Characteristics of the Photosystem II reaction centre. II. Electron donors

The properties of Photosystem II electron donation were investigated by EPR spectrometry at cryogenic temperatures. Using preparations from mutants which lacked Photosystem I, the main electron donor through the Photosystem II reaction centre to the quinone-iron acceptor was shown to be the component termed Signal II. A radical of 10 G line width observed as an electron donor at cryogenic temperatures under some conditions probably arises through modification of the normal pathway of electron donation. High-potential cytochrome b-559 was not observed on the main pathway of electron donation. Two types of PS II centres with identical EPR components but different electron-transport kinetics were identified, together with anomalies between preparations in the amount of Signal II compared to the quinone-iron acceptor. Results of experiments using cells from mutants of Scenedesmus obliquus confirm the involvement of the Signal II component, manganese and high-potential cytochrome b-559 in the physiological process leading to oxygen evolution.     

Reduction kinetics of the photo-oxidized chlorophyll a+II in the nanosecond range: Measurements of the absorption changes at 688 nm under repetitive flash excitation

The 688 nm absorption changes (ΔA₆₈₈), indicating the photochemical turnover of chlorophyll a_II (Chl a_II) have been investigated under repetitive laser flash excitation conditions in spinach chlorplasts. It was found that under steady state conditions about 50–60% of the photo-oxidized primary donor of Photosystem II (PS II), Chl a⁺_II, becomes re-reduced with a biphasic kinetics in the nanosecond time scale with half-life times of about 50 ns and 400 ns. The remaining Chl a⁺_II becomes re-reduced in the microsecond range.     

The effect of outer antenna complexes on the photochemical trapping rate in barley thylakoid Photosystem II

We have investigated the previous suggestions in the literature that the outer antenna of Photosystem II of barley does not influence the effective photosystem primary photochemical trapping rate. It is shown by steady state fluorescence measurements at the F₀ fluorescence level of wild type and the chlorina f2 mutant, using the chlorophyll b fluorescence as a marker, that the outer antenna is thermally equilibrated with the core pigments, at room temperature, under conditions of photochemical trapping. This is in contrast with the conclusions of the earlier studies in which it was suggested that energy was transferred rapidly and irreversibly from the outer antenna to the Photosystem II core. Furthermore, the effective trapping time, determined by single photon counting, time-resolved measurements, was shown to increase from 0.17±0.017 ns in the chlorina Photosystem II core to a value within the range 0.42±0.036-0.47±0.044 ns for the wild-type Photosystem II with the outer antenna system. This 2.5-2.8-fold increase in the effective trapping time is, however, significantly less than that expected for a thermalised system. The data can be explained in terms of the outer antenna increasing the primary charge separation rate by about 50%.     

Inhibition of Photosystem II by formate. Possible evidence for a direct role of bicarbonate in photosynthetic oxygen evolution

In broken chloroplasts the presence of 100 mM sodium formate at pH 8.2 will specifically lengthen the Photosystem II relaxation times of the reactions S′₂ → S₃ and S′₃ → S₀. Rates of reactions S′₀ → S₁ and S′₁ → S₂ remain unaffected. Evidence is presented which indicates the discrimination among S-states by formate cannot be attributed to a block imposed on the reducing side of Photosystem II. The results are interpreted in context of the known interaction of formate and CO₂ which is bound to the Photosystem II reaction center complex. It is proposed that those S-state transitions which show extended relaxation times in the presence of formate must result in the momentary release and rebinding of CO₂. Furthermore since formate is acting on the oxygen-evolving side of Photosystem II, it would seem that CO₂ is released in reactions that occur there. A chemical model of oxygen evolution is presented. It is based on the hypothesis that hydrated CO₂ is the immediate source of photosynthetically evolved oxygen and explains why, under certain conditions formate slows only the S-state transitions S′₂ → S₃ and S′₃ → S₀.     

Evidence for the existence of trimeric and monomeric Photosystem I complexes in thylakoid membranes from cyanobacteria

In cyanobacteria, solubilization of thylakoid membranes by detergents yields both monomeric and trimeric Photosystem I (PS I) complexes in variable amounts. We present evidence for the existence of both monomeric and trimeric PS I in cyanobacterial thylakoid membranes with the oligomeric state depending in vitro on the ion concentration. At low salt concentrations (i.e.10 mM MgSO₄) PS I is mainly extracted as a trimer from these membranes and at high salt concentrations (i.e.150 mM MgSO₄) nearly exclusively as a monomer, irrespective of the type of salt used (i.e. mono- or bivalent ions) and the temperature (i.e. 4°C or 20°C). Once solubilized, the PS I trimer is stable over a wide range of ion concentrations (i.e. beyond 0.5 M). A model is presented which suggests a monomer-oligomer equilibrium of PS I, but also of PS II and the cyt. b6/f-complex in the cyanobacterial thylakoid membrane. The possible physiological role of this equilibrium in the regulation of state transitions is discussed.Abbreviations -DM dodecyl--D-maltoside - Chl chlorophyll - cyt. b6f cytochrome b6f complex - EM electron microscopy - HPLC high performance liquid chromatography - LDAO N, N-dimethyl-N-dodecyl amine oxide - MES 4-morpholino ethane sulfonic acid - PAGE polyacrylamide gel electrophoresis - PBS phycobilisome - PS photosystem - SDS sodium dodecyl sulfate - 2D two dimensional - 3D three dimensional     

Synthetic quinones influencing herbicide binding and Photosystem II electron transport. The effects of triazine-resistance on quinone binding properties in thylakoid membranes

We have analyzed the binding of synthetic quinones and herbicides which inhibit electron transport at the acceptor side of Photosystem II (PS II) of the photosynthetic electron-transport chain in thylakoid membranes. These data show that quinones and PS II-directed herbicides compete for binding to a common binding environment within a PS II region which functions as the Q⁻ / PQ oxidoreductase. We observed that (1) synthetic quinones cause a parallel inhibition of electron transport and [¹⁴C]herbicide displacement, and (2) herbicide binding is affected both by the fully oxidized and fully reduced form of a quinone. Quinone function and inhibitor binding were also investigated in thylakoids isolated from triazine-resistant weed biotypes. We conclude the following. (1) The affinity of the secondary accepting quinone, B, is decreased in resistant thylakoids. (2) The observation that the equilibrium concentration of reduced Q after transferring one electron to the acceptor side of PS II is increased in resistant as compared to susceptible chloroplasts may be explained both by a decrease in the affinity of PQ for the herbicide / quinone binding environment, and by a decrease of the midpont redox potential of the B / B⁻ couple. (3) The binding environment regulating quinone and herbicide affinity may be divided roughly into two domains; we suggest that the domain regulating quinone head-group binding is little changed in resistant membranes, whereas the domain-regulating quinone side-group binding (and atrazine) is altered. This results in increased inhibitory activity of tetrachloro-p-benzoquinone and phenolic herbicides, which are hypothesized to utilize the quinone head-group domain. The two domains appear to be spatially overlapping because efficient atrazine displacement by tetrachloro-p-benzoquinone is observed.     

Photosystem II solubilizes as a monomer by mild detergent treatment of unstacked thylakoid membranes*

We studied the aggregation state of Photosystem II in stacked and unstacked thylakoid membranes from spinach after a quick and mild solubilization with the non-ionic detergent n-dodecyl-α,D-maltoside, followed by analysis by diode-array-assisted gel filtration chromatography and electron microscopy. The results suggest that Photosystem II (PS II) isolates either as a paired, appressed membrane fragment or as a dimeric PS II-LHC II supercomplex upon mild solubilization of stacked thylakoid membranes or PS II grana membranes, but predominantly as a core monomer upon mild solubilization of unstacked thylakoid membranes. Analysis of paired grana membrane fragments reveals that the number of PS II dimers is strongly reduced in single membranes at the margins of the grana membrane fragments. We suggest that unstacking of thylakoid membranes results in a spontaneous disintegration of the PS II-LHC II supercomplexes into separated PS II core monomers and peripheral light-harvesting complexes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of temperature on Photosystem II electron transfer reactions

The influence of temperature on the rate of reduction of P-680⁺, the primary donor of Photosystem II, has been studied in the range 5–294 K, in chloroplasts and subchloroplasts particles. P-680 was oxidized by a short laser flash. Its oxidation state was followed by the absorption level at 820 nm, and its reduction attributed to two mechanisms: electron donation from electron donor D₁ and electron return from the primary plastoquinone (back-reaction).Between 294 and approx. 200 K, the rate of the back-reaction, on a logarithmic scale, is a linear function of the reciprocal of the absolute temperature, corresponding to an activation energy between 3.3 and 3.7 kcal · mol^?1, in all of the materials examined (chloroplasts treated at low pH or with Tris; particles prepared with digitonin). Between approx. 200 K and 5 K the rate of the back-reaction is temperature independent, with $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 1.6}\text{ms}$. In untreated chloroplasts we measured a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of 1.7 ms for the back-reaction at 77 and 5 K.The rate of electron donation from the donor D₁ has been measured in darkadapted Tris-treated chloroplasts, in the range 294–260 K. This rate is strongly affected by temperature. An activation energy of 11 kcal · mol^?1 was determined for this reaction.In subchloroplast particles prepared with Triton X-100 the signals due to P-680 were contaminated by absorption changes due to the triplet state of chlorophyll a. This triplet state has been examined with pure chlorophyll a in Triton X-100. An Arrhenius plot of its rate of decay shows a temperature-dependent region (292–220 K) with an activation energy of 9 kcal · mol^?1, and a temperature-independent region (below 200 K) with $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 1.1}\text{ms}$.     

Mode of action of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Evidence that the inhibitor competes with plastoquinone for binding to a common site on the acceptor side of Photosystem II

The kinetics and concentration dependence of the binding of dichlorophenyldimethylurea (DCMU) to Photosystem II (PS II) were monitored through fluorescence measurements. According to whether the acceptor system is in the ‘odd’ state (QB⁻ ag Q⁻B) or ‘even’ state (QB), very different results are obtained. The binding to centers in the even state is rapid ( at [DCMU] = 10⁻⁵ M and [chlorophyll] = 10 μg/ml), with a pH-independent rate. The concentration curve of the bound inhibitor (at equilibrium) corresponds to an association constant of about 3.3·10⁷ M⁻¹·1. The binding of the inhibitor to centers in the odd state is slow ( at pH 7, same DCMU and chlorophyll concentrations as above), and depends on pH. In the pH range 6–8, the lower the pH, the slower the kinetics. The association constant is also diminished by a factor of approx. 20 (at pH 7) compared to the even state centers. It is shown that these effects are in good agreement with predictions from Velthuys' hypothesis (Velthuys, B.R. (1981) FEBS Lett. 126, 277–281) that the mode of action of DCMU is a competition with plastoquinone for the binding to the secondary acceptor site. A large part of PS II photochemical quenching corresponds to acceptors which seem to possess a secondary acceptor distinct from B. They were called ‘non-B-type acceptors’ (Lavergne, J. (1982) Photobiochem. Photobiophys. 3, 257–285) and may be identified with Joliot's ‘Q₂’ (Joliot P. and Joliot, A. (1977) Biochim. Biophys. Acta 462, 559–574). However, the rate at which the inhibition affects these non-B-type acceptors is similar to the rate of DCMU binding on the B site (i.e., slow in the odd state, fast in the even state).     

Investigation of the plastoquinone pool size and fluorescence quenching in thylakoid membranes and Photosystem II (PS II) membrane fragments

The efficiency of oxidized endogenous plastoquinone-9 (PQ-9) as a non-photochemical quencher of chlorophyll fluorescence has been analyzed in spinach thylakoids and PS II membrane fragments isolated by Triton X-100 fractionation of grana stacks. The following results were obtained: (a) After subjection of PS II membrane fragments to ultrasonic treatment in the presence of PQ-9, the area over the induction curve of chlorophyll fluorescence owing to actinic cw light increases linearly with the PQ-9/PS II ratio in the reconstitution assay medium; (b) the difference of the maximum fluorescence levels, F_max, of the induction curves, measured in the absence and presence of DCMU, is much more pronounced in PS II membrane fragments than in thylakoids; (c) the ratio F_max(-DCMU)/F_max(+DCMU) increases linearly with the content of oxidized PQ-9 that is varied in the thylakoids by reoxidation of the pool after preillumination and in PS II membrane fragments by the PQ-9/PS II ratio in the reconstitution assay; (d) the reconstitution procedure leads to tight binding of PQ-9 to PS II membrane fragments, and PQ-9 cannot be replaced by other quinones; (e) the fluorescence quenching by oxidized PQ-9 persists at low temperatures, and (f) oxidized PQ-9 preferentially affects the F695 of the fluorescence emission spectrum at 77 K. Based on the results of this study the oxidized PQ-9 is inferred to act as a non-photochemical quencher via a static mechanism. Possible implications for the nature of the quenching complex are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Thylakoid membrane protein phosphorylation leads to a decrease in connectivity between Photosystem II reaction centers

The room-temperature fluorescence induction transients from stroma-free chloroplast membranes (in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea) have been analyzed to determine the effects of membrane protein phosphorylation on the connectivity between Photosystem (PS) II centers. Chloroplast membranes which have been incubated in the light with ATP exhibit: (1) a decrease in the variable fluorescence as a function of the initial fluorescence, (2) a shift from a sigmoidal to an exponential fluorescence induction curve, and (3) a reduced amount of the fast () component of the induction transient. These phenomenona are completely reversible by dark incubation of the samples (leading to protein dephosphorylation). We conclude that connectivity between PS II centers is reduced as a function of thylakoid membrane protein phosphorylation. This may in turn be the mechanism which increases the amount of absorbed excitation energy available to PS I.     

Evidence that the low-potential (−700 mV) electron acceptor (X) in Photosystem I has two iron-sulphur centres

The Photosystem I reaction centre contains two groups of iron-sulphur centres: Fe-S_A and Fe-S_B with redox potentials between ?510 and ?590 mV, and Fe-S_X with redox potential about ?700 mV. Spin quantitation (Heathcote, P., Williams-Smith, D.L. and Evans, M.C.W. (1978) Biochem. J. 170, 373–378) and Mössbauer spectroscopy (Evans, E.H., Dickson, D.P.E., Johnson, C.E., Rush, J.D. and Evans, M.C.W. (1981) Eur. J. Biochem. 118, 81–84) did not show unequivocally whether Fe-S_X has one or two centres. Experiments are described which support the proposal that Fe-S_X has two centres. Fe-S_X can be photoreduced irreversibly by 210 K illumination of dithionite-reduced samples or reversibly by 7.5 K illumination of these samples. The amplitude of the Fe-S_X signal reversibly induced by illumination at 7.5 K is never more than 50% of the amplitude of the signal when Fe-S_X is prereduced by room temperature illumination or by 210 K illumination. Approx. half of the Fe-S_X is rapidly reduced by 210 K illumination, the remainder more slowly. The extent of reversible Fe-S_X reduction and P-700 photooxidation is little affected by the fast reduction of about half of the Fe-S_X. Subsequent reduction of the remaining Fe-S_X is paralleled by loss of the reversible photoreaction.     

Primary charge separation in Photosystem II

In this Minireview, we discuss a number of issues on the primary photosynthetic reactions of the green plant Photosystem II. We discuss the origin of the 683 and 679 nm absorption bands of the PS II RC complex and suggest that these forms may reflect the single-site spectrum with dominant contributions from the zero-phonon line and a pronounced ∼80 cm⁻¹ phonon side band, respectively. The couplings between the six central RC chlorins are probably very similar and, therefore, a `multimer' model arises in which there is no `special pair' and in which for each realization of the disorder the excitation may be dynamically localized on basically any combination of neighbouring chlorins. The key features of our model for the primary reactions in PS II include ultrafast (<500 fs) energy transfer processes within the multimer, `slow' (∼20 ps) energy transfer processes from peripheral RC chlorophylls to the RC multimer, ultrafast charge separation (<500 fs) with a low yield starting from the singlet-excited `accessory' chlorophyll of the active branch, cation transfer from this `accessory' chlorophyll to a `special pair' chlorophyll and/or charge separation starting from this `special pair' chlorophyll (∼8 ps), and slow relaxation (∼50 ps) of the radical pair by conformational changes of the protein. The charge separation in the PS II RC can probably not be described as a simple trap-limited or diffusion-limited process, while for the PS II core and larger complexes the transfer of the excitation energy to the PS II RC may be rate limiting. This revised version was published online in June 2006 with corrections to the Cover Date.     

Polypeptide composition of the purified Photosystem II pigment-protein complex from spinach

The Photosystem II pigment-protein complex, the chlorophyll α-protein comprising the reaction center of Photosystem II, was prepared from EDTA-treated spinach chloroplasts by digitonin extraction, sucrose-gradient centrifugation, DEAE-cellulose column chromatography, and isoelectrofocussing on Ampholine.The dissociated pigment-protein complex exhibits two polypeptide subunits that migrate in SDS-polyacrylamide gel with electrophoretic mobilities corresponding to molecular weights of approximately 43 000 and 27 000. The chlorophyll was always found in the free pigment zone at the completion of the electrophoresis. Heat-treatment of the sample (100°C, 90 s) for electrophoresis caused association of the two polypeptides into large aggregates. It is concluded that these two polypeptides, 43 000 and 27 000, are valid structural or functional components of Photosystem II pigment-protein complex.     

Characteristics of the Photosystem II reaction centre. I. Electron acceptors

The EPR characteristics of Photosystem II electron acceptors are described, in membrane and detergent-treated preparations from a mutant of Chlamydomonas reinhardii lacking Photosystem I and photosynthetic ATPase. The relationship between the quinone-iron and pheophytin acceptors is discussed and a heterogeneity of reaction centres is demonstrated such that only a minority of reaction centres were capable of secondary electron donation at temperatures below 100 K. Only these centres were therefore able to stabilise a reduced acceptor below 100 K. Parallel experiments using a barley mutant (viridis zb63) which also lacks Photosystem I, provide similar results indicating that the C. reinhardii system can provide a general model for the Photosystem II electron acceptor complex. The similarity of the system to that of the purple photosynthetic bacteria is discussed.     

Phylogenetic Analyses of the Core Antenna Domain: Investigatingthe Origin of Photosystem I

Phototrophy, the conversion of light to biochemical energy, occurs throughout the Bacteria and plants, however, debate continues over how different phototrophic mechanisms and the bacteria that contain them are related. There are two types of phototrophic mechanisms in the Bacteria: reaction center type 1 (RC1) has core and core antenna domains that are parts of a single polypeptide, whereas reaction center type 2 (RC2) is composed of short core proteins without antenna domains. In cyanobacteria, RC2 is associated with separate core antenna proteins that are homologous to the core antenna domains of RC1. We reconstructed evolutionary relationships among phototrophic mechanisms based on a phylogeny of core antenna domains/proteins. Core antenna domains of 46 polypeptides were aligned, including the RC1 core proteins of heliobacteria, green sulfur bacteria, and photosystem I (PSI) of cyanobacteria and plastids, plus core antenna proteins of photosystem II (PSII) from cyanobacteria and plastids. Maximum likelihood, parsimony, and neighbor joining methods all supported a single phylogeny in which PSII core antenna proteins (PsbC, PsbB) arose within the cyanobacteria from duplications of the RC1-associated core antenna domains and accessory antenna proteins (IsiA, PcbA, PcbC) arose from duplications of PsbB. The data indicate an evolutionary history of RC1 in which an initially homodimeric reaction center was vertically transmitted to green sulfur bacteria, heliobacteria, and an ancestor of cyanobacteria. A heterodimeric RC1 (=PSI) then arose within the cyanobacterial lineage. In this scenario, the current diversity of core antenna domains/proteins is explained without a need to invoke horizontal transfer.This article contains online-only supplementary material.Reviewing Editor: Dr. W. Ford Doolittle     

Energy transfers from Photosystem II to Photosystem I in Cryptomonas rufescens (Cryptophyceae)

In Cryptomonas rufescens (Cryptophyceae), phycoerythrin located in the thylakoid lumen is the major accessory pigment. Oxygen action spectra prove phycoerythrin to be efficient in trapping light energy.The fluorescence excitation spectra at ?196°C obtained by the method of Butler and Kitajima (Butler, W.L. and Kitajima, M. (1975) Biochim. Biophys. Acta 396, 72–85) indicate that like in Rhodophycease, chlorophyll a is the exclusive light-harvesting pigment for Photosystem I.For Photosystem II we can observe two types of antennae: (1) a light-harvesting chlorophyll complex connected to Photosystem II reaction centers, which transfers excitation energy to Photosystem I reaction centers when all the Photosystem II traps are closed. (2) A light-harvesting phycoerythrin complex, which transfers excitation energy exclusively to the Photosystem II reaction complexes responsible for fluorescence at 690 nm.We conclude that in Cryptophyceae, phycoerythrin is an efficient light-harvesting pigment, organized as an antenna connected to Photosystem II centers, antenna situated in the lumen of the thylakoid. However, we cannot afford to exclude that a few parts of phycobilin pigments could be connected to inactive chlorophylls fluorescing at 690 nm.     

Elucidating the site of action of oxalate in photosynthetic electron transport chain in spinach thylakoid membranes

The effects of oxalate on PS II and PS I photochemistry were studied. The results suggested that in chloride-deficient thylakoid membranes, oxalate inhibited activity of PS II as well as PS I. To our knowledge, this is the only anion so far known which inhibits both the photosystems. Measurements of fluorescence induction kinetics, YZ* decay, and S2 state multiline EPR signal suggested that oxalate inhibited PS II at the donor side most likely on the oxygen evolving complex. Measurements of re-reduction of P700+ signal in isolated PS I particles in oxalate-treated samples suggested a binding site of oxalate on the donor, as well as the acceptor side of PS I.     

The deactivation of Photosystem II in Chlorella as a second-order process

The kinetics of deactivation of the S₃ state in Chlorella have been observed under a variety of conditions. The S₃ state appears to decline in a dark period coming after a sequence of 30 saturating flashes in a second-order reaction, the rate constant of which is 0.132/[S*₃] s⁻¹ and which involves an electron donor, D₁, of concentration 1.25[S*₃] where [S*₃] is the concentration of the S₃ state when the oxygen yield of the light flashes is constant. If a 1 min period of 650 nm illumination is employed after the sequence of flashes, the subsequent S₃ state deactivation kinetics are more complex. There is an initial phase of S₃ state deactivation, accounting for about 35% of the original S₃ state, which is complete in less than 100 ms. The remaining 65% of the S₃ state appears to deactivate in a second-order reaction, the rate constant of which is 1.36/[S*₃] s⁻¹ and which involves an electron donor of initial concentration 0.58[S*₃]. If a 1 min period of 710 nm illumination comes after the 30 flashes, at least 98% of the S₃ state deactivates according to first-order kinetics. It is shown that this can be explained using a second-order model if there is an electron donor present of which the concentration is large compared with [S*₃]. However, S₃ state deactivation observed after 5 min of dark and two saturating flashes can be described neither by a first-order model nor a second-order model. Deactivation of the S₂ state after a 5 min dark period and one saturating flash follows second-order kinetics with a rate constant of 0.2/[S*₃] s⁻¹ and appears to involve an electron donor of initial concentration 1.3[S*₃]. Arguments are presented which tend to rule out the primary electron acceptor to Photosystem II as being any of the electron donors but it appears quite possible that the large plastoquinone pool is involved.     

Stimulation by manganese and other divalent cations of the electron donation reactions of Photosystem II

Using inside-out thylakoid membranes, it has been shown that the oxidation of water and associated reduction of dichlorophenol indophenol is partially inhibited by low concentrations of cation chelators. This inhibition correlates with a removal of two manganese ions per Photosystem II reaction centre. The chelator-induced inhibition was completely reversed by the addition of low levels of Mn²⁺ ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 20 μ}\text{M}$) and higher levels of Mg²⁺ and Ca²⁺ ($\text{C}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈ 1}\text{mM}$). Other cations were not effective, indicating that the ability to overcome the inhibition did not involve a general electrostatic screening process. The degree of inhibition by chelators was greater at lower light intensities and after treatment with glutaraldehyde. In the presence of glutaraldehyde the stimulatory effect of Mn²⁺ was lost, while pretreatment with Mn²⁺ prevented the glutaraldehyde effect. These results are discussed in terms of conformational changes of the electron donation chains involving cation- (preferentially Mn-) dependent coupling between the oxygen evolving and reaction-centre complexes of Photosystem II.