Evidence for the operation of alternative electron transport routes through photosystem I in intact barley leaves under weak and moderate white light

Abstract

The functioning of alternative routes of photosynthetic electron transport was analyzed from the kinetics of dark reduction of P700+ , an oxidized primary donor of PSI, in barley (Hordeum vulgare L.) leaves irradiated by white light of various intensities. Redox changes of P700 were monitored as absorbance changes at 830 nm using PAM 101 specialized device. Irradiation of dark-adapted leaves caused a gradual P700+ accumulation, and the steady-state level of oxidized P700 increased with intensity of actinic light. The kinetics of P700+ dark reduction after a pulse of strong actinic light, assayed from the absorbance changes at 830 nm, was fitted by a single exponential term with a halftime of 10–12 ms. Two slower components were observed in the kinetics of P700+ dark reduction after leaf irradiation by attenuated actinic light. The contribution of slow components to P700+ reduction increased with the decrease in actinic light intensity. Two slow components characterized by halftimes similar to those observed after leaf irradiation by weak white light were found in the kinetics of dark reduction of P700+ oxidized in leaves with far-red light specifically absorbed by PSI. The treatment of leaves with methyl viologen, an artificial PSI electron acceptor, significantly accelerated the accumulation of P700+ under light. At the same time, the presence of methyl viologen, which inhibits ferredoxin-dependent electron transport around PSI, did not affect three components of the kinetics of P700+ dark reduction obtained after irradiations with various actinic light intensities. It was concluded that some part of PSI reaction centers was not reduced by electron transfer from PSII under weak or moderate intensities of actinic light. In this population of PSI centers, P700+ was reduced via alternative electron transport routes. Insensitivity of the kinetics of P700+ dark reduction to methyl viologen evidences that the input of electrons to PSI from the reductants (NADPH or NADH) localized in the chloroplast stroma was effective under those light conditions.