Picosecond time-resolved fluorescence studies on photoinhibition and double reduction of Q A in photosystem II

Abstract

The influence of double reduction of Q A, the secondary electron acceptor in Photosystem (PS) II, on the primary events of PS II electron transport was studied by picosecond time-resolved fluorescence measurements in isolated PS II membranes. The double reduction was achieved either by chemical treatment or by strong anaerobic illumination. In the presence of doubly reduced Q A fluorescence decay from the PS II reaction centres showed a dominant amount of a fast phase (150–250 ps) similar to open PS II. In addition to two further components of 600 ps and 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 efficient primaru charge separation as well as the presence of a long-lived 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. Analysis of the decay curves in the framework of the exciton/radical pair equilibrium model indicates the formation of a relaxed radical pair state. Fluorescence quenching due to aerobic photoinhibition was shown to arise from the increase of a fast decaying (300–320 ps) component at the expense of the more slowly decaying components during the photoinhibitory treatment. No long-lived component in the range of about 10 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.