Decreased Ribulose-1,5-Bisphosphate Carboxylase-Oxygenase in Transgenic Tobacco Transformed with Antisense Rbcs .6. Effect on Photosynthesis in Plants Grown at Different Irradiance

Tobacco (Nicotiana tabacum L.) plants trans- formed with 'antisense' rbcS to decrease the expression of ribulose-l,5-bisphosphate carboxylase-oxygenase (Ru- bisco) have been used to investigate the contribution of Rubisco to the control of photosynthesis in plants grow- ing at different irradiances. Tobacco plants were grown in controlled-climate chambers under ambient CO2 at 20~ at 100, 300 and 750 gmol.m 2.s-1 irradiance, and at 28~ at 100, 300 and 1000 Dmol.m 2. s-I irradiance. (i) Measurement of photosynthesis under ambient condi- tions showed that the flux control coefficient of Ru- bisco A (CRubisco) was very low (0.01-0.03) at low growth irradiance, and still fairly low (0.24-0.27) at higher irra- diance. (ii) Short-term changes in the irradiance used to CRubisc o increases measure photosynthesis showed that A as incident irradiance rises. (iii) When low-light (100 Dmol'm 2's 1)-grown plants are exposed to high (750-1000 Dmol-m 2"s ~) irradiance, Rubisco is almost totally limiting for photosynthesis in wild types. How- ever, when high-light-grown leaves (750 1000 ~tmol.m 2.s t) are suddenly exposed to high and satu- rating irradiance (1500-2000Dmol.m-2-s 1), A CRubisco remained relatively low (0.23-0.33), showing that in satu- rating light Rubisco only exerts partial control over the * Present address: Institute of Field and Vegetable Crops, Universi-