Photosystem 2 (PS2)-driven electron transfer was studied in primary leaves of barley (Hordeum vulgare L.) seedlings grown under various photon fluxes (0.3-170.0 μmol m-2 s-1) of blue (BR) or red (RR) radiation using modulated chlorophyll fluorescence. The Fv/Fm ratio was 0.78-0.79 in leaves of all radiation variants, except in seedlings grown under BR or RR of 0.3 μmol m-2 s-1. The extent of the photochemical phase of the polyphasic Fv rise induced by very strong "white light" was similar in leaves of all radiation treatments. Neither radiation quality nor photon flux under plant cultivation influenced the amount of non QB-transferring centres of PS2 except in leaves of seedlings grown under BR of 0.3 μmol m-2 s-1, in which the amount of such centres increased threefold. Both BR and RR stimulated the development of photochemically competent PS2 at photon fluxes as low as 3 μmol m-2 s-1. Three exponential components with highly different half times were distinguished in the kinetics of Fv dark decay. This indicates different pathways of electron transfer from QA-, the reduced primary acceptor of PS2, to other acceptors. Relative magnitudes of the individual decay components did not depend on the radiation quality or the photon flux during plant cultivation. Significant differences were found, however, between plants grown under BR or RR in the rate of the middle and fast components of Fv dark decay, which showed 1.5-times faster intersystem linear electron transport in BR-grown leaves. and E. A. Egorova, N. G. Bukhov.
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. and E. A. Egorova, N. G. Bukhov.
Bean (Phaseolus vulgaris L. cv. Berbukskaya) seedlings were pre-treated with choline compounds, 19 mM 2-ethyltrimethylammonium chloride (Ch) or 1.6 mM 2-chloroethyltrimethylammonium chloride (CCh), during 24 h, then after 6 d the excised primary leaves were exposed to UV-B and high temperature stress. Chlorophyll (Chl) fluorescence, delayed light emission, accumulation of photosynthetic pigments, contents of thiobarbituric acid reactive substances, and activities of the active oxygen detoxifying enzymes (superoxide dismutase, ascorbate peroxidase, and glutathione reductase) were examined. Pre-treatment of plants with Ch or CCh enhanced the resistance of photosystem 2 (PS2) photochemistry to UV-B and heat injuries. The higher stress resistance can be explained by the increased activity of the detoxifying enzymes. The increased content of UV-B-absorbing pigments may also contribute to the enhanced resistance of choline-treated plants to UV-B radiation. and V. D. Kreslavski ... [et al.].