Methyl Viologen-Dependent Cyclic Electron Transport in Spinach Chloroplasts in the Absence of Oxygen

The methyl viologen (MV)-dependent, linear electron flow fromPS II to PS I was severely blocked in intact or broken, uncoupledchloroplasts when oxygen was removed from the suspension medium,as revealed by measurements of chlorophyll fluorescence andthe rate of photoreduction of MV. Kinetics of the reductionof pre-oxidized P700 by a saturating light pulse showed thatreduced MV in the absence of oxygen re-reduces P700⁺ via theintersystem electron transport chain. Since the re-reductionof P700⁺ was inhibited by 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone,the MV-mediated cyclic electron flow, in contrast to the phenazinemethosulphate-catalyzed one, involves the plastoquinone pool.However, 2-n-heptyl-4-hydro-xyquinoline-N-oxide, 2-n-nonyl-4-hydroxyquinoline-N-oxideand antimycin A did not inhibit the MV-mediated flow. Thus,the inhibition of the linear electron flow in chloroplasts underanaerobic conditions suggested the overreduction of the plastoquinonepool as a result of the MV-mediated cyclic flow (Received February 13, 1990; Accepted March 31, 1990)