The elevated anthocyanin level in the shaded peel of ‘Anjou’ pear enhances its tolerance to high temperature under high light

Pigments, chlorophyll fluorescence, dark respiration, and the antioxidant system in the shaded peel of green ‘Anjou’ pear (Pyrus communis L.) and its bud mutation, red ‘Anjou’, were compared in response to high peel temperature, high light alone or in combination to determine the protective role of anthocyanins under high temperature with or without light. Under high temperature treatment alone, no difference in the maximum quantum yield of PSII (F_V/F_M) was detected between red ‘Anjou’ and green ‘Anjou’; the superoxide dismutase activity and the glutathione pool were up-regulated in green ‘Anjou’ peel but remained unchanged in red ‘Anjou’ peel. Under high temperature coupled with high light, the F_V/F_M of green ‘Anjou’ peel was decreased to a lower value than that of red ‘Anjou’, and significant interaction was detected between temperature and light for both cultivars. Furthermore, the difference in F_V/F_M between red ‘Anjou’ and green ‘Anjou’ under high temperature coupled with high light was significantly larger than that under high light alone, indicating that this larger difference was caused by the interaction between high temperature and high light as no significant difference was detected in F_V/F_M between the two cultivars under high temperature treatment alone at any sampling point. It is concluded that the elevated anthocyanin level in the shaded peel of red ‘Anjou’ does not alter its thermotolerance in the dark, but makes it more tolerant of high temperature under high light.     

Red 'Anjou' pear has a higher photoprotective capacity than green 'Anjou'

Photoprotective function of anthocyanins along with xanthophyll cycle and antioxidant system in fruit peel was investigated in red 'Anjou' vs green 'Anjou' pear (Pyrus communis) during fruit development and in response to short-term exposure to high light. The sun-exposed peel of red 'Anjou' had higher maximum quantum yield of photosystem II (F(V)/F(M)) than that of green 'Anjou' and both the sun-exposed peel and the shaded peel of red 'Anjou' had smaller decreases in F(V)/F(M) after 2-h high light (photon flux density of 1500 mumol m(-2) s(-1)) treatment than those of green 'Anjou'. At the middle and late developmental stages, the xanthophyll cycle pool size on a chlorophyll basis, the activity of superoxide dismutase, ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) and the level of reduced ascorbate and total ascorbate pool in the sun-exposed peel were either the same or lower in red 'Anjou' than in green 'Anjou', whereas the xanthophyll cycle pool size on a chlorophyll basis and the activity of APX, catalase, MDAR, DHAR and GR in the shaded peel were higher in red 'Anjou' than in green 'Anjou'. It is concluded that red 'Anjou' has a higher photoprotective capacity in both the sun-exposed peel and the shaded peel than green 'Anjou'. While the higher anthocyanin concentration along with the larger xanthophyll cycle pool size and the higher activity of some antioxidant enzymes may collectively contribute to the higher photoprotective capacity in the shaded peel of red 'Anjou', the higher photoprotective capacity in the sun-exposed peel of red 'Anjou' is mainly attributed to its higher anthocyanin concentration.     

Significance of antioxidants and electron sinks for the cold-hardening-induced resistance of winter rye leaves to photo-oxidative stress

The contents of ascorbate and glutathione and the activities of superoxide dismutase and glutathione reductase were increased to levels as high as those in cold-hardened leaves (CHL) by incubating non-hardened leaves (NHL) of winter rye (Secale cereale L.) with the precursor substrates L -galactonic acid-γ-lactone and 2-oxothiazolidine-4-carboxylate. Reduced glutathione was rapidly depleted from NHL after application of D , L -buthionine sulfoximine, an inhibitor of its biosynthesis. In spite of greatly divergent antioxidant contents the rates of photo-inactivation of photosystem II (PSII) and catalase observed in the presence of translation inhibitors did not differ greatly. The paraquat-induced catalase inactivation and chlorophyll degradation in light were reduced in NHL with increased antioxidant levels. Paraquat-induced photo-inactivation of PSII was, however, not mitigated. The CHL had a higher capacity to prevent paraquat-induced oxidation of ascorbate and glutathione than NHL with increased antioxidant contents. Increased antioxidant contents did not establish resistance to low temperature-induced photo-inactivation of PSII and catalase in NHL. The resistance of CHL to low temperature-induced photo-inactivation of PSII and catalase required repair at low temperature and active carbon assimilation but was only little affected when photorespiration was suppressed by phosphinothricin. Protection of PSII depended also on non-photochemical quenching of excitation energy.     

The shaded side of apple fruit becomes more sensitive to photoinhibition with fruit development

Developmental changes of photochemical and non-photochemical processes and the antioxidant system in the shaded peel vs the sun-exposed peel of 'Gala' apple and their responses to sudden exposure of high light were determined to understand the susceptibility of the shaded peel to high light damage with fruit development. As fruit developed, actual PSII efficiency of the shaded peel decreased, whereas non-photochemical quenching (mainly the slow component) increased at any given PFD. Photochemical quenching coefficient of the shaded peel decreased at any given PFD with fruit development. As fruit developed, the activity of superoxide dismutase, ascorbate peroxidase and dehydroascorbate reductase and the level of reduced ascorbate and total ascorbate decreased; the activity of monodehydroascorbate reductase and glutathione reductase remained low, whereas catalase activity and the level of reduced glutathione and total glutathione increased in the shaded peel. Exposure to high light (1500 micromol m(-2) s(-1)) for 2 h significantly decreased the maximum quantum efficiency of PSII (F(V)/F(M)) in the shaded peel at each developmental stage, with the decrease being larger with fruit development. The F(V)/F(M) of the sun-exposed peel was also decreased by the high light treatment, but the decrease was much smaller than that in the shaded peel at each developmental stage. We conclude that the shaded peel of apple fruit becomes more sensitive to photoinhibition with fruit development, and this increased sensitivity is apparently related to the decease in the overall capacity for photosynthesis and photoprotection of the shaded peel with fruit development.     

Response of barley seedlings to UV-B radiation as affected by NaCl

The response of barley seedlings, subjected to 150 mmol/L NaCl for 4 days at different light regimes (4 d in the light, 4 d in darkness and a 12 h light/dark cycle) before UV-B radiation was investigated. NaCl treatment resulted in a decrease of total chlorophyll content and an increase in H2O2, free proline and lipid peroxidation, as quantified by measurement of malondialdehyde. Significantly more proline was accumulated in the light than in darkness. The combination of UV-B and NaCl treatment produced an additive effect on most of the parameters studied. UV-B radiation reduced the chlorophyll/carotenoids ratio and photochemical efficiency of PSII as estimated by chlorophyll fluorescence. NaCl pre-exposure decreased H2O2 generation and lipid peroxidation and alleviated the inhibitory effect of UV-B on PSII activity. Proline accumulated under salt stress conditions might be one of the reasons for the observed tolerance of barley seedlings to UV-B radiation.     

Examination of chlorophyll fluorescence in sulfur-deprived cells of <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>

Pulse amplitude modulation fluorimetry was used to assess chlorophyll fluorescence parameters in Chlamydomonas reinhardtii cells during sulfur deprivation. A significant (fourfold) increase in the chlorophyll fluorescence yield (parameters F ₀ and F _m) normalized to the chlorophyll concentration was shown for deprived cells. The chlorophyll content did not change during the deprivation experiments. An analysis of nonphotochemical quenching of chlorophyll fluorescence indicated a considerable modification of the energy deactivation pathways in photosystem II (PSII) of sulfur-deprived cells. For example, starved cells exhibited a less pronounced pH-dependent quenching of excited states and a higher thermal dissipation of excess light energy in the reaction centers of PSII. It was also shown that the photosynthetic apparatus of starved cells is primarily in state 2 and that back transition to state 1 is suppressed. However, these changes cannot cause the discovered elevation of chlorophyll fluorescence intensity (F ₀ and F _m) in the cells under sulfur limitation. The observed increase in the chlorophyll fluorescence intensity under sulfur deprivation may be due to partial dissociation of peripheral light-harvesting complexes from the reaction centers of PSII or a malfunction of the dissipative cycle in PSII, involving cytochrome b ₅₅₉.     

Vitamin E protects against photoinhibition and photooxidative stress in Arabidopsis thaliana

Vitamin E is considered a major antioxidant in biomembranes, but little evidence exists for this function in plants under photooxidative stress. Leaf discs of two vitamin E mutants, a tocopherol cyclase mutant (vte1) and a homogentisate phytyl transferase mutant (vte2), were exposed to high light stress at low temperature, which resulted in bleaching and lipid photodestruction. However, this was not observed in whole plants exposed to long-term high light stress, unless the stress conditions were extreme (very low temperature and very high light), suggesting compensatory mechanisms for vitamin E deficiency under physiological conditions. We identified two such mechanisms: nonphotochemical energy dissipation (NPQ) in photosystem II (PSII) and synthesis of zeaxanthin. Inhibition of NPQ in the double mutant vte1 npq4 led to a marked photoinhibition of PSII, suggesting protection of PSII by tocopherols. vte1 plants accumulated more zeaxanthin in high light than the wild type, and inhibiting zeaxanthin synthesis in the vte1 npq1 double mutant resulted in PSII photoinhibition accompanied by extensive oxidation of lipids and pigments. The single mutants npq1, npq4, vte2, and vte1 showed little sensitivity to the stress treatments. We conclude that, in cooperation with the xanthophyll cycle, vitamin E fulfills at least two different functions in chloroplasts at the two major sites of singlet oxygen production: preserving PSII from photoinactivation and protecting membrane lipids from photooxidation.     

Ultraviolet-B-induced oxidative stress and antioxidant defense system responses in ascorbate-deficient vtc1 mutants of Arabidopsis thaliana

Ultraviolet-B (UV-B) radiation has a negative impact on plant cells, and results in the generation of reactive oxygen species (ROS). In order to increase our understanding of the effects of UV-B on antioxidant processes, we investigated the response of an ascorbate-deficient Arabidopsis thaliana mutant vtc1 to short-term increased UV-B exposure. After UV-B supplementation, vtc1 mutants exhibited oxidative damage. Evidence for damage included an increase in H(2)O(2) content and the production of thiobarbituric acid reactive substances (TBARS); a decrease in chlorophyll content and chlorophyll fluorescence parameters were also reported. The vtc1 mutants had higher total glutathione than the wild type (WT) during the first day of UV-B treatment. We found reduced ratio of glutathione/total glutathione and increased ratio of dehydroascorbate/total ascorbate in the vtc1 mutants, compared to the WT plants. In addition, the enzymes responsible for ROS scavenging, including superoxide dismutase, catalase, and ascorbate peroxidase, had insufficient activity in the vtc1 mutants, compared to the WT plants. The same reduced activity in the vtc1 mutants was reported for the enzymes responsible for the regeneration of ascorbate and glutathione (including monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase). These results suggest that the ascorbate-deficient mutant vtc1 is more sensitive to supplementary UV-B treatment than WT plants and ascorbate can be considered an important antioxidant for UV-B radiation.     

Enhanced photochemical light utilization and decreased chilling-induced photoinhibition of photosystem II in cotton overexpressing genes encoding chloroplast-targeted antioxidant enzymes

The aim of this study was to determine whether increases in stromal superoxide dismutase (SOD; EC 1.15.1.1), ascorbate peroxidase (APX; EC 1.11.1.11) and glutathione reductase (GR; EC 1.6.4.2) via transformation could reduce photosystem (PS) II photoinhibition at low temperature for cotton (Gossypium hirsutum L.) plants and to determine by what mechanism this protection may be realized. During 3-h exposures of lincomycin-treated leaf discs to 10 degrees C and a photon flux density of 500 &mgr;mol m-2 s-1, all transgenic plants exhibited significantly greater PSII activity and O2 evolution than did wild-type plants. Also, the rate constant of PSII photoinactivation was significantly lower for all transgenic plants than for wild-type plants. No significant differences existed between genotypes in non-photochemical quenching of chlorophyll a fluorescence and the regulated component of the thermal dissipation of excitation energy. The relationship between changes in variable to maximum chlorophyll fluorescence (Fv/Fm) and the time-dependent averaged excessive light exposure was similar for all genotypes. This observation excluded the possibility that differences in PSII photodamage were due to improvements in the direct protection of PSII from active oxygen by antioxidant enzyme overproduction. Similar decreases in Fv/Fm during the stress treatment for all genotypes when leaves were pre-treated with 3-(3',4'-dichlorophenyl)-1,1-dimethylurea (DCMU) suggested that the effect of overproduction involved events downstream of PSII in the electron transfer pathway. Since all transgenic plants exhibited a significantly higher photochemical quenching of chlorophyll fluorescence during the chilling treatment, we concluded that, under the conditions used in this study, the enhancement of the protection of PSII from photodamage by increasing the stromal antioxidant enzyme activity in cotton leaves was due to the maintenance of a higher rate of electron transport and, consequently, a lower reduction state of QA.     

Effects of exogenous spermidine on antioxidant system of tomato seedlings exposed to high temperature stress

The effects of foliar spraying with spermidine (Spd) on antioxidant system in tomato (Lycopersicon esculentum Mill.) seedlings were investigated under high temperature stress. The high temperature stress significantly inhibited plant growth and reduced chlorophyll (Chl) content. Application of exogenous 1 mM Spd alleviated the inhibition of growth induced by the high temperature stress. Malondialdehyde (MDA), hydrogen peroxide (H₂O₂) content and superoxide anion (O₂) generation rate were significantly increased by the high temperature stress, but Spd significantly reduced the accumulation of reactive oxygen species (ROS) and MDA content under the stress. The high temperature stress significantly decreased glutathione (GSH) content and activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR), but increased contents of dehydroascorbic acid (DHA), ascorbic acid (AsA), and oxidized glutathione (GSSG) in tomato leaves. However, Spd significantly increased the activities of antioxidant enzymes, levels of antioxidants and endogenous polyamines in tomato leaves under the high temperature stress. In addition, to varying degrees, Spd regulated expression of MnSOD, POD, APX2, APX6, GR, MDHAR, DHAR1, and DHAR2 genes in tomato leaves exposed to the high temperature stress. These results suggest that Spd could change endogenous polyamine levels and alleviate the damage by oxidative stress enhancing the non-enzymatic and enzymatic antioxidant system and the related gene expression.     

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.     

Light inactivation of functional photosystem II in leaves of peas grown in moderate light depends on photon exposure

To determine the dependence of in vivo photosystem (PS) II function on photon exposure and to assign the relative importance of some photoprotective strategies of PSII against excess light, the maximal photochemical efficiency of PSII (Fv/Fm) and the content of functional PSII complexes (measured by repetitive flash yield of oxygen evolution) were determined in leaves of pea (Pisum satlvum L.) grown in moderate light. The modulation of PSII functionality in vivo was induced by varying either the duration (from 0 to 3 h) of light treatment (fixed at 1200 or 1800 mol photons · m^-2 · s^-1) or irradiance (from 0 to 3000 mol photons · m^-2 · s^-1) at a fixed duration (1 h) after infiltration of leaves with water (control), lincomycin (an inhibitor of chloroplast-encoded protein synthesis), nigericin (an uncoupler), or dithiothreitol (an inhibitor of the xanthophyll cycle) through the cut petioles of leaves of 22 to 24-day-old plants. We observed a reciprocity of irradiance and duration of illumination for PSII function, demonstrating that inactivation of functional PSII depends on the total number of photons absorbed, not on the rate of photon absorption. The Fv/Fm ratios from photoinhibitory light-treated leaves, with or without inhibitors, declined pseudo-linearly with photon exposure. The number of functional PSII complexes declined multiphasically with increasing photon exposure, in the following decreasing order of inhibitor effect: lincomycin > nigericin > DTT, indicating the central role of D1 protein turnover. While functional PSII and Fv/Fm ratio showed a linear relationship under high photon exposure conditions, in inhibitor-treated leaves the Fv/Fm ratio failed to reveal the loss of up to 25% of the total functional PSII under low photon exposure. The loss of this 25% of less-stable functional PSII was accompanied by a decrease of excitation-energy trapping capacity at the reaction centre of PSII (revealed by the fluorescence parameter, 1/Fo-1/Fm, where Fo and Fm stand for chlorophyll fluorescence when PSII reaction centres are open and closed, respectively), but not by a loss of excitation energy at the antenna (revealed by the fluorescence parameter, 1/Fm). We conclude that (i) PSII is an intrinsic photon counter under photoinhibitory conditions, (ii) PSII functionality is mainly regulated by D1 protein turnover, and to a lesser extent, by events mediated via the transthylakoid pH gradient, and (iii) peas exhibit PSII heterogeneity in terms of functional stability during photon exposure.Abbreviations D1 protein psbA gene product - DTT dithiothreitol - Fo chlorophyll fluorescence corresponding to open PSII reaction centres - Fv, Fm variable and maximum fluorescence after dark incubation, respectively - Fs, Fm steady-state and maximum fluorescence during illumination, respectively - P680 reactioncentre chlorophyll and primary electron donor of PSII - PS photosystem Financial support of this work by Department of Employment, Education and Training/Australian Research Council International Research Fellowships Program (Korea) is gratefully acknowledged.     

Cytochrome b-559 may function to protect photosystem II from photoinhibition

Although cytochrome b-559 is an integral component of the photosystem II complex (PSII), its function is unknown. Because cytochrome b-559 has been shown to be both photooxidized and photoreduced in PSII, one of several proposals is that it mediates cyclic electron transfer around PSII, possibly as a protective mechanism. We have used electron paramagnetic resonance spectroscopy to investigate the pathway of photooxidation of cytochrome b-559 in PSII and have shown that it proceeds via photooxidation of chlorophyll. We propose that this photooxidation of chlorophyll is the first step in the photoinhibition of PSII. The unique susceptibility of PSII to photoinhibition is probably due to the fact that it is the only reaction center in photosynthesis which generates an oxidant with a reduction potential high enough to oxidize chlorophyll. We propose that the function of cytochrome b-559 is to mediate cyclic electron transfer to rereduce photooxidized chlorophyll and protect PSII from photoinhibition. We also suggest that the chlorophyll(s) which are susceptible to photooxidation are analogous to the monomer chlorophylls found in the bacterial photosynthetic reaction center complex.     

The effect of acute high light and low temperature stresses on the ascorbate–glutathione cycle and superoxide dismutase activity in two Dunaliella salina strains

Dunaliella species accumulate carotenoids and their role in protection against photooxidative stress has been investigated extensively. By contrast, the role of other antioxidants in this alga, has received less attention. Therefore, the components of the ascorbate–glutathione cycle, along with superoxide dismutase (E.C. 1.15.1.1) and peroxidase (E.C. 1.11.1.11) activity were compared in two strains of Dunaliella salina. Strain IR‐1 had two‐fold higher chlorophyll and β‐carotene concentration than Gh‐U. IR‐1 had around four‐fold higher superoxide dismutase, ascorbate peroxidase and pyrogallol peroxidase activities than Gh‐U on a protein basis. Ascorbate and glutathione concentrations and redox state did not differ between strains and there was little difference in the activity of ascorbate–glutathione cycle enzymes (monodehydroascorbate reductase [E.C. 1.6.5.4], dehydroascorbate reductase [E.C. 1.8.5.1] and glutathione reductase [E.C. 1.8.1.7]). The response of these antioxidants to high light and low temperature was assessed by transferring cells from normal growth conditions (28°C, photon flux density of 100 μmol m^?2 s^?1)to 28°C/1200 μmol m^?2 s^?1; 13°C/100 μmol m^?2 s^?1; 13°C/1200 μmol m^?2 s^?1 and 28°C/100 μmol m^?2 s^?1 for 24 h. Low temperature and combined high light‐low temperature decreased chlorophyll and β‐carotene in both strains indicating that these treatments cause photooxidative stress. High light, low temperature and combined high light‐low temperature treatments increased the total ascorbate pool by 10–50% and the total glutathione pool by 20–100% with no consistent effect on their redox state. Activities of ascorbate–glutathione cycle enzymes were not greatly affected but all the treatments increased superoxide dismutase activity. It is concluded that D. salina can partially adjust to photooxidative conditions by increasing superoxide dismutase activity, ascorbate and glutathione.     

The Response of Antioxidant Enzymes in Cellular Organelles in Cucumber (Cucumis sativus L.) Leaves to Methyl Viologen-induced Photo-oxidative Stress

In order to clarify the response of antioxidant systems in various cellular organelles to photo-oxidative stress, the activities of superoxide dismutase (SOD) and enzymes of the ascorbate–glutathione (AsA-GSH) cycle were investigated in chloroplasts, mitochondria and cytosol of cucumber leaves subjected to methyl viologen (MV) treatment. Photo-oxidation by MV resulted in significant reductions in net photosynthetic rate (Pn) and increases in the ratio of the quantum efficiency of photosystem II (PSII), Φ_PSII to that of the quantum efficiency of CO₂ fixation (ΦCO₂), followed by increased activities of SOD, and a general increase of AsA-GSH cycle enzymes in chloroplasts, mitochondria and cytosol. These increases were however, most significant in chloroplasts. There were also significant increases in dehydroascorbate (DHA), reduced glutathione (GSH), and oxidized glutathione (GSSG) except that the content of ascorbate (AsA) in chloroplasts and cytosol was slightly decreased and little effected, respectively. However, GSSG in mitochondria and GSH in cytosol were little influenced by the MV treatment. The activity of ascorbate oxidase (AO) in these organelles was independent of the MV treatment while the activity of l-galactono-1,4- lactone dehydrogenase (GLDH) in mitochondria was slightly inhibited by MV treatment. These results indicate that disturbance of electron transport in chloroplasts by MV influenced the metabolism of whole cell by a crosstalk signaling system and that the AsA-GSH cycle played a primary role in sustaining the levels of AsA.     

Chloroplast photooxidation affects the accumulation of cytosolic mRNAs encoding chloroplast proteins in maize

Maize (Zea mays L.) seedlings were grown in the presence or absence of an herbicide, norflurazon (4-chloro-5-(methylamino)-2-(,,-trifluoro-m-tolyl)-pyridazinone), which prevents the accumulation of colored carotenoids. In the absence of carotenoids, plants grown in high light incur extensive photooxidative damage to their plastids, but relatively little damage elsewhere. Growth in very low light minimizes chlorophyll photooxidation and allows chloroplast development to proceed. We have previously reported that mRNA encoding light-harvesting chlorophyll a/b protein (LHCP) fails to accumulate in high-light-grown carotenoid-deficient seedlings, but accumulates normally in carotenoid-deficient seedlings grown in low light. Here we extend these results by examining the levels of translatable mRNAs encoding seven additional nuclear-encoded chloroplast proteins. When norflurazon-treated seedlings were grown in low light for 8 d and then transferred to high light for 24 h, three cytosolic mRNAs (plastocyanin, Rieske Fe–S protein, and the 33-kdalton (kDa) subunit of the photosystem II O₂-evolving complex) decreased to less than 1% the amount found in untreated seedlings. Two other mRNAs (NADP malic enzyme, EC 1.1.1.40, and the 23-kDa subunit of the photosystem II O₂-evolving complex) decreased significantly but not to levels as low as the first three. Levels of translatable mRNA for two other chloroplast proteins (pyruvate orthophosphate dikinase, EC 2.7.9.1, and ferredoxin NADP oxidoreductase, EC 1.18.1.2) were not reduced in nonflurazon-treated seedlings after 24 h in high light, but did not show the normal light-induced increase found in untreated plants. Photooxidative damage in the chloroplast thus affects the accumulation of a number of cytosolic mRNAs encoding proteins destined for the chloroplast.Abbreviations Da dalton - FNR ferredoxin NADP oxidoreductase - LHCP light-harvesting chlorophyll a/b-binding protein - poly(A)RNA polyadenylated RNA - PPDK pyruvate orthophosphate dikinase - PSII photosystem II - SDSPAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis - SSu small subunit (of ribulose-1,5-bisphosphate carboxylase)     

Brassinosteroids accelerate recovery of photosynthetic apparatus from cold stress by balancing the electron partitioning,carboxylation and redox homeostasis in cucumber

The aim of this study was to examine the role of brassinosteroids (BRs) in protecting the photosynthetic apparatus from cold‐induced damage in cucumber (Cucumis sativus) plants. Recovery at both high light (HL) and low light (LL) after a cooling at 10/7°C induced irreversible inhibition of CO₂ assimilation, photoinhibition at photosystem I (PSI) and inhibition of enzyme activities of Calvin cycle and ascorbate (AsA)‐reduced glutathione (GSH) cycle, followed by accumulation of H₂O₂ and malondialdehyde. However, cold‐induced photoinhibition at PSII was fully recovered at LL but not at HL. Meanwhile, recovery at HL increased electron flux to O₂‐dependent alternative pathway [Ja(O₂‐dependent)]. Foliar application of 24‐epibrassinolide (EBR) accelerated recovery from photoinhibition of PSII but not of PSI. EBR also significantly increased CO₂ assimilation, activity of Calvin cycle enzymes and electron flux to carbon reduction [Je(PCR)], with a concomitant decrease in Ja(O₂‐dependent); meanwhile EBR increased the activity of enzymes in AsA‐GSH cycle and cellular redox states. However, the positive effect of EBR on plant recovery was observed only at HL, but not LL. These results indicate that BR accelerates the recovery of photosynthetic apparatus at HL by activation of enzymes in Calvin cycle and increasing the antioxidant capacity, which in turn mitigate the photooxidative stress and the inhibition of plant growth during the recovery.     

Responses of<Emphasis Type="Italic"> Colobanthus quitensis</Emphasis> (Kunth) Bartl. to high light and low temperature

Antarctic plants are exposed to high light and low temperature during their natural growing season. Therefore, the maintenance of an oxidative status compatible with normal metabolism is an important determinant for their survival. We hypothesized that Colobanthus quitensis, the only dicotyledonous plant from the Antarctic, has efficient energy-dissipating mechanisms, and/or high antioxidant capacity. A short time treatment (24 h) of high light and/or low temperature reduced maximum photochemical quantum yield (Fv/Fm), photochemical quenching (qP), and the quantum yield of PSII (PSII). Concomitantly, an increase in the nonphotochemical quenching (qN) was observed. Superoxide dismutase (SOD) activity increased after cold acclimation. High light and low temperature did not significantly affect SOD, ascorbate peroxidase (APx), and glutathione reductase (GR) in nonacclimated plants. Total soluble antioxidants and carotenoids decreased after cold acclimation. Thus, it is likely that the main mechanism of C. quitensis to cope with energy imbalance is heat dissipation, as evidenced by the great increase in qN.     

Individual phenolic response and peroxidase activity in peel of differently sun-exposed apples in the period favorable for sunburn occurrence

Extreme weather events like high solar radiation can cause stress in apple fruits (Malus domestica Borkh.). The aim of the study was to make a screening of individual phenols and peroxidase activity in apple peel as a response to sunburn and different sun-exposures in the period when weather conditions are suitable for sunburn occurrence. Apple fruits of ‘Golden Delicious’ and ‘Braeburn’ were sampled. Fruit temperature and color were measured prior HPLC–MS² and peroxidase activity analyses. Sunburned peel was darker and more yellow-red in comparison to healthy peel, which appeared yellow-green. Fruit temperature, total as well as individual flavonols and dihydrochalcones, total hydroxycinnamics and perixodase activity were highest in sunburned peel in comparison with healthy sun-exposed peel, furthermore both were different than shaded sides of both fruits and peel of apples inside the tree crown; moreover in sunburned peel dihydrochalcones were determined for the first time. Chlorogenic acid was up to 2.5 times higher, 3-hydroxy-phloretin-2′-O-xyloglucoside was up to 10 times higher and quercetin-3-galactoside was up to 33 times higher in sunburned peel, comparing to shaded sided peels. Flavanols did not show a distinct pattern. A deeper insight in phenolic response against environmental stress caused by high solar radiation and high air temperatures has been made.     

Evidence for light-independent and steeply decreasing PSII efficiency along twig depth in four tree species

Recent reports have indicated a considerably inactivated PSII in twig cortices, in spite of the low light transmittance of overlying periderms. Corresponding information for more deeply located and less illuminated tissues like xylem rays and pith are lacking. In this investigation we aimed to characterize the efficiency of PSII and its light sensitivity along twig depth, in conjunction with the prevailing light quantity and quality. To that aim, optical methods (spectral reflectance and transmittance, chlorophyll fluorescence imaging, low temperature fluorescence spectra) and photoinhibitory treatments were applied in cut twig sections of four tree species, while corresponding leaves served as controls. Compared to leaves, twig tissues displayed lower chlorophyll (Chl) levels and dark-adapted PSII efficiency, with strong decreasing gradients towards the twig center. The low PSII efficiencies in the inner stem were not an artifact due to an actinic effect of measuring beam or to an enhanced contribution of PSI fluorescence. In fact, the PSII/PSI ratios in cortices were higher and those in the xylem rays similar to that of leaves. Inner twig tissues were quite resistant to photoinhibitory treatments, tolerating irradiation levels several-fold higher than those encountered in their microenvironment. Moreover, the extent of high light tolerance was similar in naturally exposed and shaded twig sides. The results indicate an increasing, inherent and light-independent inactivation of PSII along twig depth. The findings are discussed on the basis of a recently proposed model for photosynthetic electron flow in twigs, taking into account the specific atmospheric and light microenvironment as well as the possible metabolic needs of such bulky organs.