E. A. Egorova

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).

Quenching of excited states of chlorophyll molecules in submembrane fractions of Photosystem I by exogenous quinones

The ability of three substituted quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duriquinone) to quench the excited states of chlorophyll (Chl) molecules in Photosystem I (PSI) was studied. Chl fluorescence emission measured with isolated PSI submembrane fractions was reduced following the addition of exogenous quinones. This quenching progressively increased with rising concentrations of the exogenous quinones according to the Stern-Volmer law. The values of Stern-Volmer quenching coefficients were found to be 3.28 x 10(5) M(-1) (DBMIB), 1.31 x 10(4) M(-1) (DCBQ), and 3.7 x 10(3) M(-1) (duroquinone). The relative quenching capacities of the various exogenous quinones in PSI thus strictly coincided to those found for the quenching of Fo level of Chl fluorescence in isolated thylakoids, which is emitted largely by Photosystem II (PSII) [Biochim. Biophys. Acta (2003) 1604, 115-123]. Quenching of Chl excited states in PSI submembrane fractions by exogenous quinones slowed down the rate of P700, primary electron donor of PSI, photooxidation measured at limiting actinic light irradiances thus revealing a reduced photochemical capacity of absorbed quanta. The possible involvement of non-photochemical quenching of excited Chl states by oxidized phylloquinones, electron acceptors of PSI, and oxidized plastoquinones, mobile electron carriers between PSII and the cytochrome b(6)/f complex, into the control of photochemical activity of PSI is discussed.

Structural and Functional State of Thylakoids in a Halophyte Suaeda altissima before and after Disturbance of Salt–Water Balance by Extremely High Concentrations of NaCl

Halophyte plants Suaeda altissima L. were grown in water culture at different concentrations of NaCl in the medium, and their leaves were sampled to examine the ultrastructure of chloroplasts. In parallel tests, the functional state of chloroplasts was assessed from parameters of chlorophyll fluorescence. In addition the effects of NaCl on plant growth and on the contents of Na+, K+, and water in organs of S. altissima were investigated. At a wide range of external salt concentrations (0–750 mM NaCl), S. altissima plants retained the chloroplast ultrastructure and photosynthetic function in an intact condition. The impairment of thylakoid ultrastructure and the accompanying increase in nonphotochemical quenching of excited states of chlorophyll was only observed at an extremely high concentration of NaCl in the medium (1 M) that led to disruption of ionic homeostasis and lowered water content in tissues.

Identification of Ferredoxin-Dependent Cyclic Electron Transport around Photosystem I Using the Kinetics of Dark P700+ Reduction

Kinetic curves of absorbance changes induced by far-red light (FR, 830 nm) (ΔA830), 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 ΔA830 signal), 0.73 s (the middle component, 49% of ΔA830), and 0.092 s (the fast component, 26% of ΔA830). 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 ΔA830 relaxation after the abrupt offset of white light. The treatment of leaves with methyl viologen or antimycin A completely abolished the fast component of ΔA830 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.

Mechanisms and functions of photosystem I-related alternative electron transport pathways in chloroplasts

This review analyzes various alternative pathways of chloroplast electron transport mediated by photoreactions of photosystem I (PSI) and unrelated to activity of photosystem II (PSII). The mechanisms and functional significance of the alternative pathways are considered. These pathways are complexly organized and comprise ferredoxin-dependent electron recycling around PSI, as well as electron donation to noncyclic chain in the region between PSII and PSI from reduced substances localized in the chloroplast stroma. For each of the alternative pathways, the origin of corresponding enzymes and their compartmentalization in the complex membrane system of the chloroplast are discussed. It is shown that operation of alternative electron transport pathways contributes to energy transduction and cell defense function, facilitates the absorption of inorganic carbon, and is significant for chloroplast respiration. Multiple mechanisms for regulation of alternative pathways have been revealed. It is concluded that PSI-related alternative electron transport pathways constitute an integral part of entire system of photosynthetic electron transport, this system being principally responsible for energy supply of phototrophic cells and whole plants.

Ultraviolet Rays Promote Development of Photosystem II Photochemical Activity and Accumulation of Phenolic Compounds in the Tea Callus Culture (Camellia sinensis)

Effect of UV-B rays (280–320 nm) on photosynthetic electron transport and production of phenolic compounds in tea (Camellia sinensis L.) callus culture grown in white light was investigated. When white light was supplemented with UV radiation, the culture growth was retarded and morphological characteristics were modified. These conditions promoted the formation of chlorophyll-bearing cells and altered the ability of cultured cells to accumulate phenolic compounds, including flavans specific to Camellia sinensis. By the end of the culturing cycle (on the 45th day), the total content of phenolic compounds in the culture grown under supplementary UV irradiation was almost 1.5 times higher than in the control culture. The UV rays greatly stimulated photosystem II (PSII) activity in phototrophic cells of the callus culture, which was indicated by a large increase in the ratio of variable chlorophyll fluorescence to maximal fluorescence. This ratio was as low as 0.19 in cells cultured in white light and increased to 0.53 in the cell culture grown under white and UV light. The kinetics of dark relaxation of chlorophyll variable fluorescence, related to reoxidation of PSII primary acceptor, contained either two or three components, depending on the absence or presence of UV radiation, respectively. An artificial electron acceptor of PSI, methyl viologen modified the kinetics of dark decay of chlorophyll variable fluorescence in a characteristic manner, implying that photosynthetic electron transport was mediated by PSI and PSII in both treatments (culturing in white light with and without UV-B). It is concluded that stimulatory effect of UV rays on the parameters examined in phototrophic regions of Camellia tissue culture is determined by photoexcitation of a regulatory pigment that absorbs quanta in blue and long-wave UV spectral regions.

Activities of Noncyclic and Alternative Pathways of Photosynthetic Electron Transport in Leaves of Broad Bean Plants Grown at Various Light Irradiances

Activities of noncyclic and alternative pathways of photosynthetic electron transport were studied in intact leaves of broad been (Vicia faba L.) seedlings grown under white light at irradiances of 176, 36, and 18 µmol quanta/(m2 s). Electron flows were followed from light-induced absorbance changes at 830 nm related to redox transformations of P700, the photoactive PSI pigment. The largest absorbance changes at 830 nm, induced by either white or far-red light, were observed in leaves of seedlings grown at irradiance of 176 µmol quanta/(m2 s), which provides evidence for the highest concentration of PSI reaction centers per unit leaf area in these seedlings. When actinic white light of 1800 µmol quanta/(m2 s) was turned on, the P700 oxidation proceeded most rapidly in leaves of seedlings grown at irradiance of 176 µmol quanta/(m2 s). The rates of electron transfer from PSII to PSI were measured from the kinetics of dark P700+ reduction after turning off white light. These rates were similar in leaves of all light treatments studied, and their characteristic reaction times were found to range from 9.2 to 9.5 ms. Four exponentially decaying components were resolved in the kinetics of dark P700+ reduction after leaf exposure to far-red light. A minor but the fastest component of P700+ reduction with a halftime of 30–60 ms was determined by electron transfer from PSII, while the three other slow components were related to the operation of alternative electron transport pathways. Their halftimes and relative magnitudes were almost independent on irradiance during plant cultivation. It is concluded that irradiance during plant growth affects the absolute content of PSI reaction centers in leaves but did not influence the rates of noncyclic and alternative electron transport.

Origin of Multiphase Reduction of P700+ in Broad Bean Leaves after Irradiation with Far-Red Light

The kinetic curves of dark reduction of P700+ (oxidized primary donor of PSI) after far-red light irradiation were studied on broad bean (Vicia faba L.) leaves treated with antimycin A, methyl viologen, or diuron. Four components of P700+ reduction were found in untreated leaves, namely, an ultrafast component with a half-time of 25 ms, and fast (210 ms), middle (790 ms), and slow (6100 ms) components. The fast component disappeared in leaves treated with antimycin A or methyl viologen. At the same time, these substances did not affect other components of P700+ reduction. Treatment of leaves with diuron abolished both the ultrafast and fast components of P700+ reduction. As the length of far-red light exposure was increased, a lag phase appeared in the development of middle component in leaves treated with diuron, antimycin A, or methyl viologen. In thus treated leaves, an exponential pattern of the middle component was displayed with a certain delay after darkening. A conclusion was drawn that the minor ultrafast component of P700+ dark reduction in broad bean leaves was caused by electron donation to PSI from PSII, whereas the fast component of this process was determined by the operation of ferredoxin-dependent electron transport around PSI. The middle and slow components were supposed to be related to electron input to PSI from reductants localized in the chloroplast stroma.

Elevated Temperatures Inhibit Ferredoxin-Dependent Cyclic Electron Flow around Photosystem I

In experiments with barley (Hordeum vulgare L.) leaves, absorbance changes at 830 nm induced by far-red light were measured as indicator of redox conversions of primary electron donor (P700) of photosystem I (PSI). Using this method, the action of elevated temperature (45°C, 5 min) on PSI-driven electron transport through alternative pathways was examined. Thermally induced inactivation was found to transform nonmonotonic photooxidation of P700, induced by far-red light in untreated leaves, into a fast and monotonic process completed within 1-s illumination. The short-term heating of leaves fully eliminated the fast component in the kinetics of P700+ dark reduction, related to operation of ferredoxin-dependent cyclic electron transport around PSI. At the same time, thermoinactivation substantially accelerated the slow and middle components of dark P700+ reduction, i.e., the components determined by arrival of electrons to PSI from reductants located in the chloroplast stroma. The latter effect was also observed after heating of leaves pretreated with antimycin A or methyl viologen; both agents are known to inhibit the ferredoxin-dependent electron transport. It is concluded that the heat treatment of leaves inhibits the ferredoxin-dependent pathway of electron transport around PSI and activates electron transport through alternative routes providing reducing equivalents to PSI from stromal reductants.

Modulating Effect of Far-Red Light on Activities of Alternative Electron Transport Pathways Related to Photosystem I

Barley (Hordeum vulgare L.) leaves were irradiated with far-red (FR) light of various intensities after different periods of dark adaptation in order to investigate activities of alternative electron transport pathways related to photosystem I (PSI). Photooxidation of P700, the primary electron donor of PSI, was saturated at FR light intensity of 0.15 µmol quanta/(m2 s). As the photon flux density was raised in this range, the slow and middle components in the kinetics of P700+ dark reduction increased, whereas the fast component remained indiscernible. The amplitudes of the slow and middle components diminished upon further increase of FR photon flux density in the range 0.15–0.35 µmol quanta/(m2 s) and remained constant at higher intensities. The fast component of P700+ reduction was only detected after FR irradiation with intensities above 0.15 µmol quanta/(m2 s); the light-response curve for this component was clearly sigmoid. In dark-adapted barley leaves, three stages were distinguished in the kinetics of P700 photooxidation, with the steady state for P700+ achieved within about 3 min. In leaves predarkened for a short time, the onset of FR irradiation produced a very rapid photooxidation of P700. As the duration of dark exposure was prolonged, the amplitude of the first peak in the kinetic curve of photoinduced P700 photooxidation was diminished and the time for attaining the steady-state oxidation level was shortened. After a brief dark adaptation of leaves, ferredoxin-dependent electron flow did not appreciably contributed to the kinetics of P700+ dark reduction, whereas the components related to electron donation from stromal reductants were strongly retarded. It is concluded that FR light irradiation, selectively exciting PSI, suffices to modulate activities of alternative electron transport routes; this modulation reflects the depletion of stromal reductants due to continuous efflux of electrons from PSI to oxygen under the action of FR light.