EFFECT OF THE REDOX STATE OF Q(B) ON ELECTRIC-FIELD-INDUCED CHARGE RECOMBINATION IN PHOTOSYSTEM-II

Electric field-induced charge recombination in Photosystem II (PS LT) was studied in osmotically swollen spinach chloroplasts ('blebs') by m easurement of the concomitant chlorophyll luminescence emission (elect roluminescence). A pronounced dependence on the redox state of the two -electron gate Q(B) was observed and the earlier failure to detect it is explained. The influence of the Q(B)/Q(B)(-) oscillation on electro luminescence was dependent on the redox state of the oxygen evolving c omplex; at times around one millisecond after flash illumination a lar ge effect was observed in the states S-2 and S-3, but not in the state (S-4) (actually Z(+)S(3)). The presence of the oxidized secondary ele ctron donor, tyrosine Z(+), appeared to prevent expression of the Q(B) /Q(B)(-) effect on electroluminescence, possibly because this effect i s primarily due to a shift of the redox equilibrium between Z/Z(+) and the oxygen evolving complex.