EFFECTS OF LIGHT AND ATMOSPHERIC CARBON-DIOXIDE ENRICHMENT ON PHOTOSYNTHESIS AND CARBON PARTITIONING IN THE LEAVES OF TOMATO (LYCOPERSICON-ESCULENTUM L) PLANTS OVER-EXPRESSING SUCROSE-PHOSPHATE SYNTHASE

Photosynthetic carbon assimilation, carbon partitioning and foliar car bon budgets were measured in the leaves of transformed tomato plants e xpressing a maize sucrose-phosphate synthase (SPS) gene in addition to the native enzyme, and in untransformed controls. The maize SPS gene was expressed under control of either the promoter of the small subuni t of ribulose 1,5-bisphosphate carboxylase (rbcS promoter; lines 2, 9 and 18) or the 35S promoter from cauliflower mosaic virus (CaMV promot er; line 13). The rate of sucrose synthesis was increased relative to that of starch and sucrose/starch ratios were higher throughout the ph otoperiod in the leaves of all plants expressing high SPS activity. Th e leaf carbon budget over the day/night cycle in air at low irradiance (180 mu mol photon m(-2) s(-1)) was similar in all plants. Net photos ynthesis measured in air and at elevated CO2 (800-1500 mu l I-1) on wh ole plants grown in air at 400 mu mol m(-2) s(-1) irradiance was signi ficantly increased in the high SPS expressors compared to the untransf ormed controls and was highest where SPS activity was greatest. At hig h CO2 the stimulation of photosynthesis was more pronounced, We conclu de that SPS activity is a major point of control of photosynthesis par ticularly under saturating light and CO2.