Flexible coupling between light-dependent electron and vectorial proton transport in illuminated leaves of C3 plants. Role of photosystem I-dependent proton pumping.

The role of cyclic electron transport has been re-examined in leaves of C3 plants because the bioenergetics of chloroplasts (H +/e = 3 in the presence of a Q-cycle; H+/ATP = 4 of ATP synthesis) had suggested that cyclic electron flow has no function in C3 photosynthesis. After light activation of pea leaves, the dark reduction of P700 (the donor pigment of PSI) following far-red oxidation was much accelerated. This corresponded to loss of sensitivity of P700 to oxidation by farred light and a large increase in the number of electrons available to reduce P700+ in the dark. At low CO2 and O2 molar ratios, far-red light was capable of decreasing the activity of photosystem II (measured as the ratio of variable to maximal chlorophyll fluorescence, Fv/Fm) and of increasing light scattering at 535 nm and zeaxanthin synthesis, indicating formation of a trans-thylakoid pH gradient. Both the light-induced increase in the number of electrons capable of reducing far-redoxidised P700 and the decline in Fv/Fm brought about by far-red in leaves were prevented by methyl viologen. Antimycin A inhibited CO2-dependent O2 evolution of pea leaves at saturating but not under limiting light; in its presence, far-red light failed to decrease Fv/Fm. The results indicate that cyclic electron flow regulates the quantum yield of photosystem II by decreasing the intrathylakoid pH when there is a reduction in the availability of electron acceptors at the PSI level (e.g. during drought or cold stresses). It also provides ATP for the carbon-reduction cycle under high light. Under these conditions, the Q-cycle is not able to maintain a H+/e ratio of 3 for ATP synthesis: we suggest that the ratio is flexible, not obligatory.