Oxygen as an Alternative Electron Acceptor in the Photosynthetic Electron Transport Chain of C3 Plants

This study deals with effects of oxygen on the kinetics of P-700 photoinduced redox transitions and on induction transients of chlorophyll fluorescence in leaves of C-3 plants Hibiscus rosa-sinensis and Vicia faba. It is shown that the removal of oxygen from the leaf environment has a conspicuous effect on photosynthetic electron transport. Under anaerobic conditions, the concentration of oxidized P-700 centers in continuous white light was substantially lower than under aerobic conditions. The deficiency of oxygen released non-photochemical quenching of chlorophyll fluorescence, thus indicating a decrease in the trans-thylakoid pH gradient (Delta H). Quantitative analysis of experimental data within the framework of an original mathematical model has shown that the steady-state electron flux toward oxygen in Chinese hibiscus leaves makes up to similar to 40% of the total electron flow passing through photosystem 1 (PS1). The decrease in P-700(+) content under anaerobic conditions can be due to two causes: i) the retardation of electron outflow from PS1, and ii) the release of photosynthetic control (acceleration of electron flow from PS2 to P-700(+)) owing to lower acidification of the intra-thylakoid space. At the same time, cyclic electron transport around PS1 was not stimulated in the oxygen-free medium, although such stimulation seemed likely in view of possible rearrangement of electron flows on the acceptor side of PS1. This conclusion stems from observations that the rates of P-700(+) reduction in DCMU-poisoned samples, both under aerobic and anaerobic conditions, were negligibly small compared to rates of electron flow from PS2 toward P-700(+) in untreated samples.