N. G. Bukhov

Alternative Photosystem I-Driven Electron Transport Routes: Mechanisms and Functions

In addition to the linear electron transport, several alternative Photosystem I-driven (PS I) electron pathways recycle the electrons to the intersystem electron carriers mediated by either ferredoxin:NADPH reductase, NAD(P)H dehydrogenase, or putative ferredoxin:plastoquinone reductase. The following functions have been proposed for these pathways: adjustment of ATP/NADPH ratio required for CO2 fixation, generation of the proton gradient for the down-regulation of Photosystem II (PS II), and ATP supply the active transport of inorganic carbon in algal cells. Unlike ferredoxin-dependent cyclic electron transport, the pathways supported by NAD(P)H can function in the dark and are likely involved in chlororespiratory-dependent energization of the thylakoid membrane. This energization may support carotenoid biosynthesis and/or maintain thylakoid ATPase in active state. Active operation of ferredoxin-dependent cyclic electron transport requires moderate reduction of both the intersystem electron carriers and the acceptor side of PS I, whereas the rate of NAD(P)H-dependent pathways under light depends largely on NAD(P)H accumulation in the stroma. Environmental stresses such as photoinhibition, high temperatures, drought, or high salinity stimulated the activity of alternative PS I-driven electron transport pathways. Thus, the energetic and regulatory functions of PS I-driven pathways must be an integral part of photosynthetic organisms and provides additional flexibility to environmental stress.

Energy dissipation in photosynthesis: Does the quenching of chlorophyll fluorescence originate from antenna complexes of photosystem II or from the reaction center?

Abstract. Dissipation of light energy was studied in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst., and in leaves of Spinacia oleracea L. and Arabidopsis thaliana (L.) Heynh., using chlorophyll fluorescence as an indicator reaction. Maximum chlorophyll fluorescence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated spinach leaves, as produced by saturating light and studied between +5 and −20 °C, revealed an activation energy ΔE of 0.11 eV. As this suggested recombination fluorescence produced by charge recombination between the oxidized primary donor of photosystem II and reduced pheophytin, a mathematical model explaining fluorescence, and based in part on known characteristics of primary electron-transport reactions, was developed. The model permitted analysis of different modes of fluorescence quenching, two localized in the reaction center of photosystem II and one in the light-harvesting system of the antenna complexes. It predicted differences in the relationship between quenching of variable fluorescence Fv and quenching of basal, so-called F0 fluorescence depending on whether quenching originated from antenna complexes or from reaction centers. Such differences were found experimentally, suggesting antenna quenching as the predominant mechanism of dissipation of light energy in the moss Rhytidiadelphus, whereas reaction-center quenching appeared to be important in spinach and Arabidopsis. Both reaction-center and antenna quenching required activation by thylakoid protonation but only antenna quenching depended on or was strongly enhanced by zeaxanthin. De-protonation permitted relaxation of this quenching with half-times below 1 min. More slowly reversible quenching, tentatively identified as so-called qI or photoinhibitory quenching, required protonation but persisted for prolonged times after de-protonation. It appeared to originate in reaction centers.

Effect of drought on chlorophyll content and antioxidant enzyme activities in leaves of three wheat cultivars varying in productivity

Seedlings of three wheat varieties (Triticum aestivum L.)—highly productive cv. Ballada, moderately productive cv. Belchanka, and low productive cv. Beltskaya—were exposed to progressive soil drought (cessation of watering for 3, 5, and 7 days) and then analyzed for chlorophyll content and activities of ferredoxin-NADP+ oxidoreductase (FNR) and antioxidant enzymes, namely, glutathione reductase (GR) and ascorbate peroxidase (AscP). In addition, the proline content, and the extent of lipid peroxidation were examined. In the first period of water limitation, the water loss from leaves was slight for all wheat cultivars, which is characteristic of drought-resistant varieties. After 7-day drought the leaf water content decreased by 5.2–6.8%. The total chlorophyll content expressed per unit dry weight increased insignificantly during the first two periods of drought but decreased by 13–15% later on. This decrease was not accompanied by changes in chlorophyll a/b ratio. The plant dehydration did not induce significant changes in FNR activity. Activities of GR and AscP in leaves of wheat cultivars Ballada and Belchanka increased on the 3rd and 5th days of drought. Owing to the coordinated increase in GR and AscP activities, the lipid peroxidation rate remained at nearly the control level observed in water-sufficient plants. When the dehydration period was prolonged to 7 days, activities of GR and AscP in wheat cultivars reduced in parallel with the increase in malonic dialdehyde (MDA) content, indicating that the antioxidant enzyme defense system was weakened and lipid peroxidation enhanced. Unlike Ballada and Belchanka, the wheat cv. Beltskaya did not exhibit the increase in GR and AscP activities during progressive soil drought. The increase in MDA content by 16% in this cultivar was only observed after a 7-day drought period. The proline content in leaves of all wheat cultivars increased substantially during drought treatment. Thus, in wheat cultivars examined, different responses of the defense systems were mobilized to implement plant protection against water stress. The activities of antioxidant enzyme defense system depended on wheat cultivar, duration of drought, and the stage of leaf development.

Nonphotosynthetic Reduction of the Intersystem Electron Transport Chain of Chloroplasts Following Heat Stress. Steady-state Rate

Abstract The consequence of elevated temperatures in the range of 39–51°C on the steady-state rate of light-induced electron transport through photosystem I (PSI) supported by stromal reductants was studied in intact barley leaves using photoacoustic and chlorophyll fluorescence techniques. Measurable electron flow through PSI in diuron-treated leaves occurred only after exposure to temperatures above 37°C. The steady-state rate of the above diuron-insensitive electron flow with methyl viologen as electron acceptor was estimated to be 3.7 μeq m−2 s−1 or 0.018 μeq μmol chlorophyll−1 s−1 in leaves exposed for 5 min to 45°C.

Photosystem I-dependent cyclic electron flow in intact spinach chloroplasts: Occurrence, dependence on redox conditions and electron acceptors and inhibition by antimycin A

Photosystem I-dependent cyclic electron transport is shown to operate in intact spinach chloroplasts with oxaloacetate, but not with nitrite or methylviologen as electron acceptors. It is regulated by the redox state of the chloroplast NADP system. Inhibition of cyclic electron transport by antimycin A occurs immediately on addition of this antibiotic in the light. It is unrelated to a different function of antimycin A, inhibition of nonphotochemical quenching of chlorophyll fluorescence, which requires prior dissipation of the transthylakoid proton gradient before antimycin A can become effective.

Dynamic Light Regulation of Photosynthesis (A Review)

Regulatory reactions providing the photosynthetic apparatus with the ability to respond to variations of irradiance by changes in activities of the light and the dark stages of photosynthesis within a time range of seconds and minutes are considered in the review. At the light stage, such reactions are related to the changes in both distribution of light energy between two photosystems and probability of nonphotochemical dissipation of absorbed quanta in PSI and PSII. These regulatory reactions operate in a negative feedback mode, thus avoiding overreduction of electron transport chain and minimizing the probability of generation of reactive oxygen species. The crucial role in preventing the generation of reactive oxygen species belongs to dynamic regulation of electron transport activity despite the presence of complex system of their detoxification in chloroplasts. In dark reactions of Calvin cycle, the regulatory responses involve a positive feedback principle being related to redox regulation of activities of several enzymes, which is sensitive to the reduction status of PSI acceptor side. The complex of regulatory reactions based on negative and positive feedback principles provides prolonged functioning of a chloroplast and high stability of photosynthetic activity under various light conditions.

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light-dark transitions.

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F(o)) and to markedly retard the light-induced rise of variable fluorescence (F(v)). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F(o) level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F(v) at low concentration, while F(o) was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F(o) level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).

Heterogeneity of Photosystem I reaction centers in barley leaves as related to the donation from stromal reductants

The light-response curves of P700 oxidation and time-resolved kinetics of P700+ dark re-reduction were studied in barley leaves using absorbance changes at 820 nm. Leaves were exposed to 45 °C and treated with either diuron or diuron plus methyl viologen (MV) to prevent linear electron flow from PS II to PSI and ferredoxin-dependent cyclic electron flow around PSI. Under those conditions, P700+ could accept electrons solely from soluble stromal reductants. P700 was oxidized under weak far-red light in leaves treated with diuron plus MV, while identical illumination was nearly ineffective in diuron-treated leaves in the absence of MV. When heat-exposed leaves were briefly illuminated with strong far-red light, which completely oxidized P700, the kinetics of P700+ dark reduction was fitted by a single exponential term with half-time of about 40 ms. However, two first-order kinetic components of electron flow to P700+ (fast and slow) were found after prolonged leaf irradiation. The light-induced modulation of the kinetics of P700+ dark reduction was reversed following dark adaptation. The fast component (half time of 80–90 ms) was 1.5 larger than the slow one (half time of about 1 s). No kinetic competition occurred between two pathways of electron donation to P700+ from stromal reductants. This suggests the presence of two different populations of PSI.

Nonphotosynthetic Reduction of the Intersystem Electron Transport Chain of Chloroplasts Following Heat Stress. The Pool Size of Stromal Reductants

Abstract The properties of a negative transient signal (negative peak) observed during the first seconds of the induction of the photoacoustic (PA) signal in dark-adapted barley leaves treated with methyl viologen (MV) and diuron and then exposed to high temperatures have been examined. Under those conditions no electron donation from photosystem II (PSII) occurred, and electron flow through PSI could be supported only by soluble reductants located in the chloroplast stroma. The negative peak was observed only if the PA signal had been monitored at low, and not high, frequencies. The peak obviously originated from the oxygen consumption by PSI. The size of the peak increased as the temperature of preheating was raised from 39 to 45°C. The size of the peak decreased exponentially with a half-time of 3.7 s during illumination under low light. This decrease was found to be much faster under strong light. The recovery of the peak during dark acclimation required several minutes. It is concluded that the negative peak reflects the oxygen consumption supported by stromal reductants, their pool being rapidly exhausted under light in the presence of MV. The maximal size of the pool was calculated as 140 eq:P700 in dark-adapted leaves.

Changes in the structure of chlorophyll-protein complexes and excitation energy transfer during photoinhibitory treatment of isolated photosystem I submembrane particles

The activity of light-induced oxygen consumption, absorption spectra, low temperature (77 K) chlorophyll fluorescence emission and excitation spectra were studied in suspensions of photosystem (PS) I submembrane particles illuminated by 2000 microE m(-2) s(-1) strong white light (WL) at 4 degrees C. A significant stimulation of oxygen uptake was observed during the first 1-4 h of photoinhibitory treatment, which rapidly decreased during further light exposure. Chlorophyll (Chl) content gradually declined during the exposure of isolated PSI particles to strong light. In addition to the Chl photobleaching, pronounced changes were found in Chl absorption and fluorescence spectra. The position of the major peak in the red part of the absorption spectrum shifted from 680 nm towards shorter wavelengths in the course of strong light exposure. A 6-nm blue shift of that peak was observed after 5-h illumination. Even more pronounced changes were found in the characteristics of Chl fluorescence. The magnitude of the dominating long-wavelength emission band at 736 nm located in untreated particles was five times reduced after 2-h exposure, whereas the loss in absolute Chl contents did not exceed 10% of its initial value. The major peak in low-temperature Chl fluorescence emission spectra shifted from 736 to 721 nm after 6-h WL treatment. Individual Chl-protein complexes differed in the response of their absorption spectra to strong WL. Unlike light-harvesting complexes (LHC), LHCI-680 and LHC-730, which did not exhibit changes in the major peak position, its maximum was shifted from 678 to 671 nm in CPIa complex after PSI submembrane particles were irradiated with strong light for 6 h. The results demonstrated that excitation energy transfer represents the stage of photosynthetic utilization of absorbed quanta which is most sensitive to strong light in isolated PSI particles.