Apical Dominance Control in Ipomoea nil: The Influence of the Shoot Apex, Leaves and Stem

The outgrowth of lateral buds is known to be controlled by theupper shoot tissues, which include the apex, the young leavesand the upper stem. An analysis of the influence of these plantparts on axillary bud elongation in Ipomoea nil was carriedout by various treatments on these specific tissues. A restriction of elongation in the main shoot due to eitherdecapitation or shoot inversion resulted in the release of apicaldominance A non-linear type of compensating growth relationshipwas observed between the 13 cm apical growing region of thestem and the lateral buds. It was determined by decapitation,defoliation and AgNO₃ treatments that both the 13 cm stem-growthregion and the young leaves (1–5 cm in length) had a muchgreater inhibitory influence on the outgrowth of specified lateralbuds than did the stem apex (consisting of the terminal 0.5cm of the shoot). The specified lateral buds which were analyzedfor outgrowth were located a number of nodes below the shootapex. The intervening nodes were debudded. Although the importanceof young leaves in the control of apical dominance has beenpreviously recognized, the most significant result from thepresent study with Ipomoea was the strong influence of the 13cm apical growth region of the stem on the out growth of thelateral buds. Apical dominance, Ipomoea nil L., Pharbitis nil, growth region, lateral bud outgrowth, decapitation, defoliation, shoot inversion     

Effect of Cutting During Reproductive Development on the Regrowth and Regenerative Capacity of the Perennial Grass, Phalaris aquatica L., in a Controlled Environment

Two cultivars of phalaris (Phalaris aquatica L.), Australianand Sirolan, were cut at four stages of development in a controlledenvironment to study factors involved in the sensitivity ofphalaris to grazing during spring. Effects on tillering, regrowthafter cutting and regenerative capacity after an artificiallyimposed summer dormant period were observed. Compared with Australian,Sirolan cut after the commencement of stem elongation was characterizedby a higher degree of decapitation due to more synchronous elevationof its apices, and displayed a more severe reduction in regrowth,size of tiller bases and dormant buds and levels of carbohydratereserves in summer relative to plants cut before stem elongation.Suppressed bud activity in tillers of Sirolan decapitated atearly stem elongation, and the potential for profuse tilleringassociated with low bud dormancy after cutting at the earlyboot stage, could reduce persistence under field conditions. Relative to plants cut before stem elongation, regenerationgrowth after 'summer' by plants cut during reproductive developmentwas depressed more severely for Sirolan (56-70%; P 0·05)than Australian (28%; n.s), a result more closely related toregenerating tiller size than number. Regeneration growth didnot differ significantly with stage reproductive developmentat cutting in either cultivar. Regenerative capacity of phalariscut during reproductive development can be considered to dependon an increasing contribution from buds on bases of tillersdecapitated when cut and a contribution from buds on intacttiller bases which declines as the stage of cutting becomeslater. The balance between these source will depend on the environment.Copyright1993, 1999 Academic Press Phalaris aquatica L., phalaris, regrowth, persistence, defoliation, cutting, perennial grass, tillering     

去叶对水曲柳和落叶松苗木当年生长及细根动态的影响

叶片被取食会导致树木生长发育和生理代谢发生显著的变化。目前对细根动态如何对叶片损失做出响应的了解仍然有限。以生物量分配和高生长策略不同的水曲柳(Fraxinus mandschurica)和落叶松(Larix gmelinii)苗木为研究对象, 进行了不同强度的人为去叶处理(叶面积去除0% (对照)、40%和80%), 采用微根管技术对细根(直径≤2 mm)生产和死亡的季节动态进行了定量观测, 同期测定了地上部分(苗高和地径)的生长。结果表明: 1)去叶降低了两树种苗高(统计上均不显著)和地径的生长, 但是对苗高生长的影响小于地径。随着去叶强度的提高, 苗木地上生长受到的影响加大, 生长季末期水曲柳苗高比对照降低3.3%-12.1%, 地径降低5.7%-23.1%; 而落叶松苗高和地径降低相对较少(< 12%)。2)去叶显著地减少了水曲柳和落叶松细根现存量(p< 0.001), 其相对增长量((去叶后现存量高峰-去叶当日现存量)/去叶当日现存量)随着去叶强度的加大而降低。3)与对照相比, 去叶后两树种细根生产量显著减少(p< 0.05), 而细根死亡量在不同处理间没有显著差异。综合来看, 去叶对水曲柳地上部分(特别是地径)生长影响较大, 而对落叶松地下部分(主要是新根)生长影响较大。研究结果为理解冠层碳供应对根系动态影响的种间差异及其机制提供了必要的理论依据。     

Cytokinins and the Growth Responses of Seedlings of Betula pendula Roth. and Acer pseudoplatanus L. to Nitrogen and Phosphorus Deficiency

Deficiencies in the supply of nitrogen and phosphorus broughtabout the cessation of shoot extension and the formation ofterminal resting buds in seedlings of Acer pseudoplatanus maintainedunder long days, whereas seedlings of Betula pendula maintainedunder the same conditions continued extension growth of themain axis over a long period, although they showed symptomsof nitrogen deficiency, including a reduced chlorophyll content.The endogenous cytokinin levels of the base/nucleoside fractionin leaves of B. pendula were significantly lower in the nitrogen-deficientplants than in the controls only 3 d after transfer to deficientconditions. Similar, though less marked, changes were observedin response to phosphorus deficiency. Changes in the levelsof cytokinin activity in the nucleotide fraction in responseto nitrogen deficiency were much less marked. No significantchanges in cytokinin levels of the base/nucleoside or nucleotidefractions could be detected in leaf material from seedlingsof A. pseudoplatanus, even after a nitrogen or phosphorus deficiencytreatment of 4 weeks. Application of exogenous benzyladenineto seedlings of B. pendula subjected to nitrogen deficiencyresulted in a rapid regreening and to outgrowth of inhibitedlateral buds after only 2 d.     

Effects of Partial Defoliation on the Growth of Liriodendron tulipifera L. Seedlings

Partial defoliation of Liriodendron tulipifera L. by removalof leaflets as they emerged from the protective buds affecteda variety of growth parameters. Reductions occurred in totaldry weight production, the percentage of new growth devotedto foliage, leaf area ratio, and the increment of leaf areaper unit of leaf weight increment. Increases occurred in unitleaf rate and the rate of production of new leaves. Weight ofleaves near the base of the seedlings was unaffected by treatmentbut leaves developing at the end of the growing season werelarger on defoliated plants. The net effect of removing one-thirdand two-thirds of the developing leaflets was to reduce totaldry weight production by 14 and 42 per cent respectively.     

Dwarfism Caused by Floral-Bud Removal in Petunia and its Reversal by Gibberellin

The effect of floral-bud removal at different stages of developmenton the plant height and on the total number of buds of Petuniawas studied. Continuous removal of all the floral buds 2 d beforeanthesis caused a marked decrease in plant height and also increasedthe total number of floral buds formed thereafter. At otherstages of floral bud development, bud removal had a lesser effecton both phenomena. Moreover, the plants did not respond to budremoval at anthesis. GA₃ at 25 ppm applied to plants from which the buds had beenremoved, promoted stem elongation. The most pronounced effectwas on plants from which the buds were removed 2 d before anthesis,but it had no effect on plants from which the buds were removedat anthesis stage. The possible involvement of endogenous growth hormones in theresponse of Petunia plants to floral-bud removal and to applicationof GA₃ is discussed. Bud removal, bud number, dwarfness, GA₃, Petunia, plant height     

Lateral bud development and shoot growth in low-pruned Morus alba as affected by stem orientation

Two phases of bud activity were identified in the new growth of one-year-old erect coppice shoots on 11-year-old low-pruned stumps of mulberry (Morus alba L. cv. Shin-ichinose) in spring, the sprouting phase in which the majority of the buds, including the basal ones, sprout and elongate, and the dominance phase (starting 4–5 weeks after sprouting) during which the upper laterals begin to assert dominance and suppress the growth of lower laterals, becoming new leading shoots. In contrast, arching before sprouting markedly inhibited buds on the under side, leading to poor shoots. By late April, the sprouts on the upper side grew readily into new erect shoots, resulting in considerable dominance over those from the lateral sides. Of these erect shoots, those located closer to the stem base grew more in May and June. The effects of arching made during the sprouting phase (late April) on bud activity and shoot lengths were generally similar to those of earlier archings before spring bud bursting. Separation of the shoots from the upper and under sides by longitudinal, horizontal splitting of the arched stems in late April did not affect the inhibited elongation of the shoots from the under side. These results suggest that in the response to arching before and in late April, the effects are related to spring bud bursting and gravimorphism. In contrast, arching during and after the dominance phase (May) had no gravimorphic effects on growth of the shoots on the upper side, although there was a stimulation of outbreak of the buds on the upper side, which remained dormant during spring bud bursting. Continuous basal applications of abscisic acid in aqueous solution inhibited bud break and shoot growth of the postdormant erect stem segments, and defoliation of the new shoots markedly. In contrast, similar applications of an ethylene-releasing compound, Ethephon, depressed shoot elongation slightly, but enhanced defoliation greatly. Gibberellic acid (GA₃) stimulated shoot elongation, but depressed leaf enlargement.     

Effect of Severity of Defoliation on the Viability of Reproductive and Vegetative Axillary Buds of Trifolium repens L.

This glasshouse experiment was performed to assess the effectsof a range of constant defoliation regimes applied to cuttingsof a single large-leaved genotype ofTrifolium repens L. on theviability of its axillary buds. Plants were established to comprisea single main stolon (axillary branches were removed) and defoliationtreatments were applied by removing the older (basal) leavesuntil leaf complements of 1·0, 1·5, 2·0,2·5, 3·0 or all leaves (control) remained. Basalleaves were subsequently removed as necessary to maintain thetarget leaf complements. Only severe defoliation (leaf complements of 1·0 and1·5) induced a loss of viability in axillary buds. Lossof viability was greatest in reproductive buds present withinthe apical bud when the treatments were first imposed. Althoughthe most severe treatment (leaf complement 1·0) resultedin death of half the plants, in plants surviving that treatment,death of vegetative axillary buds was restricted to 21% of thevegetative buds at the three youngest node positions withinthe apical bud at the time of treatment application. No othertreatment induced any loss of viability of vegetative buds.There was no loss of viability of axillary buds at nodes formedafter the treatments were imposed. The frequency of initiationof inflorescences at nodes formed after treatments were imposeddecreased as defoliation severity increased. Severe defoliation resulted in marked changes in plant morphologyindicative of a sharp decrease in availability of intraplantresources. It was concluded that under severe defoliation: (1)the potential for vegetative growth (as represented by viablevegetative axillary buds) was maintained at the expense of reproductivegrowth; and (2) that the loss of viability of axillary budswas associated with the sudden changes in physiological processesinduced by defoliation as there was no loss of viability inbuds formed after plants had adjusted their phenotype to oneof smaller size. Trifolium repens L.; white clover; defoliation; axillary buds; viability; inflorescences     

A Mung Bean Assay for Malformin-induced Growth Stimulation

A bioassay employing green or etiolated cuttings of Phaseolus aureus Roxb. was developed for determining malformin-induced growth stimulation in light. Growth enhancement of green cuttings was more rapid and relatively greater than that of etiolated cuttings. Cuttings from green seedlings responded less as seedlings aged; those from etiolated seedlings responded more. Malformin also stimulated the growth of green or etiolated seedlings in light. Most growth enhancement induced by malformin occurred in the upper 1 cm of the stem. Using green cuttings, malformin stimulated stem elongation relatively more when cotyledons, leaves, or especially apical buds were removed. Although malformin failed to stimulate elongation of 2-cm stem sections “floated” on solutions in Petri dishes, it stimulated elongation of sections when they were upright. High concentrations of indoleacetic acid inhibited growth enhancement by malformin. When gibberellin and malformin were combined, growth enhancement was nearly additive.     

Combined Effects of Partial Defoliation and Nutrient Availability on Cloned Betula pendula Saplings: I CHANGES IN GROWTH, PARTITIONING AND NITROGEN UPTAKE

The combined effects of partial defoliation and nutrient availabilityon dry matter accumulation and partitioning, and on nitrogenuptake and partitioning, were studied in cloned Betula pendulaRoth saplings. The saplings were randomly assigned to differentnutrient levels (5, 1·5 and 0·5 mol Nm^–3)in aerated nutrient culture and to the following defoliationtreatments: (1) control (no damage), (2)damage of the developingmain stem leaves (half of the leaf lamina removed), and (3)removalof the developing main stem leaves (entire leaf lamina removed).Measured in terms of cumulative whole-plant dry weight (includingremoved leaf tissue), the birch saplings were unable to compensatefor the loss of the developing leaves (treatment 3) during the14 d study period. In response to leaf removal (treatment 3)the mean final percentage reduction in whole-plant dry weightwas actually greater than the initial mean percentage reductioncaused by the removal itself; the magnitude of the final reductionwas independent of nutrient availability. After removal of thedeveloping leaves, branch growth was favoured at the expenseof the growth of the rest of the shoot; the relative branchgrowth was most pronounced at the highest nutrient level. Atthe two highest nutrient levels the nitrogen uptake of the saplingswith the developing leaves removed was less than that of undamagedsaplings. We suggest, however, that the incapacity of the saplingsfor compensatory growth after removal of the developing leaveswas primarily due to the decreased total carbon gain of thesaplings rather than to the decreased nitrogen uptake rate. Key words: Partial defoliation, nutrient availability, birch sapling, dry matter, nitrogen     

Combined Effects of Partial Defoliation and Nutrient Availability on Cloned Betula pendula Saplings: II. CHANGES IN NET PHOTOSYNTHESIS AND RELATED BIOCHEMICAL PROPERTIES

The combined effects of partial defoliation and nutrient availabilityon net photosynthesis and related biochemical variables werestudied in cloned Betula pendula Roth saplings. The saplingswere randomly assigned to different nutrient levels (5, 1·5and 0·5 mol N m^–3) in aerated nutrient cultureand to the following defoliation treatments: (1) control (nodamage), (2) damage of the developing main stem leaves (halfof the leaf lamina removed), and (3)removal of the developingmain stem leaves (entire leaf lamina removed). The leaf immediatelybelow the damaged area in the treated plants, and the correspondingleaf in the control plants, were selected for study. Net photosynthesismeasurements and biochemical determinations were made 2, 8 and14 d after assigning the treatments. At intermediate and lownutrient levels the final net photosynthetic capacity was significantlyhigher in the saplings with the topmost leaves removed thanin the undamaged control saplings, indicating that the expressionof compensatory photosynthesis after partial defoliation isnot inhibited by nutrient deficiency. The photosynthetic enhancementwas closely associated with the increased initial activity ofribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco). However,the increased activity of Rubisco was not exclusively the resultof a higher amount of Rubisco. The expression of compensatoryphotosynthesis after partial defoliation in our study cannotunequivocally be attributed to an increased flow of nitrogento the remaining leaves. Key words: Partial defoliation, nutrient availability, net photosynthesis, nitrogen, Rubisco     

Effects of artificial and western spruce budworm (Lepidoptera: Tortricidae) defoliation on growth and biomass allocation of Douglas-fir seedlings

Artificial defoliation has been used commonly to simulate defoliation by insect herbivores in experiments, in spite of the fact that obvious differences exist between clipping foliage and natural defoliation due to insect feeding. We used a greenhouse experiment to compare the effects of artificial and western spruce budworm (Choristoneura occidentalis Freeman) defoliation on the growth and biomass allocation of 3-yr old half-sib seedlings from mature Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco variety glauca] trees that showed phenotypic resistance versus susceptibility to budworm defoliation in the forest. Artificial clipping of buds mimicked the effects of budworm feeding on total seedling biomass when 50% of the terminal buds were damaged. However, artificial defoliation decreased seedling height, relative growth rate of height, and shoot: root ratio more than budworm defoliation, whereas budworm defoliation decreased stem diameter relative growth rate more than artificial defoliation. Half-sib seedling progeny from resistant maternal tree phenotypes had greater height, diameter, biomass, and shoot: root ratio than seedlings from susceptible phenotypes. We concluded that careful artificial defoliation could generally simulate effects of budworm defoliation on total biomass of Douglas-fir seedlings, but that the two defoliation types did not have equal effects on biomass allocation between shoot and root. Further, an inherently higher growth rate and a greater allocation of biomass to shoot versus root are associated with resistance of Douglas-fir trees to western spruce budworm defoliation.     

Effects of Plant Growth Regulators and Nutrient Supply on Tiller Bud Outgrowth in Barley (Hordeum distichum L.)

Seedlings of spring barley were raised in 100 and 20% nutrientsolution and treated with a foliar application of Terpal, Cerone,TIBA, GA₃ or BAP. The growth of individual tiller buds and tillers,the main shoot and the root system was recorded over the following15 d. Terpal and Cerone stimulated tiller bud elongation within5 d at both nutrient levels and after 15 d the number of emergedtillers was increased at the higher nutrient level. Terpal characteristicallypromoted the growth of secondary tiller buds whereas Ceronepromoted the emergence of the coleoptile tiller; both thesePGRs also retarded the development of the main shoot. TIBA increasedthe number of elongating tiller buds but this did not resultin greater tillering. GA₃ reduced the number of elongating tillerbuds and restricted their growth, especially in the high nutrientregime; this was accompanied by an increase in main shoot elongation.The growth and development of tiller buds was reduced by BAPand the number of emerged tillers was reduced at 15 d in bothnutrient levels; main shoot dry weight and root elongation werealso reduced. The results are considered in relation to theoverall influence of hormonal factors and mineral supply ontiller bud outgrowth. Hordeum distichum, spring barley, tiller bud outgrowth, plant growth regulators, Terpal, Cerone, GA₃, BAP, nutrient supply, apical dominance, TIBA     

The Relationship between the Growth of the Primary Leaf and of the Coleoptile in Seedlings of Avena and Triticum

Partial inhibition of extension growth of the primary leaf occurswhen whole Triticum seedlings are immersed in aerated solutionsof IAA but is replaced by growth promotion when sucrose is addedto the external solution. In seedlings in which the coleoptilehas been excised, IAA increases the growth of the leaf bothwith and without additional sucrose. Inhibition of the leaf by moderate concentrations of IAA nolonger occurs when the seedling is detached from the endosperm.Sucrose added to the external solution raised the percentageelongation of the coleoptile almost to the level of that attainedin intact seedlings without additional carbohydrate. It alsoenabled the leaf to show a positive growth response with IAA. The results indicate that in intact seedlings treated with IAAthe growth of the primary leaf is markedly diminished owingto diversion of carbohydrate to the coleoptile if the growthof the latter is promoted as a result of the treatment. Whenthe competition of the coleoptile for carbohydrate is diminishedor eliminated, acceleration of the growth of the primary leafby IAA becomes apparent. In addition to the endogenous rhythm, with a period close to24 hours, induced in the growth-rate of the coleoptile whenseedlings of Avena are transferred from red light to darkness,a similar rhythm, with a slightly longer period, is inducedin the growth-rate of the primary leaf. This rhythm persistsin elongating leaves so long as they remain within the coleoptile.It can be recorded for at least 100 hours in deseeded seedlings. When intact seedlings of Avena are immersed for one hour inrelatively high concentrations of IAA and then transferred todistilled water for 18 hours, the elongation of the coleoptileis greater and the inhibition of the leaf is less than whenthey are transferred to humid air. Sections of the leaf of Triticum showed a slight increase inelongation in concentrations of IAA up to 5 mg./l., but no evidencewas obtained that sections of leaf and coleoptile exert any.influenceon each other's elongation when floated together on solutionsof IAA.     

Regrowth by Swards of Subterranean Clover after Defoliation. 1. Growth, Non-structural Carbohydrate and Nitrogen Content

Swards of subterranean clover (Trifolium subterraneum L.) atLAl 6 grown in N-free nutrient solution were subjected to threedefoliation treatments which removed 30, 70 and 80% of shootdry weight. Subsequent regrowth and changes in the concentrationsof carbohydrate and nitrogen in plant components were measuredat 0, 1, 5, 9 and 13 d after defoliation and compared with thosein uncut swards. The rate of shoot regrowth declined with increasing severilyof defoliation. In all defoliation treatments, growth was confinedto leaves for up to 5 d. Root growth ceased in all treatmentsfor a longer period. Reestablishment of the leaf area in severely-defoliatedswards was facilitated by the rapid opening of developing leavesand by changes in the allocation of carbon which favoured leafover branch and root, and lamina over petiole growth. Loss of carbohydrate and nitrogen from roots and branches lasting5–9 d was observed in the more severe defoliation treatments.Loss of protein (N x 6.25) exceeded that of total non-structuralcarbohydrate, and could have accounted for the nitrogen contentof new leaf during this period. Branches lost 62% of their initialcarbohydrate content compared with 25% from roots in the 80%cut swards. In contrast, roots, by virtue of their greater mass,were the principle source of mobilized nitrogen. Nitrogen accumulationceased in 80% cut swards for 9 d. However, carbohydrate levelsin the crown nodules were not severely depleted. It was concluded that partitioning of growth to lamina and themobilization of carbohydrates and nitrogen were important forrecovery from defoliation. Carbohydrates, carbon partitioning, defoliation, nitrogen, mobilization, regrowth, subterranean clover, Trifolium subterraneum L     

Shoot Apical Meristems of Picea sitchensis Seedlings Accelerate in Growth Following Bud-set

First-year seedlings of Picea sitchensis were induced to setbuds by transferring them from long days (17 h) to short days(10 h at 20 °C, 12 h night at 12 °C). After 20 shortdays, the seedlings stopped growing in height, the shoot apicaldomes began enlarging and the transverse growth rates of theshoot apical dome tissues increased. For about 25 days thereafterprimordia were produced faster within the developing buds thanat the apices of seedlings which had been kept in long daysto prevent bud-set. Accelerated primordia production after bud-setenables the largest possible buds to be formed during the frost-freeperiod before winter. Picea sitchensis, apical meristem, bud-set     

Effect of Assimilate Supply on Development and Growth Potential of Axillary Buds in Roses

The effect of assimilate supply on axillary bud developmentand subsequent shoot growth was investigated in roses. Differencesin assimilate supply were imposed by differential defoliation.Fresh and dry mass of axillary buds increased with increasedassimilate supply. The growth potential of buds was studiedeither by pruning the parent shoot above the bud, by graftingthe bud or by culturing the bud in vitro. Time until bud breakwas not clearly affected by assimilate supply during bud development,Increase in assimilate supply slightly increased the numberof leaves and leaf primordia in the bud; the number of leavespreceding the flower on the shoot grown from the axillary budsubstantially increased. No difference was found in the numberof leaves preceding the flower on shoots grown from buds attachedto the parent shoot and those from buds grafted on a cutting,indicating that at the moment of release from inhibition thebud meristem became determined to produce a specific numberof leaves and to develop into a flower. Assimilate supply duringaxillary bud development increased the number of pith cells,but the final size of the pith in the subsequent shoot was largelydetermined by cell enlargement, which was dependent on assimilatesupply during shoot growth. Shoot growth after release frominhibition was affected by assimilate supply during axillarybud development only when buds sprouted attached to the parentshoot, indicating that shoot growth is, to a major extent, dependenton the assimilate supply available while growth is taking place.Copyright1994, 1999 Academic Press Assimilate supply, axillary bud, cell number, cell size, defoliation, development, growth potential, meristem programming, pith, Rosa hybrida, rose, shoot growth     

Reactivation of Vessel Production in Ash (Fraxinus excelsior L.) Trees

Stages of earlywood vessel development have been compared withstages of bud and shoot growth throughout 12-year-old treesof ash (Fraxinus excelsior L.). Reactivation of vessel productionwas not simultaneous throughout the tree. There was evidencethat vessel expansion progressed basipetally down branches andacropetally up the main stem. The earliest expanding vesselswere found scattered around the circumference of main stem andbranches about 3 weeks before the emergence of foliage leavesfrom the buds. Other vessels expanded later between the earlierones so the whole of the first earlywood layer was expandingby 1.5 weeks before leaf emergence; this is suggested as a convenientstage to use as a baseline for a model of wood production inash. Vessel maturation progressed basipetally down the mainstem as well as branches, the first mature (presumably functional)vessels appearing in the upper stem shortly before leaf emergence.Mature vessels were not found in the lower part of the mainstem until after the beginning of rapid leaf expansion afterbudbreak, contrary to a previous assumption that functionalearlywood vessels are of necessity produced before leaf expansionin ring-porous trees. Patterns of vessel expansion are comparedbetween the ring-porous ash and the diffuse-porous sycamore;these data suggest that expansion of earlywood vessels beganat the same time in relation to budbreak in the two species,but the location of the first vessel expansion differed. ash, cambial reactivation, Fraxinus excelsior L., ring-porous hardwood, vessel expansion, vessel maturation     

Defoliation increases carbon limitation in ectomycorrhizal symbiosis of <Emphasis Type="Italic">Betula pubescens</Emphasis>

Boreal forest trees are highly dependent on root-colonizing mycorrhizal fungi. Since the maintenance of mycorrhizal symbiosis implies a significant carbon cost for the host plant, the loss of photosynthetic leaf area due to herbivory is expected to reduce the host investment in mycorrhizae. We tested this hypothesis in a common garden experiment by exposing ectomycorrhizal white birch (Betula pubescens Ehrh.) seedlings to simulated insect defoliation of 50 or 100% intensity during either the previous or the current summer or repeatedly during both seasons before harvest. The shoot and root growth of the seedlings were distinctly reduced by both 100% defoliation and repeated 50% defoliation, and they were more strongly affected by previous-year than current-year defoliation. The root to shoot ratio significantly decreased after 100% defoliation, indicating reduced proportional allocation to the roots. Ergosterol concentration (i.e. fungal biomass) in the fine roots decreased by 100% defoliation conducted either in the year of harvest or in both years. No such decrease occurred following the 100% defoliation conducted in the previous year, indicating the importance of current photosynthates for fungal symbionts. The trend was similar in the colonization percentage of thick-mantled mycorrhizae in the roots, the most marked decline occurring in the repeatedly defoliated seedlings. The present results thus support the prediction that the plant investment in ectomycorrhizae may decline as a response to foliage loss. Moreover, the colonization percentage of thick-mantled mycorrhizae correlated positively with the ratio of leaf to heterotrophic plant biomass in the defoliated birch seedlings, but not in the control ones. This tends to indicate a stronger carbon limitation of ectomycorrhizal colonization in defoliated seedlings.     

Bud Structure and Shoot Architecture of Canopy and Understorey Evergreen Broad-leaved Trees at their Northern Limit in East Asia

The morphology of winter buds, shoot growth and branching architecturewas studied in evergreen broad-leaved trees of subtropical/warm-temperaterain forests of southern and central Japan. Winter buds werecategorized into three types based on external morphology anddevelopmental processes: naked, hypsophyllary and scaled buds.Each shoot tip with intermittent growth was covered with a smallnumber of immature leaves or hypsophylls when growth ceased.Hypsophylls protect the apical meristem during its resting period,hence we termed them hypsophyllary buds. In trees with nakedbuds, immature leaves resumed their growth and developed tomature leaves the following spring; thus these trees had nospecial organs to cover shoot tips during winter. In trees withhypsophyllary buds, some hypsophylls covering the shoot tipsthrough the year were shed without further growth when new shootsstarted to grow in the spring. In trees with scaled buds, newlygrowing shoots had hypsophyllary buds at their tips in spring.After the completion of stem elongation, the buds were replacedby scaled buds (often covered with more than 30 scales) in summer.These scaled buds grew during autumn and winter until a newflush of growth the following spring. The three bud types correspondedto forest stratification in the northern-limit forest: the nakedbuds of Rubiaceae and Myrsinaceae in the ground layer; the hypsophyllarybuds of various families (e.g. Symplocaceae, Myrsinaceae) inthe understorey; and the scaled buds of Fagaceae and Lauraceaein the forest canopy. The position and activity of buds on abranch were reflected in the architectural patterns of the treesin different layers of the forest. The scaled-bud trees hadwell-protected, abundant axillary buds and are probably suitedto survive in the forest canopy (with frequent disturbances),whereas the single terminal bud of hypsophyllary-bud trees cansurvive in the less disturbed, resource-limited understoreyof the forest.Copyright 1998 Annals of Botany Company Bud structural type; bud formation; bud growth; shoot elongation; shoot-growth cycle; branching architecture; forest stratification.