Photoinhibition and light-induced cyclic electron transport in ndhB(-) and psaE(-) mutants of Synechocystis sp. PCC 6803

The ndhB^– and psaE^– mutants of the cyanobacteriumSynechocystis sp. PCC 6803 are partly deficient in PSI-drivencyclic electron transport. We compared photoinhibition in thesemutants to the wild type to test the hypothesis that PSI cyclicelectron transport protects against photoinhibition. Photoinhibitorytreatment greatly accelerated PSI cyclic electron transportin the wild type and also in both the mutants. The psaE^–mutant showed rates of PSI cyclic electron transport similarto the wild type under all conditions tested. The ndhB^–mutant showed much lower rates of PSI cyclic electron transportthan the wild type following brief dark adaptation but exceededwild type rates after exposure to photoinhibitory light. Thewild type and both mutants showed similar rates of photoinhibitiondamage and photoinhibition repair at PSII. Photoinhibition atPSI was much slower than at PSII and was also similar betweenthe wild type and both mutants, despite the known instabilityof PSI in the psaE^– mutant. We conclude that photoinhibitorylight induces sufficient PSI-driven cyclic electron transportin both the ndhB^– and psaE^– mutants to fulfill anyrole that cyclic electron transport plays in protection againstphotoinhibition. ⁴ Corresponding author: E-mail, sherbert@uwyo.edu; Fax, +1-307-766-2851;Phone, +1-307-766-4353.     

Photo-inhibition and the Effects of Protein Phosphorylation on Electron Transport in Wheat Thylakoids

The kinetics of changes in photosystem I (PSI), photosystemII (PSII), and whole chain (PSII and PSI) electron transport,chlorophyll fluorescence parameters, the capacity to bind atrazineand the polypeptide profiles of thylakoids isolated from wheatleaves on exposure to a photon flux density of 2000 µmolm^–2 s^–1 were determined. Severe and similar levelsof photo-inhibitory damage to both PSII and whole chain electrontransport occurred and were correlated with decreases in theratio of variable to maximal fluorescence, the proportionalcontribution of the rapid a phase of the fluorescence kineticsand the capacity to bind atrazine. Severe photo-inhibition ofelectron transport was not associated with a major loss of chlorophyllor total thylakoid protein. However, a small decrease in a 70kDa polypeptide together with increases in a number of low molecularmass polypeptides (8–24 kDa) occurred. Phosphorylation of thylakoid polypeptides alleviated photo-inhibitionof PSII electron transport but stimulated photoinhibitory damageto whole chain electron transport. The consequences of suchphosphorylation-induced effects on photoinhibition in vivo areconsidered. Key words: Chlorophyll fluorescence, electron transport, photo-inhibition, protein phosphorylation, thylakoid membranes, wheat (Triticum aestivum)     

Photoinhibition of Photosystem I: Its Physiological Significance in the Chilling Sensitivity of Plants

Photoinhibition was defined originally as the decrease in photosyntheticactivity that occurs upon excess illumination. The site of photoinhibitionhas generally been considered to be located in PSII. However,a novel type of photoinhibition has recently been characterizedin chillingsensitive plants. This photoinhibition occurs underrelatively weak illumination at chilling temperatures and themain site of damage is in PSI. The photoinhibition of PSI isinitiated by the inactivation of the acceptor side, with thesubsequent destruction of the reaction center and the degradationof the product of the psaB gene, which is one of the two majorsubunit polypeptides of the PSI reaction center complex. Chillingand oxidative stress (the presence of reactive species of oxygen)are characteristic requirements for the photoinhibition of PSIin vivo. (Received December 4, 1995; Accepted March 11, 1996)     

PGR5 and NDH-1 systems do not function as protective electron acceptors but mitigate the consequences of PSI inhibition

Avoidance of photoinhibition at photosystem (PS)I is based on synchronized function of PSII, PSI, Cytochrome b₆f and stromal electron acceptors. Here, we used a special light regime, PSI photoinhibition treatment (PIT), in order to specifically inhibit PSI by accumulating excess electrons at the photosystem (Tikkanen and Grebe, 2018). In the analysis, Arabidopsis thaliana WT was compared to the pgr5 and ndho mutants, deficient in one of the two main cyclic electron transfer pathways described to function as protective alternative electron acceptors of PSI. The aim was to investigate whether the PGR5 (pgr5) and the type I NADH dehydrogenase (NDH-1) (ndho) systems protect PSI from excess electron stress and whether they help plants to cope with the consequences of PSI photoinhibition. First, our data reveals that neither PGR5 nor NDH-1 system protects PSI from a sudden burst of electrons. This strongly suggests that these systems in Arabidopsis thaliana do not function as direct acceptors of electrons delivered from PSII to PSI – contrasting with the flavodiiron proteins that were found to make Physcomitrella patens PSI resistant to the PIT. Second, it is demonstrated that under light-limiting conditions, the electron transfer rate at PSII is linearly dependent on the amount of functional PSI in all genotypes, while under excess light, the PGR5-dependent control of electron flow at the Cytochrome b₆f complex overrides the effect of PSI inhibition. Finally, the PIT is shown to increase the amount of PGR5 and NDH-1 as well as of PTOX, suggesting that they mitigate further damage to PSI after photoinhibition rather than protect against it.     

Antenna Sizes of Photosystem I and Photosystem II in Chlorophyll b-Deficient Mutants of Rice. Evidence for an Antenna Function of Photosystem II Centers That are Inactive in Electron Transport

Light-harvesting capacities of photosystem I (PSI) and photosystemII (PSII) in a wild-type and three chlorophyll b-deficient mutantstrains of rice were determined by measuring the initial slopeof light-response curve of PSI and PSII electron transport andkinetics of light-induced redox changes of P-700 and Q_A, respectively.The light-harvesting capacity of PSI determined by the two methodswas only moderately reduced by chlorophyll b-deficiency. Analysisof the fluorescence induction that monitors time course of Q_Aphotoreduction showed that both relative abundance and antennasize of PSII_a decrease with increasing deficiency of chlorophyllb and there is only PSII_rß in chlorina 2 which totallylacks chlorophyll b. The numbers of antenna chlorophyll moleculesassociated with the mutant PSII centers were, therefore, threeto five times smaller than that of PSII_a in the wild type rice.Rates of PSII electron transport determined on the basis ofPSII centers in the three mutants were 60–70% of thatin the normal plant at all photon flux densities examined, indicatingthat substantial portions of the mutant PSII centers are inactivein electron transport. The initial slopes of light-responsecurves of PSII electron transport revealed that the functionalantenna sizes of the active populations of PSII centers in themutants correspond to about half that of PSII in the wild typerice. Thus, the numbers of chlorophyll molecules that serveas antenna of the oxygen-evolving PSII centers in the mutantsare significantly larger than those that are actually associatedwith each PSII center. It is proposed that the inactive PSIIserves as an antenna of the active PSII in the three chlorophyllb-deficient mutants of rice. In spite of the reduced antennasize of PSII, therefore, the total light-harvesting capacityof PSII approximately matches that of PSI in the mutants. (Received July 29, 1994; Accepted February 7, 1996)     

The Inhibition of Photosynthesis after Exposure of Bean Leaves to Various Low Levels of CO2

Mature first leaves of Phaseolus vulgaris L. were exposed tolow partial pressures of CO₂ (7, 6 and 0 Pa CO₂) for 24 h. Afterexposure of leaves to 6 Pa CO₂ for 24 h, there was a reductionin the carbon exchange rate (CER) at all partial pressures ofCO₂ at which measurements were made. After exposure to 7 PaCO₂, the CER decreased only at high partial pressures of CO₂.The rates of electron transport from water to methyl viologen,through the whole chain, decreased in parallel with the decreasein CER measured at 90 Pa CO₂. One site of inhibition in leavesexposed for 24 h to 6 Pa CO₂ appeared to be the intersystemelectron-transport chain since there were no significant changesin the activities of PSI and PSII, as determined from the levelof P-700 and measurement of fluorescence, respectively. Anotherinhibitory phenomenon appeared to be a negative change in theactivation state of Rubisco, while the level of Rubisco wasunaffected by the exposure to 6 Pa CO₂. These decreases in photosyntheticactivity caused by depletion of CO₂ explains at least in part,the inhibition of photosynthesis that is caused by rain treatment[Ishibashi and Terashima (1995) Plant Cell Environ. 18: 431]. (Received September 19, 1996; Accepted March 10, 1997)     

Demonstration of Two Sites of Inhibition of Electron Transport by Protein Phosphorylation in Wheat Thylakoids

The effect of protein phosphorylation on electron transportactivities of thylakoids isolated from wheat leaves was investigated.Protein phosphorylation resulted in a reduction in the apparentquantum yield of whole chain and photosystem II (PSII) electrontransport but had no effect on photosystem I (PSI) activity.The affinity of the D1 reaction centre polypeptide of PSII tobind atrazine was diminished upon phosphorylation, however,this did not reduce the light-saturated rate of PSII electrontransport. Phosphorylation also produced an inhibition of thelight-saturated rate of electron transport from water or durohydroquinoneto methyl viologen with no similar effect being observed onthe light-saturated rate of either PSII or PSI alone. This suggeststhat phosphorylation produces an inhibition of electron transportat a site, possibly the cytochrome b₆/f complex, between PSIIand PSI. This inhibition of whole-chain electron transport wasalso observed for thylakoids isolated from leaves grown underintermittent light which were deficient in polypeptides belongingto the light-harvesting chlorophyll-protein complex associatedwith photosystem II (LHCII). Consequently, this phenomenon isnot associated with phosphorylation of LCHII polypeptides. Apossible role for cytochrome b₆/f complexes in the phosphorylation-inducedinhibition of whole chain electron transport is discussed. Key words: Electron transport, light harvesting, photosystem 2, protein phosphorylation, thylakoid membranes, wheat (Triticum aestivum)     

Xanthophyll Cycle and Inactivation of Photosystem Ⅱ Reaction Centers Alleviating Reducing Pressure to Photosystem Ⅰ in Morning Glory Leaves under Short-term High Irradiance

Under 30-min high irradiance （1500μmol m^-2 s^-1）, the roles of the xanthophyll cycle and D1 protein turnover were investigated through chlorophyll fluorescence parameters in morning glory （Ipomoea setosa） leaves, which were dipped into water, dithiothreitol （DTT） and lincomycin （LM）, respectively. During the stress, both the xanthophyll cycle and D1 protein turnover could protect PSI from photoinhibition. In DTT leaves, non-photochemical quenching （NPQ） was inhibited greatly and the oxidation level of P700 （P700^＋） was the lowest one. However, the maximal photochemical efficiency of PSII （Fv/Fm） in DTT leaves was higher than that of LM leaves and was lower than that of control leaves. These results suggested that PSI was more sensitive to the loss of the xanthophyll cycle than PSII under high irradiance. In LM leaves, NPQ was partly inhibited, Fv/Fm was the lowest one among three treatments under high irradiance and P700^＋ was at a similar level as that of control leaves. These results implied that inactivation of PSII reaction centers could protect PSI from further photoinhibition. Additionally, the lowest of the number of active reaction centers to one inactive reaction center for a PSII cross-section （RC/CSo）, maximal trapping rate in a PSll cross-section （TRo/CSo）, electron transport in a PSll cross-section （ETo/CSo） and the highest of 1-qP in LM leaves further indicated that severe photoinhibition of PSII in LM leaves was mainly induced by inactivation of PSII reaction centers, which limited electrons transporting to PSh However, relative to the LM leaves the higher level of RC/CSo, TRo/CSo, Fv/Fm and the lower level of 1-qP in DTT leaves indicated that PSI photoinhibition was mainly induced by the electron accumulation at the PSI acceptor side, which induced the decrease of P700^＋ under high irradiance.     

Systematic analysis of the relation of electron transport and ATP synthesis to the photodamage and repair of photosystem II in Synechocystis

The photosynthetic machinery and, in particular, the photosystem II (PSII) complex are susceptible to strong light, and the effects of strong light are referred to as photodamage or photoinhibition. In living organisms, photodamaged PSII is rapidly repaired and, as a result, the extent of photoinhibition represents a balance between rates of photodamage and the repair of PSII. In this study, we examined the roles of electron transport and ATP synthesis in these two processes by monitoring them separately and systematically in the cyanobacterium Synechocystis sp. PCC 6803. We found that the rate of photodamage, which was proportional to light intensity, was unaffected by inhibition of the electron transport in PSII, by acceleration of electron transport in PSI, and by inhibition of ATP synthesis. By contrast, the rate of repair was reduced upon inhibition of the synthesis of ATP either via PSI or PSII. Northern blotting and radiolabeling analysis with [(35)S]Met revealed that synthesis of the D1 protein was enhanced by the synthesis of ATP. Our observations suggest that ATP synthesis might regulate the repair of PSII, in particular, at the level of translation of the psbA genes for the precursor to the D1 protein, whereas neither electron transport nor the synthesis of ATP affects the extent of photodamage.     

Effect of photoinhibition on algal photosynthesis: a dynamic model

Recent evidence from algal physiology and molecular biologyconfirms that photoinhibition is directly related to D1 proteindamage and recovery, and D1 protein damage leads to a decreasein electron transfer or an increase in turnover time of theelectron transfer chain. In this study, the turnover time ofthe electron transfer chain is defined as a function of therelative concentration of D1 protein in reaction centre II andthe photoinhibition processes due to D1 protein degradationare incorporated into a model of photosynthesis, initiated byDubinsky et al. (Plant Cell Physiol., 27, 1335–1349, 1986)and developed by Sakshaug et al. (Limnol. Oceanogr., 34, 198–205,1989). D1 protein damage is assumed to be both light and D1protein concentration dependent, and to be proportional to thecross-section of PSII (     

Influence of photoinhibition on electron transport and photophosphorylation of isolated chloroplasts

The effects of a photoinhibition treatment (PIT) on electron transport and photophosphorylation reactions were measured in chloroplasts isolated from triazine-resistant and susceptible Chenopodium album plants grown under high and low irradiance. Electron transport dependent on photosystem I (PSI) alone was much less affected by PIT than that dependent on both photosystem II (PSII) and PSI. There was a smaller difference in susceptibility to PIT between the photophosphorylation activitity dependent on PSI alone and that dependent on both PSII and PSI. Because in all cases photophosphorylation activity decreased faster upon PIT than the rate of electron transport, we conclude that photoinhibition causes a gradual uncoupling of electron transport with phosphorylation. Since the extent of the light-induced proton gradient across the thylakoid membrane decreased upon PIT, it is suggested that photoinhibiton causes a proton leakiness of the membrane. We have found no significant differences to PIT of the various reactions measured in chloroplasts isolated from triazine-resistant and susceptible plants. We have also not observed any significant differences to PIT of the photophosphorylation reactions in chloroplasts of plants grown under low irradiance, compared with those grown under high irradiance. However, the electron transport reactions in chloroplasts from plants grown under low irradiance appeared to be somewhat less sensitive to PIT than those grown under high irradiance.     

Strong-light photoinhibition treatment accelerates the changes of protein secondary structures in triton-treated photosystem I and photosystem II complexes

Changes in the protein secondary structure and electron transport activity of the Triton X-100-treated photosystem I (PSI) and photosystem II (PSII) complexes after strong illumination treatment were studied using Fourier transform-infrared (FT-IR) spectroscopy and an oxygen electrode. Short periods of photoinhibitory treatment led to obvious decreases in the rates of PSI-mediated electron transport activity and PSII-mediated oxygen evolution in the native or Triton-treated PSI and PSII complexes. In the native PSI and PSII complexes, the protein secondary structures had little changes after the photoinhibitory treatment. However, in both Triton-treated PSI and PSII complexes, short photoinhibition times caused significant loss of -helical content and increase of -sheet structure, similar to the conformational changes in samples of Triton-treated PSI and PSII complexes after long periods of dark incubation. Our results demonstrate that strong-light treatment to the Triton-treated PSI and PSII complexes accelerates destruction of the transmembrane structure of proteins in the two photosynthetic membranes.     

Chromatic Regulation in Chlamydomonas reinhardtii: Time Course of Photosystem Stoichiometry Adjustment Following a Shift in Growth Light Quality

Photosystem stoichiometry adjustments in Chlamydomonas reinhardtiiwere induced upon a sudden shift in the light quality duringcell growth. Reversible changes in the PSI/PSII ratio were acompensation response to changes in the balance of light absorptionby the two photosystems. Quantitations of PSII, Cyt b₆-f complexand PSI revealed a constancy in the cellular content of PSIIand the Cyt b₆-f complex, and variable amounts of PSI in C.reinhardtii. These results strengthen the notion that PSI isthe thyla-koid component subject to chromatic regulation andresponsible for the adjustment and optimization of the PSI/PSII ratio in the thylakoid of oxygenic photosynthesis. Additionalresults, obtained upon the use of protein biosynthesis translationinhibitors (chloramphenicol and cyclohex-imide), suggested thata chromatically-induced lowering of the PSI/PSII ratio in C.reinhardtii occurs by suppression of de novo biosynthesis ofPSI components and, therefore, by dilution of the PSI complexin the thylakoid membrane, rather than by active degradationof assembled PSI in chlo-roplasts. (Received November 8, 1996; Accepted December 6, 1996)     

The difficulty of estimating the electron transport rate at photosystem I

It is still a controversial issue how the electron transport reaction is carried out around photosystem I (PSI) in the photosynthetic electron transport chain. The measurable component in PSI is the oxidized P700, the reaction center chlorophyll in PSI, as the absorbance changes at 820–830 nm. Previously, the quantum yield at PSI [Y(I)] has been estimated as the existence probability of the photo-oxidizable P700 by applying the saturated-pulse illumination (SP; 10,000–20,000 µmol photons m^?2 s^?1). The electron transport rate (ETR) at PSI has been estimated from the Y(I) value, which was larger than the reaction rate at PSII, evaluated as the quantum yield of PSII, especially under stress-conditions such as CO₂-limited and high light intensity conditions. Therefore, it has been considered that the extra electron flow at PSI was enhanced at the stress condition and played an important role in dealing with the excessive light energy. However, some pieces of evidence were reported that the excessive electron flow at PSI would be ignorable from other aspects. In the present research, we confirmed that the Y(I) value estimated by the SP method could be easily misestimated by the limitation of the electron donation to PSI. Moreover, we estimated the quantitative turnover rate of P700⁺ by the light-to-dark transition. However, the turnover rate of P700 was much slower than the ETR at PSII. It is still hard to quantitatively estimate the ETR at PSI by the current techniques.

     

Changes in the Cytochrome c Oxidase Activity in Response to Light Regime for Photosynthesis Observed with the Cyanophyte Synechocystis PCC 6714

Changes in the activity of cytochrome c oxidase (EC 1.9.3.1 [EC] ,Cyt-oxidase) in response to growth conditions were studied withthe cyanophyte Synechocystis PCC 6714 in relation to changesin PSI abundance induced by light regime for photosynthesis.The activity was determined with the V_max of mammalian cytochromec oxidation by isolated membranes. The activity of glucose-6-phosphate(G-6-P):NADP⁺ oxidoreductase (EC 1.1.1.49 [EC] ) was also determinedsupplementarily. Cyt-oxidase activity was enhanced by glucoseadded to the medium even when cell growth maintained mainlyby oxygenic photosynthesis. G-6-P:NADP⁺ oxidoreductase was alsoactivated by glucose. The enhanced level of Cyt-oxidase washigher under PSII light, which causes high PSI abundance, thanthat under PSI light, which causes low PSI abundance. The levelwas intermediate under hetetrotrophic conditions. Although theactivity level was low in cells grown under autotrophic conditions,the level was again lower in cells grown under PSI light thanunder PSII light. The change of Cyt-oxidase activity in responseto light regime occurred in the same direction as that for thevariation of PSI abundance. Results suggest that in SynechocystisPCC 6714, the capacity of electron turnover at the two terminalcomponents of thylakoid electron transport system, Cyt-oxidaseand PSI, changes in parallel with each other in response tothe state of thylakoid electron transport system. ¹Present address: Institute of Botany, Academia Sinica, Beijing100044, China ²Present address: Department of Botany, Utkal University, Bhubaneswar,India 751004     

Reduced photoinhibition with stem curling in the resurrection plant Selaginella lepidophylla

Summary Selaginella lepidophylla, the resurrection plant, curls dramatically during desiccation and the hypothesis that curling may help limit bright light-induced damage during desiccation and rehydration was tested under laboratory conditions. Restraint of curling during desiccation at 25° C and a constant irradiance of 2000 mol m^–2 s^]t-1 significantly decreased PSII and whole-chain electron transport and the F_v/F_m fluorescence yield ratio following rehydration relative to unrestrained plants. Normal curling during desiccation at 37.5°C and 200 mol m^–2 s^–1 irradiance did not fully protect against photoinhibition or chlorophyll photooxidation indicating that some light-induced damage occurred early in the desiccation process before substantial curling. Photosystem I electron transport was less inhibited by high-temperature, high-irradiance desiccation than either PSII or whole-chain electron transport and PSI was not significantly affected by restraint of curling during desiccation at 25°C and high irradiance. Previous curling also helped prevent photoinhibition of PSII electron transport and loss of whole-plant photosynthetic capacity as the plants uncurled during rehydration at high light. These results demonstrate that high-temperature desiccation exacerbated photoinhibition, PSI was less photoinhibited than PSII or whole-chain electron transport, and stem curling ameliorated bright light-induced damage helping to make rapid recovery of photosynthetic competence possible when the plants are next wetted.     

Xanthophyll Cycle and Inactivation of Photosystem Ⅱ Reaction Centers Alleviating Reducing Pressure to Photosystem Ⅰ in Morning Glory Leaves under Short-term High Irradiance

investigated through chlorophyll fluorescence parameters in morning glory (Ipomoea setosa) leaves, which were dipped into water, dithiothreitol (DTT) and lincomycin (LM), respectively. During the stress, both the xanthophyll cycle and D1 protein turnover could protect PSI from photoinhibition. In DTT leaves, non-photochemical quenching (NPQ) was inhibited greatly and the oxidation level of P700 (P700+) was the lowest one. However, the maximal photochemical efficiency of PSII (Fv/Fm) in DTT leaves was higher than that of LM leaves and was lower than that of control leaves. These results suggested that PSI was more sensitive to the loss of the xanthophyll cycle than PSII under high irradiance. In LM leaves, NPQ was partly inhibited, Fv/Fm was the lowest one among three treatments under high irradiance and P700+ was at a similar level as that of control leaves. These results implied that inactivation of PSII reaction centers could protect PSI from further photoinhibition. Additionally, the lowest of the number of active reaction centers to one inactive reaction center for a PSII cross-section (RC/CSo), maximal trapping rate in a PSII cross-section (TRo/CSo), electron transport in a PSII cross-section (ETo/CSo) and the highest of 1-qP in LM leaves further indicated that severe photoinhibition of PSII in LM leaves was mainly induced by inactivation of PSII reaction centers, which limited electrons transporting to PSI. However, relative to the LM leaves the higher level of RC/CSo, TRo/CSo, Fv/Fm and the lower level of 1-qP in DTT leaves indicated that PSI photoinhibition was mainly induced by the electron accumulation at the PSI acceptor side, which induced the decrease of P700+ under high irradiance.     

Photosystem II photoinhibition-repair cycle protects Photosystem I from irreversible damage

Photodamage of Photosystem II (PSII) has been considered as an unavoidable and harmful reaction that decreases plant productivity. PSII, however, has an efficient and dynamically regulated repair machinery, and the PSII activity becomes inhibited only when the rate of damage exceeds the rate of repair. The speed of repair is strictly regulated according to the energetic state in the chloroplast. In contrast to PSII, Photosystem I (PSI) is very rarely damaged, but when occurring, the damage is practically irreversible. While PSII damage is linearly dependent on light intensity, PSI gets damaged only when electron flow from PSII exceeds the capacity of PSI electron acceptors to cope with the electrons. When electron flow to PSI is limited, for example in the presence of DCMU, PSI is extremely tolerant against light stress. Proton gradient (ΔpH)-dependent slow-down of electron transfer from PSII to PSI, involving the PGR5 protein and the Cyt b6f complex, protects PSI from excess electrons upon sudden increase in light intensity. Here we provide evidence that in addition to the ΔpH-dependent control of electron transfer, the controlled photoinhibition of PSII is also able to protect PSI from permanent photodamage. We propose that regulation of PSII photoinhibition is the ultimate regulator of the photosynthetic electron transfer chain and provides a photoprotection mechanism against formation of reactive oxygen species and photodamage in PSI.     

NaCl胁迫加重强光胁迫下超大甜椒叶片的光系统II和光系统I的光抑制

快速叶绿素荧光动力学可以在无损情况下探知叶片光合机构的损伤程度, 快速叶绿素荧光测定和分析技术(JIP-test)将测量值转化为多种具有生物学意义的参数, 因而被广泛应用于植物光合机构对环境的响应机制研究。该文研究了超大甜椒(Capsicum annuum)幼苗在强光及不同NaCl浓度胁迫下的荧光响应情况。与单纯强光胁迫相比, NaCl胁迫引起了叶绿素荧光诱导曲线的明显改变, 光系统II (PSII)光抑制加重, 同时PSII反应中心和受体侧受到明显影响, 而且高NaCl浓度胁迫下PSII供体侧受伤害明显, 同时PSI反应中心活性(P700⁺)在盐胁迫下明显降低。这些结果表明, NaCl胁迫会增强强光对超大甜椒光系统的光抑制, 并且浓度越高抑制越明显, 但对PSI的抑制作用低于PSII。高NaCl浓度胁迫易对PSII供体侧造成破坏, 且PSI光抑制严重。     

Concerning a dual function of coupled cyclic electron transport in leaves

Coupled cyclic electron transport is assigned a role in the protection of leaves against photoinhibition in addition to its role in ATP synthesis. In leaves, as in reconstituted thylakoid systems, cyclic electron transport requires “poising,” i.e. availability of electrons at the reducing side of photosystem I (PSI) and the presence of some oxidized plastoquinone between photosystem II (PSII) and PSI. Under self-regulatory poising conditions that are established when carbon dioxide limits photosynthesis at high light intensities, and particularly when stomata are partially or fully closed as a result of water stress, coupled cyclic electron transport controls linear electron transport by helping to establish a proton gradient large enough to decrease PSII activity and electron flow to PSI. This brings electron donation by PSII, and electron consumption by available electron acceptors, into a balance in which PSI becomes more oxidized than it is during fast carbon assimilation. Avoidance of overreduction of the electron transport chain is a prerequisite for the efficient protection of the photosynthetic apparatus against photoinactivation.