Photosynthesis of transgenic Pssu-ipt tobacco

Abstract

The elevated content of endogenous cytokinin (CK), as the result of the introduction of the chimeric Pssu-ipt gene in tobacco, induced changes in growth, water regime and photosynthesis of transgenic grafts and rooted plants grown in a greenhouse. Despite closed stomata remarkable water stress was detected in Pssu-ipt plants. The ABA content was lower than in wild type tobacco, indicative of disturbances in ABA transport or metabolism. The rates of net photosynthesis (measured as CO2 uptake or O-2 evolution) decreased by more than 50% in Pssu-ipt grafts and by 20% in Pssu-ipt rooted plants, respectively. The partial reactions of the electron transport chain in transgenic Pssu-ipt tobacco were differently influenced: the activity of the reaction centre of PSII was hardly affected; the PSI activity and the intersystem electron transport chain was inhibited up to 70%, particularly in the Pssu-ipt grafts. Although the slight decrease in the potential photochemical efficiency of PSII, expressed as F-v/F-m, was found in transgenic plants, the actual quantum yields, photochemical efficiencies, and q(P) under steady-state conditions indicated that transgenic plants were not seriously limited by the redox state of Q(A). The reaction centre of PSII was well preserved. In transgenic grafts, photophosphorylation capacity was strongly reduced, which could correspond with lower non-photochemical quenching. The above mentioned changes are the results of elevated CK content per se rather than the effect of altered water relations in the plants, caused by the disproportion of shoots and root system in both Pssu-ipt grafts and plants.