Mercury inhibits the non-photochemical reduction of plastoquinone by exogenous NADPH and NADH: evidence from measurements of the polyphasic chlorophyll a fluorescence rise in spinach chloroplasts

Chlorophyll a fluorescence rise kinetics (from 50 μs to 1 s) were used to investigate the non-photochemical reduction of the plastoquinone (PQ) pool in osmotically broken spinach chloroplasts (Spinacia oleracea L.). Incubation of the chloroplasts in the presence of exogenous NADPH or NADH resulted in significant changes in the shape of the fluorescence transient reflecting an NAD(P)H-dependent accumulation of reduced PQ in the dark, with an extent depending on the concentration of NAD(P)H and the availability of oxygen; the dark reduction of the PQ pool was saturated at lower NAD(P)H concentrations and reached a higher level when the incubation took place under anaerobic conditions than when it occurred under aerobic conditions. Under both conditions NADPH was more effective than NADH in reducing PQ, however only at sub-saturating concentrations. Neither antimycin A nor rotenone were found to alter the effect of NAD(P)H. The addition of mercury chloride to the chloroplast suspension decreased the NAD(P)H-dependent dark reduction of the PQ pool, with the full inhibition requiring higher mercury concentrations under anaerobic than under aerobic conditions. This is the first time that this inhibitory role of mercury is reported for higher plants. The results demonstrate that in the dark the redox state of the PQ pool is regulated by the reduction of PQ via a mercury-sensitive NAD(P)H-PQ oxidoreductase and the reoxidation of reduced PQ by an O₂-dependent pathway, thus providing additional evidence for the existence of a chlororespiratory electron transport chain in higher plant chloroplasts. This revised version was published online in June 2006 with corrections to the Cover Date.     

Resolution of components of non-photochemical chlorophyll fluorescence quenching in barley leaves

Non-photochemical chlorophyll fluorescence quenching (qN) in barley leaves has been analysed by monitoring its relaxation in the dark, by applying saturating pulses of light. At least three kinetically distinct phases to qN recovery are observed, which have previously been identified (Quick and Stitt 1989) as being due to high-energy state quenching (fast), excitation energy redistribution due to a state transition (medium) and photoinhibition (slow). However, measurements of chlorophyll fluorescence at 77 K from leaf extracts show that state transitions only occur in low light conditions, whereas the medium component of qN is very large in high light. The source of that part of the medium component not accounted for by a state transition is discussed.Abbreviations ATP adenosine 5-triphosphate - DCMU 3[3,4-dichlorophenyl]-1,1 dimethylurea - pH trans-thylakoid pH gradient - F_o, F_m room-temperature chlorophyll fluorescence yield with all reaction centres open, closed - F_v variable fluorescence = F_m–F_o - LHC II Light harvesting complex II - PS I, PS II Photosystem I, II - P700, P680 primary donor in photosystem I, II - qP photochemical quenching of variable fluorescence - qN non-photochemical quenching of variable fluorescence - qN_e, qN_t, qN_i non-photochemical quenching due to high energy state, state transition, photoinhibition - qN_f, qN_m, qN_s components of qN relaxing fast, medium, slow - q_r quenching of r relative to the dark state - tricine N-tris[hydroxymethyl]methylglycine - r ratio of fluorescence maximum from photosystem II to that from photosystem I at 77 K     

Study of the non-photochemical dark rise in chlorophyll fluorescence in pre-illuminated leaves of various C3 and C4 plants submitted to partial anaerobiosis

Changes in chlorophyll fluorescence yield were studied during a dark period in pre-illuminated leaves of various C3 and C4 plants. The oxygen content in the gaseous atmosphere was either normal (21 kPa) or low (1.5 or 0.36 kPa). C3 and C4 plants of the NAD malic enzyme subgroup showed an initial rise in fluorescence at the onset of the dark period with an amplitude depending on the O₂ level in the gas. In C4 plants belonging to the other two subgroups, the slow rise was absent or of very low size. At high [O₂], the fluorescence level decreased in some minutes to the initial F₀ level (determined in dark-adapted leaves). Conversely at low [O₂], the fluorescence yield remained higher than F₀ in all the C4 plants studied, whereas it decreased slowly to the F₀ level in the different C3 plants. At low [O₂], the fluorescence level decreased rapidly to F₀ when introducing for 30 s, a high O₂ level or when giving a 15-s far-red illumination. At the end of these treatments, the fluorescence level re-increased. These results demonstrate the presence at low [O₂] of highly fluorescent ‘closed' photosystem II centres containing Q^-_A in equilibrium with reduced plastoquinone molecules of the chloroplastic pool. Reoxidation of the plastoquinone pool would be dependent on the functioning of an oxidase probably dependent on a chlororespiration process fully active at O₂ levels higher than 2 kPa. The source of reducing equivalents for the plastoquinone pool is discussed.     

Activation of non-photochemical quenching in thylakoids and leaves

The mechanism of rapidly-relaxing non-photochemical quenching in two plant species,Chenopodium album L. andDigitalis purpurea L., that differ considerably in their capacity for such quenching has been investigated (Johnson G.N. et al. 1993, Plant Cell Environ.16, 673–679). Illumination of leaves of both species in the presence of 2% O₂ balance N₂ led to the formation of zeaxanthin. When thylakoids were isolated from leaves of each species that had been so treated it was found that inD. purpurea non-photochemical quenching was “activated” relative to the control; a higher level of quenching was found for a given trans-thylakoid pH gradient. No such activation of non-photochemical quenching was observed inC. album. Similar conclusions were drawn when comparing quenching in intact leaves. It is concluded that light activation of quenching is a process that cannot readily be induced inC. album. Measurement of the sensitivity of non-photochemical quenching in leaves ofC. album andD. purpurea to dithiothreitol (DTT; a reagent that inhibits formation of zeaxanthin) showed differences between the two species. In both cases, feeding leaves with DTT inhibited the light-induced formation of zeaxanthin. InC. album this was accompanied by complete inhibition of reversible non-photochemical quenching, whereas inD. purpurea this inhibition was only partial. Data are discussed in relation to studies on the mechanism of quenching and the role of zeaxanthin in this process.     

Inhibitor studies on non-photochemical plastoquinone reduction and H(2) photoproduction in Chlamydomonas reinhardtii

In the absence of PSII, non-photochemical reduction of plastoquinones (PQs) occurs following NADH or NADPH addition in thylakoid membranes of the green alga Chlamydomonas reinhardtii. The nature of the enzyme involved in this reaction has been investigated in vitro by measuring chlorophyll fluorescence increase in anoxia and light-dependent O(2) uptake in the presence of methyl viologen. Based on the insensitivity of these reactions to rotenone, a type-I NADH dehydrogenase (NDH-1) inhibitor, and their sensitivity to flavoenzyme inhibitors and thiol blocking agents, we conclude to the involvement of a type-II NADH dehydrogenase (NDH-2) in PQ reduction. Intact Chlamydomonas cells placed in anoxia have the property to produce H(2) in the light by a Fe-hydrogenase which uses reduced ferredoxin as an electron donor. H(2) production also occurs in the absence of PSII thanks to the existence of a non-photochemical pathway of PQ reduction. From inhibitors effects, we suggest the involvement of a plastidial NDH-2 in PSII-independent H(2) production in Chlamydomonas. These results are discussed in relation to the absence of ndh genes in Chlamydomonas plastid genome and to the existence of 7 ORFs homologous to type-II NDHs in its nuclear genome.     

The relationship between non-photochemical quenching of chlorophyll fluorescence and the rate of photosystem 2 photochemistry in leaves

It has been suggested previously that non-photochemical quenching of chlorophyll fluorescence is associated with a decrease in the rate of photosystem 2 (PS 2) photochemistry. In this study analyses of fluorescence yield changes, induced by flashes in leaves exhibiting different amounts of non-photochemical quenching of fluorescence, are made to determine the effect of non-photochemical excitation energy quenching processes on the rate of PS 2 photochemistry. It is demonstrated that both the high-energy state and the more slowly relaxing components of non-photochemical quenching reduce the rate of PS 2 photochemistry. Flash dosage response curves for fluorescence yield show that non-photochemical quenching processes effectively decrease the relative effective absorption cross-section for PS 2 photochemistry. It is suggested that non-photochemical quenching processes exert an effect on the rate of PS 2 photochemistry by increasing the dissipation of excitation energy by non-radiative processes in the pigment matrices of PS 2, which consequently results in a decrease in the efficiency of delivery of excitation energy for PS 2 photochemistry.     

Continuous recording of photochemical and non-photochemical chlorophyll fluorescence quenching with a new type of modulation fluorometer

A newly developed fluorescence measuring system is employed for the recording of chlorophyll fluorescence induction kinetics (Kautsky-effect) and for the continuous determination of the photochemical and non-photochemical components of fluorescence quenching. The measuring system, which is based on a pulse modulation principle, selectively monitors the fluorescence yield of a weak measuring beam and is not affected even by extremely high intensities of actinic light. By repetitive application of short light pulses of saturating intensity, the fluorescence yield at complete suppression of photochemical quenching is repetitively recorded, allowing the determination of continuous plots of photochemical quenching and non-photochemical quenching. Such plots are compared with the time courses of variable fluorescence at different intensities of actinic illumination. The differences between the observed kinetics are discussed. It is shown that the modulation fluorometer, in combination with the application of saturating light pulses, provides essential information beyond that obtained with conventional chlorophyll fluorometers.     

Inhibitor studies on non-photochemical plastoquinone reduction and H2 photoproduction in Chlamydomonas reinhardtii

In the absence of PSII, non-photochemical reduction of plastoquinones (PQs) occurs following NADH or NADPH addition in thylakoid membranes of the green alga Chlamydomonas reinhardtii. The nature of the enzyme involved in this reaction has been investigated in vitro by measuring chlorophyll fluorescence increase in anoxia and light-dependent O₂ uptake in the presence of methyl viologen. Based on the insensitivity of these reactions to rotenone, a type-I NADH dehydrogenase (NDH-1) inhibitor, and their sensitivity to flavoenzyme inhibitors and thiol blocking agents, we conclude to the involvement of a type-II NADH dehydrogenase (NDH-2) in PQ reduction. Intact Chlamydomonas cells placed in anoxia have the property to produce H₂ in the light by a Fe-hydrogenase which uses reduced ferredoxin as an electron donor. H₂ production also occurs in the absence of PSII thanks to the existence of a non-photochemical pathway of PQ reduction. From inhibitors effects, we suggest the involvement of a plastidial NDH-2 in PSII-independent H₂ production in Chlamydomonas. These results are discussed in relation to the absence of ndh genes in Chlamydomonas plastid genome and to the existence of 7 ORFs homologous to type-II NDHs in its nuclear genome.     

Theoretical assessment of alternative mechanisms for non-photochemical quenching of PS II fluorescence in barley leaves

The components of non-photochemical chlorophyll fluorescence quenching (qN) in barley leaves have been quantified by a combination of relaxation kinetics analysis and 77 K fluorescence measurements (Walters RG and Horton P 1991). Analysis of the behaviour of chlorophyll fluorescence parameters and oxygen evolution at low light (when only state transitions — measured as qN_t — are present) and at high light (when only photoinhibition — measured as qN_i — is increasing) showed that the parameter qN_t represents quenching processes located in the antenna and that qN_i measures quenching processes located in the reaction centre but which operate significantly only when those centres are closed. The theoretical predictions of a variety of models describing possible mechanisms for high-energy-state quenching, measured as the residual quenching, qN_e, were then tested against the experimental data for both fluorescence quenching and quantum yield of oxygen evolution. Only one model was found to agree with these data, one in which antennae exist in two states, efficient in either energy transfer or energy dissipation, and in which those photosynthetic units in a dissipative state are unable to exchange energy with non-dissipative units.Abbreviations: F_o, F_m room-temperature chlorophyll fluorescence yield with all centres open, closed - F_v variable fluorescence yield - LHC II light-harvesting chlorophyll-protein complex of PS II - PS I, PS II Photosystem I, II - P700, P680 primary donor in Photosystem I, II - Q_A primary electron acceptor of PS II - P_max maximum quantum yield of oxygen evolution - qN coefficient of non-photochemical quenching of variable fluorescence - qN_e, qN_t, qN_i coefficient of non-photochemical quenching due to high-energy-state, state transition, photoinhibition - qO coefficient of quenching of dark level fluorescence - qP coefficient of photochemical quenching of variable fluorescence - _P intrinsic quantum yield of open PS II reaction centres = _s/qP - _{PS 2} quantum yield of PS = qP × F_v/F_m - _S quantum yield of oxygen evolution = rate of oxygen evolution/light intensity     

Characterization of the photosystem II centers inactive in plastoquinone reduction by fluorescence induction

In order to characterize the photosystem II (PS II) centers which are inactive in plastoquinone reduction, the initial variable fluorescence rise from the non-variable fluorescence level F_o to an intermediate plateau level F_i has been studied. We find that the initial fluorescence rise is a monophasic exponential function of time. Its rate constant is similar to the initial rate of the fastest phase (-phase) of the fluorescence induction curve from DCMU-poisoned chloroplasts. In addition, the initial fluorescence rise and the -phase have the following common properties: their rate constants vary linearly with excitation light intensity and their fluorescence yields are lowered by removal of Mg⁺⁺ from the suspension medium. We suggest that the inactive PS II centers, which give rise to the fluorescence rise from F_o to F_i, belong to the -type PS II centers. However, since these inactive centers do not display sigmoidicity in fluorescence, they thus do not allow energy transfer between PS II units like PS II.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - DMQ 2,5-dimethyl-p-benzoquinone - F_o initial non-variable fluorescence yield - F_m maximum fluorescence yield - F_i intermediate fluorescence yield - PS II photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II     

Determination of the light pathlength elongation in leaves by measuring P700 quantitatively

P700 absorption change signals were measured at 800 nm for plant leaves and plant leaf extracts by illuminating with far-red light. The ratio of the signal amplitudes for the two sample types shows the degree of optical signal intensification. The intensification is due to optical path-length elongation, itself caused by scattering. The intensification was found to be between 6 and 15 for different leaves. For spinach leaves, the intensification varied less than 10% among samples. By replacing the air in the spongy layer of a leaf with an isotonic solution it was shown that nearly half of the intensification is due to the scattering at air-tissue interfaces. The comparison of the P700 signals of a leaf with its extract would seem to be a new technique for determining the optical scattering effects of leaves.     

Control of misses in oxygen evolution by the oxido-reduction state of plastoquinone in Dunaliella tertiolecta

The way misses happen in oxygen evolution is subject to debate (Govindjee et al. 1985). We recently observed a linear lowering of the miss probability with the flash number (Meunier and Popovic 1989). Therefore, we investigated in Dunaliella tertiolecta the link between the average miss probability and the redox state of plastoquinone after n flashes. The effect of flashes was to oxidize the plastoquinone pool; we found that the oxidation of plastoquinone highly correlated (linear regression: R ²=0.996) with the lowering of the miss probability. The flash frequency was found to affect both the miss probability and the redox state of plastoquinone. When pre-flashes were given using a high flash frequency (10 Hz), the plastoquinone pool was oxidized and misses were low; however, if long dark intervals between flashes were used, the oxidizing effect of flashes was lost and the misses were high. We could not explain our results by assuming equal misses over all S-states; but unequal misses, caused by deactivations, were coherent with our results. We deduced that chlororespiration was responsible for the reduction of plastoquinone in the dark interval between flashes. We compared oxygen evolution with and without benzoquinone, using a low flash frequency (0.5 Hz) for maximum misses. Benzoquinone lowered the misses from 34% to 3%, and raised the amplitude of oxygen evolution by more than a factor of two (2). From this we deduced that the charge carrier C postulated to explain misses (Lavorel and Maison-Peteri 1983) did not account for more than 3% of miss probability in Dunaliella tertiolecta. These results indicate that the misses in oxygen evolution are controlled by the redox state of plastoquinone, through deactivations.     

Photoinhibition of photosynthesis in intact willow leaves in response to moderate changes in light and temperature

When willow leaves were transferred from 270 to 650 μmol m^-2 s^-1 photosynthetic photon flux density (PPFD), partial photoinhibition developed over the next hours. This was manifested as roughly parallel inhibitions of the ratio of variable over maximal chlorophyll fluorescence (F_v/F_M), and of the maximal quantum yield and the capacity of photosynthesis. This occurred even though photosynthesis was operating well below its capacity and only about one fourth of the reaction centres of photosystem (PS) II were in the closed state. When the air temperature was lowered from 25 to 15°C (18°C leaf temperature) photoinhibition was markedly accelerated. This temperature effect is suggested to be mediated largely by a decrease in the rate of energy dissipation through photosynthesis and indicated by a 50% increase in the number of closed PSII reaction centres. The pool size of the carotcnoid zeaxanthin and the extent of inhibition of the F_v/F_M ratio were positively correlated during the treatment. However, the relaxation following imposition of darkness was much faster for zeaxanthin than for the F_v/F_M ratio, ruling out the possibility of a direct causal relationship. The energy distribution between PSII and PSI was unaltered upon photoinhibition. However, the functioning of the PSII reaction centres was altered, as indicated by a rise in the minimal fluorescence, Fa.     

Diurnal changes in the photochemical efficiency of the symbiotic dinoflagellates (Dinophyceae) of corals: photoprotection, photoinactivation and the relationship to coral bleaching

The photochemical efficiency of symbiotic dinoflagellates within the tissues of two reef‐building corals in response to normal and excess irradiance at water temperatures < 30 °C were investigated using pulse amplitude modulated (PAM) chlorophyll fluorescence techniques. Dark‐adapted F_v/F_m showed clear diurnal changes, decreasing to a low at solar noon and increasing in the afternoon. However, F_v/F_m also drifted downwards at night or in prolonged darkness, and increased rapidly during the early morning twilight. This parameter also increased when the oxygen concentration of the water holding the corals was increased. Such changes have not been described previously, and most probably reflect state transitions associated with PQ pool reduction via chlororespiration. These unusual characteristics may be a feature of an endosymbiotic environment, reflective of the well‐documented night‐time tissue hypoxia that occurs in corals. F_v/F_m decreased to 0·25 in response to full sunlight in shade‐acclimated (shade) colonies of Stylophora pistillata, which is considerably lower than in light‐acclimated (sun) colonies. In sun colonies, the reversible decrease in F_v/F_m was caused by a lowering of F_m and F_o suggesting photoprotection and no lasting damage. The decrease in F_v/F_m, however, was caused by a decrease in F_m and an increase in F_o in shade colonies suggesting photoinactivation and long‐term cumulative photoinhibition. Shade colonies rapidly lost their symbiotic algae (bleached) during exposure to full sunlight. This study is consistent with the hypothesis that excess light leads to chronic damage of symbiotic dinoflagellates and their eventual removal from reef‐building corals. It is significant that this can occur with high light conditions alone.     

Nigericin insensitive post-illumination reduction in fluorescence yield in Dunaliella tertiolecta (chlorophyte)

Cells of the green alga Dunaliella tertiolecta grown in a light/dark cycle were exposed to high light for about 15 min. In light, energy-dependent quenching reduced fluorescence emission and decreased PS II efficiency. Within 3 minutes after darkening fluorescence quenching largely relaxed. However, PS II fluorescence emission decreased again after further darkening. F_o and F_m decreased to the same relative extent and the PS II efficiency was not reduced. This Reduction in Fluorescence yield in Darkness, termed RFD for the purpose of this paper, lasted about 20 min. The deepoxidation state of xanthophylls remained unchanged during and after the 15-min exposure to high light. We show that RFD is insensitive to the uncoupler nigericin and thus unrelated to energy-dependent quenching. RFD correlated with a reduction of the PQ pool after darkening and low levels of far red or blue light (430 nm more than 460 nm) prevented RFD. This is in contrast to observations in higher plants, where a post-illumination reduction of the PQ pool causes and increase in F_o (Groom et al. (1993) Photosynth Res 36: 205–215). Changes in the adenylate energy charge were not correlated with RFD. Antimycin A and cyanide, both inhibitors of the PQ-oxidase, caused an increase in RFD whereas SHAM, an inhibitor of the chloroplastic glycolate-quinone oxidoreductase, caused a decrease. Low CO₂ concentrations, known to increase the oxygenase activity of Rubisco and to generate glycolate and P-glycolate in light, caused an increase in RFD. We propose that accumulated glycolate and P-glycolate reduce the PQ pool in darkness, leading to the formation of RFD. During RFD, 77 K fluorescence emission from PS II was more reduced than that from PS I, thus resembling a state I, state II transition. However, the reduction in fluorescence yield during RFD is much larger than the reduction previously attributed to state transitions and it is unclear whether RFD and state transitions are identical. The formation and relaxation of RFD increased with higher temperatures and the extent of RFD was largest at the growth temperature (25°C). RFD has to be taken into account when fluorescence is measured after darkening as it may be mistaken for energy-dependent quenching.Abbreviations F_o fluorescence, measured when PS II traps are open - F_o difference between F_o and F_o - F_m fluorescence, measured when PS II traps are temporarily closed - F_m difference between F_m and F_m - FR far red - PFD photosynthetically active photon flux density - PQ plastoquinone - RFD reduction in fluorescence in darkness - SHAM salicylhydroxamic acid - Q_A primary quinone acceptor of PS II     

Temperature dependence of violaxanthin de-epoxidation and non-photochemical fluorescence quenching in intact leaves ofGossypium hirsutum L. andMalva parviflora L.

The temperature dependence of the rate of de-epoxidation of violaxanthin to zeaxanthin was determined in leaves of chilling-sensitive Gossypium hirsutum L. (cotton) and chilling-resistant Malva parviflora L. by measurements of the increase in absorbance at 505 nm (A ₅₀₅) and in the contents of antheraxanthin and zeaxanthin that occur upon exposure of predarkened leaves to excessive light. A linear relationship between A ₅₀₅ and the decrease in the epoxidation state of the xanthophyll-cycle pigment pool was obtained over the range 10–40° C. The maximal rate of de-epoxidation was strongly temperature dependent; Q₁₀ measured around the temperature at which the leaf had developed was 2.1–2.3 in both species. In field-grown Malva the rate of de-epoxidation at any given measurement temperature was two to three times higher in leaves developed at a relatively low temperature in the early spring than in those developed in summer. Q₁₀ measured around 15° C was in the range 2.2–2.6 in both kinds of Malva leaves, whereas it was as high as 4.6 in cotton leaves developed at a daytime temperature of 30° C. Whereas the maximum (initial) rate of de-epoxidation showed a strong decrease with decreased temperature the degree of de-epoxidation reached in cotton leaves after a 1–2 · h exposure to a constant photon flux density increased with decreased temperature as the rate of photosynthesis decrease. The zeaxanthin content rose from 2 mmol · (mol chlorophyll)^–1 at 30° C to 61 mmol · (mol Chl)^–1 at 10° C, corresponding to a de-epoxidation of 70% of the violaxanthin pool at 10° C. The degree of de-epoxidation at each temperature was clearly related to the amount of excessive light present at that temperature. The relationship between non-photochemical quenching of chlorophyll fluorescence and zeaxanthin formation at different temperatures was determined for both untreated control leaves and for leaves in which zeaxanthin formation was prevented by dithiothreitol treatment. The rate of development of that portion of non-photochemical quenching which was inhibited by dithiothreitol decreased with decreasing temperature and was linearly related to the rate of zeaxanthin formation over a wide temperature range. In contrast, the rate of development of the dithiothreitol-resistant portion of non-photochemical quenching was remarkably little affected by temperature. Evidently, the kinetics of the development of non-photochemical quenching upon exposure of leaves to excessive light is therefore in large part determined by the rate of zeaxanthin formation. For reasons that remain to be determined the relaxation of dithiothreitolsensitive quenching that is normally observed upon darkening of illuminated leaves was strongly inhibited at low temperatures.Abbreviations and Symbols Chl chlorophyll - DTT dithiothreitol - EPS epoxidation state - NPQ non-photochemical chlorophyll fluorescence quenching - PFD photon flux density - PSII photosystem II - F, F_m fluorescence emission at the actual, full closure of the PSII centers C.I.W.-D.P.B. Publication No. 1092We thank Connie Shih for skillful assistance in growing the plants, for conducting the HPLC analyses, and for preparing the figures. A Carnegie Institution Fellowship and a Feodor-Lynen-Fellowship by the Alexander von Humboldt-Foundation to W.B. is gratefully acknowledged. This work was supported by Grant No. 89-37-280-4902 of the Competitive Grants Program of the U.S. Department of Agriculture to O.B.     

Compared thermoluminescence characteristics of pea thylakoids studied in vitro and in situ (in leaves). The effect of photoinhibitory treatments

Characteristics of thermoluminescence (TL) glow curves were studied in thylakoids (isolated from pea leaves) or in intact pea leaves after an exposure to very high light for 2 min in the TL device. The inhibition of photosynthesis was detected as decreases of oxygen evolution rates and/or of variable fluorescence.In thylakoids exposed to high light, then dark adapted for 5 min, a flash regime induced TL glow curves which can be interpreted as corresponding to special B bands since: 1) they can be fitted by a single B band (leaving a residual band at –5°C) with a lower activation energy and a shift of the peak maximum by –5 to –6°C and, 2) the pattern of oscillation of their amplitudes was normal with a period of 4 and maxima on flashes 2 and 6. During a 1 h dark adaptation, no recovery of PS II activity occurred but the shift of the peak maximum was decreased to –1 to –2°C, while the activation energy of B bands increased. It is supposed that centers which remained active after the photoinhibitory treatment were subjected to reversible and probably conformational changes.Conversely, in intact leaves exposed to high light and kept only some minutes in the dark, TL bands induced by a flash regime were composite and could be deconvoluted into a special B band peaking near 30°C and a complex band with maximum at 2–5°C. In the case of charging bands by one flash, this low temperature band was largely decreased in size after a 10 min dark adaptation period; parallely, an increase of the B band type component appeared. Whatever was the flash number, bands at 2–5°C were suppressed by a short far red illumination given during the dark adaptation period and only remained a main band a 20°C; therefore, the origin of the low temperature band was tentatively ascribed to recombinations in centers blocked in state S₂Q_A ^–Q_B ^2–. In vivo, the recovery of a moderately reduced state in the PQ pool, after an illumination, would be slow and under the dependence of a poising mechanism, probably involving an electron transfer between cytosol and chloroplasts or the so-called chlororespiration process.Abbreviations E_a- activation energy - FR- far-red - MV- methylviologen - pBQ- p-benzoquinone - PQ- plastoquinone - PS II- Photosystem II - Q_A- primary quinone electron acceptor of PS II - Q_B- secondary quinone electron acceptor of PS II - TL- thermoluminescence     

Mg2+ accelerates plastoquinone reduction in chloroplasts

The transient fluorescence quenching induced by the addition of a small amount of an oxidant to illuminated chloroplasts can be used to estimate the rate of electron transported by the oxidant. Using this technique, it is found that the reduction of plastoquinone by the primary acceptor of Photosystem II is sensitive to salt depletion.