Methyl viologen-dependent cyclic electron transport in spinach [Spinacia oleracea] chloroplasts in the absence of oxygen

The methyl viologen (MV)-dependent, linear electron flow from PS II to PS I was severely blocked in intact or broken, uncoupled chloroplasts when oxygen was removed from the suspension medium, as revealed by measurements of chlorophyll fluorescence and the rate of photoreduction of MV. Kinetics of the reduction of pre-oxidized P700 by a saturating light pulse showed that reduced MV in the absence of oxygen re-reduces P700+ via the intersystem electron transport chain. Since the re-reduction of P700+ was inhibited by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, the MV-mediated cyclic electron flow, in contrast to the phenazine methosulphate-catalyzed one, involves the plastoquinone pool. However, 2-n-heptyl-4-hydro-xyquinoline-N-oxide, 2-n-nonyl-4-hydroxyquinoline-N-oxide and antimycin A did not inhibit the MV-mediated flow. Thus, the inhibition of the linear electron flow in chloroplasts under anaerobic conditions suggested the overreduction of the plastoquinone pool as a result of the MV-mediated cyclic flow