Populations of photosystem 1 units rapidly and slowly reduced by stromal reductants represent photosystem 1α and photosystem 1β complexes: Evidence from irradiance-response curves of P700 photooxidation in intact barley leaves

Abstract

Photon-induced absorbance changes at 830 nm (ΔA₈₃₀) related to redox transformations of P700, primary electron donor of photosystem 1 (PS1), were examined in barley leaves treated with diuron and methyl viologen. In such leaves, only soluble reductants localized in chloroplast stroma could serve as electron donors for P700⁺. Δ A₈₃₀ were induced by 1-min irradiation of leaves with “actinic light” (AL, 700±6 nm) of various irradiances. Two exponentially decaying components with half-times of 2.75 (fast component, relative magnitude of 62 % of ΔA₈₃₀) and 11.90 s (slow one, 38 % of ΔA₈₃₀) were distinguished in the kinetics of dark relaxation of ΔA₈₃₀ after leaf irradiation with saturating AL. The components reflecting P700⁺ dark reduction in two units of PS1 differed in the rate of electron input from stromal reductants. The decline in AL irradiance reduced steady state δA₈₃₀ magnitude, which was also accompanied by a decrease in the contribution of fast component to the overall P700⁺ dark reduction kinetics. The photon-response curves were obtained separately for rapidly and slowly decaying δA₈₃₀. The values of half-saturating irradiance were 0.106 and 0.035 μmol m⁻² s⁻¹ for rapidly and slowly reduced PS1 units, respectively. The ratio of rate constants of P700⁺ dark reduction for rapidly and slowly reduced PS1 units was 1.4 times higher than the ratio of their half-saturating irradiances thus indicating higher relative antenna size in rapidly reduced PS1 units. The latter finding, taken together with higher relative amount of P700, favours the view that rapidly and slowly reduced PS1 units reflect P700⁺ reduction by stromal reductants in spatially separated PS1α and PS1β complexes.