Spatial and temporal variation in RGR and leaf quality of a clonal riparian plant: Arundo donax

Arundo donax L. is a tall perennial reed classified as an emergent aquatic plant. In California, it has invaded riparian zones, where it acts as a transformer species. Because plant growth and leaf quality influence the effectiveness of management techniques, we sought to determine if these characters varied temporally and spatially in a northern California population of A. donax. Tissue C and N content and C:N ratio varied during the growing season. Leaf N was higher in spring and in plants that were closer to a stream. It was significantly negatively related to the clump's distance from the stream but not related to its elevation relative to the stream. Plants near the stream produced taller stems with more leaves per stem than those more distant from the stream. RGR differed across time and space. It was highest in spring prior to the appearance of flowers on a few stems that were >1 year old within the clumps. Decline in RGR as the growing season progressed coincided with the appearance of branches and flowers on stems <1 year old on a few plants within the studied population. RGR was significantly related to the N content and C:N ratio of leaves on mature stems (>1 year old). This implies that the decrease in stem growth reflected changes in nutrient availability within the entire A. donax clump and not just in the growing stems (<1 year old). These findings have implications for timing of management techniques.     

Spatial and temporal distribution of the alkaloid sanguinarine in Sanguinaria canadensis L. (Bloodroot)

Acclimated and non-acclimated potted plants of Sanguinaria canadensis L. were harvested at early and late dormancy, anthesis, and immature and mature fruiting stages. Sanguinarine content and concentration were determined for rhizomes (distal, proximal, and middle sections), roots, leaves, flower, and fruit. Rhizomes had highest sanguinarine content and concentrations, and exhibited decreasing concentration gradients from the distal to proximal third. Concentrations in roots were a tenth of rhizome concentration. Concentrations in leaves, flowers, and fruit were one-thousandth of rhizome Sanguinarine content in whole acclimated plants was constant. Content in whole nonacclimated plants increased as the plant became physiologically active, but was constant during fruit maturation: content in roots, leaves, and fruit did not change. The substantial increase in whole-plant dry weight coupled with the unchanging sanguinarine content during fruit maturation suggests either a shift in photosynthate allocation from defense to growth, or a constant turnover of sanguinarine.     

Storage ability of overwintering leaves and rhizomes in a semi-evergreen fern, <Emphasis Type="Italic">Dryopteris crassirhizoma</Emphasis> (Dryopteridaceae)

Dryopteris crassirhizoma is a rhizomatous semi-evergreen fern growing in the understory of deciduous forests. Although the top portion of the overwintering leaves began to wither in early winter, intensive senescence occurred in the spring, concurrently with new leaf development. Dry weight comparisons between organs revealed that the rhizome occupied the largest proportion of the total mass, followed by the pinnae. To assess the storage ability of overwintering leaves and the rhizome, seasonal changes in nitrogen content and the dry mass of pinnae and the rhizome were measured. Nitrogen (36.6%) was resorbed from winter-withering pinnae, but not from spring-withering pinnae. In contrast, a similar decrease in dry mass per unit area occurred between winter- and spring-withering pinnae (15%). These results indicate that overwintering leaves serve as a carbohydrate storage organ, but do not serve as a nitrogen storage organ. Nitrogen was not translocated from the rhizome during the early growing season, but translocation did occur in late summer and autumn. The dry mass of the rhizome decreased by 18.4% in spring, at the time of new leaf expansion. The amount of exported dry matter from the rhizome was threefold larger than that from senescent pinnae. Therefore, the rhizome is a major carbohydrate storage organ in this species, although overwintering leaves also act as a carbohydrate storage organ.     

Non-structural carbohydrate accumulation and use in an understorey rain-forest shrub and relevance for the impact of leaf herbivory

1. Piper arieianum, an evergreen, understorey shrub of lowland moist forests of Central and South America, exhibits marked seasonal variation in reproductive activity even though climatic variation is low at the study site. Despite a lack of climatic seasonality, previous experimental leaf removal suggested that carbohydrate accumulation is seasonal, occurring prior to flowering.
2. We first tested the hypothesis that carbohydrates necessary for reproduction are accumulated prior to flowering, rather than during or after. By measuring non-structural carbohydrate production in the form of glucose and starch we found that the concentration of these reserves is greatest 1–3months before flowering, decreasing by 50% during peak fruit maturation.
3. The hypothesis that reproduction was the cause of this decrease in carbohydrate reserves was then tested by comparing reserves in plants that were prevented from flowering with those that flowered and produced fruit naturally. As predicted, reserves declined more in flowering than in non-flowering plants. A smaller decline in reserves of non-flowering plants was accompanied by greater stem and leaf production, suggesting that stored carbohydrates are also required for growth.
4. Because concentrations of non-structural carbohydrates were similar in roots, stems and leaves, and because the greatest amount of plant biomass is in stems for plants of a range of sizes, stems appear to be the main storage site of carbohydrate reserves in this plant species.
5. These results, together with previous studies, demonstrate that the impact of leaf herbivory on seed production in P. arieianum depends on the timing of that herbivory relative to the accumulation and use of non-structural carbohydrates.     

Composition of kermes oak browse as affected by shade and stage of maturity

Kermes oak (Quercus coccifera L.) is a sclerophyllous evergreen shrub of the Mediterranean region. In Greece, it grows either alone or in a mixture with other sclerophyllous species in shrublands or as the understory in Pinus brutia forest.The nutritive value and digestibility of kermes oak browse were investigated under 2 light conditions: an an understory species in a P. brutia forest and in adjacent open shrublands. The comparison was made at 3 phenological stages: during the season of rapid growth (in April), after rapid growth was terminated (in May) and when growth had ceased and stems had hardened (in June).Crude protein content of the leaves and twigs was higher in the shaded than in the unshaded plants during the growing season, while the concentrations of total non-structural carbohydrate, cell contents and soluble protein were higher in the unshaded than in the shaded plants. Tannins and lignin content were higher in shaded than in unshaded plants. Similarly, dry matter digestibility was higher in unshaded plants, but declined more drastically in the shaded plants during the growing season. Production was also higher in unshaded than in shaded plants at the end of the growing season.     

Vitis vinifera root and leaf metabolic composition during fruit maturation: implications of defoliation

Grapevine (Vitis vinifera) roots and leaves represent major carbohydrate and nitrogen (N) sources, either as recent assimilates, or mobilized from labile or storage pools. This study examined the response of root and leaf primary metabolism following defoliation treatments applied to fruiting vines during ripening. The objective was to link alterations in root and leaf metabolism to carbohydrate and N source functioning under conditions of increased fruit sink demand. Potted grapevine leaf area was adjusted near the start of véraison to 25 primary leaves per vine compared to 100 leaves for the control. An additional group of vines were completely defoliated. Fruit sugar and N content development was assessed, and root and leaf starch and N concentrations determined. An untargeted GC/MS approach was undertaken to evaluate root and leaf primary metabolite concentrations. Partial and full defoliation increased root carbohydrate source contribution towards berry sugar accumulation, evident through starch remobilization. Furthermore, root myo‐inositol metabolism played a distinct role during carbohydrate remobilization. Full defoliation induced shikimate pathway derived aromatic amino acid accumulation in roots, while arginine accumulated after full and partial defoliation. Likewise, various leaf amino acids accumulated after partial defoliation. These results suggest elevated root and leaf amino N source activity when leaf N availability is restricted during fruit ripening. Overall, this study provides novel information regarding the impact of leaf source restriction, on metabolic compositions of major carbohydrate and N sources during berry maturation. These results enhance the understanding of source organ carbon and N metabolism during fruit maturation.     

The pattern of carbon allocation supporting growth of preformed shoot primordia in Acomastylis rossii (Rosaceae)

Extreme preformation, the initiation of leaves or inflorescences more than 1 yr before maturation and function, is common in arctic and alpine habitats. This extended pattern of development provides a potential means to alleviate an apparent asynchrony between carbon supplied by photosynthesis in the summer and carbon demanded by growth in the spring. Allocation of resources to preforming organs has not been studied in herbs with multi-year patterns of preformation. Acomastylis rossii (Rosaceae) in the southern Rockies initiates leaves and inflorescences 2 yr prior to their maturation and function. Allocation to preforming organs in A. rossii was studied by means of a labeled carbon pulse chase experiment. During the summer, carbon is allocated directly to preforming organs and rhizomes from the mature leaves. Additional allocation of carbohydrate into preforming organs occurs in autumn after photosynthesis by mature leaves has ceased. Organ primordia initiated in the second year do not receive a substantial quantity of the labeled carbon from reserves stored in the rhizome the previous year. We conclude that concurrent photosynthesis is the primary source of carbon for preformation development.     

Acquiring nutrients from tree leaves: effects of leaf maturity and development type on a generalist caterpillar

The rapid growth and prolific reproduction of many insect herbivores depend on the efficiencies and rates with which they acquire nutrients from their host plants. However, little is known about how nutrient assimilation efficiencies are affected by leaf maturation or how they vary between plant species. Recent work showed that leaf maturation can greatly decrease the protein assimilation efficiency (PAE) of Lymantria dispar caterpillars on some tree species, but not on species in the willow family (Salicaceae). One trait of many species in the Salicaceae that potentially affects PAE is the continuous (or “indeterminate”) development of leaves throughout the growing season. To improve our understanding of the temporal and developmental patterns of nutrient availability for tree-feeding insects, this study tested two hypotheses: nutrients (protein and carbohydrate) are more efficiently assimilated from immature than mature leaves, and, following leaf maturation, nutrients are more efficiently assimilated from indeterminate than determinate tree species. The nutritional physiology and growth of a generalist caterpillar (L. dispar) were measured on five determinate and five indeterminate tree species while their leaves were immature and again after they were mature. In support of the first hypothesis, caterpillars that fed on immature leaves had significantly higher PAE and carbohydrate assimilation efficiency (CAE), as well as higher protein assimilation rates and growth rates, than larvae that fed on mature leaves. Contrary to the second hypothesis, caterpillars that fed on mature indeterminate tree leaves did not have higher PAE than those that fed on mature determinate leaves, while CAE differed by only 3% between tree development types. Instead, “high-PAE” and “low-PAE” tree species were found across taxonomic and development categories. The results of this study emphasize the importance of physiological mechanisms, such as nutrient assimilation efficiency, to explain the large variation in host plant quality for insect herbivores.     

Biomechanical properties of the emergent aquatic macrophyte Sparganium erectum: Implications for fine sediment retention in low energy rivers

This paper is concerned with the biomechanical properties of the emergent aquatic macrophyte, Sparganium erectum. We present observations of adjustments in the physical characteristics and biomechanical properties of S. erectum during the growing season (April-November) from the River Blackwater, UK. When a pulling device is attached to plant stems to measure their resistance to uprooting, individual plants show remarkable strength in their above- and below-ground biomass (median stem strength when stems break away from the underground biomass, 78 N, median rhizome strength, 39 N) and high resistance to uprooting (median uprooting resistance when entire plants uproot, 114 N). This provides the potential for the species to protect and reinforce the generally soft, silty sediments that it often retains and within which its rhizomes and roots develop in lower energy river environments. There is a propensity for plant stems to break before the plant is uprooted at the beginning and end of the growth season, but for the stems to have sufficient strength in mid season for plant uprooting to dominate. This ensures that rhizome and root systems remain relatively undisturbed at times when the silty sediments in which they grow are poorly protected by above-ground biomass. In contrast, rhizome strength remains comparatively invariant through the growing season, supporting the plant's potential to have a protective/reinforcing effect on fine sediments through the winter when above ground biomass is absent.     

Leaf Carbohydrate Status and Enzymes of Translocate Synthesis in Fruiting and Vegetative Plants of Cucumis sativus L

Carbon partitioning in the leaves of Cucumis sativus L., a stachyose translocating plant, was influenced by the presence or absence of a single growing fruit on the plant. Fruit growth was very rapid with rates of fresh weight gain as high as 3.3 grams per hour. Fruit growth was highly competitive with vegetative growth as indicated by lower fresh weights of leaf blades, petioles, stem internodes and root systems on plants bearing a single growing fruit compared to plants not bearing a fruit. Carbon exchange rates, starch accumulation rates and carbon export rates were higher in leaves of plants bearing a fruit. Dry weight loss from leaves was higher at night from fruiting plants, and morning starch levels were consistently lower in leaves of fruiting than in leaves of vegetative plants indicating rapid starch mobilization at night from the leaves of fruiting plants. Galactinol, the galactosyl donor for stachyose biosynthesis, was present in the leaves of fruit-bearing plants at consistently lower concentration than in leaves of vegetative plants. Galactinol synthase, and sucrose phosphate synthase activities were not different on a per gram fresh weight basis in leaves from the two plant types; however, stachyose synthase activity was twice as high in leaves from fruiting plants. Thus, the lower galactinol pools may be associated with an activation of the terminal step in stachyose biosynthesis in leaves in response to the high sink demand of a growing cucumber fruit.     

Carbon assimilation by tree stems: potential involvement of phosphoenolpyruvate carboxylase

In woody species, the photosynthesis of stems, especially young branches, occurs by refixing part of the internal respiratory CO₂. The present study aims to improve the physiological characterization of stem photosynthesis by examining enzymatic characteristics. During an entire growing season, three enzymatic activities that are linked to C₃ and C₄ metabolism were investigated in relation to the CO₂ efflux and chlorophyll content of current year stems of European beech and were compared to the corresponding characteristics of leaves. High activities of phosphoenolpyruvate carboxylase (PEPC) and NADP malic enzyme were detected in stems (up to 13 times and 30 times higher in stems than in leaves, respectively), whereas Rubisco activity remained low in comparison with leaves. Stem maximal Rubisco and PEPC activities occurred at the beginning of the season when the total chlorophyll content and the CO₂ assimilation rate were also maximal. Stems were characterized by a PEPC:Rubisco ratio that was equal to 2.5 [an intermediate value between that of C₃-plants (about 0.1) and that of C₄-plants (about 10)], whereas it was equal to 0.1 in leaves. Eight other tree species were also measured and the PEPC:Rubisco ratio was, on average, equal to 3.6. The potential role of PEPC in stem carbon assimilation is discussed in relation to its known involvement in the anaplerotic function of C₃ plants and in the carbon metabolism of the C₄ pathway.     

Carbon allocation during fruiting in Rubus chamaemorus

Background and Aims

Rubus chamaemorus (cloudberry) is a herbaceous clonal peatland plant that produces an extensive underground rhizome system with distant ramets. Most of these ramets are non-floral. The main objectives of this study were to determine: (a) if plant growth was source limited in cloudberry; (b) if the non-floral ramets translocated carbon (C) to the fruit; and (c) if there was competition between fruit, leaves and rhizomes for C during fruit development.

Methods

Floral and non-floral ramet activities were monitored during the period of flower and fruit development using three approaches: gas exchange measurements, ¹⁴CO₂ labelling and dry mass accumulation in the different organs. Source and sink activity were manipulated by eliminating leaves or flowers or by reducing rhizome length.

Key Results

Photosynthetic rates were lower in floral than in deflowered ramets. Autoradiographs and ¹⁴C labelling data clearly indicated that fruit is a very strong sink for the floral ramet, whereas non-floral ramets translocated C toward the rhizome but not toward floral ramets. Nevertheless, rhizomes received some C from the floral ramet throughout the fruiting period. Ramets with shorter rhizomes produced smaller leaves and smaller fruits, and defoliated ramets produced very small fruits.

Conclusions

Plant growth appears to be source-limited in cloudberry since a reduction in sink strength did not induce a reduction in photosynthetic activity. Non-floral ramets did not participate directly to fruit development. Developing leaves appear to compete with the developing fruit but the intensity of this competition could vary with the specific timing of the two organs. The rhizome appears to act both as a source but also potentially as a sink during fruit development. Further studies are needed to characterize better the complex role played by the rhizome in fruit C nutrition.Key words: Allocation pattern, ¹⁴C labelling, carbon translocation, carbon reserves, cloudberry, defoliation, fruit production, gas exchange, Rubus chamaemorus, source–sink relationship, flowering     

Carbon isotopic composition of legumes with photosynthetic stems from mediterranean and desert habitats

The carbon isotopic compositions of leaves and stems of woody legumes growing in coastal mediterranean and inland desert sites in California were compared. The overall goal was to determine what factors were most associated with the carbon isotope composition of photosynthetic stems in these habitats. The carbon isotope signature (d13C) of photosynthetic stems was less negative than that of leaves on the same plants by an average of 1.51 ± 0.42 ;pp. The d13C of bark (cortical chlorenchyma and epidermis) was more negative than that of wood (vascular tissue and pith) from the same plant for all species studied on all dates. Desert woody legumes had a higher d13C (less negative) and a lower intercellular CO2 concentration (Ci ) (for both photosynthetic tissues) than that of woody legumes from mediterranean climate sites. Differences in the d13C of stems among sites could be entirely accounted for by differences among site air temperatures. Thus, the d13C composition of stems did not indicate a difference in whole-plant integrated water use efficiency (WUE) among sites. In contrast, stems on all plants had a lower stem Ci and a higher d13C than leaves on the same plant, indicating that photosynthetic stems improve long-term, whole-plant water use efficiency in a diversity of species.     

Time-course of tomato whole-plant respiration and fruit and stem growth during prolonged darkness in relation to carbohydrate reserves

To evaluate the relevance of a simple carbon balance model (Seginer et al., 1994, Scientia Horticulturae 60: 55-80) in source-limiting conditions, the dynamics of growth, respiration and carbohydrate reserves of tomato plants were observed in prolonged darkness. Four days prior to the experiments, plants were exposed to high or low light levels and CO(2) concentrations. The concentration of carbohydrates in vegetative organs was 30-50 % lower in plants that were exposed to low carbon assimilation conditions compared with those exposed to high carbon assimilation conditions. During prolonged darkness, plants with low carbohydrate reserves exhibited a lower whole-plant respiration rate, which decreased rapidly to almost zero after 24 h, and carbohydrate pools were almost exhausted in leaves, roots and flowers. In plants with high carbohydrate reserves, the whole-plant respiration rate was maintained for a longer period and carbohydrates remained available for at least 48 h in leaves and flowers. In contrast, fruits maintained fairly stable and identical concentrations of carbohydrates and the reduction in their rate of expansion was moderate irrespective of the pre-treatment carbon assimilation conditions. The time-course of asparagine and glutamine concentrations showed the occurrence of carbon stress in leaves and flowers. Estimation of source and sink activities indicated that even after low carbon assimilation, vegetative organs contained enough carbohydrates to support fruit growth provided their own growth stopped. The time of exhaustion of these carbohydrates corresponded grossly to the maintenance stage simulated by the model proposed by Seginer et al. (1994), thus validating the use of such a model for optimizing plant growth.     

The Utilization of Carbohydrate and Nitrogen Reserves by Taxus During Its Spring Growth Period

The spring growth and the utilization of carbohydrate and nitrogen reserves in this growth was studied in Taxus media cv. Hicksii plants 0, 2, 4 and 6 weeks after the plants started growing in the spring. The effect of nitrogen applied the previous season on the storage and utilization of the carbohydrate and nitrogen reserves during spring growth was determined. The plants were separated into buds (all new growth), stems, needles (those produced the previous season) and roots and analyzed for changes in total nitrogen, basic and non-basic amino acids, total available carbohydrate, sugars, hemicelluloses, organic acids and chlorophyll. The bulk of the soluble nitrogen reserves were stored as arginine in the stems and old needles. With the onset of spring growth, arginine nitrogen was converted to other amino acids which accumulated in the new growth (buds). The roots, stems and needles of plants grown under high nitrogen levels always contained more total nitrogen than those grown under low nitrogen levels. The bulk of the carbohydrate reserves were stored as hemicelluloses. The plants grown under high nitrogen levels utilized the bulk of the carbohydrate reserves from the roots and smaller amounts from the stems and old needles, while plants grown under low nitrogen levels used only the reserves in the roots. In the low nitrogen plants, carbohydrates accumulated in the needles and stems. Both the carbohydrate and nitrogen reserves were important in the dry weight increase due to spring growth. However, the nitrogen reserves were the limiting factor and the high nitrogen plants grew twice as much, produced more chlorophyll, and utilized more nitrogen and carbohydrate reserve in spring growth than low nitrogen plants. The additional chlorophyll allowed the production of more carbohydrates and these additional carbohydrates were used in increased growth rates, while in the low nitrogen plants the carbohydrate produced was less and accumulated within the plant.     

Vegetative propagation and sexual reproduction in the woodland understorey pseudo-annual Circaea lutetiana L.

On the forest floor of deciduous woodlands, relative light intensity gradually declines during the early growing season. The woodland understorey pseudo-annual Circaea lutetiana L. completes its life-cycle at the end of summer. These pseudo-annuals are clonal plants which survive the winter only as seeds and as hibernacles produced by the rhizome apices. In this paper, we asked several questions related to the life-history of C. lutetiana. It was found that shoot formation in early spring did not exhaust the old hibernacle. A trade-off between rhizome + hibernacle number and weight might be expected when plants grow under resource limitation. It was hypothesised that both number and weight of rhizomes and hibernacles will be affected by light availability. Since the effect of resource supply on the size number trade-off will depend on the developmental pattern of the rhizome system, rhizome development was studied as well. Soon after the shoots emerged, 1st order rhizomes were formed in May on the nodes of the old hibernacle. First-order rhizomes branched in June and 2nd order rhizomes (side-branches) were continuously produced throughout the growing season. The phenology and developmental rate of plants growing in different light treatments were plastic. On average 30% of rhizome biomass was formed during the vegetative phase, and rhizome and fruit production were only partly separated in time. The ratio of total rhizome biomass to total fruit biomass was not affected by light. Also flower bud removal did not lead to an increase in rhizome production, which suggests that division of biomass to both reproductive modes is rather rigid. The number of 1st order rhizomes was not affected by the light treatments. Under light limitation, both rhizome number and weight of single rhizomes were reduced. In contrast, fruit number, but not weight of single fruits, was limited.     

Carbohydrates in Digitalis purpurea at various stages of development

The concentrations of carbohydrates in Digitalis purpurea have been determined at various stages of development. An increase of the carbohydrate content was observed during growth. Glucose was the major carbohydrate component in the leaves and stem, whereas fructose predominated in the inflorescence. The growing tips and other actively growing parts of the plant contained relatively high concentrations of two new compounds.     

Changes in Nonstructural Carbohydrates in Different Parts of Soybean (Glycine max [L.] Merr.) Plants during a Light/Dark Cycle and in Extended Darkness

Diurnal patterns of nonstructural carbohydrate (starch, sucrose, and hexose sugars) concentration were characterized in different parts (leaves, petioles, stems, and roots) of vegetative soybean (Glycine max [L.] Merr.) plants. Pronounced changes in all carbohydrate pools were observed in all plant parts during the normal photosynthetic period; however, starch accumulation within leaves accounted for more than 80% of the nonstructural carbohydrate accumulated by the plant during the light period. Efficiency of utilization of starch and sucrose during the normal dark period differed among organs, with leaves being most efficient in mobilizing starch reserves and roots being most efficient in utilizing sucrose reserves. The vast majority (about 85%) of the whole plant carbohydrate reserves present at the end of the photosynthetic period were utilized during the normal dark period. Sink leaf expansion ceased in plants transferred to extended darkness and the cessation in leaf expansion corresponded with carbohydrate depletion in the subtending source leaf and the remainder of the plant. Collectively, the results indicated that under the conditions employed, leaves are the whole plant's primary source of carbon at night as well as during the day.     

First flowering hybrid between autotrophic and mycoheterotrophic plant species: breakthrough in molecular biology of mycoheterotrophy

Among land plants, which generally exhibit autotrophy through photosynthesis, about 880 species are mycoheterotrophs, dependent on mycorrhizal fungi for their carbon supply. Shifts in nutritional mode from autotrophy to mycoheterotrophy are usually accompanied by evolution of various combinations of characters related to structure and physiology, e.g., loss of foliage leaves and roots, reduction in seed size, degradation of plastid genome, and changes in mycorrhizal association and pollination strategy. However, the patterns and processes involved in such alterations are generally unknown. Hybrids between autotrophic and mycoheterotrophic plants may provide a breakthrough in molecular studies on the evolution of mycoheterotrophy. We have produced the first hybrid between autotrophic and mycoheterotrophic plant species using the orchid group Cymbidium. The autotrophic Cymbidium ensifolium subsp. haematodes and mycoheterotrophic C. macrorhizon were artificially pollinated, and aseptic germination of the hybrid seeds obtained was promoted by sonication. In vitro flowering was observed five years after seed sowing. Development of foliage leaves, an important character for photosynthesis, segregated in the first generation; that is, some individuals only developed scale leaves on the rhizome and flowering stems. However, all of the flowering plants formed roots, which is identical to the maternal parent.     

The effect of assimilate supply on fruit growth and hormone levels in tomato plants

Rates of dry matter accumulation and contents of starch, sugars and abscisic acid (ABA) of tomato fruits differed significantly during development at three positions (proximal, middle and distal) on a truss. Proximal fruits, which accumulated dry matter most rapidly during early development, generally had least ABA (per g DW).Partial defoliation reduced carbon accumulation by all fruits but increased ABA, especially in distal fruit, and indoleacetic acid (IAA), particularly in proximal fruits. The ABA content of leaves in partially defoliated plants was similar to that of leaves on non-defoliated plants.Removal of distal fruits on a truss enhanced carbon movement to the remaining proximal fruits and also increased their ABA content early in development but did not affect their IAA content. On the other hand, when proximal fruit were removed there was no large or lasting increased accumulation of carbon by the remaining distal fruits and they contained less ABA and IAA than fruits on plants without fruit thinning. Leaf carbon and ABA levels showed no marked trend in response to fruit thinning.The amount of carbon in the stems was increased by fruit thinning but decreased by partial defoliation.The possible roles of ABA and IAA in regulating fruit growth are discussed.Part of this work has been presented to a Symposium on Phloem loading and related processes at Bad Grund/Oberhar, W. Germany, July 1979.