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).     

Impairment of the photosynthetic apparatus by oxidative stress induced by photosensitization reaction of protoporphyrin IX

Treatment with the herbicide acifluorfen-sodium (AF-Na), an inhibitor of protoporphyrinogen oxidase, caused an accumulation of protoporphyrin IX (Proto IX) , light-induced necrotic spots on the cucumber cotyledon within 12-24 h, and photobleaching after 48-72 h of light exposure. Proto IX-sensitized and singlet oxygen (¹O₂)-mediated oxidative stress caused by AF-Na treatment impaired photosystem I (PSI), photosystem II (PSII) and whole chain electron transport reactions. As compared to controls, the F_v/F_m (variable to maximal chlorophyll a fluorescence) ratio of treated samples was reduced. The PSII electron donor NH₂OH failed to restore the F_v/F_m ratio suggesting that the reduction of F_v/F_m reflects the loss of reaction center functions. This explanation is further supported by the practically near-similar loss of PSI and PSII activities. As revealed from the light saturation curve (rate of oxygen evolution as a function of light intensity), the reduction of PSII activity was both due to the reduction in the quantum yield at limiting light intensities and impairment of light-saturated electron transport. In treated cotyledons both the Q (due to recombination of Q_A⁻ with S₂) and B (due to recombination of Q_B⁻ with S₂/S₃) band of thermoluminescence decreased by 50% suggesting a loss of active PSII reaction centers. In both the control and treated samples, the thermoluminescence yield of B band exhibited a periodicity of 4 suggesting normal functioning of the S states in centers that were still active. The low temperature (77 K) fluorescence emission spectra revealed that the F₆₉₅ band (that originates in CP-47) increased probably due to reduced energy transfer from the CP47 to the reaction center. These demonstrated an overall damage to the PSI and PSII reaction centers by ¹O₂ produced in response to photosensitization reaction of protoporphyrin IX in AF-Na-treated cucumber seedlings.     

Kinetics of photosystem II electron transport: a mathematical analysis based on chlorophyll fluorescence induction

The OJDIP rise in chlorophyll fluorescence during induction at different light intensities was mathematically modeled using 24 master equations describing electron transport through photosystem II (PSII) plus ordinary differential equations for electron budgets in plastoquinone, cytochrome f, plastocyanin, photosystem I, and ferredoxin. A novel feature of the model is consideration of electron in- and outflow budgets resulting in changes in redox states of Tyrosine Z, P680, and Q_A as sole bases for changes in fluorescence yield during the transient. Ad hoc contributions by transmembrane electric fields, protein conformational changes, or other putative quenching species were unnecessary to account for primary features of the phenomenon, except a peculiar slowdown of intra-PSII electron transport during induction at low light intensities. The lower than F _m post-flash fluorescence yield F _f was related to oxidized tyrosine Z. The transient J peak was associated with equal rates of electron arrival to and departure from Q_A and requires that electron transfer from Q_A ^? to Q_B be slower than that from Q_A ^? to Q_B ^?. Strong quenching by oxidized P680 caused the dip D. Reduced plastoquinone, a competitive product inhibitor of PSII, blocked electron transport proportionally with its concentration. Electron transport rate indicated by fluorescence quenching was faster than the rate indicated by O₂ evolution, because oxidized donor side carriers quench fluorescence but do not transport electrons. The thermal phase of the fluorescence rise beyond the J phase was caused by a progressive increase in the fraction of PSII with reduced Q_A and reduced donor side.     

Chlorophyll a fluorescence of typical desert plant Alhagi sparsifolia Shap. at two light levels

Alhagi sparsifolia Shap. is exposed to a high-irradiance environment as the main vegetation found in the forelands of the Taklamakan Desert. We investigated chlorophyll a fluorescence emission of A. sparsifolia seedlings grown under ambient (HL) and shade (LL) conditions. Our results indicated that the fluorescence intensity in the leaves was significantly higher for LL-grown plants than that under HL. High values of the maximum quantum yield of PSII for primary photochemistry (φP_o) and the quantum yield that an electron moves further than Q_A - (φE_o) in the plants under LL conditions suggested that the electron flow from Q_A - (primary quinone electron acceptors of PSII) to Q_B (secondary quinone acceptor of PSII) or Q_B - was enhanced at LL compared to natural HL conditions. The efficiency/probability with which an electron from the intersystem electron carriers was transferred to reduce end electron acceptors at the PSI acceptor side and the quantum yield for the reduction of end electron acceptors at the PSI acceptor side were opposite to φP_o, and φE_o. Thus, we concluded that the electron transport on the donor side of PSII was blocked under LL conditions, while acceptor side was inhibited at the HL conditions. The PSII activity of electron transport in the plants grown in shade was enhanced, while the energy transport from PSII to PSI was blocked compared to the plants grown at HL conditions. Furthermore, PSII activity under HL was seriously affected in midday, while the plants grown in shade enhanced their energy transport.     

Chlorophyll a fluorescence: beyond the limits of the QA model

Chlorophyll a fluorescence is a non-invasive tool widely used in photosynthesis research. According to the dominant interpretation, based on the model proposed by Duysens and Sweers (1963, Special Issue of Plant and Cell Physiology, pp 353–372), the fluorescence changes reflect primarily changes in the redox state of Q_A, the primary quinone electron acceptor of photosystem II (PSII). While it is clearly successful in monitoring the photochemical activity of PSII, a number of important observations cannot be explained within the framework of this simple model. Alternative interpretations have been proposed but were not supported satisfactorily by experimental data. In this review we concentrate on the processes determining the fluorescence rise on a dark-to-light transition and critically analyze the experimental data and the existing models. Recent experiments have provided additional evidence for the involvement of a second process influencing the fluorescence rise once Q_A is reduced. These observations are best explained by a light-induced conformational change, the focal point of our review. We also want to emphasize that—based on the presently available experimental findings—conclusions on α/ß-centers, PSII connectivity, and the assignment of F_V/F_M to the maximum PSII quantum yield may require critical re-evaluations. At the same time, it has to be emphasized that for a deeper understanding of the underlying physical mechanism(s) systematic studies on light-induced changes in the structure and reaction kinetics of the PSII reaction center are required.     

EARLY TOXIC EFFECTS OF ZINC ON PSII OF SYNECHOCYSTIS AQUATILIS F. AQUATILIS (CYANOPHYCEAE)1

Zinc toxicity on photosynthetic activity in cells of Synechocystis aquatilis f. aquatilis Sauvageau was investigated by monitoring Hill activity and fluorescence. The oxygen‐evolving activity decreased to about 80% of the initial value after exposure to 0.1 mM ZnSO₄ for 1 h. The PSII activity was inhibited by 40% in the presence of zinc concentrations ranging from 0.5 to 5.0 mM, suggesting that the metal effect is limited by zinc uptake. The fluorescence capacity (F_max–F/F_max) decreased from 0.57 to 0.35 and 0.20 in Zn‐treated cells for 15 and 60 min, respectively, thus providing evidence for rapid inactivation of electron transport at PSII. Zinc treatment promoted a rapid increase in PSII fluorescence that was counteracted by addition of 1,4‐benzoquinone, indicating that electron transfer at the reducing side of the PSII reaction center is arrested by zinc. Furthermore, a decline in the fluorescence yield could be observed after 1 h of zinc treatment as well as when Zn‐treated cells were excited in presence of 3‐(3′,4′‐dichlorophenyl)‐1,1‐dimethylurea. Under these conditions, zinc did not affect energy transfer from phycobilisomes to PSII, and the gradual quenching of PSII fluorescence may be due to a decrease in electron flow on the donor side of PSII. However, the 20% increase in the minimal fluorescence intensity (F_o) in parallel to the absence of changes in the maximal fluorescence intensity (F_max), observed in the first hour of zinc treatment, could also suggest a metal‐induced decline in the energy transfer from PSII‐chl a antenna to the PSII reaction center.     

Site of salicylaldoxime interaction with photosystem II

The reversible inhibition of Photosystem II by salicylaldoxime was studied in spinach D-10 particles by fluorescence, optical absorption, and electron spin resonance spectroscopy. In the presence of 15 mM salicylaldoxime, the initial fluorescence yield was raised to the level of the maximum fluorescence, indicating efficient charge recombination between reduced pheophytin (Ph) and P680⁺. In agreement with the rapid (ns) backreaction expected between Ph^– and P680⁺, the optical absorption transient at 820 mm was not observed. When the particles were washed free of salicylaldoxime, the optical absorption transient resulting from the rereduction of P680⁺ was restored to the µs timescale. These results, along with the previously observed inhibition of electron transport reactions and diminution of the 515-nm absorption change in chloroplasts [Golbeck, J.H. (1980) Arch Biochem Biophys 202, 458–466], are consistent with a site of inhibition between Ph and QA in Photosystem II. ESR Signal IIf and Signal Its were abolished in the presence of 25 mM salicylaldoxime, but both signals could be recovered by washing the D-10 particles free of the inhibitor. The loss of Signal Ilf is most likely a consequence of the inhibition between Ph and Q_A; the rapid charge recombination between Ph^– and P680⁺ would preclude electron transfer from an electron donor on the oxidizing side of Photosystem II. The loss of Signal Its may be due to a change in the environment of the donor complex such that the semiquinone radical giving rise to Signal Its interacts with a nearby reductant.Abbreviations D₁ electron donor to P680⁺ in oxygen-inhibited chloroplasts - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F₀ prompt chlorophyll a fluorescence yield - F_i initial chlorophyll a fluorescence yield - F_max maximum chlorophyll a fluorescence yield - F_var variable chlorophyll a fluorescence yield - FWHM full width at half maximum - Mes 2-(N-morpholino) ethanesulfonic acid - P680 reaction center chlorophyll a of photosystem II - Ph pheophytin intermediate electron acceptor - Q_A primary quinone electron acceptor - Q_B secondary quinone electron acceptor - Tris tris(hydroxymethyl)aminomethane - Z electron donor to P680⁺     

Stress-induced chlorosis and increase in D1-protein turnover precede photoinhibition in spinach suffering under magnesium/sulphur deficiency

To test wether chlorosis is induced by photoinhibitory damage to photosystem II (PSII), onset of chlorosis and loss of PSII function were compared in young spinach (Spinaciae oleracea L.) plants suffering under a combined magnesium and sulphur deficiency. Loss of chlorophyll already occurred after the first week of deficiency and preceded any permanent functional inhibition of the photosynthetic apparatus. Permanent disturbancies of photosynthetic electron transport measured in isolated thylakoids and of PSII function, determined via the ratio of variable fluorescence to maximal fluorescence, F_v/F_m, could be detected only after the second week of deficiency. After the third week, the plants had lost about 60% of their chlorophyll; even so, fluorescence data indicated that 85% of the existing PSII was still capable of initiating photosynthetic electron transport. However, quenching analysis of steady-state fluorescence showed an early increase in non-photochemical quenching and in down-regulated PSII centres with low steady-state quantum efficiency. Together with the down-regulation of PSII centres, a 1.4-fold increase in D1-protein synthesis, measured as incorporation of [¹⁴C]leucine, could be observed at the end of the first week before any loss of D1 protein, chlorophyll or photosynthetic activity could be detected. Immunological determiation by Western-blotting did not show a change in D1-protein content; thus, at this time, D1 protein was not only faster synthesised but was also faster degraded than before the imposition of mineral deficiency. The increased turnover was high enough to prevent any loss or functional inhibition of PSII. After 3 weeks, D1-protein synthesis on a chlorophyll basis was further stimulated by a factor of 2. However, this was not enough to prevent a net loss of D1 protein of about 70%, showing that the D1-protein was now degraded faster than it was synthesised. Immunological determination and electron-transport measurements showed that together with the loss of D1 protein the other polypetides of PSII were also degraded, resulting in a specific loss of PSII centres. The degradation of PSII centres prevented a large accumulation of damaged PSII centres. We assume that the decrease in PSII centres initiates the breakdown of the other thylakoid proteins.Abbreviations Fo yield of intrinsic fluorescence when all PSII centres are open in the dark - F_m yield of maximal fluorescence when all reaction centres are closed - F_m fluorescence yield when all reaction centres are closed under steady-state conditions - F_v yield of variable fluorescence, (difference between F_o and F_m) - F yield of variable fluorescence under steady-state conditions, difference between F_m and F_t, the fluorescence yield under steady-state conditions - PFD photon flux density - Q_A primary quinone acceptor of PSII - Q_B secondary quinone acceptor of PSII - q_p photochemical quenching - q_n non-photochemical quenching This work was supported by grants from the Bundesminister für Forschung und Technologie and the German Israeli Foundation. The authors thank Prof. I. Ohad (Department of Biological Chemistry, Hebrew University, Jerusalem, Israel) for fruitful discussions.     

Recovery of photosystem I and II activities during re-hydration of lichen<Emphasis Type="Italic"> Hypogymnia physodes</Emphasis> thalli

Photochemical efficiencies of photosystem I (PSI) and photosystem II (PSII) were studied in dry thalli of the lichen Hypogymnia physodes and during their re-hydration. In dry thalli, PSII reaction centers are photochemically inactive, as evidenced by the absence of variable chlorophyll (Chl) fluorescence, whereas the primary electron donor of PSI, P700, exhibits irreversible oxidation under continuous light. Upon application of multiple- and, particularly, single-turnover pulses in dry lichen, P700 oxidation partially reversed, which indicated recombination between P700⁺ and the reduced acceptor F_X of PSI. Re-wetting of air-dried H. physodes initiated the gradual restoration of reversible light-induced redox reactions in both PSII and PSI, but the recovery was faster in PSI. Two slow components of P700⁺ reduction occurred after irradiation of partially and completely hydrated thalli with strong white light. In contrast, no slow component was found in the kinetics of re-oxidation of Q_A^–, the reduced primary acceptor of PSII, after exposure of such thalli to white light. This finding indicated the inability of PSII in H. physodes to provide the reduction of the plastoquinone pool to significant levels. It is concluded that slow alternative electron transport routes may contribute to the energetics of photosynthesis to a larger extent in H. physodes than in higher plants.Abbreviations A₀ and A₁ Primary acceptor chlorophyll and secondary electron acceptor phylloquinone - Chl a Chlorophyll a - F_m Maximal level of chlorophyll fluorescence when all PSII centers are closed - F_o Minimal level of fluorescence when all PSII centers are open after dark adaptation - FR Far-red - F_v Variable fluorescence (=F_m–F_o) - F_X, F_A, and F_B Iron–sulfur centers - MT pulse Multiple-turnover pulse - PS Photosystem - P700 Reaction center chlorophyll of PSI - Q_A Primary quinone acceptor of PSII - Q_B Secondary quinone acceptor of PSII - ST pulse Single-turnover pulse     

Age-related differences in frost sensitivity of the photosynthetic apparatus of two Plagiomnium species

Shoots of two species of moss, Plagiomnium undulatum (Hedw.) Kop. and Plagiomnium affine (Funck) Kop., were subjected to freezing at various temperatures. After thawing, the activities of different photosynthetic reactions were determined in relation to the ages of the leaves. Analysis of the fast kinetics of chlorophyll-a fluorescence of individual leaves showed that young and old tissues were considerably less frost tolerant than mature ones. In principle, the pattern of freeze inactivation of photosynthetic reactions resembles that observed in higher plants. The decreases in the amplitude of F_v (variable fluorescence) and the ratio of F_v to F_m (maximum fluorescence) with increasing freezing stress reflect a progressive inactivation of photosystem II (PSII)-mediated electron transport, i.e. inhibition of photoreaction to photochemistry and-or electron donation to the photochemical reaction, and thus a decline in the potential photochemical efficiency of PSII. The insignificant change in the F₀ (constant fluorescence) level during progressive decline of F_v indicates that the excitation-energy transfer between antenna pigments and from those to reaction centres of PSII was little impaired by lethal freezing stress. Sugar analyses of various stem sections showed that ontogenetic variation in the frost tolerance of leaves cannot be attributed to differences in the cellular levels of sucrose, glucose and fructose.Abbreviations and Symbols DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_m maximum fluorescence - F₀ constant (initial) fluorescence - F_v variable fluorescence     

Fluorescence Induction Kinetics in Membrane Preparations of Photosystem II with Heterogeneous Metal Clusters (Mn/Fe) in the Oxygen-Evolving Complex

Transport of electrons in spinach photosystem II (PSII) whose oxygen-evolving complex (OEC) contains heterogeneous metal clusters 2Mn2Fe and 3Mn1Fe was studied by measuring the fluorescence induction kinetics (FIK). PSII(2Mn,2Fe) and PSII(3Mn,1Fe) preparations were produced using Cadepleted PSII membranes (PSII(–Ca)). It was found that FIK in PSII(2Mn,2Fe) membranes is similar in form to FIK in PSII(–Ca) samples, but the fluorescence yield is lower in PSII(2Mn,2Fe). The results demonstrate that, just as in PSII(–Ca) preparations, there is electron transfer from the metal cluster in the OEC to the primary plastoquinone electron acceptor Q_A. They also show that partial substitution of Mn cations with Fe has no effect on the electron transport on the acceptor side of PSII. Thus, these data demonstrate the possibility of water oxidation either by the heterogeneous metal cluster or just by the manganese dimer. We established that FIK in PSII(3Mn,1Fe) preparations are similar in form to FIK in PSII(2Mn,2Fe) membranes but PSII(3Mn,1Fe) is characterized by a slightly higher maximal fluorescence yield, F_max. The electron transfer rate in PSII(3Mn,1Fe) preparations significantly (by a factor of two) increases in the presence of Ca²⁺, whereas Ca²⁺ has hardly any effect on the electron transport in PSII(2Mn,2Fe) membranes. In Mndepleted PSII membranes, FIK reaches its maximum (the so-called peak K), after which the fluorescence yield starts to decrease as the result of two factors: the oxidation of reduced primary plastoquinone Q _A ^? and the absence of electron influx from the donor side of PSII. The replacement of Mn cations by Fe in PSII(?Mn) preparations leads to fluorescence saturation and disappearance of the K peak. This is possibly due to the deceleration of the charge recombination process that takes place between reduced primary electron acceptor Q _A ^? and oxidized tyrosine Y _Z ^+. which is an electron carrier between the OEC and the primary electron donor P680.     

Photoinhibition and light-dependent turnover of the D1 reaction-centre polypeptide of photosystem II are enhanced by mineral-stress conditions

The function of photosystem II (PSII) and the turnover of its D1 reaction-center protein were studied in spinach (Spinacia oleracea L.) plants set under mineral stress. The mineral deficiencies were induced either by supplying the plants with an acidic nutrient solution or by strongly reducing the supply of magnesium alone or together with sulfur. After exposure for 8–10 weeks to the different media, the plants were characterized by a loss of chlorophyll and an increase in starch content, indicating a disturbance in the allocation of assimilates. Depending on the severity of the mineral deficiencies the plants lost their ability to adapt even to moderate iradiances of 400 mol photons·m^–2·s^–1 and became photoinhibited, as indicated by the decrease in F_v/F_m (the ratio of yield of variable fluorescence to yield of maximal fluorescence when all reaction centers are closed). The loss of PSII function was induced by changes on the acceptor side of PSII. Fast fluorescence decay showed a loss of PSII centers with bound Q_B, the secondary quinone acceptor of PSII, and a fast reoxidation kinetic of q _a ^- , the primary quinone acceptor of PSII, in the photoinactivated plants. No appreciable change could be observed in the amount of PSII centers with unbound Q_B and in Q_B-nonreducing PSII centers. Immunological studies showed that the contents of the D1 and D2 proteins of the PSII reaction center and of the 33-kDa protein of the water-splitting complex were diminished in the photoinhibited plants, and the occurrance of a new polypetide of 14 kDa that reacted with an antibody against the C-termius of the D1 protein. As shown by pulse-labelling experiments with [¹⁴C]leucine both degradation and synthesis of the D1 protein were enhanced in the mineral-deficient plants when compared to non-deficient plants. A stimulation of D1-protein turnover was also observed in pH 3-grown plants, which were not inhibited at growth-light conditions. Obviously, stimulation of D1-protein turnover prevented photoinhibition in these plants. However, in the Mg- and Mg/S-deficient plants even a further stimulation of D1-protein turnover could not counteract the increased rate of photoinactivation.Abbreviations amp_(f,m,s) amplitude of the fast, (medium and slow) exponential component of fluorescence decay - F_m yield of maximum fluorescenc when all reaction centers are closed - F_o yield of intrinsic fluorescence at open PSII reaction centers in the dark - F_v yield of variable fluorescence, (difference between F_m and F_o) - LHC light-harvesting complex - PFD photon flux density - Q_A primary quinone acceptor of PSII - Q_B secondary quinone acceptor of PSII Dedicated to Professor Dr. Dres. hc. Achim Trebst on the occasion of his 65th birthdayThis work was supported by grants from the BMFT and the Ministerium für Umwelt, Raumordnung and Landwirtschaft, Nordrhein-Westfalen. The authors thank H. Wietoska and M. Bronzel for skilful technical assistance.     

On the chlorophyll a fluorescence yield in chloroplasts upon excitation with twin turnover flashes (TTF) and high frequency flash trains

Chlorophyll fluorescence is routinely taken as a quantifiable measure of the redox state of the primary quinone acceptor Q_A of PSII. The variable fluorescence in thylakoids increases in a single turnover flash (STF) from its low dark level F _o towards a maximum F _m^STF when Q_A becomes reduced. We found, using twin single turnover flashes (TTFs) that the fluorescence increase induced by the first twin-partner is followed by a 20–30% increase when the second partner is applied within 20–100 μs after the first one. The amplitude of the twin response shows a period-of-four oscillation associated with the 4-step oxidation of water in the Kok cycle (S states) and originates from two different trapped states with a life time of 0.2–0.4 and 2–5 ms, respectively. The oscillation is supplemented with a binary oscillation associated with the two-electron gate mechanism at the PSII acceptor side. The F(t) response in high frequency flash trains (1–4 kHz) shows (i) in the first 3–4 flashes a transient overshoot 20–30% above the F _m^STF = 3*F _o level reached in the 1st flash with a partial decline towards a dip D in the next 2–3 ms, independent of the flash frequency, and (ii) a frequency independent rise to F _m = 5*F _o in the 3–60 ms time range. The initial overshoot is interpreted to be due to electron trapping in the S₀ fraction with Q_B-nonreducing centers and the dip to the subsequent recovery accompanying the reoxidation of the double reduced acceptor pair in these RCs after trapping. The rise after the overshoot is, in agreement with earlier findings, interpreted to indicate a photo-electrochemical control of the chlorophyll fluorescence yield of PSII. It is anticipated that the double exciton and electron trapping property of PSII is advantageous for the plant. It serves to alleviate the depression of electron transport in single reduced Q_B-nonreducing RCs, associated with electrochemically coupled proton transport, by an increased electron trapping efficiency in these centers.     

Photoinhibition in outdoor Spirulina platensis cultures assessed by polyphasic chlorophyll fluorescence transients

Photoinhibition in outdoor cultures of Spirulina platensis was studied by measuring the polyphasic rise of chlorophyll fluorescence transients, which provide information on the primary photochemistry of PSII. The maximum efficiency of PSII photochemustry (F_v/F_m) declined in response to daily increasing irradiance and recovered as daily irradiance decreased. The greatest inhibition (15%) in F_v/F_m was observed at 12:00 hr which responded to the highest irradiance. The absorption flux, the trapping flux, and the electron transport flux per PSII reaction center increased in response to daily increasing irradiance and decreased as irradiance decreased. The daily change in the concentration of PSII reaction centers followed the same pattern as F_v/F_m. However, no significant changes in the probability of electron transport beyond Q_A (Ψ_o) were observed during the day. The results suggest that the decrease in F_v/F_m induced by photoinhibition in outdoor Spirulina cultures was a result of the inactivation of PSII reaction centers. The results also suggest that the measurement of polyphasic fluorescence transients is a powerful tool to study the mechanism of photoinhibition in outdoor Spirulina cultures and to screen strains for photoinhibition tolerance. This revised version was published online in June 2006 with corrections to the Cover Date.     

On the approaches applied in formulation of a kinetic model of photosystem II: Different approaches lead to different simulations of the chlorophyll a fluorescence transients

Chlorophyll a fluorescence rise (O-J-I-P transient) was in literature simulated using models describing reactions occurring solely in photosystem II (PSII) and plastoquinone (PQ) pool as well as using complex models which described, in addition to the above, also subsequent electron transport occurring beyond the PQ pool. However, there is no consistency in general approach how to formulate a kinetic model and how to describe particular reactions occurring even in PSII only. In this work, simple kinetic PSII models are considered always with the same electron carriers and same type of reactions but some reactions are approached in different ways: oxygen evolving complex is considered bound to PSII or “virtually” separated from PSII; exchange of doubly reduced secondary quinone PSII electron acceptor, Q_B, with PQ molecule from the PQ pool is described by one second order reaction or by two subsequent reactions; and all possible reactions or only those which follow in logical order are considered. By combining all these approaches, eight PSII models are formulated which are used for simulations of the chlorophyll a fluorescence transients. It is shown that the different approaches can lead to qualitatively different results. The approaches are compared with other models found elsewhere in the literature and therefore this work can help the readers to better understand the other models and their results.     

Spermine and spermidine inhibition of photosystem II: Disassembly of the oxygen evolving complex and consequent perturbation in electron donation from TyrZ to P680 and the quinone acceptors QA to QB

Polyamines are implicated in plant growth and stress response. However, the polyamines spermine and spermidine were shown to elicit strong inhibitory effects in photosystem II (PSII) submembrane fractions. We have studied the mechanism of this inhibitory action in detail. The inhibition of electron transport in PSII submembrane fractions treated with millimolar concentrations of spermine or spermidine led to the decline of plastoquinone reduction, which was reversed by the artificial electron donor diphenylcarbazide. The above inhibition was due to the loss of the extrinsic polypeptides associated with the oxygen evolving complex. Thermoluminescence measurements revealed that charge recombination between the quinone acceptors of PSII, Q_A and Q_B, and the S₂ state of the Mn-cluster was abolished. Also, the dark decay of chlorophyll fluorescence after a single turn-over white flash was greatly retarded indicating a slower rate of Q_A⁻ reoxidation.     

Increased photosynthetic activities and thermostability of photosystem II with leaf development of elm seedlings (Ulmus pumila) probed by the fast fluorescence rise OJIP

Experiments were conducted to investigate the photosynthetic activity and thermostability of photosystem II (PSII) in elm seedling (Ulmus pumila) leaves from initiation to full expansion. During leaf development, photosynthesis, measured as CO₂ fixation, increased gradually and reached a maximum value when leaves were fully developed. In parallel with the increase of carbon assimilation, chlorophyll content increased. The chlorophyll a fluorescence measurements showed that the maximum quantum yield of PSII primary photochemistry (φ_po), the efficiency with which the energy of trapped excitons is converted into the electron transport beyond Q_A (Ψ_o) and the quantum yield of electron transport beyond Q_A (φ_Eo) increased gradually. The low light experiments confirmed these results independently. When subjected to heat stress, young leaves exhibited progressively lower φ_po and maximal fluorescence (F_m) values with considerably higher minimal fluorescence (F_o) than mature leaves, demonstrating that PSII in newly initiating leaves is more sensitive to heat stress. Further analysis revealed that PSII structure in newly initiating leaves showed a robust alteration under heat stress, which was reflected by the clear K phase in the OJIP curves. Therefore, we suggest that the enhanced thermostability of PSII in the case of leaf growth might be associated with an improvement of the stability of the oxygen-evolving complex (OEC) to heat stress during leaf development.     

Studies on the limitations to photosynthesis in leaves of the atrazine-resistant mutant ofSenecio vulgaris L.

In leaves of an atrazine-resistant mutant ofSenecio vulgaris the quantum efficiency of CO₂ assimilation was reduced by 21% compared to the atrazine-susceptible wild type, and at a light level twice that required to saturate photosynthesis in the wild type the CO₂ fixation rate in the mutant was decreased by 15%. In leaves at steady-state photosynthesis there was a measurable increase in the reduction state of the photosystem II (PSII) primary quinone acceptor,Q _A. Although this would lead to a decreased rate of PSII electron transport and may thus explain the decrease in quantum efficiency, this cannot account for the fall in the maximum rate of CO₂ fixation. The atrazine-resistant mutant showed an appreciably longer photosynthetic induction time which indicates an effect on carbon metabolism; however, the response of CO₂-fixation rate to intercellular CO₂ concentration revealed no differences in carboxylation efficiency. There were also no differences in the ability to perform a State 1–State 2 transition between the atrazine-resistant and susceptible biotypes and no difference in the profiles of phosphorylated thylakoid polypeptides. It is concluded that the alteration of the redox equilibrium between PSII quinone electron acceptors in the atrazine-resistant biotype limits appreciably the photosynthetic efficiency in non-saturating light. Additionally, there is a further, as yet unidentified, limitation which decreases photosynthesis in the resistant mutant under light-saturating conditions.Abbreviations and symbols DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F _max maximum fluorescence emission - F _o2 minimal fluorescence emission upon exposure to saturating light flash - F _v variable fluorescence emission - F _v2 variable fluorescence emission upon exposure to saturating light flash - kDa kilodalton - PSI, II photosystems I, II - Q _A primary quinone acceptor of PSH - Q _B secondary quinone acceptor of PSII - RuBP ribulose-1,5-bisphosphate     

Photosynthesis and activity of photosystem II in response to drought stress in Amur Grape (Vitis amurensis Rupr.)

The Amur Grape (Vitis amurensis Rupr.) cultivars ??shuangFeng?? and ??ZuoShanyi?? were grown in shelter greenhouse under natural sunlight and subjected to drought. Sap flow rate, net photosynthetic rate (P _N), and chlorophyll (Chl) fluorescence were measured on Amur Grape leaves subjected to different drought treatments. Significant decreases in P _N were associated with increasing intercellular CO₂ concentration (C _i), suggesting that the reduction in P _N was caused by nonstomatal limitation. Analysis of OJIP transients according to the JIP-test protocol revealed that specific (per PSII reaction center) energy fluxes for light absorption, excitation energy trapping and electron transport have significantly changed. The appearance of a pronounced K-step and J-step in polyphasic rise of fluorescence transient suggested the oxygen-evolving complex and electron transport were inhibited. Drought stress has relatively little effect on the parameter maximal quantum yield of PSII photochemistry (F_v/F_m), but the performance index (PIABS) is more sensitive in different drought treatment. There are cultivar differences in the response of PSII activity to drought, the photosynthetic apparatus of ??ZuoShanyi?? cultivar is more resistant to drought than that of ??ShuangFeng??, and JIP-test could be a useful indicator for evaluation and selection to drought tolerance.