PREFERENTIAL DEACTIVATION OF THE S(3) STATE OF THE WATER-OXIDIZING COMPLEX, FAVORED BY PLASTOQUINONE REDUCTION IN BARLEY CHLOROPLASTS

The flash patterns of oxygen evolution after pre-illumination by continuous light or by a flash sequence were compared in etiochloroplasts and mature chloroplasts of barley. In both types of plastids sub-saturating continuous illumination of some seconds strongly affected the S3 but not the S2 state of the oxygen-evolving complex. This result is ascribed to efficient charge recombination of S3 with the acceptor side of photosystem II, favoured by partial reduction of the plastoquinone pool. The increase of S3 observed in the presence of dichlorophenolindophenol in etiochloroplasts confirms this interpretation. These observations strengthen the recent hypothesis of a conformational change during the transition from S2 to S3 recently proposed to interpret the different susceptibility of these two states to hydroxylamine and hydrazine (F. Frank and G. H. Schmid, Biochim. Biophys. Acta 977, 215-218 (1989); J. Messinger, U. Wacker, and G. Renger, Biochemistry 30,7852-7862 (1991)).