Preferential Deactivation of the S3 State of the Water-Oxidizing Complex, Favoured by Plastoquinone Reduction in Barley Chloroplasts

The flash patterns o f oxygen evolution after pre-illum ination by continuous light or by a flash sequence were com pared in etiochloroplasts and m ature chloroplasts o f barley. In both types o f plastids sub-saturating continuous illum ination o f som e seconds strongly affected the S3 but not the S2 state o f the oxygen-evolving com plex. This result is ascribed to efficient charge recom bination o f S3 with the acceptor side o f photosystem II, favoured by partial re­ duction o f the plastoquinone pool. The increase o f S3 observed in the presence o f dichlorophenolindophenol in etiochloroplasts confirm s this interpretation. These observations strengthen the recent hypothesis o f a conform ational change during the transition from S2 to S3 recently proposed to interpret the different susceptibility o f these tw o states to hydroxylam ine and hydrazine (F. Frank and G. H. Schm id, Biochim . Biophys. A cta 977, 2 1 5 -2 1 8 (1989); J. M essinger, U . W acker, and G. Renger, Biochem istry 30, 7 8 5 2 -7 8 6 2 (1991)).