EXPRESSING AN RBCS ANTISENSE GENE IN TRANSGENIC FLAVERIA-BIDENTIS LEADS TO AN INCREASED QUANTUM REQUIREMENT FOR CO2 FIXED IN PHOTOSYSTEM-I AND PHOTOSYSTEM-II

It was previously shown with concurrent measurements of gas exchange a nd carbon isotope discrimination that the reduction of ribulose-1,5-bi sphosphate carboxylase/oxygenase by an antisense gene construct in tra nsgenic Flaveria bidentis (a C-4 species) leads to reduced CO2 assimil ation rates, increased bundle-sheath CO2 concentration, and leakiness (defined as the ratio of CO2 leakage to the rate of C-4 acid decarboxy lation; S. von Caemmerer, A. Millegate, G.D. Farquhar, R.T. Furbank [1 997] Plant Physiol 113: 469-477). Increased leakiness in the transform ants should result in an increased ATP requirement per mole of CO2 fix ed and a change in the ATP-to-NADPH demand. To investigate this, we co mpared measurements of the quantum yield of photosystem I and II (Phi( PSI) and Phi(PSII)) with the quantum yield of CO2 fixation (Phi(CO2)) in control and transgenic F. bidentis plants in various conditions. Bo th Phi(PSI)/Phi(CO2) and Phi(PSII)/Phi(CO2) increased with a decrease in ribulose-1,5-bisphosphate carboxylase/oxygenase content, confirming an increase in leakiness. In the wild type the ratio of Phi(PSI) to P hi(PSII) was constant at different irradiances but increased with irra diance in the transformants, suggesting that cyclic electron transport may be higher in the transformants. To evaluate the relative contribu tion of cyclic or linear electron transport to extra ATP generation, w e developed a model that links leakiness, ATP/NADP requirements, and q uantum yields. Despite some uncertainties in the light distribution be tween photosystem I and II, we conclude from the increase of Phi(PSII) /Phi(CO2) in the transformants that cyclic electron transport is not s olely responsible for ATP generation without NADPH production.