THE REGULATION OF COMPONENT PROCESSES OF PHOTOSYNTHESIS IN TRANSGENICTOBACCO WITH DECREASED PHOSPHORIBULOKINASE ACTIVITY

Tobacco plants (Nicotiana tabacum L.) transformed with an inverted cDN A encoding ribulose 5-phosphate kinase (phosphoribulokinase,PRK;EC 2.7 .1.19) were employed to study the in vivo relationship between photosy nthetic electron transport and the partitioning of electron transport products to major carbon metabolism sinks under conditions of elevated ATP concentrations and limited ribulose 1,5-bisphosphate (RuBP) regen eration. Simultaneous measurements of room temperature chlorophyll flu orescence and CO2 gas exchange were conducted on intact leaves. Under ambient CO2 concentrations and light intensities above those at which the plants were grown, transformants with only 5% of PRK activity show ed 'down-regulation' of PS II activity and electron transport in respo nse to a decrease in net carbon assimilation when compared to wild-typ e. This was manifested as a decline in the efficiency of PS II electro n transport (Phi(PS II)), an increase in dissipation of excess absorbe d light in the antennae of PS II and a decline in : total linear elect ron transport (J(1)), electron transport dedicated to carbon assimilat ion (J(A)) and electron transport allocated to photorespiration (J(L)) . The transformants showed no alteration in the Rubisco specificity fa ctor measured in vitro and calculated in vivo but had a relatively sma ller ratio of RuBP oxygenation to carboxylation rates (v(o)/v(c)), due to a higher CO2 concentration at the carboxylation site (C-c). The re lationship between Phi(PS II) and Phi(CO2) was similar in transformant s and wild-type under photorespiratory conditions demonstrating no cha nge in the intrinsic relationship between PS II function and carbon as similation, however, a novel result of this study is that this similar relationship occurred at different values of quantum flux, J(1), J(A) , J(L) and v(o)/v(c) in the transformant. For both wild-type and trans formants, an assessment was made of the possible presence of a third m ajor sink for electron transport products, beside RuBP oxygenation and carboxylation, the data provided no evidence for such a sink.