Source and sink leaf metabolism in relation to Phloem translocation: carbon partitioning and enzymology

The import-export transition in sugar beet leaves (Beta vulgaris) occurred at 40 to 50% leaf expansion and was characterized by loss in assimilate import and increase in photosynthesis. The metabolism and partitioning of assimilated and translocated C were determined during leaf development and related to the translocation status of the leaf. The import stage was characterized by C derived from either ¹⁴C-translocate or ¹⁴C-photosynthate being incorporated into protein and structural carbohydrates. Marked changes in the C partitioning were temporally correlated with the import-export conversion. Exporting leaves did not hydrolyze accumulated sucrose and the C derived from CO₂ fixation was preferentially incorporated into sucrose. Both source and sink leaves contained similar levels of acid invertase and sucrose synthetase activities (sucrose hydrolysis) while sucrose phosphate synthetase (sucrose synthesis) was detected only in exporting leaves. The results are discussed in terms of intracellular compartmentation of sucrose and sucrose-metabolizing enzymes in source and sink leaves.     

A genetic basis for the manipulation of sink–source relationships by the galling aphid <Emphasis Type="Italic">Pemphigus batae</Emphasis>

We examined how the galling aphid Pemphigus batae manipulates resource translocation patterns of resistant and susceptible narrowleaf cottonwood Populus angustifolia. Using carbon-14 (¹⁴C)-labeling experiments in common garden trials, five patterns emerged. First, although aphid galls on resistant and susceptible genotypes did not differ in their capacity to intercept assimilates exported from the leaf they occupied, aphids sequestered 5.8-fold more assimilates from surrounding leaves on susceptible tree genotypes compared to resistant genotypes. Second, gall sinks on the same side of a shoot as a labeled leaf were 3.4-fold stronger than gall sinks on the opposite side of a shoot, which agrees with patterns of vascular connections among leaves of the same shoot (orthostichy). Third, plant genetic-based traits accounted for 26% of the variation in sink strength of gall sinks and 41% of the variation in sink strength of a plant’s own bud sinks. Fourth, tree susceptibility to aphid gall formation accounted for 63% of the variation in ¹⁴C import, suggesting strong genetic control of sink–source relationships. Fifth, competition between two galls was observed on a susceptible but not a resistant tree. On the susceptible tree distal aphids intercepted 1.5-fold more ¹⁴C from the occupied leaf than did basal aphids, but basal aphids compensated for the presence of a distal competitor by almost doubling import to the gall from surrounding leaves. These findings and others, aimed at identifying candidate genes for resistance, argue the importance of including plant genetics in future studies of the manipulation of translocation patterns by phytophageous insects.     

Ecological studies on the timberline of Mt. Fuji

An ecological study of dry matter production was made in a dwarf forest dominated byAlnus maximowiczii at the timberline of Mt. Fuji. Annual gross production was estimated by two methods, namely the summation method using stem analysis and total photosynthesis calculated from leaf area and photosynthetic rate per leaf area. Seasonal changes in relative light intensity and in leaf area were measured in a quadrat. Photosynthesis and respiration rates of samples were measured in temperature-regulated assimilation chambers. The phytomass was 2,989 g d.w.m^?2, and those of stems and branches, leaves, and roots were 1,672 g, 293 g, and 1,024 g respectively. The growing period of this plant was about four months and this plant expanded leaves quickly. The maximum gross photosynthetic rate was 21 mg CO₂dm^?2 h^?1 on September 1. Annual net production estimated by examining the annual rings was 922 g d.w.m^?2 year^?1 and annual respiration was 735 g. Annual gross production estimated from photosynthetic rates was 1,747 g d.w.m^?2 year^?1. The sum of annual net production by stem analysis and respiration agree closely with gross production estimated from photosynthetic rate. Gross production of this dwarf forest is comparable to the beech forest of the upper cool temperate zone owing to the high photosynthetic rate ofAlnus maximowiczii.     

Import of 14C-Photosynthate by Developing Leaves of Sugarcane

Sink-to-source transition was studied in developing sugarcane (Saccharum interspecific variety L62–96) leaves. Fully-expanded, mature sugarcane leaves were fed ¹⁴CO₂ for 20 minutes, incorporating about 617 Bq. After five hours the leaves of each plant were cut into 1-cm-length segments that were weighed and then placed in scintillation cocktail for counting. All leaves younger than the leaf fed ¹⁴CO₂ imported labeled photoassimilate. Three to four leaves had both importing and non-importing regions within the blade and a distinct transition region between them. A transition region was observed in leaves which had expanded to between 30 and 90 % of final blade length. Radioactivity per gram fresh weight was calculated as a measure of sink strength. Sink strength was greatest in the youngest leaf and declined with leaf age. The results of this study indicate that 1) import of photosynthate by developing sugarcane leaves occurs over a longer span of developmental ages than in dicotyledonous leaves and 2) the actual tissue region undergoing transition within such a leaf can be resolved as narrow zone between the importing and non-importing regions.     

Effects of NaCl stress on the growth and photosynthetic characteristics of Ulmus pumila L. seedlings in sand culture

The effects of NaCl stress on the growth and photosynthetic characters of Ulmus pumila L. seedlings were investigated under sand culture condition. With increasing NaCl concentration, main stem height, branch number, leaf number, and leaf area declined, while Na⁺ content and the Na⁺/K⁺ ratio in both expanded and expanding leaves increased. Na⁺ content was significantly higher in expanded leaves than in those just expanding. Chlorophyll (Chl) a and Chl b contents declined as NaCl concentration increased. The net photosynthetic rate, intercellular CO₂ concentration, stomatal conductance, and transpiration rate also declined, but stomatal limitation value increased as NaCl concentration increased. Both the maximal quantum yield of PSII photochemistry and the effective quantum yield of PSII photochemistry declined as NaCl concentration rose. These results suggest that the accumulation of Na⁺ in already expanded leaves might reduce damage to the expanding leaves and help U. pumila endure high salinity. The reduced photosynthesis in response to salt stress was mainly caused by stomatal limitation.     

Sink strength regulates photosynthesis in sugarcane

The relationship in sugarcane (Saccharum spp.) between photosynthetic source tissue and sink material was examined through manipulation of the sink:source ratio of field-grown Saccharum spp. hybrid cv. N19 (N19). To enhance sink strength, all leaves, except for the third fully expanded leaf, were enclosed in 90% shade cloth for varying periods of time. Variations in sucrose, glucose and fructose concentrations were measured and the effects of shading on the leaf gas exchange and fluorescence characteristics recorded. Changes in carbon partitioning caused by shading were examined based on the uptake and translocation of fixed 14CO2. Following a decline in sucrose concentrations in young internodal tissue and shaded leaves, significant increases in the CO2-saturated photosynthetic rate (Jmax), carboxylation efficiency (CE) and electron transport rate were observed in unshaded leaves after 8 d of shading treatment. It was concluded that up-regulation of source-leaf photosynthetic capacity is correlated with a decrease in assimilate availability to acropetal culm sink tissue. Furthermore, a significant relationship was revealed between source hexose concentration and photosynthetic activity.     

Phloem Unloading in Developing Leaves of Sugar Beet : II. Termination of Phloem Unloading

Phloem unloading in developing leaves of Beta vulgaris L. (`Klein E' multigerm) occurred from successively higher order branches of veins as leaves matured. Phloem unloading was studied in autoradiographs of leaf samples taken at various times during the arrival of a pulse of ¹⁴C-labeled photoassimilate. Extension of mass flow of sieve element contents into leaf vein branches was determined from the high level of radiolabel in veins soon after first arrival of the pulse. Rapid entry, indicative of mass flow through open sieve pores, occurred down to the fourth division of veins in young, importing leaves and to the fifth or terminal branch in importing regions near the zone of transition from sink to source. The rate of unloading decreased with leaf age, as evidenced by the increased time required for the vein-mesophyll demarcation to become obscured. The rate of import per unit leaf area, measured by steady state labeling with ¹⁴CO₂ also decreased as a leaf matured. The decline in import appeared to result from progressive changes that increased resistance to unloading of sieve elements and eventually terminated phloem unloading.     

Effect of mycorrhizal-enhanced leaf phosphate status on carbon partitioning, translocation and photosynthesis in cucumber

An assessment of the effects of arbuscular mycorrhizal (AM) infection on photosynthesis, carbon (C) allocation, translocation and biomass production of cucumber, grown in sand culture, was made using a previously determined phosphorus (P) supply (0·13 mol m^?3 P) which had a significant impact on AM infection. Separation of a direct effect of AM infection from an indirect one due to an enhanced leaf P status was achieved using a comparable non‐mycorrhizal treatment (NAM + P) supplemented with extra P (0·19 mol m^?3 P). Total leaf P concentration, specific leaf mass, photosynthetic capacity, and incorporation of ¹⁴C into non‐structural carbohydrate pools were dependent on leaf age. Both maximum and ambient photosynthetic rates were significantly higher in the youngest fully expanded leaves from AM and NAM + P plants which also had the higher leaf P concentrations. There were no differences in the total concentrations of starch, sucrose, raffinose or stachyose in young or old leaves among AM, non‐mycorrhizal (NAM) and NAM + P treatments. However, younger leaves of NAM plants showed a shift in ¹⁴C‐partitioning from stachyose and raffinose synthesis to starch accumulation. Determination of ADP‐glucose pyrophosphorylase (AGPase), sucrose synthase and sucrose phosphate synthase enzyme activities revealed that only AGPase activity was correlated with the increased incorporation rate of ¹⁴C into starch in young leaves of NAM plants. Although there were significant AM‐specific effects on C translocation to the root system, AM plants had similar rate of photosynthesis to NAM + P plants. These results suggest that the increase in photosynthetic rate in leaves of AM‐infected cucumber was due to an increased P status, rather than a consequence of a mycorrhizal ‘sink’ for assimilates.     

Diurnal Pattern of Translocation and Carbohydrate Metabolism in Source Leaves of Beta vulgaris L

Transitions in carbohydrate metabolism and translocation rate were studied for evidence of control of export by the sugar beet (Beta vulgaris L. Klein E.) source leaf. Steady-state labeling was carried out for two consecutive 14-hour light periods and various quantities related to translocation were measured throughout two 24-hour periods. Starch accumulation following illumination was delayed. Near the end of the light period, starch stopped accumulating, whereas photosynthesis rate and sucrose level remained unchanged. At the beginning of the dark period there was a 75-minute delay before starch was mobilized. The rate of import to the developing sink leaves at night was similar to that during the day, whereas export decreased considerably at night.

Starch accumulation and degradation seemed to be initiated in response to the level of illumination. Cessation of starch accumulation before the end of the light period was initiated endogenously. Exogenous control appeared to be mediated by the level of sucrose in the source leaf while endogenous control seemed to be keyed to photoperiod or photosynthetic duration.

     

Phloem Unloading in Developing Leaves of Sugar Beet : I. Evidence for Pathway through the Symplast

Physiological and transport data are presented in support of a symplastic pathway of phloem unloading in importing leaves of Beta vulgaris L. (`Klein E multigerm'). The sulfhydryl reagent p-chloromercuribenzene sulfonic acid (PCMBS) at concentration of 10 millimolar inhibited uptake of exogenous [¹⁴C]sucrose by sink leaf tissue over sucrose concentrations of 0.1 to 5.0 millimolar. Inhibited uptake was 24% of controls. The same PCMBS treatment did not affect import of ¹⁴C-label into sink leaves during steady state labeling of a source leaf with ¹⁴CO₂. Lack of inhibition of import implies that sucrose did not pass through the free space during unloading. A passively transported xenobiotic sugar, l-[¹⁴C]glucose, imported by a sink leaf through the phloem, was evenly distributed throughout the leaf as seen by whole-leaf autoradiography. In contrast, l-[¹⁴C]glucose supplied to the apoplast through the cut petiole or into a vein of a sink leaf collected mainly in the vicinity of the major veins with little entering the mesophyll. These patterns are best explained by transport through the symplast from phloem to mesophyll.     

PHOTOSYNTHETIC RATE AND MESOPHYLL SURFACE AREA IN EXPANDING LEAVES OF ALTERNANTHERA PHILOXEROIDES GROWN AT TWO LIGHT LEVELS

Leaves of Alternanthera philoxeroides, alligator weed, developed at a photosynthetic photon flux density (PPFD, light energy at wavelengths of 400 to 700 nm) of 790 μmol sec⁻¹ m⁻² (High Light) had less surface area, were thicker, had a higher maximum P_n (net rate of CO₂ uptake), and required a higher PPFD for saturation of P_n, than leaves developed at 160 μmol sec⁻¹ m⁻² (Low Light). Mesophyll thickness at Low Light was within 19% of maximum 2 days after emergence but at High Light, thickness increased 79% between 2 and 16 days after leaf emergence. The ratio of mesophyll surface area to leaf surface area decreased during development in both light treatments; the ratio, however, was over 70% greater in fully expanded High Light leaves than in Low Light leaves. Maximum P_n expressed on a leaf surface area basis was 158% greater in High Light leaves than in Low Light leaves, but P_n was only 58% greater when expressed on a mesophyll surface area basis. It was estimated that fully expanded High Light leaves fixed 72% more CO₂ per leaf (P_n expressed per unit surface area times the total surface area per leaf) than fully expanded Low Light leaves when P_n was measured at the PPFD leaves expanded under. Both High and Low Light leaves would fix about the same amount of CO₂ per leaf when P_n was measured at 160 μmol sec⁻¹ m⁻² because the larger surface area of the Low Light leaves offset small differences in P_n.     

Translocation of C Sucrose in Sugar Beet during Darkness

The time-course of arrival of ¹⁴C translocate in a sink leaf was studied in sugar beet (Beta vulgaris L. cultivar Klein Wanzleben) for up to 480 minutes of darkness. Following darkening of the source leaf, translocation rapidly declined, reaching a rate approximately 25% of the light period rate by 150 minutes. Comparison of data from plants that were girdled 1 cm below the crown with data from ungirdled plants indicates that after about 150 minutes darkness the beet root becomes a source of translocate to the sink leaf. After about 90 minutes darkness, starch-like reserve polysaccharide from the source leaf begins to contribute ¹⁴C to ethanol soluble pools in that leaf. Because of a 15% isotope mass effect, sucrose, at isotopic saturation, reaches a specific activity which is about 85% of the level of the supplied CO₂. The source leaf sucrose specific activity remains at the isotopic saturation level for about 150 minutes of darkness, after which time input from polysaccharide reserves causes the specific activity to drop to about 55% of that of the supplied CO₂. Sucrose specific activity determinations, polysaccharide dissolution measurements, and pulse labeling experiments indicate that following partial depletion of the sucrose pool, source leaf polysaccharide contributes to dark translocation. Respired CO₂ from the source leaf appears to be derived from a pool which, unlike sucrose, remains at a uniform specific activity.     

Growth rates and assimilate partitioning in the elongation zone of tall fescue leaf blades at high and low irradiance

Tall fescue (Festuca arundinacea Schreb.) leaf blades elongated 33% faster at continuous low than at continuous high irradiance (60 versus 300 micromoles per second per square meter photosynthetic photon flux density) when temperature of the leaf elongation zone was held constant at 21°C. Increased rate of elongation was associated with a near proportional increase in length of the elongation zone (+38%). In contrast, growth in width and thickness was decreased at low irradiance, resulting in only a 12% increase in leaf area production and 5% less total growth-associated water deposition than at high irradiance. At low irradiance dry matter (DM) import into the elongation zone was 28% less, and 55% less DM was used per unit leaf area produced. DM use in synthesis of structural components (i.e. DM less water-soluble carbohydrates) was only 13% less at low irradiance, whereas water-soluble carbohydrates (WSC) deposition was 43% less. The lower rate of WSC deposition at low irradiance was associated with a higher net rate of monosaccharide deposition (+39%), whereas net deposition rates for sucrose (−27%) and fructan (−56%) were less than at high irradiance. Still, at low irradiance, net fructan accumulation accounted for 64% of WSC deposition, i.e. 25% of DM import, demonstrating the high sink strength of the leaf elongation zone.     

Photosynthesis in leaves and siliques of winter oilseed rape (Brassica napus L.)

The rate of photosynthesis and its relation to tissue nitrogen content was studied in leaves and siliques of winter oilseed rape (Brassica napus L.) growing under field conditions including three rates of nitrogen application (0, 100 or 200 kg N ha^-1) and two levels of irrigation (rainfed or irrigated at a deficit of 20 mm). The predominant effect of increasing N application under conditions without water deficiency was enhanced expansion of photosynthetically active leaf and silique surfaces, while the rate of photosynthesis per unit leaf or silique surface area was similar in the different N treatments. Thus, oilseed rape did not increase N investment in leaf area expansion before a decline in photosynthetic rate per unit leaf area due to N deficiency could be avoided. Much less photosynthetically active radiation penetrated into high-N canopies than into low-N canopies. The specific leaf area increased markedly in low light conditions, causing leaves in shade to be less dense than leaves exposed to ample light. In both leaves and siliques the photosynthetic rate per unit surface area responded linearly to increasing N content up to about 2 g m^-2, thus showing a constant rate of net CO₂ assimilation per unit increment in N (constant photosynthetic N use efficiency). At higher tissue N contents, photosynthetic rate responded less to changes in N status. Expressed per unit N, light saturated photosynthetic rate was three times higher in leaves than in silique valves, indicating a more efficient photosynthetic N utilization in leaves than in siliques. Nevertheless, from about two weeks after completion of flowering and onwards total net CO₂ fixation in silique valves exceeded that in leaves because siliques received much higher radiation intensities than leaves and because the leaf area declined rapidly during the reproductive phase of growth. Water deficiency in late vegetative and early reproductive growth stages reduced the photosynthetic rate in leaves and, in particular, siliques of medium- and high-N plants, but not of low-N plants.     

Whole Plant and Leaf Steady State Gas Exchange during Ethylene Exposure in Xanthium strumarium L

The effects of ethylene evolved from ethephon on leaf and whole plant photosynthesis in Xanthium strumarium L. were examined. Ethylene-induced epinasty reduced light interception by the leaves of ethephon treated plants by up to 60%. Gas exchange values of individual, attached leaves under identical assay conditions were not inhibited even after 36 hours of ethylene exposure, although treated leaves required a longer induction period to achieve steady state photosynthesis. The speed of translocation of recently fixed ¹¹C-assimilate movement was not seriously impaired following ethephon treatment; however, a greater proportion of the assimilate was partitioned downward toward the roots. Within 24 hours of ethephon treatment, the whole plant net carbon exchange rate expressed on a per plant basis or a leaf area basis had dropped by 35%. The apparent inhibition of net carbon exchange rate was reversed by physically repositioning the leaves with respect to the light source. Ethylene exposure also inhibited expansion of young leaves which was partially reversed when the leaves were repositioned. The data indicated that ethylene indirectly affected net C gain and plant growth through modification of light interception and altered sink demand without directly inhibiting leaf photosynthesis.     

Influence of Benzyladenine, Leaf Darkening, and Ringing on Movement of C-labeled Assimilates Into Expanded Leaves of Vitis vinifera L

Leaves of Vitis vinifera L., nearly fully expanded, imported only trace amounts of ¹⁴C following assimilation of ¹⁴CO₂ by a lower leaf on the same shoot, but benzyladenine (BA) application at 4.4 × 10⁻³m caused a marked increase in the movement of ¹⁴C into these leaves. Older leaves near the shoot base were less responsive; BA treatment alone had little effect on import of labeled assimilates from adjacent leaves but when the BA-treated leaves were darkened there was an increased import of labeled materials. When these 2 treatments were combined and applied to leaves on shoots with ringed bases, relatively high levels of radioactivity were detected in the BA-treated leaves but under these conditions darkening, without the application of BA, also resulted in an increased import of ¹⁴C. Accumulation of imported ¹⁴C was found to be restricted to the area of the leaf blade treated with BA. Separation of labeled compounds in ethanol extracts of treated leaves showed a lower percentage of radioactivity present in the sugar fraction from BA-treated leaves and an increased percentage present in the amino acid fraction.     

Alterations in source-sink patterns by modifications of source strength

Bean plants, trimmed to a simplified “double source, double sink” translocation system (the paired primary leaves serving as the double source and the paired lateral leaflets of the immature first trifoliate leaf as the double sink) were used to study the magnitude and short-term time course of change in the allocation ratio (partition ratio) of assimilates translocated from the labeled primary leaf to its respective “near” and “far leaflet” sinks in response to an increase or decrease in the source strength of the opposite primary leaf (the “control” leaf). If the rates of net photosynthesis in the two primary leaves were similar, assimilates from the labeled source leaf partitioned to the leaflet sinks in the ratio of 5:1 or higher, the dominant sink being the leaflet “nearer” to the labeled source leaf. If the rate of net photosynthesis in the control leaf was increased substantially above that of the labeled source leaf, the rate of translocation from the labeled source to either the near leaflet sink or far leaflet sink remained unaffected, despite, presumably, a higher translocation rate from the control leaf, and hence a higher phloem pressure gradient (or increased cross-sectional area) in the transport pathway from the control leaf to the leaflet sinks. If the control leaf was excised, thus reducing the source leaf area by about a half, the translocation rate from the remaining source leaf rapidly doubled, the partition ratio becoming equal to unity. If the control leaf was darkened, the partition ratio adjusted to an intermediate value. Although export rates from the labeled source leaf were increased either by excising or darkening the control leaf, the rate of net photosynthesis in the labeled leaf remained constant.     

Isolation and photosynthetic characteristics of mesophyll cells from developing leaves of soybean

Mesophyll cells were isolated from developing sink leaves (25 to 30 mm in length) of soybean, Glycine max (L.) Merr. cv. Will. Leaf strips were incubated for two h in a buffered medium containing osmoticum and 0.2% Pectolyase Y-23. Gently stirring the leaf strips released from 7 to 16% of the total leaf mesophyll cells. Other pectinase enzymes, effective in releasing cells from mature source leaves (70 to 75 mm in length), did not release cells from sink leaves. Sink and source cell preparations were about 50 and 95% intact, respectively, based on the exclusion of Evans Blue dye. Intact cells could not be separated from broken cells on Ficoll or metrizamide density gradients. Total protein and catalase, glyceraldehyde-3-phosphate dehydrogenase, glycolate oxidase, phosphoenolpyruvate carboxylase, and ribulose 1,5-bisphosphate carboxylase activities on a chlorophyll basis were about 50% lower in sink mesophyll cells than in sink leaf homogenates indicating that broken sink cells lost soluble protein to the medium. Source cells and source leaf homogenates had comparable amounts of protein and enzymatic activities. Enzymatic activities on a chlorophyll basis were similar in source and sink leaves with the exception of phosphenolpyruvate carboxylase, which was two times higher in sink leaves. This enzyme was also exceptionally low in source and sink cells being only 61 and 23%, respectively, of whole leaf activities. Sink cell rates of ¹⁴CO₂ fixation were only 7% of source cell rates and sink cells did not show light-dependent O₂ evolution. Both cell preparations had photosystem II activiteis which were comparable to rates of ¹⁴CO₂ fixation at satuarating light and CO₂ concentration. It was concluded that the reduced photosynthetic rate of sink cells was limited by the low photochemical capacity rather than a limitation of Calvin cycle enzymes.     

Chlorophyll, Ribulose-1,5-diphosphate Carboxylase, and Hill Reaction Activity in Developing Leaves of Populus deltoides

The synthesis of chlorophyll and ribulose diphosphate carboxylase as well as the development of Hill reaction activity were followed in expanding Populus deltoides leaves and related to photosynthetic patterns. Total chlorophyll, which was not correlated with photosynthetic rate in expanding leaves, decreased slightly with age in very young leaves, due to a decrease in chlorophyll b, but then increased linearly. The ratio of chlorophyll a to b, which rose sharply in young leaves, was highly correlated with the onset of net photosynthesis. Hill reaction activity was very low in young leaves and did not increase significantly until leaves were about half expanded. Ribulose diphosphate carboxylase activity increased in a sigmoid fashion with leaf ontogenesis and closely paralleled development of the photosynthetic system. The study demonstrates the importance of chlorophyll a and Calvin cycle enzyme synthesis to photosynthetic development in expanding leaves.     

Enhancement of [C]Sucrose Export from Source Leaves of Vicia faba by Gibberellic Acid

The effect of gibberellic acid (GA₃) on sucrose export from source leaves was studied in broad bean (Vicia faba L.) plants trimmed of all but one source and one sink leaf. GA₃ (10 micromolar) applied to the source leaf, enhanced export of [¹⁴C]sucrose (generated by ¹⁴CO₂ fixation) to the root and to the sink leaf. Enhanced export was observed with GA treatments as short as 35 minutes. When GA₃ was applied 24 hours prior to the ¹⁴CO₂ pulse, the enhancement of sucrose transport toward the root was abolished but transport toward the upper sink leaf was unchanged. The enhanced sucrose export was not due to increased photosynthetic rate or to changes in the starch/sucrose ratio within the source leaf; rather, GA₃ increased the proportion of sucrose exported. After a 10-min exposure to [¹⁴C]GA₃, radioactivity was found only in the source leaf. Following a 2 hour exposure to [¹⁴C]GA₃, radioactivity was distributed along the entire stem and was present in both the roots and sink leaf. Extraction and partitioning of GA metabolites by thin layer chromatography indicated that there was a decline in [¹⁴C]GA₃ in the lower stem and root, but not in the upper stem. This pattern of metabolism is consistent with the disappearance of the GA₃ effect in the lower stem with time after treatment. We conclude that in the short term, GA₃ enhances assimilate export from source leaves by increasing phloem loading. In the long term (24 hours), the effect of GA₃ is outside the source leaf. GA₃ accumulates in the apical region resulting in enhanced growth and thus greater sink strength. Conversely, GA₃ is rapidly metabolized in the lower stem thus attenuating any GA effect.