The rate of charge recombination in Photosystem II

Abstract

Loss by recombination of the charge separated state P680+QA- limits the performance of Photosystem II (PS II) 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 II when reduction of P680+ by the secondary electron donor Tyrosine Z cannot occur because YZ is already oxidized. The 1.4 ms recombination is seen in YZ-less mutants of the cyanobacterium Synechocystis. However, the rate of P680+QA- recombination that actually competes with the stabilization of the charge separation has not been previously reported. We have measured the kinetics 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 P680+ reduction by YZ were obtained, and the rate of the competing P680+QA- 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 YZox slightly decreased the efficiency of excitation trapping but did not seem to accelerate P680+QA- recombination. The two P680+QA- lifetimes in the literature probably reflect a significant difference between plant and cyanobacterial PS II. © 2002 Elsevier Science B.V. All rights reserved.