The history of photosynthetic thermoluminescence

A fundamental discovery of photosynthetis research in the 1950s was the detection of thermally stimulated light emission from preilluminated photosynthetic material [Arnold W and Sherwood H (1957) Proc Natl Acad Sci USA 43: 105–114]. This phenomenon, called thermoluminescence (TL), is characteristic of a wide range of materials (minerals, semiconductors, inorganic and organic crystals, and complex biological systems), which share the ability of storing radiant energy in thermally stabilized trap states. The original discovery of TL in dried chloroplasts later proved to be a phenomenon common to all photosynthetic organisms: photosynthetic bacteria, cyanobacteria, algae and higher plants, which can be observed in isolated membrane particles, intact chloroplasts and unicellular organisms, and whole leaves. Following the initial observations considerable effort has been devoted to the identification and characterization of photosynthetic TL components. This work has firmly established the participation of various oxidation states of the water-oxidizing complex, the redox-active tyrosines, and the quinone electron acceptors of Photosystem II (PS II) in the generation of photosynthetic glow curves. Since TL characteristics are very sensitive to subtle changes in the redox properties of the involved electron transport components, the TL method has become a powerful tool in probing a wide range of PS II redox reactions and their modifications by environmental stress effects. Here, the main milestones of research in photosynthetic TL are covered until the present day. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermoluminescence study of the in vivo effects of bicarbonate depletion and acetate/formate presence in the two algae Chlamydobotrys stellata and Chlamydomonas reinhardtii

We investigated the influence of CO₂/HCO₃ ^–-depletion and of the presence of acetate and formate on the in vivo photosynthetic electron transport in the two green algae Chlamydobotrys stellata and Chlamydomonas reinhardtii by means of thermoluminescence technique and mathematical glow curve analysis. The main effects of the removal of CO₂ from the algal cultures was: (1) A shift of the glow curve peak position to lower temperatures resulting from a decrease of the B band and an increase of the Q band. (2) Treatment of CO₂-deficient Chl. stellata with DCMU yielded two thermoluminescence bands in the Q band region peaking at around +12°C and +5°C; in case of Chl. reinhardtii DCMU treatment induced only one band with an emission maximum at +5°C. The presence of acetate or formate in CO₂-depleted algal cultures lowered the intensities of all of the individual TL bands but that of a HT band (TL+37). The effects of CO₂-depletion and of the presence of anions were fully reversible.Abbreviations DCMU 3-(3,4)-dichlorophenyl-1,1-dimethylurea - HT band high temperature TL band - P₆₈₀ reaction center chlorophyll of PS II - Q_A and Q_B primary and secondary quinone acceptors of PS II, respectively - PS II Photosystem II - S_2/3 redox states of the oxygen evolving complex of PS II - TL thermoluminescence     

Thermoluminescence properties of the isolated photosystem two reaction centre

Formation of thermoluminescence signals is characteristics of energy- and charge storage in Photosystem II. In isolated D1/D2/cytochrome b-559 Photosystem II reaction centre preparation four thermoluminescence components were found. These appear at -180 (Z band), between -80 and -50 (Z_v band), at -30 and at +35°C. The Z band arises from pigment molecules but not correlated with photosynthetic activity. The Z_v and -30°C bands arise from the recombination of charge pairs stabilized in the Photosystem II reaction centre complex. The +35°C band probably corresponds to the artefact glow peak resulting from a pigment-protein-detergent interaction in subchloroplast preparations (Rózsa Zs, Droppa M and Horváth G (1989) Biochim Biophys Acta 973, 350–353).Abbreviations Chl chlorophyll - Cyt cytochrome - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - D1 psbA gene product - D2 psbD gene product - P680 primary electron donor of PS II - Pheo pheophytin - PS II Photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II - RC reaction centre of PS II - TL thermoluminescence     

Thermoluminescence studies on the function of Photosystem II in the desiccation tolerant lichen Cladonia convoluta

The effect of desiccation and rehydration on the function of Photosystem II has been studied in the desiccation tolerant lichen Cladonia convoluta by thermoluminescence. We have shown that in functional fully hydrated thalli thermoluminescence signals can be observed from the recombination of the S₂₍₃₎Q_B ^– (B band), S₂Q_A ^– (Q band), Tyr-D⁺Q_A ^– (C band) and Tyr-Z⁺(His⁺)Q_A ^– (A band) charge stabilization states. These thermoluminescence signals are completely absent in desiccated thalli, but rapidly reappear on rehydration. Flash-induced oscillation in the amplitude of the thermoluminescence band from the S₂₍₃₎Q_B ^– recombination shows the usual pattern with maxima after 2 and 6 flashes when rehydration takes place in light. However, after rehydration in complete darkness, there is no thermoluminescence emission after the 1 st flash, and the maxima of the subsequent oscillation are shifted to the 3rd and 7th flashes. It is concluded that desiccation of Cladonia convoluta converts PS II into a nonfunctional state. This state is characterized by the lack of stable charge separation and recombination, as well as by a one-electron reduction of the water-oxidizing complex. Restoration of PS II function during rehydration can proceed both in the light and in darkness. After rehydration in the dark, the first charge separation act is utilized in restoring the usual oxidation state of the water-oxidizing comples.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DT desiccation tolerant - PS II Photosystem II - TL thermoluminescence - P₆₈₀ reaction center Chl of PS II - Q_A and Q_B puinone electron acceptors of PS II - S₀,...,S₄ the redox states of the water-oxidizing complex - Tyr-Z and Tyr-D redox-active tyrosine electron donors of PS II     

Water cleavage by solar radiation—an inspiring challenge of photosynthesis research

Solar energy exploitation by photosynthetic water cleavage is of central relevance for the development and sustenance of all higher forms of living matter in the biosphere. The key steps of this process take place within an integral protein complex referred to as Photosystem II (PS II) which is anisotropically incorporated into the thylakoid membrane. This minireview concentrates on mechanistic questions related to i) the generation of strongly oxidizing equivalents (holes) at a special chlorophyll a complex (designated as P680) and ii) the cooperative reaction of four holes with two water molecules at a manganese containing unit WOC (water oxidizing complex) resulting in the release of molecular oxygen and four protons. The classical work of Pierre Joliot and Bessel Kok and their coworkers revealed that water oxidation occurs via a sequence of univalent oxidation steps including intermediary redox states S_i (i = number of accumulated holes within the WOC). Based on our current stage of knowledge, an attempt is made a) to identify the nature of the redox states S_i, b) to describe the structural arrangement of the (four) manganese centers and their presumed coordination and ligation within the protein matrix, and c) to propose a mechanism of photosynthetic water oxidation with special emphasis on the key step, i.e. oxygen-oxygen bond formation. It is assumed that there exists a dynamic equilibrium in S₃ with one state attaining the nuclear geometry and electronic configuration of a complexed peroxide. This state is postulated to undergo direct oxidation to complexed dioxygen by univalent electron abstraction with Y_Z ^ox and simultaneous internal ligand to metal charge transfer.Key questions on the mechanism will be raised. The still fragmentary answers to these questions not only reflect our limited knowledge but also illustrate the challenges for future research.Abbreviations b559 cytochrome b559 - BChl bacteriochlorophyll - Chl chlorophyll - CP47 Chl a containing a 47 kDa polypeptide - D1/D2 polypeptides of the PS II reaction center - ENDOR electron nuclear double resonance - EPR electron paramagnetic resonance - ESEEM electron spin echo envelope modulation - EXAFS extended X-ray absorption fine structure - FTIR Fourier transform infrared - NMR nuclear magnetic resonance - P680, P700 photoactive Chl a of PS II and PS I, respectively - PS II Photosystem II - Q_A special plastoquinone of PS II - S_i redox states of WOC - WOC water oxidizing complex - WOS water oxidizing site - UV/VIS ultraviolet/visible - Y_D, Y_Z redox active tyrosines of polypeptides D2 and D1, respectively     

Studies on thermoluminescence,delayed light emission and oxygen evolution from photosynthetic materials: UV effects

The effect of ultraviolet light on thermoluminescence, oxygen evolution and the slow component of delayed light has been investigated in chloroplasts and Pothos leaves. All peaks including peak V (48°C) were inhibited by UV. However, the peak at 48°C which was induced by DCMU was enhanced following UV irradiation of chloroplasts at ambient temperature (23°C) whereas peak II (-12°C) and peak III (10°C) which were also induced by DCMU were inhibited. Chloroplasts treated with DCMU and dark incubated for several minutes at ambient temperature prior to recording of glow curves have also shown enhancement of peak at 48°C. A slow component of delayed light and photosystem II activity of chloroplasts were inhibited by UV whereas photosystem I activity was marginally affected. These results corroborate involvement of photosystem II in generating thermoluminescence and slow components of delayed light in photosynthetic materials.Abbreviations DCIP Dichlorophenol Indophenol - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCQ 2,6 Dichloro-p-benzoquinone - DLE delayed light emission - MOPS Morpholino propane sulfonic acid - PSI Photosystem I - PS II Photosystem II - TL thermoluminescence     

Thermoluminescence investigations on the site of action of o-phthalaldehyde in photosynthetic electron transport

Glow curves from spinach leaf discs infiltrated with o-phthalaldehyde (OPA) show significant similarity to those obtained by DCMU treatment which is known to block the electron flow from Q_A, the stable acceptor of Photosystem II (PS II). In both the cases, the thermoluminescence (TL) peak II (Q band) was intensified significantly, whereas peaks III and IV (B band) were suppressed. Total TL yield of the glow curve remained constant even when the leaf discs were infiltrated with high concentrations of OPA (4 mM) or with DCMU (100 M), indicating that even at these high concentrations no significant change in the number of species undergoing charge recombination in PS II occurred. However, studies with thylakoids revealed significant differences in the action of OPA and DCMU on PS II. Although OPA, at a certain concentration and time of incubation, reduced the B band intensity by about 50–70%, and completely abolished the detectable oxygen evolution, it still retained the TL flash yield pattern, and, thus, S state turnover. OPA is known to inhibit the oxidoreductase activity of in vitro Cyt b₆/f (Bhagwat et al. (1993) Arch Biochem Biophys 304: 38–44). However, in the OPA treated thylakoids the extent of inhibition of O₂ evolution was not reduced even in the presence of oxidized tetramethyl-p-phenylenediamine which accepts electrons from plastoquinol and feeds then directly to Photosystem I. This suggests that OPA inhibition is at a site prior to plastoquinone pool in the electron transport chain, in agreement with it being between Q_A and Q_B. However, an unusual feature of OPA inhibition is that even though all oxygen evolution was completely suppressed, a significant fraction of PS II centers were functional and turned over with the same periodicity of four in the absence of any added electron donor, an observation which appears to be similar to that reported by Wydrzynski (Wydrzynski et al. (1985) Biochim Biophys Acta 809: 125–136) with lauroylcholine chloride, a lipid analogue compound. The detailed chemistry of OPA inhibition remains to be studied. Since we dedicate this paper to William A. Arnold, discoverer of delayed light and TL in photosynthesis, we have also included in the Introduction, a brief history of how TL work was initiated at BARC (Bombay, India).Abbreviations Chl chlorophyll - Cyt b₆/f Cytochrome b₆/f - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCIP 2,6-dichloropenolindophenol - DCMU 3-(3,4-dichlorophenyl-) 1,1-dimethyl urea - HEPES (N-[2-hydroxyethyl]piperazine-N-[2-ethanesulfonic acid]) - LCC lauroylcholine chloride - OPA o-phthalaldehyde - PS I Photosystem I - PS II Photosystem II - TL thermoluminescence - TMPD 2,3,5,6-tetramethyl-p-phenylenediamine     

Effect of complete extraction and re-addition of manganese on thermoluminescence of pea Photosystem II preparations

Thermoluminescence of Photosystem II particles isolated from pea chloroplasts using digitonin and Triton X-100 was measured after 1 min illumination at a certain temperature (T(ex)) followed by illumination during cooling (40 Cdeg/min) to a lower temperature. Glow curves of the particles are characteristic of the photosynthetic oxygen-evolving material studied earlier. Complete (more than 95%) removal of Mn from the Photosystem II particles abolishes thermoluminescence bands around 0° C, related to the oxygen-evolving system, but the thermoluminescence bands peaking around -30°C (TL(-30)), -55°C (TL_ (-55)) and between-68 and -85° C, depending on Tex(TLv), remain unaltered. The bands are characterized by different dependence on T,x. The TL(-30), TL(-55) and TL v bands can also be observed in the glow curve of isolated pea and spinach chloroplasts. Re-addition of MnCI (2) (2 μM, corresponding to nearly 4 Mn atoms per reaction center of Photosystem II) to the Mn-depleted particles does not reactivate the thermoluminescence bands around 0° C. However, it does lead to suppression of TL(-30) accompanied by parallel activation of TL(-55), revealing competition of the TL (-30) and TL(-55) for charges generated by the reaction center. These data, as well as the results on the effect of inhibitors and electron donors to Photosystem II, show that positive charges contributing to the TL(-30), TL (-55) and TL v thermoluminescence bands are located on secondary electron donors of Photosystem II which do not require Mn and are located closer to the reaction center than the Mn-containing, water-oxidizing enzyme.     

Mode of action of Photosystem II herbicides studied by thermoluminescence

The mode of action of chemically different herbicides (ureas, pyridazinones, phenylcarbamates, triazines, hydroxyquinolines, hydroxybenzonitriles and dinitrophenols) on photosynthetic electron transport was investigated by measurements of oxygen evolution and thermoluminescence. Depending on the particular herbicide used the thermoluminescence band related to Q (the primary acceptor of Photosystem II) appears at +5, 0 or −14°C. It was shown that these three different peak positions can be ascribed to various redox states of Q, the shifts being due to the binding of herbicides to the chloroplast membrane. Both displacement experiments and additive inhibition of herbicide pairs measured by thermoluminescence and oxygen evolution suggested that the sites of action of these herbicides are on the same protein. However, herbicide treatment of trypsinized chloroplasts showed that there were three different binding sites on the same protein, in agreement with the classification of herbicides into three groups based on thermoluminescence measurements. Our results suggest that the primary and secondary acceptors of Photosystem II (Q and B, respectively) are in close proximity and form a common complex with the herbicide-binding protein within the chloroplast membrane.     

Thermoluminescence in plants

Recently considerable progress has been achieved in the elucidation of the origin of thermoluminescence in chloroplasts. The assignment of 2 of the thermoluminescence bands, peaking at around +5°C (Q or D band) and +30°C (B band), to the recombination of charges, originating from the oxidzed species of the oxygen evolving complex (the so-called S states) and to the reduced primary and secondary quinone acceptors Q_A and Q_B, respectively, has aided in the investigation of reactions involving both the electron donor and acceptor sides of photosystem II. In this paper we review recent thermoluminescence results concerning the deactivation of S states, temperature and pH dependence of S state transition, and the activity of the water oxidizing system after removal of Cl⁻, manganese or the 33 kDa protein. Reports on the use of thermoluminescence in investigations on the sites of action of herbicides and redox changes of Q_B conferred by herbicide resistance are also discussed. The effect of pH, bicarbonate, and Acceleration of Deactivation Reaction of enzyme "Y" (ADRY) reagents on the photosystem II reactions are presented in the light of thermoluminescence observations. Further possible applications of thermoluminescence promising better understanding of the photosynthetic processes are suggested.     

Graphical and numerical analysis of thermoluminescence and fluorescence F0 emission in photosynthetic material

A set-up for recording thermoluminescence emission together with the constant F₀ fluorescence yield is described briefly. It is driven by a microcomputer through plugged-in cards.Practical aspects of the simulation of TL bands and of decomposition of complex TL signals are examined. A reproducible and linear temperature gradient and the use of photon counting for luminescence detection are important features for further analyzing the recorded signal. The simulation procedure used is a step-by-step calculation of the number of charge recombinations, which is then substracted from the number of remaining charge pairs able to produce luminescence. This procedure consists first of a graphical fitting, followed by a numerical minimization, with a maximum of five simulated components. The quality of the simulation is evaluated by the sum of squares of differences (signal-simulation), related to the signal area. Equivalent decomposition patterns may be found for the same recording and additional information is needed for interpretation of TL data. Averaging signals is feasible, provided that maximum temperatures Tm of averaged bands are sufficiently similar (±3°C). Simultaneous measurement of the antenna fluorescence yield F₀, using an ultra-weak pulsed blue LED, gives an estimate of the luminescence yield. This has to be taken into account in the analysis of the Q band and of high temperature (>40°C) bands.The simulation parameters appear to be dependent on plant growth conditions. Quantitative analysis of thermoluminescence emission could be useful in the study of the effects of climatic factors on the photosynthetic apparatus in plants.Abbreviations PS-II Photosystem II - TL thermoluminescence - F₀ constant fluorescence emission, under ultra-low light intensity - Q_A and Q_B respectively, primary and secondary electron acceptors of Photosystem II - S₂ and S₃ respectively, the two and three positively charged states of the oxygen evolving system - SSD sum of squares of differences between the signal and a simulation (fitting) or between the signal and a smoothed curve (noise)     

Reliability of Photosystem II thermoluminescence measurements after sample freezing: Few artifacts with Photosystem II membranes but gross distortions with certain leaves

Changes of thermoluminescence (TL) properties reported for Photosystem II (PS II) membranes after removal of functional Cl^- recently have been attributed to an exposure of the experimental material to freezing temperatures in the absence of a cryoprotectant like glycerol [Krieger et al. (1998) Biochim Biophys Acta 1364: 46]. In the present study, freezing-induced modifications of the TL emissions of PS II membranes were confirmed to be a problem in TL studies, but glycerol was not always a reliable antidote. The TL measurements of this investigation lead to the conclusion that effects of sample freezing do not invalidate previous studies which have reported that Cl^- depletion shifts the TL emission to higher temperatures. Nevertheless, in agreement with the study of Krieger et al. (1998), it is shown that at pH 6.2 and in the absence of DCMU, Cl^- removal causes only a very small displacement of the TL emission peak. While the TL was affected mainly quantitatively by freezing when PS II membranes were the experimental material, substantial qualitative changes of the TL were observed with certain leaves. These are attributed tentatively to redox potential changes of the primary acceptors of PS II which allowed a reduction of Q_A by reduced Q_B via reverse electron flow. Experiments aimed at mimicking the altered TL emission in PS II membranes in vitro suggest actions of secondary metabolites and acids. Thylakoids in the leaf tissue may have become exposed to such compounds because of damage to cellular membranes during freezing. On the basis of the results of this investigation, it is recommended that sample freezing be avoided in TL studies whenever possible, regardless of the type of experimental material.     

EPR studies of the water oxidizing complex in the S1 and the higher S states: the manganese cluster and Y(Z) radical

The parallel polarization electron paramagnetic resonance (EPR) method has been applied to investigate manganese EPR signals of native S1 and S3 states of the water oxidizing complex (WOC) in photosystem (PS) II. The EPR signals in both states were assigned to thermally excited states with S=1, from which zero-field interaction parameters D and E were derived. Three kinds of signals, the doublet signal, the singlet-like signal and g=11-15 signal, were detected in Ca2+-depleted PS II. The g=11-15 signal was observed by parallel and perpendicular modes and assigned to a higher oxidation state beyond S2 in Ca2+-depleted PS II. The singlet-like signal was associated with the g=11-15 signal but not with the Y(Z) (the tyrosine residue 161 of the D1 polypeptide in PS II) radical. The doublet signal was associated with the Y(Z) radical as proved by pulsed electron nuclear double resonance (ENDOR) and ENDOR-induced EPR. The electron transfer mechanism relevant to the role of Y(Z) radical was discussed.     

Cl- channel inhibitors of the arylaminobenzoate type act as photosystem II herbicides: a functional and structural study

The Cl- channel blocker NPPB (5-nitro-2-(3-phenylpropylamino) benzoic acid) inhibited photosynthetic oxygen evolution of isolated thylakoid membranes in a pH-dependent manner with a K(i) of about 2 microM at pH 6. Applying different electron acceptors, taking electrons either directly from photosystem II (PS II) or photosystem I (PS I), the site of inhibition was localized within PS II. Measurements of fluorescence induction kinetics and thermoluminescence suggest that the binding of NPPB to the QB binding site of PS II is similar to the herbicide DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea). The effects of different arylaminobenzoate derivatives and other Cl- channel inhibitors on photosynthetic electron transport were investigated. The structure--activity relationship of the inhibitory effect on PS II shows interesting parallels to the one observed for the arylaminobenzoate block of mammalian Cl- channels. A molecular modeling approach was used to fit NPPB into the QB binding site and to identify possible molecular interactions between NPPB and the amino acid residues of the binding site in PS II. Taken together, these data give a detailed molecular picture of the mechanism of NPPB binding.     

Deletion of PsbM in tobacco alters the QB site properties and the electron flow within photosystem II

Photosystem II, the oxygen-evolving complex of photosynthetic organisms, includes an intriguingly large number of low molecular weight polypeptides, including PsbM. Here we describe the first knock-out of psbM using a transplastomic, reverse genetics approach in a higher plant. Homoplastomic Delta psbM plants exhibit photoautotrophic growth. Biochemical, biophysical, and immunological analyses demonstrate that PsbM is not required for biogenesis of higher order photosystem II complexes. However, photosystem II is highly light-sensitive, and its activity is significantly decreased in Delta psbM, whereas kinetics of plastid protein synthesis, reassembly of photosystem II, and recovery of its activity are comparable with the wild type. Unlike wild type, phosphorylation of the reaction center proteins D1 and D2 is severely reduced, whereas the redox-controlled phosphorylation of photosystem II light-harvesting complex is reversely regulated in Delta psbM plants because of accumulation of reduced plastoquinone in the dark and a limited photosystem II-mediated electron transport in the light. Charge recombination in Delta psbM measured by thermoluminescence oscillations significantly differs from the 2/6 patterns in the wild type. A simulation program of thermoluminescence oscillations indicates a higher Q(B)/Q(-)(B) ratio in dark-adapted mutant thylakoids relative to the wild type. The interaction of the Q(A)/Q(B) sites estimated by shifts in the maximal thermoluminescence emission temperature of the Q band, induced by binding of different herbicides to the Q(B) site, is changed indicating alteration of the activation energy for back electron flow. We conclude that PsbM is primarily involved in the interaction of the redox components important for the electron flow within, outward, and backward to photosystem II.     

Inactivation of photosynthetic oxygen evolution by UV-B irradiation: A thermoluminescence study

The influence of UV-B irradiation on photosynthetic oxygen evolution by isolated spinach thylakoids has been investigated using thermoluminescence measurements. The thermoluminescence bands arising from the S₂Q_B ^- (B band) and S₂Q_A ^– (Q band) charge recombination disappeared with increasing UV-B irradiation time. In contrast, the C band at 50°C, arising from the recombination of Q_A ^- with an accessory donor of Photosystem II, was transiently enhanced by the UV-B irradiation. The efficiency of DCMU to block Q_A to Q_B electron transfer decreased after irradiation as detected by the incomplete suppression of the B band by DCMU. The flash-induced oscillatory pattern of the B band was modified in the UV-B irradiated samples, indicating a decrease in the number of centers with reduced Q_B. Based on the results of this study, UV-B irradiation is suggested to damage both the donor and acceptor sides of Photosystem II. The damage of the water-oxidizing complex does not affect a specific S-state transition. Instead, charge stabilization is enhanced on an accessory donor. The acceptor-side modifications decrease the affinity of DCMU binding. This effect is assumed to reflect a structural change in the Q_B/DCMU binding site. The preferential loss of dark stable Q_B ^- may be related to the same structural change or could be caused by the specific destruction of reduced quinones by the UV-B light.Abbreviations Chl chlorophyll - DCMU 3-(3,4,-dichlorophenyl)-1,1-dimethylurea - PS II Photosystem II - Q_A first quinone electron acceptor of PS II - Q_B second quinone electron acceptor of PS II - Tyr-D accessory electron donor of PS II - S₀-S₄ charge storage states of the water-oxidizing complex     

Salicylic acid may indirectly influence the photosynthetic electron transport

Salicylic acid (SA) is a phenolic phytohormone with important roles in plant development, transpiration, endogenous signaling and defense against pathogens. One of the pathways of SA biosynthesis is located in the chloroplasts. The aim of the present work was to investigate the possible regulatory effects of SA on photosynthetic electron transport processes. Here we show that SA also affects leaf photosynthesis, via inducing stomatal closure and also by slowing down Photosystem II (PS II) electron transport. Photosynthetic CO? incorporation and stomatal conductivity (measured with an infrared gas analyzer) were much lower in SA-infiltrated tobacco leaves than in untreated or water-infiltrated controls. PS II electron transport (calculated from PAM chlorophyll fluorescence data) was more sensitive to SA than Photosystem I (PS I) (measured with far red absorption). Direct probing of PS II charge separation and stabilization (measured with thermoluminescence), however, showed that these events were less affected in isolated thylakoid membranes than in leaves, suggesting that the effect of SA on PS II is indirect and different from similar effects of phenolic herbicides.     

The grand design of photosynthesis: Acclimation of the photosynthetic apparatus to environmental cues

Dynamic acclimation of the photosynthetic apparatus in response to environmental cues, particularly light quantity and quality, is a widely-observed and important phenomenon which contributes to the tolerance of plants against stress and helps to maintain, as far as possible, optimal photosynthetic efficiency and resource utilization. This mini-review represents a scrutiny of a number of possible photoreceptors (including the two photosystems acting as light sensors) and signal transducers that may be involved in producing acclimation responses. We suggest that regulation by signal transduction may be effected at each of several possible points, and that there are multiple regulatory mechanisms for photosynthetic acclimation.Abbreviations FR far-red light - LHC I, LHC II light-harvesting chlorophyll a/b-protein complex of PS I and PS II, respectively - P700 primary electron donor of PS I - Pmax maximum photosynthetic capacity - Q_A primary quinone electron acceptor of PS II - q_N, q_P non-photochemical and photochemical quenching, respectively - R red light     

Usefulness of thermoluminescence in herbicide research

Many herbicides of different chemical structure inhibit photosynthetic electron flow by interrupting the photosyn‐thetic electron flow by interrupting the photosynthetic electron transport chain between the primary acceptor (Q_A) and the secondary acceptor (Q_B) of photosystem 2 (PS2). Thermoluminescence (TL) originates from PS2, and the bands of the glow curve can be related to the charge recombination between positively charged donors and negatively charged acceptors. The glow curve of TL is strongly influenced by addition of PS2 herbicides. The herbicide treatment shifts the peak position and activation energy of the TL band related to Q_A, suggesting that herbicide binding affects the midpoint redox potential not only of Q _B but also that of Q_A. On the basis of the band shift the herbicides of various chemical structures can be classified into different “thermodynamical” groups which relfect the differences in the binding properties of these herbicides. As a new approach TL seems to be a useful technique in studying the mechanism and site of action of herbicides that inhibit electron transport of PS2.     

The primary structure of D1 near the QB pocket influences oxygen evolution

Photosystem II (PS II) is the site of oxygen evolution. Activation of dark adapted samples by a train of saturating flashes produces oxygen with a yield per flash which oscillates with a periodicity of four. Damping of the oxygen oscillations is accounted for by misses and double hits. The mechanisms hidden behind these parameters are not yet fully understood. The components which participate in charge transfer and storage in PS II are believed to be anchored to the heterodimer formed by the D1 and D2 proteins. The secondary plastoquinone acceptor Q_B binds on D1 in a loop connecting the fourth and fifth helices (the Q_B pocket). Several D1 mutants, mutated in the Q_B binding region, have been studied over the past ten years.In the present report, our results on nine D1 mutants of Synechocystis PCC 6714 and 6803 are analyzed. When oxygen evolution is modified, it can be due to a change in the electron transfer kinetics at the level of the acceptor side of PS II and also in some specific mutants to a long ranging effect on the donor side of PS II. The different properties of the mutants enable us to propose a classification in three categories. Our results can fit in a model in which misses are substantially determined by the fraction of centers which have Q_A ^- before each flash due to the reversibility of the electron transfer reactions. This idea is not new but was more thoroughly studied in a recent paper by Shinkarev and Wraight (1993). However, we will show in the discussion that some doubts remain as to the true origin of misses and double hits.Abbreviations BQ p-benzoquinone - Chl chlorophyll - D1 and D2 proteins of the core of PS II - DCMU 3-(3,4-dichlorophenyl)-1,1 dimethyl urea - OEC oxygen evolving complex - P₆₈₀ chlorophyll center of PS II acting as the primary donor - PS II Photosystem II - Q_A and Q_B primary and secondary quinone electron acceptor - TL thermoluminescence