BLUE LIGHT-ENHANCED PHOTOSYNTHETIC OXYGEN EVOLUTION FROM LIPOSOME-BOUND PHOTOSYSTEM-II PARTICLES - POSSIBLE ROLE OF THE XANTHOPHYLL CYCLE IN THE REGULATION OF CYCLIC ELECTRON FLOW AROUND PHOTOSYSTEM-II

Light-driven electron transport in liposome-bound photosystem II (PS-I I) particles between water and ferricyanide was monitored by bare plat inum electrode oxymetry. The modification of the experimental system w ith the exogenous quinones alpha-tocopherol quinone (alpha-TQ) or plas toquinone (PQ) resulted in a pronounced effect on photosynthetic oxyge n evolution. The presence of alpha-tocopherolquinone (alpha-TQ) in PS- II samples decreased the rate of red light-induced oxygen evolution bu t increased the rate of green Iight-induced oxygen evolution. Blue lig ht applied to the assay system in which oxygen evolution was saturated by red light resulted in a further increase of the oxygen signal. The se findings are interpreted in terms of a cyclic electron transport ar ound PS-II, regulated by an excitation state of beta-carotene in the r eaction centre of PS-II. A mechanism is postulated according to which energetic coupling of beta-carotene in the reaction centre of PS-II an d that of other antenna carotenoid pigments is regulated by the portio n of the xanthophyll violaxanthin, which is under control of the xanth ophyll cycle.