. I I _____ W: 25¢ per m w m- ‘ > RETURNIE LIQWY MATERIALS: -. 33>;- , Place in bookn tore-we * 4355'” ‘ . charge from circuht‘lon "cords GROWTH, PHOTOSYNTHESIS AND TRANSLOCATION OF 14c- PHOTOSYNTHATES BY DEFOLIATED POPULUS PLANTS By John Harold Bassman A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Forestry 1980 ABSTRACT GROWTH, PHOTOSYNTHESIS AND TRANSLOCATION OF 14C-PHOTOSYNTHATES BY DEFOLIATED POPULUS PLANTS By John Harold Bassman Several field and controlled environment studies were performed to quantify the effects of partial defoliation on growth and physiology of young hybrid pOplars. Two, three- year field experiments were conducted in northern Wisconsin. The first experiment examined growth impact in relation to severity, timing and recurrence of defoliation. The combi- nation of level of first year defoliation (0%, 40%, or 80%), timing of the first year defoliation (July or August), and level of second year defoliation were important factors af- fecting growth. Results indicated no significant loss in height or diameter growth for trees receiving 40% defoliation. The most severe reduction in growth was obtained with trees receiving an 80% defoliation in August of the first year followed by an 80% defoliation in the second year. Timing of the second year defoliation was unimportant. Trees dev foliated to 80% in July of the first year displayed no sig- nificant reduction in growth regardless of the timing of John Harold Bassman second year defoliation In the second experiment, five different methods of in- flicting 75% defoliation were evaluated over three different clones. Results indicated no differences in growth based on the mechanism of defoliation. Several controlled environment studies tested the ef- ercts of a single defoliation on growth, photosynthesis and translocation of l4C-photosynthates. Plants were defoliated at PI 25 or 30 by removing all of the lamina except a 2 mm strip along each side of the midvein. In the first experi- ment, height and diameter growth was monitored in relation to five levels of defoliation plus an undefoliated control. When harvested at PI 45, defoliated plants were taller than control plants, with the heaviest defoliation producing the tallest plants. Defoliated plants also tended to have great- er stem diameters and produced more leaves and lateral branch- es than control plants. In a second eXperiment, the entire develOping leaf zone was defoliated at PI 25; plants were then harvested at PI's 29, 35, and 50. Insignificant differences in height and dia- meter growth were observed between control and defoliated plants, but the latter displayed reductions in dry weight. John Harold Bassman Defoliation stimulated a two-fold increase in lateral branch development. Leaves deveIOping subsequent to defoliation were 30% larger than complementary leaves on control plants and they showed significantly higher rates of photosynthesis. There was also a stimulation of photosynthetic rates on mature leaves remaining on the plant below the defoliated zone within 24 hr of treatment. This effect continued up to five weeks. Leaf conductance was also higher on defoli- ated plants and followed a pattern similar to photosynthesis. Translocation patterns were also altered within 24 hr after defoliation. When leaves below or the remaining tis— sue of leaves within the zone of defoliation were exposed to 14C02, more 14C-photosynthate was transported to the expand- ing shoot and lateral branches and less to the roots in de- foliated plants than in controls. Little difference between defoliated and control plants in 14C distribution occurred when leaves produced subsequent to defoliation were exposed to 14C02. After five weeks, differences in patterns of 14C distribution between defoliated and control plants were re- duced. These results substantiated biomass partitioning data. This work is dedicated to all of the wives, relatives and friends who have earned their P.h. T.'s without whose help dissertaitions and graduate education would be impossible. ii ACKNOWLEDGMENTS The author would like to express sincere thanks and appreciation to Dr. Donald 1. Dickmann for his patience, guidance and support during the course of my graduate study; and for recognition and appreciation of a good story when he heard one. Sincere thanks is also extended to Dr.'s James Hanover, Melvin Koelling and Gary Simmons for their sugges- tions and critical review of the manuscript and for serving on my graduate committee. I am very grateful to my mother, Mildred Bassman, for typing the manuscript. Finally, much thanks, gratitude and love to my wife, Nancy, for her patience, support and sacrifice in making this work possible. iii TABLE OF CONTENTS Page LIST OF TABLES . . . . . . . . . . . . . . . . . . vi LIST OF FIGURES . . . . . . . . . . . . . . . . . . x CHAPTER I. Introduction and Reviewing of Literature 1 Introduction . . . . . . . . . . . 1 Major Defoliators of Populus . . . . . . . . . . 4 Cottonwood Leaf Beetle . . . . . . . . 4 Poplar Tent Maker . . . . . . . . . . . . . . 8 Forest Tent Caterpillar . . . . . . . . . . . 10 Fall Webworm . . . . . . . . . . . 12 Night Feeding Leaf Beetles . . . . . . . . . 12 Leaf Curl Midge . . . . . . . . . . . . . . . 13 Effects of Defoliation on Growth and Physiology of Trees . . . . 14 Effects of Defoliation on Height, Diameter and Volume Growth . . . . . . . 14 Effects of Defoliation on Leaf Area Development . . . . . . . . . . . . . 21 Effects of Defoliation on Dry Weights . . . . 22 Effects of Defoliation on Root Growth . . . . 23 Physiological Effects of Defoliation . . . . 27 Effects on Photosynthesis and Stomatal Conductance . . . . . . 27 Effects of Defoliation on Translocation . 30 Effects of Defoliation on Growth Regulators . . . . . . . . . . . . . . 31 Effect of Defoliation on Nutrition . . . . . . . 32 iv Tree Resistance f0 Insect Attack CHAPTER II. The Effects of Artificial Defoliation on Growth of Hybrid Poplars in Northern Wisconsin . . . . . . . . . . . . . . . Introduction . Materials and Methods Results Discussion . . . . . CHAPTER III. Effects of Defoliation in the Developing Leaf Zone on Growth and Dry Weight Partitioning in Young Populus x Euramericana Plants- Introduction . Materials and Methods Results Discussion CHAPTER IV. Photosynthesis and Leaf Conductance of Populus x Euramericana After Defoliation in the Developing Leaf Zone Introduction . Materials and Methods Results Discussion 14 CHAPTER V. Distribution of C-Photosynthate in Populus x Euramericana After Defoliation in the Developing Leaf Zone Introduction Materials and Methods Results Discussion CHAPTER VI. Summary and Conclusions BIBLIOGRAPHY Page 34 36 36 37 4O 47 51 51 53 S8 87 94 94 96 102 112 119 119 120 125 140 147 155 LIST OF TABLES Page Effects of combination of 0% defoliation in the first year (1975) with various levels of defoliation in the second year (1976), over all levels of timing, on height and diameter (30 cm above ground) of poplar plants. Values are for August measurements. Effect of combinations of 40% defoliation in the first year (1975), timing of the first year defoliation (July or August), and various levels of defoliation in the second year (1976) on height and diameter (30 cm above ground) of poplar plants. Values are for August measurements. Effect of combinations of 80% defoliation in the first year (1975), timing of the first year defoliation (July or August), and various levels of defoliation in the second year (1976) on height and diameter (30 cm above ground) of pOplar plants. Values are for August measurements. Effect of six methods of inflicting 75% defoliation on height and diameter (30 cm above ground) of three poplar clones after two years growth. Values are for August 1977 measurements Effects of partial defoliation on height growth rates of young growth chamber- grown poplar plants for various intervals following defoliation Effects of partial defoliation on height and diameter growth above the zone of defolia- tion in the 72 hr subsequent to defoliation vi 42 43 45 46 62 Table 3.4. 3.5. 4.1. 5.1. Effects of partial defoliation of height, diameter, and biomass of young growth chamber-grown poplar plants approxi- mately 3 weeks after defoliation (PI 35). Effects of partial defoliation on height, diameter, and biomass of young growth chamber-grown poplar plants below, within and above the zone of defoliation approximately 5 weeks after defoliation (PI 50) Effects of partial defoliation on leaf area within and above the zone of de- foliation of young growth chamber— grown poplar plants at three different intervals after defoliation Leaf plastochron index (LPI) in relation to leaf serial number and plastochron index (PI) for young poplars. Photo— synthesis of all leaves above the heavy line was measured Photosynthesis, leaf conductance, PAPFD, and leaf temperature 24 hr after defoliation (PI 25) Effect of partial defoliation on specific leaf weight of young growth chamber-grown poplar plants Treatment combinations used to test the effects of partial defoliation on the translocation patterns of young growth chamber—grown plants vii Page 75 77 79 99 103 111 122 Table Page 5.2. Distribution of 14C from poplar plants when leaves below the zone of defoliation were exposed to C02 24 hr after defoliation treatments (PI 25). Figures are means of {Ho replications harvested 48 hr after CO2 treatment. . . . . . . . . . . . . . 127 5.3. Distribution of 14C from poplar plants when leaves in tIi defoliated zone were exposed to CO 24 hr after de— foliation (PI 25). figures are means of twolieplications harvested 48 hr after C02 treatment. . . . . . . . . . . 129 5.4. Distribution of 14C from poplar plants when leaves below the zone of defoliation were exposed to 14C02 three weeks after de- foliation (PI 35). Figures are means of two replications harvested 48 hr after treatment . . . . . . . . . . . . . . . . 131 5.5. Distribution of 14C from p0plar plants when leaves inlzhe defoliated zone were ex- posed to CO three weeks after defolia- tion (PI 35). Figures are means of two replications harvested 48 hr after CO2 treatment . 133 5.6. Distribution of 14C from poplar plants when new leaves produced in the three weeks subsequent to defoliation were exposed to 14CO2 (PI 35 plants). Figures are means 94 two replications harvested 48 hr after C02 treatment. . . . . . . . . . . . . . 136 5.7. Distribution of 14C from poplar plants when leaves below thelione of defolia- tion were exposed to CO five weeks after defoliation (PI 50). Figures are means of two replications harvested 48 hr after treatment . . . . . . . . . . . . . 138 viii Table Page 5.8 Distribution of 14C from poplar plants when leaves in the defoliated zone were exposed to 1 CO five weeks after defoliation (PI 50). Figures are means of two replications harvested 48 hr after 4CO2 treatment . . . . . . . . . 139 5.9 Distribution of 14C from poplar plants when new leaves produced in the five weeks subsequent to defoliation were exposed to C02 (PI 50 plants). Figures are means of twa replications harvested 48 hr after CO2 treatment. . . 141 ix Figure LIST OF FIGURES Effect of several levels of defoliation on height growth of treated poplar plants at various times after defoliation Effect of several levels of defoliation on leaf production of treated poplar plants at various times after defoliation Effect of several levels of defoliation on leaf area produced following treatment of poplar plants Effect of several levels of defoliation on above ground biomass production of treated poplar plants Illustration of defoliated and control plants immediately following defoliation (PI 25) and five weeks after defoliation (PI 50). Effects of partial defoliation on area of individual leaves produced subsequent to defoliation, 72 hr after defoliation Effects of partial defoliation on area of individual leaves produced subsequent to defoliation, approximately three weeks after defoliation. Effects of partial defoliation on area of individual leaves produced subsequent to defoliation, approximately five weeks after defoliation. Page 59 63 66 68 71 81 83 85 Figure Page 4.1. Photosynthesis, leaf conductance, PAPFD, and leaf temperature approximately three weeks after defoliation (PI 35). . . 105 4.2. Photosynthesis, leaf conductance, PAPFD, and leaf temperature approximately five weeks after defoliation (PI 50). . . . . . 108 xi CHAPTER I INTRODUCTION AND REVIEW OF LITERATURE INTRODUCTION A major task of American foresters in future years will be to supply wood fiber to a wood using industry which is increasing its demand for raw materials (185). The challenge is to supply those needs with a continuously decreasing land base (34). In the Lake States, the issue is further complicated by the generally low pro- ductivity of the northern forest land and the increased cost of owning it. Procurement of wood from distant sources in the United States and Canada is becoming a questionable alternative due to high transportation costs. To meet this challenge, more agronomic approaches to forest management have been deve10ped in the form of in- tensive, short rotation culture of fast growing trees on land close to the sites of utilization. Intensive culture systems have taken two major forms :in recent years. The first, and more intensive of the filethods, is referred to as the "silage system" and is applicable largely with hardwood species (37). It was first proposed for use with sycamore by McAlpine et al. (122) and later updated by other workers (76, 172). With this method sycamore is grown at close spacings (1.5 x 1.5 m or less) with high levels of silvicultural input on rotations from 2 to 5 years. At harvest, the entire above ground biomass is removed mechanically. Yields of nearly 10 metric tons per hectare per year have been reported (160). Similar approaches have been taken with black cottonwood and alder in the Pacific Northwest (167) and hybrid poplars in Canada (200) and the Lake States 65, 42). The second method is more conventional. Genetically superior trees are grown on somewhat longer rotations (10 years plus) for pulp and sawtimber products. Planta- tions may receive some of the same intensive cultural treatments as in the first method, but, additionally, would be thinned and pruned in some cases. This method Was presented by Schreiner (162) and discussed in terms (Df several northern species. More specific information 1:5 available for sycamore (l7, l9), cottonwood (123) aJid hybrid p0plars (161) In large measure, the success at maximizing yields in these intensive culture systems will depend on how well stresses to the growing crop are minimized. Genetic and silvicultural progress towards these ends is being made (186), but even with the best combination of genetically improved stock and cultural methods, maximum yields may not be achieved due to biotic or abiotic stresses to the plant. Of significant importance is the stress imposed by defoliating insects, which reduce the photosynthetic area available for primary production. The short rotation forest crop generally represents a monoculture, being of uniform species, age, spacing and often genetic composition, and as such, provides ideal conditions for insects to reach outbreak pr0por— tions (195). Suppression using pesticides may be necessary, but should be avoided if possible. These compounds have only short term effects, they are generally petroleum based, expensive and toxic (195). Cultural methods, which take advantage of all the .factors and relations between tree and site to maximize ‘their effects against potentially damaging insect species, Clan be developed, but more information is needed on the physiological effects of defoliation (195). The objective of the studies described in this dissertation was to in- vestigate the effects of partial defoliation on the growth, photosynthesis and translocation patterns of young Populus plants. MAJOR DEFOLIATORS OF POPULUS About 60% of the insect species attacking poplars are defoliators (195). The life histories and impacts of the major poplar defoliators will now be discussed. Ef- fects of defoliation on the growth and physiology of trees will follow. Cottonwood Leaf Beetle The cottonwood leaf beetle (Chrysomela soripta F.) is one of the most serious pests of young poplars in nurseries and plantations throughout the eastern United States. In nurseries, the cottonwood leaf beetle stunts Iieight growth and reduces the yield of cuttings. One- E12nd two-year-old plantations are weakened by early e€rtle with chemical insecticides. Abrahamson et al. (1) reported that systemic insecticides, especially carbofuran, are the most promising candidates for chemical control in nurseries and plantations. Other chemical control strategies are necessary when dry weather prevents ade- quate systemic transport (140). Because of the toxicity, need for precise timing and repeated applications, high costs, loss of de- sirable predators and probable develOpment of insecti— cide-resistant strains of the leaf beetle, genetically resistant poplar clones may be a better answer. Oliveria 21nd C00per (139) found wide variation in tolerance of CC)ttonwood to damage by the cottonwood leaf beetle in 141(30 eastern cottonwood clones originating from 36 Iua‘tllral stands along the Mississippi river. Generally, nc>1¢1:hern clones were more resistant to damage that southern cl.c>11es, though heritability for tolerance was low (0.18). CaL]_c1beck et a2. (23) tested 33 pOplar clones for relative SIJSSCZGPtlbllltY to the leaf beetle near Ames, Iowa. Clones leaasst damaged had Populus aZba parentage. Shoot height ETOWth was reduced by as much as 80% in severely defoliated clones . PRDPZaP Tent Maker The poplar tent maker (Ichthyura inclusa an.) dfif0113tes poplars and willows from southern Canada to the Gulf of Mexico and West to Colorado (132). It may seriously defoliate young trees in nurseries and planta- tions, especially during the first year. Height growth is stunted, resulting in lower yields of cuttings in nurseries and stunted growth in plantations (132). Its occurrence in stands of cottonwood and aspen usually is not important, but occasional outbreaks have caused serious defoliation (29,133). Small groups of trees, especially trees growing more or less in the open are most seriously defoliated (10). There are two or more generations per year in the .Sc311th. Adults appear in the spring and then again in midsummer. Eggs are laid in clusters on the undersides oi? Ileaves. The larvae live in tents or webs and feed firc>nn May to October, then crawl to the ground and pupate in loose cocoons over the winter (10,132). Parasites and Predators usually control tent makers in natural stands, ‘blltl rapid buildups in plantations may require chemical treatment (132). In Texas, where such a buildup occurred, ‘3111)’ the second generation caused extensive damage (29). At present, the p0plar tent maker is considered Cnnly a minor pest, however, it is expected that 10 increased planting of cottonwood could bring this pest into prominence (29,133). Forest Tent Caterpillar The forest tent caterpillar (Malacosoma disstria an.) occurs throughout most of the united States and Canada and feeds on a wide variety of hardwoods (10, 16). This insect has been a serious pest of aspens,but few reports on injury to cottonwoods is available. Heavy clefoliation on large natural cottonwoods and a 4-year-old crattonwood plantation did, however, occur in Louisiana .111 1966 (133). Generally, it is believed that these defoliations kill few trees except those that are supressed, although growth can be appreciably reduced (131.,14,44,45,154,l76). There is only one generation of the forest tent caterpillar per year. Winter is spent in the egg stage and hatching occurs in the spring at the time of bud SVvefill on the host tree. Young larvae feed on expanding buC18; older ones devour the foliage, often completely defOIiating the tree. Larvae feed in clusters at :fjJFSt. then later disperse. The caterpillars do not spin 11 a tent, instead, they form a silken mat on the trunk or branches on which they congregate in masses to rest or molt. They also lay down strands of silk along which they travel. Pupation occurs in pale yellow cocoons spun in folded leaves, in bark crevices, on shrubs or other vegetation and occasionally on buildings, five to six weeks after hatching. Adults appear from late May in the South to late June and July in the North, and live for only a few days during which time eggs are dezposited in masses in the upper crown branches (10,16). Several natural control measures may keep pOpula— tzic>zls in check. Freezing weather shortly after hatch Hui)? kill a large number of larvae. Excessively high temperatures later in the spring may kill a large number (NE zidults and seriously reduce viability of newly laid eggs . Mortality of late larval stages may be severe or Complete as a result of starvation in heavily defoliated $118J1ds. A polyhedrus virus may kill large numbers in late stages of outbreaks. Several species of flies and wasps parasitize the eggs, larvae and pupae, the most imp‘n‘tant being large gray flies, Sarcophaga aldrichi Parker. Predatory beetles, ants, bugs, spiders, birds and 12 small mammals also help. Several insecticides are registered for use on this insect (10,16). At present, this insect is of incidental importance in cottonwood and hybrid poplar plantations but it could become a potential threat (133). Fall Webworm The fall webworm (Hyphantria cunea Drury) occurs throughout the United States and southern Canada. It feeds (on more than 100 species of forest and shade trees, LLsually understory species of no great value. Outbreaks nnzxy'occur, however, sometimes encompassing tracts of s everal miles (10). The fall webworm was observed defoliating deltoid 1>c>131ars in Colorado (133) and serious damage to poplars €111mH va ucmuwmwaw maucmofimwcwfim uoa mum umuuma memm mnu kn cmBOHHow modam> mmo:H*« .vmuomump mum3 vmuwmu monoHo msu mo %cm now mquEummuu wnu mo mam cmmBumn mmocmumwwwv unmofimacwfim ozx no.HH no.- m~.ma ammfi ommH mmmm ««mzoamu wmma mafiaHmEmu comm m.mH m.- a.q~ NRA mNH mfim «0 N mwm>OEmu mo>mma mo x doe m.NH m.HH m.m~ 00H NNH mom vm>oEmn mmma some «0 «\m vm>oamu mama wGHCHmEmu comm NO x 0.33 m.- m.m~ oma 3N3 3mm nem>oemn mm>mma N Aum>m mo H m.o~ o.HH m.m~ wqfi omfi omm vm>oEmu mm>me mo «\m yoga: o.m m.NH n.wH ONH qu wmm pm>oEmu mm>mwa m mum>m mo q AGOHuwHHowmv oav m.~H m.NH m.wH Hofi HmH «mm Homezoo Hmmm comm Nmmm Hmmm comm mmmm Aeev AEUV «mmymz . m ufimamhflmmme .nusoum mama» oBu umumm mmcoao umamoa mmMSu mo Avcaouw m>onm Bo omv umumsmfiu can uswfimn co coauMfiHowmv Nmm wcwuowamaw mo muonuma Nam mo nommmm .q.~ manna 47 Furthermore, there was no difference in response of the various clones, i.e., no clone x treatment interaction. There was, however, a highly significant difference in height growth among the three clones (Table 2.4). DISCUSSION The results of this study are generally consistent with previous experiments showing that heavy defoliation causes re« duced height and diameter growth (100). Although there were some reductions in height and diameter growth with in« creased levels of defoliation up to 80% in the present ex- periment, 40% defoliation treatments were not statistically different from no defoliation. When the effects of defoliation in the first or second year of establishment are considered separately, it appears that first year defoliation is no more significant than second year defoliation in causing reduced growth of hybrid poplars. When the effects of first and second year defolia- tion are combined as repeated defoliation, timing of the first year defoliation becomes an important factor. This was demonstrated when an 80%, July, first year defoliation, 48 followed by 80% defoliation in the following year resulted in little, if any, growth loss. If the first year, 80% de- foliation occured in August, followed by a second 80% defoli- ation the next year, sustantial growth reductions occur. Apparently timing of the second year defoliation is unimport- ant. It is curious that plants receiving two back to back 80% defoliations would show such different growth responses. That the 80% July first year defoliation followed by another 80% defoliation the second year did not reduce growth, may be due to several factors. If these plants were sufficiently established and had vigorous root and shoot systems at the time of early defoliation, they still would have had the re- mainder of the growing season to respond with compensatory growth. In fact, injury of this nature can cause a stimu- lation of growth (28,54,72,99,l44) This compensatory growth is possible because the normal photosynthetic capacity of plants probably exceeds that required in growth, giving the plants a considerable foliar reserve (67). In addition, Chapters III and IV of this dissertation show that leaf area production and photosynthetic rates of old and new leaves increase following defoliation. Thus, the defoliated tree 49 may be able to return to a fairly normal state of vigor by the end of the growing season and continue to catch up in following seasons, even if repeated defoliations occur. A late season, first year defoliation of 80% followed by heavy defoliation in the second year, is more likely to reduce growth, as trees do not have time during the growing season for compensatory growth. At the time of defoliation, many trees were setting bud, a period when photosynthate is channeled to storage locations or wood production. Most of the leaves on the plant were mature and probably transporting photosynthate predominantly to roots and subtending stem wood (105). Defoliation at this time removes a source of photo- synthate for transport to storage regions, consequently re« ducing tree vigor and early growth the next season. There is considerable evidence showing that defoliation causes reductions or cessation of root growth and reduces starch, carbohydrate, and amino acid levels in roots (143,145,146, 152,192,193,l94). An additional consequence of defoliation just prior to leaf fall is the loss of nutrients and other compounds that would normally be transported back into the plant during senescence of the leaves (64). These studies provided some basic quantitative data on 50 the physiological effects of partial defoliation on intensive- ly cultured poplars. They also pointed out the need for more intensive investigations into partitioning of growth follow- ing defoliation. The remaining chapters report the results of such studies. CHAPTER III EFFECTS OF DEFOLIATION IN THE DEVELOPING LEAF ZONE ON GROWTH AND DRY WEIGHT PARTITIONING IN YOUNG POPULUS X EURAMERICANA PLANTS INTRODUCTION Partial defoliation of tree species has been shown to reduce height, diameter, and dry matter production (100). Field studies with Populus hybrids at Rhinelander, Wisconsin (12, Chapter 11), however, showed that trees subjected to 80% defoliation in two successive years had little, if any, reduction in height and diameter growth, provided the first year defoliation occured early in the growing season. When first year defoliation occured late in the season, substan- tial reductions in height and diameter growth were observed. These field experiments provided some basic quantitative information on the effects of partial defoliation on inten- sively cultured hybrid p0plars, but the results were incon- clusive. To more fully understand these field responses to defoliation, studies were needed in which the environmental factors could be controlled, thus allowing isolation of the effects of defoliation. 51 52 Two experiments are reported. The first tested the ef- fects of several levels of defoliation in the developing leaf zone on growth of young hybrid poplar trees and was, in effect, an extension of the field experiments t6 the growth chamber. In the second study, only one treatment level was used with defoliation of the entire developing leaf zone and more attention given to partitioning of growth throughout the tree. The deve10ping leaf zone was selected for treatments based on the observed habit of the cottonwood leaf beetle (Chrysomela scripta F), a major poplar defoliator, to feed on immature foliage. Field studies have shown that immature foliage was preferred by a ratio of 33:1 over mature foliage (20,22,23). In addition, this zone has been shown to be im— portant in shoot ontogeny. As leaves in this zone mature, net photosynthesis reaches a maximum, COZ compensation point a minimum, and dark respiration a minimum (36,39,105). Leaves in this zone also change from net importers of photosynthate to predominantly exporters (105). Finally, secondary vas— cularization usually begins in the internode associated with the first mature leaf (108). 53 MATERIALS AND METHODS Unrooted hardwood cuttings of Populus x euramerieana cv. 'Negrito de Granada' were selected for uniformity in length, diameter and bud size. They were raised in a growth chamber with temperatures maintained at 25 C during an 18 hr day and 15 C at night. Relative humidity was held between 75% and 85%. Illumination, supplied by cool white fluor- escent tubes and incandescent lamps, ranged from 600 pE m'zs'1 at the upper level of tree growth, to about 100 pE m-Zs-1 at pot level. Plants were irrigated daily and fertilized at weekly intervals with 3 15-30-15 (N-P-K) commercial, water soluble plant food supplemented with micronutrients. The plastochron index (PI) and leaf plastochron index concepts, as deve10ped for cottonwood by Larson and Isebrands (106), were used as a morphological time scale for gauging measurements and treatments. The first leaf at the apex of a plant to attain 20 mm in length was designated the index leaf and assigned a LPI of 0. The next leaf below the index leaf was then assigned a LPI of l, and so on down the plant. The PI of any plant refers to the number of leaves below the index leaf, including those which may have abscised. 54 Experiment 1. Dormant, 20 cm hardwood cuttings were planted in 4 liter pots containing a mixture of soil-perlite- vermiculite (2:1:1). The cutting was inserted in the soil mix leaving two viable buds above the soil surface. Within 8 to 10 days, shoots had begun expanding. If more than one shoot developed, the shortest one was removed. When plants reached the 8- to lO-leaf stage, the first four basal leaves were removed. These small, preformed leaves are usually malformed and abscise early during shoot develOpment. All subsequent leaf counts were made by start- ing from the first remaining basal leaf. Beginning at a PI of 10 to 12, height, diameter, and total number of leaves were measured at weekly intervals, Length of each leaf in the expanding zone was added to the regimen beginning at a PI of 25. Defoliation treatments were imposed when plants had ate tained a PI of 30. At this stage, the developing leaf zone consisted of the first 13 to 14 leaves from the apex (LPI 0 to 12 or 13). Size of the developing leaf zone was de— termined from measurement of leaf lengths. Stabilization of leaf length expansion indicates maturity of the leaf (84). Defoliation was accomplished by using scissors to remove all 55 of the lamina except a 2 mm strip on each side of the mid- vein. There were two reasons for leaving some tissue along the midvein. First, field observations indicate that de- foliating insects will not consume the entire lamina. If adequate food supplies are available, they will leave some tissue surrounding the midvein and major veins (20,22,23). The exception occurs with very young leaves which may be consumed entirely. Leaving some leaf tissue has physiologi- cal significance since young leaves are known production sites of important growth regulators such as auxins, ethyl« ene, gibberellins and abscisic acid, and sinks for cytokinins (157,190). Five levels of defoliation were tested: removal of a) LPI's 0 thru 3, b)LPI's 0 thru 6, c) LPI's 0 thru 9, d) LPI's 0 thru 13, e) LPI's 4 thru 8, and f) control, no defoliation. Each treatment was replicated four times in a randomized complete block design. Effects of treatments were examined with a two-way analysis of variance. Differences between means were compared by using the least significant difference (LSD) (168,171). All plants were grown and measured until they reached 56 a PI of 45, at which time they were harvested and partitioned into stem, petioles and leaves. If lateral branches were present, they were also partitioned separately. Area of each main stem leaf and area of leaves on lateral branches were measured with a Li Cor LI-3000 leaf area meter. All harvested material was then oven-dried at 75 C. Experiment 2. The objective of this study was to more carefully define how growth was partitioned within the plant after defoliation treatment of the entire developing leaf zone. Cultural treatments were basically the same as in the first experiment. Dormant 25 cm cuttings were grown in a growth chamber under the same conditions as experiment 1, ex- cept 15 liter pots were used and a soil-sand (2:1) potting mix was substituted for the soil-perlite-vermiculite mix. Plants were irrigated at regular intervals and fertilized weekly with the commercial plant food used in the first ex— periment. In this study, the first four basal leaves were not removed, as had been done earlier. Height, diameter, and total number of leaves were monitored beginning at the 8— to lO-leaf stage. In this experiment, only one treatment was tested, de- foliation of the entire developing leaf zone. As plants 57 approached a PI of 20, pairs were selected from a larger pool. Pairings were based on uniformity of height, diameter, estimated leaf area*, and PI (total number of leaves). Care was taken to assure that leaves at individual nodes were of equal size, thus eliminating the effects of malformed lower leaves. At a PI of 25, one plant from each pair was randomly selected and defoliated. Previous experiments with the same clone had indicated that at a PI of 25, the developing leaf zone reliably consisted of LPI's 0 thru 10 (i.e., the first 11 leaves greater than or equal to 2 cm in length below the apex). Defoliation was accomplished by using paired razor blades to remove all of the lamina except a 2 mm strip on both sides of the midvein. Area of leaf tissue removed was determined, and then it was dried in an oven at 75 C for 48 hr, and weighed. Plants were harvested 72 hr (ca. PI 29), three weeks (PI 35), and five weeks (PI 50) after defoliation. At har- vest, each plant was partitioned into leaves, petioles, stems, lateral branches, roots and cutting. The above- ground portions were further separated into sections below, *Estimated from a regression equation similar to that of Larson and Isebrands (107) using leaf length, leaf location and leaf area. 58 within, and above the defoliated zone. Soil was removed from the :root system by gentle washing with a stream of water. Recovery of roots was good. Leaves were excised at the junction of the lamina and petiole. Area of each main stem leaf and cummulative area of leaves on lateral branches were determined with a Li Cor LI-3000 leaf area meter. All har- vested material was then dried in an oven at 75 C for at least 48 hr. Experimental design was a paired comparison using the Student's t-test to compare means (171). Four replications were available for the 72 hr comparison, and 6 replications were used for three week and five week comparisons. RESULTS Experiment 1. Heights of all plants remained the same until about one week after defoliation. Heavily defoliated plants then began to show increased height growth over con- trols (Figure 3.1). Two weeks following defoliation, all defoliated plants showed highly significant (Ps.01) in- creases in height over controls, with a trend towards greater height with increased levels of defoliation. At the end of 59 Figure 3.1. Effect of several levels of defoliation on height growth of treated pOplar plants at various times after defoliation. 6 0 LPI Defol iated Control H to 4 H to 7 H to 10 O-—O to 13 H f0 10 H 0.0000 Height (cm) o 10 20 30 Days Since Defoliation Figure 3.1. 61 the experiment, all defoliated plants except treatment (b) were significantly (P1§.05) taller than controls, with the heaviest defoliation treatment (d) having the greatest height. Rates of height growth were computed for defoliated and control plants following defoliation (110). Defoliated plants. generally maintained higher growth rates than controls until three weeks after defoliation when all plants returned to about the same rate of growth (Table 3.1). The most severely defoliated plants generally had higher growth rates than con— trols and less severely defoliated plants. Diameter growth showed trends similar to that of height growth, although differences among treatments were nonsignif— icant. The two most severely defoliated treatments had the largest diameters by the end of the experiment. Leaf production increased after defoliation; thus number of leaves on defoliated plants was greater than control plants until the end of the experiment (Figure 3.2). The general trend was towards a greater number of leaves with increased levels of defoliation, with treatments (c) and (d) having significantly (P§.05) greater PI’s than controls at all dates except the one immediately following defoliation. The rates of leaf production on defoliated plants (number of 62 Table 3.1. Effects of partial defoliation on height growth rates of young growth chamber-grown poplar plants for various inter— vals following defoliation. TREATMENT PERIOD (Days Since Defoliation) -2 to 3 4 to 8 9 to 14 15 to 22 Actual Growth Rate (cm/day) CONTROL 2.21 1.79 1.42 2.04 LPI 0 to 4 2.16 2.01 1.68 2.02 LPI 0 to 7 1.99 1.88 2.01 1.82 LPI 0 to 10 1.83 2.13 1.84 1.80 LPI 0 to 13 2.07 2.25 1.86 1.97 LPI 5 to 10 2.20 1.86 1.39 1.85 LSD* 0.26 0.25 0.28 NS Relative Growth Rate (cm/cm day) . . . . . CONTROL .034 .024 .017 .018 LPI 0 to 4 .032 .025 .019 .017 LPI 0 to 7 .031 .025 .024 .016 LPI 0 to 10 .027 .027 .021 .016 LPI 0 to 13 .030 .028 .021 .016 LPI 5 to 10 .031 .023 .015 .016 LSD* .004 .004 .004 NS * 5% level Least Significant Difference 63 Figure 3.2. Effect of several levels of defoliation on leaf production of treated pOplar plants at various times after defoliation. ()4 50.. LPI Defoliated ControlH 0104 H 0107 H 45— I 0'0100——€ Oio 13H 5'0 lOA—A I (0 O ’ C .3 _35_ I e I- s I E :- I30»- 25_. I l l l l 1 I 1 0 IO 20 30 Days Since Defoliation Figure 3.2. 65 new leaves 22 cm in length produced per day) was signifi- cantly higher (P $.01) than control plants almost immediate- ly following defoliation. These rates then dropped off to levels about equal to controls just prior to harvest. Total leaf area at the time of harvest was, of course, less for defoliated plants; however, a highly significant (P $.01) increase in leaf area produced above the defoliated zone was observed with the greatest levels of defoliation (Figure 3.3). For the heaviest defoliation treatment, (d), this area represented 52% of the total leaf area at the time of harvest, whereas this same proportion for controls was only 22%. No significant differences between treatments in stem dry weights occurred (Figure 3.4), but a general reduction in the leaf plus petiole component with increased levels of defoliation was observed due to removal of leaf tissue. Pro- duction of lateral branches on defoliated plants increased significantly (Figure 3.4). Controls and treatment (e) pro- duced no lateral branches. Greatest lateral branch dry weight was observed for treatment (b), an intermediate level of defoliation. The reduced above-ground dry weight of defoliated plants was caused soley by the loss in leaf 66 Figure 3.3. Effect of several levels of defoliation on leaf area produced following treatment of poplar plants. 67 1400 1300 '1' as _==_==_====_=__=__=_=_==_===_===_========_ = 2.. n n—o. O .mwhrmvnum~qnmfiu~mimwmm‘lim‘wllflnflWL—wflamunmnuw“Humannwnu ~mum u n .. . .... .ss. 4. H h.. .... . . a _Lu."1.w.1:"Inf-:1.»—..:T..:::::::3:F...“-.:- — O— 0+0 7.7%.; :3 I .02coU 0 0 0 0 .l 9 I $.53 22 :3 1200 1000 800 700 600 Defoliated [Pl Figure 3.3. 68 Figure 3.4. Effect of several levels of defoliation on above ground biomass production of treated p0plar plants. Leaves and Petknes Lateral Branches Stem 69 ,lllllllllllllllllllIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIII 9L . . mmImml'mnnmumnumm r IIIIIIIIIIIIIII mm 1 1 I 1 1 1 1 I 4— qoqqeeeee goiwtonvmm 1 l l l l l l o o O o o o o o . 6 o a N o In V n a .— (3) "mm m Ol°‘§ 'closo 0l°‘0 (0‘0 7°00 '01 we) 01°09 £1000 61°00 1°10 roso '0“qu 010:9 €l°60 010:0 1°60 7°10 Iouuog lPI Defoliated Figure 3.4. 70 tissue removed during defoliation. When dry weights of leaf tissue removed during defoliation were included in the total plant dry weights, defoliated plants actually showed greater, though nonsignificant, dry weights than controls. Experiment 2. A diagramatic representation of plants at P1 25 (time of defoliation) and at PI 50 is shown in Fig- ure 3.5. At the time of treatment, plants designated for defoliation had an average PI of 26.7. Before defoliation, the zone to be treated contained a leaf area of 485 cmz. De- 2 of this area, or about foliation removed an average 432 cm 90%. Average area removed as a percent of total plant leaf area was 46.8%. Defoliation resulted in an average loss of 2.3 g dry weight of leaf tissue. Table 3.2 shows that the effect of defoliation on sub- sequent shoot growth was immediate. Within 72 hr after de- foliation, internodal elongation and diameter growth above the zone of defoliation was significantly increased. The weight of leaves produced was also increased over that of control plants. Table 3.3 summarizes the impact on growth three weeks after defoliation (PI 35). Stem lengths and diameters were identical on defoliated and control plants in the section 71 Figure 3.5. Illustration of defoliated and control plants immediately following defoliation (PI 25) and five weeks after defoliation (PI 50). 72 Control Dolollatod - 1" age; 3% e : {GEEE 883% . I It I ((Wd'i (Mia 11 5'55 .oo>aou mu. 51:10.00 o>oa< oc0N Control Dotollatod xon< \\\\\w\ 30:... p: gaze-.300 .o ocoN .a.s..3 v.. coza:o*oa 30.00 econ unsung 0:30.260 . 1.. Tormlnation of Experlmont ol Dolollatlon (T:O) Tlmo (T=5 Weeks) Figure 3.5. 73 Table 3.2. Effects of partial defoliation on height and diameter growth above the zone of defoliation in the 72 hr subsequent to defoliation. TREATMENT STEM DIAMETER DRY WEIGHT ELONGATION (mm) (cm) LEAVES STEMa O O O O O 0 g 0 0 O O DEFOLIATED 4.4* 3.8* 0.2** 0.12* CONTROL 3.7 3.2 0.1 0.09 aIncludes petiole dry weights. *Significantly different than controls 5% level Student's t—test. **Significantly different than controls 1% level Student's t-test. 74 below the defoliated zone but were very significantly reduced on defoliated plants in the defoliated zone. There was also an apparent reduction in stem elongation in the section above the zone of defoliation on defoliated plants, contrary to the response 72 hr after defoliation. Thus, total height of de- foliated plants was significantly reduced as compared to con- trols. Diameter growth both within and above the defoliated zone was greater on defoliated plants. Leaf biomass was reduced 83% on defoliated plants in the defoliated zone, but showed a 25% increase on defoliated plants above the zone of defoliation. The net effect was a 35% reduction in total leaf biomass three weeks after treat— ment. Biomass of the stem plus petiole component was reduced 28% in the defoliated zone on defoliated plants as compared to controls. Flushing of axial buds was stimulated by defoliation, with defoliated plants showing greater lateral branch dry weights both below and within the zone of defoliation. Neither defoliated or control plants produced any lateral branches above the defoliated zone. Cutting and root bio« mass were also reduced by defoliation, with roots showing a 37% reduction. 75 Table 3.3. Effects of partial defoliation on height, diameter, and biomass of young growth chamber—grown poplar plants ap— proximately 3 weeks after defoliation (PI 35). PARAMETER LOCATION IN RELATION TO ZONE OF DEFOLIATION BELOW WITHIN ABOVE TOTAL Da Cb D c D c D c HEIGHT 16.7 16.7 18.7** 20.7 19.7* 21.7 55.5* 59.2 (cm) DIAMETER 6.7 6.7 6.2* 5.9 5.7* 5.4 (mm) DRY WEIGHT (g) LEAVES 3.1 2.8 1.2** 7.1 3.8** 2.9 8.3* 12.8 STEMSC 1.8 1.9 1.3** 1.8 0.9 0.9 3.7* 4.5 LATERALS 0.07* 0.01 0.19 0.12 0.0 0.0 0.25 0.13 CUTTING 7.2 8.8 ROOTS S.7** 9.1 TOTALS 5.0 4.7 2.7** 9.0 4.8* 3.9 25.1* 35.3 aDefoliated Plant *Significantly different than controls, 5% level Student's bControl Plant t-test. CIncludes petiole dry weight **Significantly different than controls, 1% level Student's 76 The net effect three weeks after defoliation was a 29% reduction in total plant dry weight. The component of total biomass produced after defoliation, however, increased 23% on defoliated plants. Growth impacts five weeks after defoliation (PI 50) are summarized in Table 3.4. At this stage, there was generally no differences in height or diameter between defoliated and control plants. Diameter at the base of the plant was re- duced slightly by defoliation. Leaf biomass was still reduced by 81% on defoliated plants in the defoliated zone after five weeks, but biomass of leaves produced above the zone of defoliation was greater on defoliated plants, giving a net reduction in total leaf biomass of only 20%. Stem plus petiole biomass was reduced on defoliated plants in the defoliated zone, but this component was un- affected in other zones. Lateral branch production was greater on defoliated plants, but this effect was confined mainly to the section below the defoliated zone. Lateral branch production in the other two zones was about equal. Total plant dry weight was 17% less on defoliated plants, with a slight increase (5%) in the section produced after 77 Table 3.4. Effects of partial defoliation on height, diameter, and biomass of young growth chamber—grown poplar plants below, within and above the zone of defoliation approximately 5 weeks after defoliation (PI 50). PARAMETER LOCATION IN RELATION TO ZONE OF DEFOLIATION BELOW WITHIN ABOVE TOTAL Da 0b D c D c D c HEIGHT 15.9 15.9 19.0 20. 52.0 51.7 86.9 87.7 ( cm) DIAMETER 8.7* 9.1 7.7 8. 7.3 7.4 (mm) DRY WEIGHT (.8) LEAVES 3.3 3.7 1.7** 8. 18.7* 17.3 23.7** 29.8 STEMC 3.3 3.8 2.7¥* 3. 5.0 5.2 11.0** 12.6 LATERALS 1.5* 0.0 1.5 1. 0 1 0 1 3.1 1 8 CUTTING 9 1* 10 4 ROOTS 12.4** 16.7 TOTALS 8.1 7.5 5.9** 14. 23.8 22.6 59.3* 71.3 aDefoliated Plant bControl Plant CIncludes dry weights of petioles. *Significantly different than controls, 5% level Student's t-test. **Significantly different than controls, 1% level Student's t-test. 78 defoliation. DeveIOpment of leaf area within and above the defoliated zone 72 hr, three weeks, and five weeks after treatment is presented in Table 3.5. The remaining immature leaf tissue in the upper portion of the developing leaf zone continued to expand for some time, whereas leaves in the lower portion of the developing leaf zone, which were nearly mature at the time of defoliation, did not expand to any extent. As a re- sult, the upper leaves in this zone had a larger leaf area than the lower leaves at the termination of the experiment. The net effect was a doubling of leaf area in the defoliated zone by the fifth week after defoliation. None-the-less, de- foliated plants still had 81% to 88% less leaf area in the defoliated zone than control plants. Leaf area developing subsequent to defoliation was much greater on defoliated plants and this effect was noticeable 72 hr after defoliation. At this time, defoliated plants had 139% more leaf area above the defoliated zone than con- trol plants. The increase in leaf area in this zone on defoliated plants after three weeks and five weeks was 41% and 17% respectively. Stimulation of lateral branch growth on defoliated 79 Table 3.5. Effects of partial defoliation on leaf area within and above the zone of defoliation of young growth chamber-grown poplar plants at three different intervals after defoliation. TREATMENT LEAF AREA ( cmz) MAIN SHOOT LATERAL TOTAL SHOOTS WITHIN ABOVE .72 hr After Defoliation . . . . . . . . . DEFOLIATED 95.5** 34.5** - 644.0** CONTROL 781.4 14.4 - 1383.0 . . .3 Weeks After Defoliation . . . . . . . DEFOLIATED 181.2** 717.2** 26.0 1447.0** CONTROL 1113.2 507.8 16.8 2119.4 . . . . .5 Weeks After Defoliation . . . . . . . DEFOLIATED 205.6 2442.3* 378.9** 3505.3* CONTROL 1103.0 2080.0 213.5 3925.4 ‘ *Significantly different than controls, 5% level Student's t—test. **Significantly different than controls, 1% level Student's t—test. 80 plants also increased the total leaf area of defoliated plants, There was no lateral branch production 72 hr after defoliation, but defoliated plants showed 55% and 77% greater leaf area on lateral branches than controls after three weeks and five weeks respectively. The net effect of defoliation was to reduce total leaf area on defoliated plants by 53% after 72 hr, 32% after three weeks, and 11% after five weeks. One of the most consistent and striking growth responses to defoliation was an increase in the size of leaves develop— ing subsequent to defoliation. New leaves on defoliated plants were up to 33% larger than complementary leaves on control plants. Figure 3.6 shows this response for the har- vest three days after defoliation. These plants had expanded 5 to 6 new leaves since defoliation, and leaves on defoliated plants were significantly larger at all positions than com- plementary leaves on control plants. A similar response was observed three weeks after defoliation (Figure 3.7). Only those very immature leaves near the apex were not signifi- cantly larger. After five weeks (Figure 3.8), differences in individual leaf areas were less pronounced, particularly near the apex. None—the-less, most leaves produced after defoliation were larger on defoliated plants. 81 Figure 3.6. Effects of partial defoliation on area of individual leaves produced subsequent to defoliation, 72 hr after defoliation. 82 4 I _____=.._________._.___________ w oo =................. m .. m e 2 .I m m __________ x m. m f 1 p m m .5. M - =. _ o 6 4 2 O 8 6 4 2 1| 1| 4| 1| NED moc< Coo; Figure 3.6. 83 Figure 3.7. Effects of partial defoliation on area of individual leaves produced subsequent to defoliation, approximately three weeks after defoliation. I Control 200 E De foliated 180 84 13 12 llllllllllllllllllll ' ‘ F llllllllllllll " O Illlllllllllllllll ’ " o: Illllllllll , Q lllllllllll h lllllll 0 III! I!) II I Q I "’ N O O O 8 8 2 8 8 6 v A 1- v- 1- '- zwo 291V 1391 Approximate LPI Figure 3.7. 85 Figure 3.8. Effects of partial defoliation on area of individual leaves produced subsequent to defoliation, approximately five weeks after defoliation. I Control DotoHatod 200 86 24 22 20 18 16 14 12 10 N O O O O O O O O O Q to V N O m CO V N P F F 1- '- 3W3 331v 1331 Approximate LPI Figure 3.8. 87 DISCUSSION These results show that partial defoliation of young Populus plants can have an immediate and strong impact on subsequent shoot ontogeny. The plant apparently compensates for the loss of leaf area as a result of defoliation by in- creased leaf production. This increased leaf production is reflected in greater size and number of main stem leaves pro- duced above the defoliated zone, and greater leaf area pro- duced on lateral branches. Evidence for a growth stimulating effect as a result of partial defoliation has been presented by several researchers. Such defoliations have been shown to induce more rapid leaf expansion (3,62), increase leaf production and leaf area (3,117,148), and increase leaf dry weight (3). Kulman (99) found that complete disbudding of Aeer saccharum apparently caused a stimulation of height growth. Churchill et a2. (28) reported that certain aspen stands defoliated for two to three years showed increased height and basal area growth compared to undefoliated stands. In the present studies, the effect of partial defoli- ation on height growth was inconclusive. In the first 88 experiment there was an apparent stimulation of height growth on defoliated plants that was still evident at the termin- ation of the experiment three weeks after defoliation. In the second experiment, stem elongation above the defoliated zone was stimulated within 72 hr of defoliation, but was re- tarded by three weeks after defoliation. By five weeks after defoliation, stem elongation above the defoliated zone in both defoliated and control plants were the same. Data from a preliminary study (not presented) showed height growth to be stimulated by defoliation 72 hr, three weeks and five weeks after defoliation. This growth stimulation could have been caused by in- creased levels of gibberellins. Young leaves are major sites of gibberellin synthesis and gibberellins have been shown to increase height growth in many tree species (86,127,153) Elevated levels of gibberellins have been found associated with spider mite infested plum trees (8) and decapitated apple and pear trees (90). The increased height growth in these studies was often associated with reduced root growth, root-top ratio, and total tree weight (18,48,153,l63) Some of these studies have shown that differences in gibberellin— induced height growth diminished with time, such that control 89 plants eventually catch up (18,48). Many of these same ef- fects accompanied defoliation in the present studies and are consistent with a role for gibberellins. Gibberellins are also known to increase stem elongation with no concurrent increase in dry weight, the growth stimu- lation resulting mainly from enhanced water uptake (157). Some species do show an increase in dry weight, resulting mainly from greater leaf development and more photosynthesis (174). Where increased stem elongation occurred in the pre- sent studies, there was also no increase in dry weight, ex- cept for the leaf component. These results are also COHSiS* tent with a role for gibberellins. Pollard (148) reported that defoliation in aspen caused increased leader growth. He reasoned that the increased growth may have been due to a reduced transpiration load. The increased height growth observed in the present study could also be a result of reduced transpiration load on the defoliated plants. Auxins are known to be produced in young leaves of poplar plants (50,52,53,9l) and defoliation could have re- duced their production. Defoliation also resulted in a reduced supply of photosynthate from the defoliated leaves. 90 Translocation studies have shown that a cottonwood leaf preferentially feeds the subtending internode and the con— tribution to other internodes declines basipetally (105). Larson (102) reported that auxin controls cell diameter and the amount of photosynthate controls cell wall thick- ness in developing xylem elements. Thus, reduced auxin and photosynthate to xylem elements in the internodes sub— tending the defoliated leaves could have produced small diameter, thin walled cells of lower dry weight. According to Larson (102) "Mutilations, such as partial defolia- tion, that seriously interfere with photosynthetic ability evoke almost immediate alterations in wall development, and cells with extremely tenuous walls can be produced". Dry weights in other parts of the plant were not reduced by defoliation, nor was there any reduction in stem diameter. Apparently, defoliation in Popuius does not significantly affect wood production outside the defoliated zone. Of significance is the immediate and consistent reduction in growth of cuttings and roots on defoliated plants throughout the experiment. It is likely that this reduced growth results from a shift in allocation of 91 available photosynthate from the roots to the shoot. Eliasson (49) found that the rate of root growth in Populus tremula was determined by the supply of carbohyv drates from the leaves and related to the light intensity impingent on the leaves. A later study (51) with the same species indicated a competitive effect for photosynthetic products between roots and shoots. Rapid shoot growth was accompanied by decreased root growth. Similar results were reported by Richardson (152). Reduced transport of photosynthate to roots is also indicated by lowered carbOv hydrate levels in the roots following defoliation treatments (87,143,145,146,194). A defoliation—induced increase in gibberellin production could also have resulted in reduced root growth (18,117). The increased size of leaves produced after defoliation also implicates a shift in allocation of photosynthate to the apical region of the plant. This shift allows defoliated plants to quickly begin to rec0ver lost leaf area. For example, at the time of treatment, leaf area was reduced on defoliated plants by almost 50%. Three days later, defolia- ted plants responded by producing 139% more leaf area than control plants. Continued production of larger leaves on 92 the main stem resulted in defoliated plants having only 32% less main stem leaf area three weeks after defoliation and 16% at five weeks. Additionally, stimulated production of lateral branches as a result of defoliation cut the differences in total leaf area to only 11% after five weeks. Leaf biomass differences were reduced from 29% right after defoliation to only 17% after five weeks. Increased size and number of leaves has occurred following defoliation in both woody and herbaceous species (3,117,148). There is some evidence that both gibberellins and cytokinins can cause enlargement of leaf surface area (127,157). Lateral branch production is probably stimulated by a release of lateral buds from apical dominance. As stated above, auxin production is probably reduced by defoliation in the developing leaf zone, and continued supply of auxin from the apex is necessary to prevent release of lateral buds from apical control (155,156). Additionally, cytokinins and gibberellins are needed for growth of the lateral shoots (47,77,155,156). Several studies have shown that auxins, gibberellins and cytokinins interact in con- trol of bud dormancy in PopuZus (50,52,53,77,78,79). Both gibberellins and cytokinins have also been shown to be 93 increased as a result of insect attack or decapitation (8,56,57,90). The mechanisms of the defoliation effects discussed above are complex and may involve altered source—sink relationships and translocation patterns, changes in photo- synthetic efficiency, and production and distribution of growth regulators. It is likely that all of these physio- logical factors are involved in the growth responses to de- foliation observed in the present experiments. Further experiments were carried out to investigate the photo- synthetic and translocation patterns of defoliated plants and are reported in Chapters IV and V. CHAPTER IV PHOTOSYNTHESIS AND LEAF CONDUCTANCE OF POPULUS X EURAMERICANA AFTER DEFOLIATION IN THE DEVELOPING LEAF ZONE INTRODUCTION Intensive culture forestry systems strive to maximize fiber or biomass production per unit land area in the shortest time possible. Two key physiological components contributing to increased wood yields are (l) the rates of leaf photo- synthesis and (2) the develOpment of photosynthesizing leaf area (38). The amount of photosynthate available for tree growth ultimately depends on the amount of leaf area available for photosynthesis. Total leaf area is an important determinant of growth and yield as demonstrated in pOplar by the close correlation between leaf area and above ground biomass (104, 107). High productivity is generally reflected by a large amount of leaf area per unit ground area, or leaf area index (LAI), close spaced hybrid pOplars showing LAI values up to 40 m-Zm.2 (4,38,85). Of equal importance is the length of time that photosynthetically active foliage remains on the tree, or leaf area duration. There are important possibilities 94 95 for genetic selection in this regard (24,85,147,l98) but leaf disease and defoliating insects may minimize or negate any gains in leaf area duration. Defoliating insects are particularly harmful because they show preference for immature foliage when adequate food supplies are available. For example, the cottonwood leaf beetle (Chrysomela scripta F.), a major poplar defoliator, prefers immature foliage by a ratio of 33:1 over mature fol- iage (20,22,23). The effects of this foliage destruction are apparent when the importance of the developing leaf zone to buildup of LAI is considered. Recent studies of partial defoliation in crop plants and tree species have shown increased growth (3,28,72,99, 117,148) and increased photosynthetic rates of leaves or leaflets remaining on the plant (3,136,157,175,191). Defoli« ation in the deve10ping leaf zone of hybrid poplars has also resulted in a stimulation of leaf area production and, in some cases, height and diameter growth (13, Chapter III). The purpose of this study was to determine the photosynthetic response of hybrid poplars to partial defoliation in the de- veloping leaf zone. 96 MATERIALS AND METHODS Cultural. Unrooted hardwood cuttings of Populus x euramericana cv. 'Negrito de Granada' were selected for uni- formity in length, diameter and bud size. They were raised in a growth chamber in a 2:1 soil-sand mix. Temperatures were maintained at 25 C during an 18 hr day and 15 C at night. Relative humidity was maintained between 75% and 85%. Cool white fluorescent tubes and incandescent lamps supplied a photosynthetically active photon flux density (PAPFD) which ranged from 600 DE m'zs.1 at the upper level of tree growth to 100 DE m.zs'1 at pot level. Plants were watered often enough to maintain soil moisture at or near field cap— acity. A 15-30-15 (N-P-K) commercial water soluble plant food, supplemented with micronutrients, was used to fertilize the plants weekly. The plastochron index (PI) and leaf plastochron index (LPI) concepts, as developed for cottonwood by Larson and Isebrands (106) were used to time treatments and measure- ments, since a morphological time scale is more reliable than a chronological scale in studies relating to physiological development (101). A reference length for the lamina of the 97 index leaf of 20 mm was used. Height, diameter and number of new leaves 20 mm or great- er in length were recorded at weekly intervals beginning at a PI of 10 to 12. Length of the index leaf and the leaf im— mediately above it were measured for calculation of PI and LPI.I At a PI of 12, the index leaf (LPI 0), the 12th leaf from the base, would be exactly 20 mm long. The next older leaf below the index leaf has a LPI of 1 and so on down the plant. Defoliation Treatments. As plants approached a PI of 20, pairs were selected from a larger pool. Pairings were based on uniformity in height, diameter, number of leaves (PI), and estimated leaf area. At a PI of 25, one plant from each pair was randomly selected and defoliated. Defoliation was accomplished by using paired razor blades to remove all of the lamina except a 2 mm strip along each side of the mid- vein. All leaves in the developing leaf zone (LPI 0 to 10) were similarly treated. Area of leaf tissue removed in de- foliation was measured, and then it was dried in an oven set at 75 C for 48 and weighed. Photosynthesis and Conductance. Measurements were made on plants 24 hr, 3 weeks, and 5 weeks after defoliation, 98 corresponding to approximate PI's of 25, 35, and 50, respect- ively. At each of these PI's determinations were made on leaf number (from the base of the plant) 10, 14, 24, 25, 26, 30, 35, 40, and 45, if their LPI was greater than or equal to 5. The resulting sampling scheme is shown in Table 4.1 and consists of both a horizontal or ageing series, and a vert- cal series. The horizontal series allows comparisons of the same leaf position as it gets older, while the vertical ser— ies allows comparison of leaves in similar states of deve10p- ment (36). Photosynthetic measurements were made in the growth chamber in which the plants were grown. Prior to determi- nations, leaf conductance was measured with a Li Cor LI-6S AutOporometer equipped with a Kanemasu-type sensor (93). Leaf temperature was measured with the bead thermistor built into the porometer by appressing it to the lower leaf surface. PAPFD was determined with 3 LI Cor LI-185 light meter equip— ped with a quantum sensor. Measurement of PAPFD was made parallel to the leaf surface, assuring of in situ radiation conditions by maintaining continuity of current canopy archi— tecture (98,130). Leaf photosynthesis was measured using a portable gassing 99 Table 4.1. Leaf plastochron index (LPI) in relation to leaf serial number and plastochron index (PI) for young poplars. Photo— synthesis of all leaves above the heavy line was measured. LEAF NUMBER PLASTOCHRON INDEX (PI) (from base) 25 35 50 LPI . . . . . . . . . . . . 10 15 25 40 14 11 21 36 24a 1 11 26 25a 0 10 25 26 9 24 30 5 20 35 0 15 40 10 45 5 50 0 3 Leaves in the defoliated zone. 100 device similar to that described by McWilliam et al. (124). -1 14 Air containing 332 ppm C02 (5.0 uCi 1 C02 ) was passed over both surfaces of a leaf by clamping the treatment cham- 1 bers over the leaf for 20 seconds. Flow rate was 80 ml min' , sufficient to minimize the boundary layer resistance and pre- vent depletion of C02 in the chambers. The exposed area (0.5 cm2) was then immediately cut from the leaf with a 1 cm cork borer and placed into a scintillation vial containing 1.5 ml NCS tissue solubilizer (Amersham Corp.). Scintillation vials containing leaf discs and NCS were placed in an oven set at 50 C for 24 hours. After cooling for 30 minutes, samples were bleached by the addition of 0.5 ml H202 . Twenty four hours later, 17 ml of a scintillation fluid containing 1000 m1 toluene, 400 ml methyl cellosolve and 60 ml Spectrafluor (Amersham Corp.) was added. After an additional 24 hr dark equilibration period, samples were counted in a Packard TriCarb 2002 liquid scintillation spec- trometer at room temperature. The rate of photosynthesis, expressed as the weight of C02 taken up by the leaf per unit time and leaf area, was calculated using a formula similar to that of McWilliam et al. (124). Following photosynthetic measurements, plants were used in a 14C-translocation study (see Chapter V) lasting an 101 additional 48 hr, then harvested. At harvest, each plant was partitioned into leaves, petioles, stems, lateral branch- es, roots and cutting. The above-ground portions were fur- ther separated into sections below, within and above the de- foliated zone. Soil was removed from the root system by gentle washing with a stream of water. Area of each main stem leaf and cummulative area of lateral branches was mea- sured with a Li Cor LI-3000 leaf area meter. All harvested material was then dried in an oven at 75 C for at least 48 hr, then weighed. Leaves that had been used for photosyn— thetic measurements were weighed individually for calculation of specific leaf weight (SLW), an index of the relative density of a leaf and a sensitive morphological index of physiological state (110). SLW was monitored to determine whether morphological changes occurred in leaves subsequent to defoliation. The experimental design was a paired comparison using the Student's t-test to compare means (171). Four repli- cations were available for testing the PI 25 plants and 6 replications were used for each the PI 35 and PI 50 plants. 102 RESULTS Patterns of photosynthesis, leaf conductance, light in- tensity and leaf temperature 24 hr after defoliation (ca. PI 25) are given in Table 4.2. It was impossible to determine photosynthetic rates on any leaves in the defoliated zone at this stage because not enough lamina remained on leaves. Thus, only two data points were obtained; on the 10th and 14th leaves from the base of the plant (approximately LPI 15 and 11 respectively), with the latter representing the first leaf below the defoliated zone. Photosynthetic rates were higher at both leaf positions on defoliated plants within 24 hr after defoliation. These differences were significant (P‘EIOS) at LPI 11, but not sig- nificant at LPI 15 due to increased variability. In both cases there was a decline in photosynthesis in the older leaf position (LPI 15). Values for leaf conductance showed trends opposite those of photosynthesis. Leaves on defoli- ated plants had lower conductances (greater leaf resistances) than complementary leaves on controls. Significant differ— ences occurred only at LPI 11. Leaf conductance decreased with increased leaf age on both defoliated and control plants. 103 o.mm mmm mmm. c.ma AomHzoo «.mm mmm mmm. q.HN QmHmH NH .mHouucoo amnu ucouomwfiv %Huamoawacwfim«k .mucmaa Houucoo mo cmoz n .umouiu m.u:meoum Ho>oa Nm .maouucoo cmnu uaouommav >Husmofiwficwfimk .mucmHQ Emumwaomov mo cmoz m wm.o ew.o m I I I I I I no mq.m oq.n OH I I I I I I 06 mm.w «Hm.w ma I I I I I I mm mH.m «o~.~ om mo.m mm.m m I I I om mn.w «om.m 6N mm.m mm.m m I I I om m~.m Nq.m om w~.o om.© Hm mm.m mm.o HH 6H No.0 m~.n oq mm.m mq.o mm ow.m «xem.o ma OH 0 a qu o a Hma no we Hma om Hm mm Hm mm Hm mmom< owls mmuz mmuz mzom ameom< szHHB 304mm m>om< szHH3 Soqmm UZHHHDU whoom mzmem mm>um£ mcoaumOHHnou oBu mo memos mum mouswfim .AmN Hmv mucoaumouu cowumfiHOMOO nouwm a; «N Noqu ou vomoaxo mums coaumeOMOw mo moon ecu aegme OO>OOH coza mucmfia moaned scum cod no cowuonfiuumfia .N.n manna 128 leaves on control plants. The results of feeding leaves in the defoliated zone are presented in Table 5.3. Both defoliated and control plants retained a large percentage of their total activity in this zone, as most of these leaves were immature and still strong sinks (105). Defoliated plants translocated a greater percentage of radioactivity from treated leaves (40%) than did controls (24%), a pattern consistent with that of leaves treated be- low the defoliated zone. As a percent of the total 14C re- covered, defoliated plants allocated over five times more material for new leaves above the defoliated zone than did controls. Twice as much activity was allocated to the stem plus petiole component in the fed zone on defoliated plants, but only about 20% as much was recovered in the same com- ponent below the defoliated zone. Defoliated plants allo- cated double the amount of photosynthate, on a percentage basis, to the stem and petioles above the defoliated zone. In contrast to control plants, roots and cuttings received only a small percentage of total 14C on defoliated plants. Similar patterns of translocation are indicated by the specific activity data (Table 5.3). Though leaves in the 129 .mOHOHuOQ new name moosaocH m Hmm NHN Hock mqmm NHm comm Oflqw 0N AcmBZOU m m moqw ommm mm NNNmH mmmm ma DMH0m< ZHEHHE 304mm M>Om< ZHIHH3 BOAmm UZHHHDU mBOOm MEMHm mm>umn mSOfiumOHHaOH oSu mo momma mum mmuswfih .Amm Hmv coaumHHOwov noumm an em Nooqfi Ou womoaxo ouo3 econ cmumfiHOWOO Gnu Ev mo>mOH :o:3 madman Headed Scum oq~ we coauonfiuumfin .m.m manna 130 defoliated zone had only 15% less total activity in defoli- ated plants, their specific activity was only about half that of controls. The highest specific activity was recorded for new leaves above the defoliated zone on defoliated plants. They had almost double the specific activity of the new leaves in control plants. Distribution of 140 Within PI 35 Plants. Plants reached PI 35 about three weeks after defoliation. At this stage, about 10 new leaves had formed above the defoliated zone. The deve10ping leaf zone essentially coencided with the section above the defoliated zone, though the upper 2 to 3 leaves in the defoliated zone were not quite mature. These leaves on defoliated plants in the defoliated zone had ac- cumulated some photosynthetic surface through expansion of tissue left adjacent to the midvein. Leaves in the section below the defoliated zone consisted entirely of mature leaves, most of which had been mature for three weeks or longer. There were no detectible signs of senescence on any of these leaves at the time they were treated. Leaves exposed to 14C02 below the defoliated zone ex— ported about 50% of the total activity recovered (Table 5.4), with little difference between defoliated and control plants. 131 .mOHOfiuom pom Oumw mowaaocH m mNN mom mmw me «ma HNOH 50H we «mam AOMHZOU wofi mmm qum mam mHm Homfi One an momm QMHom< szHHB 304mm m>om< ZHEHHZ 30amm whoomm UZHHHDU meoom Aumn maoauOOHHaou 03» mo momma one newsman .Amm Hmv coaumfiaomow neuwm mx063 moucu Nov3 ou womoaxo mama aowumfiaomov we meow onu BONOQ mm>moa eons mucmam Headed Eoum 03 mo coauonfiuumfia q.m OHQOH As a percentage of the total 14C recovered, leaves above the defoliated zone received four times more 14C—photosynthate in defoliated plants than in control plants. The stem plus petiole component on defoliated plants contained a greater percentage of activity recovered in all three zones, with over 80% more translocated to the section above the defoli- ated zone. By PI 35, defoliated plants had produced lateral branch- es, mainly in the lower portion of the defoliated zone and in the upper portion of the section below it. Control plants had produced few lateral branches. Defoliated plants allo— cated a much greater portion of their 14C-photosynthate for lateral branch production, whereas allocations to roots and cutting on defoliated plants were only about half that shown by controls. Specific activities reflected a similar pattern. Shoots above the defoliated zone and lateral branches on defoliated plants were stronger sinks, and roots and cutting were weaker sinks than the same components in control plants. Leaves in the defoliated zone exposed to 14 C02 three weeks after defoliation contained 42% to 54% of the total activity recovered (Table 5.5). Defoliated plants exported 133 .mmaoauoa new wump movaaocH m 05H «mo wmoq @mHH mmw woe qu quN Hm Homezoo on Hm mmmofi comm mmmm «Ne awe mmmw mH QMHom< szBHB 304mm m>om< szHHS 304mm mHoomm . UZHHHDU mHoom Auma meoquOfiHaou can we momma mum mouowfim .Amm Hmv coaumfiHOMOv umuwm mxoo3 mounu Nov3 ou wowedxo mums moon moumfiaomoo onu rm mo>moa cons mucmam headed Beam Dem we aoausnauumwm .m.m OHan 134 a greater percentage (58%) than controls (46%). Defoliated plants allocated a much greater percentage of their total activity to leaves above the defoliated zone than did con- trols Also, a greater percentage of the total activity in defoliated plants was contained in the stem plus petiole component above the defoliated zone than for the same section in control plants. The stem plus petiole component in the defoliated zone on defoliated plants received 3 times more photosynthate, as a percentage of total 14C recovered, than the same component on controls. Lateral branches received a much greater proportion of the total incorporated activity on defoliated plants, whereas roots and cutting received substantially lower amounts on defoliated plants. Specific activity data indicated the defoliated zone to be a strong sink for assimilates on defoliated plants. Leaves in this zone retained a_significantly higher concen— tration of activity than did the complementary leaves on control plants. Specific activity in the subtending stem and petioles was nearly five times higher. The region above the defoliated zone and lateral branches were also stronger sinks for assimilates on defoliated plants, but roots and cuttings on defoliated plants were apparently weak sinks as 135 compared to control plants. Leaves fed with 14C02 above the defoliated zone con— tained 92% to 95% of the total activity recovered (Table 5.6). Most of the 14C transported from treated leaves was recovered from the subtending internodes and petioles, with slightly more transported by defoliated plants. Except for treated leaves, specific activities were higher in defoliated than control plants. Distribution of 140 Within PI 50 Plants. Plants attain- ed a PI of 50 some five weeks after defoliation. At this stage, the deve10ping leaf zone consisted of the first 13 to 15 leaves from the apex. Thus, the section above the defoliated zone contained not only the developing leaf zone, but also 10 or more recently mature leaves. Leaves in the other two zones were fully mature. The lowest leaves on the plant were beginning to senesce, though this was occurring to a much greater degree on control plants. Both control and defoliated plants were beginning to produce some lateral shoots at the base of the section above the defoliated zone. These were slightly larger and more numerous on control plants than in defoliated plants. In general, defoliated plants exported more 136 .mOHOHan pom mumw mowsaocH m e N OH omq~ we 0 mmHmH m mm AMOHZOU o mH wNN Hmma mm~ ma mmwm «H mm QMHom< ZHSHHB 304mm m>om< szHHZ 304mm mHOOEm UZHHHDU whoom Aums meoquOHHQOH oBu mo momma mum mouswfim .Amuamaa mm Hmv mooS ou vmmoaxo muo3 cofiumfiaowov Ou ucmsvmmnsm mxmos Gonna mnu ca wouovoua mo>moa 36: cons madman weaned Beam o.»H mo coauonfiuumfin .o.m manna 137 l4C-photosynthate from treated leaves than did control plants, and specific activities paralleled the pattern of percentage of total 14C recovered. Patterns of translocation in defoliated plants were becoming similar to that of control plants, but important differences occurred. When leaves were exposed to 14CO2 below the defoliated zone (Table 5.7), the major differences occurred in the amounts allocated to lateral branches and roots. Defoliated plants incorporated over 13% of their total activity (24% of that exported from fed leaves) into lateral branches, whereas controls allocated almost nothing for this component. Later- al branches on defoliated plants were also the most active sinks for transported material, showing considerably higher specific activities than this component on control plants. Defoliated plants incorporated far less activity into root biomass than did controls, but amounts incorporated into the cutting were similar. Defoliated plants also incorporated a greater percentage of 14C-photosynthate into lateral shoots, but less into roots and cutting than controls when leaves were exposed to 14C02 in the defoliated zone (Table 5.8). Specific activity of the treated leaves on defoliated plants was almost five times 138 .mOHoauoa now must mowoaocH m HmH «mm mm as mm «me am NH ofiqm qomezou now «am Hmo~ me mm 0mm «N «H omom QmHom< szHHz zoqmm m>om< szHHz 304mm whoomm ozHHHDo meoom Aum£ macaumo IHHQOH o3u mo momma mum mouswfim .Aom Hmv coaumfiaowmc umumm mxoos o>wm moo3 ou vowedxo ones coaumHHOMOv mo meow ecu SONOQ wo>mOH c053 mucmHa Headed Eoum cod no cowuonfiuumfio .n.m OHQOH 139 .mmaofiuom new mumw mOODHOcH w «mm qu mmmm mm mam 00m mm mwNH Hm 4009200 mmm 00m “mom 00 00mm mmm 5m 00H0 Hm 0mH0m< szHHz 300mm m>0m< szHHB 304mm meoomm I UZHHHDU meoom Aumn waoauOOHaaou ozu mo momma mum mouswfim .Aom Hmv coaumHHOwov amuwm mxoo3 O>Hw N00j ou pomoaxo whoa econ woumfiaowov Gnu rm mm>eoa c003 madman weaned Eoum 06H mo cowuanfiuumfin .w.m manna 140 greater than in the same leaves on control plants. The stem plus petiole component in the defoliated zone also showed much higher specific activities on defoliated plants, indi- cating it, too, was a much stronger sink than the same com— ponent on controls. Leaves fed with 14C02 above the defoliated zone (Table 5.9) contained about 80% of the total activity recovered. Most of the material transported from these leaves was re- covered in the subtending stem and petioles, slightly more being transported from leaves on defoliated plants. General- ly there were no diffeerences between defoliated and control plants in allocation of 14C-photosynthate, though specific activities were higher in all components, except lateral shoots, on defoliated plants. DISCUSSION It is generally accepted that distribution of photo- synthate in plants follows a source-sink relationship, with assimilates moving from sources to sinks (25). Photosynthe- sizing leaves typically constitute the source and any growing, storing or metabolizing tissue which can attract 141 .mOHOfiuoa Mom mumm mowsaocH m 00 OMH N00 H00 mofi 0m «moH w e AOMHZOU HNH mma OHN won NqH mNH 000a mH mH 0mHom< szHHS 304mm m>om< szHHB 304mm mHoomm UZHHHDU mHoom Aumn mcoquOfianou oBu mo mamoa one newsman .Amuaman 0m H00 N006” Ou vomoaxo ouo3 :OHuOfiHOMOv ou acosv Immnsm mxooa o>Hm may :a noosnoua mo>moa 30C :053 muamaa HOHQOQ 80pm qu we coausnuuuwaa .m.n manna 142 photosynthate in competition with other plant parts may be the sink. It has previously been established that transport in cottonwood conforms to this model (43,103,105). The distribution of 14Cephotosynthate in the pOplar plants used in this study was similar to that described for eastern cottonwood (105). When mature leaves were exposed to 14C02, as in the zone below the defoliated zone, they transported primarily downward to roots and the cutting or incorporated photosynthates into adjacent stem wood. When both mature and immature leaves together were exposed to 14C02, there was considerable bidirectional movement. The proportions exported downward, upward, or remaining in the treated zone depended on the proportion of mature and de- veloping leaves treated. At P1 35, about 75% of the leaves in the defoliated zone were mature or recently mature and equal amounts of activity were exported downward, upward, and to stem and petioles in the treated zone. At PI 50, the section above the defoliated zone contained about two thirds immature leaves and one third mature. About 25% of the ex— ported material was translocated downward, 25% upward, and about 50% was incorporated into the stem and petioles in the treated zone. 143 When only deve10ping leaves were treated, as in the defoliated zone at PI 25, or the zone above defoliation at P1 35, much of the 14C was incorporated into plant parts in the treated zone. Most remained in the treated leaves, and of that exported, the majority was incorporated into subtending stem and petioles. Defoliation caused a redistribution of assimilates, with certain sink regions taking on greater importance, although the general patterns of distribution outlined by Larson and Gordon (105) were not affected. Defoliation altered the quantity of assimilates translocated from treated leaves, the amount transported to the region above the defoliated zone, and the amounts allocated to lateral branches and roots. 14C from exv Defoliated plants exported relatively more posed leaves than did control plants, and they also incor~ porated a greater percentage of acitivity into the growing region above the defoliated zOne. The relative strength of this apical sink was also demonstrated in the higher spec« ific activities in this zone on defoliated plants. Rangnekar and Forward (150) reported a similar effect in defoliated Pinus resinosa trees. Slightly more 14C activity was 144 incorporated into leaves than stems and petioles, particular- ly right after defoliation. This effect seemed to be independ- ent of the zone treated with 14CO2 or age of the plant. Leaves treated below the defoliated zone exported less as- similate to the growing region above the defoliated zone on both controls and defoliated plants, the roots being in closer proximity and consequently stronger sinks (105). The observed translocation patterns are very supportive of the growth data reported in Chapter III, which showed in- creased stem elongation and leaf growth above the defoli- ated zone. Expanding lateral branches became very strong sinks on defoliated plants. Not only was a greater percentage of fixed 14C allocated to them, but specific activities were also higher. Defoliated plants generally produced more lateral branches than controls (Chapter III). Defoliation causes increased lateral branch production by interrupting the supply of auxin which is normally produced in young leaves, therefore releasing lateral buds from apical con- trol (50,52,53). These growing laterals result in new sinks and consequently a greater amount of photosynthate is 3110- cated to them. 145 Defoliation caused a reduction in assimilate incorp- oration into roots and cuttings. Though this effect was not evident in plants exposed to 14C02 24 hours after defoli- ation, it was an important factor by three weeks after de- foliation and was still evident on plants harvested five weeks after defoliation. Only about half as much assimilate was allocated to roots and cuttings on defoliated plants at these stages. Defoliation did result in reduced root bio- mass (Chapter III), and the reduced translocation of assimiv lates to the roots seems the probable cause. Numerous stud— ies have shown that defoliation results in reduced root carbohydrate levels (87,143,145,146,194). That translo- cation to the roots and cutting after exposure to 14C02 at P1 25 caused a greater portion to be allocated to roots on defoliated plants is difficult to reconcile in light of the reduced root and cutting biomass of these plants. The first changes in translocation patterns occurred within a few days of defoliation and were mainly confined to increased allocation to the region above the defoliated zone. Several weeks after defoliation, the maximum effects were observed. By five weeks after defoliation, however, translocation patterns were becoming similar again in both 146 defoliated and control plants. There was still considerably less 14C-assimilate being allocated to the roots and cut- tings at this time, and lateral shoots were still a stronger sink on defoliated plants, but other differences were now minimal. This study indicates that young poplars respond to partial defoliation by shifting normal translocation pat- terns such that less photosynthate is allocated for storage in, or growth of, the root system in favor of compensatory growth in the above ground portions. The long term effects of this response on growth and survival in the field is un- known, but needs to be investigated. It seems likely that the shortfall of photosynthate translocated to the root system of defoliated plants would put those plants at a dis- advantage with competing vegetation, especially if more than one defoliation occurred. It is also probable that COppicing ability could be reduced. CHAPTER VI SUMMARY AND CONCLUSIONS Intensive, short rotation culture of forest trees for maximum fiber production may contribute to the alleviation of predicted wood shortages, particularly in the Lake States. Success of these systems depends on minimizing all stresses to the growing crop. Most Operations are entirely mechanized and resulting profit margins are low. Any factor which re- duces growth below its maximum potential in a crop rotation may render the operation uneconomical. Defoliating insects are a particular problem in this re- gard, removing the photosynthesizing leaf tissue that is the basis for wood production. At the very minimum, periodic grazing by these insects could reduce growth below its max- imum potential, and at the maximum, it could result in heavy mortality. Repulsion of insect attack using pesticides may be necessary, but a better and less expensive alternative would be to develop cultural methods which allow trees to take advantage of site and their own resistance factors to keep damage by defoliating insects at an acceptable level. The purpose of the present research was to quantify the 147 148 effects of defoliation on the growth of young hybrid pOplars and to define its phySiological impact. Data were obtained from several field and controlled environment studies. Two, three year field studies were conducted in northern Wisconsin. The first experiment examined growth impact on young Populus plants in relation to severity, timing and re- currence of defoliation. The combination of first year de- foliation (0%,40%, or 80%), timing of the first year defoli- ation (July or August) and level of the second year defoli— ation were the most important factors affecting growth. Re- sults indicated that height growth of trees receiving a 40% defoliation was unaffected. The most severe reduction in growth was observed when trees received an 80% defoliation in August of the first year followed by 80% defoliation in the second year. Timing of the second year defoliation was un— important. Trees defoliated to 80% in July of the first year followed by 80% defoliation in the second year displayed no significant reduction in growth, regardless of timing of the second year defoliation. In a second experiment, five different methods of in- flicting 75% defoliation were evaluated for three different Populus clones. No growth differences among the defoliation 149 methods were observed. Since defoliation was performed early in the season, it reaffirmed results of the first study. Several controlled environment studies were performed to test the effect of a single defoliation on growth, photo- synthesis and translocation of 14C-photosynthates. Plants were defoliated at a plastochron index (PI) of 25 or 30 by removing all of the lamina except a 2 mm strip on each side of the midvein. In the first experiment, height, and dia- meter growth was monitored in relation to five levels of defoliation plus an undefoliated control. When harvested at a PI of 45, defoliated plants were slightly taller than controls with the heaviest defoliation treatment having the tallest plants. Defoliated plants also tended to have greater stem diameters and produced more leaves and lateral branches than control plants. In a second experiment, only one level of defoliation was tested; removal of lamina along the entire developing leaf zone. Plants were defoliated at P1 25 and harvested either three days, three weeks, or five weeks after defoli- ation. Insignificant differences in height and diameter growth was observed between defoliated and control plants, 150 but the latter displayed a reduction in dry weight. The defoliation treatment stimulated a two—fold increase in lateral branch development over control plants. Leaves de- ve10ping subsequent to defoliation were 30% larger than com- plementary leaves on control plants. Photosynthesis, leaf conductance and 14C-translocation patterns were also determined on these plants. Photosynthe- sis was calculated from uptake of a 20 5 pulse of 14C02 ad- ministered by a portable gassing device. Leaf conductance was measured with a diffusion porometer. Translocation pat- terns were determined by exposing leaves below, within, or above the defoliated zone to 14C02 and determining the 14C distribution within the plant after 48 hr. Leaves on defoliated plants had significantly higher rates of photosynthesis than complementary leaves on control plants. There was a stimulation of photosynthetic rates on mature leaves remaining on the plants below the defoliated zone within 24 hr of treatment. This effect continued up to five weeks. New leaves produced after defoliation were larger and also had higher photosynthetic rates than new leaves on control plants. Leaf conductance was also higher on defoliated plants and followed a pattern similar to 151 photosynthesis. Specific leaf weight was increased slightly on leaves below the defoliated zone, but was significantly reduced on leaves produced after defoliation. Translocation patterns were also altered within 24 hr after defoliation. When leaves below or remaining leaf tis- sue within the zone of defoliation were exposed to 14C02, more 14C-photosynthate was transported to the expanding shoot and lateral baranches and less to the roots in defoliated plants than in controls. No differences were observed be- 14 tween defoliated and control plants in C distribution when leaves produced subsequent to defoliation were exposed to 14C02. After five weeks, differences in patterns of 14C distribution between defoliated and control plants were re- duced. These results substantiated the biomass partitioning data. It appears that young poplar plants can withistand a single, relatively heavy defoliation with little or no loss in shoot growth. In fact, at least one study showed shoot growth to be slightly stimulated by defoliation. The most evident response to defoliation was increased leaf area pro- duction, with the size of leaves substantially larger than normal. Accompanying this increased leaf growth was an 152 increased shunting of assimilates to the apical region of the plant and increased photosynthetic rates on the leaves and leaf tissue remaining after defoliation. Later, defoli- ated plants responded by increased lateral branch production and continued production of larger leaves above the defoli- ated zone. More assimilate was translocated to the lateral branches and to the shoot above the defoliated zone, and photosynthetic rates remained higher on defoliated plants. Increases in the above ground growth apparently occur at the expense of root growth, which was significantly recuced three weeks after defoliation. Lower root biomass was ac- companied by reduced translocation of assimilates to these sinks. By five weeks after defoliation, defoliated plants were returning more to a state of normalcy in terms of growth, however, root growth was still reduced and photosynthetic rates remained higher. The findings of the research reported here indicate the need for continued investigation to elucidate the defense mechanism of young Populus plants to defoliation. For ex- ample, it would be instructive to learn if anatomical changes accompany the increased leaf size of leaves produced after 153 defoliation. Also left to test is whether increased rates of photosynthesis in leaves of defoliated plants are ac- companied by increased dark respiration and photorespiration rates. Thus, some form of experiment on net gas exchange is indicated. In this regard, it would also be informative to have data on changes in leaf chlorOphyll and protein content, particularly changes in RuBP carboxylase/oxygenase. Trans- location patterns should also be examined on an individual leaf and location within leaf basis to more specifically isolate the effects of defoliation on redistribution of as- similates. Some role for growth regulators in the observed respon- ses is likely and should be investigated. Auxins are known to be important in shoot elongation. Increased lateral branch growth observed in these experiments as a result of defoliation probably resulted from removal of apical con- trol exerted over lateral buds by young deve10ping leaves. Young leaves are known to be major sites of gibberellin syn- thesis, and these compounds are known to increase stem elongation with no concominant increase in dry weight. Abscisic acid is produced in leaves in response to stress and may increase plant resistance to such stresses. 154 Cytokinins are also abundant in young expanding leaves, though they are probably produced in the roots. These com- pounds are known to promote cell division and lateral bud development, promote chloroplast development, and delay senescence. Cytokinins have also been implicated in the production of RuBP carboxylase/oxygenase and may be the direct cause of increased photosynthetic rates Observed in defoliated plants. BIBL IOGRAPHY BIBLIOGRAPHY Abrahamson, L.P, R.C. Morris and N.A. Overgaard. 1977. Control of certain insect pests in cottonwood nurser- ies with the systemic insecticide Carbofuran. J. Econ. Entomol. 70:89-91. Alderfer, R.G. 1974. Photosynthesis in deve10ping plant canopies. pp 227-228 in D.M. Gates, ed. Per- spectives in BiOphysical Ecology. Springer Verlag, New York. Alderfer, R.G. and C.F. Eagles. 1976. The effect of partial defoliation on growth and photosynthetic ef- ficiency of bean leaves. Bet. 032. 137:351-355. Anderson, H.W. 1979. Biomass production of hybrid poplar grown in minirotation. Report 11 in D.C.F. Fayle, L. Zsuffa and H.W. Anderson, eds. Poplar Research, Management and Utilization in Canada. Ont. Min. Natur. Resour., Forest Res. Inform. Pap. No. 102. 13 pp. Arru. G.M. 1964-65. Studies on the morphology and biology of Pygaera anastomosis (L.). Bull. 2001. Agrar. Bachicoltura 2:206-272. Arru, G.M. 1975. Report of the working party on in- sects, FAO/Int. Poplar Comm. F0:CIP/75/41. Rome, Italy. Austin, R.B. and P.C. Longden. 1967. A rapid method for the measurement of rates of photosynthesis using 14002. Ann. Bot, N.S. 31:245-253. Avery, D.J. and J. Lacey. 1968. Changes in the growth regulator content of plum infested with fruit tree red spider mite, Panonychus ulmi (Koch). J. Exp. Bot. 19:760-769. 155 10. 11. 12. 13. 14. 15. l6. l7. 18. 156 Baker, W.L. 1941. Effect of gypsy moth defoliation on certain trees. J. For. 39:1017-1022. 1972. Eastern Forest Insects. USDA Forest Service Misc. Publ. 1175. 642 pp. Barter, G.W. and 0.6. Cameron. 1955. Some effects of Defoliation by the forest tent caterpillar. Can. Dept. Agric., Bi-mon. Prog. Rep. ll(6):1. Bassman, J.H. and W.L. Myers. 1978. Growth response of three Populus clones subjected to simulated de- foliation. Abstract in C.A. Hollis and A.E. Squil- lace, eds. Proc. 5th North Amer. For. Biol. Work- shop, March 13-15, 1978, Gainesville, Fla. p 413. and D.I. Dickmann. 1979. Growth and dry weight of young Populus trees subjected to de— foliation in the developing leaf zone. Abstract in Plant Physiology (supplement) 63:106. Batzer, H.0. 1955. Effects of defoliation by the forest tent caterpillar. Entomol. Soc. Amer. North Cental Branch Proc. 10:27-28. , A.C. Hodson, and A.E. Schneider. 1954. Results of an inquiry into the effects of defoliation of aspen trees by the forest tent caterpillar. Minn. Forest. Note 31. 2 pp. , and R.C. Morris. 1978. Forest tent caterpillar. USDA Forest Service, Forest Insect and Disease Leaflet 9. 8 pp. Belanger, R.P. 1973. Volume and weight tables for plantation grown sycamore. USDA For. Serv. Res. Pap. SE 107. Bilan, M.V. and A.K. Kemp. 1962. Effect of gibberellin on growth and development of loblolly pine seedlings. J. For. 60:391-394. 19. 20. 21. 22. 23. 24. 25. 26. 27. 157 Briscoe, C.B. 1969. Establishment and early care of sycamore plantations. USDA For. Serv. Res. Pap. SO-50. Brown, W.S. 1956. The new world species of Chryso- mela L. (Coleoptera:Chrysomelidae). Can. Entomol. 88:suppl. 3. 54 pp. Bruner, L. 1890. Insects injurious to young trees in tree claims. Nebr. Agric. Exp. Sta. Bull. 14:83-149. Burkot, T.R. and D.M. Benjamin. 1979. The bionomics of the cottonwood leaf beetle, Chrysomela scripta Fab., on tissue culture hybrid poplars. Pp. 131-135 in Proc. 13th Lake States For. Tree Improv. Conf., U.S. For. Serv. Gen. Tech. Rep. NC-SO. Caldbeck, E.S., H.S. McNabb, Jr., and E.R. Hart. 1978. Poplar clonal preferences of the cottonwood leaf beetle. J. Econ. Entomol. 71:518-520. Cannell, M.G.R. and S.C. Willett. 1976. Shoot growth phenology, dry matter distribution and rootzshoot ratios of provenances of Populus trichocarpa, Picea sitchensis and Pinus contorta growing in Scotland. Silv. Genet. 25:49-59. Canny, M.J.P. 1975. Mass transfer. Pp 139-153 in M.H. Zimmermann and J.A. Milburn, eds. Transport in Plants 1. Phloem Transport. EncyclOpedia of Plant Physiology. Springer Verlag, New York. Carlisle, A., A.H.F. Brown, and E.J. White. 1966. Litterfall, leaf production and the effect of de- foliation by Turtix vitidana in a senila oak (Quercus petraea) woodland. J. Ecology 54:65-86. Cazacu, I., and A. Fratian. 1966. The need for eco- nomic calculations in Operations to control insect defoliators (in Rumanian, English summary). Rev. Padurilor. 81:466-472. (Forestry Abstr. 28 N0. 4138). 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 158 Churchill, C.B., H.H. Johns, D.P. Duncan and A.C. Hodson. 1964. Long term effects Of defoliation of Aspen by the forest tent caterpillar. Ecology 45:630-633. Coster, J.E. 1973. Systemic insecticides effective against pOplar tentmaker in cottonwood plantations. Tree Planters Notes 24:33-34. Coulombe, L.-J., and R. Paquin. 1959. Effects de l'acide gibberellique sur le metabolisme des plantes. Can. J. Bot. 37:897-901. Curtis, J.D. and N.R. Lersten. 1974. Morphology, seasonal variation, and function of resin glands on buds and leaves of Populus deltoides (Salicaceae). Am. J. Bot. 61:835-845. Das, T.M. 1968. Physiological changes with leaf senescence: kinins on cell ageing and organ sene- scence. Pp 91-102 in S.M. Sircar, ed. Proc. Int. Symp. Plant Growth Substances. Calcutta Univ. Davidson, A.G. and R.M. Prentice. 1968. Insects and diseases. Pp 116-144 in J.S. Maini and J.H. Cayford, eds. Growth and Utilization of POplars in Canada. Can. Dept. For. Rural Dev. Publ. 1205. Dawson, D.H. 1976. History and oganization of the maximum wood yield program. Pp 1-4 in Intensive Plantation Culture: Five Years Research. USDA For. Serv. Gen. Tech. Rep. NC-Zl. , and J.B. Crist. 1972. Dry weight yields and wood anatomy Of 2 Populus clones. 6th TAPPI Forest Biol. Conf. Abstr. III-3. Dickmann, D.I. 1971. Photosynthesis and respiration by developing leaves of cottonwood (Populus deltoides Bartr.). Bot. Gaz. 132:253-259. 1975. Plant materials apprOpriate for intensive culture of wood fiber in the North Central region. Iowa State J. Res. 49:281-286. 38. 39. 40. 41. 42. 43. 44. 45. 159 Dickmann, D.I. 1979. Physiological determinants of poplar growth under intensive culture. Report 12 in D.C.F. Fayle, L. Zsuffa, and H.W. Anderson, eds. Poplar Research, Management and Utilization in Canada. Ont. Min. Natur. Resour. Forest Res. Inf. Pap. No. 102. , and D.H. Gjerstad. 1973. Application to woody plants of a rapid method for determining leaf C02 compensation concentrations. Can. J. For. Res. 3:237-242. ' , D.H. Gjerstad, and J.C. Gordon. 1974. DevelOpmental patterns of C02 exchange, diffusion resistance and protein synthesis in leaves of Populus x euramericana. Pp. 171-181 in R. Marcelle, ed. Environmental and Biological Control of Photosynthe- sis. Dr. W. Junk b.v., Publishers, The Hague. , and J.G. Gordon. 1975. Incorporation of 1ZIC-photosynthate into protein during leaf dev- elopment in young Populus plants. Plant Physiology 56:23-27. , K.W. Gottschalk, and J.H. Bassman. 1979. Physiological studies of growth and develop- ment of young hybrid pOplars. Pp 123-132 in Proc. 1979 North American Poplar Council Meeting, Thomp- sonville, Michigan. Dickson, R.B. and P.R. Larson. 1975. Incorporation of 14C-photosynthate into major chemical fractions of source and sink leaves of cottonwood. Plant Physiology 56:185-193. Dils, R.B. and M.W. Day. 1950. Effect of defoliation upon the growth of aspen. Mich. Agric. Exp. Sta. Quarterly Bull. 33:111-113. Duncan, D.P. and A.C. Hodson. 1958. Influence of forest tent caterpillar upon the aspen forests of Minnesota. For. Sci. 4:71-93. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 160 Duncan, D.P., A.C. Hodson, A.E. Schneider, H. Batzer, R. Froelich, D Myer, and C. Shive. 1956. In- fluence of the forest tent caterpillar (Malacosoma disstria an.) upon the aspen forest of Minnesota. Minn. Off. Iron Range Res. Rehab. Publ. 45 pp. Eagles, C.F. and P.F. Wareing. 1964. The role of growth substances in the regulation of bud dormancy. Physiol. Plant. 17:697-709. Einspahr, D.W. and J.P. van Buijtenen. 1961. The influence of gibberellic acid on growth and fiber length in quaking aspen. For. Sci. 7:43-51. Eliasson, L. 1968. Dependence of root growth on photo- synthesis in Populus tremula. Physiol. Plant. 21:806-810. 1969. Growth regulators in Populus tremula I. Distribution of auxin and growth in- hibitors. Physiol. Plant. 22:1288-1301. 1971. Adverse effect of shoot growth on root growth in rooted cuttings of aspen. Physiol. Plant. 25:268-272. 1971. Growth regulators in Populus tremula III. Variation in auxin and inhibitor level in roots in relation to root sucker formation. Physiol. Plant. 25:118-121. 1971. Growth regulators in Populus tremula IV. Apical dominance and suckering in young plants. Physiol. Plant. 25:263-267. Ellison, L. 1960. Influence of grazing on plant suc- cession of rangelands. Bot. Rev. 26:1-78. Embree, D.G. 1967. Effects of the winter moth on growth and mortality of red oak in Nova Scotia. For. Sci. 13:295-299. 56. 57. 58. S9. 60. 61. 62. 63. 64. 161 Engelbrecht, L. 1971. Cytokinin activity in larval infected leaves. Biochem. Physiol. Pflanzen. 162:9-27. , U. Orban, and W. Hesse. 1969. Leaf miner caterpillars and cytokinins in the green islands of autumn leaves. Nature 223:319-331. Fransen, J.J. and G. Houtzagers. 1946. Loss of in- crement as a result of defoliation, and the season growth of poplars (Dutch). Ned. Boschb. Tijdschrift 18:36-39. (Forestry Abstracts 11, No. 608). Froelich, R., A.C. Hodson, A.E. schneider, and D.P. Duncan. 1951. Influence of aspen defoliation by the forest tent caterpillar in Minnesota on the radial growth of associated balsam fir. Minn. Forest. Note 45. 2 pp. , C. Shive, D.P. Duncan, and A.C. Hodson. 1956. The effect of rainfall on the basal area growth of aspen as related to defoliation by the forest tent caterpillar. Minn. Forest. Note 48. 2 pp. Furukawa, A. 1975. Comparison of photosynthesis, post- illumination C02 outburst, and C02 compensation in poplar varieties, sunflower and bean. J. Jap. For. Soc. 57:268-274. Goodwin, R.H. 1937. Role of auxin in leaf deve10p- ment in Solidago species. Am. J. Bot. 24:43 Graham. S.A., R.P. Harrison, Jr., and C.B. Westell,Jr. 1963. Aspens: Phoenix trees of the Great Lakes region. Univ. Michigan Press, Ann Arbor, Michigan 272 pp. Grigal, D.P., L.F. Ohmann, and R.B. Brander. 1976. Seasonal dynamics of fall shrubs in northeastern Minnesota: Biomass and nutrient element changes. For. Sci. 22:195-208. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 162 Gross, H.L. and M.J. Larsen. 1971. Nutrient content of artificially defoliated branches of Betula paperifera. Phytopath. 61:631-635. Habeshaw, D. 1973. Translocation and the control of photosynthesis in sugar beet. Planta 110:313-226. Hackett, C. 1973. An exploration of the carbon econ- omy of the tobacco plant I. lnferences from a simulation. Aust. J. Biol. Sci. 26:1057—1071. Hadzi-Greogiev, K. 1974. Effect of attack by Gypsonoma acerianan on the growth and increment of poplar stems (Servo-croatian, French summary). TOpola 18:23-34. Haissig, B.E. 1972. Near total recovery of small tissue samples after milling. For. Sci. 18:262-262. Hanover, J.W. 1975. Physiology of tree resistance to insects. Annual Review of Entomology 20:75-95. Harlow, R.P. and L.K. Hills. 1972. Response of yellow poplar and dogwood seedlings to clipping. J. Wildl. Mgmt. 36:1076-1080. Harris, P. 1973. Insects in the population dynamics of plants. Pp 201-209 in Insect Plant Relationships. Symp. Royal Entomol. Soc. Lond. 6. Hartt, C.B., H.P. Kortschak and G.O. Burr. 1964. Effects of defoliation, deradication and darkening the blade upon translocation or C14 in sugar cane. Plant Physiology 39:15-22. Haukioja, E. and P. Niemela. 1979. Birch leaves as a resource for herbivores: Seasonal occurence of increased resistance in foliage after mechanical damage of adjacent leaves. Oecologia (Berl.) 39:151-159. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 163 Head, R.H. and W.W. Neel. 1973. The cottonwood leaf beetle: Observations on the biology and reproductive potential in Mississippi. J. Econ. Entomol. 66:1327- 1328. Herrick, A.M. and C.L. Brown. 1967. A new concept in cellulose production: Silage sycamore. Agric. Sci. Rev. 5:8-13. Hewett, E.W. and P.F. Wareing. 1973. Cytokinins in Populus robusta : Changes during chilling and bud burst. Physiol. Plant. 28:393-399. and P.F. Wareing. 1973. Cytokinins in Populus robusta : Qualitative changes during devel- Opment. Physiol. Plant. 29:386-389. and P.F. Wareing. 1973. Cytokinins in Populus robusta : A complex in leaves. Planta 112:225-233. Hildahl, V. and W.A. Reeks. 1960. Outbreaks of the forest tent caterpillar, Malacosoma disstria an., and their effects on stands of trembling aspen in Manitoba and Saskatchewan. Can. Entomol. 92:199- 209. House, W.P. 1963. Gypsy moth-white pine damage study. In N. Turner, ed. Effect of defoliation by the gypsy moth. Conn. Agric. Exp. Sta. Bull. 658. 30 pp. Incoll, L.D. 1977. Studies of phtosynthesis: monitor- ing with 14C02 . Pp. 137-155 in J.J. Landsberg and C.V. Cutting, eds. Environmental Effects on CrOp Physiology. Academic Press, London. and L.D. Wright. 1969. A field technique for measuring photosynthesis using l4-carbon dioxide. Spec. Soils Bull. Conn. Agric. Exp. Sta. No. 30. Isebrands, J.G. and P.R. Larson. 1973. Anatomical changes during leaf ontogeny in Populus deltoides. Am. J. Bot. 60:199-208. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 164 Isebrands, J.G., L.C. Promnitz and D.H. Dawson. 1977. Leaf area development in short rotation intensive cultured Populus plots. Pp. 201-210 in Proc. TAPPI Forest Biol. Wood Chem. Conf. Jensen, K.F., and L.S. Dochinger. 1972. Gibberellic acid and height growth of white pine seedlings. For. Sci. 18:196-197. and R.C. Masters. 1973. Effect of de- foliation on carbohydrate content of yellow pOplar seedlings. USDA For. Serv. Res. Note NE-l79. 2 pp. Johnson, B.E. and W.E. Brun. 1966. The effect of simulated caterpillar damage on C02 fixation and translocation in banana leaves. Physiol. Plant. 19:417-421. Joly, R. 1959. The influence of forest defoliators on increment (French). Rev. Forest. Fr. 11:775-784. (Forestry Abstracts 21, No. 2046). Jones, O.P. and H.J. Lacey. 1968. Gebberellin-like substances in the translocation stream of apple and pear trees. J. Exp. Bot. 19:526-531. Kamiéhska, A. 1971. Auxin content in developing leaves of black poplar trees (Populus nigra L.). Roczniki Nauk Rolniczych, Seria A, T. 97:13-21. Kamilovski, M. 1966. Determining the best time to destroy defoliators of poplar. God. Zborn. Zemi. Sum. Fak. Univ. SkOpje No. 19 157-86. (Forestry Abstracts 29, No. 2599). Kanemasu, E.T., G.W. thurtell, and C.B.Tanner. 1969. Design, calibration and field use of a stomatal diffusion porometer. Plant Physiology 44:881-885. Kimmins, J.P. 1972. Relative contributions of leach- ing, litter-fall and defoliation by Neodiprion sertifer (Hymenoptera) to the removal of cesium-134 from red pine. Oikos 23:226-234. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 165 King, R.W., J.P. Wardlaw, and L.T. Evans. 1967. Effect of assimilate utilization on photosynthetic rate in wheat. Planta 77:261-276. Kocher, H. and C.A. Leonard. 1971. Translocation and metabolic conversion of 14C-labled assimilates in detached and attached leaves of Phaseolus vulgaris L. in different phases of leaf expansion. Plant Physiology 47:212-216. Kozlowski, T.T. 1969. Tree physiology and forest pests. J. For. 67:118-123. Kriedemann, P.E., T.F. Neales, and D.H. Ashton. 1964. Photosynthesis in relation to leaf orientation and light interception. Aust. J. Biol. Sci. 17:591-600. Kulman, H.M. 1965. Effects of disbudding on the shoot mortality, growth and bud production in red and sugar maples. J. Econ. Entomol. 58:23-26. 1971. Effects of insect defoliation on growth and mortality of trees. Ann. Rev. Entomol. 16:289-324. Lamoreaux, R.J., N.R. Chaney, and K.M. Brown. 1978. The plastochron index: A review after two decades of use. Am. J. Bot. 65:586-593. Larson, P.R. 1969. Wood formation and the concept of wood quality. Yale University School of Forestry Bull. No. 74. and R.B. Dickson. 1973. Distribution of imported i4C in developing leaves of eastern cotton- wood according to phyllotaxy. Planta 111:95-112. , R.B. Dickson and J.G. Isebrands. 1976. Some physiological applications for intensive cul- ture. Pp 10-18 in Intensive Plantation Culture: Five Years Research. USDA For. Serv. Gen. Tech. Rep. NC-Zl. 105. 106. 107. 108. 109. 110. 111. 112. 113. 166 Larson, P.R. and J.C. Gordo 1969. Leaf develop- ment, photosynthesis and C distribution in Populus deltoides seedlings. Am. J. Bot. 56:1058-1066. and J.G. Isebrands. 1971. The plasto- chron index as applied to developmental studies of cottonwood. Can. J. For. Res. 1:1-ll. and J.G. Isebrands. 1972. The relation between leaf production and wood weight in first year sprouts of two populus clones. Can. Jl For. Res. 2:98-104. and J.G. Isebrands. 1974. Anatomy of the primary-secondary transition zone in stems of Populus deltoides. Wood Sci. and Tech. 8:11-26. , J.G. Isebrands, and R.B. Dickson. 1972. Fixation patterns of 14C within developing leaves of eastern cottonwood. Planta 107:301-314. Ledig, F.T. 1974. Concepts of growth analysis. Pp. 166-182 in Proc. 3rd North American Forest Biology Workshop, Fort Collins, Colorado. Lester, D.C., O.G. Carter, F.M. Kelleher, and D.R. Laing. 1972. The effect of gibberellic acid on apparent photosynthesis and dark respiration of simulated swards of Pennisetum cZaudestinum Hochst. Aust. J. Agric. Res. 23:205-213. Llewellyn, M. 1975. The effects of the lime aphid (EucaZZipterus tiZZiae L) (Aphidae) on growth of the lime (Tilia x vulgaris Hayne). II. The pri- mary production of saplings and mature trees, the energy drain imposed by the aphid populations and revised standard deviations of aphid population energy budgets. J. Applied Ecology 12:15-23. Ludwig, L.J. and D.T. Canvin. 1971. An open gas ex- change system for the simultaneous measurement of the C02 and 14C02 fluxes from leaves. Can. J. Bot. 49:1299-1313. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 167 Ludwig, L.J., T.Saeki, and L.T. Evans. 1965. Photo- synthesis in artificial communities of cotton plants in relation to leaf area. I. Experiments with pro- gressive defoliation of mature plants. Aust. J. Biol. Sci. 18:1103-1117. Luitjes, J. 1973. The effect of defoliation by Leucoma salicis on the growth of poplar. Nederlands Bosbouw Tijdschrift 45:45-53. Luukkanen, O. and T.T. Kozlowski. 1972. Gas exchange in six populus clones. Silv. Genet. 21:220-229. Madgwick, H.A.I. 1975. Effects of partial defoliation on the growth of Liriodendron tulipifera L. seed- lings. Ann. Bot. 39:1111-1115. Magnoler, A. and A. Cambini. 1970. Effects of arti- ficial defoliation on the growth of cork oak. For. Sci. 16:363-366. and Cambini. 1973. Radial growth of cork oak and the effects of defoliation caused by larvae of Lymantria dispar and Malacosoma neustria Boletin do Institute das Products Florestais, Cortica 35(413):53-59. Marcou, B., and I. Catrina. 1962. Experimental re- search on the causes of dieback in oak. Proc. Congr. IUFRO, 13th, Part 2, Sect. 24/13. 16 pp. Mattson, W.J. and N.D. Addy. 1975. Phytophagous insects as regulators of forest primary production. Science 190:515-522. McAlpine, R.C., C.L. Brown, A.M. Herrick and H.H. Ruark. 1966. Silage sycamore. Forest Farmer 26:6-7, 16 McKnight, J.S. 1970. Planting cottonwood cuttings for timber production in the South. USDA For. Serv. Res. Pap. SO-60. 124. 125. 126. 127. 128. 129. 130. 131. 132. 133. 168 McWilliam, J.R., P.J. Phillips and R.R. Parkes. 1973. ‘Measurement of photosynthetic rate using labeled carbon dioxide. CSIRO Aust. Div. Pl. Ind. Tech. Paper No. 31:1-12. Meidner, H. 1969. Rate limiting resistances and photosynthesis. Nature 222:876-877. 1970 . Effects of photoperiodic induction and debudding in Xanthium pennsylvanicum and Phase- oZus vulgaris on rates of net photosynthesis and stomatal conductances. J. Exp. Bot. 21:164-169. Melchior, G.H. and R. Knapp. 1962. Gibberellin- Wirkungen an Baumen. Silvae Genet. 11:29-39. Michael, D., D. Dickmann, and N. Nelson. 1979. Photosynthesis, C0 compensation and stomatal con- ductance of young poplar plants grown under inten- sive culture. Plant Physiology (supplement) 63:121. Minott, G.W. and I.T. Guild. 1925. Some results of defoliation of trees. J. Econ. Entomol. 18:345- 348. Monsi, M., Z. Uchijima and T. Oikawa. 1973. Structure of foliage canopies and photosynthesis. Ann. Rev. Ecol. Syst. 4:301-327. Morris, R.C. 1956. Leaf Beetle damage cottonwood trees in the Delta. MS Agric. Exp. Sta. Inf. Sheet 537. , T.H. Filer, J.D. Solomon, F.I. McCracken, N.A. Overgaard and M.J. Weiss. 1975. Insects and diseases of cottonwood. USDA For. Serv. Gen. Tech. Rep. S0-8. and F.L. Oliveria. 1976. Insects of periodic importance in cottonwood. Pp. 280-285 in C.B.A. Thielges and S.B. Land, Jr., eds. Proc. Symp. on Cottonwood and Related Species, Lousiana State University, Baton Rouge, LA. 169 134. Nagarajah, S. 1975. Effect of debudding on photo- syntheses in leaves of cotton. Physiol. Plant.~ 33:28-31. ' 135. Neales, T. F. and L. D. Incoll. 1968. The control of leaf photosynthesis rate by the level of as- similate concentration in the leaf: A review of- the hypothesis. Bot. Rev. 34:107-125. 136. , K. J. Treharne, and P. F. Wareing.197l. A relationship between net photosynthesis, dif- fusive resistance and carboxylating enzyme acti- — vity in bean leaves. Pp. 89-96 in M.D. Hatch, C.B. Osmond and R.B. Slatyer, eds. Photosynthesis and Respiration. Wiley Interscience, New York. 137. Neel, W. W., R.C. Morris and R.B. Head. 1976. Biol- ogy and natural control of the cottonwood leaf beetle, Chrysomela scripta (Fab.) (Colepoptera: Chrysomelidae). Pp. 264-271 in B.A. Thielges and S. B. Land, Jr., eds. Proc. Symp. on Cotton- wood and Related Species. Louisiana State Univer- sity, Baton Rouge, LA. 138. Okafo, O. and J. W. Hanover. 1978. Comparative photosynthesis and respiration of trembling and bigtooth aspens in relation to growth and develop- ment. For. Sci. 24:103-109. 139. Olivera, F.L. and D.T. Cooper. 1977. Tolerance of cottonwood to damage by the cottonwood leaf beetle. South For. Tree Improv. Conf. 14:213-217. 140. Page, M. and R.L. Lyon. 1977. Contact toxicity of insecticides applied to cottonwood leaf beetle. J. Econ. Entomol. 69:147-148. 141. Pallardy, S. G. and T.T. Kozolowski. 1979. Relation- ships of leaf diffusion resistance of Populus clones to leaf water potential and enviornment. Oecologia (Berl.) 40:371-380. 142. 143'. 144. 145. 146. 147. 148. 149. 150. 170 and T.T. Kozlowski. 1979. Response of Populus clones to light intensity and vapor pressure deficit. Parker, J. 1974. Effects of defoliation, girdling, and severing of sugar maple trees on root starch and sugar levels. USDA For. Serv. Res. Paper NE-306. 1978. Effects of fertilization on shoot growth of defoliated and undefoliated red oak seedlings. USDA For. Serv. Res. Paper NE-399. and D. R. Houston. 1971. Effects of re- peated defoliation on root and root collar extractives of sugar maple trees. For. Sci. 17:91-95. and R.L. Patton. 1975. Effects of drought and defoliation on some metabolites in roots of black oak seedlings. Can. J. For. Res. 5:457-463. Pauley, S.S. and T.O. Perry. 1954. Ecotypic varia- tion of the photOperiodic response of Populus. J. Arnold Arboretum 35:167-188. Pollard, D.F.W. 1970. Effect of partial defolia- tion on leader growth. Canadian Forestry Service Bimonthly Research Notes 26:10-11. 1970. The effect of rapidly changing light on the rate of photosynthesis in large- tooth aspen (Populus grandidentata). Can. J. Bot. 48:824-829. 1972. Above ground dry matter pro- duction in three stands of trembling aspen. Can. J. For. Res. 2:27-33. 151. 152. 153. 154. 155. 156. 157. 158. 159. 171 Rangnekar, P.V. and D.P. Forward. 1973. Foliar nutrition and wood growth in red pine: Effects of darkening and defoliation on the distribution of 14C photosynthate in young trees. Can. J. Bot. 51:103-108. ' - Richardson, S.D. 1953. Studies on root growth in Acer saccharinum II. Factors affecting root growth when photosynthesis is curtailed. Proc. Kon. Ned. Akad. v. Wetensch. Amsterdam C 56:346-353. Roberts, B.R., P. J. Kramer, and C.M. Carl. 1963. Long term effects of gibberellin on the growth of loblolly pine seedings. For. Sci. 9:202-205. Rose. A. H. 1958. The effect of defoliation on foliage production and radial growth of quaking aspen. For. Sci. 4:335-342. Sachs, T. and K.V. Thimann. 1964. Release of lateral buds from apical dominance. Nature 201:939-940. and K.V. Thimann. 1967. The role of auxins and cytokinins in the release of buds from dominance. Am. J. Bot. 54:136-144. Salisbury, F.B. and G.W. Ross. 1978. Plant Physiol- ogy. Wadsworth Publishing Co. Pp. 240-241. Sanders, T.H., D.A. Ashley and R.H. Brown. 1977. Effects of partial defoliation on petiole phloem area, photosynthesis and 14C Translocation in deve10ping soybean leaves. CrOp Sci. 17:548-550. Satoh, M., P.E. Kriedemann and B.R. Loveys. 1977. Changes in photosynthetic activity and related processes following decapitation in mulberry trees. Physiol. Plant. 41:203-210. 160. 161. 162. 163. 164. 165. 166. 167. 168. 169. 172 Saucier, J. R., A. Clark III, and R.G. McAlpine. 1972. Above bround biomass yields of short rota- tion sycamore. Wood Sci. 5:1-6. Schreiner, E.J. 1959.. Production of poplar timber in Europe and its significance and appli- cation in the United States. USDA For. Serv. Agric. Handbook No. 150. 1970. Mini Rotation Forestry. USDA For. Serv. Res. Paper NE-l74. Scurfield, G. 1967. Effects of gibberellic acid on woody perennials with special reference to species of Eucalyptus. For. Sci. 8:168-179. Shimshi, D. 1969. A rapid method for measuring photosynthesis with labeled carbon dioxide. J. Exp. Bot. 20:381-401. Smith, D.W. and G.E. Gatherum. 1974. Effect of moisture and clone on photosynthesis and growth of an aspen poplar hybrid. Bot. Gaz. 135:293-296. Smith, J.H.G. 1972. Tree size and yields in juven- ile red alder stands. Paper presented at the Forest Section, N.W. Scientific Assoc., March 23-25, 1972, Bellingham, Washington. 35 pp. and D.S. Debell. 1973. Opportunities for short rotation culture and complete utiliza- tion of seven northwestern tree species. For. Chron. 49:31-34. Snedecor, G.W. and W.G. Cochran. 1967. Statistical Methods. Iowa State University Press. 593 pp. Solomon, J.D., J.R. Cook, F.L. Oliveria and T. H. Filer. 1976. Insect and canker disease impact in cottonwood nurseries. Pp. 301-307 in C.B.A. Thielges and S.B. Land, Jr., eds. Proc. Symp. on Cottonwood and Related Species, Louisiana State University, Baton Rouge, LA. 170. 171. 172. 173. 174. 175. 176. 177. 178. 173 I Spence, J.A. and E.C. Humphries. 1972. Effect of moisture supply, root temperature, and growth regulators on photosynthesis of isolated leaves of sweet potato. Ann. Bot. 36:115-121. Steel, R.G.D. and J.H. Torrie. 1960. Principles and procedures of statistics. McGraw Hill, New York 481 pp. Steinbeck, K., R.G. McAlpine and J.T. May. 1972. Short rotation culture of sycamore: A status report. J. For. 70:210-213. Stephens, G.R., N.C. Turner and H.C. de R00. 1972. Some effects of defoliation by gypsy moth (Parthetria dispar L.) and elm spanworm (Ennomos subsignarius an.) on water balance and growth of deciduous forest trees. For. Sci. 18:326-330. Stuart, N.W. and H.M. Cathey. 1961. Applied aspects of the gibberellins. Ann. Rev. Plant Physiol. 12:369-394. Sweet, G.B. and P.F. Wareing. 1966. Role of plant growth in regulating photosynthesis. Nature 210:77-79. Thomas, J.B. 1978. A review of the economic impact of insects on the genus Populus in Ontario. Can. For. Serv., Sault Ste. Marie, Ont. Report 0-X-271. 45 pp. , and A.H. Rose. 1979. Insect damage to hybrid poplar plantings. Report 21 in D.C.F. Fayle, L. Zsuffa and H.W. Anderson, eds. Poplar Research, Management, and Utilization in Canada. Ontario Ministry Nat. Resour. For Inf. Paper No. 102. Thorne, J.H. and H.R. Koller. 1974. Influence of assimilate demand on photosynthesis, diffusive re- sistances, translocation and carbohydrate levels of soybeans. Plant Physiology 54:201-207. 179. 180. 181. 182. 183. 184. 185. 186. 187. 174 Thrower, S.L. 1962. Translocation of labeled assimilates in the soybean II. The pattern of translocation in intact and defoliated plants. Aust. J. Biol. Sci. 15:629-649. 1967. The patterns of translocation during leaf ageing. Symp. Soc. Exp. Biol. 21:483-506. Tiezen, L. L., D.A. Johnson, and M.M. Caldwell. 1974. A portable system.for the measurement of photosynthesis using 14- carbon dioxide. Photo- synthetica 8:151-160. Treharne, K.J. and J.L. Stoddart. 1968. Effects of gibberellin on photosynthesis in red clover (Tri- foZium pratense L.). Nature 220:487-488. Treharne, K. J. 1972. Biochemical limitations to photosynthetic rates. Pp. 285-302 in A.R. Rees, K.E. Cockshull, D.W. Hand. and R.G. Hurd, eds. Crop Processes in Controlled Environments. Academic Press, London. Turcenskaja, I.A. 1963. Effect of Lymantria dispar and other defoliators on the growth of oaks (Russian, English summary). Zool. Zh Mouskva 42:248-255. (Rev. Appl. Entomol. Ser. B. 52:594). USDA Forest Service. 1973. The outlook for timber in the United States. USDA For. Serv. For. Res. Paper 20. ' USDA Forest Service. 1976. Intensive plantation culture: Five years research. USDA For. Serv. Gen. Tech. Rep. NC-Zl. 117 pp. Varga, F. 1964. Loss in increment due to damage caused by Lymantria dispar in turkey oak stands (Hungarian, English summary). Sci. Pub. Univ. Forest. Ind. 2:219-226.‘ 188. 189. 190. 191. 192. 193. 194. 195. 196. 197. 175 Wallace, J.B. and M.S. Blum. 1969. Redefined de- fensive mechanisms in Chrysomela scripta. Ann. Entomol. Soc. Am. 62:503-506. Wallace, P.P. 1945. Certain effects of defoliation of deciduous trees. Conn. Agric. Exp. Sta. Bull. 488: 358-373. Wareing, P.F. and I.D.J. Phillips. 1970. The con- trol of growth and differentiation in plants. Permangon Press, New York. 303 pp. Wareing P.F., M.M. Khalifa and K.J. Treharne. 1968. Rate-limiting processes in photosynthesis at saturating light intensitites. Nature 220:453-457. Wargo, P.M. 1971. Enhanced growth of AmiZZaria mellea on extracts from roots of defoliated sugar maple trees. Abstract in Phytopathology 61:915. 1972. Defoliation induced chemical changes in sugar maple roots stimulate growth of Armillaria mellea. Phytopathology 62:1278-1282. , J. Parker, and D.R. Houston. 1972. Starch content in roots of defoliated sugar maple. For. Sci. 18:203-204. Wilson, L.F. 1976. Entomological problems of forest crops grown under intensive culture. Iowa State J. Res. 50:277-286. 1979. Insect pests of Populus in the Lake States. Pp. 75-81 in Proc. 1979. North American Poplar Council Meeting, Thompsonville, Michigan. Woolhouse, H.W. 1967. The nature of senescence in plants. Symp. Soc. Exp. Biol. 21:179-213. 176 198. Ying, Ch-. Ch-. and W.T. Bagley. 1976. Genetic variation of eastern cottonwood in an eastern Nebraska provenance study. Silvae Genet. 25:67. 199. Zimmermann, M.H. and C.L. Brown. 1971. Trees: Structure and function. Springer Verlag, New York. Pp. 221-275. 200. Zsuffa, L., H.W. Anderson, and P. Jacine. 1977. Trends and prospects in Ontario's poplar planta- tion management. For. Chron. 53:195-200.