PICOSECOND TIME-RESOLVED FLUORESCENCE STUDIES ON PHOTOINHIBITION AND DOUBLE REDUCTION OF Q(A) IN PHOTOSYSTEM-II

The influence of double reduction of Q(A) the secondary electron accep tor in Photosystem (PS) II, on the primary events of PS II electron tr ansport was studied by picosecond time-resolved fluorescence measureme nts in isolated PS II membranes. The double reduction was achieved eit her by chemical treatment or by strong anaerobic illumination. In the presence of doubly reduced Q(A) fluorescence decay from the PS II reac tion centres showed a dominant amount of a fast phase (150-250 ps) sim ilar to open PS II. In addition to two further components of 600 ps an d 2-3 ns, a long-lived component of about 10 ns was observed which is characteristic of the doubly reduced state only. The data indicate eff icient primary charge separation as well as the presence of a long-liv ed radical pair when Q(A) is doubly reduced. From these results it is concluded that the electrostatic effect of the two electrons on Q(A)(2 -), which is expected to strongly suppress primary charge separation, is neutralized, most likely by protonation at or near the Q(A) site. A nalysis of the decay curves in the framework of the exciton/radical pa ir equilibrium model indicates the formation of a relaxed radical pair state. Fluorescence quenching due to aerobic photoinhibition was show n to arise from the increase of a fast decaying (300-320 ps) component at the expense of the more slowly decaying components during the phot oinhibitory treatment. No long-lived component in the range of about 1 0 ns was observed in case of aerobic photoinhibition. It is concluded that, in contrast to the anaerobic photoinhibition, no doubly reduced Q(A) is accumulated during the aerobic photoinhibitory treatment.