INFLUENCE OF DIFFERENT UNCOUPLERS ON PHOTOSYSTEM-II PHOTOCHEMISTRY AND FLUORESCENCE

Energy-dependent quenching of chlorophyll fluorescence, its reversal by uncouplers and NADP reduction were studied in isolated thylakoids as functions of light intensity and the de-epoxidation of carotenoids. Lipophilic uncouplers, but not NH4Cl, enhance fluorescence whether or not quenching has previously occurred, and increase PS II limited electron transport under low light intensity, independently of the occurrence of carotenoid de-epoxidation. Energy dependent fluorescence quenching, which is highly enhanced by de-epoxidation, is reversed by all uncouplers. We interpret these observations to indicate that membrane localized protons are involved in fluorescence quenching and the regulation of PS II efficiency, and that the stimulation of PS II fluorescence by lipophilic uncouplers results of two processes, only one of them associated with the reversal of the light induced fluorescence quenching (qE) which is enhanced by carotenoid de-epoxidation.