Maximum H+/hνPSI Stoichiometry of Proton Transport during Cyclic Electron Flow in Intact Chloroplasts Is at Least Two, but Probably Higher than Two

Effects of antimycin A on 9-aminoacridine (9AA) fluorescence quenching by intact chloroplasts during light-dependent electron flow to different electron acceptors indicated that considerable cyclic electron flow occurs concurrently with linear electron transport already at low PFDs, when oxygen supported electron flow, but not, when nitrite or methylviologen (MV) were present. Quantum efficiencies of the use of 696 and 675 nm light were calculated for oxygen-, nitrite- and MV-dependent linear electron flows. Since H+/e=3 during linear electron transport [Ivanov (1993) Photosynthesis, p. 111; Kobayashi et al. (1995) Plant Cell Physiol. 36: 1613] and comparable 9AA fluorescence quenching indicates comparable transthylakoid proton gradients, total proton transport could be calculated and part of it could be assigned to linear and the remainder to cyclic electron transport when oxygen was electron acceptor. Quanta of 696 nm light not used to support linear electron flow to oxygen at hν/e=2 were assumed to be available for coupled proton transport during cyclic electron flow. H+/hν ratios for cyclic electron transport obtained on this basis were consistently higher than 1 and occasionally approached 3. No allowance was made in these calculations for oxidized P700 in the reaction center of PSI, which could not donate electrons to the cyclic pathway, and for reduced QA in the reaction center of PSII. It therefore appears likely that maximum H+/hν ratios in cyclic electron transport are higher than values calculated in this work. Our observations with intact chloroplasts agree in principle with those of [Heath (1972) Biochim. Biophys. Acta 256: 645] with thylakoids, who also reported high H+/ e ratios in cyclic electron transport. These ratios are briefly discussed in relation to the H+/ATP stoichiometry of ATP production during carbon assimilation of leaves and to protection of chloroplasts against photoinactivation.