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 transformed with 'antisense' rbc S to decrease the expression of ribulose-1,5-bisphosphate carboxylase- oxygenase (Rubisco) have been used to investigate the contribution of Rubisco to the control of photosynthesis in plants growing at differen t irradiances. Tobacco plants were grown in controlled-climate chamber s under ambient CO2 at 20-degrees-C at 100, 300 and 750 mumol.m-2.s-1 irradiance, and at 28-degrees-C at 100, 300 and 1000 mumol.m-2.2-1 irr adiance. (i) Measurement of photosynthesis under ambient conditions sh owed that the flux control coefficient of Rubisco (C(Rubisco)A) was ve ry low (0.01-0.03) at low growth irradiance, and still fairly low (0.2 4-0.27) at higher irradiance. (ii) Short-term changes in the irradianc e used to measure photosynthesis showed that C(Rubisco)A increases as incident irradiance rises. (iii) When low-light (100 mumol.m-2.s-1)-gr own plants are exposed to high (750-1000 mumol-m-2.s-1) irradiance, Ru bisco is almost totally limiting for photosynthesis in wild types. How ever, when high-light-grown leaves (750-1000 mumol.m-2-s-1) are sudden ly exposed to high and saturating irradiance (1500-2000 mumol.m-2.s-1) , C(Rubisco)A remained relatively low (0.23-0.33), showing that in sat urating light Rubisco only exerts partial control over the light-satur ated rate of photosynthesis in ''sun'' leaves; apparently additional f actors are co-limiting photosynthetic performance. (iv) Growth of plan ts at high irradiance led to a small decrease in the percentage of tot al protein found in the insoluble (thylakoid fraction), and a decrease of chlorophyll, relative to protein or structural leaf dry weight. As a consequence of this change, high-irradiance-grown leaves illuminate d at growth irradiance avoided an inbalance between the ''light'' reac tions and Rubisco; this was shown by the low value of C(Rubisco)A (see above) and by measurements showing that non-photochemical quenching w as low, photochemical quenching high, and NADP-malate dehydrogenase ac tivation was low at the. growth irradiance. In contrast, when a leaf a dapted to low irradiance was illuminated at a higher irradiance, Rubis co exerted more control, non-photochemical quenching was higher, photo chemical quenching was lower, and NADP-malate dehydrogenase activation was higher than in a leaf which had grown at that irradiance. We conc lude that changes in leaf composition allow the leaf to avoid a one-si ded limitation by Rubisco and, hence, overexcitation and overreduction of the thylakoids in high-irradiance growth conditions. (v) 'Antisens e' plants with less Rubisco contained a higher content of insoluble (t hylakoid) protein and chlorophyll, compared to total protein or struct ural leaf dry weight. They also showed a higher rate of photosynthesis than the wild type, when measured at an irradiance below that at whic h the plant had grown. We propose that N-allocation in low light is no t optimal in tobacco and that genetic manipulation to decrease Rubisco may, in some circumstances, increase photosynthetic performance in lo w light.