Effects of Exogenous γ-Aminobutyric Acid (GABA) on Photosynthesis and Antioxidant System in Pepper (Capsicum annuum L.) Seedlings Under Low Light Stress

The effects of γ-aminobutyric acid (GABA) treatment on parameters of photosynthesis and antioxidant defense system were measured in pepper (Capsicum annuum L.) leaves under low-light (LL) stress. Seedlings exposed to LL stress showed increased chlorophyll content as well as decreased net photosynthetic rate (P _n), stomatal conductance (g _s), maximum quantum yield of PSII (F _v/F _m), actual PSII photochemical efficiency (Φ_PSII), electron transport rates and photochemical quenching coefficient (q _p). However, almost all the photosynthetic parameters above were enhanced markedly in seedlings treated with GABA under LL stress. Moreover, LL stress increased malondialdehyde (MDA) content, superoxide anion radical (O₂ ^·?) and hydrogen peroxide (H₂O₂) production. GABA-treated, LL-stressed seedlings exhibited lower MDA, O₂ ^·? and H₂O₂ production, and showed an activated antioxidant defense system, including increased activities of superoxide dismutase, catalase, ascorbate peroxidase, glutathione peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, ascorbate and glutathione. Moreover, seedlings subjected to LL stress showed increased endogenous GABA levels, and the level was further improved by application of exogenous GABA. These results suggest that GABA mitigates the LL-induced stress via regulating the antioxidant defense system and maintaining a high level of photochemical efficiency in pepper seedlings.

     

Increased activity of ATP-sulfurylase and increased contents of cysteine and glutathione reduce high cadmium-induced oxidative stress in mustard cultivar with high photosynthetic potential

Cadmium is known to reduce photosynthesis and overall growth of plants. Plants adopt several mechanisms of Cd detoxification, such as accumulation of sulfur-rich compounds, like glutathione (GSH) and its precursor cysteine. The accumulation of GSH is regulated by the activity of ATP-sulfurylase, a rate-limiting enzyme in sulfur assimilation. The carbon of Cys is provided through photosynthesis. Thus, a plant with the higher photosynthetic potential and ATP-sulfurylase activity may have the higher contents of Cys and GSH and therefore may provide for a greater tolerance to Cd stress. Mustard (Brassica juncea L. Czern and Coss.) cvs. Varuna (high photosynthetic potential) and RH30 (low photosynthetic potential) were subjected to 0 and 200 mg Cd/kg soil, and the activity of ATP-sulfurylase, the contents of Cys and GSH, oxidative stress, and activities of antioxidant enzymes were studied. Under 200 mg Cd/kg soil, cv. Varuna showed an increased ATP-sulfurylase activity, the higher contents of Cys and GSH, and the net photosynthetic rate than cv. RH30. In contrast, the activity of superoxide dismutase, the contents of thiobarbituric acid-reactive substances, and H₂O₂, and electrolyte leakage were found to be greater in cv. RH30 showing an increased oxidative stress than cv. Varuna. However, the activities of ascorbate peroxidase and glutathione reductase were greater in cv. Varuna than cv. RH30. The results show that a greater ATP-sulfurylase activity, an enhanced production of Cys and GSH, and an efficient antioxidant enzyme system in the high photosynthetic mustard cv. Varuna helped to the reduce the oxidative stress maintaing high photosynthesis.     

Selenium Promotes the Growth and Photosynthesis of Tomato Seedlings Under Salt Stress by Enhancing Chloroplast Antioxidant Defense System

Tomato (Lycopersicon esculentum Miller) cv. Jiahe No. 9 (a salinity-resistant cultivar) and cv. Shuangfeng 87-5 (a salinity-sensitive cultivar) were used as experimental materials to investigate the effects of exogenous selenium (Na₂SeO₃ 0.05 mM) on plant growth, chlorophyll fluorescence, photosynthetic rate, and antioxidative metabolism of chloroplasts in tomato seedlings under NaCl (100 mM) stress. Salt stress significantly inhibited plant growth, net photosynthetic rate (P _n), maximum quantum yield of PSII (F _v/F _m), actual photochemical efficiency of PSII (Φ _PSII), photochemical quenching coefficient (q _P), and non-photochemical quenching coefficient (q _N) of both cultivars, whereas application of Se reversed the negative effects of salt stress. Furthermore, application of Se significantly decreased the levels of hydrogen peroxide (H₂O₂) and malondialdehyde. Application of Se increased the activities of superoxidase dismutase, glutathione reductase, dehydroascorbate reductase, monodehydroascorbate reductase, glutathione peroxidase, and thioredoxin reductase, and the contents of ascorbate, glutathione (GSH) and NADPH, and the ratios of GSH/GSSH, AsA/DHA, and NADPH/ NADP⁺ in the salt-stressed chloroplasts of both cultivars. These results suggest that Se alleviates salt-induced oxidative stress through regulating the antioxidant defense systems in the chloroplasts of tomato seedlings, which is associated with the improvement of the photochemical efficiency of PSII, thereby maintaining higher photosynthetic rates. In addition, the salt tolerance of Jiahe No. 9 is closely related with high reactive oxygen species scavenging activity and reducing power levels in the chloroplasts.     

Photosynthesis is improved by exogenous calcium in heat-stressed tobacco plants

Effects of exogenous calcium chloride (CaCl₂) (20 mM) on photosynthetic gas exchange, photosystem II photochemistry, and the activities of antioxidant enzymes in tobacco plants under high temperature stress (43 °C for 2 h) were investigated. Heat stress resulted in a decrease in net photosynthetic rate (P_n), stomatal conductance as well as the apparent quantum yield (AQY) and carboxylation efficiency (CE) of photosynthesis. Heat stress also caused a decrease of the maximal photochemical efficiency of primary photochemistry (F_v/F_m). On the other hand, CaCl₂ application improved P_n, AQY, and CE as well as F_v/F_m under high temperature stress. Heat stress reduced the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), whereas the activities of these enzymes either decreased less or increased in plants pretreated with CaCl₂; glutathione reductase (GR) activity increased under high temperature, and it increased more in plants pretreated with CaCl₂. There was an obvious accumulation of H₂O₂ and O₂⁻ under high temperature, but CaCl₂ application decreased the contents of H₂O₂ and O₂⁻ under heat stress conditions. Heat stress induced the level of heat shock protein 70 (HSP70), while CaCl₂ pretreatment enhanced it. These results suggested that photosynthesis was improved by CaCl₂ application in heat-stressed plants and such an improvement was associated with an improvement in stomatal conductance and the thermostability of oxygen-evolving complex (OEC), which might be due to less accumulation of reactive oxygen species.     

A DESD-box helicase functions in salinity stress tolerance by improving photosynthesis and antioxidant machinery in rice (Oryza sativa L. cv. PB1)

The exact mechanism of helicase-mediated salinity tolerance is not yet understood. We have isolated a DESD-box containing cDNA from Pisum sativum (Pea) and named it as PDH45. It is a unique member of DEAD-box helicase family; containing DESD instead of DEAD/H. PDH45 overexpression driven by constitutive cauliflower mosaic virus-35S promoter in rice transgenic [Oryza sativa L. cv. Pusa Basmati 1 (PB1)] plants confers salinity tolerance by improving the photosynthesis and antioxidant machinery. The Na⁺ ion concentration and oxidative stress parameters in leaves of the NaCl (0, 100 or 200 mM) treated PDH45 overexpressing T₁ transgenic lines were lower as compared to wild type (WT) rice plants under similar conditions. The 200 mM NaCl significantly reduced the leaf area, plant dry mass, net photosynthetic rate (P_N), stomatal conductance (gs), intercellular CO₂ (Ci), chlorophyll (Chl) content in WT plants as compared to the transgenics. The T₁ transgenics exhibited higher glutathione (GSH) and ascorbate (AsA) contents under salinity stress. The activities of antioxidant enzymes viz. superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX) and glutathione reductase (GR) were significantly higher in transgenics; suggesting the existence of an efficient antioxidant defence system to cope with salinity induced-oxidative damage. Yeast two-hybrid assay indicated that the PDH45 protein interacts with Cu/Zn SOD, adenosine-5′-phosphosulfate-kinase, cysteine proteinase and eIF(4G), thus confirming the involvement of ROS scavenging machinery in the transgenic plants to provide salt tolerance. Furthermore, the T₂ transgenics were also able to grow, flower, and set viable seeds under continuous salinity stress of 200 mM NaCl. This study provides insights into the mechanism of PDH45 mediated salinity stress tolerance by controlling the generation of stress induced reactive oxygen species (ROS) and also by protecting the photosynthetic machinery through a strengthened antioxidant system.     

Regulation of photosynthesis and antioxidant metabolism in maize leaves at optimal and chilling temperatures: review

Maize (Zea mays L.) is a chilling (below 15 °C) sensitive plant that shows little capacity to acclimate to low growth temperatures. Maize leaves are extremely sensitive to chilling injury, which usually results in premature leaf senescence. Leaves exposed to temperatures below 10 °C in the light show substantial inhibition of CO₂ assimilation and down-regulation of photosynthetic electron transport. However, the intrinsic relationships between the quantum efficiencies of photosystems I and II are not modified by chilling. Moreover, the integral relationships between non-cyclic electron transport and CO₂ fixation are similar in chilled and unchilled leaves. In this review we examine the roles and importance of photosynthetic regulation, carbon metabolism and antioxidant metabolism in determining the sensitivity of maize leaf photosynthesis to chilling. The distinct cellular localisation patterns of antioxidant enzymes such as glutathione reductase (EC 1.6.4.2) and dehydroascorbate reductase (EC 1.8.5.1) can restrict the recycling of antioxidants associated with photosynthesis during chilling. Disruption of circadian regulation of metabolism and insufficient antioxidant defence are postulated to cause chilling sensitivity.     

5-Aminolevulinic acid enhances photosynthetic gas exchange, chlorophyll fluorescence and antioxidant system in oilseed rape under drought stress

This study evaluates the role of exogenous foliar application of 5-aminolevulinic acid (ALA) on water relations, gas exchange, chlorophyll fluorescence, and the activities and gene expression patterns of antioxidant enzymes in leaves of oilseed rape under drought stress and recovery conditions. Seedlings at four-leaf stage were imposed to well-watered condition (80 % of water-holding capacity) or drought stress (40 % of water-holding capacity) and subsequently foliar sprayed with water or ALA (30 mg l^?1). Drought suppressed the accumulation of plant biomass and decreased chlorophyll content and leaf water status (relative water content and water potential). The actual quantum yield of photosystem II and electron transport rates were hampered in parallel to net photosynthetic rate. However, drought stress induced the accumulation of malondialdehyde (MDA) and hydrogen peroxide, enhanced the activities of catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR) and superoxide dismutase and up-regulated the expression of APX and GR. After rehydration for 4 days, the growth of drought-treated seedlings was restored to normal level for most of the physiological parameters. Foliar application of ALA maintained relatively higher leaf water status and enhanced chlorophyll content, net photosynthetic rate, actual quantum yield of photosystem II, photochemical quenching, non-photochemical quenching and electron transport rates in stressed leaves. Exogenous ALA also alleviated the accumulation of MDA and hydrogen peroxide, increased the activities of antioxidant enzymes and enhanced the expression of CAT and POD in drought-treated plants. These results indicate that ALA may effectively protect rapeseed seedlings from damage induced by drought stress.     

Drought stress induces oxidative stress and the antioxidant defense system in ascorbate-deficient vtc1 mutants of Arabidopsis thaliana

Drought stress has a negative impact on plant cells and results in the generation of reactive oxygen species (ROS). To increase our understanding of the effects of drought stress on antioxidant processes, we investigated the response of the ascorbate-deficient Arabidopsis thaliana vtc1 mutant to drought stress. After drought stress, vtc1 mutants exhibited increases in several oxidative parameters, including H₂O₂ content and the production of thiobarbituric acid reactive substances. Decreases in chlorophyll content and chlorophyll fluorescence parameters were also observed. The vtc1 mutants had higher total glutathione than did wild-type (WT) plants after 48 h of drought stress. A reduced ratio of glutathione/total glutathione and an increased ratio of dehydroascorbate/total ascorbate were observed in the vtc1 mutants compared with the WT plants. In addition, the activities of enzymes that are responsible for ROS scavenging, including superoxide dismutase, catalase, and ascorbate peroxidase, were decreased in the vtc1 mutants compared with the WT plants. Similar reductions in activity in the vtc1 mutant were observed for the enzymes that are responsible for the regeneration of ascorbate and glutathione, including monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase. These results suggest that low intrinsic ascorbate and impaired ascorbate–glutathione cycling in the vtc1 mutant induced a decrease in the reduced form of ascorbate, which enhanced sensitivity to drought stress.     

Chickpea glutaredoxin (CaGrx) gene mitigates drought and salinity stress by modulating the physiological performance and antioxidant defense mechanisms

Glutaredoxins (Grxs) are short, cysteine-rich glutathione (GSH)-mediated oxidoreductases. In this study, a chickpea (Cicer arietinum L.) glutaredoxin [LOC101493651 (CaGrx)] gene has been selected based on screening experiments with two contrasting varieties of chickpea, PUSA-362 (drought-tolerant) and ICC-1882 (drought-sensitive) under drought and salinity. The tolerant variety showed higher CaGrx gene expression, as compared to less in the sensitive variety, under both the stresses. The CaGrx gene was then over-expressed in Arabidopsis thaliana and were exposed to drought and salinity. The over-expression of CaGrx elevated the activity of glutaredoxin, which induced antioxidant enzymes (glutathione reductase; GR, glutathione peroxidase; GPX, catalase; CAT, ascorbate peroxidase; APX, glutathione-S-transferase; GST, superoxide dismutase; SOD, monodehydroascorbate reductase; MDHAR, and dehydroascorbate reductase; DHAR), antioxidants (GSH and ascorbate) and stress-responsive amino acids (cysteine and proline). Enhancement in the antioxidant defense system possibly administered tolerance in transgenics against both stresses. CaGrx reduced stress markers (H₂O₂, TBARS, and electrolyte leakage) and enhanced root growth, seed germination, and survival against both stresses. The physiological parameters (net photosynthesis; P_N, water use efficiency; WUE, stomatal conductance; g_s, transpiration; E, electron transport rate; ETR, and photochemical quenching; qP), chlorophylls and carotenoids, were improved in the transgenics during both stresses, that maintained the photosynthetic apparatus and protected the plants from damage. The enhanced activity of the cysteine biosynthesis enzyme, o-acetylserine (thiol) lyase (OAS-TL), increased the cysteine level in the transgenics, which elevated glutathione biosynthesis to maintain the ascorbate–glutathione cycle under both stresses. This investigation verified that the CaGrx gene provides tolerance against salinity and drought, maintaining physiological and morphological performances, and could be exploited for genetic engineering approaches to overcome both the stresses in various crops.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12298-021-00999-z.     

Antioxidant responses of Atlantic Forest native tree species as indicators of increasing tolerance to oxidative stress when they are exposed to air pollutants and seasonal tropical climate

Tropical forest ecosystems may be subjected to climate-origin oxidative stress as it is observed in Southeast Brazil. The region is characterized by seasonal climate with well-defined wet and dry periods. Anthropogenic air pollutants are additional oxidative stress sources in these ecosystems. However, the tolerance of Brazilian tree species to oxidative stress is still unknown. Thus, the current field study aims to comparatively establish the range of antioxidant responses of ascorbate-glutathione cycle in Astronium graveolens, Croton floribundus and Piptadenia gonoacantha adult trees exposed to air pollutants and seasonal tropical climate as indicators of their increasing tolerance to oxidative stress. These are the most abundant species in the semideciduous Atlantic Forest, in Southeast Brazil. Variations in biochemical leaf traits (antioxidant defenses: ascorbate peroxidase, catalase, superoxide dismutase, glutathione reductase, ascorbate and glutathione; pigments: chlorophyll a, b and carotenoid; oxidative damage indicators: hydrogen peroxide and lipid peroxidation indicator) were determined. The native Brazilian tree species revealed distinct biochemical patterns in response to environmental oxidative stress during the wet and dry seasons. Biochemical leaf traits changed mainly in response to photoxidative stress, during the wet season. This variation was stimulated by better climate conditions to photosynthesis and plant growth, such as high light energy, water availability and temperatures. Catalase seemed to be the biochemical leaf tolerance indicator in all species during the wet season. The environmental conditions during the dry season, either of natural or anthropogenic origin, were stressful to the Brazilian tree species. They induced several changes in their biochemical leaf traits. Such changes were indicated by multilinear regression analyses. Oxidative/antioxidative imbalances, such as increased lipid peroxidation and decreased glutathione as well as the chlorophyll contents, were the most appropriate oxidative stress indicators during the dry season. C. floribundus was the most efficient species in terms of oxidative stress tolerance and it was followed by A. graveolens and P. gonoacantha.     

Girdling induces oxidative damage and triggers enzymatic and non-enzymatic antioxidative defences in Citrus leaves

The effects of girdling on oxidative damage, antioxidant enzyme activity, antioxidant metabolites and proline (Pro) were studied in leaves arising from different shoot types of potted 2-year-old ‘Loretina’ mandarin (Citrus reticulata Blanco) trees during the spring flush period. Girdling increased malonyldialdehyde (MDA) and basal chlorophyll (Chl) a fluorescence (F_o) in young leaves 30 days after girdling but not in the mature leaves (ML) suggesting a disruption of photosynthetic apparatus and oxidative damage in young leaves. This phenomenon was accompanied by increasing levels of Pro. Paralleling these changes, an increase of all antioxidant enzyme activities occurred in leaves from vegetative (VG) and multiflowered leafy shoots (MLY) of girdled trees. Similarly, in ML of girdled trees, ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR) activity also increased. However, dehydroascorbate reductase (DHAR) activity decreased and superoxide dismutase (SOD) activity remained unchanged. Total leaf carbohydrate content and starch also increased as a result of girdling in all shoot types. Whilst soluble sugars increased markedly in young leaves, they increased only slightly in ML. In conclusion, this study provides evidence that girdling gives rise to oxidative damage in Citrus during carbohydrate accumulation, triggering enzymatic and non-enzymatic defence mechanisms.     

Antioxidant system and photosynthetic characteristics responses to short-term PEG-induced drought stress in cucumber seedling leaves

The effect of short-term drought stress on the water content, antioxidant system and photosynthetic characteristics was investigated using cucumber (Cucumis sativus L.) seedlings. The results indicated that polyethylene glycol induced water stress reduced water content in shoots of cucumber seedling after treatment of 36 hours, and caused obvious reductions in net photosynthetic rate, stomatal conductance, intercellular CO₂ concentration and transpiration of leaves. In addition, water stress significantly reduced the photosynthetic pigment content and inhibited photochemical activity, including actual photochemical efficiency, maximal quantum yield of photosystem II photochemistry and coefficient for photochemical quenching. Meanwhile non-photochemical quenching increased. As responses to drought stress, significant increases in electrolyte leakage, malondialdehyde, superoxide anion and hydrogen peroxide levels were detected in leaves. The superoxide dismutases, catalase, glutathione reductase and dehydroascorbate reductase activities, protein, ascorbate and glutathione content, all decreased and peroxidases activity increased, while ascorbate peroxidase and monodehydroascorbate reductase activities exhibited different trend under different degree of water stress. Therefore, it can be concluded that water stress strongly disrupted the normal metabolism of leaves and restrained water absorption.     

Alleviation of exogenous 6-benzyladenine on two genotypes of eggplant (Solanum melongena Mill.) growth under salt stress

Cytokinins were recently shown to control plant adaptation to environmental stresses. To characterize the roles of cytokinins in the tolerance of eggplant (Solanum melongena Mill.) to salt stress, the protective effects of 6-benzyladenine (6-BA) on the growth, photosynthesis, and antioxidant capacity in the leaves of two eggplant cultivars Huqie12 (salt-sensitive) and Huqie4 (salt-tolerant) were investigated. Under 90 mM NaCl stress, Huqie4 showed higher biomass accumulation and less oxidative damage compared to the Huqie12. Application of exogenous 10 μM 6-BA significantly alleviated the growth suppression caused by salt stress in two eggplant genotypes. In parallel with the growth, 6-BA application in salt-stressed plants resulted in enhanced chlorophyll contents, as well as photosynthetic parameters such as net CO₂ assimilation rate (P _n), stomatal conductance (g _s), transpiration rate (E), and intercellular CO₂ concentration (C _i). Furthermore, exogenous 6-BA also significantly reduced the O₂ ^? production rate and malondialdehyde content and markedly increased the antioxidant enzymes superoxide dismutase and peroxidase, the antioxidant metabolites ascorbate and reduced glutathione (GSH), and proline in both genotypes under salt stress. The results indicate that exogenous 6-BA is useful to improve the salt resistance of eggplant, which is most likely related to the increase in photosynthesis and antioxidant capacity.     

Effects of ALA on Photosynthesis,Antioxidant Enzyme Activity,and Gene Expression,and Regulation of Proline Accumulation in Tomato Seedlings Under NaCl Stress

This study examined the effects of 5-aminolevulinic acid (ALA) application on photosynthesis, activity and gene expression of key antioxidant enzymes, and on proline accumulation in tomato (Lycopersicon esculentum Mill. ‘Hezuo 903’) seedlings under NaCl stress. NaCl stress significantly decreased the net photosynthetic rates and inhibited the activity of photosystem II, whereas exogenous ALA application significantly restored the net photosynthetic rates, quantum yield of electron transport, and energy conversion efficiency of photosystem II of tomato under NaCl stress. Production of superoxide, hydrogen peroxide, and malondialdehyde strongly increased in response to NaCl stress, and these increases were significantly counteracted by ALA. ALA increased the activity of reactive oxygen species (ROS) scavenging antioxidant enzymes, including superoxide dismutase, catalase, ascorbate peroxidase, and peroxidase, and upregulated the expression of SOD, APX, and POD, genes that encode these enzymes in NaCl-treated plants. ALA simultaneously increased proline accumulation in tomato seedlings under NaCl stress by regulating the expression of genes that encode ALA biosynthetic enzymes and that control proline biosynthesis and metabolism, for example, expression of GluRS and GluTR was downregulated, accompanied by a significant increase in the expression of P5CS and decline in the expression of ProDH. ALA provided protection against NaCl stress by increasing photosynthetic capacity, regulating antioxidant enzyme gene expression and proline accumulation, and decreasing ROS accumulation and lipid peroxidation in tomato.     

Girdling induces oxidative damage and triggers enzymatic and non-enzymatic antioxidative defences in Citrus leaves

The effects of girdling on oxidative damage, antioxidant enzyme activity, antioxidant metabolites and proline (Pro) were studied in leaves arising from different shoot types of potted 2-year-old ‘Loretina’ mandarin (Citrus reticulata Blanco) trees during the spring flush period. Girdling increased malonyldialdehyde (MDA) and basal chlorophyll (Chl) a fluorescence (F_o) in young leaves 30 days after girdling but not in the mature leaves (ML) suggesting a disruption of photosynthetic apparatus and oxidative damage in young leaves. This phenomenon was accompanied by increasing levels of Pro. Paralleling these changes, an increase of all antioxidant enzyme activities occurred in leaves from vegetative (VG) and multiflowered leafy shoots (MLY) of girdled trees. Similarly, in ML of girdled trees, ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR) activity also increased. However, dehydroascorbate reductase (DHAR) activity decreased and superoxide dismutase (SOD) activity remained unchanged. Total leaf carbohydrate content and starch also increased as a result of girdling in all shoot types. Whilst soluble sugars increased markedly in young leaves, they increased only slightly in ML. In conclusion, this study provides evidence that girdling gives rise to oxidative damage in Citrus during carbohydrate accumulation, triggering enzymatic and non-enzymatic defence mechanisms.     

Antioxidant response of wheat roots to drought acclimation

Wheat (Triticum aestivum L.) seedlings of a drought-resistant cv. C306 were subjected to severe water deficit directly or through stress cycles of increasing intensity with intermittent recovery periods. The antioxidant defense in terms of redox metabolites and enzymes in root cells and mitochondria was examined in relation to membrane damage. Acclimated seedlings exhibited higher relative water content and were able to limit the accumulation of H₂O₂ and membrane damage during subsequent severe water stress conditions. This was due to systematic up-regulation of superoxide dismutase, ascorbate peroxidase (APX), catalase, peroxidases, and ascorbate–glutathione cycle components at both the whole cell level as well as in mitochondria. In contrast, direct exposure of severe water stress to non-acclimated seedlings caused greater water loss, excessive accumulation of H₂O₂ followed by elevated lipid peroxidation due to the poor antioxidant enzyme response particularly of APX, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and ascorbate–glutathione redox balance. Mitochondrial antioxidant defense was found to be better than the cellular defense in non-acclimated roots. Termination of stress followed by rewatering leads to a rapid enhancement in all the antioxidant defense components in non-acclimated roots, which suggested that the excess levels of H₂O₂ during severe water stress conditions might have inhibited or down-regulated the antioxidant enzymes. Hence, drought acclimation conferred enhanced tolerance toward oxidative stress in the root tissue of wheat seedlings due to both reactive oxygen species restriction and well-coordinated induction of antioxidant defense.     

Hydroponic treatment with salicylic acid decreases the effects of chilling injury in maize (Zea mays L.) plants

The addition of 0.5 mM salicylic acid (SA) to the hydroponic growth solution of young maize (Zea mays L.) plants under normal growth conditions provided protection against subsequent low-temperature stress. This observation was confirmed by chlorophyll fluorescence parameters and electrolyte leakage measurements. In addition, 1 d of 0.5 mM SA pre-treatment decreased net photosynthesis, stomatal conductivity and transpiration at the growth temperature (22/20 °C). Since there was only a slight decrease in the ratio of variable to maximal fluorescence (F_v/F_m) the decrease in photosynthetic activity is not due to a depression in photosystem II. The analysis of antioxidant enzymes showed that whereas SA treatment did not cause any change in ascorbate peroxidase (EC 1.11.1.11) and superoxide dismutase (EC 1.15.1.1) activities, there was a decrease in catalase (EC 1.11.1.6) activity, and an increase in guaiacol peroxidase (EC 1.11.1.7) and glutathione reductase (EC 1.6.4.2) activities after the 1-d SA treatment at 22/20 °C. In native polyacrylamide gels there was, among the peroxidase isoenzymes, a band which could be seen only in SA-treated plants. It is suggested that the pre-treatment of maize plants with SA at normal growth temperature may induce antioxidant enzymes which lead to increased chilling tolerance. Received: 4 June 1998 / Accepted: 23 November 1998     

Ethylene reverses photosynthetic inhibition by nickel and zinc in mustard through changes in PS II activity,photosynthetic nitrogen use efficiency,and antioxidant metabolism

We investigated the influence of exogenously sourced ethylene (200 μL L^?1 ethephon) in the protection of photosynthesis against 200 mg kg^?1 soil each of nickel (Ni)- and zinc (Zn)-accrued stress in mustard (Brassica juncea L.). Plants grown with Ni or Zn but without ethephon exhibited increased activity of 1-aminocyclopropane carboxylic acid synthase, and ethylene with increased oxidative stress measured as H₂O₂ content and lipid peroxidation compared with control plants. The oxidative stress in Ni-grown plants was higher than Zn-grown plants. Under metal stress, ethylene protected photosynthetic potential by efficient PS II activity and through increased activity of ribulose-1,5-bisphosphate carboxylase and photosynthetic nitrogen use efficiency (P-NUE). Application of 200 μL L^?1 ethephon to Ni- or Zn-grown plants significantly alleviated toxicity and reduced the oxidative stress to a greater extent together with the improved net photosynthesis due to induced activity of ascorbate peroxidase and glutathione (GSH) reductase, resulting in increased production of reduced GSH. Ethylene formation resulting from ethephon application alleviated Ni and Zn stress by reducing oxidative stress caused by stress ethylene production and maintained increased GSH pool. The involvement of ethylene in reversal of photosynthetic inhibition by Ni and Zn stress was related to the changes in PS II activity, P-NUE, and antioxidant capacity was confirmed using ethylene action inhibitor, norbornadiene.     

Boron excess affects photosynthesis and antioxidant apparatus of greenhouse Cucurbita pepo and Cucumis sativus

This study aimed to evaluate the behavior of zucchini (Cucurbita pepo L.) and cucumber (Cucumis sativus L.) under boron (B) excess. Plants were grown under greenhouse conditions in a sandy soil–peat mixture using a nutrient solution containing 0.2 (control), 10 and 20 mg L^?1 B. Visible symptoms were quantified and leaf B accumulation, gas exchanges, chlorophyll (Chl) a fluorescence, malondialdehyde by-products and antioxidants were investigated 20 days after the beginning of the treatments. Boron toxicity induced oxidative load and leaf necrotic burns coupled with the reduction of leaf growth and biomass accumulation in both species. Boron excess resulted in a decrease of Chl a/b ratio, potential (F_v/F_m) and actual (Φ_PSII) PSII quantum efficiency, photosynthetic rate (Pn), stomatal conductance (g_s), and transpiration (E) as well. A general stimulation of the antioxidant enzymes ascorbate peroxidase, catalase and superoxide dismutase was observed, and a significant increase in the oxidized form of ascorbate and glutathione was evidenced for treated plants of both species. A difference between the two species was observed: C. pepo appeared to be more sensitive to B stress being damaged at all B concentration. C. sativus grown at 10 mg L^?1 B in nutrient solution showed some down-regulated mechanisms, i.e. increase in Chl b content and a good photochemical PSII efficiency as well as a higher amount of constitutive antioxidant molecules, that, however, are not sufficient to contrast the negative effects of B.     

外源H_2S对NO_3~-胁迫下番茄幼苗生理生化特性的影响

采用营养液水培方法,通过外源施加H2S供体NaHS(100μmol/L),研究了信号分子H2S对100mmol/L NO3-胁迫下番茄幼苗生理生化特性的影响。结果表明:(1)NO3-胁迫下,随着处理时间的延长,番茄幼苗的株高、根长、鲜重和干重显著降低,叶绿素(a、b)含量、净光合速率、气孔导度、蒸腾速率均显著降低,而胞间CO2浓度以及丙二醛(MDA)、H2O2含量增加,超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)和抗坏血酸过氧化物酶(APX)活性显著降低,抗坏血酸(AsA)和还原性谷胱甘肽(GSH)含量显著降低。(2)与NO3-胁迫处理相比,外源NaHS处理1、3、5d后,番茄幼苗的株高、根长、鲜重和干重显著增加,叶绿素(a、b)含量、净光合速率、气孔导度、蒸腾速率均显著升高,而胞间CO2浓度显著降低;MDA和H2O2含量降低,SOD、POD、CAT和APX活性显著增强,AsA和GSH含量显著增加,而且幼苗的硝酸还原酶、谷氨酰胺合成酶、谷氨酸合酶的活性显著增强;L-半胱氨酸脱巯基酶活性和内源H2S含量增加。研究认为,外源H2S可能通过提高抗氧化物酶的活性和增加抗氧化物质含量来缓解NO3-对番茄幼苗造成的伤害,从而增强其对NO3-胁迫耐性。