Electron flow to photosystem I from stromal reductants in vivo: the size of the pool of stromal reductants controls the rate of electron donation to both rapidly and slowly reducing photosystem I units

Electron donation from stromal reductants to photosystem I (PSI) was studied using the kinetics of P700(+) (the oxidized primary donor of PSI) reduction in the dark after irradiation of barley ( Hordeum vulgare L.) leaves. The leaves were treated with diuron and methyl viologen to abolish both the electron flow from PSII and PSI-driven cyclic electron transport. The redox state of P700 was monitored using the absorbance changes at 830 nm (Delta A(830)). Two exponentially decaying components with half-times of about 3 s (the slow component) and about 0.6 s (the fast one) were distinguished in the kinetic curves of Delta A(830) relaxation after a 1-s pulse of far-red light. The complex kinetics of P700(+) reduction thus manifested two types of PSI unit differing in the rate of electron input from stromal reductants. The rates of both kinetic components assayed after 1-s pulses were increased about 20-fold by a short (2-5 min) heat-pretreatment of leaves, indicating the accelerated input of electrons to both types of PSI unit. The increased rates of electron flow to P700(+) were even observed 1.5 h after the action of heat had been completed. Both kinetic components were dramatically slowed down upon irradiation of heat-treated leaves for 20-30 s. Their rates were restored after a short (20-30 s) period of darkness. A 5-min leaf exposure at 38 degrees C was sufficient to stimulate by severalfold the reduction of P700(+) pre-oxidized by a brief light pulse. In contrast, the acceleration of P700(+) reduction after a 1-min irradiation was observed only if leaves were subjected to temperatures above 40 degrees C. Neither heat treatment of leaves nor light-dark modulations in the rates of the fast and the slow components of P700(+) dark reduction influenced the relative magnitudes of the two kinetic components, providing strong additional evidence in favor of two distinct types of PSI existing per se in barley leaves. The key role in the control of the activity of electron donation to P700(+) in both rapidly and slowly reducing PSI units was attributed to the amount of stromal reductants available for P700(+) reduction. The latter was expected to be reduced under illumination in the presence of methyl viologen, while increased again in the dark. The regeneration of the pool of stromal reductants in the dark was likely provided by starch breakdown within the chloroplast stroma, but not by import of reducing equivalents from the cytosol. This was evidenced by much lower rates, compared with 1-h dark-adapted leaves, of dark reduction of both components of P700(+) in leaves stored for 24 h in the dark and thus depleted of starch but containing large amounts of glucose, the respiratory substrate.     

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.     

Blue, red and blue plus red light control of chlorophyll content and CO2 gas exchange in barley leaves: Quantitative description of the effects of light quality and fluence rate

The dependence of the Chl content and the rate of CO₂ gas exchange (RGE) on both blue and red quanta fluence rates have been studied in primary leaves of barley ( Hordeum vulgare L. cv. Viner). Empirical equations connecting the two photosynthetic indices with fluence rates of blue or red light were developed. These equations consist of 3 (Chl content) or 2 (RGE) terms, each reflecting the involvement of a specific reaction in the long-term light control of the development of the photosynthetic apparatus. On the basis of the equations the effects of mixed blue plus red light on both the Chl content and RGE were calculated. An additive mode of the co-action of blue and red light in the range of high PFDs (10–170 μmol m⁻² s⁻¹) becomes evident from the comparison of the experimental results and calculated data. The results indicate the involvement of phytochrome, cryptochrome and chlorophyll in the long-term regulation of the Chl content and RGE.     

Endogenous Control of Photosynthetic Activity during Progressive Drought: Influence of Final Products of Photosynthesis

Photosynthetic activities and the redox states of photosystem I (PSI) and photosystem II (PSII) in intact leaves of cucumber plants (Cucumis sativus L.), as well as the sucrose and starch contents were examined under conditions of ongoing soil water deficit imposed by the cessation of watering. As the soil drought progressed, the maximum rate of photosynthetic CO₂ fixation was shown to decrease. These changes in the maximum photosynthetic rate occurred synchronously with changes in the maximum quantum yield of photosynthesis. Under soil water deficit, the reduced form of PSII primary acceptor Q _A was accumulated only at photon flux densities of about 100 mol/(m² s). At such photon flux densities, the changes in nonphotochemical quenching (qN) induced by soil water deficit were opposite to changes in photochemical quenching parameter (1 – qP). Irrespective of the duration of soil drought, the relationship between steady-state concentrations of photochemically inactive reaction centers of PSI and PSII (the fractions of P700 and Q _A in the oxidized and reduced state, respectively) was almost linear, which provides evidence for the concerted operation of both photosystems. The conditions of soil water deficit promoted sucrose accumulation in the source leaf, which was paralleled by a substantial decrease in the amount of starch in the same leaf. The highest content of sucrose in the leaf after a 7-day drought was correlated with the largest decrease in photosynthetic activity. It is concluded that the progressive drought triggers an endogenous mechanism that regulates photosynthesis through feedback relations, namely, the inhibition of photosynthesis by its end products.     

Modeling of Alternative Pathways of Electron Transport to Photosystem I in Isolated Thylakoids

Kinetics of dark decay of absorbance changes at 830 nm (₈₃₀) was examined in thylakoids isolated from leaves of pea seedlings at various concentrations of exogenous NADPH or NADH. Absorbance changes were induced by far-red light to avoid electron donation from photosystem II. In the presence of either biological reductant, the kinetics of ₈₃₀ decay reflecting dark reduction of 700⁺, the primary electron donor of photosystem I, was fitted by a single exponential term. The rate of 700⁺ reduction increased with the rise in the concentration of both NADPH and NADH. The values of K _M and V _max for 700⁺ reduction estimated from concentration dependences were 105 ± 21 M and 0.32/s for NADPH or 21 ± 8 M and 0.12/s for NADH. The rate of P700⁺ reduction by either NADPH or NADH significantly increased in the presence of rotenone, a specific inhibitor of chloroplast reductase. The value of V _max was changed only in the presence of rotenone, whereas K _m was practically unaffected. Unlike the chloroplasts of intact leaves, the only enzyme mediating the input of reducing equivalents from NADPH or NADH to the electron transport chain was concluded to be present in thylakoids.     

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. __________ Translated from Fiziologiya Rastenii, Vol. 52, No. 6, 2005, pp. 805–813. Original Russian Text Copyright ? 2005 by Egorova, Drozdova, Bukhov.     

Redox states of photosystems I and II in irradiated leaves of wheat seedlings grown under different conditions of nitrogen nutrition

Changes in the redox states of photosystem I (PSI) and PSII in irradiated wheat leaves were studied after growing seedlings on a nitrogen-free medium or media containing either nitrate or ammonium. The content of P700, the primary electron donor of PSI was quantified using the maximum magnitude of absorbance changes at 830 nm induced by saturating white light. The highest content of P700 in leaves was found for seedlings grown on the ammonium-containing medium, whereas its lowest content was observed on seedlings grown in the presence of nitrate. At all irradiances of actinic light, the smallest accumulation of reduced Q_A was observed in leaves of ammonium-grown plants. Despite variations in light-response curves of P700 photooxidation and Q_A photoreduction, the leaves of all plants exposed to different treatments demonstrated similar relationships between steady-state levels of P700⁺ and Q_A ^–. The accumulation of oxidized P700 up to 40% of total P700 content was not accompanied by significant Q_A photoreduction. At higher extents of P700 photooxidation, a linear relationship was found between the steady-state levels of P700⁺ and Q_A ^–. The leaves of all treatments demonstrated biphasic patterns of the kinetics of P700⁺ dark reduction after irradiation by far-red light exciting specifically PSI. The halftimes of corresponding kinetic components were found to be 2.6–4 s (fast component) and 17–22 s (slow component). The two components of P700⁺ dark reduction were related to the existence of two PSI populations with different rates of electron input from stromal reductants. The magnitudes of these components differed for plants grown in the presence of nitrate, on the one hand, and plants grown either in the presence of ammonium or in the absence of nitrogen, on the other hand. This indicates the possible influence of nitrogen nutrition on synthesis of different populations of PSI in wheat leaves. The decrease in far-red light irradiance reduced the relative contribution of the fast component to P700⁺ reduction. The fast component completely disappeared at low irradiances. This finding indicates that the saturating far-red light must be applied to determine correctly the relative content of each PSI population in wheat leaves.Translated from Fiziologiya Rastenii, Vol. 52, No. 2, 2005, pp. 165–171.Original Russian Text Copyright © 2005 by Dzhibladze, Polesskaya, Alekhina, Egorova, Bukhov.This revised version was published online in April 2005 with a corrected cover date.     

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. This revised version was published online in June 2006 with corrections to the Cover Date.     

Relaxation of variable chlorophyll fluorescence after illumination of dark-adapted barley leaves as influenced by the redox states of electron carriers

Kinetics of the dark relaxation of variable chlorophyll fluorescence, Fv, were studied after brief illumination of dark-adapted barley leaves in order to understand the rapid reversibility of pulse-induced fluorescence increases, which is observed even when fast linear electron transport to an external electron acceptor is not possible. Four kinetically distinct components were observed which reveal complexity in the oxidation of the reduced primary quinone acceptor of Photosystem II, Q_A ⁻: the slowest component accounted for 4–5% of maximal Fv and had a life-time of several seconds. It is suggested to represent a minor population of inactive Photosystem II centers. The other three components displayed first-order kinetics with half-time of 6–8 ms (`fast' component), 60–80 ms (`middle' component) and 650–680 ms (`slow' component). The fast component dominated Fv when methyl viologen or far-red light accelerated oxidation of plastohydroquinone. It shows rapid oxidation of Q_A ⁻ during electron flow to plastoquinone commensurate with maximum linear electron flow through the electron transport chain. The other two components were observed under conditions of restricted electron flow and excessive reduction of electron carriers. Unexpectedly, the slow component, which is interpreted to reflect the recombination between Q_A ⁻ and an intermediate on the oxidizing side of Photosystem II, saturated already at low irradiances of actinic light when plastoquinone was not yet strongly reduced suggesting that dark-adaptation of leaves results not only in the loss of activity of light-regulated enzymes of the carbon cycle but affects also electron flow from Q_A ⁻ to plastoquinone. KCN poisoning or high temperature treatment of leaves produced a nonexponential pattern of slow Fv relaxation. This effect was largely (heat treatment) or even completely (KCN) abolished by far-red light. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.