Photoinduced changes in the redox state of photosystem I (PSI) primary donor, chlorophyll P700 were studied by measuring differential absorbance changes of pea leaves at 810 nm minus 870 nm (ΔA 810). The kinetics of ΔA 810 induced by 5-s pulses of white light were strongly affected by preillumination. In dark-adapted leaves, the light pulse caused a transient oxidation of P700 and its subsequent reduction. An identical pulse, applied after 30-s preillumination with white light, induced sequential appearance of two peaks of P700 oxidation. These kinetic differences of ΔA 810 reflect regulatory changes of electron flow on the donor and acceptor sides of PSI induced by illumination of leaf for 20–40 s. The amplitude of ΔA 810 second peak depended nonmonotonically on the dark interval preceding illumination: it increased with the length of dark period in the range 3–10 s and decreased upon longer dark intervals. The second wave of ΔA 810 disappeared after the treatment with combination of ionophores preventing ΔpH and electric potential formation at the thylakoid membrane. In leaves treated with monensin eliminating ΔpH only, the ΔA 810 signals become incompletely reversible and were characterized by slow relaxation in darkness. The results indicate an important role of electrochemical proton gradient in generation of the second wave of light-induced P700 oxidation.
Russian Journal of Plant Physiology – Springer Journals
Published: Jul 12, 2008
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