The rate of charge recombination in Photosystem II

Loss by recombination of the charge separated state P M,Q limits the performance of Photosystem 11 (PS 11) as a photochemical energy converter. Time constants reported in literature for this process are mostly either near 0.17 ms or near 1.4 ms. The shorter time is found in plant PS 11 when reduction of P, by the secondary electron donor Tyrosine Z cannot occur because Y-Z is already oxidized. The 1.4 ins recombination is seen ill Y-Z-less mutants of the cyanobacterium Synechocystis. However, the rate of P(680)(+)Q(A)(-) recombination that actually competes with the stabilization of the charge separation has not been previously reported. We have measured the kinetic, of the flash-induced fluorescence yield changes in the microsecond time domain in Tris-washed spinach chloroplasts. In this way the kinetics and yield of P reduction by Y-Z were obtained and the rate of the competing P(680)(+)Q(A)(-) recombination could be evaluated. The recombination time was less than 0.5 ms the best-fitting time constant was 0.1 ms. The presence of Y-Z(08) slightly decreased the efficiency of excitation trapping but did not seem to accelerate P(680)(+)Q(A)(-) recombination. The two P(680)(+)Q(A)(-) lifetimes in the literature probably reflect a significant difference between plant and cyanobacterial PS II. (C) 2002 Elsevier Science B.V. All rights reserved.