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.