Inhibition of photosynthetic reactions by light

Illumination of isolated intact chloroplasts of Spinacia oleracea L. for 10 min with 850 W m^-2 red light in the absence of substrate levels of bicarbonate caused severe inhibition of subsequently measured photosynthetic activities. The capacity of CO₂-dependent O₂ evolution and of non-cyclic electron transport were impaired to similar degrees. This photoinactivation was prevented by addition of bicarbonate which allowed normal carbon metabolism to proceed during preillumination. Photoinhibition of electron transport was observed likewise upon illumination of intact or broken chloroplasts when efficient electron acceptors were absent. Addition of uncouplers did not influence the extent of inhibition. Studies of partial electron-transport reactions indicated that the activity of both photosystems was affected by light. In addition, the water-oxidation system or its connection to photosystem II seemed to be impaired. Preillumination did not cause uncoupling of photophosphorylation. Chlorophyll-fluorescence data obtained at room temperature and at 77 K are consistent with the view that photosystem-II reaction centers were altered. Addition of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6) or 1,4-diazabicyclo(2,2,2)octane to isolated thylakoids prior to preillumination substantially diminished photoinhibition. This result shows that reactive oxygen species were involved in the damage. It is concluded that bright light, which normally does not damage the photosynthetic apparatus, may exert the described destructive effects under conditions that restrict metabolic turnover of photosynthetic energy.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - PSI photosystem I - PSII photosystem II     

Photoinactivation of Catalase Occurs under Both High- and Low-Temperature Stress Conditions and Accompanies Photoinhibition of Photosystem II

Severe photoinactivation of catalase (EC 1.11.1.6) and a decline of variable fluorescence (F_v), indicating photoinhibition of photosynthesis, were observed as rapid and specific symptoms in leaves exposed to a high heat-shock temperature of 40°C as well as in leaves exposed to low chilling temperatures in white light of only moderately high photosynthetic photon flux density of 520 μE m⁻² s⁻¹. Other parameters, such as peroxidase (EC 1.11.1.7), glycolate oxidase (EC 1.1.3.1), glutathione reductase (EC 1.6.4.2), or the chlorophyll content, were hardly affected under these conditions. At a compatible temperature of 22°C, the applied light intensity did not induce severe photoinactivations. In darkness, exposures to high or low temperatures did not affect catalase levels. Also, decline of F_v in light was not related to temperature sensitivity in darkness. The effective low-temperature ranges inducing photoinactivation of catalase differed significantly for chilling-tolerant and chilling-sensitive plants. In leaves of rye (Secale cereale L.) and pea (Pisum sativum L.), photoinactivation occurred only below 15°C, whereas inactivation occurred at 15°C in cucumber (Cucumis sativus L.) and maize (Zea mays L.). The behavior of F_v was similar, but the difference between chilling-sensitive and chilling-tolerant plants was less striking. Whereas the catalase polypeptide, although photoinactivated, was not cleaved at 0 to 4°C, the D1 protein of photosystem II was greatly degraded during the low-temperature treatment of rye leaves in light. Rye leaves did not exhibit symptoms of any major general photodamage, even when they were totally depleted of catalase after photoinactivation at 0 to 4°C, and catalase recovered rapidly at normal temperature. In cucumber leaves, the decline of catalase after exposures to bright light at 0 to 4°C was accompanied by bleaching of chlorophyll, and the recovery observed at 25°C was slow and required several days. Similar to the D1 protein of photosystem II, catalase differs greatly from other proteins by its inactivation and high turnover in light. Inasmuch as catalase and D1 protein levels depend on continuous repair synthesis, preferential and rapid declines are generally to be expected in light whenever translation is suppressed by stress actions, such as heat or chilling, and recovery will reflect the repair capacity of the plants.     

Effect of varying NaCl doses on flavonoid production in suspension cells of Ginkgo biloba: relationship to chlorophyll fluorescence,ion homeostasis,antioxidant system and ultrastructure

Ginkgo suspension cells were used to investigate the mechanism that governs the shift between primary and secondary metabolism under NaCl elicitation. The production of three flavonol glycosides, chlorophyll fluorescence, ion content, the antioxidant system, and the cellular ultrastructure in the presence of NaCl doses from 5 to 175 mM were examined. At low salt doses (5–50 mM), cell growth and flavonol glycosides accumulation were stimulated without damaging cell structure or inducing oxidative stress by maintaining high K⁺ and chlorophyll content. At moderate salt doses (75–125 mM), the cells could withstand the salt stress without an impact on survival by changing internal cellular structure, maintaining high levels of K⁺ and Ca²⁺ and increasing anti-oxidative enzyme activities rather than flavonol glycosides to counteract the inhibition of the photosystem II, the accumulation of Na⁺ and hydrogen peroxide (H₂O₂) in the cells. This allowed cells to divert their metabolism from growth to defense-related pathways and tolerate NaCl stress. At higher salinity (150–175 mM), the cellular structure was damaged, and the high Na⁺ and low K⁺ content led to osmotic stress, and therefore, the stimulation of peroxidase (POD) and catalase (CAT) was not enough to cope with high H₂O₂ accumulation. The high production of flavonol glycosides may be a response of elicitation stimulation to serious damage at 175 mM NaCl. In conclusion, the use of 175 mM NaCl may be desirable for the induction of flavonol glycoside production in Ginkgo suspension cells.     

Phosphorylation of chloroplast membrane proteins partially protects against photoinhibition

Thylakoids isolated from peas (Pisum sativum cv. Kelvedon Wonder) and phosphorylated by incubation with ATP have been compared with non-phosphorylated thylakoids in their sensitivity to photoinhibition by exposure to illumination in vitro. Assays of the kinetics of fluorescence induction at 20° C and the fluorescence emission spectra at-196° C indicate a proportionally larger decrease in fluorescence as a result of photoinhibitory treatment of non-phosphorylated compared with phosphorylated thylakoids. It is concluded that protein phosphorylation can afford partial protection to thylakoids exposed to photoinhibitory conditions.Abbreviations and symbols DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F ₀ Level of chlorophyll fluorescence when photosystem 2 traps are open - F _m Level of chlorphyll fluorescence when photosystem 2 traps are closed - P Maximum level of fluorescence reached in the absence of DCMU - PSI (II) photosystem I(II)     

Photorespiration is More Effective than the Mehler Reaction in Protecting the Photosynthetic Apparatus against Photoinhibition

I. Isolated intact chloroplasts: Photosystem II, but not photosystem I, of the electron transport chain is rapidly photoinactivated even by very low intensities of red light when no large proton gradient can be formed and the electron transport chain becomes over-reduced in the absence of oxygen and other reducable substrates. Electron acceptors including oxygen provide protection against photoinactivation. Nevertheless, photosystem II is rapidly, and photosystem I more slowly, photoinactivated by high intensities of red light when oxygen is the only electron acceptor available. Increased damage is observed at increased oxygen concentrations although catalase is added to destroy H₂O₂ formed during oxygen reduction in the Mehler reaction. Photoinactivation can be decreased, but not prevented by ascorbate which reduces hydrogen peroxide inside the chloroplasts and increases coupled electron flow. II. Leaves: Simple measurements of chlorophyll fluorescence permit assessment of damage to photosystem II after exposure of leaves to high intensity illumination. In contrast to isolated chloroplasts, chloroplasts suffer more damage in situ at reduced than at elevated oxygen concentrations. The difference in the responses is due to photorespiration which is active in leaves, but not in isolated chloroplasts. After photosynthesis and photorespiration are inhibited by feeding glyceraldehyde to leaves, photoinactivation is markedly increased, although oxygen reduction in the Mehler reaction is not affected by glyceraldehyde. In the presence of reduced CO₂ levels, photorespiratory reactions, but not the Mehler reaction, can prevent the overreduction of the electron transport chain. Over-reduction indicates ineffective control of photosystem II activity. Effective control is needed for protection of the electron transport chain against photoinactivation. It is suggested to be made possible by coupled cyclic electron flow around photosystem I which is facilitated by the redox poising resulting from the interplay between photorespiratory carbohydrate oxidation and the refixation of evolved CO₂.     

Alteration in the acceptor side of photosystem II of chloroplast by high light

The effect of high light on the acceptor side of photosystem II of chloroplasts and core particles of spinach was studied. BothV _max and apparentK _m for DCIP were altered in photoinhibited photosystem II core particles. The double reciprocal plot analysis as a function of actinic light showed increased slope in chloroplasts photoinhibited in the presence of DCMU. Exposure of chloroplasts to high light in the presence of DCMU did not protect the chloroplast against high light induced decrease in F_m, level. Further the high light stress induced decrease inF _m level was not restored by the addition of DCMU. These results suggest that the high light stress induced damage to chloroplast involves alteration in the binding site forQ _B on the DI protein on the acceptor side of photosystem II     

Differential response to paraquat induced oxidative stress in two rice cultivars on antioxidants and chlorophyll a fluorescence

The responses of antioxidative system and photosystem II photochemistry of rice (Oryza sativa L.) to paraquat induced oxidative stress were investigated in a chilling-tolerant cultivar Xiangnuo no. 1, and a chilling-susceptible cultivar, IR-50. Electrolyte leakage and malondialdehyde (MDA) content of Xiangnuo no. 1 were little affected by paraquat, but they increased in IR-50. After paraquat treatment, superoxide dismutase (SOD) activity remained high in Xiangnuo no. 1, while it declined in IR-50. Activities of catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) declined with oxidative stress in both cultivars, but Xiangnuo no. 1 had higher GR activity than IR-50. Under paraquat induced oxidative stress, ascorbic acid (AsA) and reduced glutathione (GSH) concentrations remained high in Xiangnuo no. 1, but decreased in IR-50. The results indicated that higher activities of SOD and GR and higher contents of AsA and GSH in Xiangnuo no. 1 under paraquat induced oxidative stress were associated with its tolerance to paraquat, while paraquat induced damage to IR-50 was related to decreased activities of SOD, APX and GR and contents of AsA and GSH. F _v/F _m, Φ _PSII, and qP remained high in Xiangnuo no. 1, while they decreased greatly in IR-50 under paraquat induced oxidative stress.     

The photodynamic action of eosin,a singlet-oxygen generator

Eosin, a known generator of singlet oxygen, applied to leaf discs of Pisum sativum L. sensitized the inhibition of photosynthesis. Analysis of partial photosynthetic electron-transport reactions and of the kinetics of variable chlorophyll fluorescence located the damage at photosystem II. This injury required the presence of oxygen and was also caused by the irradiation of eosin-treated leaf tissue with green light. The role of oxygen and photodynamic reactions in the susceptibility of photosystem II to damage by environmental stresses is discussed.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCPIP 2,6-dichlorophenolindophenol - DPC 1,5-diphenylcarbazide - PSI photosystem I - PSII photosystem II - ¹O₂ singlet oxygen - Tricine N-[2-hydroxyl-3,1-bis(hydroxymethyl)ethyl]-glycine     

Mode of Action Studies on Nitrodiphenyl Ether Herbicides : II. The Role of Photosynthetic Electron Transport in Scenedesmus obliquus

The nitrodiphenyl ether herbicide 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitroacetophenone oxime-o-(acetic acid, methyl ester) (DPEI) induces light- and O₂-dependent lipid peroxidation and chlorophyll (Chl) bleaching in the green alga Scenedesmus obliquus. Under conditions of O₂-limitation, these effects are diminished by prometyne and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), both inhibitors of photosynthetic electron transport. Mutants in which photosynthetic electron transport is blocked are also resistant to DPEI under conditions of O₂-limitation. Light- and O₂-dependent lipid peroxidation and Chl bleaching are also induced by 5-[2-chloro-4-(trifluoromethyl)phenoxy]-3-methoxyphthalide (DPEII), a diphenyl ether whose redox properties preclude reduction by photosystem I. However, these effects of DPEII are also inhibited by DCMU. Under conditions of high aeration, DCMU does not protect Scenedesmus cells from Chl bleaching induced by DPEI, but does protect against paraquat. DPEI, but not paraquat, induces tetrapyrrole formation in treated cells in the dark. This is also observed in a mutant lacking photosystem I but is suppressed under conditions likely to lead to O₂ limitation. Our results indicate that, in contrast to paraquat, the role of photosynthetic electron transport in diphenyl ether toxicity in Scenedesmus is not to reduce the herbicide to a radical species which initiates lipid peroxidation. Its role is probably to maintain a sufficiently high O₂ concentration, through water-splitting, in the algal suspension.     

Fluorescence and Delayed Light Emission from Mesophyll and Bundle Sheath Cells in Leaves of Normal and Salt-Treated Panicum miliaceum

A modified fluorescence microscope system was used to measure chlorophyll fluorescence and delayed light emission from mesophyll and bundle sheath cells in situ in fresh-cut sections from leaves of Panicum miliaceum L. The fluorescence rise in 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU)-treated leaves and the slow fluorescence kinetics in untreated leaves show that mesophyll chloroplasts have larger photosystem II unit sizes than do bundle sheath chloroplasts. The larger photosystem II units imply more efficient noncyclic electron transport in mesophyll chloroplasts. Quenching of slow fluorescence also differs between the cell types with mesophyll chloroplasts showing complex kinetics and bundle sheath chloroplasts showing a relatively simple decline. Properties of the photosynthetic system were also investigated in leaves from plants grown in soil containing elevated NaCl levels. As judged by changes in both fluorescence kinetics in DCMU-treated leaves and delayed light emission in leaves not exposed to DCMU, salinity altered photosystem II in bundle sheath cells but not in mesophyll cells. This result may indicate different ionic distributions in the two cell types or, alternatively, different responses of the two chloroplast types to environmental change.     

Gibberellic-acid-responsive protoplasts from mature aleurone of Himalaya barley

The role of oxygen in the photoinactivation of the photosynthetic apparatus of Spinacia oleracea L. was investigated. Moderate irradiation (1200 mol photons m^-2s^-1) of spinach leaves in an atmosphere of pure nitrogen caused strong inhibition of subsequently measured net CO₂ assimilation, whereas considerably less photoinhibition was observed in the presence of low partial pressures (10–20 mbar) of O₂. The decrease in activity caused by anaerobiosis in the light was not based on stomatal closure; the decline of assimilation represents a photoinhibition, as activity was not impaired by low irradiation (80 mol photos m^-2s^-1). In contrast, gassing with pure N₂ in the dark caused strong inhibition. Electron-transport rates and chlorophyll-fluorescence data of thylakoids isolated from photoinhibited leaves indicated damage to the electron-transport system, in particular to photosystem II reaction centers. In vitro, photoinhibition in isolated thylakoid membranes was also strongly promoted by anaerobiosis. Photoinhibition of electron-transport rates under anaerobic conditions was characterized by a pronounced increase in the initial fluorescence level, F₀, of chlorophyll-fluorescence induction, in contrast to photoinhibition under aerobic conditions. The results are discussed in terms of two mechanisms of photoinhibition, one that is suppressed and a second that is promoted by oxygen.Abbreviations Chl chlorophyll - DCMU 3-(3, 4-dichlorophenyl)-1,1-dimethylurea - PSI, II photosystem I, II     

Multiple effects of salinity on photosynthesis of the protist Euglena gracilis

The effect of NaCl in the culture medium on growth, photosynthesis and cell content of chlorophyll, K⁺, Na⁺, Ca²⁺ and Mg²⁺ in Euglena gracilis was studied. O₂ production, quantum yield of photosystem II (PSII), the non-photochemical quenching of chlorophyll fluorescence (q_N) and the chlorophyll alb ratio all diminished by 0.2 M NaCl. Respiration and chlorophyll a and b increased, whereas the photochemical quenching (q_p) of chlorophyll fluorescence was not affected by 0.2 M NaCl. Salt stress also induced an increase in cell volume and in K⁺ and Na⁺ concentrations, but decreased the concentrations of Ca²⁺ and Mg²⁺. Except for a protective effect on O₂ production, additional Ca²⁺ in the culture medium did not attenuate the salt effect on the parameters measured. The addition of HCO₃^? restored the PSII quantum yield of O₂ production in cells grown in high salt. Salt stress promoted a decrease in the apparent rate of quinone A (Q_A) reduction and an apparent obstruction of Q_B reduction, which were not prevented by excess HCO₃^?; the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) did not increase chlorophyll fluorescence in salt-grown cells. These results indicate that photosynthesis in Euglena grown under salt stress exhibits: (1) diminution of the HCO₃^? dependent water-splitting activity of PSII; (2) inhibition of the electron transfer at the quinone pool level; (3) probable increase in thylakoid stacking (as indicated by the effect on the chlorophyll alb ratio); and (4) dissipation of the H⁺ gradient across the thylakoid membranes (as indicated by the decrease of q_N).     

Osmotic and Atmospheric Dehydration Effects in the Lichens Hypogymnia Physodes,Lobaria Pulmonaria,and Peltigera Aphthosa: An in vivo Study of the Chlorophyll Fluorescence Induction

Inactivation of photosynthesis during atmospheric and osmotic (highly concentrated NaCl or sucrose solutions) dehydration was monitored by measurement of chlorophyll fluorescence induction (OIP-phase, Kautsky-curves) in three lichen species. The induction curves were changed in a very similar way by all three treatments. All dehydration effects were rapidly reversible after rehydration. At relatively mild water stress, the rise time to the transient peak F_p was prolonged, and the variable part of fluorescence was diminished. In addition, at severe water stress, a considerable decline of the F₀ value was observed. For NaCl treatment this effect started at water potentials <-8.5 MPa in P. aphthosa, <-12 MPa in H. physodes, and <-21 MPa in L. pulmonaria. Above these water potentials, our observations are in agreement with values from desiccation-tolerant algae, higher plants, and lichens, where an inactivation on the photosystem 2 (PS2) donor side has been postulated. At very low water potentials, the decrease in F₀ probably monitors changes in the organization of the antenna apparatus of PS2. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photoinactivation of catalase at low temperature and its relevance to photosynthetic and peroxide metabolism in leaves

Abstract In green as well as in etiolated leaves of rye (Secale cereale L. ev. ‘Halo’), exposed to strong light at low temperature (0.4°C) catalase was inactivated. Other heme-containing enzymes (peroxidases) and various enzymes of photosynthetic, photorespiratory or peroxide metabolism were not photoinactivated. After returning plants from a low to a physiological temperature (22°C), catalase activity recovered within 12 h through new synthesis. The leaf contents of H₂O₂ and organic peroxides were not affected by the photoinactivation of catalse. The content of malondialdehyde generally increased after exposure to a higher light intensity. High-light-induced increases of ascorbate, and particularly of glutathione, were more marked in catalase-deficient than in normal leaves. Photoinactivation of catalase was accompanied by severe inhibition of photosynthesis. Photoinhibition of photosynthesis was not related to the lack of catalase because photosynthesis was not impaired when catalase activity was kept low by growing the plants under non-photorespiratory conditions. Photoinhibition appeared to result from photodamage in primary photochemistry of photosystem II, as indicated by a decrease of the maximal variable fluorescence. Photoinhibition of photosynthesis and of catalase have in common that in both instances proteins are involved that are continuously inactivated in light and, therefore, particularly sensitive to stress conditions that prevent their replacement by repair synthesis.     

Sensitivity of photosynthesis by spinach chloroplast membranes to osmotic stress in vitro: Rapid inhibition of O2 evolution in presence of magnesium

Thylakoids prepared from spinach (Spinacea oleracea L.) chloroplasts were exposed to osmotic stress in vitro in the presence or absence of different inorganic salts. By an hour after incubation in 1.0 M sorbitol and 10 mM (or more) MgCl₂, the thylakoids lost approximately 80% of their photosystem (PS) II activity, but not PS I. The inhibition occurred only in presence of magnesium as indicated by the combinations of several cations/anions. The PS II activity was relatively insensitive to osmotic stress in the presence of diphenyl carbazide. We therefore conclude that under conditions of water stress in the presence of 10 mM or higher Mg²⁺, the oxygen evolving system in chloroplasts is rapidly inactivated.Abbreviations DCMU 3(3,4-dichlorophenyl)-1,1-dimethylurea - DCPIP 2,6-dichlorophenol indophenol - DPC diphenyl carbazide - MV methyl viologen - PS photosystem Part of this work was included in the thesis submitted by the first author of M.Phil.degree.     

Photochemical activities,pigment distribution and photosynthetic unit size of subchloroplast particles isolated from synchronized cells of Scenedesmus obliquus

Photosystem II (PS II) reactions of chloroplast particles show the same variations during the synchronous life cycle of Scenedesmus obliquus, strain D₃ (Gaffron Biol. Zbl. 59, 302 1939), as the whole cells they derived from. Photosystem I (PS I) reactions of whole cells and of subchloroplast particles show little or no variation in their activity, whereas PS I reactions of chloroplast particles vary like PS II reactions during the life cycle. The variation in chloroplast particles could be attributed to the change in the reoxidation capacity of plastoquinone still attached to PS I. Digitonin-treatment of chloroplast particles from Scenedesmus and subsequent sucrose density gradient separation yielded 3 distinct fractions: Fraction I contained pure PS I particles with the most efficient PS I-mediated methylviologen (MV) reduction with subsequent oxygen uptake (3 mmol O₂/mg Chl·h); no Hill reaction; and a high chlorophyll a/b ratio, and a vast amount of unbound protein xanthophyll complexes. Fraction II is enriched in PS II particles, with little PS I activity (less than 10% of the PS I particles) and a low chlorophyll a/b ratio. The activity of the water-splitting system was completely lost. This fraction must also contain most of the light-harvesting pigment system. Fraction III is also enriched in PS II with even less PS I activity, but the ratio of chlorophyll a/b is slightly higher than in whole cells and the water-splitting system is intact. -carotene was part of all fractions whereas functional xanthophylls seemed to be restricted to the PS II particles. From the constant chlorophyll P/700 ratio we had to conclude that size of the photosynthetic unit does not change during the life cycle of a synchronized Scenedesmus obliquus culture.Abbreviations DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl-urea - DCPIP dichlorphenolindophenol - MV methylviologen (paraquat) - PS I photosystem I - PS II photosystem II - DPC diphenyl-carbazide     

Electron transport to oxygen mitigates against the photoinactivation of Photosystem II in vivo

The role of electron transport to O₂ in mitigating against photoinactivation of Photosystem (PS) II was investigated in leaves of pea (Pisum sativum L.) grown in moderate light (250 mol m^–2 s^–1). During short-term illumination, the electron flux at PS II and non-radiative dissipation of absorbed quanta, calculated from chlorophyll fluorescence quenching, increased with increasing O₂ concentration at each light regime tested. The photoinactivation of PS II in pea leaves was monitored by the oxygen yield per repetitive flash as a function of photon exposure (mol photons m^–2). The number of functional PS II complexes decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O₂ concentration. The results suggest that electron transport to O₂, via the twin processes of oxygenase photorespiration and the Mehler reaction, mitigates against the photoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excitation. Photoprotection via electron transport to O₂ in vivo is a useful addition to the large extent of photoprotection mediated by carbon-assimilatory electron transport in 1.1% CO₂ alone.Abbreviations Fm, Fo, Fv- maximal, initial (corresponding to open PS II traps) and variable chlorophyll fluorescence yield, respectively - NPQ- non-photochemical quenching - PS- photosystem - Q_A- primary quinone acceptor - qP- photochemical quenching coefficient     

Photosystem I shows a higher tolerance to sorbitol‐induced osmotic stress than photosystem II in the intertidal macro‐algae Ulva prolifera (Chlorophyta)

The photosynthetic performance of the desiccation‐tolerant, intertidal macro‐algae Ulva prolifera was significantly affected by sorbitol‐induced osmotic stress. Our results showed that photosynthetic activity decreased significantly with increases in sorbitol concentration. Although the partial activity of both photosystem I (PS I) and photosystem II (PS II) was able to recover after 30 min of rehydration, the activity of PS II decreased more rapidly than PS I. At 4 M sorbitol concentration, the activity of PS II was almost 0 while that of PS I was still at about one third of normal levels. Following prolonged treatment with 1 and 2 M sorbitol, the activity of PS I and PS II decreased slowly, suggesting that the effects of moderate concentrations of sorbitol on PS I and PS II were gradual. Interestingly, an increase in non‐photochemical quenching occurred under these conditions in response to moderate osmotic stress, whereas it declined significantly under severe osmotic stress. These results suggest that photoprotection in U. prolifera could also be induced by moderate osmotic stress. In addition, the oxidation of PS I was significantly affected by osmotic stress. P700⁺ in the thalli treated with high concentrations of sorbitol could still be reduced, as PS II was inhibited by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU), but it could not be fully oxidized. This observation may be caused by the higher quantum yield of non‐photochemical energy dissipation in PS I due to acceptor‐side limitation (Y(NA)) during rehydration in seawater containing DCMU.     

Photorespiration is Essential for the Protection of the Photosynthetic Apparatus of C3 Plants Against Photoinactivation Under Sunlight

CO₂ assimilation, transpiration and modulated chlorophyll fluorescence of leaves of Chenopodium bonus-henricus (L.) were measured in the laboratory and, at a high altitude location, in the field. Direct calibration of chlorophyll fluorescence parameters against carbon assimilation in the presence of 1 or 0.5% oxygen (plus CO₂) proved necessary to calculate electron transport under photorespiratory conditions in individual experiments. Even when stomata were open in the field, total electron transport was two to three times higher in sunlight than indicated by net carbon gain. It decreased when stomata were blocked by submerging leaves under water or by forcing them to close in air by cutting the petiole. Even under these conditions, electron transport behind closed stomata approached 10 nmol electrons m^?2 leaf area s^?1 at temperatures between 25 and 30 °C. No photoinactivation of photosystem II was indicated by fluorescence analysis after a day's exposure to full sunlight. Only when leaves were submerged in ice was appreciable photoinactivation noticeable after 4 h exposure to sunlight. Even then almost full recovery occurred overnight. Electron transport behind blocked stomata was much decreased when leaves were darkened for 70 min (in order to deactivate light-regulated enzymes of the Calvin cycle) before exposure to full sunlight. Brief exposure of leaves to HCN (to inhibit photoassimilation and photorespiration) also decreased electron transport drastically compared to electron transport in unpoisoned leaves with blocked stomata. Non-photochemical fluorescence quenching and reduction of Q_A, the primary electron acceptor of photosystem II was increased by HCN-poisoning. Very similar observations were made when glyceraldehyde was used instead of HCN to inhibit photosynthesis and photorespiration. In HCN-poisoned leaves, residual electron transport increased linearly with temperature and showed early light saturation revealing characteristics of the Mehler reaction. During short exposure of these leaves to photon flux densities equivalent to 25% of sunlight, no or only little photoinactivation of photosystem II was observed. However, prolonged exposure to sunlight caused inactivation even though non-photochemical quenching of chlorophyll fluorescence was extensive. Simultaneously, oxidation of cellular ascorbate and glutathione increased. Inactivation of photosystem II was reversible in dim light and in the dark only after short times of exposure to sunlight. Glyceraldehyde was very similar to HCN in increasing the sensitivity of photosystem II in leaves to sunlight. We conclude from the observations that the electron transport permitted by the interplay of photoassimilatory and photorespiratory electron transport is essential to prevent the photoinactivation of photosynthetic electron transport. The Mehler and Asada reactions, which give rise to strong nonphotochemical fluorescence quenching, are insufficient to protect the chloroplast electron transport chain against photoinactivation.     

The use of in vivo microscopy to image the cnidarian stress response in three octocoral species

Energy‐converting symbionts and organelles may be central to the cnidarian stress response. Stress may damage photochemistry in the endosymbiotic dinoflagellate Symbiodinium spp., leaving photosystems I and II relatively reduced and increasing reactive oxygen species (ROS). Alternatively, ROS may emanate from mitochondria of either the host or the symbiont, or both. These sources of ROS can be distinguished by using microscopy to examine the effects of light on stressed cnidarians incubated in the dark with a fluorescent, ROS‐detecting probe. Experiments were carried out with three species of alcyonacean octocoral, Phenganax parrini, Sarcothelia sp., and Sympodium sp. After incubation of colonies for 1 h at elevated temperature, imaging and illumination (excitation 450–490 nm, emission 515–565 nm) were begun simultaneously. Formation of ROS largely corresponded to the onset of illumination. On the other hand, chlorophyll fluorescence (excitation 530–580 nm, emission 620–690 nm) did not conform to this pattern. This difference is consistent with the expected rates of reaction. Remarkably, treatment with the inhibitor 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) resulted in dramatically higher levels of light‐induced ROS. Chlorophyll fluorescence was higher in the DCMU treatment but not significantly so. By controlling for variation between individual symbionts, however, DCMU produced significantly greater levels of chlorophyll fluorescence, indicating the expected greater reduction of photosystem II. A brief exposure to light and thermal stress produced a similar effect in all three species. In addition to ROS being initiated by light, these results indicate that: (1) a brief period of stress shifts photosystem redox state toward reduction, (2) photosystem II can donate electrons to oxygen when blocked with DCMU, and (3) chlorophyll fluorescence is highly variable among individuals of Symbiodinium. Imaging of individual symbionts in hospite thus provides a powerful method for understanding the initial steps of the cnidarian stress response.