Enhanced rates of P700 < sup > + < /sup > dark-reduction in leaves of Cucumis sativus L. photoinhibited at chilling temperature

Abstract

The changes in electron transport within photosystem I (PSI) were studied in detached leaves of Cucumis sativus L. during the course of irradiation with moderate white light (300 μmol photons m-2 s-1) at 4°C. When intact leaves were exposed to the combination of moderate light and low temperature, the amplitude of far-red light-induced P700 absorbance changes at 820 nm (ΔA820), a relative measure of PSI, progressively decreased as the light treatment time increased. Almost no oxidation of P700 was noticeable after 5 h. Methyl viologen accelerated the oxidation of P700 to a steady-state level and also increased the magnitudes of ΔA820 changes in photoinhibited leaves, reflecting the rapid removal of electrons from native carriers. Photoinhibition under moderate light and chilling temperature also accelerated the rate of P700+ reduction after far-red light excitation as the half-times of the two exponential components of P700+ decay curves decreased relative to the control ones. A detailed analysis of the kinetics of P700+ reduction using diuron alone or the combination of diuron and methyl viologen strongly favours an increased rate of electron donation from stromal reductants to PSI through the plastoquinone pool following photoinhibitory treatment. Importantly, the marked acceleration of P700+ re-reduction is the consequence of the irradiation of leaf segments at low temperature and not caused by chilling stress alone.