Flexible coupling between light-dependent electron and vectorial proton transport in illuminated leaves of C-3 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 presenc e of a Q-cycle. H+/ATP = 4 of ATP synthesis) had suggested that cyclic elec tron 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 sensi tivity of P700 to oxidation by farred light and a large increase in the num ber 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 ph otosystem II (measured as the ratio of variable to maximal chlorophyll fluo rescence, F-v/F-m) and of increasing light scattering at 535nm and zeaxanth in synthesis, indicating formation of a transthylakoid pH gradient. Both th e 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 l eaves were prevented by methyl viologen. Antimycin A inhibited CO2-dependen t O-2 evolution of pea leaves at saturating but not under limiting light; i n its presence, far-red light failed to decrease F-v/F-m. The results indic ate that cyclic electron flow regulates the quantum yield of photosystem II by decreasing the intrathylakoid pH when there is a reduction in the avail ability 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 r atio of 3 for ATP synthesis: we suggest that the ratio is flexible, not obl igatory.