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 C-3 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 C-3 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 O-2 molar ratios. far-red light was capable of decreasing the activity of photosystem II (measured as the ratio of variable to maximal chlorophyll fluorescence, F-v/F-m) and of increasing light scattering at 535nm and zeaxanthin synthesis, indicating formation of a transthylakoid pH gradient. Both the light-induced increase in the number of electrons capable of reducing far-red-oxidised P700 and the decline in F-v/F-m brought about by far-red in leaves were prevented by methyl viologen. Antimycin A inhibited CO2-dependent O-2 evolution of pea leaves at saturating but not under limiting light; in its presence, far-red light failed to decrease F-v/F-m. 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 accepters at the PST 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.