedlings (Minotti et al. 1968). Excision of shoots usually causes a greatly diminished rate of nitrate uptake, and nitrate leakage from previous absorption may occur (Minotti and Jackson 1970). Supplying an energy source such as glucose in the nutrient solution aids in the maintenance of uptake activity by excised roots (Minotti and Jackson 1970). Absorption of nitrate appears to be more sensitive to decapitation of shoots and to the removal of an energy source than that of other ions, such as chloride or phosphate (Jackson et al. 1973; Koster 1963). A continual supply of energy appears to be essential for maintenance of nitrate uptake. Light and photosynthesis appear to be the sources of energy in intact plants. Nitrate reduction and its assimilation into organic compounds are closely related to photosynthesis in green plants (Huffaker and Rains 1978; Schrader 1978). Light reportedly has a role in mobilizing nitrate from storage tissue or cell components (Beevers et al. 1965; Huffaker and Rains 1978; Jackson 1978). Light also provides for the synthesis of carbon compounds which generate the reductant for nitrate assimilation and may provide reductant directly for nitrate assimilation (Neyra and Hagemen 1974; Magalhaes et al. 1974; Schrader 1978). Nitrate reductase is activated by light (Beevers and Hageman 1969, 1972; Jordan and Huffaker 1972). Short periods of illumination are sufficient for activation (Jones and Sheard 1972, 19751, and the same kinds of effects on nitrate uptake have been observed (Jones and Sheard 1975). Since activation of nitrate reductase and stimulation of nitrate


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uptake are red/far-red reversible, phytochrome may be involved in mobilizing an inducer or in mobilizing nitrate from a storage pool to a metabolic pool (Jones and Sheard 1975; Briggs and Rice 1972). Nitrate reductase is postulated as having both retention and transport functions (Butz and Jackson 1977). Therefore, activation of nitrate reductase through the mobilization of inducers or by a general stimulation of protein synthesis would enhance nitrate uptake if the absorption and reduction systems coincided (Travis and Key 1971). d. Effects of Carbon Dioxide.-Reduction of nitrate requires the presence of carbon dioxide, as well as light and nitrate, for nitrate reductase activity is diminished in carbon dioxide-free air (Kannangara and Woolhouse 1967; Klepper et al. 1971). On the other hand, nitrate uptake has been shown to be greater in the absence of carbon dioxide than in its presence (Neyra and Hageman 1976; Huffaker and Rains 1978). The effects of carbon dioxide on nitrate uptake are greater a t high light intensities than a t low intensities (Huffaker and Rains 1978). The inhibitory effect of carbon dioxide may be due to the competition of carbon dioxide reduction with nitrate uptake for energy or reducing power generated by light or due to stomata1 closure in the presence of carbon dioxide. The latter effect results in a lessening of transpiration and water flux through the roots to the shoots.

B. Factors Affecting Acquisition of Ammonium Nitrogen by Plants Plants have evolved in soils in which nitrates are the primary form of inorganic nitrogen available for their nutrition; consequently, they have little tolerance for high levels of ammonium nitrogen in their root environment. Ammonium ions are readily absorbed by plant roots, but they must not be absorbed more rapidly than they can be utilized in the cell; otherwise, toxic reactions occur (Barker et al. 1967; Ajayi et al. 1970; Maynard and Barker 1969; Maynard et al. 1966). Theoretically, ammonium-nitrogen should be the preferred form (Reisenauer 1978). It should be used more efficiently in the plant, for it need not be reduced before incorporation into organic matter. In the soil, ammonium is less subject to leaching and to denitrification losses than is nitrate. The ammonium intake by a plant must be carefully regulated, for the tolerance range of plants to ammonium nitrogen is quite narrow and is dependent upon the presence of nitrate in the medium (Mills et al. 1976 a,b; McElhannon and Mills 1977). 1. Environmental Factors Affecting Acquisition.-a. Ammonium Concentration.-As with nitrate, the most important factor affecting the uptake of ammonium ions by plants is the ions’ concentration in the en-

408


vironment of the roots (Munn and Jackson 1978); increasing the total supply of ammonium-nitrogen in a medium may increase its uptake to the point of toxicity in the plant. Toxicity of ammonium ultimately decreases root and total plant growth sufficiently so that total nitrogen intake by a plant nourished with ammonium-nitrogen may be far less than that of plants cultured of nitrate-nitrogen or when the toxic reactions are averted (Barker et al. 1966a; Barker 1967; Maynard and Barker 1969). The proportion of ammonium-nitrogen relative to nitrate in the medium is an important factor governing its acquisition and plant growth response (Mills et al. 1976a,b). Concentrations of ammonium in excess of that required to induce toxicity symptoms in plants can be maintained without adverse effects when nitrate supplies part of the nitrogen form (McElhannon and Mills 1977). If most or all of the nitrogen nutrition of a plant is supplied as ammonium, simply reducing the concentration of ammonium does not eliminate the toxicity. Plants apparently deficient in nitrogen will exhibit symptoms of ammonium toxicity when grown in dilute solutions in which all of the nitrogen is ammoniacal (Barker, unpublished data). Ammonium toxicity symptoms, lesions, and severe wilting developed with snap bean and southernpea plants within 14 days when they were cultured with all-ammonium nutrition a t deficient and sufficient N levels (McElhannon and Mills 1977).

b. pH.-With ammonium nutrition, plants absorb cations in excess of anions and the pH of the growth medium drifts downwardly (Raven and Smith 1976), while nitrate absorption causes an alkaline drift (Smiley 1974). The predominant form of nitrogen supplied to field- and container-grown plants resulted in differences in the rhizosphere pH between nitrate- and ammonium-fed plants of up to 2.2 units (Smiley 1974). Also, the pH of the rhizosphere was lower with ammonium and higher with nitrate in comparison to the bulk soil pH. In nutrient solutions or in sand culture, pH values may fall as low as 2.8 with ammonium nutrition (Maynard and Barker 1969). The decline in pH increases the toxicity of ammonium-nitrogen, for the most favorable pH for its utilization is near neutrality. Even when all of the nitrogen is ammoniacal, nearly normal growth can be obtained if the pH of the medium is buffered near neutrality (Barker et al. 1966a,b; Barker 1967; Sander and Barker 1978). It is interesting that solution pH did not appear to influence the trends of nitrate and ammonium absorption with lima beans as 100% nitrate absorption occurred with solution pH’s ranging from 3.5 to 7.5, while with ammonium, different trends in ammonium absorption occurred a t similar pH’s (McElhannon and Mills 1978). These results suggest that ammonium’s adverse effects on plant growth occur after absorption.


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c. Other Ions.-Nitrate in the presence of ammonium enhances plant growth and increases the total acquisition of nitrogen by plants (Mills et al. 1976a,b). Attempts to overcome the toxicity of ammonium-nitrogen by altering the cationic or anionic composition, with the exception of nitrate, have been unsuccessful and are in part based on the influences which ammonium nutrition has on plant composition. Calcium and magnesium contents are lowered sharply by ammonium nutrition, with these reductions being proportionately greater than those observed for potassium (Barker and Maynard 1972; Barker et al. 1966a; Harada et al. 19681, whereas phosphorus and sulfur concentrations are increased relative to those in plants grown with nitrate nutrition (Barker et al. 1966a; Blair et al. 1970). The decreases in cation uptake have been explained in various ways, ranging from cation competition for absorption sites (Blair et al. 1970) to cation-anion balances including organic and inorganic anions (Kirkby and Hughes 1970; Hiatt 1978). The mechanisms involved by increasing phosphorus and sulfur uptake are also poorly understood but have been postulated as being associated with enhanced cation (NH,') intake (Blair et al. 1970) or, to an effect, on anion uptake mechanisms (Miller 1965). In soils, fixation of K' by NH,' may occur so that potassium uptake by plants is reduced by ammonium nutrition due to the restricted availability of potassium (Barker et al. 1967; Maynard et al. 1968; Ajayi et al. 1970). d. Light and Carbohydrate Status.-Ammonium uptake by plants shows a wide diurnal variation (Van Egmond 1978). The diurnal pattern can be disturbed by providing continuous light or by supplying glucose to the nutrient medium during darkness. Ammonium and nitrate uptake are greater in light than in darkness and increase with increases in light intensity (Van Egmond 1978). The decline in ammonium uptake in darkness is apparently due to the depletion of carbohydrate reserves in roots, for the assimilation of ammonium has high energy requirements (Reisenauer 1978). Absorption and utilization of ammonium-nitrogen are affected by carbohydrate supply and plant age (Street and Sheat 1958). Plants well supplied with carbohydrates are better able to utilize ammonium-nitrogen than are energy-starved plants. Young plants with active photosynthetic mechanisms may be more tolerant than older plants which are declining in photosynthetic capacity; however, older plants with adequate carbohydrate reserves may be quite tolerant of ammonium nutrition, particularly if they have large leaf areas. Seedlings and germinating seeds are very sensitive to ammonium toxicity because of their low carbohydrate contents and inabilities to assimilate ammonium-nitrogen sufficiently rapidly to prevent its internal accumulation.

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Ketoacids, such as a-ketoglutarate, are essential for the initial complexation of ammonium absorbed by roots (McKee 1962; Hewitt 1970). Plants rich in carbohydrates are able to supply the necessary ketoacids for the assimilation of ammonium-nitrogen into amides and other amino acids. Plants which are grown on ammonium nutrition accumulate larger amounts of amides than those grown on nitrate nutrition, and the predominant amide is asparagine rather than glutamine (Barker and Bradfield 1963). Also, plants receiving ammonium-nitrogen have been shown to have higher ratios of aspartate:glutamate than those receiving nitrate-nitrogen (Barker and Bradfield 1963; Richter et al. 1975). These phenomena may be related to the metabolism of the plant being directed toward the conservation of carbon compounds by complexation of ammonium-nitrogen into 4-carbon compounds rather than into 5-carbon compounds. The tolerance of plants for external supplies or for internal accumulation of ammonium-nitrogen is low, whereas the tolerance for nitrate is high. Toxic reactions occur when ammonium nutrition is excessive, but plants will accumulate nitrate and transport it throughout the plant with few toxic effects. On the other hand, ammonium accumulation in plants cannot be tolerated, and its translocation to shoots is especially deleterious (Barker et al. 1966b; Puritch and Barker 1967). Ammonium assimilation into amides within the roots appears to be a detoxification mechanism for plants to survive on high levels of ammonium nutrition (Barker et al. 1966a,b; Maynard and Barker 1969). Proper pH control is essential for assimilation for ammonium-nitrogen into amides in the roots (Barker et al. 1966a,b; Maynard and Barker 1969). Reisenauer (1978) proposed that the ammonium status of a plant can be characterized by the ratio of carboxylates to amides. A rapid drop in carbohydrate level in roots occurs with the initiation of ammonium nutrition (Michael et al. 1970; Reisenauer 1978). Nitrate nutrition does not deplete carbohydrate levels to the same extent, for nitrates can be translocated to the shoots or into vacuoles and stored, processes which cannot occur with ammonium-nutrition without toxic effects. The assimilation of ammonium-nitrogen into amides must be rapid to avoid the toxicity. Therefore, according to Reisenauer (1978) and others (Cox and Reisenauer 1973; Kirkby and Hughes 1970), carboxylates in roots decrease, and amides increase with increases in the level of ammonium supply, producing low carboxy1ate:amide ratios. 2. Genetic Factors Affecting Acquisition of Nitrate and Ammonium Nitrogen.-Plants differ in their abilities to acquire nitrate and ammonium from a medium and in their tolerance of ammonium-nitrogen. Interspecific differences are to be expected. For example, annual range grasses, Auena, Bromus, and Lolium species, were shown to differ in


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abilities to absorb nitrate from a nutrient solution (Huffaker and Rains 1978). Corn (Zea mays L.), soybean (Glycine max Merr.), sorghum (Sorghum bicolor L.), and bromegrass (Bromus inermis L.) also have been shown to differ in capacity to absorb nitrate (Warncke and Barber 1974). Differences in root development and morphology undoubtedly affect the abilities of plants to acquire nitrogen from the soil as well as to affect the efficiency of the uptake mechanism (Huffaker and Rains 1978). Differences for nitrate acquisition within species have been observed for corn (Hoener and Deturk 1938), wheat (Triticurn dururn L.) (Brunetti et al. 1971), and barley (Hordeurn uulgare L.) (Smith 1973). Generally, cultivars with high protein contents absorbed more nitrate than those with low protein contents. Differences among species and cultivars with respect to nitrate accumulation have been documented (Barker et al. 1971, 1974; Maynard and Barker 1974; Maynard et al. 1976). These variations appear to be due to differing abilities of plants to assimilate nitrates (Olday et al. 1976a,b). Plants appear to differ widely in their abilities to reduce nitrate in their roots (Olday et al. 1976b; Pate 1973; Wallace and Pate 1965). Most cultivated plants exhibit some intolerance to ammonium nutrition (Pardo 1935). The tolerance of a plant to ammonium nutrition always should be evaluated by any investigator under prescribed conditions, for tolerance can vary with experimental conditions such as pH of the medium, presence of nitrate, activity of nitrifying organisms, and age of plants. Plants such as the Ericaceae which have evolved or are grown in acidic peaty soils are reported to have a preference for ammonium nitrogen (Cain 1952, 1954; Colgrove and Roberts 1956; Greidanus et al. 1972; Oertli 1963; Townsend 1969). These plants, however, possess the ability to assimilate nitrate (Dirr et al. 1972a) and exhibit toxic reactions to ammonium-nitrogen when grown with nutrient solutions in soil-less culture (Dirr et al. 1972b, 1973). Due to their ability to assimilate ammonium-nitrogen into amides in the roots and bulb, onions have a remarkable tolerance to ammonium nutrition (Maynard and Barker 1969). The assimilation of ammonium nitrogen into amides in onion roots and bulbs parallels that found in the roots of plants grown in a medium buffered a t a neutral pH (Barker et al. 1966a,b). Many of the favorable growth responses of plants to ammonium nutrition have been observed under conditions where the pH of the soil was alkaline (Lorenz et al. 1972, 1974).

C. Crop Responses to Form of Nitrogen 1. Nitrate Versus Ammonium Form.-a. Ammonium Toxicity.-Nitrate has been the primary source of nitrogen in the successful culture of most

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cultivated crops. However, since the work of Muntz (1893) and Maze (1900), it has been known that either nitrate or ammonium salts will serve as nitrogen sources for plant growth. Many studies on the relative merits of nitrate and ammonium nitrogen have been made, and Street and Sheat (1958) reviewed the early work in this field. Reports frequently show that severe injury to plants results from ammonium nutrition in excess of the needs of the plants, for the conditions for optimum utilization differ widely. Often the conditions for optimum utilization of ammonium were not provided (Street and Sheat 1958). On the other hand, Prianishnikov (1951) noted that if the optimum conditions for utilization of each source could be provided, nitrate and ammonium forms of nutrition would be equivalent. Toxicity from ammonium fertilizers occurs when the ammonium ion remains in the root zone in large quantities and when ammonium rather than nitrate is the dominant form of inorganic nitrogen present in acidic media (Barker et al. 1966a, 1967; Maynard and Barker 1969; Maynard et al. 1966, 1968). Conditions which lead to a predominance of ammonium-nitrogen are cool, spring soil conditions which inhibit nitrification (Alexander 1965) or when chemical nitrification inhibitors are added with heavy applications of ammoniacal fertilizers. Some workers have identified ammonia toxicity as that occurring when gaseous NH3 is released from fertilizer bands of urea or diammonium phosphates (Adams 1966; Bennett and Adams 1970; Court et al. 1964a,b).The p H of these bands exceeds 9, resulting in the release of the free ammonia (Allred and Ohlrogge 1964). The phytotoxicity of ammonia is identified as brown and necrotic roots or root tips and death of germinating seedlings near the fertilizer bands. T h e toxicity from ammonium ions, which is exhibited a t low pH, is characterized by greatly restricted root growth which is also discolored (Maynard and Barker 1969). Aerial manifestations of ammonium toxicity are chlorosis and necrosis of leaves, epinasty, and stem lesions (Maynard and Barker 1969; Maynard et al. 1966, 1968). Secondary problems such as potassium deficiency (Barker et al. 1967) and calcium deficiency (Adams 1966) often occur with ammonium nutrition of plants. Germinating seeds are also severely damaged and impaired in further growth by ammonium ions (Barker et al. 1970; Patnaik et al. 1972).

b. Nitrate Toxicity.-Plants can tolerate very high tissue levels of nitrates. Nitrate nitrogen concentrations may rise by several percentage points before phytotoxicity is apparent (Maynard and Barker 1971), whereas a few milligrams of ammonium per gram of tissue (dry weight basis) are sufficient to kill the tissue (Barker et al. 1966a,b; Maynard and Barker 1969). Excess nitrate nutrition is toxic, but the mechanism of toxicity is unknown. Whiptail of czuliflower, a manifestation of molyb-


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denum deficiency, is apparently due to the accumulations of large quantities of nitrate in the leaf margins (Candella et al. 1957). 2. Physiology of Ammonium Toxicity.-Several manifestations of ammonium toxicity have been described. Nonparasitic rots, corkiness, and other damage to roots have been associated with the accumulation of ammonium-nitrogen in fumigated or steam-sterilized soils (Uljee 1964). Similar lesions on the stems of tomato and eggplant (Hohlt et al. 1970) are symptoms of ammonium injury which occurs when potassium is insufficient in the medium (Barker et al. 1967; Barker 1978; Maynard et al. 1968). The development of these lesions occurs only when both factors, excessive ammonium and deficient potassium, are present a t once. Neither factor alone causes the lesions to form, although ammonium ions can induce potassium deficiency through potassium fixation in the clay (Barker et al. 1967). Leaf lesions also form during ammonium toxicity. These lesions appear a s darkened, water-soaked areas or as areas of collapsed tissue which becomes necrotic (Maynard and Barker 1969; Maynard et al. 1966). Marginal burning of leaves as in potassium deficiency is often evident, but supplying potassium does not alleviate the foliar symptoms entirely. T h e necrosis and loss of leaf tissue appear to be one of the most ruinous effects of ammonium toxicity. The appearance of stem lesions is more rapid than that on the leaves, and the development of stem lesions does not imply that lesions will form later on the leaves. Therefore, if plants are fertilized to the point of stem lesion initiation, a signal is given that no further nitrogen fertilization is necessary. In soils where nitrification proceeds normally, yields will not be reduced by stem lesion development a t this stage. T h e root plays a key role in the assimilation of ammonium nitrogen. Plants which complex inorganic ammonium-nitrogen into organic nitrogen in the roots have a much greater range of tolerance to ammonium nutrition than those which translocate ammonium freely to the shoots (Barker et al. 196613; Maynard and Barker 1969). The maintenance of a neutral pH in the root environment favors the detoxification of ammonium in the roots and limits its transport to the shoots. Roots, although injured by ammonium toxicity, are apparently able to tolerate ammonium nutrition as long as an abundant supply of carbohydrate is available (Reisenauer 1978). Once ammonium ions reach the shoots, the biochemistry and physiology of the plant are greatly disrupted. Ammonium ions may inhibit photosynthesis through their uncoupling of photophosphorylation (Krogmann et al. 1959). With the incidence of symptoms of ammonium toxicity in leaves, Barker et al. (1966b) correlated the breakdown of organic nitrogen compounds and the accumulation of uncombined ammonium in the leaves. They showed t h a t the majority of the ammonium-nitrogen accumulating in leaves was from

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an internal source. McElhannon and Mills (1978) found t h a t total nitrogen of the vegetative plant components of lima beans was not a n adequate indicator of the nitrogen available for pod development when ammonium supplied a significant portion of the nitrogen form. Puritch and Barker (1967) showed t h a t the chloroplasts of ammonium-toxic leaves were severely disrupted and t h a t the tissue had impaired photosynthetic capabilities. Similar studies with plants under the stresses of excessive nitrate nutrition have not been made. Ideally, ammonium-nitrogen should be the preferred source, for i t should be used more efficiently in the plant than would nitrate. However, when sufficient nitrogen is supplied to meet the goals of maximum crop production and nitrogen is supplied only in the ammonium form, the toxic reactions of the accumulation of uncomplexed ammonium override the potential increased efficiency of assimilation. Even when the optimum conditions for assimilation of ammonium-nitrogen are provided, yields may not be equivalent to those obtained with nitrate nutrition. With ammonium nutrition, much of the energy production of the plant must go into carbon skeletons for the incorporation of ammonium-nitrogen and its detoxification. This process diverts energy and carbohydrates away from growth. Thus, even when conditions are ideal for ammonium assimilation, growth may be limited relative to t h a t produced with nitrate nutrition. A balance between nitrate and ammonium nutrition normally gives the best of both regimes.

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SKOROPANOU, S.G. 1968. Reclamation and cultivation of peat-bog soils. U S . Dept. Commerce, Springfield, Va. SMILEY, R.W. 1974. Rhizosphere pH as influenced by plant, soil, and nitrogen fertilizers. Soil Sci. SOC. Amer. Proc. 38:795-799. SMITH, F.A. 1973. The internal control of nitrate uptake into excised barley roots with differing salt contents. New Phytol. 72:769-782. SPARKS, D. and D.H. BAKER. 1975. Growth and nutrient response of pecan seedlings, Carya illinoensis Koch, to nitrate levels in sand culture. J. Amer. SOC. Hort. Sci. 100:392-399. STANFORD, G., C.B. ENGLAND, and A.W. TAYLOR. 1969. Fertilizer use and water quality. USDA/ARS 41-168. STEEN, W.C. and B.J. STOJANOVIC. 1971. Nitric oxide volatilization from a calcareous soil and model aqueous solutions. Soil Sci. SOC. Amer. Proc. 35: 277-282. STEWART, B.A., F.G. VIETS, and G.L. HUTCHINSON. 1968. Agriculture’s effect on nitrate pollution of ground water. J. Soil Water Conserv. 23:13-15. STREET, H.E. and D.E.G. SHEAT. 1958. The absorption and availability of nitrate and ammonia. p. 150-166. In W. Ruhland (ed.) Encyclopedia of plant physiology, Vol. VIII. Springer-Verlag, Berlin. TERMAN, G.L. and J.C. NOGGLE. 1973. Nutrient concentration changes in corn as affected by dry matter accumulation with age and response to applied nutrients. Agron. J. 65:941-945. TOWNSEND, L.R. 1969. Influence of form of nitrogen and pH on growth and nutrient levels in leaves and roots of the lowbush blueberry. Can. J . Plant Sci. 49:333-338. TRAVIS, R.L. and J.L. KEY. 1971. Correlation between polyribosome level and the ability to induce nitrate reductase in dark-grown corn seedlings. Plant Physiol. 48:617-620. ULJEE, A.H. 1964. Ammonium nitrogen accumulation and root injury to tomato plants. New Zealand J. Agr. Res. 7:343-356. VAN EGMOND, F. 1978. Nitrogen nutritional aspects of the ionic balance of plants. p. 171-189. In D.R. Nielsen and J.G. MacDonald (eds.) Nitrogen in the environment, Vol. 2. Academic Press, New York. WALLACE, W. and J.S. PATE. 1965. Nitrate reductase in the field pea (Pisum aruense L.). Ann. Bot. 29:655-671. WARD, P.C. 1970. Existing levels of nitrate in waters-The California situation. p. 14-26. I n V. Snoeyink and V. Griffin (eds.) Nitrate and water supply: source and control. 12th Sanitary Eng. Conf. Proc., College of Eng., Univ. of Illinois, Urbana-Champaign. Engineering Publishing Office, University of Illinois, Urbana. WARNCKE, D.D. and S.A. BARBER. 1974. Nitrate uptake effectiveness of four plant species. J. Environ. Qual. 3:28-30. WOLDENDORP, J.W. 1962. The quantitative influence of the rhizosphere on denitrification. Plant Soil 17:267-270. WRIGHT, M.J. and K.L. DAVIDSON. 1964. Nitrate accumulation in crops and nitrate poisoning in animals. Adv. Agron. 16:197-247.


9 The Distribution and Effectiveness of the Roots of Tree Crops David Atkinson Department of Pomology, East Malling Research Station, Maidstone, Kent, U.K.

I. Introduction 425 11. Methods Used to Investigate Tree Root Systems 426 A. Excavation 426 B. Sampling 427 C. Observation 429 D. RootActivity 430 E. Indirect 433 111. The Development of Individual Roots 434 434 A. Growth and Changes with Age B. Root Soil Contact 435 C. Secondary Thickening 437 D. The Functions of Different Root Types 437 E. The Longevity of Roots in the Soil 439 IV. The Seasonal Periodicity of Root Growth 445 A. Variations Among Species 446 448 B. Effects of Pruning and the Vigor of Shoot Growth C. Effects of Cropping 449 D. OtherFactors 449 V. The Distribution of the Roots of Tree Crops 453 A. Apple 453 B. Pear 456 C. PrunusSpecies 456 D. Other Tree Crops 457 E. Conclusions 457 VI. Root Density in Tree Crops 457 458 VII. Root Activity and Effectiveness in Relation to Distribution VIII. The Effect of Environmental and Management Factors on the Distribu462 tion and Efficiency of Tree Roots 424

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IX.

X. XI. XII.

A. SoilType 462 B. Fertilizers 463 C. Irrigation 464 D. Soil Management 465 E. Planting Density and Orchard Systems 469 The Influence of Rootstock and Scion Genotype on the Root System A. Rootstock Effects 471 B. Scion Cultivar Effects 472 Root-Shoot Interactions 472 Conclusions 474 Literaturecited 475

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471

1. INTRODUCTION

The aerial parts of crops are easily visible and are surrounded by a medium which allows for both simple and complex measurements of growth and activity to be made. In contrast, the root system of a plant is encased in a medium which precludes easy examination of even the simplest growth parameters, and so the available information on the functioning of root systems does not parallel that on shoots. Much of the information available on root function derives from solution culture studies in the laboratory and requires considerable qualification before it can be applied to whole systems growing in the variable environment found under field conditions. Despite these difficulties, the realization of the impact of root performance on shoot growth and crop production has led to a recent surge of interest and the publication of a number of texts which discuss or review the plant root system, e.g., Carson (19741, Clarkson (1974), Torrey and Clarkson (1975), Russell (1977), Nye and Tinker (1978), and Harley and Russell (1979). The emphasis in most of these texts, however, is on the roots of cereals and other annual plants; little attention is given to the unique features of perennial root systems or to the physiology of other than primary roots. Studies on the roots of tree crops have been reviewed only infrequently, although Rogers (1939a) listed 118 and Kolesnikov (1971) about 300 relevant publications in their reviews of this subject. In addition, parts of the subject have been treated by Rogers (1952), Rogers and Booth (1959), Rogers and Head (1966), and Head (1973), and reviews dealing with the root system of trees as a whole, with emphasis on forest trees, have been published by Lyr and Hoffmann (1967) and Bilan (1971). T h e root systems of conifers have also been reviewed (Sutton 1969). In view of the material already available, this review concentrates on papers published in the last 15 years, with reference to some earlier papers where necessary for balance and completeness of treatment.

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11. METHODS USED TO INVESTIGATE TREE ROOT SYSTEMS

Information obtained is a function of the techniques available, and its applicability is influenced by the means of data collection. A wide range of methods for the investigation of plant root systems has been reviewed by Bohm (19791, but with major emphasis on field crops. The techniques used in studies of tree crops can be divided as follows: 1. 2. 3. 4. 5.

Whole tree excavation techniques. Sampling methods. Observation techniques. Measurements of root activity. Indirect methods.

A. Excavation

Despite the size of many tree root systems and the weight of soil which needs to be removed to expose the roots (commonly 60 Mg (megagrams or tonnes), and for a tree planted a t 10 m X 10 m with roots to 2 m depth 300 Mg), a large number of these excavations have been performed. In one paper alone Rogers (1935) described the excavation of 177 tree root systems. One of the best descriptions, of perhaps the most frequently used excavation method, is given by Rogers and Vyvyan (1934). In summary, a trench is dug beyond the root area and soil is removed in sections of 50 cm across the ground occupied by the root system until the excavation is complete. T h e soil is gently removed in small pieces from the side of the vertical soil face and the position of each root marked upon a plan which enables the root system to be reconstructed. Following excavation, roots either can be left intact (skeleton method) or cut off within 125 liter units to allow for the recording of root weight, length, etc., within specific soil areas, before, in some cases, reassembly of the root system. The advantages and disadvantages of this method have been discussed by Bohm (1979). Excavation is the only method which gives a clear picture of the entire root system of a plant as it grows in the field. T h e length, weight, volume, surface area, shape, color, 3-dimensional distribution, and other characteristics of both the individual roots and the root system as a whole can be recorded. Because of the small number of really large roots in a root system, this is the only accurate way of estimating the weight of a root system or the distribution of major (> 5 mm diameter) roots. This method also allows for studies of the interrelationships of competing root systems (i.e., Coker 1959; Atkinson et al. 1976) and for studies of the relationships between soil condition, soil cracks, worm holes, etc., and root growth (i.e., Rogers and Vyvyan 1934;

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Atkinson 1973a). However, it requires a large amount of labor, which often can be prohibitively expensive, and can irreparably disturb the soil and so increase variability in a field to be used for other experiments. In addition, there is usually a limitation to the number of specimens which can be removed so the population is seldom sampled adequately. Another serious limitation is the relatively high potential losses of fine roots during the field extraction. Dudney (1972) estimated that 22 to 37% of root weight remained in the soil after a partial excavation. The weight remaining normally would be much less than this with a full excavation.

B. Sampling These include monolith, auger and core sampling, profile wall, partial excavation, and other methods where only a portion of the soil volume is examined to estimate the performance of the root system as a whole. Little use has been made of monolith methods (including needle boards, box samples, etc.) in studies of trees, perhaps because the detailed visual presentation of only part of the root system is of limited value. The use of auger and core sampling methods has been discussed by Weller (1971) who counted the numbers of root tips in 10 replicate 4.0 cm diameter samples. He showed large variations among replicate samples. For example, under a 35-year-old tree of ‘Boskoop’ apple on seedling stock growing in a soil with intermittent waterlogging, the mean number of tips a t 30 to 40 cm depth (the horizon with the highest density of tips) was 203 and the range was 4 to 463. This range paralleled those for the mean numbers of root tips present a t 10 cm intervals from 0 to 150 cm depth, 2 to 203. However, despite this variability, the average patterns of root distribution appeared to be related closely to both soil condition and applied treatments. In a study involving core sampling around 26-yearold apple trees of ‘Fortune’/M 9 (Atkinson 1974a), 100 core samples taken from close to the trunk of a small number of trees gave a standard error of 10% of the mean for roots -= 1 mm in diameter. When a similar number of samples was taken (1) from a larger number of apparently similar trees, (2) a t a greater distance from the trunk, or (3) of roots of a greater diameter, the variation rose sharply so that differences reaching statistical significance were rare. Similarly, in a study of Douglas fir (Pseudotsuga taxifolia (Poir) Britt), Reynolds (1970) found that the “percent standard error” for root weight, estimated from 10 replicate core samples, averaged 30% and ranged up to 95%. Thus, he was unable to detect significant differences among different horizontal zones or depths. However, Roberts (1976) for Pinus sylvestris L. and Ford and Deans (1977) for Picea sitchensis (Bong.) Carr. found lower variability and were able to confirm significant differences in root density with

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depth and horizontal position. In these latter studies, the estimated mean root densities, LA (cm/cm’ soil surface), were 126 and 68, respectively, and, therefore, much higher than that of 7 found by Atkinson and Wilson (1980) for mature trees of ‘Fortune’ apple/M 9. This lower value of LA may have influenced the higher apparent variability. Statistical analysis of core sampling results has been discussed by Persson (1978). Despite these problems, core sampling has been used frequently in studies of fruit and other trees and can allow relatively rapid comparisons of positions and treatments without the disturbance caused by total excavation. Profile wall methods also have been used by a number of investigators, e.g., Oskamp and Batjer (1932) and Atkinson and White (1980). With this method a soil profile is exposed either mechanically or by hand, and then a further layer of soil is carefully removed to expose any roots present a t the surface of the profile. Roots present are recorded with the aid of a grid and a diagram of distribution produced, with each root identified by a dot. This method has been discussed in detail by Bohm (1979). In applying the method to fruit and other trees, the siting of the trench is critical. Root distribution differs between row and interrow areas (Atkinson and White 1980) and with distance from the trunk (Gurung 1979).T h e siting of the trench will thus influence the density of roots recorded, and, if the treatments to be compared have affected horizontal distribution, will interact with assessed treatment effects. To counteract some of these problems, Huguet (1973) suggested using a logarithmic spiral trench, rather than a simple straight trench. T h e modification is based on the hypothesis that under homogeneous conditions the tree root system is a “radient” system with the plantation line as a symmetrical axis. In view of this, a logarithmic spiral will provide the most information, as it samples all parts of the tree area and different areas in proportion to the volume of soil present a t distances from the trunk. Data obtained from both this type of trench and from a straight trench have been compared by Gurung (1979). H e found no significant difference in the mean density of roots detected by the 2 methods, although the overall variability among replicate trees was such that only very large differences (> 60%) would have been detected. Variability among replicates was, however, higher for data obtained from straight trenches. Partly as a result of this, the spiral trench method was better a t detecting significant differences among applied treatments. Partial excavations have been used as means of sampling by some workers. Coker (1958) excavated quarter sections of the root volume, rather than all of it. Rogers and Vyvyan (1934) and Atkinson (1973a) showed that the major roots of some trees are unevenly distributed. In


429

extreme cases most of the root system can be found in half of the soil volume; thus, the excavation of only one segment can give misleading results unless replication is good. Dudney (1972) used a partial excavation technique which involved removing the soil from an area a t the base of a tree within a circle of approximately 1.5 m diameter and a depth of about 30 cm. Following this the root system was strained with a tractormounted jack to expose major roots, and those attached to them unearthed by hand to a depth of about 50 cm. Atkinson et al. (1976) showed that this type of method couId recover about half of the weight of roots obtained by a conventional excavation and could give good relative data. The proportion recovered clearly will be influenced by the root distribution with depth, the soil type (which will influence the ease with which roots free themselves from the soil), and root strength (which will influence the length of root pulled from the soil). Thus, method may interact with treatments. With all of these methods variability within the volume of the root system provides a major problem with respect to easy sampling. With sparse root systems like apple (Atkinson and Wilson 1980) the number of cores found without roots and the non-normal distribution usually obtained also complicate the statistical treatment of results.

C. Observation Total excavation does not allow the growth and development of the root system to be followed easily, although this has been done by frequent core sampling (Weller 1971; Roberts 1976; Ford and Deans 1977). T h e development of the root system and the periodicity of its growth are probably most easily followed using observation windows in the soil. T h e disadvantages of this type of method were listed by Rogers (1934) as: (1) only a small sample of the roots of the plant is seen, although this can be compensated for by replication; (2) the installation of a window against an established tree causes the cutting of some large roots which may induce abnormal patterns of growth; (3) the observed roots are growing against a sheet of glass which may modify water, mineral, and air movement; and (4) the roots are exposed to light during recording. However, the method does allow a series of direct and detailed measurements to be made on the same roots or the same part of the root system over a period of time. In addition, photographic methods such as time-lapse cinematography may be used. T h e measurements obtained can be used for correlation studies with environmental variables in the same way as is done for shoot growth. While early investigations with this type of method were in chambers

430


dug in the field, usually against established trees, some later studies involved trees planted adjacent to a permanent installation (Rogers 1969). This negates the abnormal effects of a severe root pruning of one face of the root system and allows environmental variables to be related to growth and development more conveniently. Measurements obtained with this technique on fruit trees compare favorably with those obtained by other methods, e.g., tracer uptake (Atkinson 1974b), excavation (Atkinson 1978), or water depletion (Atkinson 1978). Roberts (1976) compared the use of core sampling methods and observation trenches in recording the seasonal periodicity of root growth in Pinus syluestris and found that the peak number of root tips seen against the windows of his observation pit occurred two to three months later than peak numbers in the core samples. In addition, the yearly trends were not the same with the two methods. These differences are difficult to explain, but could be due to a number of causes. They emphasize the need for care in interpreting any information on root activity. Two of the principal disadvantages of using permanent root observation laboratories are that trees have to be brought to the root facility (i.e., they cannot be used to investigate problems in the orchard) and that only limited replication is possible. Both of these disadvantages can be overcome by the use of observation tubes as suggested by Waddington (1971) and developed for use in the orchard by Gurung (1979). This method, called the mini-rhizotron method by Bohm (1974), involves the installation of transparent tubes in the orchard soil. Root growth a t the soil tube interface is then observed with either a ‘fibrescope’ (Waddington 1974), a lens and mirror (Bohm 1974), or an industrial tank viewing endoscope (Gurung 1979). Results obtained with this method were correlated significantly with those obtainable in a root laboratory (Gurung 1979). The principal disadvantage of the method seems to be the variation in the length of root visible against the tube, necessitating the use of large numbers of tubes.

D. Root Activity T h e determination of root activity or potential for activity is the most frequent objective of root studies. T h e absence of activity a t any given time or place can be due to either an absence of roots or to conditions which prevent their functioning. T h e effects of a low soil water potential can be transient (Huxley et al. 1974), ar.d while estimates of root activity can positively indicate activity a t a given time and place, they do not reflect the system’s potential for activity under all conditions. Root activity most commonly has been measured with radioactive tracers,


431

usually ‘12P.Tracer is injected into the soil a t a number of points with fixed geometry (distance, position, depth) in relation to the tree. The activity in similar trees, but with either different treatments or placements, is compared. The method has been used successfully in a number of crops, including citrus, coconut, cocoa, coffee, and rubber (Nethsinghe et al. 1968; Soong et al. 1971; Huxley et al. 1974). I t has not always been completely successful in apple, largely because of the high variability among replicates (Broeshart and Nethsinghe 1972; Atkinson 1974). In apple, Broeshart and Nethsinghe (1972) found a similar pattern of activity using both 32Pand 15N as tracers; variation (as coefficient of variation, 30 days after injection) was 53% for 32P,but only 27% for I5N. As a result of this, a greater than 2-fold difference in the rate of uptake from 2 different depths was not statistically significant using 32P, but was with IsN. In a study of variations between replicate trees of Betula and Fraxinus they found “variation coefficients” of 125% and 10096, respectively. In a study of young apple trees, Atkinson (1974b) also found high variability (cv = 50%), as well as very low rates of uptake for apple, relative to that for grass and weed species. Like Broeshart and Nethsinghe (1972), Atkinson (1977) found a similar pattern but reduced variation and more efficient uptake (Atkinson et al. 1979) when 15N, rather than :{*P,was used as a tracer. Patterns of translocation from root to shoot clearly vary during the season. To use the technique as an assay for root activity, one must either standardize on a particular organ for assay or understand the changes in translocation. Nethsinghe (1970) found statistically significant leaf type X root position interactions for both citrus and cocoa, although the effects of these interactions were relatively small compared with those of placement and time. Differences in the levels of activity absorbed into leaves of different ages have been reported in coconut palm (Nethsinghe 19661, Heuea brasiliensis (Soong et al. 1971), and apple (Atkinson 1974b). Leaf age did not always interact with root position, but sampling of similar aged leaves is advisable. Atkinson and White (1980) found that the pattern of apparent 32Puptake from different depths was different for leaves and fruit, with much less apparent effect on fruit. The number of tracer injections needed around a tree has been discussed by Nethsinghe (1970), Huxley et al. (1974), and Patel and Kabaara (1975). Nethsinghe (1970) found that variation among replicates was as high with 32 as with 16 injection points, although the level of activity in leaves was increased probably because of the larger amount of tracer used. Patel and Kabaara (1975) noted maximum uptake of 32P with 30 injection points per tree, with 45 and 60 points having little additional effect.

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T h e pattern of placement of the injection points can be varied. Most workers have used a ring so that all points are equidistant from the trunk, but banding on one side of the tree may be as efficient as a similar quantity of tracer applied in a circular pattern (Pate1 and Kabaara 1975). T o compare activity in herbicide-treated row and grassed interrow areas practically, placements of ,'I2Pand l 5 N along a row, rather than in a circle, have been used with apparently satisfactory results (Atkinson 1977; Atkinson et a l . 1979). Despite the modifications of the tracer method which have been tested, its major limitation remains the high variability among replicates. T h e most probable causes of this variation are: (1) Unequal probability of individual roots containing the applied tracer because of the limited number of injection points. However, the failure to reduce variation by increasing the number of injection points (Nethsinghe 1970) tends to refute this as the main cause, unless the number of points is still too small relative to the volume of the root system. (2) Soil heterogeneity. This is known to occur for P and can have a large effect because of its limited mobility in soil. (3) Variability among trees. This could be a major factor, but large variations occur even where trees are carefully selected for homogeneity (Nethsinghe 1970). (4) Variations among leaves. This is potentially a major factor, although considerable variation can remain even when multiple standardized samples from individual trees are used (Atkinson 1974b).

The use of isotopic placements is probably the most convenient method of directly assessing root activity a t different points within the root volume and over a season. However, like other sampling techniques, it is limited by variation among replicate samples. Root activity also has been determined using the rate of soil water depletion from different zones within the soil as an index of activity. Although the method can indicate activity a t a given time in the season, like the tracer method it does not predict potential for activity. If a plant has a root system which is non-uniformly distributed through the soil (as is normal), but which has more than adequate capacity to supply water to the tree, then the initial pattern of water depletion will reflect root density. This assumes no great variations in soil hydraulic conductivity a t different positions within the profile and low axial resistances of roots, so that roots close to the trunk are not favored. However, as the soil dries, the rate of water depletion will proportion-


433

ately and perhaps absolutely increase from those areas of soil with relatively low root densities (which remained relatively moist) and decrease from the drier areas of higher root density. The rate of water depletion from the former areas (per unit root length and possibly per unit soil volume) then will be higher than that found initially and possibly higher than that from the now drier areas of high root density. This is because of the higher flow rates needed from an effectively restricted root system to maintain transpiration in balance with evaporative demand. Despite these potential problems, patterns of water depletion have been used as indicators of root activity by Cahoon and Stolzy (1959), Atkinson (1978), and Atkinson and White (1980). Cahoon and Stolzy (1959) found good agreement between water depletion and root density for citrus, although they obtained different rates of water depletion/unit length of root in different soil types and a t different depths within one soil type. T h e highest rates of depletion were from deep in the profile, a feature attributed by Taylor and Klepper (1973) to differences in the relative ages of roots. Clearly, this also could be due to differences in soil physical properties. Atkinson (1978) showed good general agreement between root distribution and the pattern of water depletion. Water depletion from below 50 cm depth was highest by trees with a high proportion of deep roots. While initial depletion was related to overall root density, the relationship a t any specific time was less exact, probably as a result of the difficulties outlined above.

E. Indirect Root distribution and density have been assessed by a variety of other techniques. The force needed to remove a tree from the ground often has been used (cf. Fraser and Gardiner 1967) as an index of the size of the root system. Obviously the force needed also will vary with soil type and soil water content. Root weight also has been estimated by using a fixed rootlshoot ratio, from measurements of shoot weight. Any type of indirect method will need careful calibration and can be even more prone to difficulties of interpretation than direct methods. Thus, a review of the literature on methods of assessing the size and activity of tree root systems suggests that major problems are the time and destruction involved in total excavation and the high variability encountered with any type of sampling system. Therefore, the best reflection of root activity and distribution may be given by a combination of methods which reinforce one another.

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111. THE DEVELOPMENT OF INDIVIDUAL ROOTS A. Growth and Changes with Age The mature fruit tree root system includes roots which differ in age, diameter, and degree of suberization. The development of young roots on fruit trees has been described by Rogers (1939b, 1968), Rogers and Booth (1959), Rogers and Head (1962,1966,1969),Head (1968a, 19701, Mason et al. (1970), Bhar et al. (1970), Hilton and Khatamian (1973), Atkinson, Lewis and Jones (19771, Atkinson and Lewis (1979), and Hilton (1979). The young root, as seen through the windows of an observation laboratory, is initially white and succulent with short root hairs. After between 1 week and 4 weeks, during most of the year, it begins to turn brown and the root hairs shrivel (Rogers 1939b). Browning takes an average of 2 to 3 weeks during the period of May through September, but can be as long as 1 2 weeks during winter (Head 1966). The browning spreads as a wave from older regions toward the tip. The irregular waves, sometimes days apart, can spread a t 2 to 3 mm hr - l (Head 1968). The browning of the cortex is followed by its decay and disintegration, due largely to the feeding of soil fauna such as collembola, millipedes, symphilids, mites, and enchytraeid worms (Head 1968a). After the loss of the cortex, secondary thickening occurs in some roots which become part of the perennial root system. Other roots, usually laterals, either remain unthickened or disappear completely (Head 1968b). Horsley and Wilson (1971) suggested that the fate of a root tip is related to its relative primary xylem diameter (P X D), with only large roots, those where the P X D is '25% of that of their parent root, surviving. In apple, roots usually can be divided (Rogers and Head 1969) into extension roots and lateral roots. The thicker extension roots survive while the thinner lateral roots tend to be ephemeral, although this class of roots can be infected by mycorrhizal fungi (Mosse 1957).Head (1970)has shown that under some circumstances they may branch highly and remain with a brown, but intact, cortex for several years without any further development. Growing white roots of apple vary from 0.3 to 2 mm diameter. Extension roots of cherry can be even larger. The maximum rate of growth for apple roots seems to be approximately 1 cm day - l , although lower rates are more usual (Rogers 1939b). Head (1968a) reported a similar rate for cherry, as did Hilton and Khatamian (1973) for apple, quince, and grape. Rates of growth can be affected by many factors. Atkinson and Lewis (1979) found that old living roots in the path of a new root reduced its growth rate by about half. Head


435

(1965) noted that cherry roots grew faster between 1600 and 2400 than a t other times during the day. Hilton and Khatamian (1973) also observed higher rates of root growth during the night, 1800 to 0600, for a range of fruit species. Mason et al. (1970) and Bhar et al. (1970) found that for Mugho pine and plum (Shiro/Myrobalan), respectively, the rates of both elongation and suberization were related to root diameter, with elongation being more rapid in roots of large diameter. The root hairs on apple are much shorter than those found in many other plants, e.g., black currant, rarely being longer than 0.075 mm and more commonly 0.025 to 0.05 mm (Rogers 1939b). In Prunus species (Head 1968a) the root hairs are longer but tend to be irregularly distributed, being most abundant close to soil particles. In apple (Rogers 1939; Head 1964, 1968a) the root hairs appear to exude droplets of liquid, but the mechanism for the liquid’s formation and its function is not known.

B. Root Soil Contact During its life, the contact between a root and the surrounding soil varies substantially. This clearly will affect effectiveness in relation to the uptake of water and mineral nutrients. T h e effect of root contact on radial resistance to water flow has been reviewed by Tinker (1976), and changes in contact and their significance to fruit trees have been discussed by Atkinson and Wilson (1979). Initially, particularly if the root deforms the soil during its growth rather than utilizing an existing channel, contact between the root and soil should be good. However, Sanders (1971) found that 40% of apple roots visible in a root laboratory were not in contact with the soil, while others were in incomplete contact. Atkinson and Wilson (1979) showed that even while roots remained white, the movements of soil fauna, particularly non-parasitic nematodes, adjacent to the root surface could reduce root/soil contact. Water stress caused substantial diurnal fluctuations in the diameter of maize roots (Huck et al. 1970), but was less effective on cherry roots (Atkinson and Wilson 1979). Where shrinkage occurs, it will affect contact, the greatest influence occurring a t times of maximum demand. After the loss of the cortex, contact between root and soil is poor. Rogers (1968) showed that this could cause a 50% reduction in root diameter, leaving an unthickened stele suspended in a root cavity (Head 1968a,b). If the root becomes secondarily thickened, then contact will improve. Head (1968b) found a maximum rate of thickening of 3.7 mm year - I , and Atkinson and Wilson (1979) concluded that within a season an average apple root of diameter 1.5 mm could decrease to 0.75 mm on the loss of the primary cortex and then reestablish contact with the soil. This contact would

436


probably remain good (Head 1968b) as long as the root continued to function. Tinker (1976) suggested th at when uptake per unit root length was low, i.e., 0.015 cm:l cm d - I , then contact would be unimportant, but that when uptake was high, i.e., 0.3 cm:' c m - ' d - I , then contact would be critical. Because the total root length in a fruit tree is relatively short (Table 9.1), Atkinson and Wilson (1979) calculated th a t rates of uptake, within Tinker's high range, would occur when transpiration was high. TABLE 9.1. VALUES OF LA(ROOT LENGTH UNDER A UNIT AREA OF SOIL SURFACE) AND Lv (LENGTH IN UNIT VOLUME) FOR TREES GROWN UNDER FIELD CONDITIONS

Species APPLE Fortune/M 9 26-vear Herbicide square

Deuth of SaApling L, (cm) (cm cm - l )

L" (cm cm -'I

Reference

75

6.8

0.09

75 30

3.4 1.9-2.2

0.05 0.06-0.07

Atkinson and Wilson 1980 Ibid White 1977

120

0.8-4.3

0.01-0.04

Atkinson etal. 1976

0.05

Krysanov 1969

60 80

2.9-3.4 7.5-9.1

0.04 0.05-0.06 0.08-0.11

Ibid Reckruhm 1974 Farre 1979

120 120

3.6-23.8 5.2

0.03-0.20 0.04

Atkinson 1978 Atkinson 1977

91 or 122 26-69

0.29-0.56

Cockroft and Wallbrink 1966

60

7-8.2

0.12-0.14

Reckruhm 1974

Merton Glory Sweet Cherry/ F 12/1-2-year

120

15.3

0.13

Peach

91 or 122 17-68

0.29-0.56

Atkinson and Wilson 1980 Cockroft and Wallbrink 1966

CONIFERS Loblolly pine

10

5

0.5

Douglas fir Scots pine Sitka spruce

107 183 -

77 126 68

0.72 0.69 -

Grass interrow Cox/M 26 12-year Golden DelicioudM 9 5-year Pepin SafrannyVForest stock Pepin Safrannyll Red-leaved paradise Golden Pearmain/M 4 Cox/M 26 Golden Delicious/M 9 1-3-year* Cox/M 26* PEAR Williams/Keiffer Beurre Bosc, Conference/ Scedling

PR UNUS

* Estimated from root laboratory measurements-maximum

Kramer and Bullock 1966 Reynolds 1970 Roberts 1976 Ford and Deans 1977 values.


437

C. Secondary Thickening

The development of secondary root thickening has been discussed by Head (1968b) for fruit trees and by Fayle (1975) for forest trees. Head (196813) found that both apple and cherry roots may thicken for a period of consecutive years and then stop and, once having stopped, may not start thickening again for several more years. He illustrated this with an apple root which increased in diameter 0.52 mm in 1963 and 0.48 mm in 1964, and then showed no further increase between 1964 and 1967. Some roots survive in the soil as isolated steles for many years without visible thickening. Knight (1961) found that activity in old roots moved as a wave from the junction with the stem to the root tip. However, Head (1968b), in a more extensive study, could find no definite pattern; maximum rates of thickening occurred in July through August, the highest measured rate being 1.84 mm in 27 days on a root originally 3.18 mm in diameter. Root thickening coincided with the late season peak of new root growth, but occurred later than either shoot growth or most trunk thickening. D. The Functions of Different Root Types

The absorption of water and mineral nutrients by plants is often assumed to occur exclusively through the younger parts of the root system, i.e., root tips and areas with root hairs. This type of root has been termed “absorbing root” by Kolesnikov (1971) and this logic underlines Weller’s (1971) principle of counting root tips in core samples as a means of determining root activity. Both basal and apical segments of the seminal roots of barley were able to absorb and translocate both and 32P(Clarkson et al. 1968). Subsequent studies on cereals and annual plants (Harrison-Murray and Clarkson 1973; Graham et al. 1974; Ferguson and Clarkson 1975) have shown that, to a greater or lesser extent, most of the root system is able to function as an absorbing surface, although the rate of absorption is greater in apical areas. For forest trees, Kramer and Bullock (1966) showed that the permeability of suberized roots was 7 to 82% of that of comparable unsuberized roots. T h e absorption of water and minerals by fruit tree roots has been discussed by Atkinson and Wilson (1979, 1980) and Wilson and Atkinson (1978, 1979). Atkinson and Wilson (1979) found that woody roots could function in absorption. T h e uptake of 32Pby white and woody roots (which were of a larger diameter) of F12/1 cherry stocks was similar on a basis of surface area (ng P mm - 2 h -*), but higher in white roots on a volume (ng P mm - 3 h - l ) basis. Both types of roots translocated a similar proportion of absorbed material and absorbed similar amounts of water. Sur-

438


prisingly, they found that the variation between replicate samples of woody roots (on a basis of surface area) was less than that for white roots. This suggested that injury points or lenticels, which had been suggested as possible entry points into old roots (Addoms 1946), were unlikely to be the major pathway because these, being of irregular distribution, would be expected to lead to relatively high variation. Wilson and Atkinson (1979) found that the uptake of 45Cawas higher in woody roots of F12/1 cherry, while that of RGRb was higher in white roots. White roots translocated about half of the amount of both elements absorbed, while woody roots translocated a higher proportion of RGRb, but a very variable amount of 45Ca.Atkinson and Wilson (1980) found that woody roots of both F12/1 cherry and M 27 apple were able to absorb calcium, and suggested that their ability to function in absorption was likely to be general among fruit tree species. T h e ability of different root types to function in absorption is probably related to their anatomy. MacKenzie (1979) found that most, but not all, of the events occurring in the development of the endodermis, a layer critical in relation to absorption, happened nearer to the tip in apple roots than in those of annual species. T h e early stages in the development of the casparian strip occurred simultaneously opposite both xylem and phloem poles 4 to 5 mm from the tip, as in annual species. However, 16 mm from the tip suberin lamellae developed on endodermal cell walls opposite the phloem poles, and 30 mm from the tip an additional cellulosic layer was formed internally to the suberin. Opposite the xylem poles this change occurred 100 mm behind the tip. Both of these occur a t about 320 mm in barley with most wall thickening occurring 50 to 200 mm from the tip. These processes are unrelated to color changes. One hundred to 150 mm behind the tip the root begins to turn brown, coinciding with the early division of the phellogen and the production of secondary xylem. Rosaceous plants develop, in addition to the endodermis, an inner layer of the cortex known as the phi layer. The lignification of its walls occurs simultaneously with that of the casparian strip. These cells have many plasmodesmata in their radial and outer tangential walls but relatively few in the inner tangential wall. The earlier development of the endodermis in apple must influence the balance of apoplastic and symplastic movement in roots of different ages. T h e function of the phi layer is unclear, although it seems well equipped to move substances to the xylem via “gaps” in the endodermis. It may increase root resistance. Taylor and Klepper (1978) suggested that diurnal root shrinkage would not occur if radial resistance to flow was higher in the endodermal zone than in the cortex. T h e phi layer may therefore prevent diurnal shrinkage in cherry (Atkinson and Wilson 19791, and aid in conservation of water. Passioura (1972) showed that a high root resistance helped to conserve water by distributing it


439

more equally over the season. Landsberg and Fowkes (1978) suggested that the optimum length and total resistance of a root were functions of the ratio of axial and radial resistances. As roots of most fruit trees are often long (Rogers 1939a) (Tables 9.2 to 9.51, this implies substantial radial resistances, perhaps created by the combination of the phi and endodermal layers. As in apple, the cherry endodermis appears to become suberized within 5 mm of the root tip (Atkinson and Wilson 1980). In this species nutrient uptake appears to decline more rapidly with increasing distance from the tip (Clarkson and Sanderson 1971) than in cereals. In a secondarily thickened root the increase in root diameter following the loss of tissues external to the pericycle is due to production of phellem and phelloderm (bark) and xylem (wood). To reach the xylem of a woody root, water or ions must cross both the periderm (phellem, consisting of approximately four to six suberized cells, phellogen and phelloderm) (Wilson and Atkinson 19791, and the phloem, although the latter is permeated by parenchymatous rays. Clarkson et d.(1978) demonstrated that suberin lamellae alone need not prevent uptake, while Atkinson and Wilson (1980) suggested that the failure of the periderm to act as a barrier to movement might be related to the deposition of the suberin on the inside of the cellulose cell wall, rather than within the wall as in the casparian strip of the endodermis. Therefore, this should leave the apoplastic path viable in the phellogen, although symplastic movement might be affected unless substantial numbers of plasmodesmata are present. The effectiveness, or contribution to nutrient and water uptake, of the different root types will depend upon the relative amounts present, inherent rates of absorption, contact with the soil, and differential environmental effects on different roots (Atkinson and Wilson 1980). However, in tree crops all roots, rather than just those newly produced, are apparently effective to some extent.

E. The Longevity of Roots in the Soil The survival of roots in the soil has been reviewed in detail by Head (1973) and discussed by Kolesnikov (1968). Root death is part of a natural cyclic process which returns mineral nutrients to the soil and feeds the soil flora and fauna, whose activities are important in relation to soil structure. Copeland (1952) found that the proportion of dead roots in Pinus species increased with age, rising from 2.9 to 3.7% a t 15 years to 6.3 to 18% a t 35 years. With younger trees most dead roots were < 6 mm diameter, but on the older trees some roots > 25 mm were dead. For fruit trees, Kolesnikov (1966) found that in apple, pear, and sour cherry seedlings, 2 to 4.8%of root tips were dead. The rate of root

Unworked 5-year-old M2 M3 M4 M9 MM 106 A2 4-year-old MM 102 Boskoop, Zuccalmaglio, Ontario, Ananas Reinette/M 2 mature trees Papirovka, Antonovka/M 3 12-year-old Papirovka, Shafran, Lenii, Golden Winter Pearmain,

Cultivar and Age Cox/M 1, M 2, M 9 16-year-old

Various soil types

Soil Type Fine sandy loam Clay with flints Clay with impeded drainage Compact sand Brick earth over sand

3.8

71 36 (year 6)

0.85 0.80 0.80 0.40 0.50 0.80 0.75

0-30 0-30 30-100

1.0 1%> 1.0

25 34 Range 1.8 (M 9)9.6(M 3)

0-30 30-100

1%>1.0

28 62% 32% 67%

38% 61%

62%

0-40 (2-year-old) 80-100 (12-year-old)

Framework: below 40 cm

Fine roots: 0-50 5% 50-100 48%

0-30

12%>1.0

26

Babuk 1971

Kolesnikov 1971

Weller 1965

Kovall977

Depth of Main Root Zone (cm) and % Roots Contained (When Stated) Reference 0-30 73% Coker 1958

Vertical Spread (m) 6%> 1.0

Radial S read YmZ) 29

TABLE 9.2. THE DISTRIBUTION OF APPLE ROOTS

.p 0

SI

Calcareous Clayey Chernozem

Cox/M 9 6-year-old Boskoop, Golden Winter Pearmain/M 9

Sandy loam

Golden Winter Pearmain/M 4 Golden Delicious. Red Delicious, Jonathan/M 4 10-year-old Pseudopdzolic Newton Pippin, Cinnamon Wellington/M 4 Forest and 20-year-old Alluvial Meadow London Pippin, Reinette de Champagne/M 2, M 4 Pseudopodzolic Newton Pippin (NP), Cinnamon Wellington (Well)/M 4 Forest and Alluvial Meadow Sandy soil Jonathan/M 4 with high water table Cinnamon Golden DelicioudM 7 Heavy clay Jonathan, Ch ern ozem Richared Delicious Mantuaner/M 9 3-year-old Sandy loam Golden Delicious/M 9 5-year-old

White Winter, Calville/M 4 Jonathan/M 4 4-year-old

67% (HDP')50% (LDP2)0.3 Greater than branch spread,

0-25 >50 (LDP)* 0-25 >50 (HDP)' 0-30

2 (high density 1.2 planting')-,5 lower density planting2)

10-30

49% 25%

54% 15%

73%

Diasamidze and Soziashuiti 1976

20-40

Weller 1966

Atkinson 1976

Atkinson 1978

Doichev 1977 Perstneva 1977

Tamasi 1964

Stoichkov et al. 1975

Stoichkov et al. 1974

20-80

0-60 0-60

2.4

3 (NP)-2.6 (Well)

1.4

Reckruhm 1974 Angelov 1976

0-30 10-60 80%

Tanas'ev and Balan 1976

20-80

Most 12.6

Highest coicentration 0.8

1.4

Vertical Spread (m)

Prize Wagener/M 9 Papirovka; Antonovka/Seedling 12-year-old Boskoop/Seedling

Snow Calville,

Deep loam

98-104

Marly Rendzina

Zuccalmaglio/M 9

2

4.2-4.4

1.o

0.8-1.0

Marly Rendzina

20-170

Moderately high density 0-25, moderate density 25-50 Moderate densitv 0-25. lowmoderate 25-50 11-30

Atkinson 1973b

Fine roots: 0-30 75% Main roots: 0-45 90% 20-80

Ananas Reinette/M 9

Atkinson 1973a

0-30

1.5

Weller 1971

Polikarpov and Adaskalilsii 1977 Kolesnikov 1971

Weller 1965

Nurmanbetov and Andronov 1976 Weller 1965

Coker 1959

0-30 59-7596

5%>1.0

4

10-14 67-SO%, (7 3Xbranch spread 47

Weller 1971

0-70

1.5

Weller 1971

Kolesnikov 1971

Reference

10-70

Depth of Main Root Zone (cm) and % Roots Contained (When Stated)

1.6

Pm2) highest density beside trunk 38-43 3.4-3.6

Radial

S read

CultivadM 9

Cultivar and Age Soil Type 6-year-old Seedling (mature) PaDirovka. AntonovkalM 9 12-year-old Golden Winter Pearmain/M 9 Pseudogley 7-year-old Boskooa/M 9 Pseudodev - 7-year-old Cox/M 9 Sandy loam 16-year-old Sandy loam Fortune/M 9 26-year-old Fortune/M 9 Sandy loam 26-year-old


P

z ! L

=!

XI

3:

U

N

4 &

~

Prostrate forms 5-year-old

Reinette de Champagne/ paradise stock Cultivard Malus sieuersii Cultivars 22-year-old Cultivars 5-year-old

Boskoop, Zuccalmaglio, Ontario, Ananas Reinette/Seedling Boskoop, Golden Winter Pearmaidseedling Jonathan/ Malus sylvestris 5 cultivadcrab stock

Jonathan/Seedling

3 cultivadforest stock

Golden Winter Pearmain/Seedling 30-year-old BoskoodSeedlinp

35-year-old

Dernopodzol

Degraded Chernozem Light soil

Drift sand

Loam

Range of soil types

Loess loam with Parabraunerde Pseudogley on clay

Intermittently wet loam Loess loam

36 53% in 1 3

Most 0.8-1.8

64

Most 12.5

2.5 X crown spread 2 X crown spread

8.6

1.5

2

30-60

1

Tamasi 1965

Weller 1966

Weller 1965

Tamasi 1965

Babaev 1968

Weller 1971

Weller 1971

0-40 80-88% (conventional cultivation) 59-81% (deep ploughing) 10-50 69%

20-80

Ryzhkov 1972

Krayushkina et al. 1977

Nurmanbetov and Andronov 1976 Kolesnikov 1966

20-40 (with cover Ghena 1965 crop) 0-40 (no cover) 20-40 Ghena 1964a

Below 25

0-50

Fine roots: 0-50 37% 50-100 33%

0-30 21-42%

20-75

0-50

0-100

0-50

2

2

2

w

rp

51

m

B

z

4

~

1

~~

HDP= High density planting. LDP = Low density planting.

Cultivars

Cultivars

Reinette Simirenko

Delicious/vigorous stock Pepin Shafrannyl 3 cultivars

Cultivar and Age


Terrace

Soil a t 80% water holding capacity 50% capacity 2-3 X crown projection

Inner terrace: 0-20 Outer: 20-40

Deeper than above

20-60

20-90 cm

To impenetrate subsoil

Sandy soil

65%

De th of Main Root &ne (cm) and % Roots Contained (When Stated) 0-40 15-45

Vertical Spread (m)

Terrace

Soil Type

Radial S read ?mz)

Ryhakov and Dzavakjanc 1967 Kairov eta1.1977

Danov et al. 1967

Zerebcov 1966

Luchkov 1971 Potapov 1971

Reference


445

TABLE 9.3. THE DISTRIBUTION OF THE ROOTS OF PEAR TREES, SOME ON QUINCE STOCKS

Cultivar Cultivad quince stock, wild pear Williams/ Keiffer Abate Fetel/ quince Beurre Bosc, Conference/ Seedling 6 cultivad Quince A Quince Quince Quince

Soil Type Degraded Chernozem

Radial Spread (m2)

Sandy soil Sandy loam Shallow soil Highest density 0.3

Depth of Main Root Zone Vertical (cm) and % Spread Roots Contained (m) (When Stated) Reference 2-3 20-40 Ghena 1964a 3.5 40-60 1 1.2

Terrace soil 0.6

0-50 0-60 Fine roots: 25-50 0-30

Cockroft and Wallbrink 1965 Manzo and Nicotra 1967 Reckruhm 1974

10-50

Milanov 1977

Inner area: 0-20 Outer area: 20-40

Kairov et al. 1977

Fine: 0-20 40-60

52% 19%

Kovall977 Mursalov 1966

shedding is influenced by environmental conditions. In gooseberry 25 to 72% of the roots are normally shed in a year (Kolesnikov 1968). In a dry year, irrigation reduced the proportion of dead roots from 85 to 75%. In fir and pine trees, Orlov (1966) estimated the average life of a root to be 3.5 to 4 years, while in fruit trees Head (1973) showed that significant numbers of roots survive for over 3 years. There seems to be no available data on the maximum survival of roots or information on the extent to which very old roots are able to continue with either absorption or translocation. IV. THE SEASONAL PERIODICITY OF ROOT GROWTH

Regardless of whether white roots alone or all roots function in absorption and metabolism under field conditions, the growth of new roots is important to the system. All brown and woody roots originate from white roots, so that any factor affecting new root production ultimately will influence total root length. In addition, the presence of a flush of new growth will greatly increase total root length. This may be critical during some periods in the year and allows the possibility, as a result of variation in the position and depth of new growth, for adaptation to prevailing environmental conditions. Studies of the periodicity of new growth have been reviewed by Rogers (1939b1, Rogers and Head (1969), and Lyr and Hoffmann (1967).

446


A. Variations Among Species

Lyr and Hoffmann (1967) noted considerable variability to exist among tree species. The basic seasonal pattern, apart from the complicating effects of the cultural practices reviewed later, will be influenced by major environmental variables, e.g., soil temperature and soil moisture. Rogers (1934) and Atkinson (1973c), among others, have observed the effects of short-term fluctuations in soil temperature on root growth. Lyr and Hoffmann (1967) concluded that most authors (beginning with Theophrastus 372 to 287 BC) agreed that in the spring, root growth begins before shoot growth. Richardson (1958) found that the roots of Acer saccharinum began growth at 5”C, while bud expansion began a t 10°C, and suggested that a lower temperature optimum for root growth was a partial explanation for this pattern. In apple, the onset of root growth seems to occur a t a temperature of 6.2”C (Rogers 1939b). After its initiation, root growth follows an irregular time course (Lyr and Hoffmann 1967) with periods of active growth alternating with less active periods. Engler (1903) concluded that, in central Europe, all tree species have a maximum period of root growth in May through June, followed by a rest period and then a second growth period in the autumn. However, other work, reviewed by Lyr and Hoffmann (1967), has not always shown this clear two-peak curve. These reviewers, however, concluded that maximum growth most frequently occurred in early summer. The root growth of fruit trees has been studied for many years using observation laboratories a t East Malling Research Station. Rogers (1939b), studying established trees of ‘Lanes’ Prince Albert’ on M 1, M 9, and M 16, found little root growth during the winter and a major peak of growth in June/July. In some years, and with some stocks, there were also peaks in spring (May through June) and autumn (September). For ‘Worcester Pearmain’/MM 104, Head (1966) noted a small peak in May and a larger peak beginning in July and continuing until October. The end of the initial peak corresponded approximately with the beginning of active shoot growth and that of the second peak with leaf-fall. Root growth in ‘James Grieve’ and ‘Crawley Beauty’, both on M 7, exhibited a similar periodicity, with a single peak in June/July (i.e., as usually found by Rogers (1939b)), despite large differences between cultivars in the time of bud burst. Subsequent studies (Head 1967; Rogers and Head 1969) confirmed the general pattern of a peak of growth in May through June ending a t the time of vigorous shoot growth, and a second peak in August through October beginning after shoot growth had ended. Head (1967) also reported that the periodicity of root growth a t a number of different distances from the trunk was similar. This bimodal periodicity for root growth may be due to competition between shoots and roots for carbohydrate reserves. Quinlan (1965) showed that photosynthates from the eight youngest leaves on a shoot


447

were exported mainly to the shoot; only when there were more than eight leaves did photosynthates begin to reach the roots. Priestley et ul. (1976) found that when 14C02was fed to the basal leaves of extension shoots virtually none of the 14Cwas transferred to the root region before shoot extension was under way, while 45% of translocated 14Cmoved to the roots during and after the main period of shoot growth. Similarly, Katzfuss (1973), in a study of a number of cultivars on M 4, found little 14C-labelled assimilate moving to the roots, but large amounts to the shoots, in trees of ‘Golden Delicious’ where root dry matter production was poor. In the other cultivars incorporation into roots exceeded that into shoots after July, reaching a maximum in September when incorporation into leaves was minimal. Lucic (1967) found that reducing relative light intensity from 1300 to 740 relative units reduced by 45% the rate of root growth in 1-year-old pear seedlings within 2 to 3 days. T h e growth rate increased again within three days of re-illumination. However, this interrelationship between shoot and root growth is probably not due entirely to carbohydrate reserves, for growth hormones also are likely to be involved (Richardson 1957). As previously reported, Voronova (1965) noted 5 peaks of root growth in apple, Ryhakov and Dzavakjanc (1967) 3 peaks in 1-year-old trees and 2 peaks in 5-year-old trees, and Zakotin and Atanasov (1972) 4 peaks which were related to flowering, seed development, flower initiation, and fruit ripening. Kolesnikov (1966) observed that root growth extended over a period of five to nine months. Like Head (1966) and Abramenko (1977), Koseleva (1962) found in apple a peak of activity in April through May. Head (1967) found that in unpruned plum trees, in contrast to apple, there was only one major peak of root growth which extended from early May until late July. H e attributed this to weaker shoot growth in plum. Atkinson and Wilson (1980) reported a single major peak of root growth, May through August, in unworked trees of the plum stocks St. Julien A and Pixy. Bhar et al. (1970) observed the highest growth in July through September, while Skripka (1977) found two peaks (spring and autumn), and Voronova (1965) four peaks of growth. For trees of ‘Merton Glory’ sweet cherry on both F12/1 and Colt rootstocks, Atkinson and Wilson (1980) reported a single peak of growth extending from May until mid-July (F12/1) or mid-August (Colt). With Prunus cerusifera Ehrh, Bulatovic and Lucic (1972) found one to three peaks of activity, with the main peak in July/August. Here the number of peaks was influenced by soil condition, reduced by drought, waterlogging, and high temperatures. In apricot (Iglanov 1976) root growth continued for most of the year, with maximum activity in June/ July and September/October. The periodicity of root growth in pear trees has been described in detail

448


by Head (1968a). He found root growth beginning later in the spring than for apple, and with only a single peak of growth as in plum and cherry. Again, Head attributed this to the relatively weak shoot growth that he found in pear. In contrast, Mursalov (1966) found the most active root growth in MarchIApril, MayIJune, and August through October, with its onset preceding shoot growth. Similar patterns of growth to those found in fruit trees have been noted in other tree species. Ovington and Murray (1968) found a single major peak of activity in birch which began after the end of leaf expansion and ended before leaf-fall. In tea, Fordham (1972) described periods of maximum shoot growth associated with minimal root growth. Here root growth was also severely reduced by drought. Mason et al. (1970) showed that in Mugho pine growth extended from April through November, but was most active in the summer, while for Pinus syluestris, Roberts (1976) found the highest activity to be in March through August, earlier than the July through September period given as most active by Ford and Deans (1977) for Sitka spruce. In addition to other internal factors, the pattern of root growth also seems to vary with tree age. Atkinson and Wilson (1980) showed that in newly planted trees of ‘Golden Delicious’IM 9 one major peak of root growth coincided with shoot growth. However, when the same trees were 3 years old the main peak of root growth was delayed until the rate of extension shoot growth decreased. Age effects in addition will be complicated by the effects of cropping. T h e basic patterns of root growth appear to differ among species, perhaps because of the wide range of environmental conditions under which the studies reviewed were conducted. These, and some cultural factors which influence them, are reviewed subsequently.

B. Effects of Pruning and the Vigor of Shoot Growth Head (1967) showed for both apple and plum that the vigor of shoot growth, as influenced by pruning, affected both the periodicity and elitent of new root growth. In plum, late spring pruning stimulated strong shoot growth in JuneIJuly while reducing root growth, and induced a second peak of root growth in AugustISeptember. Similarly, Head found that pruning of apple stimulated shoot growth, reduced root growth during the most active phase in JuneIJuly, and prolonged inactivity in summer. In pear pruning reduced and delayed the onset of root growth (Head 1968a). Similar effects of pruning were reported by Haas and Hein (1973) and Knight (1934) for apple. In black currant shoot removal in late July caused an immediate reduction in the length of white root present (Atkinson 19721, while in tea pruning caused root growth to cease


449

for three months (Fordham 1972). Root growth seems to be retarded during active shoot growth. However, the usual balance between tree shoot vs. root weights (Rogers and Vyvyan 1934; Atkinson et al. 1976), implies either increased growth during other periods, or survival of more of the roots produced so as to maintain the overall balance. Schumacher et al. (1972) found th at foliar applications of “Alar” (2,2-dimethylhydrazide of succinic acid) to apple retarded both shoot and root growth. I n raspberry, root growth in the initial year was best in those plants with the best shoot growth (Atkinson 1 9 7 3 ~ ) . C. Effects of Cropping

Head (1969a) investigated the effect of cropping on the periodicity of new root growth. From July onwards even light crops of fruit reduced root growth; in some years fruiting eliminated the major growth peak observed in deblossomed trees in July through September. Weller (1967) noted t ha t cropping reduced the length of fine roots present and th a t variation in root length was reduced in vegetative trees. Ryhakov and Dzavakjanc (1967) observed one peak of growth in cropping trees and two in vegetative trees, while Dzhavakyants (1971) reported root growth in spring-summer in cropping trees and, additionally, in autumn in noncropping trees. Adverse effects of cropping on root growth also have been detailed by Semina (1971), Golikova and Grachev (1973), Atkinson (19771, and Zakotin and Atanasov (1972). Avery (1970) investigated the effect of cropping on growth in a range of rootstocks of varying vigor, including M 2, M 26, 3430 (very vigorous), and 3426 (more dwarfing than M 9). Th e latter had the largest effect, particularly on root weight. In this stock fruiting caused a net decrease in root volume, presumably because roots lost due to natural cycles (Kolesnikov 1968) were not replaced. Chalmers and van den Ende (1975) found t ha t the proportion of the annual increment of dry weight going to the root system decreased from 20% in a young tree to 1% in a mature tree. Corresponding values for percentages of dry weight production going to the fruit were 30 and 70%, respectively. I n addition, the ratio of top to root growth increased from 1 to 4 with increasing age. These studies suggest t ha t cropping is likely to be responsible for much of the yearto-year variation in root growth.

D. Other Factors T h e periodicity of root growth can be affected by a range of other potential factors, both internal and external to the tree. Defoliation of apple trees four to six weeks before natural leaf-fall reduced the length of

Cultivar/Mahaleb

Subkhany, Isfarak/Seedling Cultivars Krasnoscekij 6- 7-year-old Meilleur d'Hongrie, Paviot, Luiset, Tardive de Bucharest/ 6 stocks CHERRY Sour cherry/Mahaleb Germersdorfer/Mahaleb Sour cherry/Mahaleb

Cultivars/Myrobalan

APRICOT Ungarische Beste, Timpurii de Thyrnau, Paviot, Tirzii de Bucuresti/Myrobalan 12-year-old Paviot/Myrobalan, apricot, peach, plum stock

Species and Cultivar ALMOND

Degraded Chernozem

2.1 (1-year-old)10 (5-year-old)

Sandy

1.7-3.4 Few vertical roots Few>O.9

Ghena 1964b

Tamasi 1975 Tamasi 1973

11-40 21-50 0-20 20-60

8%

Tamasi 1976

Iglanov 1976 Popescu 1963 Bespecalnaja and Smykov 1965 Lupescu 1965

Ghena 1965a

Lupescu 1961

Reference Dziljanov and Penkovl964b Ghena and Tertcell962

1-40 87%

Fine roots: 20-40

Superficial

0-25

30%

Fine roots: 19-40 Framework: 20-60

Depth of Main Root Zone (cm) and % Roots Contained (When Stated) 20-60

0-20 2.85-3.90 depending on cultivar 2-2.5

2.4 (1-year-old) 0.8 13.3 (5-year-old)

Most 12.5

Greater than crown

Vertical Spread (m) 97% 2 mm diameter

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a t 0 to 25 cm depth, but only 35% of roots < 2 mm. A more superficial distribution of the root framework, as compared with the finer roots, has been described by Atkinson (1973b1, Weller (1965), and Kolesnikov (1971). B. Pear

T h e root distribution of pear trees and pears on quince stocks has been studied less than that of apple, although a thorough description is given by Rogers (1933). In contrast to apple, some pear cultivars have a much more vertically orientated root system. The extent varies among rootstocks and is greater for seedling pear than for quince stocks, although even (pear) stocks lack a true tap root. They have, however, many roots descending to 1 m and a smaller number to greater depths. The horizontal component of the root system is better developed on quince stocks. Information on pear root distribution is given in Table 9.3. Horizontal root spread ranged from 7 to 28 m2 (Rogers 1933), depending upon cultivar, and was 2 to 3 times that of the branches, as in apple. Many roots (62 to 80%) were found in the 1 m2 nearest the trunk and 92 to 100% within 4 m2. Manzo and Nicotra (1967) reported most roots to be in the central 0.3 m2 area. The reported maximum depths of penetration for pear roots range from 0.6 to 3.5 m with, as in apple, the zone containing most roots being much closer to the surface, i.e., 0 to 60 cm depth and with as much as 52% (Mursalov 1966) between 0 cm and 20 cm depth.

C. Prunus Species The form of the sour cherry root system has been described by Kolesnikov (1971) and is similar to that of apple and pear in having both horizontal and vertical components which can vary in their relative development. Information on root distribution in almond, apricot, cherry, peach, and plum is given in Table 9.4. T h e area of soil exploited by sour cherry roots varied from 2 (Tamasi 1973) to 20 m2 (Kolesnikov 1971) and was greater than that of the branch system. Radial spread increased with age and varied among soil types (Tamasi 1973,1976).T h e maximum depth of root growth ranged from 0.8 to 5 m, with values of 1 to 2 m being most common. However, as in other fruit types, the majority of roots were found in a more restricted zone (0 to 60 cm depth), with significant numbers found a t 0 to 25 cm. Again the distribution of the root framework seemed to be more superficial than that of fine roots. There was no large difference in root distribution among the different Prunus types.


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D. Other Tree Crops Information on the root distribution of a wide range of other tree crops, some grown for fruits and some for wood, is given in Table 9.5. Horizontal root spread ranged from 2 to 63 m2 and usually exceeded that of the branches. T h e depth of root penetration varied from 1 to 3.7 m, although most roots seemed to be a t 0 to 50 cm and in many cases, 0 to 30 cm. T h e depth of root growth and that of the zone containing most roots appeared to increase with age (Inforzata and De Carvalho 1967) and to be affected by soil type (Atanasov 1965). E. Conclusions

The root systems of tree crops have many similarities. In the species discussed, the horizontal spread ranged from 2 to 100 m2, most commonly 10 to 20 m2, the vertical spread from 1to 9 m, commonly 1 to 2 m, and the zone containing most roots was 0 to 50 cm depth. However, root distribution merely indicates a potential for activity and need not be the same as effectiveness. Distribution changes with age, differs among varieties, cultivars, and soil types, and can be modified by soil management and orchard factors such as pruning and spacing. These aspects are discussed in detail in the remainder of this review. VI. ROOT DENSITY IN TREE CROPS The density of roots in the soil is important for the absorption of water and mineral nutrients, and has been discussed in relation to annual plants by Newman (1969), Andrews and Newman (1970), and Newman and Andrews (1973), and for tree crops by Atkinson and Wilson (1979, 1980). Root density can be expressed relative to either soil surface area (LA = cm cm - 2 ) or soil volume (Lv = cm cm - 3 ) . Some estimates of these parameters for fruit trees are given in Table 9.1. Reported values of LA for apple range from 0.8 to 23.8, with 2 to 6 being most common. In pear and peach the maximum value reported, 69, was much higher and similar to some reported for conifers (68 to 126). Newman (1969) reviewed data for a wide range of species and found that, in Gramineae, reported values were in the range of 100 to 4000 and in herbs 52 to 310, i.e., both considerably higher than in fruit trees. The consequences of a low L A value have been discussed by Atkinson and Wilson (1979,19801, who described the situation as follows. When a plant transpires, it withdraws water from the soil. This will come initially from soil immediately adjacent to the root with this zone being replenished from bulk soil. If the rate of withdrawal exceeds the rate of water movement through the soil to the

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root, i.e., the rate of uptake exceeds soil hydraulic conductivity, then the soil adjacent to the root will become drier than the bulk of soil and the rate of water flow into the root will decrease and may result in water stress. This process is important both to water and nutrient supply. Localized drying, and thus gradients of water potential a t the root surface, will reduce the uptake both of minerals thought to be moved by mass flow, e.g., calcium, and those where the diffusive characteristics of the soil are important, e.g., potassium and nitrate. If root density is high, flow rates always will tend to be low and gradients a t the root surface will be rare. Where root density is low, as in fruit trees, the contrary will be true. Newman (1969) calculated that soil resistance a t the root surface would become high, i.e., > 0.2 MPa, only when LAwas < 10. In orchards this seems likely to occur. In addition, Atkinson and Wilson (1980) emphasize that root density varies with depth, so reduced soil water potentials will not be the same a t all depths and this will, therefore, affect the balance of nutrient uptake from different parts of the soil profile a t different times during the season. When plants compete with one another, as often occurs in an orchard, nitrogen uptake will depend upon the relative amounts of root on different plants (Andrews and Newman 1970). In addition, the uptake of phosphorus, because of its limited diffusion in soil, always will be dependent upon root length. In fruit trees relative root length seems to be limited, and so rates of uptake per unit root length are likely to be high. In a review of nutrient flow rates into roots, Brewster and Tinker (1972) concluded that average rates of nutrient inflow for a range of species were 1 pmol cm - l s - * for N and 0.1 pmol cm - l s - l for P. For fruit trees Atkinson and Wilson (1980) suggested that comparable values would be 8.5 pmol cm - l s - l for N and 0.56 pmol cm - l s -1 for P. As a result of these high flow rates, fruit trees are liable to be more susceptible to both competition, particularly from species with high LA values, and to adverse soil conditions, than species which have lower relative flow rates. VII. ROOT ACTIVITY AND EFFECTIVENESS IN RELATION TO DISTRIBUTION

T h e previously reviewed studies (p. 437) of the ability of woody and other older roots to function in absorption suggest that the whole of the tree root system and all of its roots should be considered with respect to the system’s effectiveness. New growth is, however, important in increasing the size of the root system (so exploiting new soil volumes and re-exploiting others), giving the flexibility to adapt to changing conditions and to the production of growth substances. Atkinson (1974) found a close correlation in 2-year-old apple trees of


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‘Cox’IM 9 between the length of white root present and the uptake of :3zP injected into the soil a t a number of depths, both growth and activity peaking in AugustISeptember. As the season progressed, both the distribution of new growth and relative uptake from depth increased. In 26-year-old apple trees of ‘Fortune’/M 9 a substantial amount of was absorbed from 90 cm depth where excavation showed a substantial root presence. The translocation of absorbed tracer from any given depth seemed to be equally efficient a t different times during the season. Nicolls et al. (1969), Broeshart and Nethsinghe (1972), Atkinson (1974b1, and Shorokhov (1976) all found maximal uptake from relatively superficial placements, 10 to 20 cm depth, in young apple trees. In older trees (Atkinson 1974b; Atkinson and Wilson 1980) there was much more activity a t depth. Atkinson (1977) and Atkinson and White (1976a) showed a good relationship between the horizontal distribution of white apple roots as seen in a root laboratory or by excavation and the distribution of the uptake of both 32P and 15N. Moreover, Atkinson et al. (1979) and Atkinson and White (1980) compared the horizontal distribution of roots exposed by a profile wall technique with the uptake of 15N.Although root distribution was a reasonable guide to root activity averaged over a season, the presence of roots a t any given point in time, even under conditions apparently favorable for activity, did not assure root activity. The mechanism by which activity was switched off and on was unclear. Using similar trees, Farre (1979) found a pattern of root distribution with depth similar to that described by Atkinson and White (1980) on the basis of the uptake of 32P,This relationship was not as strong for trees under herbicide than for those under grass. In coffee the position of maximum root activity within the soil volume can change rapidly within a single season (Huxley et al. 1974). Many workers have tried to relate root distribution to water absorption. Because the amount of water held in soil is finite, the relationship may change as the season progresses (p. 4321, and as trees age. Atkinson (1978) showed that the relationship between new growth and water depletion was good in young trees a t a range of spacings, but poorer for older trees. Detailed results on root activity and water depletion are discussed in later sections with respect to applied treatment effects (p. 464-469). Faust (1980) suggested that new root growth, photosynthesis, and calcium uptake were causally related. H e found that the application of simazine, known to inhibit photosynthesis, to apple seedlings in water culture decreased both new root growth and calcium uptake. These adverse effects were overcome by feeding sucrose to the plants. However, suberized and woody roots are able to absorb calcium (Atkinson and Wilson 1980), and the need for an adequate length of root, energy for

460


calcium uptake, or an additional factor produced in parallel with photosynthesis may be more important in affecting calcium uptake than photosynthesis itself. T h e relative lengths of white and woody roots present a t different times in a season and on trees of different ages have been discussed by Wilson and Atkinson (1979). The length of white root varies during the season (see p. 445). As a result the relative length of brown root on 2-year-old apple trees could fluctuate from approximately 15% a t the height of the main peak of new growth to approximately 100% in winter or a t times of no new root production, as in mid-summer. Atkinson and Wilson (1979) found no association between the periodicity of new root production and the depletion of soil water, which provides additional evidence for the role of brown roots in the absorption of water. Root growth will be affected by the incidence of pests and diseases in the soil. A detailed review of this is outside the scope of this paper, but the following examples illustrate potential effects and the need to consider the effects of other organisms which are present in real soil situations. Rogers and Head (1969) reviewed the effects of specific apple replant disease, which prevents normal growth on a site where trees of the same species recently have grown. The roots of affected trees are darker in color than healthy roots, grow less vigorously, and develop fewer lateral branches. The effect is most obvious where new trees have been planted, but probably also occurs within the root system of an established tree. Rogers and Head (1969) pointed out that the type of root growth in apple changes, in both amount and morphology, with tree age. Sewell (1979) has shown that species of Pythium, which commonly occur in the soil, can have a major effect on root development, while Zentmyer (1979) has discussed the effect of Phytophthora cinnamomi on Persea indica and Mircetich et al. (1976) the effect of Phytophthora on cherry roots. Pitcher and Flegg (1965) described the effects of nematodes of the species Trichodorus viruliferus Hooper on individual apple roots. The activity of apple roots also can be beneficially influenced by microorganisms. Mosse (1957) showed that apple seedlings infected with an endogenous mycorrhiza grew better than uninfected seedlings, while Atkinson and White (1980) suggested that this might be the cause of the very high rate of phosphorus uptake found with apple trees grown under grass, but with abundant irrigation. T h e effectiveness of roots is influenced by soil temperature. Tromp (1978, 1980) found that apple trees on M 9, M 26, or M M 106 rootstocks had a minimal Ca/K ratio a t 18" to 24"C, as a t this temperature K uptake was maximal, while that of calcium was less affected by temperature. Gur Hepner and Mizrahi (1976) showed that temperatures above 25°C had an adverse effect on both root and total growth, al-


461

though some of the adverse effects could be overcome by potassium applications. Gur, Mizrahi and Samish (1976) reported differences among rootstocks in their reactions to supra-optimal temperatures. They found an optimum temperature of 25°C for M 1, M 2, M 9, M 25, MM 109, and some local stocks, and 30°C for M 7. Susceptibility to supraoptimal temperatures was variable and influenced by the scion cultivar. As soil temperature varies both with depth and time of year, under field conditions, this is likely to influence the relative activity of roots a t different depths in the soil and to interact to give changes within a season. Because of their distribution with depth, under field conditions, different parts of the tree root system are exposed to different nutritional conditions. The effect of treating different parts of the root system with different concentrations of nutrient solutions has been discussed for apple by Taylor and Goubran (1976). They found that non-fed root parts could be supplied with enough phosphorus by redistribution to maintain active growth. However, the proportion of the root system fertilized influenced the plants’ P content, although uptake by any part of the system was independent of the activity of other parts. The pattern of P absorption from areas differing in P concentration was related to the relative concentrations, while the concentration of P in non-fed roots remained low irrespective of the concentration in the roots in well fed areas. Atkinson and Wilson (1980) have verified this latter point for N, but not for K under field conditions. Variations in nutrient concentrations among parts of the root system have been studied in Pinus contorta by Coutts and Philipson (1976, 1977) and Philipson and Coutts (1977). They found that when different nutrient concentrations were applied to the two halves of a root system, root growth was stimulated only in the half exposed to the higher concentration, although nutrient concentrations increased in both halves. Thus, translocated nutrients had little effect on growth which differs from Taylor and Goubran’s (1976) results. Philipson and Coutts (1977) found that different parts of the root system seemed to compete for assimilates, with the enhanced growth of one part of the root system being accompanied by the reduced growth of another. Atkinson and White (1980) have suggested that this occurs when different horizontal sections of the root system are subjected to different soil water potentials, while a similar effect with respect to vertical parts of the root system has been demonstrated by Atkinson et al. (1976). Coutts and Philipson (1977) have shown that if gradients of nutrient supply among different parts of the root system are removed, then previously deprived parts are able to respond to improved nutrient supply. Thus, the perennial root system retains the plasticity to respond to changing nutrient conditions. Although one of the major requirements of a tree root system’s ef-

462


fectiveness is physical support, this seems to have received little attention in recent years. Rootstocks are, however, known to vary in their abilities to support trees (Brase and Way 1959).Rogers and Parry (1968) investigated the effects of deep planting of trees of M 7 on anchorage and performance. They found that deep planted trees developed a new root system a t the surface, but initially maintained enough roots a t depth (although these were replaced by the surface roots) to give good anchorage. These trees were better anchored than the control trees which were planted normally. Root distribution is likely to influence the effectiveness of support. The relationship between root distribution and activity or effectiveness is not simple. Different root distributions will vary in their effectiveness for different purposes. The optimum root system in the field is likely to be a result of a series of interactions. VIII. THE EFFECT OF ENVIRONMENTAL AND MANAGEMENT FACTORS ON THE DISTRIBUTION AND EFFICIENCY OF TREE ROOTS

A. Soil Type Many papers deal with general effects of particular, often local, soils or soil types upon root growth, e.g., Weller (1971). Clearly, soil type can influence the tree root system. Rogers and Vyvyan (1934) described the effects of loam, light sand, and heavy clay soils on trees on a range of rootstocks. Depth of rooting increased from sand to loam to clay. In loam and sand the root system had the same general conformation, a shallow scaffold with vertically descending roots. In clay, most roots sloped down and grew in the subsoil, although with a marked depth boundary (90 cm) where a seasonal water table occurred. The total weight of root on a given scion/stock combination was in the order loam > clay > sand. The ratio of stem to root varied from 2.0 to 2.5 for loam and clay to 0.7 to 1.0 on sand. Coker (1958) also investigated the effect of a range of soil types on a range of rootstocks. Impeded drainage and compaction a t depth checked downward growth of roots. Like Rogers and Vyvyan (19341, he found that the main scaffold was a t greater depth on the heavier soils. Root branching in the top soil was more prevalent in sandy loam than in clay loam. These results have been confirmed by other studies. Dziljanov and Penkov (1964a,b) and Ghena (1966) both found large effects of soil type upon root growth, while Hoekstra (1968) reported poor root development on sandy soils and Tamasi (1964b) the adverse effects of a shallow water table. Weller (1971) studied the distribution of root tips on mature trees of either ‘Golden Winter Pearmain’ or ‘Boskoop’ apple on seedling root-


463

stocks in relation to soil type and profile. Root distribution a t depth (below 100 cm) was reduced by impeded drainage. Comparing distribution on a brown loess with that on a similar soil but where clay particles had migrated from the upper layers to the B horizon (Parabraunerde), he found that the number of root tips decreased sharply a t the point of clay accumulation where the minimum percentage of soil, air was found. Webster (1978) investigated the relation between soil physical conditions and root development. He found that the abundance of small apple roots (< 5 mm diameter) was related to porosity. Below a given boundary porosity, roots were sparse or absent; above this they increased with increasing porosity. The boundary porosity was from 29 to 39%, depending upon soil texture. Root growth was poor if less than 10% of soil volume was air-filled a t -10 KPa tension. As the relationship between total porosity and pore size distribution is not constant (Atkinson and Herbert 19791, Webster’s observed relationship may not hold for all soil conditions.

B. Fertilizers Studies of the effects of mineral fertilizers on the root growth of fruit trees probably have been fewer than with some other crops because of the limited response of tree crops to fertilizers (Greenham 1976; Atkinson and White 1980), a feature emphasized since the introduction of herbicides. Rogers (1933) found that pears receiving an application of farmyard manure (FYM) a t planting had a larger root system, a higher ratio of stem to root weight, and a more restricted root spread. Bziava (1966) showed that in the absence of fertilizers, 80% of tea roots occurred a t 0 to 20 cm depth, compared with 57% in plants receiving NPK fertilizer. Farmyard manure, however, promoted growth a t depth to a greater extent. In addition, the fertilizer increased the weight of large roots five to six times and small roots two to three times. Krasnoshtan (1975) found that fertilizers could increase root length by 13 to 170% for apple trees on M 3 stocks, while Tanas’ev and Balan (1977) reported that the combination of FYM and PK produced the longest roots. Weller (1966a) demonstrated that the addition of a mineral fertilizer close to the tree trunk increased root density there, but a t the expense of root growth elsewhere within the system. Similarly, Smith (1965) showed that if only half of a citrus tree’s root system was fertilized, root growth was enhanced in the fertilized part, but that excessively high fertilizer rates and NaN03 reduced root growth. Head (196913) showed that in the absence of applied fertilizers ‘Worcester’IMM 104 apple trees did not show new root activity in the spring. Goode et al. (1978a), however, could detect no effect of either rate or timing of nitrogen application on

464


root growth of ‘Cox’/MM 104. Response to fertilizers will interact with those to irrigation and soil management.

C. Irrigation T h e reduction in apple root growth under warm conditions in the summer usually coincides with the drying of the soil (Rogers 1939b),with growth reduced a t a soil water potential of -40 to -50 K P a or lower. Similarly, the root growth of peach trees in Australia (Richards and Cockroft 1975) was enhanced by keeping the soil moist by frequent irrigation (every three to four days). They suggested that soil drying determined the growth of roots in the surface soil. Here the high concentration of roots in the surface soil, combined with a low frequency of irrigation and a high transpiration rate, resulted in rapid soil drying and in slower root growth. Conversely, the combination of slow drying and a low root concentration resulted in good root growth. Goode and Hyrycz (1970) found that irrigation increased the weight of black currant roots a t several distances from the bush. There was no effect on distribution and soil moisture deficit was related to root density. Goode et al. (1978a) showed that irrigation increased root density in apple trees ‘Cox’/MM 104, although the effect was significant only a t 0 to 15 cm depth. In contrast (Goode and Hyrycz 1964), there was no significant effect of irrigation on the total amount of fine root ( < 1 mm diameter) on trees of ‘Laxton’s Superb’!M 2, although here the treatments affected root distribution both a t 1 m and 2 m from the tree. Irrigation increased root weight a t 0 to 15 cm and reduced it a t 1 5 to 30 cm depth. Yakushev (1972) and Ponder and Kenworthy (1976) also increased root production by irrigating. Doichev (1977), however, found no effect of irrigation on root distribution, nor did Sidorenko (1973) or Cahoon and Stolzy (1966). Thus, the effects of irrigation are variable, probably as a result of variations in other factors which influence root growth, e.g., tree growth, soil condition. Also, soil water is limiting to a varying extent. T h e method of irrigation can also influence root distribution. Huguet (1976) found that drench irrigation, which was wasteful of water, limited rooting to a superficial zone, while localized irrigation, with one application point a t the tree trunk, resulted in poor root growth because of waterlogging and excessive leaching. His best results came from localized irrigation with 2 application points, each 50 cm from the tree. This gave a dense and regular pattern of root growth. Doichev (1977) compared furrow and sprinkler irrigation with apple trees of ‘Golden Delicious’/ M 7. With both methods, most roots were a t 0 to 60 cm depth, equally distributed 0.5 to 2.0 m from the trunk. Doichev et al. (1974) observed,


465

however, that the main horizontal roots were deeper and growth was enhanced more with furrow irrigation than with overhead sprinkler irrigation. Taylor (1974) investigated the effect of trickle (drip) irrigation on mature peach trees, and found fine roots in the wetted zone, but only where the drainage was good. Roots were concentrated within a 30- to 40-cm radius, but with none under the drippers. In contrast, Goode et al. (1978b) observed 4 to 5 times more fine root, both in the vicinity of (30 cm radius) and beneath the nozzle. In the absence of irrigation, most roots were present a t 0 to 30 cm depth but with irrigation a t 0 to 60 cm depth. Away from the wetted zone there was little effect on root growth and a t 180 cm from the tree there was an apparent reduction in root density in the irrigated trees. Ponder and Kenworthy (1976) found that trickle irrigation had no effect on root system depth, but increased root weight in sugar maple, honey locust, and pin oak. The effects of trickle irrigation on apple root growth in Israel have been reviewed by Levin et al. (1980). Root distribution depended upon the volume of wetted soil, which was related to soil hydraulic conductivity and the rate and duration of water application. T h e wetted soil volume was usually 30 to 50% of the whole. T h e root system adapted to this by becoming restricted to within 60 cm of the nozzles. A higher root density in a smaller soil volume may necessitate extensive nutrient feeding. Under sprinkler irrigation 80% of the apple root system occurred a t 0 to 60 cm depth (Levin et al. 1980). They attributed this to excessive water a t depth. Root distribution a t 60 to 120 cm depth was greater when a relatively low moisture threshold was maintained in this zone during the main period of root growth. Thus, as for irrigation in general, specific systems interact with growing conditions and climate to affect response. D. Soil Management

Top fruit soil is cultivated, grassed, or treated with herbicides. Coker (1959) studied the effects of grass and cultivation on the root systems of apple trees of ‘Cox’/M 9. The general conformation of the main roots and the depth of root penetration were similar, although with grass the basic form was modified by: (1)the absence of cultivation which allowed tree roots to grow to the soil surface; (2) direct competition for water, nutrients, etc.; and (3) indirect effects on soil structure and nutrient availability. Under cultivation roots growing above 1 2 cm depth were pruned annually. In general, the spread of the root system was wider under grass, and there was more extensive branching, an increase in fine root weight, and a decrease in the larger root weight, a t all depths. Schultz (1972) also found that cultivation reduced the length of roots at 0 to 15

466


cm depth. The total root surface area under cultivation and grass was similar. Mitchell and Black (1968) also found a wider distribution of peach roots under a grass sod than under cultivation. In contrast with Coker’s work, Weller (1971) found a reduced number of ‘Boskoop’/M 9 apple roots tips a t 0 to 50 cm depth under grass, compared with bare soil. T h e number of tips a t 0 to 40 cm depth was greatest under a mulch. Sechi (1975), for peach, recorded a reduced number of roots under grass, compared with cultivation, while Ghena (1965) found a deeper root distribution in plum under a cover crop. Weller (1966b) found most roots of mature ‘Boskoop’/seedling apple trees a t 0 to 50 cm under cultivation, compared with 5 to 20 for grass. A deeper root system under a cover crop has been reported by Bjorkman and Lundeberg (1971) for pine and Hill (1966) for peach. Slowik (1962) noted more roots under the cultivated alley, than under the grassed tree row in an apple orchard, although soil was generally less compact under grass (Slowik 19681, which might have been expected to encourage root growth. Goode and Hyrycz (1976) compared the effects of grass and cultivation on irrigated, unworked M 2 rootstocks which received either a heavy soil application of nitrogen fertilizer or urea sprays. Root weight was generally higher under cultivation, where the nitrogen additions had no effect. Under grass the weights of both fine and large roots were increased by supplemental fertilization, while large roots also were increased by urea sprays. Response to cultivation varies with method. Root growth in apple was increased by deep cultivation prior to planting (Druchek and Zakotin 1972) or subsoiling (Kolesnikov 1963). Bogdan (1977) compared different depths of cultivation with cultivating either part or all of the soil. T h e response varied among apple cultivars, but cultivation of only a strip or the surface alone was best. Krayushkina et al. (1977) found that with deep plowing (65 to 70 cm), 59 to 81% of the roots were a t 0 to 40 cm compared with 80 to 88% with conventional cultivation. Morettini (1974) showed that successive cultivations of peach trees had adverse effects, although the roots regrew following cultivation. Gurung (1979) found that apple root density was highest under herbicide, lowest with grass, and intermediate with cultivation. Roots a t 0 to 5 cm and 5 to 10 cm depth were abundant with herbicide and grass, but almost absent under cultivation. Similar results have been reported by Catzeflis (1972). At 0 to 20 cm depth root density was greatest under herbicide, least under cultivation, and intermediate under grass. At 20 to 40 cm herbicide again produced most roots, although numbers were similar under grass and cultivation where density was highest a t 40 to 80 cm depth. Although a grass cover eliminates mechanical damage, it competes with the trees. White and Holloway (1967) showed that 1.44 m2 herbicide-


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treated squares around newly planted trees of ‘Cox’/M 26 apple greatly increased the number of roots (1 mm diameter, compared with trees under grass, although herbicide was less effective than a mulch. However, the herbicide had little effect on distribution with depth. Atkinson and White (197613, 1980) observed that 5-year-old trees of ‘Cox’/M 26 had more roots (both < and > 2 mm diameter) under overall herbicide than under grass, with a herbicide strip treatment being intermediate. Major differences between the treatments were a t the surface. A similar increase in average root density was observed in 12-year-old trees. In contrast Farre (1979) found the root length per unit soil area of trees of ‘Cox’/M 26 to be higher under grass than under herbicide, the differences being greatest a t 50 to 80 cm depth. Subsequently, Atkinson and White (1980) showed a greater uptake of :j2Pfrom 90 cm depth by trees under grass, compared to trees under overall herbicide or a herbicide strip. In western Europe, most fruit trees are grown in weed-free strips of bare soil separated by grassed alleyways. The system presents the trees with two dissimilar environments, one with and the other without interspecific competition. The effect of this type of treatment on root distribution and nutrient uptake from the two areas has been discussed by Atkinson and White (1976a,b), Atkinson (1977), Atkinson et al. (1977, 1979), Gurung (1979), and Atkinson and White (1980). In young apple trees, root growth is higher under the herbicide strip than the grassed alley, and begins earlier in the year. As a result the majority of roots are within the herbicide strip (Atkinson and White 1976a1, which is the major zone of nutrient uptake. Four-year-old trees of ‘Cox’/M 26 apple did not absorb l5No3applied 10 cm deep in the grass alley, while young trees of ‘Cox’/MM 106 absorbed no l5NO3from 15 cm, but a little 32P04 from 25 cm depth late in the season (Atkinson 1977). As the tree ages, more use is made of the grassed alleys, although even a t 12 years (Atkinson et al. 1977, 1979) the uptake of 15N03from under the grassed alley is small compared with that from the herbicide strip. The surface soil (0 to 10 cm depth) becomes less important as the season continues and soil moisture deficits increase. Even with large mature trees of ‘Crispin’/MM 111, uptake of l5NO3from the herbicide strip was much higher than that from the grassed alley (Atkinson and White 1980). In young trees most roots are near the trunk, so little use is made of the grassed alley. The alley is exploited as the trees age, although never to the extent of the herbicide strip. This may be due to the differences in soil water potential which exist under the two management areas for most of the season. In very high density plantings where the herbicide strip becomes drier than the grassed alley, root activity, as indicated by soil moisture depletion, is stimulated under the alley (Atkinson and White 1980).

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The absence of activity under the alley is not due solely to distance from the tree. Atkinson et al. (1979) compared the uptake of 15N0, from 10 cm and 25 cm depth under the alleyway of trees under (1) overall herbicide, (2) overall grass, or (3) herbicide strip with a grassed alley. With overall herbicide there was uptake from both depths, while with the other treatments apparent root activity was limited. Gurung (1979) compared apple root density a t a number of distances from the tree under either a wide herbicide strip or overall herbicide management. H e found that mean density was higher under overall herbicide, mainly as a result of larger tree size. At most distances, but particularly 0 to 50 cm from the trunk and in mid-alley, there were more roots under total herbicide, particularly a t 0 to 10 cm depth. There were more roots a t 150 cm from the trunk in the line of the tree row in the herbicide strip than under the grassed alley. An increased number of roots under a herbicidetreated, rather than a grassed alley, has been observed by Catzeflis (1972), while a number of papers (Duperrex 1964; Mel’nik 1975; Cockroft and Wallbrink 1966; Catzeflis 1972)have reported increased root growth a t the surface. This part of the root system seems to be most sensitive to soil management treatments. Fritzsche and Nyfeler (1974) investigated the effects of sward management on apple root growth. When grass mowings were left on the soil, apple root growth was 32% higher than when mowings were removed. There was no reduction in root growth under the wheel track marks in the orchard. T h e use of mulches as part of orchard management has been widely investigated. Young apple trees under a straw mulch produced more roots, of all diameters, particularly a t 0 to 8 cm depth, than did trees under grass, herbicide, or cultivation (White and Holloway 1967). Similarly, Reckruhm (1974) found in 500 cm3 soil samples 680 mm of pear root under mulch and 580 mm under grass. Comparable values for apple were 280 mm and 240 mm. Three-year-old trees of ‘Jonathan’/seedling apple under mulch had 42% of their roots a t 0 to 30 cm compared with 27% for cultivation, and the trees were 1.8 times the size of the cultivated trees (Tamasi 1965). A mulch increased surface rooting in chestnuts (Chiba 1966) and in peaches (Hill 1966). T h e effect of pre-planting soil management on subsequent tree root growth was investigated by Gurung (1979). Root growth was better where a total herbicide regime was created by killing a grass sward, rather than from cultivation. The presence of a thin layer of straw (not a mulch) after planting improved root growth. Both effects were attributed to better rain penetration resulting from improved soil structure. Rhee (1975) found that adding worms to soil improved soil structure and increased the length of roots < 1mm and 1to 5 mm diameter by 75% and 55%, respectively .


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There have been few studies of the direct effects of weed competition on tree growth compared to the many of grass and cover crops. Bergamini (1965) grew peach trees in tubs with half of the root system bare and half covered by legumes or grass. H e found th a t legumes inhibited root growth. Atkinson and Holloway (1976) looked a t the effect of allowing a number of weeds to become established by manipulation of the herbicide program in the herbicide strip around trees of ‘Cox’/M 26 apple. T h e presence of even small annual weeds like Senecio vulgaris and Poa a n n u a reduced root activity, as indicated by the uptake of 32P a t depths from 5 to 40 cm, but particularly a t 20 cm. Atkinson and White (1980) reported th at competition from grass apparently stopped root activity in apple during a dry summer. Herbicides are of vital importance to modern soil management. There have, however, been few critical studies of their direct effects upon the root systems of trees growing under normal orchard conditions when the effects of weed competition or cultivation damage have been excluded. Gurung (1979) found th at the application of mecoprop to mature apple trees in grass did not affect total root growth, but modified root distribution. No roots were present a t 0 to 5 cm and numbers were reduced a t 5 to 10 cm. There was, however, compensating growth below this depth. With newly planted trees of ‘Cox’/MM 106 in overall grass, total root growth was reduced by mecoprop. T h e elimination of root growth near the surface might, however, improve the balance of calcium to potassium in the tree, with advantages for fruit storage (Delver and Rooyen 1972). Atkinson and Petts (1978) were able to modify the distribution of root activity in grasses (measured as water uptake) by application of growth regulators. This also would be of practical value in tree crops.

E. Planting Density and Orchard Systems Although there have been many studies of the effects of tree density on growth and cropping, and numerous evaluations of training methods, etc., there have been few studies of the impact of these on root distribution and activity. Atkinson et al. (1976) found th a t a t wider tree spacings the root system was composed mainly of horizontal roots with relatively few vertical sinkers (Fig. 9.1), but a t high densities mainly of vertical sinkers. Th e degree of intermingling of adjacent root systems increased with density of planting, while the weight, length, volume, and surface area of roots on an individual tree decreased. T h e relative density of roots in the soil, however, increased with increasing density of planting. T he root/shoot ratio was unaffected by spacing. In addition to effects on total root length, distribution with depth was changed. In a very high density planting 25% of total weight occurred below 50 cm

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depth, compared with 15% a t a wide spacing. At all depths root density increased as spacing decreased. The significance of these changes has been discussed by Atkinson (1978).Trees a t the higher planting densities made greater use of the subsoil a t a much earlier point in their lives. An appreciable number of roots occurred below 80 cm depth at the highest densities in the initial year. At the wider spacings this did not occur until year 3, a t which time as much as 75% of new growth could be found a t this depth in the higher density plantings. Although the amount and distribution of root growth were affected, the time during the year when new growth occurred was not. As a consequence of these effects upon root distribution, root activity (indicated by water absorption) occurred a t relatively greater depths in the high density plantings, and the soil moisture deficits produced a t all depths were much higher. The effect of planting density in ‘Washington’ Navel oranges has been investigated by Kaufmann et al. (1972) and Boswell et al. (1975). They found that root distribution was affected by spacing and that root density was much higher in a high density planting where, for much of the profile, actual root densities seemed close to the maximum density. Intermixing of adjacent root systems occurred only a t high densities. Kemmer (1964) also reported that spacing influenced vertical root penetration in apple, while Fraser and Gardiner (1967) found that sinkers were initiated a t an earlier stage in Sitka spruce planted a t a high density. Spacing also affected lateral extension in both species. Perstneva (1977) found a reduced weight of roots on trees of ‘Jonathan’, ‘Richared Delicious’, and ‘Mantuaner’/M 9 apple a t 4 m X 1 m, in comparison with those a t 4 m X 2.5 m; Potapov (1971) reported that 7-year-old trees of ‘Pepin Shafrannyl’ apple a t 8 m X 4 m had 40% more root per m2 than those a t 8 m X 8 m. In the higher density planting 1.4 times more root was found in the tree row than in the alleyway. Yakushev (1972) recorded an even development of roots in apple trees a t 10 m X 10 m, but a concentration in the interrow areas in trees a t 10 m X 5 m. Manzo and Nicotra (1967) also noted better pear root growth between rows and a relationship between the development in the interrow areas and the vigor of the trees in contiguous rows. This latter effect also has been reported by Atkinson and White (1980) for apple. The effect of the branch training system on the root system has been investigated for palmette grown trees. Both Nicotra (1967) and Ponomarchuk and Golovanov (1973) found that palmette trees had a normal radial root system. The way that the soil around the tree is managed and the orchard laid out obviously has a large effect upon the basic pattern of root distribution and function.


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IX. THE INFLUENCE OF ROOTSTOCK AND SCION GENOTYPE ON THE ROOT SYSTEM A. Rootstock Effects

The effects of rootstock on the root system has been reported on a number of occasions. Rogers and Vyvyan (1934) compared the root systems of apple trees on the rootstocks M 1,M 2, M 9, M 16, and OF 5. For M 9 and M 2 50% of the total weight and 75% of the fine roots were below 33 cm depth. On the more vigorous M 1 and M 16, the root systems were shallower with only 25% of total weight and 50% of the fine roots below 33 cm. The trees on the different rootstocks were planted a t different spacings, related to differences in vigor, and this may have interacted with rootstock effects. On a poor sandy soil the maximum depth of rooting was highest for OF 5 and least for M 9 and M 1. Thus, while root spread and depth can be large for trees on vigorous rootstocks (i.e., M 1 or OF 5), this is not automatically so, and the root systems of trees on dwarf stocks are not always shallow. Coker (1958) found that differences in the depth of rooting appeared only when the soil was sufficiently deep. In a deep soil, M 2 roots were more dense below 120 cm than M 1 roots, with M 9 being intermediate. Root spread was greatest for M 1,least for M 9, and intermediate for M 2. De Haas and Jurgensen (1963) compared 57 apple cultivar/rootstock combinations, and found both rootstock and scion effects on form of the root system. Rootstock also affects the depth of rooting. Ghena and Tertecel (1962) reported a deeper root system which resulted in enhanced drought resistance in apricot trees of ‘Ungarische Besle’/myrobalan than in other combinations. Weller (1965) compared the root systems of four apple cultivars on M 2, M 9, or seedling stocks. With M 9 and seedling, the main roots were horizontal; in M 2 they sloped. For seedling, 37%, 33%, 21%, and 9% of the root system were present a t 0 to 50 cm, 50 to 100 cm, 100 to 150 cm, and 150 to 200 cm depth, respectively. The corresponding values for M 2 were 5%, 48%, 28%, and 19%. In contrast, Ghena (1966) found the deepest root systems to be associated with the most vigorous stocks in several species, while Hoekstra (1968) observed no difference in the distribution of fine roots between M 4 and M 9 apple. Lupescu (1965) measured the sizes of root systems of four apricot cultivars on six rootstocks and determined the following order of vigor: ‘Rosior de Voinesti’ plum > peach > black plum > ‘Rouge de Simlev’ plum > myrobalan > seedling apricot. Tanas’ev and Balan (1977) found that M 4 had double the root weight of M 9, while Pilshchikova and Pilshchikova (1978) found differences between rootstocks in both distribution with depth and the ability to regenerate after root pruning.

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Rogers (1939) compared the periodicity of root growth in apple trees on M 1, M 9, and M 16. H e found little difference in the appearance of individual roots, but M 16 reached peak production more quickly, its distribution of new growth was deeper, and its rate of root browning slower. There were no clear differences among stocks in the rates of growth of individual roots, so differences in root density must have been due to the number of roots developing. Gurung (1979), using ‘Cox’ apple on several rootstocks, found that root growth was generally most vigorous with the stronger stocks, i.e., M M 106 and M M 111greater than M 9 and M 27, with M 26 intermediate. However, in both years of his study M M 106 produced more root than the more vigorous M M 111. Differences in the periodicity of root growth also were apparent, with the stronger stocks showing more growth in the autumn and in distribution with depth, as M M 106 had most growth a t 30 to 50 cm. Various plum and cherry stocks also differ in periodicity of growth (Atkinson and Wilson 1980). Atkinson (1973d) observed that uptake of 32Pper unit weight of root was greater in stocks of M M 111 than in those of M 9 or M 26. Thus, with rootstocks, root system size, distribution, periodicity of growth, and activity appear to be potentially variable.

B. Scion Cultivar Effects Although the choice of scion can influence tree size, few studies have addressed scion effects on the root system. However, scion cultivar has been shown to affect root system form in plum (Ghena 1964a), and root density in apple (Weller 1965), both maximum depth and lateral root spread (Angelov 1976), and formation of fibrous roots (Kemmer 1964) in apple. Head (1966) compared the periodicity of root growth of trees of ‘Crawley Beauty’ and ‘James Grieve’/M 7 which differed greatly in time of bud burst. H e found little difference, although the onset of growth was slightly earlier in the earlier flowering ‘James Grieve’. Atkinson (1973e) compared the root growth of ‘Cox’ and ‘Golden Delicious’/M 9 a t a range of planting densities. H e found only small differences between cultivars, although root growth by ‘Delicious’ seemed to be relatively better a t high densities. Variation in the choice of a scion cultivar may thus affect the root system, although available examples suggest it as having less effect than the rootstock. X. ROOT-SHOOT INTERACTIONS

Rogers and Vyvyan (1934) estimated the ratio of root to shoot (R/S) for ll-year-old trees of ‘Lanes’ Prince Albert’ on M 1, M 2, M 9,and


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M 16 to be 0.5 to 0.4 for a loam soil and 1.0 to 1.4 for a sandy soil. For 16-year-old trees of ‘Cox’/M 9, Coker (1959) gave values of 0.36 to 0.42. For 5-year-old trees of ‘Golden Delicious’/M 9, Atkinson et al. (1976) found a value of 0.13 to 0.16 irrespective of spacing. These lower values may be due to age or to improved cultural conditions. Avery (1970) and Tamasi (1965), however, noted a tendency for R / S to increase with increasing age. For l-year-old trees of ‘Worcester’/M 26, R / S was 0.26, while for 4-year-old trees it was 0.5 (Avery 1970).Comparable values for trees of ‘Worcester’/3430 were 0.30 and 0.59. Avery (1970) found R/S to range from 0.15 (1-year-old ‘Worcester’/M 2) to 0.59 (4-year-old ‘Worcester’/3430). Rootstock also affected R / S with ratios highest for 3430 and lowest for M 2. A comparison of Coker’s (1959) data with that of Atkinson et al. (1976) suggests that R / S changed as a result of changes in cultural practice, i.e., the use of herbicides. Atkinson and White (1976b) showed that R / S was highest for trees under grass and lowest for those under total herbicide. Atkinson and White (1980) have suggested that this relative reduction in root length may partially explain the reduced uptake of phosphorus under total herbicide management trees, i.e., root systems adapted to favorable water supply have an inadequate length of root available for phosphorus uptake, which depends greatly on root surface area. The amount of root extracted from soil has varied with investigator. Although values may be comparable within one study, comparison of the values resulting from different studies is difficult. Cripps (1971) studied the effects of moisture stress, fluctuating soil moisture availability, and waterlogging on R/S. All increased R / S and reduced the total growth of trees of ‘Granny Smith’/MM 115 apple. Gur, Hepner and Mizrahi (1976) found that R/S decreased with an increasing soil temperature. R / S seems likely to increase when trees are in a stress situation. For a range of herbaceous plants, Atkinson (1973f) showed that R / S increased progressively with increasing phosphorus and nitrogen deficiencies. Atkinson and Davison (1973) found that the change in R / S was closely correlated with the reduction in growth produced by nutrient stress. In annual plants, where most of the root and shoot tissue is active in either synthesis or absorption, R / S is related to the balance of activity between root and shoot. Hunt (1975) showed a close relationship between the mass ratio of root and shoot and their activity ratios calculated as: specific absorption rate for potassium (pg K mg root - 1 day - I ) / unit shoot rate (increase in plant weight per unit shoot weight (mg mg - l day -’)). The importance for peach trees in this type of situation has been discussed by Richards (1976) and Richards and Rowe (1977). They showed that the change in plant weight was related to the amount of water

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absorbed by the roots. For a given decrease in R/S there was a n increase in water uptake per unit length of root, indicating that the root system has the capacity to increase uptake as demand increases. They also showed that leaf area was related to water uptake, plant weight to nutrient uptake (in a similar way to water uptake), and leaf number to root number. As a result, shoot demand appeared to control the uptake of many substances. T h e above experiments, however, were conducted on small plants which, in functional terms, approximate annual plants. Using data for root length (Atkinson et al. 1976) and for leaf areas of the same trees (Atkinson 1978), the ratio leaf area (cm')/root length (cm) can be calculated for trees growing a t a range of densities. Values of 1.3 to 1.7 are obtained for trees with leaf area indexes (LAI) of < 1.3. For high density plantings where LA1 is 5.8 to 9.7, comparable values are 2.1 to 3.4. Assuming that a LA1 of only 2 is functional in orchard light interception, i.e., it intercepts most of the available light, these values become 0.4 to 1.2. This implies that in very high density plantings twice the length of root is needed to supply a given amount of evaporative surface. This may be related to the rapid depletion of soil water near the soil surface in trees a t very high densities (Atkinson 1978), which probably makes much of the root system non-functional. Variation in the ratio of leaf area/root length occurs where R/S is constant and it may be, therefore, a more realistic appraisal of activity in trees where much tissue has no synthetic or absorptive activity. Clearly, additional information is needed about the control of the relative amounts of growth and activity in the root and the shoot, and their interrelationships. XI. CONCLUSIONS

Although there are many basic similarities in the root systems of tree crops with respect to patterns of growth, timing, and distribution of growth, and the ways in which they function, there are also major differences. These can arise from genetic variation in the planting material, but also can be brought about by cultural factors, i.e., practices which we can control. To make best use of both of these sources of variation, greater understanding of relationships among root growth, density, and effectiveness is needed. Many of the studies reported here were purely observational and as such often provide little understanding of mechanisms of activity and effectiveness. Based on observed effects, studies of root function in relation to internal metabolic and external factors are needed to raise our understanding of the tree root system to a level already available for many annual and field crops.


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XII. LITERATURE CITED ABRAMENKO, N.F. 1977. Seasonal rhythm of root growth of apple trees in hilly unirrigated orchards (in Russian). Vest. S K h Naoki Katah. 3:58-60. ADDOMS, R.M. 1946. Entrance of water into suberized roots of trees. Plant Physiol. 2 1:109-111. AIYAPPA, K.M. and K.C. SRIVASTAVA. 1965. Studies on root system of Coorg Mandarin seedling trees. Indian J. Hort. 22:122-130. ANDREWS, R.E. and E.I. NEWMAN. 1970. Root density and competition for nutrients. Ecologia Plant. 5:319-334. ANGELOV, T . 1976. Root system distribution in bearing apple trees and methods of irrigation (in Bulgarian). Ovoshcharstvo 55:33-37. ATANASOV, G. 1965. The distribution of the root system of the Kazanlik rose (Rosa damascena Mill.) in diluvial meadow and leached cinnamon forest soils (in Bulgarian). Rasten. Nauki 2:lOl-108. ATKINSON, D. 1972. Seasonal periodicity of black currant root growth and the influence of simulated mechanical harvesting. J. Hort. Sci. 47:165-172. ATKINSON, D. 1973a. T h e root system of Fortune/M 9. Rpt. East Malling Res. Sta. for 1972. p. 72-78. ATKINSON, D. 1973b. Field studies on root systems and root activity. Rpt. East Malling Res. Sta. for 1972. p. 56-58. ATKINSON, D. 1973c. Seasonal changes in the length of white unsuberized root on raspberry plants grown under irrigated conditions. J. Hort. Sci. 48: 413-419. ATKINSON, D. 1973d. Structure and physiology of individual roots and root systems. Rpt. East Malling Res. Sta. for 1972. p. 56. ATKINSON, D 1973e. Root competition in tree spacing experiments. Rpt. East Malling Res. Sta. for 1972. p. 58-59. ATKINSON, D. 1973f. Some general effects of phosphorus deficiency on growth and development. New Phytol. 72:lOl-111. ATKINSON, D. 1974a. Field studies on root systems and root activity. Rpt. East Malling Res. Sta. for 1973. p. 69. ATKINSON, D. 1974b. Some observations on the distribution of root activity in apple trees. P l a n t & Soil 40:333-342. ATKINSON, D. 1976. Preliminary observations on the effect of spacing on the apple root system. Sci. Hort. 4:285-290. ATKINSON, D. 1977. Some observations on the root growth of young apple trees and their uptake of nutrients when grown in herbicide strips in grassed orchards. Plant & Soil 49:459-471. ATKINSON, D. 1978. The use of soil resources in high density planting systems. Acta Hort. 65:79-89. ATKINSON, D. and A.W. DAVISON. 1973. The effects of phosphorus deficiency on water content and response to drought. New Phytol. 72:307-313. ATKINSON, D. and R.F. HERBERT. 1979. A review of long-term effects of

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NURMANBETOV, T.T. and I.G. ANDRONOV. 1976. Root system of the apple cultivar: apart in relation to different types of dwarfing rootstock (in Russian). Vestnik S - K h N a u k i Kazakhstana 6:57-60. NYE, P. and P.B. TINKER. 1978. Solute movement in the soil-root system. Blackwell, Oxford. ORLOV, A. JA. 1966. Significance of dying feeding tree roots on the organic cycle of the forest (in Russian). Zhurnut Obshch. Biol. 27:40-46. OSKAMP, J. and L.P. BATJER. 1932. Soils in relation to fruit growing in New York. 11. Size production and rooting habit of apple trees on different soil types in the Helton and Morton areas, Monroe Country. Cornell Univ. Agr. Expt. Sta. Bul. 55O:l-45. OVINGTON, J.D. and G. MURRAY. 1968. Seasonal periodicity of root growth of birch trees. p. 146-154. I n M.S. Ghilarov, V.A. Kovda, L.N. NovichkovaJuanova, L.E. Rodin, and V.M. Sveshnikova (eds.) Methods of Productivity Studies in Root Systems and Rhizosphere Organisms. USSR Academy of Sciences. PASSIOURA, J.B. 1972. The effect of root geometry on the yield of wheat growing on stored water. Austral. J. Agr. Res. 23:745-752. PATEL, R.Z. and A.M. KABAARA. 1975. Isotope studies on the efficient use of P fertilizers by Caffea arabica in Kenya. I. Uptake and distribution of PIL from labelled KH2 PO4. Expt. Agr. 11:l-11. PERSSON, H. 1978. Root dynamics in a young Scots pine stand in central Sweden. Oikos 30:508-519. PERSTNEVA, T.A. 1977. The root system of young apple trees on dwarfing rootstocks in an intensive orchard (in Russian). Pouysh Produktiun Plodou Nascz denii u Moldauii Kishineu Moldavian SSR 27-31. PHILIPSON, J.J. and M.P. COUTTS. 1977. T h e influence of mineral nutrition on the root development of trees. 11. The effect of specific nutrient elements on the growth of individual roots of Sitka spruce. J. Expt. Bot. 28:864-871. PILSHCHIKOVA, F.N. and N.V. PILSHCHIKOVA. 1978. Regeneration of roots of different apple rootstocks (in Russian). Izu. Timiryuzeu. Sel’kokh. Ahad. 2:145-150. PITCHER, R.S. and J.J.M. FLEGG. 1965. Observations of root feeding by the nematode Trichodoros viruliferus Hooper. N u t u r London 207:317. POLIKARPOV, V.P. and M.M. ADASKALILSII. 1977. Root systems of bearing palmette apple trees (in Russian). Vestn. Sel’kokh. N u u k i 6:112-114. PONDER, H.G. and A.L. KENWORTHY. 1976. Trickle irrigation of shade trees growing in the nursery. 11. Influence on root distribution. J. Amer. SOC. Hort. Sci. 101:104-107. PONOMARCHUK, V.P. and I S . GOLOVANOV. 1973. The root system of apple trees with flat crowns (in Russian). Vestn. Sel’kokh. N u u k i Kuzakhstana 9:87-91. POPESCU, M. 1963. The effect of planting methods on the development of the root system of apricot trees grown in sand (in Romanian). Grad. via Liu. 12:55-60.

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POTAPOV, V.A. 1971. The root system and soil management in young apple orchards with close tree spacing in the rows (in Russian). Sb. Nauch. Rub. Vsesoyuznyi Nauchno-lssledouatel’skiiInst. Sadouod. I. Michurina 15:88-90. PRIESTLEY, C.A., P. B. CATLIN, and E.A. OLSSON. 1976. The distribution of “C labelled assimilates in young apple trees as influenced by doses of supplementary nitrogen. I. Total 14C radioactivity in extracts. Ann. Bot. 40: 1163-1170. QUINLAN, J.D. 1965. The pattern of distribution of I4carbonin a potted apple rootstock following assimilation of 14carbon dioxide by a single leaf. Rpt. East Malling Res. Sta. for 1964. p. 117-118. RECKRUHM, I. 1974. Direct root analysis in orchards with different soil management systems (in German). Archiu f u r Gartenbau 22:17-26. REYNOLDS, E.R.C. 1970. Root distribution and the cause of its spatial variability in PseudotoLga toxifolia (Poir) Brit. P l a n t & Soil 32:501-517. RHEE, J.A. VAN. 1975. T h e effect of earthworms on production in orchards. Fruitteelt 65:204-206. RICHARDS, D. 1976. Root-shoot interactions: a functional equilibrium for water uptake in peach ( P r u n u s persica (L)) Batsch. Ann. Bot. 41:279-281. RICHARDS, D. and B. COCKROFT. 1974. Soil physical properties and root concentrations in an irrigated peach orchard. Austral. J. Expt. Agr. Anim. HUsb. 14:103- 107. RICHARDS, D. and B. COCKROFT. 1975. The effect of soil water on root production of peach trees in summer. Austral. J. Agr. Res. 26:173-180. RICHARDS, D. and R.N. ROWE. 1976. Root-shoot interactions in peach: the function of the root. Ann. Bot. 41:1211-1216. RICHARDSON, S.D. 1957. Studies of root growth of Acer saccharinum L. VI. Further effects of the shoot system upon root growth. Kon Ned. Akad. Wetensch. Ser. C 60:624-629. RICHARDSON, S.D. 1958. Bud dormancy and root development in Acer saccharinum. p. 409-425. In K.V. Thimann (ed.) T h e physiology of forest trees. Ronald Press, New York. ROBERTS, J. 1976. A study of root distribution and growth in a P i n u s syluestris L. (Scots pine) plantation in East Anglia. P l a n t & Soil 44:607-621. ROGERS, W.S. 1933. Root Studies 111. Pears, gooseberry and black currant root systems under different soil fertility conditions with some observations on root stock and scion effect in pears. J. Pomol. Hort. Sci. 11:l-18. ROGERS, W.S. 1934. Root Studies IV. A method of observing root growth in the field; illustrated by observations in an irrigated apple orchard in British Columbia. Rpt. East Malling Res. Sta. for 1933. p. 86-91. ROGERS, W.S. 1935. Root Studies VI. Apple roots under irrigated conditions with notes on use of a soil moisture meter. J. Pomol. Hort. Sci. 13 (3):190-201. ROGERS, W.S. 1939a. Root Studies VII. A survey of the literature on root growth, with special reference to hardy fruit plants. J. Pomol. Hort. Sci. 17:67-84.


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ROGERS, W.S. 1939b. Root Studies VIII. Apple root growth in relation to rootstock, soil, seasonal and climatic factors. J Pomol. Hort. Sci. 17:99-130. ROGERS, W.S. 1952. Fruit plant roots and their environment. Rpt. 13th Intern. Hort. Congr. 1952. p. 1-4. ROGERS, W.S. 1968. Amount of cortical and epidermal tissue shed from roots of apple. J. Hort. Sci. 43:527-528. ROGERS, W.S. 1969. The East Malling root-observation laboratory. p. 361376. I n W.J. Whittington (ed.) Root growth. Butterworths, London. ROGERS, W.S. and G.A. BOOTH. 1959. The roots of fruit trees. Sci. Hort. 14:27-34. ROGERS, W.S. and G.C. HEAD. 1962. Studies of growing roots of fruit plants in a new underground root observation laboratory. Proc. 16th Intern. Hort. Congr. p. 311-318. ROGERS, W.S. and G.C. HEAD. 1966. The roots of fruit plants. J. R. Hort. S I C . 4 1:199-205. ROGERS, W.S. and G.C. HEAD. 1969. Factors affecting the distribution and growth of roots of perennial woody species. I n W.J. Whittington (ed.) Root growth. Butterworths, London. ROGERS, W.S. and M.S. PARRY. 1968. Effects of deep planting on anchorage and performance of apple trees. J Hort. Sci. 43:103-106. ROGERS, W.S. and M.C. VYVYAN. 1934. Root studies V. Root stock and soil effect on apple root systems. J. Pomol. Hort. Sci. 12:llO-150. ROSATI, P., W. FAEDI, and M. FAEDI. 1976. Studies on the root systems of various peach rootstocks in Romagna (in Italian). Frutticoltura 38:9-13. RUSSELL, R.S. 1977. Plant root systems-their function and interaction with the soil. McGraw-Hill, London. RYBAKOV, A.A. and Z.L. DZAVAKJANC. 1967. Root growth and development under irrigation (in Russian). Sudouodstuo 7:34-35. RYZHKOV, A.P. 1972. The characteristics of growth development and distribution of the root system in prostrate forms of apple trees (in Russian). Nauchnife Chteniya Pamyati Akademii MA Lisavenko (Barnaul USSR) 178185. SANDERS, F.E.T. 1971. The effect of root and soil properties on the uptake of nutrients by competing roots. D. Phil. Thesis, University of Oxford. SCHULTZ, R.P. 1972. Root development of intensively cultivated slash pine. Soil Sci. Soc. Amer. Proc. 36 (1):158-162. SCHUMACHER, R., F. FANKHAUSER, and E. SCHLAPFER. 1972. Development of apple roots: influence of the growth retardant Alar on root development (in German). Schweiz. Zeilschrift f u r obst und weinbuu 107:438-452. SECHI, A. 1975. Observations on the root system of peaches under different cultured treatments (in Italian). Centro Regionale Agrario Sperimentale-Cagliari, Publ. 48. p. 1-15. SEMINA, N.P. 1971. The growth characteristics of the active part of the root system in relation to the compatability of the graft components (in Russian).

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Sbornik Nauchnykh Rabot Vsesoyuznyi Nauchno-Issledouatel’<skii Institut Sadovodstua. I V Mich urina 15:112- 119. SEWELL, G.W.F. 1979. Effect of Pythium spp on growth of apple seedlings. Rpt. East Malling Res. Sta. for 1978. p. 92. SHOROKHOV, S.S. 1976. Uptake of 32Pby apple trees from super phosphate placed a t different depths (in Russian). Selektsiya Sortoizuchenie Agrotekhnika Poldouykh i Yagodnykh K u l t u r Ore1 USSR 7:155-161. SIDORENKO, V.M. 1973. The characteristics of distribution of the root system of apple trees in relation to soil management and irrigation (in Russian). Yuzhnoe Stepnoe Sadovodstuo (1973). p. 98-103. SKRIPKA, I.S. 1977. Changes in the growth of plum tree root system in relation to the cultivation and fertilization of orchard soil (in Russian). Soderzhi udobr Pochuy u Plodou Nasazhdeniyakh Kishineu Moldavian SSR. p. 57-86. SLOWIK, K. 1962. The distribution of the root systems of 11 year old apple trees grown in grass strips in the rows with clean cultivation between the rows (in Polish). Prace Inst. Sadow. Skierniew. 6:lOl-117. SLOWIK, K. 1968. The effect of compaction by machinery on the physical properties of the soil and on apple tree growth (in Polish). Prace Inst. Sadow. Skierniew. p. 79. SMITH, P.F. 1965. Effect of nitrogen source and placement, on the root development of Valencia orange trees. Proc. Flu. State Hort. SOC.78:55-59. SOONG, N.K., E. PUSHPARAJAH, M.M. SINGE, and 0. TALIBUDEEN. 1971. Determination of active root distribution of Hevea Brasiliensis using radioactive phosphorus. Proc. Intern. Symp. Soil Fert. Eual. 1:309-319. SPINA, P. 1966. Observations on the root systems of Citrus (in Italian). Tecnica. Agricola 18:31-54. STOICHKOV, I., M. PENKOV, A. ANDREEV, S. KEREMIDARSKA, B. MILANOV, and T.S. TSOLOV. 1974. The influence of pseudopodzolic cinnamon forest and alluvial meadow soils on the root distribution of Newton Pippin and Wellington apple cultivars grafted on M 4 (in Bulgarian). Grad. Lozar. N a u k a 11:ll-25. STOICHKOV, I., T.S. TSOLOV, B. MILANOV, A. ANDREEV, S. KEREMIDARSKA, and M. PENKOV. 1975. Architectonics of the root system of Doucin IV apple rootstock in relation to the soil and the scion cultivar (in Russian). Grad. Lozar. N a u k a 12:3-14. SUTTON, R.F. 1969. Form and development of conifer root systems. Tech. Comm. 7. Commonw. For. Bur., Commonw. Agr. Bur., Farnham Royal, U.K. TAMASI, J. 1964a. The effect of the soil water table on the root system of apple trees (in Hungarian). Magy. T u d . Akad. Kozlem. 23:25-41. TAMASI, J. 1964b. The effect of different cultural practices on the formation of the root system of apple trees (in Hungarian). Kiserl. Kozl. Sect C 57:43-63. TAMASI, J. 1965. Location of the roots of 1-6 year-old Jonathan apple trees grafted wild stock in an orchard on sandy soil (in Hungarian). Novenytermeles 14:115-148.


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TAMASI, J. 1973. Root development in closely planted sour cherry trees grafted on Mahaleb rootstocks in sandy soil in relation to cultural practices (in Hungarian). Gyumolcstermesztes 8:29-50. TAMASI, J. 1974. Cultural practices in relation to root development in young plum trees (in Hungarian). Gyumolcstermesztes 1:23-46. TAMASI, J. 1975. Investigation on the root development of young cherry trees planted in clay soil (in Hungarian). Gyumolcstermesztes 2:47-68. TAMASI, J . 1976. Investigation on early root development in sour cherries on Mahaleb rootstocks closely planted in a clay soil (in Hungarian). Gyumolcstermesztes 3:81-98. TANAS’EV, U.K. and V.V. BALAN. 1976. Development of the root system of palmette trained Jonathan apple trees grafted on M 4 in relation to the rate of tree planting fertilization (in Russian). T r u d y Kishineu. Sel-Khoz Znst. 154: 46-50. TANAS’EV, V.K. and V.V. BALAN. 1977. The effect of rootstock and high rates of deep pre-planting fertilization on the development of the apple tree root system (in Russian). Sadou. Vinogr. Vinod. Mold. 2:17-20. TAYLOR, A. 1974. Trickle irrigation experiments in the Goulburn Valley. Vict. Hort. Dig. 61:4-8. TAYLOR, B.K. and F.H. GOUBRAN. 1976. The phosphorus nutrition of the apple tree 11. Effects of localized phosphate placement on the growth and phosphorus content of split root trees. Austral. J. Agr. Res. 27:533-539. TAYLOR, H.M. and B. KLEPPER. 1973. Rooting density and water extraction patterns for corn (Zea mays L). Agron. J. 65:965-968. TAYLOR, H.M. and B. KLEPPER. 1978. The role of rooting characteristics in the supply of water to plants. Adu. Agron. 30:99-128. THAGUSEU, N.A. 1968. On the root systems of filberts (in Russian). Sel’ kokh. Biol. 3:623-626. TILL, M.R. and J.B. COX. 1965. A guide to cultural practices for young citrus trees. J. Agr. Sci. Austral. 68:232-233. TINKER, P.B. 1976. Roots and water: transport of water to plant roots in soil. Phil. Trans. R. SOC.London, Ser. B 273:445-461. TORREY, J.G. and D.T. CLARKSON. 1975. The development and function of roots. Academic Press, London. TROMP, J. 1978. The effect of root temperature on the absorption and distribution of K, Ca and Mg in three rootstock clones of apple budded with Cox’s Orange Pippin. Gartenbau. 43:49-54. TROMP, J. 1980. Mineral absorption and distribution in young apple trees under various environmental conditions. p. 173-182. Zn D. Atkinson, J.E. Jackson, R.O. Sharples, and W.M. Waller (eds.) The mineral nutrition of fruit trees. Butterworths, Borough Green, U.K. VORONOVA, T.G. 1965. The rhythm of root growth in fruit crops in relation to the development of their aerial parts (in Russian). Agrobiologija 2:291-293.

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10 Light and Lighting Systems for Horticultural Plants Henry M. Cathey and Lowell E. Campbell U.S. Department of Agriculture, Beltsville, Maryland 20705 I. Introduction 492 11. Measurement of Radiation and Temperature 493 A. Radiant Energy 493 1.Wavelength Classification 494 2. Wavelength Characteristics of Sources 494 3. Irradiance-Quantities, Units and Symbols 495 B. Measuring Units 500 1.Photon Units 500 2. Solar Radiation Units 504 3. Radiometric Units 504 4. Photometric Units 504 C. Measurement Methods 505 1. Basic Requirements for Measurement 505 2. Types of Light and Radiation Meters 508 3. Types of Sensors and Detectors 508 4. Calibration 511 5. Guidelines 51 1 6. Recommended Methods 512 a. Illumination Meter-Photometric-($50, $150 and Up) 512 b. Irradiance Meter-Radiornetric-($500 and Up) 512 c. Photon Meter-($750 and Up) 512 7. Spectral Distributions 512 D. Infrared and Thermal Radiation 513 111. Spectral Radiant Power of Lamps 514 A. Photometric Data (Table 10.4) 515 B. Wavelength Intervals (Table 10.4) 516 IV. Generic Responses of Plants to Lamps 516 V. Selection of Efficient Light Sources by Plant Responses 520 A. Practical Plant Lighting 521 1.Display: 0.3 W/m2 521 2. Photoperiod: 0.9 W/m* 524 3. Survival: 3.0 W/m2 524 491

492


4.Maintenance: 9.0 W/m2 524 5. Propagation: 18.0 W/m2 525 a. Greenhouse: 24.0 W/m2 526 527 b. Growth Chamber: 50.0 W/mZ 527 VI. Comparison of Light Sources A. Incandescent Lamps 527 B. Fluorescent Lamps 528 528 C. High Intensity Discharge Lamps VII. Summary 529 VIII. Literature Cited 532

I. INTRODUCTION

Intervention of the signals exerted by nature and the interposing of desired plant growth through the use of light and temperature manipulation have been one of the earliest concerns of horticulturists (Bailey 1893).I t began when estate gardeners developed growing methods which permitted flowering or harvesting of horticultural plants out or ahead of season. Forcing was added as a modifier to greenhouses and low temperature storage facilities to identify accelerated or year-round culture. The assumption was made that the previous growth cycle acted as a reservoir for storing sufficient energy (carbohydrates and other compounds in stems, roots, leaves) to ensure the development of green leaves and flowers without additional photosynthetic activity. Only a few woody and bulbous plants responded to these procedures. Many other plant species had to be held in glass-covered, sun-heated pits to ensure their survival over winter. They were expected to grow and flower only during the bright, long, warm days of spring to fall. Plant and engineering scientists have done much to bring about major changes as to which plants are grown, what structures are to be used for their production, and where the new plant products are to be consumed. We use the word Photo-regulation to describe these new procedures of growing plants (Garner and Allard 1920). We assume the use of pestfree, clonal material which is grown in environments which exert nearmaximum control of photosynthesis and photomorphogenesis. This review deals with an area of research where most of the physical environment, particularly radiation, has not been regulated previously. Artificial lamps used in plant studies previously have been regarded primarily as light sources in the region of 400 to 700 nm; the other regions of radiation often were neglected or ignored (Gaastra 1959). We will present tables listing the electrical, photometric, and radiometric characteristics of various types of lamps. We will offer suggestions as to how the properties of various types of lamps can be measured when utilized in various environments. We will describe the plant responses (seed ger-

LIGHT AND LIGHTING SYSTEMS

493

mination, photoperiod control, growth) to various types of lamps. And, we will review recent research and offer an overview of how the various kinds of lamps can be utilized to create energy-efficient growing systems by controlling the distance, intensity, duration, absence, or presence of sunlight . This review is written to bridge the many engineering activities designed to conserve energy in growing structures and the theoretical actions of the two major photo-triggering pigments-chlorophyll and phytochrome. T he structural engineering aspects have been covered in detail by Aldrich and White (1969) and White (1979); the horticultural aspects have been covered by Cathey and Campbell (1975); and the plant physiology aspects have been reviewed by Meijer (1971) and Bickford and Dunn (1972). These research areas (engineering, physiology) have used many different methods of measuring the energy output and uses from the various light sources. Th e measurements are further complicated by the interactions among the lamps, the fixtures, and the environment (Carlson et al. 1964). In this review we have attempted to treat the lamps and the plant responses generically and to avoid the minor variations among lamps from various manufacturers. We also wish to provide a basis for the rapid evaluation of any new light source a s a n energy source for growing any specific species. Measurements of research findings must be compatible with or transferable to engineering and architectural design. Since measurements require some correction for plant regulation, we have attempted to show t ha t such corrections for spectral power distributions can be made on the basis of generic types of lamps with established instruments and units. For each generic-type lamp a conversion factor will be given. 11. MEASUREMENT OF RADIATION AND TEMPERATURE

A. Radiant Energy T he radiation of major concern to biologists is optical or non-ionizing radiation to distinguish it from the remainder of the electromagnetic spectrum. Measurements are concerned with intensity and spectral distribution. I t is nearly impossible in practical observations to distinguish between the effects of direct radiation and the indirect effects of heat resulting from the radiation. This difficulty is minimum a t low radiation levels and increases in proportion to the intensity of radiation. For many years the pyroheliometer which measures total radiation and illumination meters (light meters) which measure light in terms of human eye response were the main instruments of measurements. In recent years new instruments have been developed. These new units vary widely in characteristics and price.

494


Careful review and evaluation of the existing literature on light and plants reveal contradictory claims and hypotheses, obvious errors in light and radiation measurement and evaluation, observation attributed to hypothesis without valid experimental data, and lack of consideration of ultraviolet, infrared, and heat radiation in the environment. Some of the confusion can be attributed to the inherent difficulties of measurement. Different interest groups (biologists, engineers, manufacturers) approach the measurement problem with varying disciplines and with different instruments, resulting in variations in measurement procedures and in reported results. Several systems of terminology are used. 1. Wavelength Classification.-The regions of the electromagnetic spectrum are arbitrarily divided as follows:

Classification U1traviole t (UV)

uv-c

UV-B UV-A Visible Infrared (IR) or Infrared (IR) Thermal

Wavelength 100-380 nm 100-280 nm 280-320 nm 320-380 nm 380-780 nm 780-lo5 nm 780-2500 nm 2500 + nm

Traditionally engineers and physics authorities divided the ultraviolet spectrum into 100 nm bands from 100 to 400 nm. The classification of UV into A, B, and C is from the International Commission on Illumination (CIE) from use by photo-biologists. The spectral limits vary with authorities. The biological response is a gradual transition or overlap among regions without sharp delineation. The A, B, C system fits the peak emission of light sources better than 100 nm bands point, wherein the light sources often may peak a t the wavelength division between point between two bands. UV and UV-A bands frequently use 400 nm as a limit. This is inconsistent with the established definition of visible light (380 to 780 nm); however, the biological response continues from UV-A into the visible region above 400 nm. Many portions of these regions are referred to as “light” at times. Strictly speaking the term “light” refers only to the visible portion of the electromagnetic spectrum, but all regions, ultraviolet, visible, and infrared, are electromagnetic radiation. 2. Wavelength Characteristics of Sources.-The sources of radiation, both natural and artificial, are not limited to the individual limits of the regions’ wavelength. The sun emits energy throughout all the wavelength


495

regions, although not a t the same level. Fluorescent lamps emit mainly in the visible region but have some energy in adjacent ultraviolet and infrared bands (Campbell et al. 1975). Incandescent lamps emit relatively more infrared and a lower amount of visible than sunlight or fluorescent lamps. Figure 10.1illustrates the spectral emission of radiant power from the sun. Figure 10.2 shows the spectral power emission of typical light sources used in horticulture. I t should be noted that all lamps have infrared and thermal radiation not shown in the visible spectral power distributions. Figure 10.3 shows both the visible and infrared radiation from high pressure sodium lamps. A graphic display of the power conversion of cool-white fluorescent and the HID lamps, highpressure sodium, metal halide, and mercury, is shown in Fig. 10.4. 2.5

h C iI&

2 . 0 t

1.5

SOLAR SPECTRAL IRRADIANCE OUTSIDE ATMOSPHERE SOLAR SPECTRAL IRRADIANCE AT SEA LEVEL (m = 1)

1 , CURVE FOR BLACKBODY AT 5900 K

, -03

H10

w d

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 1.6 1.8 WAVELENQTH ( p m )

2.0

2.2

2.4

2.6

2.8

3.0

C

FIG. 10.1. SOLAR SPECTRAL IRRADIANCE

The shaded areas indicate absorption at sea level due to the atmospheric constituents shown.

3. Irradiance-Quantities, Units and Symbols.-We should first consider several concepts which may help in the comprehension of radiant power measurement. Radiometry and radiometric terms are valid throughout the entire electromagnetic spectrum. “Light,” photometry, and photometric terms are restricted to the measurement of light in the


496

100

-

RADIANT POWER PER LUMEN INSECT CONTROL

50 -

-

200

WAVELENGTH (NANOMETER)

400

600

860 '

'

I

1000


U

+

RADIANT POWER PER LUMEN FLUORESCENT COOL WHITE (FCW)

9

94 F

RADIANT POWER PER LUMEN FLUORESCENT WARM WHITE (FWW)

v)

t

400

600

800

1000



1 5 0 ~

E~a

RADIANT POWER PER LUMEN PLANT GROWTH A (PGA)

100 +

= I w

RADIANT POWER PER LUMEN PLANT GROWTH B (PGB)

!toot ?

F

e C 0

c 200 WAVELENGTH (NANOMETER)


From Campbell et al. (1975) FIG. 10.2. SPECTRAL RADIANT POWER CURVES FOR HORTICULTURAL LAMPS


150

r

497

150

U

+ RADIANT POWER PER LUMEN MERCURY (CLEAR) (Hg)

5 100

RADIANT POWER PER LUMEN MERCURY DELUXE WHITE (Hg /DX)

z

F

Llg +

sp

600

400

800

50

2

1000



150

5

U

2 100

RADIANT POWER PER LUMEN METAL HALIDE A (MHA)

s

+

Y P

F

?

I-

+

5

u)

4

0

50

50

0 2

,

,

I

I 400

'

I

600

800

1

1000


RADIANT POWER PER LUMEN LOW PRESSURE SODIUM (LPS)

RADIANT POWER PER LUMEN METAL HALIDE B (MHB)


150,-

I 5 O F

f

t WAVELENGTH (NANOMETER)

FIG. 10.2. (Confinued)


498


n

HIGH PRESSURE SODIUM 400W

E

c

0

cu

II

w

a. [r

5

:

740- 2500nrn

80W

W

L I-

2500nrn+ 142W

n

4w

II I

700

1000

1500 2000 WAVELENGTH (nm)

2500

HIGH PRESSURE SODIUM 400W

E

7

II W

a. II

5

380 -740nm

2

118W

w

L I-

4W

a 250

300

350

”

400

450 500 550 WAVELENGTH (nrn)

600

650

700

Adapted from Jack and Koedam (1974)

FIG. 10.3. SPECTRAL POWER DISTRIBUTION HIGH PRESSURE SODIUM 400 WATT LAMP 250 TO 2500 NM

region from 380 to 780 nm. Light is a weighted response based on the relative stimulation of the human eye. Figure 10.5 shows the response for photopic vision (luminous efficiency, V) known as the “CIE (Commission Internationale de 1’Eclairage) curve” or eye sensitivity curve. The three systems of units for irradiance per unit area are: Illumination E, lumen per square meter = lux (1x1 Irradiance E, watt per square meter (W/mz) or (W em - 2 )

(photometric) (radiometric)


C.

INPUT 400W POWER IN ARC COLUMN

I/

499

INPUT 18OW POWER IN ARC COLUMN

ELEC-

ELEC-

DISCHARGE RADIATION

DISCHARGE RADIATION

59W

60W OUTERJACKET

I

1

I

281W

74W

1\

207W

A i

\ D.

Adapted from Jack and Koedam (1974) FIG. 10.4. POWER CONVERSION OF LAMPS A. Fluorescent. B. High Pressure Sodium. C.Metal Halide. D. Mercury. E.Low Pressure Sodium.

500


FIG. 10.5. STANDARD LUMINOUS EFFICIENCY CURVE (CIE)

Photon-flux density E, quantum per second and square meter (q s - 1 m - z )

(photon radiome try)

Notes Subscript refers to photometric quantities wherein luminous efficiency is included. Subscript refers to radiometric quantities. Subscript refers to photon quantities. The recommended SI units for these systems of measurement are shown in Tables 10.1 to 10.3, developed by the National Bureau of Standards, U.S. Department of Commerce. B. Measuring Units Considerable confusion and disagreement exist about what units should be used to measure radiation for plant science. Photometric units (lux, foot-candle) are used by lighting engineers for describing irradiance of radiation sources and in specification or evaluation of lighting applications. This system is widely used by manufacturers and design engineers. It is generally agreed that plant response to radiation is different from that of humans and that these photometric units are unsuitable for directly describing photosynthetic response. However, photometric measurements can be converted to useful units. 1. Photon Units.-Photon units are based on number of photons or quanta which vary in energy with wavelength. For example, 4 photons at 400 nm have the same energy as 7 photons a t 700 nm. Photon meters’ response thus approximates the number of photons a t each wavelength.

Symbol

Definition1

I

dQ/dX dL/dX

d*+/(dA.cose.dw); d*@/da-dw) dL,/ds; dI,/dV

f L-dw

d+/da;

f LcosO.dw

d@/dA;

dQ/dV dQ/dt d@/dw

f +.dt dQ/dA; f f [email protected] dQ/da; f f L.dw.dt

watt per square meter and steradian watt per cubic meter and steradian joule per nanometer watt per square meter, steradian, and nanometer

watt per square meter

watt per square meter

joule per cubic meter watt watt per steradian

joule per square meter

Unit joule joule per square meter

[W-m - 2 1

Unit Symbol

Source: Adapted from Nicodemus (1978). dA = element of (directed) surface; da = cross-sectional area of spherical element; dL, = element of generated (emitted or scattered into ray) radiance; dI, = element of generated radiant intensity; ds = element of distance along ray; dV = element of volume; and dt = element of time. * I n September 1977 a t Berlin, the CIE Technical Committee TC 1.2 on Photometry and Radiometry adopted a number of recommendations for additions and changes in the upcoming edition of the International Lighting Vocabulary. Among those recommendations, the terms “spherical exposure” and “spherical irradiance” were given as the preferred terms for what have been called here, respectively, “fluence” and “fluence rate,” although the latter also were recognized as acceptable alternates, widely used in photobiology.

(Other soectral auantities are similarlv treated)

spectral radiant energy spectral radiance

radiant sterisent

radiant exitance radiant (omni-directional) fluence rate* radiance

irradiance

Quantity radiant energy radiant (directedsurface) exposure radiant (omni-directional) fluencez radiant (volume) density radiant power or flux radiant intensity radiant flux (directedsurface) density

TABLE 10.1. SI DERIVED UNITS FOR RADIOMETRY

Symbol

Unit lumen-second; (candela-steradian-second) lumen-second; (talbot) lux-second (candela-steradian-secondper square meter) lumen-second per square meter lux-second (candela-steradian-second per square meter) lumen-second per square meter lumen; (candela-steradian) lumen candela lumen per steradian lumen (candela-steradian) per square meter lumen per square meter lux; (candela-steradian per square meter) lumen per square meter lumen (candela-steradian) per square meter lumen per square meter lux; (candela-steradian per square meter) lumen per square meter candela per square meter lumen per square meter and steradian candela per cubic meter lumen per cubic meter and steradian [cd.m -3] L1m.m -3-sr - 1 1

Unit Symbol

Source: From Nicodemus (1978). Note: The first entry or entries for each quantity give the SI units, including, in every case, units in terms of the candela [cd] as the base unit. The last entry for each quantity is the same unit in terms of the lumen llml or lumen-second llm.sl, that parallels the corresponding radiometric unit in terms of the watt [W] or joule [Jl, respectively, for the corresponding quantity in Table 10.1. The definitions (defining expressions) in that table, and the footnotes there, also apply to the corresponding quantities listed here.

luminous sterisent

luminous (omni-directional) fluence rate luminance

luminous exitance

luminous flux (directedsurface) density illuminance (illumination)

luminous intensity

luminous flux

luminous (omni-directional) fluence

Quantity luminous energy; quantity of light luminous (directedsurface) exposure

TABLE 10.2. SI DERIVED UNITS FOR PHOTOMETRY E3 0

cn

Symbol

[q.s -1.m - 2 1 [q.s-l.m-z.sr -11 [qs-l.m-3.sr-l]

quantum per second and square meter quantum per second, square meter, and steradian quantum per second, cubic meter, and steradian

-21

[q.s -1.m

'1

quantum per second and square meter

[qs

[q.s-'.sr-1]

[ql [ q m -'I [qm-']

Unit Symbol

quantumZ quantum per square meter quantum per square meter quantum per second quantum per second and steradian

Unit

Note: The einstein [El = NA*[q] (where N, is the Avogadro constant, the number of molecules [particles] per mole [moll of any substance) and is widely used as a (much larger) unit of photon flux. (The latest value of the Avogadro constant in NBS Pub'. 398 is given as N, = (6.022045 f 0.000031) X loz3[particle~mol-~l.) efinitions (defining expressions) are the same as for corresponding quantities in Table 10.1. Also, spectral quantities are formed as shown in that table. The number of photons or quanta in a beam of radiation is frequently regarded as a pure (dimensionless) number, the ratio between the energy in that beam and the energy (hu) of an individual photon or quantum. However, that number is certainly a measure of the "amount of radiation" in the beam and it is not just a number, but is a number of a distinctive physical quantity, just as the number of joules is a physical quantity. Accordingly, it is useful to assign the quantum per second [q.s-'] as the unit of photon flux. Then all of the other geometrical quantities and their interrelationships and units parallel exactly those for radiant flux, luminous flux, or any other flux of a physical quantity propagated in rays that obey the laws of geometrical optics.

:r.,

Quantity' photon-flux energy; number of photons photon-flux exposure photon-flux fluence photon-flux photon-flux intensit photon-flux (surfacer density incident photon-flux density photon-flux exitance photon-flux fluence rate photon-flux sterance (radiance) photon-flux sterisent

TABLE 10.3. UNITS FOR PHOTON-FLUX RADIOMETRY

W

0

cn

504


In 1976 the Crop Science Society of America (Shibles 1976) defined the following:

Photosynthetically Active Radiation (PAR): Radiation in the 400 to 700 nm waveband (McCree 1971, 1972a,b). Photosynthetic Photon F l u x Density (PPFD): Photon flux density of PAR. The number of photons (400 to 700 nm) incident per unit time on a unit surface. (Suggested units: nE(einsteins1 * s - l * cm - 2 ) . Photosynthetic Irradiance (PI): Radiant energy flux density of PAR. The radiant energy (400 to 700 nm) incident per unit time on a unit surface. mW * cm - 2 . (Suggested units compatible with the SI convention would be Wm - 2 and Es - l m -z.) These definitions permit PAR to be reported in either quantum or energy units (McCree 1971, 1972a,b). In the past and in some current literature PAR usage does not conform to these definitions. 2. Solar Radiation Units.-Measurement of outdoor solar radiation (in Langleys), reported periodically by the U.S. Weather Bureau (now NOAA) from 1950 until 1972, was discontinued when errors of up to 100% in reported values were discovered. With improved calibration this reporting was reinstated in 1978 in both W/m2 and Langleys. This radiation information is needed for all solar energy utilization. NOAA also reports percentage of sunshine, temperature, and other environmental parameters. (A large package describing available information is available from the National Climatic Center, Federal Climatic Center, National Oceanic and Atmospheric Administration, Asheville, North Carolina 28801.)

3. Radiometric Units.-Irradiance or radiometric units which indicate energy in watts per square meter (W/m2) have equal response a t all wavelengths. This is an absolute measurement but not easily accomplished in a simple direct measurement because sensors with the appropriate spectral response usually are not available. Radiometric units are sometimes referred to as energy units. Nearly all standards and basic calibrations are in radiometric units (watt). Spectral power distributions of lamp manufacturers are also in radiometric units. We have adopted the viewpoint of using radiometric units W/m2 for photoregulation of plants in the subsequent portions of this chapter. 4. Photometric Units.-Lamp manufacturers rate lamps’ output in lumens. On request, most manufacturers will provide a visual spectral power distribution which shows the emitted radiation in microwatts per nanometer per lumen emission. With this information, the absolute radiation in W/m2 can be determined by summing the radiation over the interval desired.


505

C. Measurement Methods We define and use photoregulation to denote plant response to all wavelengths of radiation, UV, visible, and infrared. Outdoors we are concerned with wavelengths from 300 to about 3000 nm. High intensity discharge lamps (HID) and incandescent emit radiation extending over about the same wavelength region but with different spectral power distribution. Fluorescent lamps emit mainly in the visible region but have some longer wave radiation. No single measurement or instrument is adequate to describe the radiant energy of light sources in relation to crop production. Pyranometers or solarimeters will measure the total radiation but take no account of the spectral distribution. Other meters such as illumination and photon quantities have spectral limitations. 1. Basic Requirements for Measurement.-In any measurement, if the spectral sensitivity of the sensor or detector and the spectral power distribution of the source are known, both the spectral and total radiation can be calculated. At each wavelength relative values of the sensor sensitivity and source emission multiplied together give the weighted radiant flux density. These are summed for all wavelength bands within the limits of the known wavelength response. Since most light sources including the sun have almost a generic spectral power distribution, projections can be accurately calculated beyond the sensor wavelength limits. This permits the use of standard measuring instruments wherein the measurement can be converted to total radiation in whatever wavelength region is desired. Table 10.4 was developed to determine the radiation in various wavelength bands using illumination meters. Basic rules to be followed with measurements include:

a. Make measurements once a month. b. Check measurements occasionally with another meter. c. Check meter calibration a t least once a year. In addition, certain information always must be recorded and reported: type and number of lamp (as etched on lamp); distance of sensor from lamp; make and model of meter; sensor type or model number. Ambient temperature a t point of measurement and date and time of measurement also should be reported. PAR measurements, by definition, whether PPFD or PI, reflect the total radiation between 400 nm and 700 nm and do not indicate differences in the spectral emission of sources. The differences between PI and PPFD are less than 10%. This is less than the normal instrument total error, not including user error.

LampIdentification Incandescent (INC) lOOA Fluorescent Cool white FCW Cool white FCW Warmwhite FWW PlantgrowthA PGA PlantgrowthB PGB Infrared FIR HID Discharge Clearmercury HG Mercurydeluxe HG/DX Metalhalide MH High-pressure sodium HPS Low-pressure sodium LPS

3

4 Output

5

6

7 8 9 Radiation per Unit of Luminous Flux

10

40 3200 70 215 15700 64 40 3250 71 40 925 20 40 1700 37 40 170 3.7 400 21000 50 400 22000 50 400 40000 85 400 50000 105 180 33000 143

46 245 46 46 46 46

440 440 460

470

230

1740 17

100

100

183

125

52 55 100

80 73 81 23 42 4.2

17

1.92

2.45

2.60 2.62 3.05

2.93 2.93 2.81 6.34 3.96 4.30

3.97

2.18

3.38

2.77 2.81 3.42

2.99 2.99 2.86 6.41 4.37 24.00

8.63

1.89

1.58

0.14 0.73 1.17

1.02 1.02 1.23 3.39 1.95 0.56

2.53

0.26

0.93

0.17 0.19 0.37

0.06 0.06 0.05 0.08 0.41 20.00

4.66

0.25

0.72

0.06 0.05 0.25

0.009 0.009 0.006 0.007 0.03 2.10

1.69

Total Lamp Total Total Lamp 400-700 nm 400-850 nm 580-700 nm 700-850 nm 800-850 nm mW/lm mW/lm W W lm Im/W lm/W mW/lm mW/lm mW/lm

1 2 Input Power

TABLE 10.4. ELECTRICAL, PHOTOMETRIC AND RADIOMETRIC CHARACTERISTICS OF SELECTED LAMPS

Column Number

11

12

13 Radiation Output

14

15

16

17 18

Efficiency

19

20

Source: Radiation data revised April, 1980 by R.W. Thimijan, USDA-SEA-AR, Beltsville, Md.

Lamp 400-700 nm 400-850 nm 580-700 nm 700-850 nm 800-850 nm 400-700 nm 400-850 nm 580-700 nm 700-850 nm 800-850 nm Identification W W W W W mW/W mW/W mW/W mW/W mW/W Incandescent lOOA 6.90 15.00 4.41 8.11 2.94 69 150 44 66 29 F1 uorescen t FC W 9.38 9.56 3.27 0.18 0.03 204 208 71 4 0.6 FC W 46.00 47.00 16.00 0.88 0.14 188 192 65 4 0.6 FWW 9.13 9.28 4.00 0.15 0.02 199 202 87 3 0.4 PGA 5.86 5.93 3.13 0.07 0.01 127 129 68 2 0.1 PGB 6.73 7.42 3.32 0.69 0.05 146 161 72 15 1 FIR 0.73 4.13 0.10 3.40 0.35 16 90 2 74 8 HID Discharge 3 HG 55 58 3 3.6 1.2 124 132 6 8 3 36 9 HG/DX 58 62 16 4.1 1.1 131 140 22 MH 122 137 47 14.8 10.0 265 297 102 32 ___ ~. 77 168 99 261 360 79 46.5 36.0 123 169 HPS 36 271 37 8.3 276 313 62 8.6 63 72 LPS

Column Number

>

X c3

0

E:

508


If measurements are determined in units other than W/m2 or lux, conversions must be available for engineering or design specifications. Some published conversions assume a flat spectral distribution of the source rather than actual source spectra (Biggs and Hansen 1979). When fluorescent lamps alone are used in a growth chamber, PAR measurements from 400 to 700 nm will correlate reasonably well with crop production. This is because fluorescent lamps have very low emission beyond 700 nm. However, when HID lamps such as high-pressure sodium, metal halide, and incandescents are used either alone or in combination with other sources, crop production correlates more closely with radiation if the wavelengths from 700 to 850 nm are included. This is believed to be a response combining photomorphogenesis and photosynthesis along with other undefined photo responses. It is consistent with our observations that plants in the greenhouse and outdoors have increased performance (growth rates, flowering time) that cannot be attributed solely to the energy measurements in the 400 to 700 nm region. 2.

Types of Light and Radiation Meters.-

Type of Meter Illumination Photon Irradiance Pyranomometer (solarimeter) Ultraviolet Infrared Solar cells Total irradiance Spectroradiometers

Unit -

lux (foot-candle) quantum watt per square meter watt per square meter watt per square meter watt per square meter watt per square meter watt per square meter watt per square meter and nanometer

Wavelength (nm) 380-780 400-700

*

320-4200 250-400*

*

400-800

*

300-1100

Low sensitivity and poor stability are no longer an inherent problem in meters (sensors not included) due to the advent of solid state electronics. With proper design there should be little or no temperature instability of the meter. Battery operation is feasible and desirable except for meters used in continuous recording. Electrically, the basic parts of all meters are essentially the same. The differences are in the sensors and the calibration units used. 3. Types of Sensors and Detectors.-There are four basic types of sensors or detectors. Their characteristics are briefly noted. Spectral response curves for typical sensors or detectors are shown in Fig. 10.6 through 10.10. *No accepted standard-variance in spectral sensitivity and wavelength range.

LIGHT AND LIGHTING SYSTEMS W

v)

z

2

-

SCANNER

v)

CIE OBSERVER CURVE

W

I__

U

W

I I-

d

i W

U

400

600

800

1000

WAVELENGTH (NANOMETERS) FIG., 10.6. PHOTOMETRIC DETECTOR CIE CURVE

100

-

IDEAL QUANTUM RESPONSE

W

a W I Id

i W

I

U

I

400

I

I

I

I

I

I

I

600 800 WAVELENGTH (NANOMETERS)

I

I

I

I

J

1000

FIG. 10.7. COMMERCIAL PHOTON DETECTOR SPECTRAL RESPONSE (PHOTON-FLUX DENSITY)

W

v)

2

2 v)

W W

I I-

3

w

U

100 -

-

\

-

1

60 -

20 I

I

I

I

I

I

I

FIG. 10.8. COMMERCIAL VISIBLE-IR SPECTRAL RESPONSE

I

1

1

1

I

I

I

RADIOMETRIC DETECTOR

I

509

510


z

W

8

100

v)

W

a W 2 I-

4

W

60 20

a WAVELENGTH (NANOMETERS) FIG. 10.9. COMMERCIAL DETECTOR FOR IR NARROW BANDWIDTH

z

W

8

100

v)

w

a

60

W

2 c

5

20

W

a WAVELENGTH (NANOMETERS) FIG. 10.10. COMMERCIAL SILICON PYRANOMETER CALIBRATED FOR TOTAL SOLAR IRRADIANCE

Not useable for other sources without correction.

a. Thermopile or bolometer-flat response to all wavelengths; low signal level, slow response, requires ambient temperature correction. b. Photovoltaic cells-Selenium or silicon; range: 300 to 900 nm, response varies with wavelength, medium to high signal output, used with filters, temperature stable a t low impedance. c. Phototubes-photomul tipliers-200 to 1000 nm-response varies with wavelength. No single tube covers entire wavelength. Requires DC power supply, temperature sensitive, high sensitivity. Used mainly in laboratory instruments and spectroradiometers.

LIGHT AND LIGHTING SYSTEMS 511

d. Photodiodes-Similar to photovoltaic-some respond in IR region, long time stability unknown, high temperature coefficient, may require bias voltage or current.

For measurements to be reported on an absolute basis, only the following sensors can be used readily with sources of different spectral content: a. Photometric-silicon or selenium-calibrated in lux or foot-candles. b. Flat response sensors-calibrated in quanta or watts per square meter. When either of these sensors is used and the generic type of light source is reported, the radiation a t various wavelengths can be calculated and comparisons can be made with other reports. 4. Calibration.-Radiation sources used in calibration are known as standard lamps or standard sources. A primary standard is a source from which the values of other standards are derived. Primary standards usually are found in national physical laboratories such as a National Bureau of Standards. A secondary standard or reference standard is a radiation source calibrated from a primary standard. Secondary standards also are maintained in national physical laboratories and photometric and radiation laboratories, and by meter manufacturers. Working standards are sources calibrated from secondary standards for regular day-to-day use. These are usually special incandescent lamps. Standard lamps may be calibrated in total irradiance (watts per square meter) with wavelength limits, or in watts per square meter per nanometer, or both. Calibrated accuracy is f3% in the visible region, f8% in the UV region, and f4% in the IR region. Standard lamps require a precise power supply accurate to 5 ppm. Most plant scientists have neither time nor resources for precise calibration checks. Return of meters to the manufacturer or utilization of calibration labs may be a simpler way to check on equipment operation. There are several methods to approximately check calibration: (1)compare with other meters, preferably one recently calibrated; (2)purchase a second meter and keep it on the shelf as a standard; or, (3) for the meter alone without sensor, make voltage checks according to the manufacturer’s calibration instructions.

5. Guidelines.-Uncertainty with any meter will be about f5% and is likely to be f10% or more in practical day-to-day measurements. There are several recognized causes: a. Calibrations traceable to National Bureau of Standards have up to f5% uncertainty . b. Sensors and meters are seldom entirely free of temperature error. c. Cosine diffusion will vary with wavelength of source.

512


d. Meter and sensor are calibrated with an incandescent lamp as a point source. Measured sources differ in both spectral power distribution and extension in space. Radiation in growth chambers or in sunlight is almost never constant. Measurements show conditions a t a particular moment. In growth chambers with fluorescent lamps, lamp output may change f5% over a few minutes, and in the sun irradiance may vary considerably more than this. Sensors without cosine correction (incident light correction) result in relative values that cannot be compared to cosine corrected measurements. A 360” correction also will be confusing. Radiation measurements of the same growth chamber requirements taken independently by 2 individuals with the same equipment are likely to be different by 5 to 10%. 6. Recommended Methods.-In

order of complexity and cost.

a. Illumination Meter-Photornetric-($50, $150 and Up).-Meter measures radiation emitted in lux (foot-candle) for each generic type source, and converts to irradiance (watts per square meter) from tables for the wavelength region desired. (See Table 10.4.) b. Irradiance Meter-Radiornetric-($500 and Up).-Meter is used directly with any available sensor for specific wavelength intervals. Narrow bandwidth sensors which have interference filters which change calibration with change of direction of radiation and temperature are available. Wide band sensors may not have flat spectral response. Field of view is frequently only a 10 degree cone since cosine correction introduces a wavelength error. Pyranometer (Eppley type) thermopile has flat response but drifts with temperature. c. Photon Meter-($750 and Up).-These meters have the same basic silicon sensor and electrical circuit of other meters but have filters for approximate desired spectral response of 400 to 700 nm. Calibration check is difficult because standard sources are calibrated in watts. It is best to return to manufacturer for calibration. 7. Spectral Distributions.-Spectroradiometers are used to determine spectral distributions as in Fig. 10.2 and 10.3. In years past, the required equipment and expertise limited spectral measurements to a few lamp manufacturers and commercial testing laboratories. Commercial portable units in the past have lacked the precision and accuracy needed for characterizing the radiation environment of plant growth environments “in situ.” Recent developments of an automated spectroradiometer by the Instrumentation Research Laboratory a t the Beltsville Agricultural Research Center are promising for obtaining spectral irradiance as com-


513

mercial models are developed. Spectroradiometric measurements are extremely difficult and time-consuming. Even with the anticipated improvement in equipment, the time and attention to calibration will limit spectroradiometric measurements to larger facilities where a part or fulltime individual can be assigned this responsibility. Additional information on spectroradiometry is contained in the Self-study Manual of Optical Radiation of the National Bureau of Standards (Nicodemus 1978).

D. Infrared and Thermal Radiation It is convenient to divide the IR region into a region from 780 to 2500 nm as Infrared and 2500 to 10 - 5 nm as Thermal, since the 2500 and above is not transmitted by glass. Solar radiation is mainly below 2500 (Fig. 10.1) and transmitted by glass. Transparent plastic materials transmit some of the thermal wavelengths. The plant response to this IR radiation is mainly unknown except that heating results from the radiation. Table 10.5 shows the emitted radiation of various sources in the visible, infrared, and thermal regions per 100 watts of input power. TABLE 10.5. RADIATION POWER DISTRIBUTION OF LIGHT SOURCES PER 100 WATT OF TOTAL RADIATION

uv

Light Source FC W HG/DX MH

1 4 0 0 nm W 2 3 4

IR 400-850 nm 850-2500 nm W W 36 19 41

1 18 8

Thermal 2500 + nm W 61 60 47

Total Radiation

W

100 100

100

Source: Radiation data developed or revised by R.W. Thimijan, USDA-SEA-AR, Beltsville, Md.

In a growth chamber, radiant energy including the visible region, 100 watts of radiation raises the temperature of 300 cfm (150 literdsec) of air 0.5"C. Thus, in a growth chamber (1 mz) with 20,000 lux CWF and 5000 lux INC and 150 l i t e d s e c (300 cfm) the air temperature will rise nearly 2°C. If no glass barrier exists between the lamps and the chamber, the rise will be nearly 3°C. Outdoors, in similar light levels and similar air movement, the air temperature rise would be about 1°C from sunlight. Table 10.6 shows the radiation in a typical growth chamber with and without a glass barrier. The method and location of temperature sensors for either measurement or control have been a problem. With the lights on and the sensor

514


TABLE 10.6. RADIATION FROM LIGHT SOURCES

Through Glass Light Source FC W INC FCW+INC SUN HPS LPS

klx 20 5 25 20 20 25

400-850 nm W/m2 60 43 103 110 68 55

850-2500 nm 3 192 195 63 17 3

No GlassTotal 166 260 426 186 136 98


unshielded, the temperature indicated in a growth chamber will rise 4" to 6°C. If the sensor is shielded and aspirated, the circulated air temperature will be indicated. This may not be as near the effective plant temperature as an unshielded sensor in the direct radiation. When the circulated air is exhausted a t the top of a chamber which has a physical barrier between the lamps and the chamber, the temperature of the exhaust air may more closely reflect the effective plant temperature. The fact must be faced that in a radiation environment transpiring plants have an effective temperature which is somewhere between that of the ambient air and that of radiation-absorbing mass. Remote sensing as well as contact methods of temperature has not indicated a precise relation to plant response. At present, due to the lack of definitive precedent 'measurements or equipment, the investigator should consider possible effects of plant temperature due to radiation. Temperature measurements, in a chamber without barriers, tend to be misleading even in shielded enclosures. Plastic barriers are less effective than glass in shielding plants from thermal radiation since they transmit greater amounts of infrared than glass.

111. SPECTRAL RADIANT POWER OF LAMPS The spectroradiometric system was used to determine the light-source emissions in graphic form (Campbell et al. 1975). The graphs then were normalized to a per-lumen basis. The curves were compared with manufacturers' data and with published information. Most such comparisons were made within the wavelength region of 500 to 600 nm, since this region provides the most sensitive and accurate measurement. Published information and manufacturers' data on wavelengths above 700 nm are extremely limited because these wavelengths are less important than are those in vision lighting. After the data in graphic displays were determined to be in agreement with other known information a t as many points as possible, data were


515

compiled on irradiances in the wavelength intervals that are of concern in plant response. The data shown in the graphs and tables are essentially extensions of existing information, but they are presented in a manner that should be useful in horticultural lighting. Table 10.4 shows the electrical, photometric, and radiometric properties of a range of lamps that are important in horticultural lighting. The lamps selected are typical of commercially available light sources that have high efficiency or spectral radiation unique to plant lighting. Table 10.7 shows the energy balance in the lamps. Input energy equals the sum of visible radiation, heat radiation, conduction, convection, and ballast loss. The power conversions are for lamps without luminaires or enclosures. Enclosures are expected to decrease slightly visible radiation and to increase heat radiation, conduction, and convection. TABLE 10.7. INPUT POWER CONVERSION OF LIGHT SOURCES'

Lamp Identification Incandescent (INC) lOOA Fluorescent Cool white FC W Cool white FCW Warm white FWW Plant growth A PGA Plant growth B PBB Infrared FIR Disc ha rge HG Clear mercury Mercury deluxe HG/DX Metal halide MH High-pressure sodium HPS Low-pressure sodium LPS

Total Input Radiation Other Power (400-850 nm) Radiation (Watts) (%) (%I

Conduction and Convection

Ballast Loss

(%)

(%)

100

15

75

10

00

46 225 46 46 46 46

21 19 20 13 16 09

32 34 32 35 34 39

34 35 35 39 37 39

13 12 13 13 13 13

440 440 460

13 14 30

61 59 42

17 18 15

09 09 13

470

36

36

13

15

230

31

25

22

22

Source: Radiation data developed or revised by R.W. Thimijan, USDA-SEA-AR, Beltsville, Md. 1 Conversion efficiency is for lamps without luminare. Values com iled from manufacturer data, published information, and unpublished test data by W.W. Thimijan, USDASEA-AR, Beltsville, Md.

A. Photometric Data (Table 10.4) Columns 1 through 5 give the standard electrical and lumen ratings of the lamps. The lumen rating shown for the lamp FIR is an estimated value (de Boer 1974; Kaufman and Christiensen 1972; Kaufman 1973). Columns 6 through 20 list 5 repetitive wavelength intervals that describe the emission in 3 ways. Columns 6 through 10 are given in milliwatts per

516


lumen, which can be used with illumination-meter measurements (footcandle or lux) to determine radiation in milliwatts per unit of area. Lux times milliwatts per lumen equals milliwatts per square meter. Footcandles times milliwatts per lumen equals milliwatts per square foot. When generic types of lamps are used in combination, measurements must be taken individually, with only one type of lamp in operation a t a time; the measurements are then summed. Columns 11 through 15 give the total radiation output in the indicated wavelength interval for each lamp. The values should be rounded off to two significant figures for practical use. The overall efficiency of the lamps-watts emitted per watt of electrical energy used, including ballasts-is shown in columns 16 through 20 for the wavelength interval indicated. These columns allow a comparison of the lamps’ relative efficiency for emissions a t specific wavelength intervals.

B. Wavelength Intervals (Table 10.4) The 400 to 70 nm wavelength interval is similar to the region of plant response in photosynthesis. The commonly accepted photosynthesis action spectra peaks in the blue and red region of the spectrum. Recent reports indicate that all wavelengths between 400 nm and 850 nm may be effective in photosynthesis (Cathey and Campbell 1977; Meijer 1971). In photomorphogenesis plants respond to emissions in the red and far red portions a t wavelengths of 580 to 700 nm (red), 700 to 850 nm (far red), and 400 to 500 nm (blue). These wavelength intervals are important in photoperiod control and/or in the control of flowering, bulbs, and other light-mediated plant responses (Cathey and Campbell 1974). The information on radiation emitted in the 800 to 850 nm wavelength interval, shown in Table 10.9, permits comparisons to be made of lamps that have emissions above 800 nm with those which do not. The importance of emissions above 800 nm is not known. IV. GENERIC RESPONSES OF PLANTS TO LAMPS All human vision lamps can be used as an additional source to grow plants. All lamps, however, have inherent limitations in their use as a sole source for growing. The limitations are: (1) Irradiance source and its distribution over an area. (2) Irradiance without the creation of a cone of heat or other growthmodifying radiation.


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(3) Irradiance insufficiency or surplus which modifies growth responses. (4) Irradiance lacking energy in essential region(s) or the photoconver-

sion of factors regulating growth. ( 5 ) Irradiance performance (output and maintenance) in various environments. (6) Irradiance capability in space available for installation of lamps and fixtures. (7) Irradiance level and duration sensitivity based on growth stage and responses desired. When these limitations and others are considered, many lamps can be eliminated for use in many lighting situations. I t also means that the variations among special lamps can be extremely difficult to identify when so many kinds of plants and plant responses are considered. Sunlight, unlike all of the lamps described, is a continuous spectrum source (Fig. 10.1) from the ultraviolet into the infrared. The photo-action of sunlight is not described in Table 10.8. Experiments with sunlight often use neutral filters to reduce the intensity in the comparable energy range of artificial light sources. These experiments are complicated by hourly, daily, and seasonal shifts in intensity, heat, and light quality (Frankland and Letendre 1978; deLint and Klapwijk 1973). The information from such experiments often is not transferrable from one section of the country to another and has led to many disappointing or inefficient facilities for growing plants (Hammer et al. 1978; Morris 1973; Enoch et al. 1973). The effects of sunlight on plants are similar to the ones suggested for INC and associated lamps. The paling of foliage, the lengthening of stems, the expansion of leaf blades, and the suppression of lateral branching are extremely sensitive interactions between sunlight and temperature, moisture-stress, and mineral ions. Sunlight in this review is considered to be an everchanging energy source for which artificial light can substitute or supplement (Holmes and Smith 1975). The regulating action of intense sunlight and its effects on phytochrome in tissue filled with chlorophyll still await analysis (Morgan and Smith 1976). Table 10.8 describes how the different types of lamps regulate seed germination, photoperiod control, and growth. The standard light source for many years has been cool white (CWF) or warm white (WWF) fluorescent and is used as the standard to which the other sources are compared. The lamps are described in generic types. Special lamps combining the features of several of the lamps are available. In our experience, these lamps cost the most, are less efficient, and require more complex equipment to operate them than standard, widely available lamps.

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TABLE 10.8. LAMPS AND PLANT RESPONSE

Lamp Fluorescent - Cool White(CW)

Plant Responses Growth Green foliage which expands to parallel to the surface of the lamp Stems elongate slowly Multiple side shoots develop Flowering occurs over a long period of time

Seed Germination Prompt uniform response Photoperiod Short Day Plants: Interruption, 4 to 8 hours Long Day Plants: Relatively ineffective Fluorescent Gro Lux (GL) and Plant Light

Growth Deep green foliage which expands, often larger than on plantsgrownunderCWor WW Stems elongate very slowly, extra stems develop Multiple side shoots develop Flowering occurs late, flower stalks do not elongate Seed Germination Prompt, seedlings shorter than those grown under CW or WW Pho toperiod Short Day Plants: Interruption 4 to 8 hours Long Day Plants: Relatively ineffective

Fluorescent Gro Lux WS Plant Light WS(GL-WS) Vita-lite (VITA) Agro-lite (AGRO),and Wide Spectrum Lamps

Growth Light green foliage which tends toward thelamp Stems elongate rapidly, distances between the leaves Suppresses development of multiple side shoots Flowering occurs soon, flower stalks elongated, plants mature and age rapidly Seed Germination Prompt, seedlings taller than those grown under CW or WW Photoperiod Short Day Plants: Interruption 3 to 6 hours Long Day Plants: Extension of 8-hour day to 18 to 20 hours daily

High Intensity Discharge Mercury (HG)or Metal Halide (MH)

Growth Similar to CW and WW fluorescent lamps compared on equal energy Green foliage which expands Stemselongate slowly Multiple side shoots develop Flowering occurs over a long period of time Seed Germination Prompt, seedlings similar to ones germinated under CW andWW Photoperiod Short Day Plants: Ineffective Long Day Plants: Ineffective

LIGHT AND LIGHTING SYSTEMS TABLE 10.8. (Continued) Lamp

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Plant Responses

Growth High Intensity Discharge High Pressure Sodium (HPS) Similar to Gro-Lux and other improved fluorescent compared on equal energy Deep green foliage which expands, often larger than on plantsgrown under HG and MH Stems elongate very slowly, extra thick stems develop Multiple side shoots develop Flowering occurs late, flower stalks do not elongate Seed Germination Prompt, seedlings shorter than those grown under HG and MH Pho toperiod Short Day Plants: Extension or interruption for 4 to 16 hours Long Day Plants: Ineffective Low Pressure Sodium (LPS)

Growth Extra deep green foliage, bigger and thicker than on plantsgrown under other light sources Stem elongation is slowed, very thick stems develop Multiple side shoots develop even on secondary shoots Floweringoccurs, flower stalks do not elongate Exceptions: Saintpaulia, lettuce, and Impatiens must have supplemental sunlight or incandescent to ensure development of chlorophyll and reduction of stem elongation Seed Germination Prompt, uniform response Pho toperiod Short Day Plants: Ineffective Long Day Plants: Ineffective

Incandescent (INC) and Incandescen t-Mercury (INC-HG PLANT LIGHT) (TUNGSTEN-HALOGEN)

Growth Paling of foliage, thinner and longer than on plants grown under other light sources Stem elongation is excessive, eventually becomes spindly and breaks easily Side shoot development is suppressed, plants expand only in height Flowering occurs rapidly, the plants mature and senescence occurs Exceptions: Rosette and thick leaved plants such as S a n seueria may maintain themselves for many months. The new leaves which eventually develop will elongate and will not have the typical characteristics of the species Seed Germination Inhibitsgermination of some species Photoperiod Short Day Plants: Effective as extension or interruption, given continuously or intermittently (6 sec/min to 3 m i d 3 0 min for 4 to 8 hours) Long Day Plants: Effective as extension or intermittently (12 sec/min for 4 to 8 hours)

Note: Mention of a trademark name or a proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture, and does not imply approval of it to the exclusion of other products that also may be suitable.

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V. SELECTION OF EFFICIENT LIGHT SOURCES BY PLANT RESPONSES Plants are widely adapted to growing in highly varied light levels. We seldom see them growing, however, under optimum conditions. The technology developed in growth chambers which combined the simultaneous enhancement of light, temperature, humidity, water, and nutrition is seldom transferred into our traditional growing facilities. We have been forced to adapt the use of light-transmitting structures and neutral filter to reduce the energy available to the plants to a workable intensity. However, the true energy level under which we have grown the plant is extremely difficult to decipher when so much energy is applied and removed simultaneously. A clue to our problem is that most plants cannot be grown in unshaded and unventilated transparent structures, but yet can be grown out-ofdoors. The growers’ dilemma of plants exposed to the so-called “greenhouse effect” of the visible light energy (short wavelengths) entering but being trapped as heat (long wavelengths) or not being irradiated in covered structure is still unresolved. Plants must have visible radiation, but we must design systems to eliminate or utilize the surplus long wavelength radiation. In the approaching age of energy shortages and conservation, all energy must be trapped from the sun and must be utilized to produce acceptable plants for commercial growers, biomass converters, small farm producers, and urban (home-grown) gardeners. Lighting to substitute for or supplement the available sunlight can be used to accomplish some goals: (1) Reduce the time required to produce the desired stage of growth. (2) Utilize the lamps as energy sources rather than just for their light-use several criteria for measurement and installation of lamps. (3) Establish the light intensity and duration required for the plant processes to proceed. Overlighting (called “overshoot”) wastes energy (visible light and heat). (4) Utilize light properly throughout the growth so that the plants will not require “acclimatization” for their successful use by the consumer. (5) Learn how to compare one light source to another to create equally effective lighting systems for growing plants. ( 6 ) Develop alternative and lowered-energy-requiring structures for growing plants. We have arranged the energy requirements for displaying, handling, and growing plants into six levels. All are well below what is recorded (whatever sensor and value system) when plants are grown out-of-doors


521

(McCree 1971, 1972a,b). Much of the energy in the out-of-doors occurs in “overshoot,” supra-optimal levels of natural light (Menz et al. 1969). Due to the constantly changing light intensities resulting from clouds, rain, daylength, and the orientation of the sun, these levels are seldom sustained over extended periods of time (Evans 1963). Further, re-radiation of the shortwave energy from the plants back into space occurs without interference from the covering (glass, plastic, films), thus effectively reducing the total energy that plants must tolerate. We do not believe that any growth systems for plants should or can be designed to mimic out-of-door conditions in its spectral or energy distribution (Balegh and Biddulph 1970; Singh et al. 1974). Supplemental or substitute lighting systems are thus, a t best, a simulation of only part of what actually occurs in nature. We are fortunate, however, that lighting systems for plants can be designed which afford, in many cases, simpler and easier environments to standardize and to regulate. Horticultural research scientists must maintain the perspective, however, that they are still creating only an approximation of the natural environment (Evans 1963). One then should anticipate that some species, cultivars, or breeding lines will exhibit aberrant growth characteristics when grown in a regulated photo-environment which may not be apparent when the same plants are grown in the natural environment. Even the traces or the absence of one or several parts of the spectrum (290 to 2500 nm) may limit the growth of a few species or several subtypes. The majority of species tested, however, did not exhibit abnormal growth characteristics and developed plants typical for the type (Canham 1974; Cathey and Campbell 1977). As we wander away from our three traditional light systems-INC for photoperiod (Downs and Borthwick 19561, CWF for acclimatization chambers (Biran and Kofranek 1976; Cathey et al. 19781, and CWF + INC for growth chambers (Tibbitts et al. 1976)-we can expect more and more examples of unusual growth problems (Brown, Cathey, Bennett and Thimijan 1979; Brown, Foy, Bennett and Christiansen 1978). First we shall discuss the lighting from a generic sense-when the various types of lamps can be used interchangeably. Then, we shall discuss energy-efficient lighting systems which satisfy specific spectral requirements and/or combinations of lamps for most prompt and rapid growth.

A. Practical Plant Lighting 1. Display: 0.3 W/mz.-Plants will exist a t an intensity of 0.3 W/mZ (Table 10.9). By tradition, the lamp of preference has changed with technological advances in efficiency and distribution. The emphasis, how-

Lamp Type, Illumination, l l o l u x Lamps per Square Meter and Distance from Plants, Meters Fluorescent-Cool White 40W single lamp 4 f t 3.2 klm Illumination, kilolux Lamps per square meter Distance from plants, meter 40W 2-lamp fixtures (4f t ) 6.4 klm Illumination, kilolux Fixtures per square meter Distance from plants, meter 215 W 2-8 ft lamps 31.4 klm Illumination, kilolux Fixtures per square meter Distance from plants, meter High Intensity Discharge Mercury (1)400W Parabolic Reflector Illumination, kilolux Lamps per square meter Distance from plants, meter 0.30 0.36 1.7 0.30 0.18 2.4 0.30 0.04 5.1 0.32 0.05 4.4

0.10 0.12 2.9 0.10 0.06 4.1 0.10 0.01 + 8.8 0.1 0.02 7.6

0.3

1.1 0.17 2.4

3.2 0.52 1.4

3.0 0.39 1.6

3.0 1.8 0.75

1.o 0.60 1.3

1.0 0.13 2.8

3.0 3.6 0.53

1.o 1.2 0.92

6.4 1.o 1.o

6.0 0.77 1.1

NA

NA

8.6 1.4 0.8

8.0 1.o 1.o

NA

NA

Radiant Power 400-850 nm a t Plant Level Watts per Square Meter, W.m - 2 0.9 3 9 18 24

TABLE 10.9. LIGHTING DESIGN GUIDE FOR RADIANT ENERGY LEVELS 0.3 TO 50 W.m-2

18.0 2.9 0.6

16.7 2.2 0.7

NA

NA

50

u,

r

m

z

N N

cn

1.4 0.088 3.4 0.33 0.56 1.3 0.33 0.35 1.7 0.50 0.74 1.2

0.41 0.026 6.2 0.10 0.17 2.4 0.098 0.10 3.1 0.15 0.22 2.1 0.16

0.14 0.009 10.7 0.033 0.056 4.2 0.033 0.035 5.4 0.050 0.07 3.7 0.054 0.54

0.89 0.05 4.5

0.27 0.015 8.2

0.089 0.005 14.2

0.88 0.08 3.6

0.26 0.02 6.5

0.09 0.01 11.3

1.6

1.5 2.2 0.67

3.2

3.O 4.5 0.47

2.0 2.1 0.7

2.0 3.4 0.54

1.o 1.7 0.77

1.0 1.0 1.o

8.3 0.53 1.4

5.3 0.30 1.a

5.3 0.47 1.5

4.1 0.26 2.0

2.7 0.15 2.6

2.6 0.24 2.1


Metal Halide (1) 400W Illumination, kilolux Lamps per square meter Distance from plants, meter High Pressure Sodium 400W Illumination, kilolux Lamps per square meter Distance from plants, meter Low Pressure Sodium 18OW Illumination, kilolux Lamps per square meter Distance from plants, meter Zncandescen t Incandescent lOOW Illumination, kilolux Lamps per square meter Distance from plants, meter Incandescent 150W Flood Illumination, kilolux Lamps per square meter Distance from plants, meter Incandescent-Hg 160W Illumination, kilolux Lamps per square meter Distance from plants, meter Sunlight Illumination, kilolux 4.3

4.0 6.0 0.41

2.6 2.8 0.6

2.7 4.5 0.47

11.0 0.70 1.2

7.1 0.39 1.6

7.0 0.63 1.3

8.9

8.3 12.0 0.28

5.5 5.8 0.4

5.6 9.4 0.33

23.0 1.46 0.83

15.0 0.82 1.1

15.0 1.3 0.87

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ever, always has been directed toward color rendering and the type of atmosphere created in the living spaces. Low wattage INC and FLUOR have been the lamps of preference. At this intensity the plants can be displayed (seen), but little or no significant impact on plants can be expected. Also, timing (light-dark durations) and temperature interaction would not be of concern.

Photoperiod: 0.9 W/mz.-Plant growth can be regulated a t an intensity of 0.9 W/m2 (Table 10.9). By tradition, this intensity has been tagged as the so-called “low light intensity” systems which are triggered by the photo-reversible blue pigment-phytochrome (Downs and Borthwick 1956; Downs et al. 1958; Deutch and Deutch 1978; Downs and Piringer 1958; Whalley and Cockshull 1976; Jose and Vince-Prue 1978). The range of plant responses (promote or delay flowering, promote growth) which can be regulated is extensive and is widely demonstrated and practiced by commercial growers (Nitsch 1957a,b; Perry 1971; Withrow 1958; Withrow and Richman 1933; Withrow and Withrow 1947). Cathey and Campbell (1975) reported the relative order of activity in regulating photoperiod responses as incandescent (INC) > high-pressure sodium (HPS) >> metal halide (MH) = cool white fluorescent (F)>> clear mercury (Hg) from the major types of sources tested. Later they found (unreported) that LPS was as effective as F in photoregulation of the daylength responses of plants. The effectiveness of any lighting system was increased by the use of reflective aluminum soil mulch (Cathey et al. 1975). 2.

3. Survival: 3.0 W/mz.-Plants can survive a t an intensity of 3.0 W/m2 (Table 10.9). By tradition, this intensity creates an environment where many green plants can maintain their green color. Stem lengthening and reduction of leaf size and thickness, however, occur almost immediately following placement of plants under this intensity. In time, the overall development of the plants falls behind that of other plants grown under higher intensities. Photoperiod responses do not function well a t this intensity since all plants lengthen and seldom develop green foliage. There are, however, strong interactions between this intensity and temperature, watering frequency, and nutrition. Cooler temperatures (less than 17°C)tend to help conserve the previously stored material while frequent watering and fertilization aggravate the stem lengthening and aging of the older foliage.

4. Maintenance: 9.0 W/mz.-Plants can maintain growth over many months when exposed to an intensity of 9 W/m2 (Table 10.9). By tradition, this is the intensity a t which many indoor gardeners (Boodley 1970; Dunn 1975) (professional or hobbyist) grow their plants when


525

starting them from seeds, cuttings, or meristems. It has become a convenient base and energy balance, particularly for those who use fluorescent lamps as a sole source for growing plants (Biran and Kofranek 1976; Boodley 1970). As anticipated, interactions with the environment (temperature, airflow, relative humidity, pollutants) may vary greatly from installation to installation. When the concentration of lamps is limited and air exchange is provided for, simple facilities to grow a wide range of plant species can be constructed (Stoutemyer and Close 1946). The rate of development, particularly as the plants grow in size, can be slow, compared to plants grown a t higher intensities (Cathey et al. 1978). During the development of the seedling and the rooting of the cutting, there appears to be little response to photoperiod. In fact, for most plants during the initial phases of development, continuous light (and heat) should be used to help compensate, in some part, for the limited irradiance. Most plant species develop deep green foliage and large leaves, and may accelerate the transfer of nutrients and stored materials from their older to their younger, rapidly developing leaves. The plants eventually begin to drop or lose an old leaf for every new leaf that develops. Adjustment of the lighting regime to a 12-hour light-12-hourday cycle, coupled with reduced frequencies of watering and fertilization, creates an environment where growth is slowed and few new leaves are formed while most older leaves are retained. Most container-grown foliage plants are now “acclimatized” for 4 to 16 weeks under an intensity of 9 W/m2 and are sold to the consumer (Fonteno and McWilliams 1978; Cathey et al. 1978). An “acclimatizated” plant can be readily identified by its slowed growth, few if any new leaves, deep green leaves which are broad and flat, and persistent leaves to the soil line. 5. Propagation: 18.0 W/mZ.-Plants can be propagated rapidly when exposed to an intensity of 18 W/m2 for a minimum of 6 to 8 hours daily (Table 10.9). By tradition, this is the intensity a t which many propagators attempt to shade their greenhouses with one or several layers of neutral filters (films on coverings, plastic or other fabrics) to restrict the entry of light (and heat) into the propagation area. At least 50% of the incident sunlight already has been lost by reflection of the covering (glass, plastic, polyethylene) and by absorption or interference of the framing and supports of the greenhouse. Cuttings rooted at this intensity maintain a growth rate similar to that of the cuttings attached to the stock plant. Stem length, branching, and leaf color, however, can be regulated by manipulating the temperature, moisture stress, and nutrients (Klueter and Krizek 1972). Most plants grown for their flowers and fruits can be brought to maturity, usually by increasing the daylength to 16 to 18 hours for flower initiation (or rapid growth) and then reducing

526


the daylength to 8 to 12 hours for development. The growth rate, however, is relatively slow (Krizek et al. 1968, 1972). For most prompt development (leaf number, number of branches, early initiation of flower initiation), the plants must be transferred to a lighting regime which is higher-24 to 50 W/m2. a. Greenhouse: 24.0 Wlm'.-Plants can be grown year-round in a greenhouse in which the natural light is supplemented with 24 W/m2 for 8 to 16 hours daily (Table 10.9). By tradition, this is the intensity, when coupled with the ambient sunlight (shaded by clouds, greenhouse structures, and lamp fixtures), which can stimulate many of the growth responses and rates which have been associated with growth chamber studies (Cathey and Campbell 1979; Duke et al. 1975). The photomorphogenetic activity of sunlight, even under dim light conditions of midwinter, is essential to regulate many unknown or yet-to-be detected growth responses. The supplementary intensity of 24 W/m2, from a wide range of artificial light sources, is sufficient to boost growth rates and create a growing environment for rapid growth and early flowering (Carpenter 1976; Carpenter and Beck 1973; White 1974; Carpenter and Anderson 1972). The different phototypes (short-, long-, and dayneutral) and growth systems (regulation of flowering and dormancy) exhibit a wide array of responses. Since the most widely grown species and cultivars are quantitative in their responses to daylength, supplemental lighting tends to lump the growth responses into one type of response-accelerated growth and early flowering (Austin and Edrich 1974). The plants grown in the greenhouse without the supplemental lighting grow much more slowly and flower much later than the lighted ones. Duration (in hours) and placement (day-night) are extremely critical (Downs et al. 1973). Supplemental lighting for 8 hours during the day (0800 to 1600) is nowhere near as effective as lighting a t night (2000 to 0400) (Cathey and Campbell 1979). Neither of these lighting regimes, however, is as effective as lighting for 16 hours from morning to midnight (0800 to 2400). Lighting of the short-day plants such as soybeans, chrysanthemum, and poinsettia is relatively inefficient because they can be lighted only during the 8- to 12-hour day, followed by the obligatory 12- to 16-hour daily dark period (Anderson and Carpenter 1974). Deciduous trees lighted with INC (at 0.9 W/m2) maintain vegetative growth over many months. On the other hand, deciduous trees lighted with HID lamps, regardless of spectral composition, go dormant or develop abnormally colored leaves. Species vary widely in their sensitivity to lighting with INC and HID lamps. Continuous lighting of most plants initially induces a paling of the foliage, then an abrupt loss of all visible pigments in the top-most leaves. The plants, however, do not die but survive many


527

weeks under such bleached conditions. The condition is corrected in part by giving the plants a t least 4 hours of dark each day, by increasing and/ or lowering the temperature 2" to 4"C, by maintaining high relative humidity, and by spraying the foliage with minor element solutions.

b. Growth Chamber: 50.0 Wlrn*.-Plants can be grown in growth chambers if the light intensity is a minimum of 50 W/m2 (Table 10.9). This intensity is approximately one-fourth of that recorded out-of-doors. This intensity can be used to simulate many growth conditions (daylength, temperature range, relative humidity, airflow, carbon dioxide concentrations), and has become the standard growth chamber (Krizek and Zimmerman 1973; Zimmerman et al. 1970; Krizek et al. 1968; Krizek 1972; ASHS Special Comm. Growth Chamber Environments 1977). There is no one source used to light these chambers (Frank and Barker 1976; Wilson et al. 1978). For convenience, cool white fluorescent lamps have been widely used for more than 30 years (Patterson et al. 1977; Tibbitts et al. 1976). More recently, HID lamps have been substituted for fluorescent lamps (Buck 1973; Roper and Thomas 1978). All require, for most consistent results, a barrier of glass or other material between the lamp and the plants and separate ventilating systems to help remove the heat which can build up rapidly in such enclosed spaces. Since water filters or airflow cannot completely remove IR (infrared), the chambers are difficult to standardize from different manufacturers (Downs and Bonaminio 1976). It often leads to confusing information on plant growth and flowering in relation to what is observed with plants grown in greenhouses and out-of-doors (Tibbitts et al. 1977). When the total irradiance is 50 (80) W/m2 and 10 to 20%of the total watt input is provided with INC lamps, we find that most kinds of plants can be grown successfully (Bailey et al. 1970; Tibbitts et al. 1976). We observe the typical plant forms and flowering and fruiting responses when the plants are subjected to daylength (8 to 24 hours), temperature (9" to 35"C), carbon dioxide (300 to 5000 ppm), relative humidity (20 to loo%), and airflow. Growing plants in chambers constructed to provide intensities greater than 50 W/m2 becomes progressively more difficult, and the uncontrolled aspects become too complex to solve (Wareing et al. 1968; Measures et al. 1973). VI. COMPARISON OF LIGHT SOURCES A. Incandescent Lamps The standard light source to regulate the photoperiod responses of plants is INC, providing equal red (660 nm) and far red (730 nm) (Lane et

528


al. 1965; Piringer 1962) (Table 10.10). We are unable to detect differences in the plant responses to the INC when its basic 120 volt-frosted covering is modified (Downs 1977). Similar photoperiodic effects are observed when the covering is changed from the traditionally frosted one to clear, ceramic-coated (yellow, bug, orange, red) and colored glass (red, ruby, blue). These changes alter the human vision aspects in the yellowgreen region, but have a slight effect on the red (600 nm)-far red (730 nm) regions or ratio. The lamps of rated lives of 750 hours, 2500 hours, and 8000 hours also are equally effective as a light interruption or given as cyclic lighting. Most photoperiod responses could be regulated with 0.9 W/m2 for 1 or 2 to a t most 4 hours given continuously or cyclic (1 to 30 minutes) during the middle of a 12- to 16-hour dark period. Other light sources (fluorescent or HID) were never as effective (intensity) or efficient (rapid cycling and long life) as INC lamps.

B. Fluorescent Lamps Extensive testing has been conducted to determine the relative effectiveness of fluorescent lamps which emit more red, blue, infrared, and ultraviolet radiation than the traditional cool white and warm white lamps (Pallas 1964). Although there are reports of exceptional performance of a specific plant under a special lamp (Corth et al. 1973), the prevailing conclusion is that total lumen output is a much better criterion for plant growth than any special spectral distribution in the visible range (Table 10.10). Evaluating the effectiveness of a new fluorescent lamp can become very complex (Cathey et al. 1978). Fluorescent tubes are extremely sensitive to interactions with the environment. The lamps are coated with phosfors which create the fluorescence. They vary greatly in their spectral output, shift, and life. The glass used to make the lamp can alter the light emitted to the plants (LaCroix et al. 1966). Overall cool white and warm white fluorescent lamps are anticipated to be the standard fluorescent light source (Biran and Kofranek 1976; Dunn and Went 1959; Helson 1963; Newton 1973; Cathey et al. 1978).

C. High Intensity Discharge Lamps The high intensity discharge lamps (HID) were generally ignored for plant lighting until the insides of mercury lamps were coated with phosfors. The efficiency of these lamps (color improved mercury) finally equalled or exceeded that of tubular fluorescent lamps (Swain 1964). They were soon superseded with lamps enriched with various metals. No single type of lamp was satisfactory for plant growth. Finally, the sodium lamps (HPS and LPS) with much greater efficiency (as measured by


529

lumens per watt) than the other types of HID lamps were made available for plant lighting (Buck 1973; Cathey and Campbell 1974). We have observed that many kinds of plants may be grown under HPS and LPS lamps as a sole source or as a supplement in greenhouses. When there was a special requirement for spectral composition for plant growth, the HPS lamps were more satisfactory than LPS lamps (Morgan and Cooke 1971). The abnormal growth characteristics observed in plants growing under LPS could be reduced by adding INC lamps and/or increasing the ambient temperature (Brown et al. 1979). HPS lamps apparently provided the required visible and infrared radiation to grow a wide range of plants. Even plants lighted with HPS benefit from the addition of INC. Again, the mixture of visible and IR more successfully simulates the action of sunlight .

VII. SUMMARY People always have been willing to test new light sources for growing plants, anticipating that different and accelerated cultural systems can be developed. This review presents the “no nonsense view” that most “human vision’’ light sources can be used for regulating or growing plants. We have described the anticipated growth-regulating performances of various artificial light sources. With the basic information on the use of sensors to measure irradiation, we have suggested conversion factors with which to convert numbers into various systems. We believe, however, that these measurements alone, without a detailed analysis of the variations without the lighted area, can lead to very confusing results. We urge workers to continue to present the physical measurements of distance and spacing employed for the different types of lamps. We also recommend utilizing W/mz in the region of 400 to 850 nm as the basis for 6 intensities for showing, maintaining, propagating, and growing plants. We believe that these six base-line intensities should serve as the beginning of the construction of any facilities for plant growth studies. In one table we have shown how to achieve these intensities with most of the basic types of lamps (Table 10.9). Coupled with the statements on how the lamps regulate plants (Table 10.71, we believe that research workers and growers can systematically decide which lamps would be the most energy-efficient system for a specific growing situation. We need to build lighting (plus energy) facilities to provide for plant growth. In Table 10.9 we have shown the design information to estimate the lighting required to achieve the various energy levels. For each type of lamp in each column is shown: (1)the equivalent illumination in kilolux (1000 lux); (2) the number of lamps per unit area; and (3) approximate distance from lamp fixture to plants. These are approximate values

CWF, WWF MH

available available available available

Pretransplanted Seedlings and Cuttings

Interior Survival

CWF, WWF MH LPS INC

CWF, WWF MH HPS LPS

none (and/or available) none none none

none (and/or available) none none available daylight

CWF, WWF MH HPS LPS

available available available available

Daylength and photosynthesis supplement (propagation)

Pre-interior Preparation

INC

available

Transplanted Seedlings and Cuttings Daylength only

LPS

HPS

Artificial Light CWF, WWF

Natural Light available

Growth Stage Seed Germination

TABLE 10.10. LIGHTING BY STAGE OF GROWTH

45 40 65 25

280 245 245 385

560 490 490 770

20

280 245 245 385

1.5

9

18

20

9

Intensity fc W/m2 140 4.5

-

0800-1600 (0800-1600 or any 8-hour period)

0600- 1800 (0600-1800 or any 12-hour period)

2000-0400 2000-0400 2000-0400 2000-0400

2 m i d 1 0 min)

2000-0400

( 12 sec/min or

0800-2400 0800-2400 0800-2400 0800-2400

Duration (hours) 0000-2400

cn

8

z

0

X

0

w

INC HPS

available

available

HPS

available

25

10

25

10

1565 1390 2175

710

560 490 490

280 245 245 385 150

0.9

0.9

0.9

0.9

50

18

9

Source: Radiation data developed or revised by R.W. Thimijan, USDA-SEA-AR, Beltsville, Md. Note: Lamp Code CFW = Cool White Fluorescent WWF = Warm White Fluorescent M H = MetalHalide HPS = High Pressure Sodium LPS = Low Pressure Sodium INC = Incandescent

Daylength and photosynthesis supplement (cover soil with reflecting aluminum coated DaDer)

INC

CWF,WWF HPS LPS

none none none

Growth Chamber

available

CWF,WWF MH HPS LPS

none none none none

Propagation

Field Daylength only

CWF,WWF MH HPS LPS -

none (and/or available) none none none available

Maintenance

2000-0400 (12 sec/min or 2 m i d 1 0 min) 2000-0400

2000-0400 (12 sec/min or 2 m i d 1 0 min) 2000-0400

Any period required

0600-1800 (0600-1800or any 12-hour period)

-

0600-1800 (0600-1800 or any 12-hour period)

532


based on multiple lamp installations of four or more lamps. For a single lamp or fixture, the kilolux and distance from plants will be reduced by one-third to one-half. This table is intended as a planning guide only. Actual installations should be planned using photometric data from the fixture (luminair) manufacturer. VIII. LITERATURE CITED ALDRICH, R.A. and J.W. WHITE. 1969. Solar Radiation and Plant Growth in Greenhouses. Trans. Amer. SOC. Agr. Eng. 12:l. ANDERSON, G.A. and W.J. CARPENTER. 1974. High intensity supplementary lighting of chrysanthemum stock plants. HortScience 9:58-60. ANON. International lighting vocabulary, 1970, 3rd edition. Commission International de l'Eclairage, Paris. ASHS SPECIAL COMM. GROWTH CHAMBER ENVIRONMENTS. 1977. Revised guidelines for reporting studies in controlled environment chambers. HortScience 12:309-310. AUSTIN, R.B. and J.A. EDRICH. 1974. A comparison of six sources of supplementary light for growing cereals in glasshouses during winter time. J.Agr. Eng. Res. 19:339-345. BAILEY, L.H. 1893. Greenhouse notes: Third report upon electrohorticulture. Cornell Uniu. Agr. Expt. Sta. Bul. 55:127-238. BAILEY, W.A., H.H. KLUETER, D.T. KRIZEK, and N.W. STUART. 1970. COz systems for growing plants. Trans. Amer. SOC. Agr. Eng. 13(3):263-268. BALEGH, S.E. and 0. BIDDULPH. 1970. The photosynthetic action spectrum of the bean plant. Plant Physiol. 46:l-5. BICKFORD, E.D. and S. DUNN. 1972. Lighting for plant growth. Kent State Univ. Press, Kent, Ohio. BIGGS, W.W. and M.C. HANSEN. 1979. Instrument letter on radiation measurements. LI-COR, Lincoln, Neb. BIRAN, I. and A.M. KOFRANEK. 1976. Evaluation of fluorescent lamps as an energy source for plant growth. J. Amer. SOC. Hort. Sci. 101(6):625-628. BOODLEY, J.W. 1970. Artificial light sources for gloxinia, african violet, and tuberous begonia. Plants & Gardens 26:38-42. BROWN, J.C., H.M. CATHEY, J.H. BENNETT, and R.W. THIMIJAN. 1979. Effect of light quality and temperature on Fe3+ reduction, and chlorophyll concentration in plants. Agron. J. 71:1015-1021. BROWN, J.C., C.D. FOY, J.H. BENNETT, and M.N. CHRISTIANSEN. 1978. Two light sources (LPS & FCW) differentially affected Fe3+ and growth of cotton. Plant Phvsiol. 63:692-695. BUCK, J.A. 1973. High intensity discharge lamps for plant growth application. Trans. Amer. SOC. Agr. Eng. 16(1):121-123.


533

CAMPBELL, L.E., R.W. THIMIJAN, and H.M. CATHEY. 1975. Spectral radiant power of lamps used in horticulture. Trans. Amer. SOC. Agr. Eng. 18(5):952-956. CANHAM, A.E. 1974. Some recent developments in artificial lighting for protected crops. Proc. XIX Intern. Hort. Congr., Warsaw. Sept. 11-18, 1974. p. 267-276. CARLSON, G.E., G.A. MOTTER, JR., and V.C. SPRAGUE. 1964. Uniformity of light distribution and plant growth in controlled environment chambers. Agron. J. 56 2 4 2 -243. CARPENTER, W.J. 1976. Photosynthetic supplementary lighting of spray pompom, Chrysanthemum morifolium. Ramat. J. Amer. SOC. Hort. Sci. 101: 155-158. CARPENTER, W.J. and G.A. ANDERSON. 1972. High intensity supplementary lighting increases yields of greenhouse roses. J. Amer. SOC. Hort. Sci. 97:331-334. CARPENTER, W.J. and G.R. BECK. 1973. High intensity of supplementary lighting of bedding plants after transplanting. HortScience 8(6):482-483. CATHEY, H.M. and L.E. CAMPBELL. 1974. Lamps and lighting-a horticultural view. Lighting Design & Application 4(11):41-51. CATHEY, H.M. and L.E. CAMPBELL. 1975. Effectiveness of five visionlighting sources on photo-regulation of 22 species of ornamental plants. J. Amer. SOC. Hort. Sci. 100(1):65-71. CATHEY, H.M. and L.E. CAMPBELL. 1977. Plant productivity: new approaches to efficient sources and environmental control. Trans. Amer. SOC. Agr. Eng. 20(2):360-371. CATHEY, H.M. and L.E. CAMPBELL. 1979. Relative efficiency of high- and low-pressure sodium and incandescent filament lamps used to supplement natural winter light in greenhouses. J. Amer. SOC. Hort. Sci. 104:812-825. CATHEY, H.M., L.E. CAMPBELL, and R.W. THIMIJAN. 1978. Comparative development of 11 plants grown under various fluorescent lamps and different duration of irradiation with and without additional incandescent lighting. J. Amer. SOC. Hort. Sci. 103:781-791. CATHEY, H.M., G.G. SMITH, L.E. CAMPBELL, J.G. HARTSOCK, and J.U. MCGUIRE. 1975. Response of Acer rubrum L. to supplemental lighting reflective aluminum soil mulch, and systemic soil insecticide. J. Amer. SOC. Hort. Sci. 100:234-237. CORTH, K., G.M. JIVIDEN, and R.J. DOWNS. 1973. New fluorescent lamp growth applications. J. Illurn. Eng. SOC. 9(2):139-142. DE BOER, J.B. 1974. Modern light sources for highways. J. Zllum. Eng. SOC. 3(4):142-152. DELINT, P.J.A.L. and D. KLAPWIJK. 1973. Observations on growth and development of tomato seedlings. Acta Hort. 32:161-172. DEUTCH, B. and B.I. DEUTCH. 1978. Spectral dependence of a single and a subsequent second light pulse inducing barley leaf unfolding. Photochem. & Pho tobiol. 27 :14 1- 146.

534


DOWNS, R.J. 1977. Incandescent lamp maintenance in plant growth chambers. HortScience 12:330-332. DOWNS, R.J. and V.P. BONAMINIO. 1976. Phytotron procedural manual for controlled-environment research a t the Southeastern Plant Environment Laboratory. North Carolina Agr. Expt. Sta. Tech. Bul. 244. DOWNS, R.J. and H.A. BORTHWICK. 1956. Effects of photoperiod on growth of trees. Bot. Gaz. 117:310-326. DOWNS, R.J., H.A. BORTHWICK, and A.A. PIRINGER, JR. 1958. Comparison of incandescent and fluorescent lamps for lengthening photoperiods. Proc. Amer. SOC. Hort. Sci. 71:568-578. DOWNS, R.J. and A.A. PIRINGER, J R . 1958. Effects of photoperiod and kind of supplemental light on vegetative growth of pines. For. Sci. 4(3):185-195. DOWNS, R.J., W.T. SMITH, and G.M. JIVIDEN. 1973. Effect of light quality during the high-intensity period of growth of plants. ASAE Pap. 73-4525. ASAE, St. Joseph, Mich. DUKE, W.B. et al. 1975. Metal halide lamps for supplemental lighting in greenhouses. Crop response and spectral distribution. Agron. J. 67:49-63. DUNN, S. 1975. Lighting for plant growth or maintenance. Flor. Rev. 156 (4054):41, 86-90. DUNN, S. and F.W. WENT. 1959. Influence of fluorescent light quality on growth and photosynthesis of tomato. Lloydia 22:302-324. ENOCH, H.Z., V. ZIESLIN, Y. BIRAN, A.H. HALEVY, M. SCHWARZ, B. KESLER, and D. SHIMSI. 1973. Principles of COz nutrition research. Acta Hort. 32:97-117. EVANS, L.T. 1963. Extrapolation from controlled environments to the field. p. 421-437. I n L.T. Evans (ed.) Environmental control of plant growth. Academic Press, New York. FONTENO, W.C. and E.L. MCWILLIAMS. 1978. Light compensation points and acclimatization of four tropical foliage plants. J. Amer. SOC.Hort. Sci. 103:52-56. FRANK, A.B. and R.E. BARKER. 1976. Rates of photosynthesis and transpiration and diffusive resistance of six grasses grown under controlled conditions. Agron. J. 68:487-490. FRANKLAND, B. and R.J. LETENDRE. 1978. Phytochrome and effects of shading on growth of woodland plants. Photochem. & Photobiol. 27:223-230. GAASTRA, P. 1959. Photosynthesis of crop plants as influenced by light, carbon dioxide, temperature, and stomata1 diffusion resistance. Meded v.d. LBHS to Wageningen 59(13):11. GARNER, W.W. and H.A. ALLARD. 1920. Flowering and fruiting of plants a s controlled by the length of day. USDA Yearb. 1920, U.S. Dept. Agr., Washington, D.C. p. 377-400. GOVINDGEE, R. 1974. The absorption of light in photosynthesis. Sci. Amer. 231(6):68-80, 82.


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the action spectrum for photosynthesis. Plant Physiol. 49:704-706. MEASURES, M., P. WEINBERGER, and H. BAER. 1973. Variability of plant growth within controlled-environment chambers as related to temperature and light distribution. Can. J. Plant Sci. 53:215-220. MEIJER, G. 1971. Some aspects of plant irradiation. Acta Hort. 22:103-105. MENZ, K.M., D.N. MOSS, R.Q. CANNELL,and W.A. BRUN. 1969. Screening for photosynthetic efficiency. Crop Sci. 9:692-694. MORGAN, S.F. and J.I. COOKE. 1971. Supplementary light sources for greenhouse crops. 1-lettuce. Rpt. Ref. ECRC/R392. Elec. Counc., London. MORGAN, D.C. and H. SMITH. 1976. Linear relationship between phytochrome photsequilibrium and growth in plants under simulated natural radiation. Nature 262:210-212. MORRIS, L.G. 1973. Enclosures for research on control of aerial environment for protected crops. Acta Hort. 32:73-88. NEWTON, P. 1973. Growing rooms. Symposium on greenhouse climate: Evaluation of research methods. Acta Hort. 32:89-95. NICODEMUS, F.E. (ed.) 1978. Self-study manual of optical radiation measurements part 1. Tech. Notes 910-2 and 910-4, National Bureau of Standards, U S . Dept. Commerce, Washington, D.C. NITSCH, J.P. 1957a. Growth responses of woody plants to photoperiodic stimuli. Proc. Amer. Soc. Hort. Sci. 70:512-525. NITSCH, J.P. 1957b. Photoperiodism in woody plants. Proc. Amer. SOC. Hort. Sci. 70:526-544. PALLAS, J.E. 1964. Comparative bean and tomato growth and fruiting under two fluorescent lamp sources. Bioscience 14:44-45. PATTERSON, D.T., M.M. PEET, and J.A. BUNCE. 1977. Effect of photoperiod and size at flowering on vegetative growth and seed yield of soybean. Agron. J. 69:631-635. PERRY, T.O. 1971. Dormancy of trees in winter. Science 171(3966):29-36. PIRINGER, A.A. 1962. Photoperiodic response of vegetable plants. p. 173185. I n Proc. Plant Sci. Symp., Mar. 5-10, 1962. Campbell Soup Company, Camden, N.J. ROPER, C.D., JR. and J.F. THOMAS. 1978. Photoperiodic alteration of dry matter partitioning and seed yield in soybeans. Crop Sci. 18:654-656. SHIBLES, R. 1976. Committee report terminology pertaining to photosynthesis. Crop Sci. 16:437-439. SINGH, M., W.L. OGREN, and J.M. WIDHOLM. 1974. Photosynthetic characteristics of several Cs and C4 plant species grown under different light intensities. Crop Sci. 14:563-566. STOUTEMYER, V.T. and A.W. CLOSE. 1946. Rooting cuttings and germinating seeds under fluorescent and cold cathode lighting. Proc. Amer. SOC. Hort. Sci. 48:309-315. SWAIN, G.S. 1964. The effect of supplementary illumination by mercury va-


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Index (Volume 2)

Adzuki bean,genetics, 373 Aluminum, deficiency and toxicity symptoms in fruits and nuts, 154 Apple, and light, 240-248 replant disease, 3 root distribution, 453-456 Arsenic, deficiency and toxicity symptoms in fruits and nuts, 154 Asexual embryogenesis, 268- 3 10

Bacteria, and tree short life, 46-47 Boron, deficiency and toxicity symptoms in fruits and nuts, 151-152

Calcium, deficiency and toxicity symptoms in fruits and nuts, 148-149 Chlorine, deficiency and toxicity symptoms in fruits and nuts, 153 Cold hardiness, 33-34 injury, 26-27 Controlled-atmosphere storage, seeds, 134135 Copper, deficiency and toxicity symptoms in fruits and nuts, 153 Cowpea, genetics, 317-348

Deficiency symptoms, in fruit and nut crops, 145-154 Disease, in lettuce, 187-197 Dormancy, 27-30

Embryogenesis, 268- 3 10

Fertilizer, in lettuce, 175-176 nitrogen, 401-404 Fruit crops, nutritional ranges, 143-164 roots, 453-457 short life and replant problem, 1-116 Fungi, and tree short life problem, 47-49

Genetics and breeding, in lettuce, 185-187 and tree short life, 66-70 nitrogen nutrition, 410-411 of Vigna, 311-394 Germination, seed, 117-141, 173-174 Growth substances, 60-66 in embryogenesis, 277-281

Harvesting, in lettuce, 176-181

In uitro embryogenesis, 268-310

propagation, 268-310 Insects, in lettuce, 197-198 short life problem, 52 Iron, deficiency and toxicity symptoms in fruits and nuts, 150 Irrigation, in lettuce industry, 175 root growth, 464-465

Lamps, for plant growth, 514-531 Lettuce, industry, 164-207 Light, and nitrogen nutrition, 406-407 for plant growth, 491-537 in orchards, 208-267 Lighting, for plant growth, 491-537

539

540


Magnesium, deficiency and toxicity symptoms in fruits and nuts, 148 Manganese, deficiency and toxicity symptoms in fruits and nuts, 150-151 Metabolism, seed, 117-141 Moisture, and seed storage, 125-132 Moth bean, genetics, 373-374 Mung bean, genetics, 348-364 Mycoplasma-like organisms, short life problem, 50-51

Nematodes, in lettuce, 197-198 short life problem, 49-50 Nitrogen, deficiency and toxicity symptoms in fruits and nuts, 146 in embryogenesis, 273-275 nutrition of horticultural crops, 395-423 Nut crops, nutritional ranges, 143-164 Nutrient, concentration in fruit and nut crops, 154-162 media in embryogenesis, 273-281 Nutrition, fruit and nut crops, 143-164

Orchards systems, and light, 208-267 and root growth, 469-470

Peach, short life, 4 Pear, decline, 11 root distribution, 456 short life, 6 Phosphorus, deficiency and toxicity symptoms in fruits and nuts, 146-147 Photosynthesis, and light, 237-238 Physiology, of embryogenesis, 268-310 of seed, 117-141 Phytotoxins, 53-56 Plant protection, short life, 79-84 Postharvest physiology, in lettuce, 181-185 seed, 117-141 Potassium, deficiency and toxicity symptoms in fruits and nuts, 147-148 Pruning, and light interception, 250-251 Prunus, root distribution, 456

Replant problem, deciduous fruit trees, 1116 Rice bean, genetics, 375-376 Roots, and tree crops, 424-490 Rootstock, and light interception, 249-250 and root systems, 471-474 and short life, 70-75

Seed, research in lettuce, 166-174 viability and storage, 117-141 Short life problem, fruit crops, 1-116 Sodium, deficiency and toxicity symptoms in fruits and nuts, 153-154 Soil management, and root growth, 465-469 Storage, of seed, 117-141 Stress on plants, 34-37 Sulfur, deficiency and toxicity symptoms in fruits and nuts, 154 Symptoms, deficiency and toxicity of fruits and nuts, 145-154

Temperature, plant growth, 36-37 and seed storage, 132-133 Tissue culture, 268-310 Toxicity symptoms, in fruit and nut crops, 145-154 Tree crops, roots, 424-490 Tree decline, 1-116

Urd bean, genetics, 364-373

Vigna, genetics, 31 1-394 Viruses, short life problem, 50-51

Water, and light in orchards, 248-249 Weed in lettuce, lg8

Zinc, deficiency and toxicity symptoms in fruits and nuts, 151


Cumulative Index (Volumes 1-2 Inclusive)

Abscission, anatomy and histochemistry, 1: 172-203 Adzuki bean, genetics, 2:373 Alternate bearing, chemical thinning, 1: 285-289 Aluminum, deficiency and toxicity symptoms in fruits and nuts. 2:154 Anatomy and morphology, embryogenesis, 1:4-21, 35-40 fruit abscission, 1:172-203 fruit storage, 1:314 petal senescence, 1:212-216 Angiosperms, embryogenesis in, 1:l-78 Apple, CA storage, 1:303-306 chemical thinning, 1:270-300 fertilization, 1:105 fire blight control, 1:423-474 light, 2:240-248 replant disease, 2:3 root distribution, 2:453-456 yield, 1:397-424 Apricot, CA storage, 1:309 Arsenic, deficiency and toxicity symptoms in fruits and nuts, 2:154 Artichoke, CA storage, 1:349-350 Asexual embryogenesis, 1:1-78; 2:268-310 Asparagus, CA storage, 1:350-351 Avocado, CA storage, 1:310-311

Bacteria, short life problem, 2:46-47 Bacteriocides, fire blight, 1:450-459 Bacteriophage, fire blight control, 1:449450 Banana, CA storage, 1:311-312 fertilization, 1:105 Bean, CA storage, 1:352-353 Bedding plants, fertilization, 1:99-100 Beet, CA storage, 1:353 Begonia (Rieger), fertilization, 1:104 Boron, deficiency and toxicity symptoms in fruits and nuts, 2:151-152

Broccoli, CA storage, 1:354-355 Brussels sprouts, CA storage, 1:355

Cabbage, CA storage, 1:355-359 fertilization, 1:117-118 Calcium, deficiency and toxicity symptoms in fruits and nuts, 2:148-149 Carnation, fertilization, 1:100 Carrot, CA storage, 1:362-366 Cauliflower, CA storage, 1:359-362 Celeriac, CA storage, 1:366-367 Celery, CA storage, 1:366-367 Cherry, CA storage, 1:308 Chicory, CA storage, 1:379 Chlorine, deficiency and toxicity symptoms in fruits and nuts, 2:153 Chrysanthemum fertilization, 1:100-101 Citrus, CA storage, 1:312-313 fertilization, 1:105 rootstock, 1:237-269 Cold hardiness, 2:33-34 injury, 2:26-27 Controlled-atmosphere storage, fruits, 1: 301-336 seeds, 2:134-135 vegetables, 1:337-394 Copper, deficiency and toxicity symptoms in fruits and nuts, 2:153 Cowpea, genetics, 2:317-348 Cranberry, fertilization, 1:106 Cucumber, CA storage, 1:367-368

Deficiency symptoms, in fruit and nut crops, 2:145-154 Delicious apple, 1:397-424 Disease, in lettuce, 2:187-197 Dormancy, 2:27-30

541

542


Embryogenesis, 1:l-78; 2:268-310 Energy, efficiency in controlled environment agriculture, 1:141- 171 Environment, controlled for energy efficiency, 1:141-171 fruit set, 1:411-412 in embryogenesis, 1:22, 43-44 Erwinia amylouora, 1:423-474 Ethylene, CA storage, 1:317-319, 348

Fertilizer, controlled-release, 1:79-139 in lettuce, 2:175-176 nitrogen, 2:401-404 Fire blight, 1:423-474 Floricultural crops, fertilization, 1:98-104 postharvest physiology, 1:204-236 senescence, 1:204-236 Flower, senescence, 1:204-236 Foliage plants, fertilization, 1:102-103 Frost, and apple fruit set, 1:407-408 Fruit, abscission, 1:172-203 set (apple), 1:397-424 size and thinning, 1:293-294 Fruit crop fertilization, 1:104-106 nutritional ranges, 2: 143-164 roots, 2:453-457 short life and replant problem, 2:l-116 Fungi, short life problem, 2:47-49 Fungicide, and apple fruit set, 1:416

In uitro embryogenesis, 1:l-78; 2:268-310

Insects, in lettuce, 2:197-198 short life problem, 2:52 Iron, deficiency and toxicity symptoms in fruits and nuts, 2:150 Irrigation, in lettuce industry, 2:175 root growth, 2:464-465

Lamps, for plant growth, 2:514-531 Leek, CA storage, 1:375 fertilization, 1:118 Lemon, rootstock, 1:244-246 Lettuce, CA storage, 1:369-371 fertilization, 1:118 industry, 2:164-207 Light, and fruit set, 1:412-413 and nitrogen nutrition, 2:406-407 for plant growth, 2:491-537 in orchards, 2:208-267

Magnesium, deficiency and toxicity symptoms in fruits and nuts, 2:148 Mandarin, rootstock, 1:250-252 Manganese, deficiency and toxicity symptoms in fruits and nuts, 2:150-151 Mango, CA storage, 1:313 Metabolism, flower, 1:219-223 seed, 2:117-141 Moisture, and seed storage, 2:125-132 Moth bean, genetics, 2:373-374 Mung bean, genetics, 2:348-364 Mushroom, CA storage, 1:371-372 Muskmelon, fertilization, 1:118-119 Mycoplasma-like organisms, short life problem. 2:50-51

Garlic, CA storage, 1:375 Genetics and breeding, and embryogenesis, 1:23 and short life, 2:66-70 fire blight resistance, 1:435-436 flower longevity, 1:208-209 in lettuce, 2:185-187 nitrogen nutrition, 2:410-411 Nectarine, CA storage, 1:309-310 of Vigna, 2:311-394 Nematodes, in lettuce, 2:197-198 Germination, seed, 2:117-141, 173-174 short life problem, 2:49-50 Grape, CA storage, 1:308 Nitrogen, deficiency and toxicity symptoms Greenhouse, energy efficiency, 1:141-171 in fruits and nuts, 2:146 Growth substances, 2:60-66 in embryogenesis, 2:273-275 apple fruit set, 1:417 nutrition of horticultural crops, 2:395-423 apple thinning, 1:270-300 Nursery crop, fertilization, 1:106-112 CA storage in vegetables, 1:346-348 Nut crops, fertilization, 1:106 in embryogenesis, 1:41-43; 2:277-281 nutritional ranges, 2:143-164 Nutrient, concentration in fruit and nut crops, 2:154-162 media, in embryogenesis, 2:273-281 Nutrition, and embryogenesis, 1:40-41 Harvesting, flower stage, 1:211-212 and fire blight, 1:438-441 in lettuce, 2:176-181 and fruit set, 1:414-415 Histochemistry, fruit abscission, 1:172-203 fruit and nut crops, 2:143-164 Horseradish, CA storage, 1:368 slow-release fertilizers, 1:79-139

CUMULATIVE INDEX (VOLUMES 1-2 INCLUSIVE) Okra, CA storage, 1:372-373 Onion, CA storage, 1:373-375 Orange, sour, rootstock, 1:242-244 sweet, rootstock, 1:252-253 trifoliate, rootstock, 1:247-250 Orchard systems, and light, 2:208-267 and root growth, 2:469-470 Ornamental plants, fertilization, 1:98-104, 106-116

Papaya, CA storage, 1:314 Parsley, CA storage, 1:375 Peach, CA storage, 1:309-310 short life, 2:4 Pear, CA storage, 1:306-308 decline, 2: 11 fire blight control, 1:423-474 root distribution. 2:456 short life, 2:6 Pecan, fertilization, 1:106 Pepper, CA storage, 1:375-376 fertilization, 1:119 Persimmon, CA storage, 1:314 Pest control, fire blight, 1:423-474 Pesticide, and fire blight, 1:450-461 Phosphorus, deficiency and toxicity symptoms in fruits and nuts, 2:146-147 Photosynthesis, and light, 2:237-238 Physiology, cut flower, 1:204-236 of embryogenesis, 1:21-23; 2:268-310 of seed, 2:117-141 Phytotoxins, 2:53-56 Pigmentation, flower, 1:2 16-219 Pineapple, CA storage, 1:314 Plant protection, short life, 2:79-84 Plum, CA storage, 1:309 Poinsettia, fertilization, 1:103-104 Pollination, and embryogenesis, 1:21-22 apple, 1:402-404 Postharvest physiology, cut flower, 1:204236 fruit, 1:301-336 in lettuce, 2:181-185 seed, 2:117-141 vegetables, 1:337-394 Potassium, deficiency and toxicity symptoms in fruits and nuts, 2:147-148 Potato, CA storage, 1:376-378 fertilization, 1:120-121 Pruning, and light interception, 2:250-251 and training, on apple yield, 1:414 on fire blight, 1:441-442 Prunus, root distribution, 2:456

Radish, fertilization, 1:121 Replant problem, deciduous fruit trees, 2: 1-116

543

Respiration, fruit in CA storage, 1:315-316 vegetables in CA storage, 1:341-346 Rice bean, genetics, 2:375-376 Roots, and tree crops, 2:424-490 Rootstock, and fire blight, 1:432-435 and light interception, 2:249-250 and root systems, 2:471-474 and short life, 2:70-75 apple, 1:405-407 citrus, 1:237-269 Rose, fertilization, 1:104

Scoring, and fruit set, 1:416-417 Seed, abortion, 1:293-294 research in lettuce, 2:166-174 viability and storage, 2:117-141 Senescence, cut flower, 1:204-236 petal, 1:212-216 Short life problem, fruit crops, 2:l-116 Small fruit, CA storage, 1:308 Sodium, deficiency and toxicity symptoms in fruits and nuts, 2:153-154 Soil management, and root growth, 2:465469 Storage, of seed, 2:117-141 Strawberry, fertilization, 1:106 Stress on plants, 2:34-37 Sulfur, deficiency and toxicity symptoms in fruits and nuts, 2:154 Sweet potato, fertilization, 1:121 Symptoms, deficiency and toxicity of fruits and nuts, 2:145-154

Temperature, and apple fruit set, 1:408411 and CA storage of vegetables, 1:340-341 and fire blight forecasting, 1:456-459 and seed storage, 2:132-133 plant growth, 2:36-37 Thinning, apple, 1:270-300 Tissue culture, 1:l-78; 2:268-310 Tomato, CA storage, 1:380-386 fertilization, 1:121-123 Toxicity symptoms, in fruit and nut crops, 2:145-154 Tree crops, roots, 2:424-488 Tree decline, 2:l-116 Turfgrass, fertilization, 1:112-117 Turnip, fertilization, 1:123-124

Urd bean, genetics, 2:364-373

544


Vegetable crops, CA storage, 1:337-394 fertilization, 1:117-124 Vigna, genetics, 2:311-394 Viruses, short life problems, 2:50-51

Water, and light in orchards, 2:248-249

Watermelon, fertilization, 1:124 Weed research, in lettuce, 2:198

Zinc, deficiency and toxicity symptoms in fruits and nuts, 2:151

Horticultural Reviews Volume 2

Volume 13, Horticultural Reviews

Horticultural Reviews, Volume 31