Primary Electron Acceptor Of Photosystem Research Articles

Investigating the electronic properties of the primary electron acceptor of photosystem I

Investigating the electronic properties of the primary electron acceptor of photosystem I

Maximum fluorescence and electron transport kinetics determined by light-induced fluorescence transients (LIFT) for photosynthesis phenotyping

Photosynthetic phenotyping requires quick characterization of dynamic traits when measuring large plant numbers in a fluctuating environment. Here, we evaluated the light-induced fluorescence transient (LIFT) method for its capacity to yield rapidly fluorometric parameters from 0.6 m distance. The close approximation of LIFT to conventional chlorophyll fluorescence (ChlF) parameters is shown under controlled conditions in spinach leaves and isolated thylakoids when electron transport was impaired by anoxic conditions or chemical inhibitors. The ChlF rise from minimum fluorescence (Fo) to maximum fluorescence induced by fast repetition rate (Fm−FRR) flashes was dominated by reduction of the primary electron acceptor in photosystem II (QA). The subsequent reoxidation of QA− was quantified using the relaxation of ChlF in 0.65 ms (Fr1) and 120 ms (Fr2) phases. Reoxidation efficiency of QA− (Fr1/Fv, where Fv = Fm−FRR − Fo) decreased when electron transport was impaired, while quantum efficiency of photosystem II (Fv/Fm) showed often no significant effect. ChlF relaxations of the LIFT were similar to an independent other method. Under increasing light intensities, Fr2′/Fq′ (where Fr2′ and Fq′ represent Fr2 and Fv in the light-adapted state, respectively) was hardly affected, whereas the operating efficiency of photosystem II (Fq′/Fm′) decreased due to non-photochemical quenching. Fm−FRR was significantly lower than the ChlF maximum induced by multiple turnover (Fm−MT) flashes. However, the resulting Fv/Fm and Fq′/Fm′ from both flashes were highly correlated. The LIFT method complements Fv/Fm with information about efficiency of electron transport. Measurements in situ and from a distance facilitate application in high-throughput and automated phenotyping.

Unique chlorophylls in picoplankton Prochlorococcus sp. "Physicochemical properties of divinyl chlorophylls, and the discovery of monovinyl chlorophyll b as well as divinyl chlorophyll b in the species Prochlorococcus NIES-2086".

In this review, we introduce our recent studies on divinyl chlorophylls functioning in unique marine picoplankton Prochlorococcus sp. (1) Essential physicochemical properties of divinyl chlorophylls are compared with those of monovinyl chlorophylls; separation by normal-phase and reversed-phase high-performance liquid chromatography with isocratic eluent mode, absorption spectra in four organic solvents, fluorescence information (emission spectra, quantum yields, and life time), circular dichroism spectra, mass spectra, nuclear magnetic resonance spectra, and redox potentials. The presence of a mass difference of 278 in the mass spectra between [M+H]+ and the ions indicates the presence of a phytyl tail in all the chlorophylls. (2) Precise high-performance liquid chromatography analyses show divinyl chlorophyll a' and divinyl pheophytin a as the minor key components in four kinds of Prochlorococcus sp.; neither monovinyl chlorophyll a' nor monovinyl pheophytin a is detected, suggesting that the special pair in photosystem I and the primary electron acceptor in photosystem II are not monovinyl but divinyl-type chlorophylls. (3) Only Prochlorococcus sp. NIES-2086 possesses both monovinyl chlorophyll b and divinyl chlorophyll b, while any other monovinyl-type chlorophylls are absent in this strain. Monovinyl chlorophyll b is not detected at all in the other three strains. Prochlorococcus sp. NIES-2086 is the first example that has both monovinyl chlorophyll b as well as divinyl chlorophylls a/b as major chlorophylls.

Spectral Resolution of the Primary Electron Acceptor A0 in Photosystem I

The reduced state of the primary electron acceptor of Photosystem I, A(0), was resolved spectroscopically in its lowest energy Q(y) region for the first time without the addition of chemical reducing agents and without extensive data manipulation. To carry this out, we used the menB mutant of Synechocystis sp. PCC 6803 in which phylloquinone is replaced by plastoquinone-9 in the A(1) sites of Photosystem I. The presence of plastoquinone-9 slows electron transfer from A(0) to A(1), leading to a long-lived A(0)(-) state. This allows its spectral signature to be readily detected in a time-resolved optical pump-probe experiment. The maximum bleaching (A(0)(-) - A(0)) was found to occur at 684 nm with a corresponding extinction coefficient of 43 mM(-1) cm(-1). The data show evidence for an electrochromic shift of an accessory chlorophyll pigment, suggesting that the latter Q(y) absorption band is centered around 682 nm.

The nature of the photosystem II reaction centre in the chlorophyll d-containing prokaryote, Acaryochloris marina

Pigment-protein complexes enriched in photosystem II (PS II) have been isolated from the chlorophyll (Chl) d containing cyanobacterium, Acaryochloris marina. A small PS II-enriched particle, we call 'crude reaction centre', contained 20 Chl d, 0.5 Chl a and 1 redox active cytochrome b-559 per 2 pheophytin a, plus the D1 and D2 proteins. A larger PS II-enriched particle, we call 'core', additionally bound the antenna complexes, CP47 and CP43, and had a higher chlorophyll per pheophytin ratio. Pheophytin a could be photoreduced in the presence of a strong reductant, indicating that it is the primary electron acceptor in photosystem II of A. marina. A substoichiometric amount of Chl a (less than one chlorophyll a per 2 pheophytin a) strongly suggests that Chl a does not have an essential role in the photochemistry of PS II in this organism. We conclude that PS II, in A. marina, utilizes Chl d and not Chl a as primary electron donor and that the primary electron acceptor is one of two molecules of pheophytin a.

Simultaneous analysis of prompt and delayed chlorophyll a fluorescence in leaves during the induction period of dark to light adaptation

An attempt is made to reveal the relation between the induction curves of delayed fluorescence (DF) registered at 0.35–5.5 ms and the prompt chlorophyll fluorescence (PF). A simple formulation was proposed to link the ratio of the transient values of delayed and variable fluorescence with the redox state of the primary electron acceptor of Photosystem II—Q A, and the thylakoid membrane energization. The term luminescence potential ( U L ) was introduced, defined as the sum of the redox potential of Q A and the transmembrane proton gradient. It was shown that U L is proportional to the ratio of DF to the variable part of PF. The theoretical model was verified and demonstrated by analysing induction courses of PF and millisecond DF, simultaneously registered from leaves of barley—wild-type and the chlorophyll b-less mutant chlorina f2. A definitive correlation between PF and DF was established. If the luminescence changes are strictly due to U L , the courses of DF and PF are reciprocal and the millisecond DF curve resembles the first derivative of the PF( t) function.

Discovery of pheophytin function in the photosynthetic energy conversion as the primary electron acceptor of Photosystem II.

This minireview describes the discovery of participation of pheophytin, a metal-free derivative of chlorophyll, in the early steps of photosynthetic solar energy conversion as the primary electron acceptor of Photosystem II.

Separation and determination of minor photosynthetic pigments by reversed-phase HPLC with minimal alteration of chlorophylls.

Reversed-phase HPLC conditions for separation of chlorophyll (Chl) a, Chl a' (the C132-epimer of Chl a), pheophytin (Pheo) a (the primary electron acceptor of photosystem (PS) II), and phylloquinone (PhQ) (the secondary electron acceptor of PS 1), have been developed. Pigment extraction conditions were optimized in terms of pigment alteration and extraction efficiency. Pigment composition analysis of light-harvesting complex II, which would not contain Chl a' nor Pheo a, showed the Chl a'/Chl a ratio of 3-4 x 10(-4) and the Pheo a/Chl a ratio of 4-5 x 10(-4), showing that the conditions developed here were sufficiently inert for Chl analysis. Preliminary analysis of thylakoid membranes with this analytical system gave the PhQ/Chl a' ratio of 0.58 +/- 0.03 (n = 4), in line with the stoichiometry of one molecule of Chl a' per PS I.

Normal-phase HPLC separation of possible biosynthetic intermediates of pheophytin a and chlorophyll a'.

Normal-phase HPLC conditions have been developed for separating the C17(3) isoprenoid isomers, which are expected to be formed as biosynthetic intermediates of chlorophyll (Chl) a, Chl a' (C13(2)-epimer of Chl a), pheophytin (Pheo) a and protochlorophyll (PChl). The application of these conditions to pigment composition analysis of greening etiolated barley leaves allowed us to detect, for the first time, the C17(3) isomers of Chl a', a possible constituent of the primary electron donor of photosystem (PS) I, P700, and those of Pheo a, the primary electron acceptor of PS II, in the very early stage of greening. The C17(3) isomer distribution patterns were approximately the same between Chl a and Chl a', but significantly different between Pheo a and Chl a', probably reflecting the similarity and difference, respectively, in the biosynthetic pathways of these pigment pairs.

Specific mutations of the PsaB methionine axial ligand to chlorophyll (A0) of PSI in Chlamydomonas reinhardtii

The primary electron acceptor in photosystem I is a chlorophyll a monomer termed A0. Because of its location in the structure, A0 may play a role both in excitation energy transfer and electron transfer. Methionine residues in the helix X domain of PsaA and PsaB are positioned close to A0 and may provide axial ligands, or occlude axial ligation by water. Met-664 of PsaB has been changed to His and Ser. Both the MH(B664) and MS (B664) mutants accumulate similar levels of PSI to wild type and can grow heterotrophically when acetate is supplied. Phototrophic growth is severely impaired in both mutants: MH(B664) will not grow under high light and grows only slowly under low light, whereas MS(B664) can grow slowly under high light. Femtosecond transient absorbance measurements of both mutants show a significantly increased contribution of long lived components (>>100 ps) compared to WT control. This indicates a significant decrease in the coupling between the antenna and electron transfer core, suggesting a role for A0 in excitation energy transfer. No significant change was found in the excitation trapping time, suggesting that the rate of primary charge separation was unchanged. Similar mutants of PsaA are currently under investigation.

Chlorophyll fluorescence during CAM-phases in Clusia minor L. under drought stress

Some species of Clusia show a high flexibility in regulating carbon uptake during the day-night cycles in response to environmental conditions. In this study, individuals of the C 3 -CAM intermediate plant Clusia minor were subjected to drought. The characteristics of chlorophyll fluorescence, gas exchange and organic acid content were investigated in individuals performing CAM under controlled laboratory conditions. The organic acid content increased after 16d of drought, however, the malate/citrate ratio showed a 2.6-fold decrease. After 13 d of drought, phase IV of CAM was completely suppressed. The highest levels of non-photochemical quenching (measured as q N and NPQ) were observed on day 16. However, increased capacity to dissipate the energy in excess to drive photosynthesis was not enough to maintain a low reduction state of the primary electron acceptor of photosystem II (measured as 1-q p ) at late afternoon under drought stress. Sustained decreases in predawn F V /F M ratio were observed even though organic acid accumulation increased after 16 d without irrigation. Despite non-photochemical quenching remaining high after rewatering, the decline in F V /F M ratio was relatively rapidly reversible. Considering the partitioning of q N into its fast (q F ) and slow (q S ) components, it was observed that the proportion of the two components was dependent on both the number of days without watering and the different CAM phases.

Photorespiration is Essential for the Protection of the Photosynthetic Apparatus of C3 Plants Against Photoinactivation Under Sunlight

Abstract:CO2 assimilation, transpiration and modulated chlorophyll fluorescence of leaves of Chenopodium bonus‐henricus (L.) were measured in the laboratory and, at a high altitude location, in the field. Direct calibration of chlorophyll fluorescence parameters against carbon assimilation in the presence of 1 or 0.5% oxygen (plus CO2) proved necessary to calculate electron transport under photorespiratory conditions in individual experiments. Even when stomata were open in the field, total electron transport was two to three times higher in sunlight than indicated by net carbon gain. It decreased when stomata were blocked by submerging leaves under water or by forcing them to close in air by cutting the petiole. Even under these conditions, electron transport behind closed stomata approached 10 nmol electrons m−2 leaf area s−1 at temperatures between 25 and 30 °C. No photoinactivation of photosystem II was indicated by fluorescence analysis after a day's exposure to full sunlight. Only when leaves were submerged in ice was appreciable photoinactivation noticeable after 4 h exposure to sunlight. Even then almost full recovery occurred overnight. Electron transport behind blocked stomata was much decreased when leaves were darkened for 70 min (in order to deactivate light‐regulated enzymes of the Calvin cycle) before exposure to full sunlight. Brief exposure of leaves to HCN (to inhibit photoassimilation and photorespiration) also decreased electron transport drastically compared to electron transport in unpoisoned leaves with blocked stomata. Non‐photochemical fluorescence quenching and reduction of QA, the primary electron acceptor of photosystem II was increased by HCN‐poisoning. Very similar observations were made when glyceraldehyde was used instead of HCN to inhibit photosynthesis and photorespiration. In HCN‐poisoned leaves, residual electron transport increased linearly with temperature and showed early light saturation revealing characteristics of the Mehler reaction. During short exposure of these leaves to photon flux densities equivalent to 25% of sunlight, no or only little photoinactivation of photosystem II was observed. However, prolonged exposure to sunlight caused inactivation even though non‐photochemical quenching of chlorophyll fluorescence was extensive. Simultaneously, oxidation of cellular ascorbate and glutathione increased. Inactivation of photosystem II was reversible in dim light and in the dark only after short times of exposure to sunlight. Glyceraldehyde was very similar to HCN in increasing the sensitivity of photosystem II in leaves to sunlight. We conclude from the observations that the electron transport permitted by the interplay of photoassimilatory and photorespiratory electron transport is essential to prevent the photoinactivation of photosynthetic electron transport. The Mehler and Asada reactions, which give rise to strong nonphotochemical fluorescence quenching, are insufficient to protect the chloroplast electron transport chain against photoinactivation.

The effect of light levels on daily patterns of chlorophyll fluorescence and organic acid accumulation in the tropical CAM treeClusia hilariana

Sandy plains are characteristic of the coastal region of Brazil. We investigated the diel patterns of changes in organic acid levels, leaf conductance and chlorophylla fluorescence for sun-exposed and shaded plants ofClusia hilariana, one of the dominant woody species in the sandy coastal plains of northern Rio de Janeiro state. Both exposed and shaded plants showed a typical CAM pattern with considerable diel oscillations in organic acid levels. The degradation of both malic and citric acids during the midday stomatal closure period could lead to potential CO2 fixation rates of 28 μmol m-2 s-1 in exposed leaves. Moreover, exposed leaves exhibited large increases in total non-photochemical quenching (qN) accompanied by a substantial decrease in effective quantum yield during the course of the day. However, these potential high rates of CO2 fixation and the increases inqn of exposed plants were not enough to maintain the primary electron acceptor of photosystem II (qA) in a low reduction state, similar to that of shaded plants. As a result, there was a moderate increase in the reduction state of qA throughout the day. Most of the decline in photochemical efficiency of exposed leaves ofC. hilariana was reversible, as evidenced by the high levels of pre-dawn potential quantum yields (Fv/Fm) and their rapid recovery after sunset. However, the depletion of the organic acid pool in the afternoon resulted in an accentuated subsequent drop in Fv/Fm, suggesting that prolonged periods of water stress accompanied by high irradiance levels may expose plants ofC. hilariana in unprotected habitats to the danger of photoinhibition.

Temperature Effects on Chlorophyll Fluorescence Induction in Tomato

Chlorophyll fluorescence induction of tomato leaf discs was measured at a low actinic light intensity of 10 mu mol.m(-2).s(-1) and at decreasing temperatures from 30 degrees to 0 degrees C. F-o remained constant within the temperature range assessed. In contrast, the peak of fluorescence induction, F-p, showed a strong temperature dependence. F-p increased by lowering the temperature within two temperature regions, from 30 degrees to 22 degrees C and from 14 degrees to 0 degrees C, respectively. F-p was hardly affected by temperatures between 14 degrees and 22 degrees C. Consequently, two temperature breakpoints of F-p were observed at 14 degrees and 22 degrees C. F-p reflected a maximum reduction state of the primary electron acceptor of photosystem II, Q(A). The relation between F-p and maximal Q(A) reduction was used to explain the observed temperature effects on F-p. The reduction state of Q(A) depended on both photosystem I oxidizing and photosystem II reducing activity. Consequently, breaks in the temperature response of F-p were attributed to dissimilar effects of temperature on photosystem I and photosystem II functioning. The steep increase of F-p below 14 degrees C was attributed to a low temperature induced impaired electron transport. This was sustained by the absence of a low temperature break when electron transport was inhibited by DCMU. A limitation of Q(A) oxidizing activity below 14 degrees C could be deduced from the temperature dependence of F-p of control discs. The second break in the temperature response of F-p at 22 degrees C was not caused by temperature effects on electron transport. The break of F-p at 22 degrees C was also found for F-m of electron transport inhibited leaf discs. While F-m decreased above 22 degrees C, the ratio of F-m determined at 687 and 730 nm (F(m)730/F(m)687) increased. This indicates a redistribution of excitation energy between the photosystems. Kinetic analysis of fluorescence induction traces showed a temperature effect on PSII heterogeneity. From 22 degrees to 30 degrees C, the PSII alpha fraction decreased from 70 to 50 % while the PSII beta fraction increased from 35 to 50 %. These results also suggest a temperature induced redistribution of excitation energy above 22 degrees C in favour of PSI. Implications of temperature effects on thylakoid membrane associated processes and plant functioning are discussed.

Observation of the reduction and reoxidation of the primary electron acceptor in photosystem I.

Femtosecond transient absorption spectroscopy has been used to investigate the primary charge separation in a photosystem II deletion mutant from the cyanobacterium Synechocystis sp. PCC 6803. These cells contain only the photosystem I reaction center and have a pigment content of approximately 100 chlorophylls per P700. Utilizing relatively high excitation intensities, the difference spectrum for the reduction of primary electron acceptor [(A0(-)-A0) difference spectrum] was obtained from experiments performed under both reducing and oxidizing conditions. Both approaches yield very similar results with the (A0(-)-A0) difference spectrum displaying a maximum bleaching at 687 nm. The shape of the difference spectrum suggests that the primary electron acceptor in photosystem I may be a chlorophyll a molecule. The observed rate of primary radical pair formation depends on the overall rate of decay of excitations in the antenna; the radical pair state forms as the antenna decays. The decay of the primary radical pair state is characterized by a 21-ps time constant. Under conditions that avoid annihilation effects, the mean lifetime for excitations in the antenna is 28 ps [Hastings, G., Kleinherenbrink, F.A.M., Lin, S., & Blankenship, R.E. (1994) Biochemistry (preceding paper in this issue)]. This indicates that the reduced acceptor decays faster than it forms. Therefore, only a low concentration of the reduced acceptor will accumulate under most conditions.

Iron-Induced Changes in Light Harvesting and Photochemical Energy Conversion Processes in Eukaryotic Marine Algae

The role of iron in regulating light harvesting and photochemical energy conversion processes was examined in the marine unicellular chlorophyte Dunaliella tertiolecta and the marine diatom Phaeodactylum tricornutum. In both species, iron limitation led to a reduction in cellular chlorophyll concentrations, but an increase in the in vivo, chlorophyll-specific, optical absorption cross-sections. Moreover, the absorption cross-section of photosystem II, a measure of the photon target area of the traps, was higher in iron-limited cells and decreased rapidly following iron addition. Iron-limited cells exhibited reduced variable/maximum fluorescence ratios and a reduced fluorescence per unit absorption at all wave-lengths between 400 and 575 nm. Following iron addition, variable/maximum fluorescence ratios increased rapidly, reaching 90% of the maximum within 18 to 25 h. Thus, although more light was absorbed per unit of chlorophyll, iron limitation reduced the transfer efficiency of excitation energy in photosystem II. The half-time for the oxidation of primary electron acceptor of photosystem II, calculated from the kinetics of decay of variable maximum fluorescence, increased 2-fold under iron limitation. Quantitative analysis of western blots revealed that cytochrome f and subunit IV (the plastoquinone-docking protein) of the cytochrome b(6)/f complex were also significantly reduced by lack of iron; recovery from iron limitation was completely inhibited by either cycloheximide or chloramphenicol. The recovery of maximum photosynthetic energy conversion efficiency occurs in three stages: (a) a rapid (3-5 h) increase in electron transfer rates on the acceptor side of photosystem II correlated with de novo synthesis of the cytochrome b(6)/f complex; (b) an increase (10-15 h) in the quantum efficiency correlated with an increase in D1 accumulation; and (c) a slow (>18 h) increase in chlorophyll levels accompanied by an increase in the efficiency of energy transfer from the light-harvesting chlorophyll proteins to the reaction centers.

Zeaxanthin Formation and Energy-Dependent Fluorescence Quenching in Pea Chloroplasts under Artificially Mediated Linear and Cyclic Electron Transport

Artificially mediated linear (methylviologen) and cyclic (phenazine methosulfate) electron transport induced zeaxanthin-dependent and independent (constitutive) nonphotochemical quenching in osmotically shocked chloroplasts of pea (Pisum sativum L. cv Oregon). Nonphotochemical quenching was quantitated as Stern-Volmer quenching (SV(N)) calculated as (F(m)/F'(m))-1 where F(m) is the fluorescence intensity with all PSII reaction centers closed in a nonenergized, dark-adapted state and F'(m) is the fluorescence intensity with all PSII reaction centers closed in an energized state. Reversal of quenching by nigericin and electron-transport inhibitors showed that both quenching types were energy-dependent SV(N). Under light-induced saturating DeltapH, constitutive-SV(N) reached steady-state in about 1 minute whereas zeaxanthin-SV(N) continued to develop for several minutes in parallel with the slow kinetics of violaxanthin deepoxidation. SV(N) above the constitutive level and relative zeaxanthin concentration showed high linear correlations at steady-state and during induction. Furthermore, F(o) quenching, also treated as Stern-Volmer quenching (SV(O)) and calculated as (F(o)/F'(o))-1, showed high correlation with zeaxanthin and consequently with SV(N) (F(o) and F'(o) are fluorescence intensities with all PSII reaction centers in nonenergized and energized states, respectively). These results support the view that zeaxanthin increases SV(N) above the constitutive level in a concentration-dependent manner and that zeaxanthin-dependent SV(N) occurs in the pigment bed. Preforming zeaxanthin increased the rate and extent of SV(N), indicating that slow events other than the amount of zeaxanthin also affect final zeaxanthin-SV(N) expression. The redox state of the primary electron acceptor of photosystem II did not appear to determine SV(N). Antimycin, when added while chloroplasts were in a dark-adapted or nonenergized state, inhibited both zeaxanthin-SV(N) and constitutive-SV(N) induced by linear and cyclic electron transport. These similarities, including possible constitutive F(o) quenching, suggest that zeaxanthin-dependent and constitutive SV(N) are mechanistically related.

Effects of temperature on the regulation of photosynthetic carbon assimilation in leaves of maize and barley

The aim of this work was to examine the effect of temperature in the range 5 to 30 ° C upon the regulation of photosynthetic carbon assimilation in leaves of the C4 plant maize (Zea mays L.) and the C3 plant barley (Hordeum vulgare L.). Measurements of the CO2-assimilation rate in relation to the temperature were made at high (735 μbar) and low (143 μbar) intercellular CO2 pressure in barley and in air in maize. The results show that, as the temperature was decreased, (i) in barley, pools of phosphorylated metabolites, particularly hexose-phosphate, ribulose 1,5-bisphosphate and fructose 1,6-bisphosphate, increased in high and low CO2; (ii) in maize, pools of glycerate 3-phosphate, triose-phosphate, pyruvate and phosphoenolpyruvate decreased, reflecting their role in, and dependence on, intercellular transport processes, while pools of hexose-phosphate, ribulose 1,5-bis phosphate and fructose 1,6-bisphosphate remained approximately constant; (iii) the redox state of the primary electron acceptor of photosystem II (QA) increased slightly in barley, but rose abruptly below 12° C in maize. Non-photochemical quenching of chlorophyll fluorescence increased slightly in barley and increased to high values below 20 ° C in maize. The data from barley are consistent with the development of a limitation by phosphate status at low temperatures in high CO2, and indicate an increasing regulatory importance for regeneration of ribulose 1,5-bisphosphate within the Calvin cycle at low temperatures in low CO2. The data from maize do not show that any steps of the C4 cycle are particularly cold-sensitive, but do indicate that a restriction in electron transport occurs at low temperature. In both plants the data indicate that regulation of product synthesis results in the maintenance of pools of Calvin-cycle intermediates at low temperatures.

Analysis of chlorophyll a fluoresence changes in weak light in heat treated Amaranthus chloroplasts

After preheating of Amaranthus chloroplasts at elevated temperatures (up to 45°C), the chlorophyll a fluorescence level under low excitation light rises as compared to control (unheated) as observed earlier in other chloroplasts (Schreiber U and Armond PA (1978) Biochim Biophys Acta 502: 138-151). This elevation of heat induced fluorescence yield is quenched by addition of 0.1 mM potassium ferricyanide, suggesting that with mild heat stress the primary electron acceptor of photosystem II is more easily reduced than the unheated samples. Furthermore, the level of fluorescence attained after illumination of dithionite-treated samples is independent of preheating (up to 45°C). Thus, these experiments indicate that the heat induced rise of fluorescence level at low light can not be due to changes in the elevation in the true constant F0 level, that must by definition, be independent of the concentration of QA. It is supposed that the increase in the fluorescence level by weak modulated light is either partly associated with dark reduction of QA due to exposure of chloroplasts to elevated temperature or due to temperature induced fluorescence rise in the so called inactive photosystem II centre where QA are not connected to plastoquinone pool. In the presence of dichlorophenyldimethylurea the fluorescence level triggered by weak modulated light increases at alkaline pH, both in control and heat stressed chloroplasts. This result suggests that the alkaline pH accelerates electron donation from secondary electron donor of photosystem II to QA both in control and heat stressed samples. Thus the increase in fluorescence level probed by weak modulated light due to preheating is not solely linked to increase in true F0 level, but largely associated with the shift in the redox state of QA, the primary stable electron acceptor of photosystem II.

A resonance Raman characterization of the primary electron acceptor in photosystem II

A resonance Raman characterization of the primary electron acceptor in photosystem II