The origin of nonmonotonic changes in the redox state of P700, the primary electron donor of PSI, was investigated on predarkened barley (Hordeum vulgare L.) leaves exposed to far-red light. To accomplish this, the relaxation kinetics of absorbance changes at 830 nm, reflecting the dark reduction of P700+, were measured at different stages of the induction curve. The onset of far-red light resulted in rapid oxidation of P700, which was followed by its partial reduction and subsequent slow oxidation of P700 to a steady-state level. This steady-state level was usually attained within 10 s under far-red light. The relative contribution of the slow kinetic component of P700+ reduction decreased in parallel with the transient photoreduction of P700+ and increased upon a subsequent stage of P700 photooxidation. The contribution of the middle component to the dark reduction of P700+ increased monotonically with the length of far-red light irradiation. The relative amplitude of the fast component of P700+ reduction increased sharply during the first 3 s of irradiation and decreased upon longer light exposures. The rates of fast and slow components of dark reduction of P700+ remained constant upon illumination of dark-adapted leaves with far-red light for 1 s and longer periods. Thus, nonmonotonic changes in the redox state of P700 in barley leaves exposed to far-red light reflect variable contributions of few alternative electron transport pathways characterized by different rates of electron donation to PSI. The results show the principle possibility of switching-over between alternative pathways of PSI-related electron transfer within one complex of this photosystem. Such switching may occur irrespective of active operation or inhibition of ferredoxin-dependent electron transport.
Russian Journal of Plant Physiology – Springer Journals
Published: Sep 28, 2005
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