COUPLING RATIOS H+ E=3 VERSUS H+/E=2 IN CHLOROPLASTS AND QUANTUM REQUIREMENTS OF NET OXYGEN-EXCHANGE DURING THE REDUCTION OF NITRITE, FERRICYANIDE OR METHYLVIOLOGEN/

Using intact and osmotically ruptured chloroplasts, ratios of coupling between deposition of protons in the intrathylakoid space and light-d ependent transport of electrons from water to an external acceptor wer e determined. The data indicate coupling between proton and electron t ransport at a ratio of H+/e = 3 with methylviologen as electron accept or in thylakoids and with nitrite as electron acceptor in intact chlor oplasts. With ferricyanide as electron acceptor in thylakoids, values close to H+/e=2 were observed. Evidence is discussed that H+/e=3 is a fixed value in intact chloroplasts at levels of thylakoid energization sufficient for supporting effective carbon assimilation. In the prese nce of methylviologen and ascorbate, the minimum quantum requirement o f oxygen uptake by thylakoids was about 2.7 quanta of 675 nm light per O-2 indicating an e/O-2 ratio of 1.33. In the absence of ascorbate, a nd with KCN present in addition to methylviologen, e/O-2 ratios up to 4 were observed. The minimum quantum requirement of oxygen evolution b y thylakoids in the presence of ferricyanide and by intact chloroplast s in the presence of nitrite was about 8 quanta/O-2.