It has been hypothesized that photosynthesis can be feedback limited when the phosphate concentration cannot be both low enough to allow starch and sucrose synthesis at the required rate and high enough for ATP synthesis at the required rate. We have measured the concentration of phosphate in the stroma and cytosol of leaves held under feedback conditions. We used nonaqueous fractionation techniques with freeze-clamped leaves of Phaseolus vulgaris plants grown on reduced phosphate nutrition.Feedback was induced by holding leaves in low 02 or high CO2 partial pressure. We found 7 millimolar phosphate in the stroma of leaves in normal oxygen but just 2.7 millimolar phosphate in leaves held in low oxygen. Because 1 to 2 millimolar phosphate in the stroma may be metabolically inactive, we estimate that in low oxygen, the metabolically active pool of phosphate is between negligible and 1.7 millimolar. We conclude that halfway between these extremes, 0.85 millimolar is a good estimate of the phosphate concentration in the stroma of feedback-limited leaves and that the true concentration could be even lower. The stromal phosphate concentration was also low when leaves were held in high C02, which also induces feedback-limited photosynthesis, indicating that the effect is related to feedback limitation, not to low oxygen per se. We conclude that the concentration of phosphate in the stroma is usually in excess and that it is sequestered to regulate photosynthesis, especially starch synthesis. The capacity for this regulation is limited by the coupling factor requirement for phosphate.During photosynthesis, phosphate is required by the coupling factor for the production of ATP from ADP. At the same time, phosphate inhibits starch (15) and so the phosphate level need not fall to such low levels to limit photosynthesis.The concentration of phosphate in the stroma has been measured in the past (3, 21, 36), but it has never been measured in feedback-limited leaves, that is leaves which exhibit 02-insensitive photosynthesis. We decided to measure the concentration of phosphate in the stroma and cytosol of leaves exhibiting feedback-limited photosynthesis. This condition can be induced by feeding the phosphate sequestering agent mannose (9) and can also occur under natural conditions ( 13,20).The measurement of stromal phosphate concentration is difficult because plants grown on luxuriant levels of phosphate, as is common practice in research, usually have a large amount of metabolically inactive phosphate in the vacuole (4, 39). This problem can be overcome by growing plants with more realistic phosphate nutrition. When the phosphate supply is restricted, the phosphate concentration in the vacuole can be substantially reduced with little or no effect on photosynthesis (5, 22

The atmospheric hydrocarbon budget is important for predicting ozone episodes and the effects of pollution mitigation strategies. Isoprene emission from plants is an important part of the atmospheric hydrocarbon budget. We measured isoprene emission capacity at the bottom, middle, and top of the canopies of a white oak (Quercus alba L.) tree and a red oak (Quercus rubra L.) tree growing adjacent to a tower in the Duke University Forest. Leaves at the top of the white oak tree canopy had a three- to fivefold greater capacity for emitting isoprene than leaves at the bottom of the tree canopy. Isoprene emission rate increased with increasing temperature up to about 42 degrees C. We conclude that leaves at the top of the white oak tree canopy had higher isoprene emission rates because they were exposed to more sunlight, reduced water availability, and higher temperature than leaves at the bottom of the canopy. Between 35 and 40 degrees C, white oak photosynthesis and stomatal conductance declined, whereas red oak (Quercus rubra) photosynthesis and stomatal conductance increased over this range. Red oak had lower rates of isoprene emission than white oak, perhaps reflecting the higher stomatal conductance that would keep leaves cool. The concentration of isoprene inside the leaf was estimated with a simplified form of the equation used to estimate CO(2) inside leaves.

Intramolecular deuterium distributions of the carbonbound hydrogens of glucose were measured using deuterium nuclear magnetic resonance. Glucose isolated from leaf starch of common bean (Phaseolus vulgaris cv. Linden) or spinach (Spinacia oleracea cv. Giant nobel) was depleted in deuterium in the C(2) position, compared with glucose isolated from leaf sucrose or bean endosperm starch. In beans, the depletion of C(2) was independent of the light intensity during growth (150 or 700 µmol photons s -1 m -2 ). The ratio of glucose-6-phosphate to fructose-6-phosphate ([G6P]/[F6P]) in bean chloroplasts was 0·9 in high light, indicating that the phosphoglucose isomerase reaction was not in equilibrium ([G6P]/[F6P]) ≈ 3). This implies that the kinetic isotope effect of phosphoglucose isomerase depleted deuterium in the C(2) position of G6P. Because the depletion was the same, the chloroplastic ([G6P]/[F6P]) ratio was in disequilibrium irrespective of the light intensity. If the ([G6P]/[F6P]) ratio was in equilibrium, a large chloroplastic pool of G6P would be unavailable for regeneration of ribulose-1,5-bisphospate. We argue that chloroplast phosphoglucose isomerase activity is regulated to avoid this. The deuterium depletion of C(2) explains the known low overall deuterium abundance of leaf starch. This example shows that measurements of intramolecular deuterium distributions can be essential to understand overall deuterium abundances of plant material.

Abstract. Photosynthesis, leaf assimilate partitioning, flowering, and fruiting were examined in two lines of Lycopersicon esculentum Mill. transformed with a gene coding for sucrose-phosphate synthase (SPS) (EC 2.3.1.14) from Zea mays L. expressed from a tobacco ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco) small subunit promoter. Plants were grown at either 35 or 65Pa CO 2 and high light (1000gmol photons.m-2.s-1). Limiting and maximum SPS activities were significantly greater (up to 12 times) in the leaves of SPS-transformed lines for all treatments. Partitioning of carbon into sucrose increased 50% for the SPS transformants. Intact leaves of the control lines exhibited CO2-insensitivity of photosynthesis at high CO2 levels, whereas the SPS transformants did not exhibit CO2-insensitivity. The O2-sensitivity of photosynthesis was also greater for the SPS-transformed lines compared to the untransformed control when measured at 65 Pa CO 2. These data indicate that the SPS transformants had a reduced limitation on photosynthesis imposed by endproduct synthesis. Growth at 65 Pa CO 2 resulted in reduced photosynthetic capacity for control lines but not for SPS-transformed lines. When grown at 65 Pa CO2, SPS transformed lines had a 20% greater photosynthetic rate than controls when measured at 65 Pa COz and a 35% greater rate when measured at 105 Pa CO 2. Photosynthetic rates were not different between lines when grown at 35 Pa CO z. The time to 50% blossoming was reduced and the total number of inflorescences was significantly greater for the SPS transformants when grown * Present addresses: Department of Biology, Queen's University, Kingston, Ontario K7L 3N6, Canada ** Department of Biology, Keimyung University, 1000 SindangDong, Taegu 704-701, Korea Abbreviations: DAS=days after seeding; nptlI=neomycin phosphotransferase; Rubisco=ribulose-l,5-bisphosphate carboxylase/ oxygenase; RuBP=ribulose-l,5-bisphosphate; SPS=sucrose-phosphate synthase; SSU=Rubisco small subunit Correspondence to: T.D. Sharkey; FAX: 1 (608) 262 7509; E-mail: Sharkey @ macc.wisc.edu at either 35 or 65 Pa CO 2. At 35 Pa CO 2, the total fruit number of the SPS transformants was up to 1.5 times that of the controls, the fruit matured earlier, and there was up to a 32% increase in total fruit dry weight. Fruit yield was not significantly different between the lines when grown at 65 Pa CO 2. Therefore, there was not a strict relationship between yield and leaf photosynthesis rate. Flowering and fruit development of the SPS-transformed lines grown at 35 Pa CO 2 showed similar trends to the controls grown at 65 Pa CO 2. Incidences of blossom-end rot were also reduced in the SPS-transformed lines. These data indicate that altering starch/sucrose partitioning by increasing the capacity for sucrose synthesis can affect acclimation to elevated CO 2 partial pressure and fowering and fruiting in tomato.

We report on the export capability and structural and ultrastructural characteristics of leaves of the sucrose export defective1 (sed1; formerly called sut1) maize mutant. Whole-leaf autoradiography was combined with light and transmission electron microscopy to correlate leaf structure with differences in export capacity in both wild-type and sed1 plants. Tips of sed1 blades had abnormal accumulations of starch and anthocyanin and distorted vascular tissues in the minor veins, and they did not export sucrose. Bases of sed1 blades were structurally identical to those of the wild type and did export sucrose. Electron microscopy revealed that only the plasmodesmata at the bundle sheath-vascular parenchyma cell interface in sed1 minor veins were structurally modified. Aberrant plasmodesmal structure at this critical interface results in a symplastic interruption and a lack of phloem-loading capability. These results clarify the pathway followed by photosynthates, the pivotal role of the plasmodesmata at the bundle sheath-vascular parenchyma cell interface, and the role of the vascular parenchyma cells in phloem loading.