The spectroscopic measurements of the slow phase of the electrochromic effect and the redox kinetics of cytochrome b 6 and f provide strong evidence that a Q cycle operates in chloroplasts under conditions of non-cyclic electron transport. The effect of HQNO and DBMIB on the extent and kinetics of these light-induced changes places several constraints on the mechanism of quinol oxidation by the cyt. b/f—FeS complex: for each electron removed from the cyt. b/f—FeS complex by P700 an additional charge is transferred across the membrane; the cyclic pathway of electrons involved in quinol oxidation by the cyt. b/f—FeS complex includes at least one of the two b6 cytochromes; the electrogenic step associated with quinol oxidation is subsequent to the reduction of at least one cytochrome b6 quinol oxidation may proceed in a stepwise manner, with the first electron going to cytochrome b6 and the second electron going to the FeS center and cytochrome f.
