Photosynthetic leaf area modulates tiller bud outgrowth in sorghum

Shoot branches or tillers develop from axillary buds. The dormancy versus outgrowth fates of buds depends on genetic, environmental and hormonal signals. Defoliation inhibits bud outgrowth indicating the role of leaf‐derived metabolic factors such as sucrose in bud outgrowth. In this study, the sensitivity of bud outgrowth to selective defoliation was investigated. At 6 d after planting (6 DAP), the first two leaves of sorghum were fully expanded and the third was partially emerged. Therefore, the leaves were selectively defoliated at 6 DAP and the length of the bud in the first leaf axil was measured at 8 DAP. Bud outgrowth was inhibited by defoliation of only 2 cm from the tip of the second leaf blade. The expression of dormancy and sucrose‐starvation marker genes was up‐regulated and cell cycle and sucrose‐inducible genes was down‐regulated during the first 24 h post‐defoliation of the second leaf. At 48 h, the expression of these genes was similar to controls as the defoliated plant recovers. Our results demonstrate that small changes in photosynthetic leaf area affect the propensity of tiller buds for outgrowth. Therefore, variation in leaf area and photosynthetic activity should be included when integrating sucrose into models of shoot branching.     

APICAL DOMINANCE AND FORM IN WOODY PLANTS: A REAPPRAISAL

The form of woody plants is commonly interpreted in terms of apical dominance. Trees with the decurrent or deliquescent branching habit are said to have weak apical dominance, whereas excurrent branching is associated with strong apical dominance. A close examination of many decurrent species such as the oaks, hickories, and maples reveals that almost all of the lateral buds on the current year's twigs are completely inhibited. This complete inhibition of lateral buds by definition and common usage of the term is an expression of strong apical dominance. In trees possessing the excurrent branching habit, such as most conifers and some angiosperms, many of the lateral buds on the current year's twigs elongate to varying degrees. This is usually interpreted as an expression of weak apical dominance. The relationship between bud inhibition and form in woody perennials is much more complex than bud inhibition in herbaceous plants because of the time sequence in the formation and release of lateral buds. For example, it is only after a period of rest or dormancy in the decurrent forms that one or more of the uppermost lateral buds are released, and these may outgrow the currently elongating terminal shoot resulting in forking. Conversely, in the excurrent forms, it seems that the initial expression of weak apical dominance enables the terminal leader to outgrow the currently elongating lateral branches so that it exerts complete control over their subsequent growth and development in later years. An examination of the levels of diffusible auxin at different points along the twigs of excurrent and decurrent species indicates that the balance of growth factors at any given locus, and not the absolute quantity of auxin, exerts primary control over bud inhibition and shoot elongation.     

Seasonal Changes in Auxin Effect on Rooting of Douglas-Fir Stem Cuttings as Related to Bud Activity

The relation of seasonal bud activity to the periodicity of rooting in Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, stem cuttings was studied in combination with auxin and cold storage treatments. Cuttings were collected in all months except April and May, for 3 years. Rooting was least in September and October when bud dormancy was most pronounced, greatest in December and January if exogenous auxin was applied, or in February and March if no auxin was used. The buds contributed significantly to rooting from January to April, and were responsible for differences in rooting of terminal and lateral cuttings during this period. Auxin did not enhance rooting in September and October, but at other times it replaced or supplemented the role of vegetative buds in promoting rooting. Auxin also removed the differences in rooting between lateral and terminal cuttings. Cold treatment in October and November removed bud dormancy and enhanced rooting. After November the need for auxin or cold treatment diminished and rooting without either treatment reached a maximum in February and March. Auxin did not change the seasonal pattern of rooting but broadened and enhanced the rooting response in favor of earliness. These results are discussed in relation to the effect of bud activity on auxin response and root initiation. The hypotheses are proposed that cambial dormancy or auxin deficiency is not the limiting factor during bud dormancy, and that cold treatments have the effect of bringing inhibitors and promoters into proper balance for optimum rooting response.     

Branch development in Lupinus angustifolius L.#I. Not all branches have the same potential growth rate

Although the basal and uppermost lateral branches of Lupinus angustifolius L. frequently grow and contribute to yield, buds formed in the axils of leaves 6-12 (referred to as middle buds) rarely grow. This may be due to an inherent limitation of these buds, or some form of apical dominance or competition imposed by the plant. The hypothesis that middle buds have the full capacity to grow, but remain suppressed on intact plants was tested. The main stem apex and buds from the axils of leaves 1 and 8 (bud 1 and bud 8) were excised and cultured on sterile agar. The buds were removed from culture and weighed every 2-3 d for 21 d. The growth rate of apices from the main stem was approximately 5.8 mg d^-1, compared to 2.4 mg d^-1 for bud 1 and 0.9 mg d^-1 for bud 8. Buds in the axils of leaves 6-10 on intact plants were painted six times with a synthetic cytokinin, benzylaminopurine, from 40 d after sowing. This promoted rapid elongation and thickening of these buds, visible as early as 5 d after painting began. The rapid growth of these branches was associated with a reduction in the length of the remaining branches on the plant. However, excision of lower branches did not increase the growth of the middle buds. It is concluded that buds 6-12 of Lupinus angustifolius L. have a partial potential to grow. This potential appears to be limited by innate factors in the bud, and may be structural and/or hormonal. The limitation appears to develop very early in the plant, and potential growth is not modified by subsequent nutrition of the plant.     

Analytical studies on the responses of sunflower (Helianthus annuus) to various defoliation treatments

The effects of defoliation treatments on plant growth in sunflower (Helianthus annuus) were studied in the field. Four defoliation treatments, 0 (control), 37.4, 56.1 and 93.4% of the total leaf dry weight, were applied to plants that had small third leaves. Decreased leaf weight/whole plant weight (F/W) ratios in defoliated plants rapidly recovered to almost the same ratio as that observed in the control within 12 to 16 days after defoliation according to the degree of defoliation. The mechanism involved in the recovery of the F/W ratio in defoliated plants mainly consisted of three parameters: enhancement of (1) carbon distribution ratios in the leaves, (2) photosynthetic activity in the remaining leaves, and (3) retranslocation of carbon from the stem and/or roots to leaves. Inhibitive effects of defoliation on relative growth rate and net assimilation rate were seen at an early stage, but subsequently both rates became larger in defoliated plants than in controls. Defoliated plants tended to show rapid development and expansion of new leaves, and to show increased specific leaf area and protein synthesis in individual leaves. The sugar content of leaves in defoliated plants was higher than that in controls, while the content in both stem and roots was lower. These responses seem to be advantageous for development of the photosynthetic system. Heights of defoliated plants were clearly depressed according to the degree of defoliation, and this was attributed largely to differences in the elongation rates of the internodes resulting from defoliation.     

Tiller hierarchy and defoliation frequency determine bud viability in the grass <Emphasis Type="Italic">Poa ligularis</Emphasis>

Bud viability after various defoliation frequency treatments was determined in the perennial bunchgrass Poa ligularis under arid field conditions from 2002 to 2005. Bud respiratory activity was examined on various stem base hierarchies using the tetrazolium test, as validated with the vital stain Evan’s blue. The hypothesis of this work was that the total and viable axillary bud numbers on stem bases of all study stem base hierarchies are reduced as defoliation frequency increases. Interpretation of the results differed when they were expressed as a percentage rather than on a number per stem base basis. The total number of axillary buds per stem base was similar in all defoliation frequencies. When the results were expressed on a percentage basis, the order on stem bases having metabolically active buds was daughter tillers > stem bases with green tillers > stem bases without green tillers in all defoliation frequencies. The reverse order was found when considering dead buds. How the results are expressed thus deserves our attention when reporting results on bud viability in perennial grasses. An increased defoliation frequency increased the percentage of dead and dormant buds after the third or fourth defoliation of P. ligularis during the 1st study year. These percentages of bud viability, however, increased after the first defoliation during the 2nd study year. Bud viability was affected not only by the cumulative effects of defoliation but also by climatic variables throughout the seasons. However, our results show that P. ligularis can be defoliated up to twice a year without affecting bud viability, and thus its potential capacity for regrowth after defoliation.     

The effect of leaf primordia and auxin on leaf initiation rate in strawberry

Summary Removal of the leaf primordia hastens the rate of leaf initiation at the apex, and a paste containing 0.2% IAA in lanoline will substitute for the effect of the leaf primordia. Physical factors involved in the alteration in bud structure resulting from defoliation, such as gaseous diffusion and shading from light, have only negligible effect on the rate of leaf initiation, and the compsition of the internal atmosphere of the intact buds is not very different from the external atmosphere.The evidence suggests that developing leaf primordia inhibit cell division of the apical meristem through their production of auxin which is discharged into the stem at points which are morphologically basal to the apical cells; this, therefore, could be another case of correlative inhibition by auxin, comparable with the inhibition of lateral buds by the terminal apex.     

Induction of Cold Acclimation in Cornus stolonifera Michx

A warm (20 to 15 Celsius day or night) preconditioning treatment enhanced cold acclimation of Cornus stolonifera bark under short-day conditions when plants were preconditioned for at least 4 weeks. Warm preconditioning inhibited the acclimation of plants subjected to long photoperiods. Removing leaves from plants exposed to low temperatures and short days inhibited acclimation. Removal of buds did not affect acclimation. Plants did not acclimate unless they were exposed to at least 4 weeks of short photoperiods prior to defoliation. Plants began to acclimate to cold at the time of growth cessation but not before. When half of the leaves were removed from plants, the defoliated and foliated branches both acclimated as well as branches on completely foliated plants. Girdling the phloem between foliated and defoliated branches prevented acclimation of the latter regardless of the position of the girdle in relation to the root system and the defoliated branch. When all of the leaves of plants were covered with aluminum foil to exclude light after 0 or 4 weeks of exposure to short days, the results resembled a defoliation study, i.e., plants with leaves covered at the start of the experiment failed to acclimate, and those covered after 4 weeks acclimated to some extent but less than uncovered control plants. Under longday conditions plants with all leaves covered failed to acclimate, and plants with none or half of their leaves covered acclimated equally and to a limited extent. Under short-day conditions, however, the covered branches of partially covered plants acclimated more than their uncovered counterparts or branches of totally uncovered plants.     

Evidence that the mature leaves contribute auxin to the immature tissues of pea (<Emphasis Type="Italic">Pisum sativum</Emphasis> L.)

In plants such as the garden pea (Pisum sativum L.), it is widely thought that the auxin indole-3-acetic acid (IAA) is synthesised mainly in the immature tissues of the apical bud and then transported basipetally to other parts of the plant. Consistent with this belief are results showing that removal of the apical bud markedly reduces the IAA content in the stem. However, it has also been suggested that the mature leaves may synthesise substantial amounts of IAA, which enters the basipetal transport stream after being transported to the shoot apex in the phloem (Cambridge and Morris in Planta 99:583–588, 1996). To examine this theory, we defoliated pea plants and measured the effect on IAA content in the remaining shoot tissues. IAA levels were reduced in the internodes, and to a lesser extent in the apical bud, after defoliation, suggesting that mature leaves are indeed an important source of auxin for the shoot. Consistent with this idea, we have demonstrated that mature, fully expanded leaves are capable of de novo IAA synthesis. Furthermore, we report evidence for the presence of IAA in the phloem sap of pea. Together these results support those of Cambridge and Morris, suggesting that mature leaves are a source of the IAA in the basipetal transport stream.     

Changes in endogenous hormones in apple during bud burst induced by defoliation

Under the tropical conditions of East Java, terminal buds of apple burst at any time of the year in response to removal of the subtending leaves. Following two such defoliations, two weeks apart on separate trees, there was a decrease in abscisic acid (ABA), a three-fold increase in gibberellin-like substances (GAs) and only a slight increase in cytokinin-like substances (CKs) in the apex tissue of closed buds. These changes preceded bud opening and the associated increases in fresh and dry weight, and may be causally related to bud burst. In open buds (i.e. young expanding leaves) the concentration of CKs was greater, and the concentrations of ABA and GAs less, than the concentrations in closed buds. As the leaves expanded, ABA increased and GAs and CKs decreased in concentration. The decrease in concentration of GAs and CKs, however, was due to the rise in dry weight of the expanding tissue; the amounts of all three hormones (per apex) increased. During bud burst there was a concurrent decrease in the CKs of subtending stems, suggesting a transfer into the expanding bud tissues. Removal of the old leaves by defoliation may remove the source of ABA and allow the amount of GAs in the apex to rise, bud burst following. Stem CKs may be utilized in the expansion of the new leaves in the bursting buds.     

AXILLARY AND DICHOTOMOUS BRANCHING IN THE PALM CHAMAEDOREA

In both Chamaedorea seifrizii Burret and C. cataractarum Martius each adult foliage leaf subtends one axillary bud. The proximal buds in C. seifrizii are always vegetative, producing branches (= new shoots or suckers); and the distal buds on a shoot are always reproductive, producing inflorescences. The prophyll and first few scale leaves of a vegetative branch lack buds. Transitional leaves subtend vegetative buds and adult leaves subtend reproductive buds. Both types of buds are first initiated in the axil of the second or third leaf primordia from the apex, P2 or P3. Later development of both types of bud tends to be more on the adaxial surface of the subtending leaf base than on the shoot axis. Axillary buds of C. cataractarum are similarly initiated in the axil of P2 or P3 and also have an insertion that is more foliar than cauline. However, all buds develop as inflorescences. Vegetative branches arise irregularly by a division of the apex within an enclosing leaf (= P1). A typical inflorescence bud is initiated in the axil of the enclosing leaf when it is in the position of P2 and when each new branch has initiated its own P1. No scale leaves are produced by either branch and the morphological relationship among branches and the enclosing leaf varies. Often the branches are unequal and the enclosing leaf is fasciated. The vegetative branching in C. cataractarum is considered to be developmentally a true dichotomy and is compared with other examples of dichotomous (= terminal) branching in the Angiospermae.     

Relative importance of nodal roots and apical buds in the control of branching in Trifolium repens L.

Clonal species are characterised by having a growth form in which roots and shoots originate from the same meristem so that adventitious nodal roots form close to the terminal apical bud of stems. The nature of the relationship between nodal roots and axillary bud growth was investigated in three manipulative experiments on cuttings of a single genotype of Trifolium repens. In the absence of locally positioned nodal roots axillary bud development within the apical bud proceeded normally until it slowed once the subtending leaf had matured to be the second expanded leaf on the stem. Excision of apical tissues indicated that while there was no apical dominance apparent within fully rooted stems and very little in stems with 15 or more unrooted nodes, the outgrowth of the two most distal axillary buds was stimulated by decapitation in stems with intermediate numbers of unrooted nodes. Excision of the basal branches from stems growing without local nodal roots markedly increased the length and/or number of leaves on 14 distally positioned branches. The presence of basal branches therefore prevented the translocation of root-supplied resources (nutrients, water, phytohormones) to the more distally located nodes and this caused the retardation in the outgrowth of their axillary buds. Based on all three experiments we conclude that the primary control of bud outgrowth is exerted by roots via the acropetal transport of root-supplied resources necessary for axillary bud outgrowth and that apical dominance plays a very minor role in the regulation of axillary bud outgrowth in T. repens.     

Execution of the auxin replacement apical dominance experiment in temperate woody species

The classic Thimann-Skoog or auxin replacement apical dominance test of exogenous auxin repression of lateral bud outgrowth was successfully executed in both seedlings and older trees of white ash, green ash, and red oak under the following conditions: (1) decapitation of a twig apex and auxin replacement were carried out during spring flush, (2) the decapitation was in the previous season's overwintered wood, and (3) the point of decapitation was below the upper large irrepressible lateral buds but above the lower repressible lateral buds. Although it has been suggested that neither auxin, the terminal bud, nor apical dominance have control over the outgrowth of the irrepressible buds during spring flush, there is evidence in the present study that indicates that such control over the repressible buds exists. In seedlings, second-order branching, which resulted from decapitation of elongating current shoots, was also inhibited by exogenous auxin in the three species. Hence, the auxin replacement experiments did work on year-old proleptic buds (of branches of older trees) that would have entered the bud bank and also on current buds of seedlings. Cytokinin treatments were ineffectual in promoting bud growth.     

Floral determination in the terminal and axillary buds of Nicotiana tabacum L

The terminal and axillary buds of the day-neutral plant, Nicotiana tabacum cv. Wisconsin 38, become determined for floral development during the growth of the plant. This state of determination can be demonstrated with a simple experiment: buds determined for floral development produce the same number of nodes in situ and if rooted. After several months of growth and the production of many leaves, the terminal bud became determined for floral development within a period of about 2 days. After the terminal bud became florally determined, it produced four nodes and a terminal flower. The buds located in the axils of leaves borne just below the floral branches became florally determined 5 to 9 days after the terminal bud became florally determined. Since florally-determined axillary buds were not clonally derived from a florally-determined terminal meristem, axillary buds and the terminal bud acquired the state of floral determination independently. These data indicate that a pervasive signal induced a state of floral determination in competent terminal and axillary buds.     

Histochemical Studies of Sclereid Induction in the Shoot of Rauwolfia Species: I. CYTOCHROME OXIDASE

Results of histochemical tests for cytochrome oxidase activityin four species of Rauwolfia have been reported. The cells beneaththe terminal shoot meristem constitute the pith. Histochemicaltests showed intensified enzymatic activity in those cells ofthe pith which would differentiate as sclereid initials. Similarcytochrome oxidase activity also occurred in the sclereid initialsand the developing sclereids. The cytochrome oxidase activitywas associated with two types of particulate formation, thegranular and rod-shaped. The ground parenchymatous cells ofthe pith and the leaf-base showed very little enzymatic activityof cytochrome oxidase. The characteristic indophenol blue colorationdue to cytochrome oxidase activity did not appear in controlsections. Physiological changes correlated with morphogeneticexpression of some pith cells demonstrate that the physiologicalchanges occur before the initiation of sclereids in the morphologicallyhomogeneous parenchymatous cells of the pith. Intensified cytochromeoxidase activity was also recorded in the meristematic tissuesof shoot apex, procambium, axillary buds and the laticiferouscells of Rauwolfia.     

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     

Hormones and Apical Dominance in the Fern Davallia

Lateral buds of the fern Davallia trichomanoides are releasedfrom inhibition by the removal of the main shoot apex. However,auxin is not capable of substituting for the apex in decapitatedshoots nor can auxin in shoot tips be detected by bioassay orextraction and chromatography. Expanding leaves of this speciescontain auxin, but these organs are not responsible for inhibitionof lateral bud growth. The response of lateral buds to an exogenouslyapplied cytokinin does not result in initial bud break. It isconcluded that the hormonal factors known to govern apical dominancein seed plants are not responsible for the regulation of differentialbud expansion in this fern.     

Variation in shoot mortality within crowns of severely defoliated <Emphasis Type="Italic">Betula maximowicziana</Emphasis> trees in Hokkaido,northern Japan

To clarify mortality patterns of current-year shoots within the crown of Betula maximowicziana Regel after severe insect herbivory in central Hokkaido, northern Japan, we investigated the degree of defoliation, pattern of shoot development, shoot mortality, and leaf tissue-water relations. One hundred current-year long shoots growing in a B. maximowicziana plantation were observed for defoliation and mortality in June 2002. An outbreak of herbivorous insects (Caligula japonica and Lymantria dispar praeterea) occurred in the stand in mid-to-late June, and the monitored shoots were defoliated to various degrees. Within 1 month of defoliation, some of the severely defoliated shoots had produced new leaves on short shoots that had emerged from axillary buds. Stepwise logistic regression revealed that the probability that current-year long shoots would put out axillary short shoots with leaves is closely related to the degree of defoliation. To evaluate the water relations of the leaves, we determined pressure–volume curves for the leaves that survived the herbivorous insect outbreak and the new leaves that emerged after defoliation. The water potential at turgor loss (Ψ_l,tlp) and the osmotic potential at full turgidity (Ψ_π,sat) were higher for the new leaves than for the surviving leaves, indicating a lower ability to maintain leaf cell turgor against leaf dehydration in the new leaves. Of the 100 shoots, 13 died after the emergence of new leaves. Stepwise logistic regression revealed that the probability that the long shoots would die generally increased with the emergence of new leaves, with increasing shoot height. This result suggests that the combined effect of the vulnerability of newly emerged leaves and low water availability, associated with higher shoot positions within the crown, caused shoot mortality. Based on our results, some possible mechanisms for mortality in severely defoliated B. maximowicziana are discussed.     

ONTOGENY AND DIFFERENTIATION OF SCLEREIDS IN RAUWOLFIA

An interesting anatomic feature of Rauwolfia is the occurrence of a remarkable type of sclereid in the stem and root. The initials of the sclereids in the stem arise in the ground tissue element of the pith in a region between 50 and 70μ below the surface of the shoot apex. This region of the shoot remains surrounded by a whorl of either 3 or 4 leaves. Sclereids initiate in succession in association with each whorl of leaves. Thus, the sclereids are restricted to the nodes. The sclereids in the stem arise as a primary element of the shoot from the ground tissue of the pith. In the root, they differentiate from the cells of the phelloderm and are secondary in origin. Morphologically, the sclereids in these 2 organs are basically the same, except that the sclereids in the stem are larger in size than those in the root. A solitary cell, or 2 to several cells in a longitudinal cell file (originated from a single mother cell), may differentiate into sclereid initials. The growth of the sclereids through relatively compact ground tissue of the pith is possibly accomplished by a combination of gliding growth and apical intrusive process. The sclereid initials grow rapidly and force their way between the parenchymatous cells. As a result, the neighboring cells lose their original surface contacts. Sclereids increase in size rapidly, and, therefore, very enlarged sclereids with thin primary walls may be observed in the second node. They mature progressively in basipetal direction in the subjacent nodes. In the fifth or sixth node, mature sclereids with massive secondary walls are most common. The secondary walls of sclereids contain much lignin as determined by the phloroglucinol-HCl test. The walls of sclereids at maturity show a variable number of lamellae ranging from 10 to 15 in the lateral walls. A remarkable feature of the sclereids is their canal-like pits in the secondary walls. Two adjacent pits may coalesce uniquely to form a Y-like configuration directed centrifugally from the lumen of the sclereids. The sclereids are ventrically symmetrical, joined end-to-end by their transverse walls like 2 superimposed young fibers.     

Woody tissue photosynthesis and its contribution to trunk growth and bud development in young plants

Stem photosynthesis can contribute significantly to woody plant carbon balance, particularly in times when leaves are absent or in ‘open’ crowns with sufficient light penetration. We explored the significance of woody tissue (stem) photosynthesis for the carbon income in three California native plant species via measurements of chlorophyll concentrations, radial stem growth, bud biomass and stable carbon isotope composition of sugars in different plant organs. Young plants of Prunus ilicifolia, Umbellularia californica and Arctostaphylos manzanita were measured and subjected to manipulations at two levels: trunk light exclusion (100 and 50%) and complete defoliation. We found that long‐term light exclusion resulted in a reduction in chlorophyll concentration and radial growth, demonstrating that trunk assimilates contributed to trunk carbon income. In addition, bud biomass was lower in covered plants compared to uncovered plants. Excluding 100% of the ambient light from trunks on defoliated plants led to an enrichment in ¹³C of trunk phloem sugars. We attributed this effect to a reduction in photosynthetic carbon isotope discrimination against ¹³C that in turn resulted in an enrichment in ¹³C of bud sugars. Taken together our results reveal that stem photosynthesis contributes to the total carbon income of all species including the buds in defoliated plants.