Effects of dark- and light-induced proton gradients in thylakoids on the Q and B thermoluminescence bands

Moderate drought stress stabilizes the primary quinone acceptor QA and the secondary quinone acceptor QB in photosystem II

Drought induces stomata closure and lowers the CO₂ concentration in the mesophyll, limiting CO₂ assimilation and favoring photorespiration. The photosynthetic apparatus is protected under drought conditions by a number of downregulation mechanisms like photosynthetic control and activation of cyclic electron transport leading to the generation of a high proton gradient across the thylakoid membrane. Here, we studied photosynthetic electron transport by chlorophyll fluorescence, thermoluminescence (TL), and P₇₀₀ absorption measurements in spinach exposed to moderate drought stress. Chlorophyll fluorescence induction and decay kinetics were slowed down. Under drought conditions, an increase of the TL AG-band and a downshift of the maximum temperatures of both, the B-band and the AG-band, were observed when leaves were illuminated under conditions that maintained the proton gradient. When leaves were frozen prior to the TL measurements, the maximum temperature of the B-band was upshifted in drought-stressed leaves. This shows a stabilization of the Q_B /Q_B ^•- redox couple in accordance with the slower fluorescence decay kinetics. We propose that during drought stress, photorespiration exerts a feedback control on photosystem II via the binding of a photorespiratory metabolite at the non-heme iron at the acceptor side of photosystem II. According to our hypothesis, an exchange of bicarbonate at the non-heme iron by a photorespiratory metabolite such as glycolate would not only affect the midpoint potential of the Q_A /Q_A ^•- couple, as shown previously, but also that of the Q_B /Q_B ^•- couple.

Identification of the AG afterglow thermoluminescence band in the cyanobacterium Synechocystis PCC 6803

The so-called afterglow, AG, thermoluminescence (TL) band is a useful indicator of the presence of cyclic electron flow (CEF), which is mediated by the NADH dehydrogenase-like (NDH) complex in higher plants. Although NDH-dependent CEF occurs also in cyanobacteria, the AG band has previously not been found in these organisms. In the present study, we tested various experimental conditions and could identify a TL component with ca. +40°C peak temperature in Synechocystis PCC 6803 cells, which were illuminated by far-red (FR) light at around -10°C. The +40°C band could be observed when WT cells were grown under ambient air level CO₂ , but was absent in the M55 mutant, which is deficient in the NDH-1 complex. These experimental observations match the characteristics of the AG band of higher plants. Therefore, we conclude that the newly identified +40°C TL component in Synechocystis PCC 6803 is the cyanobacterial counterpart of the plant AG band and originates from NDH-1-mediated CEF. The cyanobacterial AG band was most efficiently induced when FR illumination was applied at -10°C and its contribution to the total TL intensity declined when cells were illuminated above and below this temperature. Based on this phenomenon we also conclude that CEF is blocked by low temperatures at two different sites in Synechocystis PCC 6803: (1) Below -10°C at the level of NDH-1 and (2) below -30°C at the donor or acceptor side of Photosystem I.

The afterglow photosynthetic luminescence

The afterglow (AG) photosynthetic luminescence is a long-lived chlorophyll fluorescence emitted from PSII after the illumination of photosynthetic materials by FR or white light and placed in darkness. The AG emission corresponds to the fraction of PSII centers in the S_2/3 Q_B non-radiative state immediately after pre-illumination, in which the arrival of an electron transferred from stroma along cyclic/chlororespiratory pathway(s) produces the S_2/3 Q_B ^- radiative state that emits luminescence. This emission can be optimally recorded by a linear temperature gradient as sharp thermoluminescence (TL) band peaking at about 45°C. The AG emission recorded by TL technique has been proposed as a simple non-invasive tool to investigate the chloroplast energetic state and some of its metabolism processes as cyclic transport of electrons around PSI, chlororespiration or photorespiration. On the other hand, this emission has demonstrated to be a useful probe to study the effect of various stress conditions in photosynthetic materials.

Low-pH induced reversible reorganizations of chloroplast thylakoid membranes - As revealed by small-angle neutron scattering

Energization of thylakoid membranes brings about the acidification of the lumenal aqueous phase, which activates important regulatory mechanisms. Earlier Jajoo and coworkers (2014 FEBS Lett. 588:970) have shown that low pH in isolated plant thylakoid membranes induces changes in the excitation energy distribution between the two photosystems. In order to elucidate the structural background of these changes, we used small-angle neutron scattering on thylakoid membranes exposed to low p2H (pD) and show that gradually lowering the p2H from 8.0 to 5.0 causes small but well discernible reversible diminishment of the periodic order and the lamellar repeat distance and an increased mosaicity - similar to the effects elicited by light-induced acidification of the lumen. Our data strongly suggest that thylakoids dynamically respond to the membrane energization and actively participate in different regulatory mechanisms.

Iron Deficiency Induces a Partial Inhibition of the Photosynthetic Electron Transport and a High Sensitivity to Light in the Diatom Phaeodactylum tricornutum

Iron limitation is the major factor controlling phytoplankton growth in vast regions of the contemporary oceans. In this study, a combination of thermoluminescence (TL), chlorophyll fluorescence, and P700 absorbance measurements have been used to elucidate the effects of iron deficiency in the photosynthetic electron transport of the marine diatom P. tricornutum. TL was used to determine the effects of iron deficiency on photosystem II (PSII) activity. Excitation of iron-replete P. tricornutum cells with single turn-over flashes induced the appearance of TL glow curves with two components with different peaks of temperature and contributions to the total signal intensity: the B band (23°C, 63%), and the AG band (40°C, 37%). Iron limitation did not significantly alter these bands, but induced a decrease of the total TL signal. Far red excitation did not increase the amount of the AG band in iron-limited cells, as observed for iron-replete cells. The effect of iron deficiency on the photosystem I (PSI) activity was also examined by measuring the changes in P700 redox state during illumination. The electron donation to PSI was substantially reduced in iron-deficient cells. This could be related with the important decline on cytochrome c 6 content observed in these cells. Iron deficiency also induced a marked increase in light sensitivity in P. tricornutum cells. A drastic increase in the level of peroxidation of chloroplast lipids was detected in iron-deficient cells even when grown under standard conditions at low light intensity. Illumination with a light intensity of 300 μE m(-2) s(-1) during different time periods caused a dramatic disappearance in TL signal in cells grown under low iron concentration, this treatment not affecting to the signal in iron-replete cells. The results of this work suggest that iron deficiency induces partial blocking of the electron transfer between PSII and PSI, due to a lower concentration of the electron donor cytochrome c 6. This decreased electron transfer may induce the over-reduction of the plastoquinone pool and consequently the appearance of acceptor side photoinhibition in PSII even at low light intensities. The functionality of chlororespiratory electron transfer pathway under iron restricted conditions is also discussed.

Assessment of photosystem II thermoluminescence as a tool to investigate the effects of dehydration and rehydration on the cyclic/chlororespiratory electron pathways in wheat and barley leaves

Thermoluminescence emission from wheat leaves was recorded under various controlled drought stress conditions: (i) fast dehydration (few hours) of excised leaves in the dark (ii) slow dehydration (several days) obtained by withholding watering of plants under a day/night cycle (iii) overnight rehydration of the slowly dehydrated plants at a stage of severe dessication. In fast dehydrated leaves, the AG band intensity was unchanged but its position was shifted to lower temperatures, indicating an activation of cyclic and chlororespiratory pathways in darkness, without any increase of their overall electron transfer capacity. By contrast, after a slow dehydration the AG intensity was strongly increased whereas its position was almost unchanged, indicating respectively that the capacity of cyclic pathways was enhanced but that they remained inactivated in darkness. Under more severe dehydration, the AG band almost disappeared. Rewatering caused its rapid bounce significantly above the control level. No significant differences in AG emission could be found between the two drought-sensitive and drought-tolerant wheat cultivars. The afterglow thermoluminescence emission in leaves provides an additional tool to follow the increased capacity and activation of cyclic electron flow around PSI in leaves during mild, severe dehydration and after rehydration.

The role of the high potential form of the cytochrome b559: Study of Thermosynechococcus elongatus mutants

Cytochrome b559 is an essential component of the photosystem II reaction center in photosynthetic oxygen-evolving organisms, but its function still remains unclear. The use of photosystem II preparations from Thermosynechococcus elongatus of high integrity and activity allowed us to measure for the first time the influence of cytochrome b559 mutations on its midpoint redox potential and on the reduction of the cytochrome b559 by the plastoquinone pool (or QB). In this work, five mutants having a mutation in the α-subunit (I14A, I14S, R18S, I27A and I27T) and one in the β-subunit (F32Y) of cytochrome b559 have been investigated. All the mutations led to a destabilization of the high potential form of the cytochrome b559. The midpoint redox potential of the high potential form was significantly altered in the αR18S and αI27T mutant strains. The αR18S strain also showed a high sensitivity to photoinhibitory illumination and an altered oxidase activity. This was suggested by measurements of light induced oxidation and dark re-reduction of the cytochrome b559 showing that under conditions of a non-functional water oxidation system, once the cytochrome is oxidized by P680(+), the yield of its reduction by QB or the PQ pool was smaller and the kinetic slower in the αR18S mutant than in the wild-type strain. Thus, the extremely positive redox potential of the high potential form of cytochrome b559 could be necessary to ensure efficient oxidation of the PQ pool and to function as an electron reservoir replacing the water oxidation system when it is not operating.

Pitfalls, artefacts and open questions in chlorophyll thermoluminescence of leaves or algal cells

Thermoluminescence of intact photosynthetic organisms, leaves or algal cells, raises specific problems. The constitutive S2/3Q B (-) B bands constitute major probes of the state of photosystem II in vivo. The presence of a dark-stable acidic lumen causes a temperature downshift of B bands, specially the S3 B band, providing a lumen pH indicator. This is accompanied by a broadening of the S3 B band that becomes an envelope of elementary B bands. The occasional AT, Q and C bands are briefly examined in an in vivo context. It is emphasized that freezing below the nucleation temperature is not necessary for physiological studies, but a source of artefacts, hence should be avoided. In intact photosynthetic structures, a dark-electron transfer from stroma reductants to the quinonic acceptors of photosystem II via the cyclic/chlororespiratory pathways, strongly stimulated by moderate warming, gives rise to the afterglow (AG) luminescence emission that reflects chloroplast energy status. The decomposition of complex TL signals into elementary bands is necessary to determine the maximum temperature T m and the area of each of them. A comparison of TL signals after 1 flash and 2 flashes prevents from confusing the three main bands observed in vivo, i.e. the S2 and S3 B bands and the AG band. Finally, the thermoluminescence bands arising sometimes above 50 °C are mentioned. The basic principles of (thermo)luminescence established on isolated thylakoids should not be applied directly without a careful examination of in vivo conditions.

Restricted capacity for PSI-dependent cyclic electron flow in ΔpetE mutant compromises the ability for acclimation to iron stress in Synechococcus sp. PCC 7942 cells

Exposure of wild type (WT) and plastocyanin coding petE gene deficient mutant (ΔpetE) of Synechococcus cells to low iron growth conditions was accompanied by similar iron-stress induced blue-shift of the main red Chl a absorption peak and a gradual decrease of the Phc/Chl ratio, although ΔpetE mutant was more sensitive when exposed to iron deficient conditions. Despite comparable iron stress induced phenotypic changes, the inactivation of petE gene expression was accompanied with a significant reduction of the growth rates compared to WT cells. To examine the photosynthetic electron fluxes in vivo, far-red light induced P700 redox state transients at 820nm of WT and ΔpetE mutant cells grown under iron sufficient and iron deficient conditions were compared. The extent of the absorbance change (ΔA(820)/A(820)) used for quantitative estimation of photooxidizable P700(+) indicated a 2-fold lower level of P700(+) in ΔpetE compared to WT cells under control conditions. This was accompanied by a 2-fold slower re-reduction rate of P700(+) in the ΔpetE indicating a lower capacity for cyclic electron flow around PSI in the cells lacking plastocyanin. Thermoluminescence (TL) measurements did not reveal significant differences in PSII photochemistry between control WT and ΔpetE cells. However, exposure to iron stress induced a 4.5 times lower level of P700(+), 2-fold faster re-reduction rate of P700(+) and a temperature shift of the TL peak corresponding to S(2)/S(3)Q(B)(-) charge recombination in WT cells. In contrast, the iron-stressed ΔpetE mutant exhibited only a 40% decrease of P700(+) and no significant temperature shift in S(2)/S(3)Q(B)(-) charge recombination. The role of mobile electron carriers in modulating the photosynthetic electron fluxes and physiological acclimation of cyanobacteria to low iron conditions is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Thermoluminescence and P700 redox kinetics as complementary tools to investigate the cyclic/chlororespiratory electron pathways in stress conditions in barley leaves

Cyclic electron flow around photosystem I drives additional proton pumping into the thylakoid lumen, which enhances the protective non-photochemical quenching and increases ATP synthesis. It involves several pathways activated independently. In whole barley leaves, P700 oxidation under far-red illumination and subsequent P700(+) dark reduction kinetics provide a major probe of the activation of cyclic pathways. Two 'intermediate' and 'slow' exponential reduction phases are always observed and they become faster after high light illumination, but dark inactivation of the Benson-Calvin cycle causes the emergence of both a transient in the P700 oxidation and a 'fast' phase in the P700(+) reduction. We investigate here the afterglow (AG) thermoluminescence emission as another tool to detect the activation of cyclic electron pathways from stroma reductants to the acceptor side of photosystem II. This transfer is activated by warming, yielding an AG band at about 45°C. However, treatments that accelerate the 'intermediate' and 'slow' P700(+) reduction phases (brief anoxia, hexose infiltration, fast dehydration of excised leaves) also produced a downshift of this AG band. This pathway ascribable to NADPH dehydrogenase (NDH) would be triggered by a deficit in ATP, while the 'fast' reduction phase corresponding to the ferredoxin plastoquinone reductase pathway is triggered by an overreduction of the photosystem I acceptor pool and is undetected in thermoluminescence. Contrastingly, slow dehydration of unwatered plants did not cause faster reduction of P700(+) nor temperature downshift of the AG band, that is no induction of the NDH pathway, whereas an increased intensity of the AG band indicated a strong NADPH + ATP assimilatory potential.

Peculiar properties of chlorophyll thermoluminescence emission of autotrophically or mixotrophically grown Chlamydomonas reinhardtii

The microalgae Chlamydomonas reinhardtii and Chlorella sp. CCAP 211/84 were grown autotrophically and mixotrophically and their thermoluminescence emissions were recorded above 0 °C after excitation by 1, 2 or 3 xenon flashes or by continuous far-red light. An oscillation of the B band intensity according to the number of flashes was always observed, with a maximum after 2 flashes, accompanied by a downshift of the B band temperature maximum in mixotrophic compared to autotrophic grown cells, indicative of a dark stable pH gradient. Moreover, new flash-induced bands emerged in mixotrophic Chlamydomonas grown cells, at temperatures higher than that of the B band. In contrast to the afterglow band observed in higher plants, in Chlamydomonas these bands were not inducible by far-red light, were fully suppressed by 2 μM antimycin A, and peaked at different temperatures depending on the flash number and growth stage, with higher temperature maxima in cells at a stationary compared to an exponential growth stage. These differences are discussed according to the particular properties of cyclic electron transfer pathways in C. reinhardtii.

Thermoluminescence: experimental

Thermoluminesence measurements are useful for the study of Photosystem II electron transport in intact leaves, in algal and cyanobacterial cells, as well as in isolated membrane complexes. Here an overview of the experimental approaches is provided. In the present review, instruments and the experimental procedures for measuring thermoluminescence emission from photosynthetic systems of various origins are summarized and discussed. Major pitfalls frequently encountered in measurements with isolated membranes, suspensions of intact organisms or solid leaf samples are highlighted. Analytical and numeric methods for the analysis of measured thermoluminescence curves are also discussed.

Photosynthesis limitations during water stress acclimation and recovery in the drought-adapted Vitis hybrid Richter-110 (V. berlandierixV. rupestris)

The hybrid Richter-110 (Vitis berlandierixVitis rupestris) has the reputation of being a genotype strongly adapted to drought. A study was performed with plants of R-110 subjected to sustained water-withholding to induce acclimation to two different levels of water stress, followed by rewatering to induce recovery. The goal was to analyse how photosynthesis is regulated during acclimation to water stress and recovery. In particular, the regulation of stomatal conductance (g(s)), mesophyll conductance to CO(2) (g(m)), leaf photochemistry (chlorophyll fluorescence and thermoluminescence), and biochemistry (V(c,max)) were assessed. During water stress, g(s) declined to 0.1 and less than 0.05 mol CO(2) m(-2) s(-1) in moderately and severely water-stressed plants, respectively, and was kept quite constant during an acclimation period of 1-week. Leaf photochemistry proved to be very resistant to the applied water-stress conditions. By contrast, g(m) and V(c,max) were affected by water stress, but they were not kept constant during the acclimation period. g(m) was initially unaffected by water stress, and V(c,max) even increased above control values. However, after several days of acclimation to water stress, both parameters declined below (g(m)) or at (V(c,max)) control values. For the latter two parameters there seemed to be an interaction between water stress and cumulative irradiance, since both recovered to control values after several cloudy days despite water stress. A photosynthesis limitation analysis revealed that diffusional limitations and not biochemical limitations accounted for the observed decline in photosynthesis during water stress and slow recovery after rewatering, both in moderately and severely stressed plants. However, the relative contribution of stomatal (SL) and mesophyll conductance (MCL) limitations changes during acclimation to water stress, from predominant SL early during water stress to similar SL and MCL after acclimation. Finally, photosynthesis recovery after rewatering was mostly limited by SL, since stomatal closure recovered much more slowly than g(m).

Effects of low temperature stress on excitation energy partitioning and photoprotection in Zea mays

Analysis of the partitioning of absorbed light energy within PSII into fractions utilised by PSII photochemistry (Φ_PSII), thermally dissipated via ΔpH- and zeaxanthin-dependent energy quenching (Φ_NPQ) and constitutive non-photochemical energy losses (Φ_f,D) was performed in control and cold-stressed maize (Zea mays L.) leaves. The estimated energy partitioning of absorbed light to various pathways indicated that the fraction of Φ_PSII was twofold lower, whereas the proportion of thermally dissipated energy through Φ_NPQ was only 30% higher, in cold-stressed plants compared with control plants. In contrast, Φ_f,D, the fraction of absorbed light energy dissipated by additional quenching mechanism(s), was twofold higher in cold-stressed leaves. Thermoluminescence measurements revealed that the changes in energy partitioning were accompanied by narrowing of the temperature gap (ΔT_M) between S_2/3Q_B^- and S₂Q_A^- charge recombinations in cold-stressed leaves to 8°C compared with 14.4°C in control maize plants. These observations suggest an increased probability for an alternative non-radiative P680⁺Q_A^- radical pair recombination pathway for energy dissipation within the reaction centre of PSII in cold-stressed maize plants. This additional quenching mechanism might play an important role in thermal energy dissipation and photoprotection when the capacity for the primary, photochemical (Φ_PSII) and zeaxanthin-dependent non-photochemical quenching (Φ_NPQ) pathways are thermodynamically restricted in maize leaves exposed to cold temperatures.

The induction of CP43′ by iron-stress in Synechococcus sp. PCC 7942 is associated with carotenoid accumulation and enhanced fatty acid unsaturation

Comparative lipid analysis demonstrated reduced amount of PG (50%) and lower ratio of MGDG/DGDG in iron-stressed Synechococcus sp. PCC 7942 cells compared to cells grown under iron sufficient conditions. In parallel, the monoenoic (C:1) fatty acids in MGDG, DGDG and PG increased from 46.8%, 43.7% and 45.6%, respectively in control cells to 51.6%, 48.8% and 48.7%, respectively in iron-stressed cells. This suggests increased membrane dynamics, which may facilitate the diffusion of PQ and keep the PQ pool in relatively more oxidized state in iron-stressed compared to control cells. This was confirmed by chlorophyll fluorescence and thermoluminescence measurements. Analysis of carotenoid composition demonstrated that the induction of isiA (CP43') protein in response to iron stress is accompanied by significant increase of the relative abundance of all carotenoids. The quantity of carotenoids calculated on a Chl basis increased differentially with nostoxanthin, cryptoxanthin, zeaxanthin and beta-carotene showing 2.6-, 3.1-, 1.9- and 1.9-fold increases, respectively, while the relative amount of caloxanthin was increased only by 30%. HPLC analyses of the pigment composition of Chl-protein complexes separated by non-denaturating SDS-PAGE demonstrated even higher relative carotenoids content, especially of cryptoxanthin, in trimer and monomer PSI Chl-protein complexes co-migrating with CP43' from iron-stressed cells than in PSI complexes from control cells where CP43' is not present. This implies a carotenoid-binding role for the CP43' protein which supports our previous suggestion for effective energy quenching and photoprotective role of CP43' protein in cyanobacteria under iron stress.

Influence of the Redox Potential of the Primary Quinone Electron Acceptor on Photoinhibition in Photosystem II

We report the characterization of the effects of the A249S mutation located within the binding pocket of the primary quinone electron acceptor, Q(A), in the D2 subunit of photosystem II in Thermosynechococcus elongatus. This mutation shifts the redox potential of Q(A) by approximately -60 mV. This mutant provides an opportunity to test the hypothesis, proposed earlier from herbicide-induced redox effects, that photoinhibition (light-induced damage of the photosynthetic apparatus) is modulated by the potential of Q(A). Thus the influence of the redox potential of Q(A) on photoinhibition was investigated in vivo and in vitro. Compared with the wild-type, the A249S mutant showed an accelerated photoinhibition and an increase in singlet oxygen production. Measurements of thermoluminescence and of the fluorescence yield decay kinetics indicated that the charge-separated state involving Q(A) was destabilized in the A249S mutant. These findings support the hypothesis that a decrease in the redox potential of Q(A) causes an increase in singlet oxygen-mediated photoinhibition by favoring the back-reaction route that involves formation of the reaction center chlorophyll triplet. The kinetics of charge recombination are interpreted in terms of a dynamic structural heterogeneity in photosystem II that results in high and low potential forms of Q(A). The effect of the A249S mutation seems to reflect a shift in the structural equilibrium favoring the low potential form.

Light emission originating from photosystem II radical pair recombination is sensitive to zeaxanthin related non-photochemical quenching (NPQ)

We have used chlorophyll fluorescence, delayed luminescence and thermoluminescence measurements to study the influence of an artificial DeltapH in the presence or absence of zeaxanthin on photosystem II reactions. Energization of the pea thylakoid membranes induced non-photochemical fluorescence quenching and an increase in the overall luminescence emission of PSII during delayed luminescence and thermoluminescence measurements. This DeltapH-induced overall luminescence increase was caused by a strongly enhanced delayed luminescence in the seconds range before sample heating. In the subsequent thermoluminescence measurements the intensity of the B-band decreased after one and increased after two or more single turnover flashes. We propose that strong membrane energization shifted the redox potential of photosystem II radical pairs to more negative values causing the high delayed luminescence. The zeaxanthin-dependent non-photochemical fluorescence quenching component, however, did not alter thermoluminescence B-bands but decreased the delayed luminescence intensity by 30%. To our knowledge this is the first report that the radiative radical pair recombination, exhibited as delayed luminescence but not thermoluminescence emission, is sensitive to the antenna located zeaxanthin related non-photochemical fluorescence quenching. Our data can be interpreted within the frame of the exciton/radical pair equilibrium model that describes photosystem II as a shallow trap and incorporates the transfer of energy from the re-excitated reaction centre to the antenna of photosystem II.

Cytochrome c550 in the Cyanobacterium Thermosynechococcus elongatus

Cytochrome c(550) is one of the extrinsic Photosystem II subunits in cyanobacteria and red algae. To study the possible role of the heme of the cytochrome c(550) we constructed two mutants of Thermosynechococcus elongatus in which the residue His-92, the sixth ligand of the heme, was replaced by a Met or a Cys in order to modify the redox properties of the heme. The H92M and H92C mutations changed the midpoint redox potential of the heme in the isolated cytochrome by +125 mV and -30 mV, respectively, compared with the wild type. The binding-induced increase of the redox potential observed in the wild type and the H92C mutant was absent in the H92M mutant. Both modified cytochromes were more easily detachable from the Photosystem II compared with the wild type. The Photosystem II activity in cells was not modified by the mutations suggesting that the redox potential of the cytochrome c(550) is not important for Photosystem II activity under normal growth conditions. A mutant lacking the cytochrome c(550) was also constructed. It showed a lowered affinity for Cl(-) and Ca(2+) as reported earlier for the cytochrome c(550)-less Synechocystis 6803 mutant, but it showed a shorter lived S(2)Q(B)(-) state, rather than a stabilized S(2) state and rapid deactivation of the enzyme in the dark, which were characteristic of the Synechocystis mutant. It is suggested that the latter effects may be caused by loss (or weaker binding) of the other extrinsic proteins rather than a direct effect of the absence of the cytochrome c(550).

Effects of UV irradiation on barley and tomato leaves: thermoluminescence as a method to screen the impact of UV radiation on crop plants

The effect of different UV intensities and irradiation times on barley and tomato leaves was investigated by analysis of thermoluminescence (TL) and chlorophyll (chl) fluorescence measurements. Epifluorescence microscopy was used to estimate the epidermal UV transmittance of leaves. In barley a strong supression of TL emission from the S₂Q_B^- (B-band) and the S₂Q_A^- (Q-band) charge recombination was observed increasing with prolonged UV exposure. Primary barley leaves were more sensitive to UV than secondary leaves. In tomato plants a decrease in the B-band only takes place at very high UV intensities and after prolonged exposure times (4 h). The impact of UV in cotyledons was more pronounced than in pinnate leaves of tomato plants. The strong differences in sensitivity to UV in the investigated barley and tomato variety may be due to different concentrations of UV screening pigments in the epidermal layer as demonstrated by epifluorescence measurements. The results show that TL has the same potential to analyse the sensitivity or tolerance of crop plants to UV irradiation as routine fluorescence techniques. Furthermore, TL is directly monitoring the radical pair states of PSII and can distinguish between UV-induced donor and acceptor site-related damage.

The role of transmembrane electrochemical potential and phosphorylation of PS II proteins in temperature induced light emission from ATP-treated lettuce thylakoids

Changes in characteristics of flash-induced thermoluminescence (TL) glow curves in thylakoids of lettuce following incubation of the organelles with ATP, under illumination or in the dark, were investigated. TL bands were induced by 1 or 2 flashes fired at -10°C or 1°C in thylakoids: TL curves in control thylakoids which were dark-adapted or submitted to an illumination without ATP, can be deconvoluted as the sum of one single B band and minor contributions. In thylakoids incubated for 90 s with 0.5 mM ATP, either under light or in the dark (after a 90 s preillumination), bands presented complex shapes; after deconvolution, they appeared composed of a B band with a low Ea (activation energy): 0.6 e.v. as compared to 0.75 in control, and a supplementary band peaking at about 10°C. The band at low temperature was suppressed by low concentrations (10-20 nM) of valinomycin, nigericin or FCCP as well as by 10 mM ammonium chloride, leaving B bands with the same characteristics as in control material. Finally with higher nigericin concentrations, the bands became single B bands with high Ea (0.9 e.v.). These characteristics would define 3 different energized states (in the form of a transmembrane electrochemical potential) for thylakoids based upon the presence of the 10°C band and the value of the activation energy for the B band component. The presence of a large 10°C band was also correlated to the existence of a larger transmembrane pH gradient, in the dark, after an ATP-treatment, than in controls. The 10°C band was specifically suppressed by the action of low concentrations of alkaline phosphatase with minor changes in characteristics of the remaining B band suggesting that phosphorylation of PS II proteins is also involved in the appearance of this low temperature band. The main mechanism at the origin of the low temperature band would be a destabilization of S2/3QB (-) charge pairs in energized membranes.