Photosynthesis of transgenic Pssu-ipt tobacco

The elevated content of endogenous cytokinin (CK), as the result of the int roduction of the chimeric Pssu-ipt gene in tobacco, induced changes in grow th, water regime and photosynthesis of transgenic grafts and rooted plants grown in a greenhouse. Despite closed stomata remarkable water stress was d etected in Pssu-ipt plants. The ABA content was lower than in wild type tob acco, indicative of disturbances in ABA transport or metabolism. The rates of net photosynthesis (measured as CO2 uptake or O-2 evolution) decreased b y more than 50% in Pssu-ipt grafts and by 20% in Pssu-ipt rooted plants, re spectively. The partial reactions of the electron transport chain in transg enic Pssu-ipt tobacco were differently influenced: the activity of the reac tion centre of PSII was hardly affected; the PSI activity and the intersyst em electron transport chain was inhibited up to 70%, particularly in the Ps su-ipt grafts. Although the slight decrease in the potential photochemical efficiency of PSII, expressed as F-v/F-m, was found in transgenic plants, t he actual quantum yields, photochemical efficiencies, and q(P) under steady -state conditions indicated that transgenic plants were not seriously limit ed by the redox state of Q(A). The reaction centre of PSII was well preserv ed. In transgenic grafts, photophosphorylation capacity was strongly reduce d, which could correspond with lower non-photochemical quenching. The above mentioned changes are the results of elevated CK content per se rather tha n the effect of altered water relations in the plants, caused by the dispro portion of shoots and root system in both Pssu-ipt grafts and plants.