MAXIMUM H+ H-NU(PSI) STOICHIOMETRY OF PROTON TRANSPORT DURING CYCLIC ELECTRON FLOW IN INTACT CHLOROPLASTS IS AT LEAST 2, BUT PROBABLY HIGHER THAN 2/

Effects of antimycin A on 9-aminoacridine (9AA) fluorescence quenching by intact chloroplasts during light-dependent electron flow to differ ent electron accepters indicated that considerable cyclic electron flo w occurs concurrently with linear electron transport already at low PF Ds, when oxygen supported electron flow, but not, when nitrite or meth ylviologen (MV) were present. Quantum efficiencies of the use of 696 a nd 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 C ell Physiol. 36: 1613] and comparable 9AA fluorescence quenching indic ates comparable transthylakoid proton gradients, total proton transpor t could be calculated and part of it could be assigned to linear and t he remainder to cyclic electron transport when oxygen was electron acc eptor. Quanta of 696 nm light not used to support linear electron flow to oxygen at h nu/e=2 were assumed to be available for coupled proton transport during cyclic electron flow. H+/h nu ratios for cyclic elec tron transport obtained on this basis were consistently higher than 1 and occasionally approached 3. No allowance was made in these calculat ions for oxidized P700 in the reaction center of PSI, which could not donate electrons to the cyclic pathway, and for reduced Q(A) in the re action center of PSII. It therefore appears likely that maximum H+/h n u ratios in cyclic electron transport are higher than values calculate d in this work. Our observations with intact chloroplasts agree in pri nciple with those of [Heath (1972) Biochim. Biophys. Acta 256: 645] wi th thylakoids, who also reported high H+/e ratios in cyclic electron t ransport. 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.