Photoreduction of pheophytin at the reaction centres of photosystem II of intact cells of green algae and cyanobacteria in anaerobic conditions

The reversible photoreduction of pheophytin (Pheo) has been detected; it is accompanied by fall in the yield of fluorescence of chlorophyll, in photosystem II of the intact cells of green algae and cyanobacteria in anaerobic conditions. It is shown that this photoreaction is inhibited by diurone and reactivated on subsequent addition of ascorbate or dithionite. It is assumed that donation of an electron from the system of oxidation of water present in the S₃ state to the reaction centre of system II in the {A figure is presented}Q^- state after the primary photoreaction results in the formation of a longlived state [P₆₈₀Pheo{A figure is presented}]Q^-. It was found that oxidized NADP, benzyl viologen and methyl viologen speed up oxidation of Pheo reduced in the light indicating the ability of these electron acceptors, typical of photosystem I, to accept an electron from Pheo{A figure is presented} in the photosystem II.