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 ofcoupling between deposition of protons in the intrathylakoidspace and light-dependent transport of electrons from waterto an external acceptor were determined. The data indicate couplingbetween proton and electron transport at a ratio of H⁺/e=3 withmethylviologen as electron acceptor in thylakoids and with nitriteas electron acceptor in intact chloroplasts. With ferricyanideas electron acceptor in thylakoids, values close to H⁺/e=2 wereobserved. Evidence is discussed that H⁺/e=3 is a fixed valuein intact chloroplasts at levels of thylakoid energization sufficientfor supporting effective carbon assimilation. In the presence of methylviologen and ascorbate, the minimumquantum requirement of oxygen uptake by thylakoids was about2.7 quanta of 675 nm light per O₂ indicating an e/O₂ ratio of1.33. In the absence of ascorbate, and with KCN present in additionto methylviologen, e/O₂ ratios up to 4 were observed. The minimumquantum requirement of oxygen evolution by thylakoids in thepresence of ferricyanide and by intact chloroplasts in the presenceof nitrite was about 8 quanta/O₂. (Received May 1, 1995; Accepted October 2, 1995)