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

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I. V. Kuvykin, A. V. Vershubskii, V. V. Ptushenko, and A. N. Tikhonov^*

Faculty of Physics, Lomonosov Moscow State University, 119992 Moscow, Russia; E-mail: an_tikhonov@mail.ru

^* To whom correspondence should be addressed.

Received January 18, 2008; Revision received May 6, 2008

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This study deals with effects of oxygen on the kinetics of P₇₀₀ photoinduced redox transitions and on induction transients of chlorophyll fluorescence in leaves of C₃ 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 P₇₀₀ 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 (ΔpH). 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 ~40% of the total electron flow passing through photosystem 1 (PS1). The decrease in P₇₀₀⁺ 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 P₇₀₀⁺) 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 P₇₀₀⁺ reduction in DCMU-poisoned samples, both under aerobic and anaerobic conditions, were negligibly small compared to rates of electron flow from PS2 toward P₇₀₀⁺ in untreated samples.

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KEY WORDS: C₃ plants, chloroplasts, electron transport, EPR, non-photochemical fluorescence quenching, mathematical modeling

DOI: 10.1134/S0006297908100027

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