An active Mehler-peroxidase reaction sequence can prevent cyclic PS I electron transport in the presence of dioxygen in intact spinach chloroplasts

Higher Reduced State of Fe/S-Signals, with the Suppressed Oxidation of P700, Causes PSI Inactivation in Arabidopsis thaliana

Environmental stress increases the risk of electron accumulation in photosystem I (PSI) of chloroplasts, which can cause oxygen (O₂) reduction to superoxide radicals and decreased photosynthetic ability. We used three Arabidopsis thaliana lines: wild-type (WT) and the mutants pgr5^hope1 and paa1-7/pox1. These lines have different reduced states of iron/sulfur (Fe/S) signals, including F_x, F_A/F_B, and ferredoxin, the electron carriers at the acceptor side of PSI. In the dark, short-pulse light was repetitively illuminated to the intact leaves of the plants to provide electrons to the acceptor side of PSI. WT and pgr5^hope1 plants showed full reductions of Fe/S during short-pulse light and PSI inactivation. In contrast, paa1-7/pox1 showed less reduction of Fe/S and its PSI was not inactivated. Under continuous actinic-light illumination, pgr5^hope1 showed no P700 oxidation with higher Fe/S reduction due to the loss of photosynthesis control and PSI inactivation. These results indicate that the accumulation of electrons at the acceptor side of PSI may trigger the production of superoxide radicals. P700 oxidation, responsible for the robustness of photosynthetic organisms, participates in reactive oxygen species suppression by oxidizing the acceptor side of PSI.

Redox changes of ferredoxin, P700, and plastocyanin measured simultaneously in intact leaves

Properties and performance of the recently introduced Dual/KLAS-NIR spectrophotometer for simultaneous measurements of ferredoxin (Fd), P700, and plastocyanin (PC) redox changes, together with whole leaf chlorophyll a (Chl) fluorescence (emission >760, 540 nm excitation) are outlined. Spectral information on in vivo Fd, P700, and PC in the near-infrared region (NIR, 780-1000 nm) is presented, on which the new approach is based. Examples of application focus on dark-light and light-dark transitions, where maximal redox changes of Fd occur. After dark-adaptation, Fd reduction induced by moderate light parallels the Kautsky effect of Chl fluorescence induction. Both signals are affected analogously by removal of O₂. A rapid type of Fd reoxidation, observed after a short pulse of light before light activation of linear electron transport (LET), is more pronounced in C4 compared to C3 leaves and interpreted to reflect cyclic PS I (CET). Light activation of LET, as assessed via the rate of Fd reoxidation after short light pulses, occurs at very low intensities and is slowly reversed (half-time ca. 20 min). Illumination with strong far-red light (FR, 740 nm) reveals two fractions of PS I, PS I (LET), and PS I (CET), differing in the rates of Fd reoxidation upon FR-off and the apparent equilibrium constants between P700 and PC. Parallel information on oxidation of Fd and reduction of P700 plus PC proves essential for identification of CET. Comparison of maize (C4) with sunflower and ivy (C3) responses leads to the conclusion that segregation of two types of PS I may not only exist in C4 (mesophyll and bundle sheath cells), but also in C3 photosynthesis (grana margins plus end membranes and stroma lamellae).

Chloroplastic ATP synthase builds up a proton motive force preventing production of reactive oxygen species in photosystem I

Over-reduction of the photosynthetic electron transport (PET) chain should be avoided, because the accumulation of reducing electron carriers produces reactive oxygen species (ROS) within photosystem I (PSI) in thylakoid membranes and causes oxidative damage to chloroplasts. To prevent production of ROS in thylakoid membranes the H⁺ gradient (ΔpH) needs to be built up across the thylakoid membranes to suppress the over-reduction state of the PET chain. In this study, we aimed to identify the critical component that stimulates ΔpH formation under illumination in higher plants. To do this, we screened ethyl methane sulfonate (EMS)-treated Arabidopsis thaliana, in which the formation of ΔpH is impaired and the PET chain caused over-reduction under illumination. Subsequently, we isolated an allelic mutant that carries a missense mutation in the γ-subunit of chloroplastic CF₀ CF₁ -ATP synthase, named hope2. We found that hope2 suppressed the formation of ΔpH during photosynthesis because of the high H⁺ efflux activity from the lumenal to stromal side of the thylakoid membranes via CF₀ CF₁ -ATP synthase. Furthermore, PSI was in a more reduced state in hope2 than in wild-type (WT) plants, and hope2 was more vulnerable to PSI photoinhibition than WT under illumination. These results suggested that chloroplastic CF₀ CF₁ -ATP synthase adjusts the redox state of the PET chain, especially for PSI, by modulating H⁺ efflux activity across the thylakoid membranes. Our findings suggest the importance of the buildup of ΔpH depending on CF₀ CF₁ -ATP synthase to adjust the redox state of the reaction center chlorophyll P700 in PSI and to suppress the production of ROS in PSI during photosynthesis.

Photorespiration provides the chance of cyclic electron flow to operate for the redox-regulation of P700 in photosynthetic electron transport system of sunflower leaves

To elucidate the molecular mechanism to oxidize the reaction center chlorophyll, P700, in PSI, we researched the effects of partial pressure of O2 (pO2) on photosynthetic characteristic parameters in sunflower (Helianthus annuus L.) leaves. Under low CO2 conditions, the oxidation of P700 was stimulated; however the decrease in pO2 suppressed its oxidation. Electron fluxes in PSII [Y(II)] and PSI [Y(I)] showed pO2-dependence at low CO2 conditions. H(+)-consumption rate, estimated from Y(II) and CO2-fixation/photorespiration rates (JgH(+)), showed the positive curvature relationship with the dissipation rate of electrochromic shift signal (V H (+) ), which indicates H(+)-efflux rate from lumen to stroma in chloroplasts. Therefore, these electron fluxes contained, besides CO2-fixation/photorespiration-dependent electron fluxes, non-H(+)-consumption electron fluxes including Mehler-ascorbate peroxidase (MAP)-pathway. Y(I) that was larger than Y(II) surely implies the functioning of cyclic electron flow (CEF). Both MAP-pathway and CEF were suppressed at lower pO2, with plastoquinone-pool reduced. That is, photorespiration prepares the redox-poise of photosynthetic electron transport system for CEF activity as an electron sink. Excess Y(II), [ΔY(II)] giving the curvature relationship with V H (+) , and excess Y(I) [ΔCEF] giving the difference between Y(I) and Y(II) were used as an indicator of MAP-pathway and CEF activity, respectively. Although ΔY(II) was negligible and did not show positive relationship to the oxidation-state of P700, ΔCEF showed positive linear relationship to the oxidation-state of P700. These facts indicate that CEF cooperatively with photorespiration regulates the redox-state of P700 to suppress the over-reduction in PSI under environmental stress conditions.

Post-illumination transient O2 -uptake is driven by photorespiration in tobacco leaves

This study aims to elucidate the molecular mechanism for the transient increase in the O₂ -uptake rate in tobacco (Nicotiana tabacum cv Xanthi) leaves after turning off actinic lights (ALs). The photosynthetic O₂ evolution rate reaches a maximum shortly after the onset of illumination with ALs and then decreases to zero in atmospheric CO₂ /O₂ conditions. After turning off the ALs, tobacco leaves show a transient increase in the O₂ -uptake rate, the post-illumination transient O₂ -uptake, and thereafter, the O₂ -uptake rate decreases to the level of the dark-respiration rate. Photosynthetic linear electron flow, evaluated as the quantum yield of photosystem II [Y(II)], maintained a steady-state value distinct from the photosynthetic O₂ -evolution rate. In high-[CO₂ ] conditions, the photosynthetic O₂ -evolution rate and Y(II) showed a parallel behavior, and the post-illumination transient O₂ -uptake was suppressed. On the other hand, in maize leaves (a C4 plant), even in atmospheric CO₂ /O₂ conditions, Y(II) paralleled the photosynthetic O₂ -evolution rate and the post-illumination transient O₂ -uptake was suppressed. Hypothesizing that the post-illumination transient O₂ -uptake is driven by C3 plant photorespiration in tobacco leaves, we calculated both the ribulose 1,5-bisphosphate carboxylase- and oxygenase-rates (Vc and Vo) from photosynthetic O₂ -evolution and the post-illumination transient O₂ -uptake rates. These values corresponded to those estimated from simultaneous chlorophyll fluorescence/O₂ -exchange analysis. Furthermore, the H⁺ -consumption rate for ATP synthesis in both photosynthesis and photorespiration, calculated from both Vc and Vo that were estimated from chlorophyll fluorescence/CO₂ -exchange analysis, showed a positive linear relationship with the dissipation rate of the electrochromic shift signal. Thus, these findings support our hypothesis.

Regulation of NAD(P)H dehydrogenase-dependent cyclic electron transport around PSI by NaHSO₃ at low concentrations in tobacco chloroplasts

Although bisulfite at low concentrations (L-NaHSO₃) has been found to increase the cyclic electron transport around PSI (CET), its regulative mechanism remains unknown. In this work, the role of L-NaHSO₃ (0.1-500 μM) in NAD(P)H dehydrogenase-dependent CET (the NDH pathway) was investigated. After treatment of tobacco leaves with L-NaHSO₃, the NDH pathway, as reflected by a transient post-illumination increase in Chl fluorescence, the dark reduction of P700+ after far-red light and the amount of NDH, was increased after the light-dark-light transition, but was slightly lowered under continuous light. Meanwhile, the linear electron transport (LET) was accelerated by L-NaHSO₃ under both the light regimes. Experiments in thylakoids further demonstrated that both LET, monitored by light-dependent oxygen uptake, and CET, as determined from the NADPH-dependent oxygen uptake and dark reduction of P700+, were enhanced by L-NaHSO₃ and the enhancements were abolished by superoxide dismutase. Furthermore, L-NaHSO₃-induced CET was partially impaired in thylakoids of the ΔndhCKJ mutant, while L-NaHSO₃-induced LET was not affected. Based on these results, we propose that the photooxidation of L-NaHSO₃ initiated by superoxide anions in PSI regulates NDH pathway to maintain efficient photosynthesis.

A non-invasive assay of the plastoquinone pool redox state based on the OJIP-transient

The plastoquinone (PQ) pool of the photosynthetic electron transport chain becomes reduced under anaerobic conditions. Here, anaerobiosis was used as a tool to manipulate the PQ-pool redox state in darkness and to study the effects of the PQ-redox state on the Chl-a fluorescence (OJIP) kinetics in pea leaves (Pisum sativum L.). It is shown that the F(J) (fluorescence intensity at 3 ms) is linearly related to the area above the OJ-phase (first 3 ms) representing the reduction of the acceptor side of photosystem II (PSII) and F(J) is also linearly related to the area above the JI-phase (3-30 ms) that parallels the reduction of the PQ-pool. This means that F(J) depends on the availability of oxidized PQ-molecules bound to the Q(B)-site. The linear relationships between F(J) and the two areas indicate that F(J) is not sensitive to energy transfer between PSII-antennae (connectivity). It is further shown that a approximately 94% reduced PQ-pool is in equilibrium with a approximately 19% reduction of Q(A) (primary quinone acceptor of PSII). The non-linear relationship between the initial fluorescence value (F(20 micros)) and the area above the OJ-phase supports the idea that F(20 mus )is sensitive to connectivity. This is reinforced by the observation that this non-linearity can be overcome by transforming the F(20 micros)-values into [Q(A) (-)]-values. Based on the F(J)-value of the OJIP-transient, a simple method for the quantification of the redox state of the PQ-pool is proposed.

A dip in the chlorophyll fluorescence induction at 0.2-2 s in Trebouxia-possessing lichens reflects a fast reoxidation of photosystem I. A comparison with higher plants

An unusual dip (compared to higher plant behaviour under comparable light conditions) in chlorophyll fluorescence induction (FI) at about 0.2-2 s was observed for thalli of several lichen species having Trebouxia species (the most common symbiotic green algae) as their native photobionts and for Trebouxia species cultured separately in nutrient solution. This dip appears after the usual O(J)IP transient at a wide range of excitation light intensities (100-1800 micromol photons m(-2) s(-1)). Simultaneous measurements of FI and 820-nm transmission kinetics (I(820)) with lichen thalli showed that the decreasing part of the fluorescence dip (0.2-0.4 s) is accompanied by a decrease of I(820), i.e., by a reoxidation of electron carriers at photosystem I (PSI), while the subsequent increasing part (0.4-2 s) of the dip is not paralleled by the change in I(820). These results were compared with that measured with pea leaves-representatives of higher plants. In pea, PSI started to reoxidize after 2-s excitation. The simultaneous measurements performed with thalli treated with methylviologen (MV), an efficient electron acceptor from PSI, revealed that the narrow P peak in FI of Trebouxia-possessing lichens (i.e., the I-P-dip phase) gradually disappeared with prolonged MV treatment. Thus, the P peak behaves in a similar way as in higher plants where it reflects a traffic jam of electrons induced by a transient block at the acceptor side of PSI. The increasing part of the dip in FI remained unaffected by the addition of MV. We have found that the fluorescence dip is insensitive to antimycin A, rotenone (inhibitors of cyclic electron flow around PSI), and propyl gallate (an inhibitor of plastid terminal oxidase). The 2-h treatment with 5 microM nigericin, an ionophore effectively dissipating the pH-gradient across the thylakoid membrane, did not lead to significant changes either in FI nor I(820) kinetics. On the basis of the presented results, we suggest that the decreasing part of the fluorescence dip in FI of Trebouxia-lichens reflects the activation of ferredoxin-NADP(+)-oxidoreductase or Mehler-peroxidase reaction leading to the fast reoxidation of electron carriers in thylakoid membranes. The increasing part of the dip probably reflects a transient reduction of plastoquinone (PQ) pool that is not associated with cyclic electron flow around PSI. Possible causes of this MV-insensitive PQ reduction are discussed.

Effect of CO2 supply on formation of reactive oxygen species in Arabidopsis thaliana

Light-induced generation of reactive oxygen species (ROS) in 2-week-old leaves of Arabidopsis thaliana was studied by means of the ROS-sensitive dyes nitroblue tetrazolium (NBT) and 5-(and-6)-carboxy-2',7'-dichlorodihydrofluorescein diacetate (DCF-DA). Superposition of pictures of chlorophyll fluorescence and DCF fluorescence indicated that the origin of ROS was in the chloroplasts. Experiments were done with zero, 0.1, or 10 mM NaHCO3 in the infiltration medium. Energy quenching in photosystem II was higher under low CO2 concentrations as measured by chlorophyll fluorescence. DCF fluorescence showed that CO2 deficiency led to an increase of ROS generation. In contrast, the photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea reduced the light-induced increase of DCF fluorescence. This indicates that ROS production does not primarily result from over-reduction of photosystem II as caused by impeding electron flow in the electron transfer chain. More likely, it is an effect of diverting electron flux normally aimed at carboxylation in the Calvin cycle to other sinks more prone to the generation of toxic radicals. There was no significant effect of salicyl hydroxamate (a blocker of the alternative oxidase), showing that the mitochondrial electron transfer chain seems to play a minor role as already indicated by the superposition of chlorophyll and DCF fluorescence.

Simultaneous measurement of deltapH and electron transport in chloroplast thylakoids by 9-aminoacridine fluorescence

Electron transport and the electrochemical proton gradient across the thylakoid membrane are two fundamental parameters of photosynthesis. A combination of the electron acceptor, ferricyanide and the DeltapH indicator, 9-aminoacridine, was used to measure simultaneously electron transport rates and DeltapH solely by changes in the fluorescence of 9-aminoacridine. This method yields values for the rate of electron transport that are comparable with those obtained by established methods. Using this method a relationship between the rate of electron transport and DeltapH at various uncoupler concentrations or light intensities was obtained. In addition, the method was used to study the effect of reducing the disulfide bridge in the gamma-subunit of the chloroplast ATP synthase on the relation of electron transport to DeltapH. When the ATP synthase is reduced and alkylated, the threshold DeltapH at which the ATP synthase becomes leaky to protons is lower compared with the oxidized enzyme. Proton flow through the enzyme at a lower DeltapH may be a key step in initiation of ATP synthesis in the reduced enzyme and may be the way by which reduction of the disulfide bridge in the gamma-subunit enables high rates of ATP synthesis at low DeltapH values.

Homeostasis of adenylate status during photosynthesis in a fluctuating environment

This review describes and assesses pathways likely to influence and stabilize the ATP/reductant balance during whole cell photosynthesis. The sole reductive step of the Calvin cycle occurs during the conversion of 3-phosphoglycerate to triose phosphate. Photophosphorylation linked to this reaction can undoubtedly supply most of the ATP required by the Calvin cycle and other chloroplastic reactions. Small but crucial contributions must come from several other pathways, some of which involve co-operation between the chloroplast and the rest of the cell. Extrachloroplastic compartments can contribute to chloroplastic ATP requirements by supplying ATP directly or, probably more significantly, by accepting reducing equivalents and so supporting ATP synthesis within the chloroplast.

State transitions, cyclic and linear electron transport and photophosphorylation in Chlamydomonas reinhardtii

The relationship between state transitions and the kinetic properties of the electron transfer chain has been studied in Chlamydomonas reinhardtii. The same turnover rate of cytochrome f was found in state 1 and 2. However, while DBMIB was inhibitory in both states, DCMU was effective only in state 1. These observations suggest that linear electron transport was active only in state 1, while a cyclic pathway around photosystem (PS) I operated in state 2. The reversible shift from linear to cyclic electron transport was modulated by changes of PSII antenna size, which inactivated the linear pathway, and by oxygen, which inhibited the cyclic one. Attainment of state 2, under anaerobiosis in the dark, was associated with the decline of the ATP/ADP ratio in the cells and the dark reduction of the intersystem carriers. Upon illumination of the cells, the ATP/ADP ratio increased in a few seconds to the aerobic level. Then, several minutes later, the F(m) returned to the state 1 level, and O(2) evolution was reactivated. This suggests that ATP, though required for photosynthesis, is not the rate-limiting factor in the reactivation of photosynthetic O(2) evolution, which is rather controlled by the redox state of the electron carriers.

Proton to electron stoichiometry in electron transport of spinach thylakoids

According to the concept of the Q-cycle, the H+/e- ratio of the electron transport chain of thylakoids can be raised from 2 to 3 by means of the rereduction of plastoquinone across the cytochrome b6f complex. In order to investigate the H+/e- ratio we compared stationary rates of electron transport and proton translocation in spinach thylakoids both in the presence of the artificial electron acceptor ferricyanide and in the presence of the natural acceptor system ferredoxin+NADP. The results may be summarised as follows: (1) a variability of the H+/e- ratio occurs with either acceptor. H+/e- ratios of 3 (or even higher in the case of the natural acceptor system, see below) are decreased towards 2 if strong light intensity and low membrane permeability are employed. Mechanistically this could be explained by proton channels connecting the plastoquinol binding site alternatively to the lumenal or stromal side of the cytochrome b6f complex, giving rise to a proton slip reaction at high transmembrane DeltapH. In this slip reaction protons are deposited on the stromal instead of the lumenal side. In addition to the pH effect there seems to be a contribution of the redox state of the plastoquinone pool to the control of proton translocation; switching over to stromal proton deposition is favoured when the reduced state of plastoquinone becomes dominant. (2) In the presence of NADP a competition of both NADP and oxygen for the electrons supplied by photosystem I takes place, inducing a general increase of the H+/e- ratios above the values obtained with ferricyanide. The implications with respect to the adjustment of a proper ATP/NADPH ratio for CO2 reduction are discussed.

Flux control of the malate valve in leaf cells

The coupled processes of the chloroplast trans-envelope transport of malate and oxaloacetate and their interconversion as catalyzed by the stromal NADP-linked malate dehydrogenase are quantitatively analyzed by means of a steady-state model. The equation for the NADP-malate dehydrogenase reaction is developed. The empirical dependence of enzyme activity on NADPH and NADP+ is used to determine its actual activity. The trans-envelope counter exchange of malate and oxaloacetate is described by a kinetic model of the translocator. Kinetic parameters are derived from known data, except for the Km value and the maximum rate for oxaloacetate transport, which are estimated from oxaloacetate-dependent malate formation in isolated intact chloroplasts. Using the kinetic properties of the system and the known metabolite concentrations, the model demonstrates that photosynthetically generated NADPH can be exported efficiently from the chloroplasts to the cytosol by the malate-valve system. The transfer capacity of the malate valve is estimated not to exceed 20 mumol (mg Chl)-1 h-1 (or 5% of the electron transport) under normal physiological conditions. The possible role of the malate valve in leaf cells under normal conditions and during stress is discussed.

Short-term responses of Photosystem I to heat stress : Induction of a PS II-independent electron transport through PS I fed by stromal components

When 23°C-grown potato leaves (Solanum tuberosum L.) were exposed for 15 min to elevated temperatures in weak light, a dramatic and preferential inactivation of Photosystem (PS) II was observed at temperatures higher than about 38°C. In vivo photoacoustic measurements indicated that, concomitantly with the loss of PS II activity, heat stress induced a marked gas-uptake activity both in far-red light (>715 nm) exciting only PS I and in broadband light (350-600 nm) exciting PS I and PS II. In view of its suppression by nitrogen gas and oxygen and its stimulation by high carbon-dioxide concentrations, the bulk of the photoacoustically measured gas uptake by heat-stressed leaves was ascribed to rapid carbon-dioxide solubilization in response to light-modulated stroma alkalization coupled to PS I-driven electron transport. Heat-induced gas uptake was observed to be insensitive to the PS II inhibitor diuron, sensitive to the plastocyanin inhibitor HgCl2 and saturated at a rather high photon flux density of around 1200 μE m(-2) s(-1). Upon transition from far-red light to darkness, the oxidized reaction center P700(+) of PS I was re-reduced very slowly in control leaves (with a half time t1/2 higher than 500 ms), as measured by leaf absorbance changes at around 820 nm. Heat stress caused a spectacular acceleration of the postillumination P700(+) reduction, with t1/2 falling to a value lower than 50 ms (after leaf exposure to 48°C). The decreased t1/2 was sensitive to HgCl2 and insensitive to diuron, methyl viologen (an electron acceptor of PS I competing with the endogenous acceptor ferredoxin) and anaerobiosis. This acceleration of the P700(+) reduction was very rapidly induced by heat treatment (within less than 5 min) and persisted even after prolonged irradiation of the leaves with far-red light. After heat stress, the plastoquinone pool exhibited reduction in darkness as indicated by the increase in the apparent Fo level of chlorophyll fluorescence which could be quenched by far-red light. Application (for 1 min) of far-red light to heat-pretreated leaves also induced a reversible quenching of the maximal fluorescence level Fm, suggesting formation of a pH gradient in far-red light. Taken together, the presented data indicate that PS I responded to the heat-induced loss of PS II photochemical activity by catalyzing an electron flow from stromal reductants. Heat-stress-induced PS I electron transport independent of PS II seems to constitute a protective mechanism since block of this electron pathway in anaerobiosis was observed to result in a dramatic photoinactivation of PS I.

Coupled cyclic electron transport in intact chloroplasts and leaves of C3 plants: Does it exist? if so, what is its function?

Transthylakoid proton transport based on Photosystem I-dependent cyclic electron transport has been demonstrated in isolated intact spinach chloroplasts already at very low photon flux densities when the acceptor side of Photosystem I (PS I) was largely closed. It was under strict redox control. In spinach leaves, high intensity flashes given every 50 s on top of far-red, but not on top of red background light decreased the activity of Photosystem II (PS II) in the absence of appreciable linear electron transport even when excitation of PS II by the background light was extremely weak. Downregulation of PS II was a consequence of cyclic electron transport as shown by differences in the redox state of P700 in the absence and the presence of CO2 which drained electrons from the cyclic pathway eliminating control of PS II. In the presence of CO2, cyclic electron transport comes into play only at higher photon flux densities. At H(+)/e=3 in linear electron transport, it does not appear to contribute much ATP for carbon reduction in C3 plants. Rather, its function is to control the activity of PS II. Control is necessary to prevent excessive reduction of the electron transport chain. This helps to protect the photosynthetic apparatus of leaves against photoinactivation under light stress.

Inhibition by ethoxyzolamide of a photoacoustic uptake signal in leaves: Evidence for carbonic anhydrase catalyzed CO2-solubilisation

A photoacoustic pulse-modulation technique is applied for the study of a CO2-stimulated gas uptake signal in leaves (Reising and Schreiber, Photosynthe Res 31: 227-238, 1992). It is shown that this uptake signal can be substantially suppressed by application of the carbonic anhydrase inhibitor, ethoxyzolamide, to leaf discs. This inhibitor does not affect the O2-evolution signal in air or the chlorophyll fluorescence induction pattern at high CO2, when non-saturating light intensities are used. On the basis of these findings it is concluded that at least a major part of the CO2-stimulated photoacoustic uptake signal results from light-modulated CO2-solubilisation catalysed by carbonic anhydrase. Modulated CO2-solubilisation appears likely to be induced by light driven H(+)-translocation from the stroma into the thylakoid lumen. Comparison of the induction patterns of chlorophyll fluorescence quenching and the uptake signal suggests a correlation between membrane energisation and CO2-uptake. The importance of O2-dependent electron flow as a major cause of membrane energisation is discussed. It is proposed that in the absence of CO2 the combination of Mehler- and ascorbate peroxidase reactions does not result in a photobaric signal, as O2-uptake and O2-evolution components cancel each other. Two main conclusions, which are of considerable importance for future practical applications of the photoacoustic method, are drawn from these findings: (1) When high CO2 is applied to leaves, the photobaric uptake component may provide a unique means of monitoring the function of stromal carbonic anhydrase in vivo. (2) Brief flushing of the photoacoustic cell with air may prevent the occurrence of an uptake signal, thus allowing a straight-forward deconvolution into photothermal and O2-evolution components.

Rates of vectorial proton transport supported by cyclic electron flow during oxygen reduction by illuminated intact chloroplasts

The light-dependent quenching of 9-aminoacridine fluorescence was used to monitor the state of the transthylakoid proton gradient in illuminated intact chloroplasts in the presence or absence of external electron acceptors. The absence of appreciable light-dependent fluorescence quenching under anaerobic conditions indicated inhibition of coupled electron transport in the absence of external electron acceptors. Oxygen relieved this inhibition. However, when DCMU inhibited excessive reduction of the plastoquinone pool in the absence of oxygen, coupled cyclic electron transport supported the formation of a transthylakoid proton gradient even under anaerobiosis. This proton gradient collapsed in the presence of oxygen. Under aerobic conditions, and when KCN inhibited ribulose bisphosphate carboxylase and ascorbate peroxidase, fluorescence quenching indicated the formation of a transthylakoid proton gradient which was larger with oxygen in the Mehler reaction as electron acceptor than with methylviologen at similar rates of linear electron transport. Apparently, cyclic electron transport occured simultaneously with linear electron transport, when oxygen was available as electron acceptor, but not when methylviologen accepted electrons from Photosystem I. The ratio of cyclic to linear electron transport could be increased by low concentrations of DCMU. This shows that even under aerobic conditions cyclic electron transport is limited in isolated intact chloroplasts by excessive reduction of electron carriers. In fact, P700 in the reaction center of Photosystem I remained reduced in illuminated isolated chloroplasts under conditions which resulted in extensive oxidation of P700 in leaves. This shows that regulation of Photosystem II activity is less effective in isolated chloroplasts than in leaves. Assuming that a Q-cycle supports a H(+)/e ratio of 3 during slow linear electron transport, vectorial proton transport coupled to Photosystem I-dependent cyclic electron flow could be calculated. The highest calculated rate of Photosystem I-dependent proton transport, which was not yet light-saturated, was 330 μmol protons (mg chlorophyll h)(-1) in intact chloroplasts. If H(+)/e is not three but two proton transfer is not 330 but 220 μmol (mg Chl H)(-1). Differences in the regulation of cyclic electron transport in isolated chloroplasts and in leaves are discussed.