Kinetic curves of absorbance changes induced by far-red light (FR, 830 nm) (ΔA 830), which reflect redox transformations of PSI primary electron donor, P700, were examined in intact barley (Hordeum vulgare L.) leaves. In intact leaves, FR induced the biphasic increase in absorbance related to P700 photooxidation. Leaf treatment with methyl viologen or antimycin A suppressed the slow phase of P700 photooxidation, which was attained in such leaves within the first second of light exposure. With FR turned off, the previously increased absorbance at 830 nm dropped down to its initial level, thus reflecting P700+ reduction. In the control leaves, the kinetics of P700+ reduction consisted of three exponentially decaying components, with the corresponding half-times of 8.8 s (the slow component, with its magnitude comprising 24% of the total ΔA 830 signal), 0.73 s (the middle component, 49% of ΔA 830), and 0.092 s (the fast component, 26% of ΔA 830). The rate of the fast component of P700+ reduction, following FR irradiation of leaves, was about ten times lower than that of the noncyclic electron transfer from PSII to PSI computed from ΔA 830 relaxation after the abrupt offset of white light. The treatment of leaves with methyl viologen or antimycin A completely abolished the fast component of ΔA 830 relaxation after FR exposure. It was concluded that the fast component is determined by the operation of ferredoxin-dependent cyclic electron transport around PSI. This study represents the first report on the identification of this pathway of electron transport in vivo and the estimation of its rate.
Russian Journal of Plant Physiology – Springer Journals
Published: May 19, 2005
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