Overexpression of sucrose-phosphate synthase in tomato plants grown with CO2 enrichment leads to decreased foliar carbohydrate accumulation relative to untransformed controls

Tomato plants expressing the maize sucrose-phosphate synthase (SPS) cDNA under the control of the promoterof the small subunit of ribulose-1,5-bisphosphate carboxylase oxygenase (rbcS) promoter were grown 5 weeks in air (450 μmol.m–2.s–1 irradiance, 350 ppm CO2) and then either maintained in air or exposed to CO2 enrichment (1 000 ppm CO2) for 8 d. A linear relationship between the foliar sucrose to starch ratio and maximal extractable SPS activity was found both in air and high CO2. Starch accumulation was dramatically increased in all plants subjected to CO2 enrichment but the CO2-dependent increase in foliar starch accumulation was much lower in the leaves of the SPS transformants than in those of the untransformed controls in the same conditions. Maximal extractable ribulose-1,5-bisphosphate carboxylase/oxygenase activity was reduced by growth at high CO2 to a similar extent in both plant types. The carbon/nitrogen ratios were similar in both plant lines in both growth conditions after 20 d exposure to high CO2. A small (5 %) increase in carbon export capacity was observed at high CO2 in the leaves of transformed plants compared to leaves from untransformed controls. Increased foliar SPS activity did not, however, prevent acclimation of photosynthesis in plants grown with long-term CO2 enrichment.