Antioxidant enzyme responses to chilling stress in differentially sensitive inbred maize lines

Antioxidant enzyme activities were determined at the first,third and fifth leaf stages of four inbred lines of maize (Zeamays L.) exhibiting differential sensitivity to chilling. Plantswere exposed to a photoperiod of 16:8 L:D for one of three treatments:(a) control (25C), (b) control treatment plus an exposure toa short-term chilling shock (11C 1 d prior to harvesting),and (c) long-term (11 C constant) chilling exposure. Catalase(CAT; EC 1.11.1.6 [EC] ), ascorbate peroxidase (ASPX; EC 1.11.1.11 [EC] ),superoxide dismutase (SOD; EC 1.15.1.1 [EC] ), glutathione reductase(GR; EC 1.6.4.2 [EC] ), and mono-dehydroascorbate reductase (MDHAR;EC 1.6.5.4 [EC] ) activities were assessed. Reducing and non-reducingsugars and starch concentrations were determined as generalmetabolic indicators of stress. Reduced activities of CAT, ASPX,and MDHAR may contribute to limiting chilling tolerance at theearly stages of development in maize. Changes in levels of sugarand starch indicated a more rapid disruption of carbohydrateutilization in comparison to photosynthetic rates in the chilling-sensitiveline under short-term chilling shocks and suggested a greaterdegree of acclimation in the tolerant lines over longer periodsof chilling. Key words: Antioxidant enzymes, differential chilling sensitivity, maize, soluble carbohydrates, Zea mays     

Synthesis and Secretion of Acid Phosphatase in Halotolerant Zygosaccharomyces rouxii

Changes in the contents of defensive substances against the active oxygen in water-stressed spinach plants were examined. The contents of ascorbate peroxidase (AP; EC 1.11.1.7), glutathione reductase (GR; EC 1.6.4.2) and α-tocopherol increased remarkably in water-stressed spinach leaves, while those of Superoxide dismutase (SOD; EC 1.15.1.1), dehydroascorbate reductase (EC 1.8.5.1), ascorbate and glutathione changed little. The content of α-tocopherol in chloroplast thylakoid membranes isolated from water-stressed leaves was higher than that from normal leaves. It is, therefore, conceivable that GR, AP and α-tocopherol might be related to the tolerance of plants to water deficiency.     

Antioxidative responses of Calendula officinalis under salinity conditions.

To gain a better insight into long-term salt-induced oxidative stress, some physiological parameters in marigold (Calendula officinalis L.) under 0, 50 and 100 mM NaCl were investigated. Salinity affected most of the considered parameters. High salinity caused reduction in growth parameters, lipid peroxidation and hydrogen peroxide accumulation. Under high salinity stress, a decrease in total glutathione and an increase in total ascorbate (AsA + DHA), accompanied with enhanced glutathione reductase (GR, EC 1.6.4.2) and ascorbate peroxidase (APX, EC 1.11.1.11) activities, were observed in leaves. In addition, salinity induced a decrease in superoxide dismutase (SOD, EC 1.15.1.1) and peroxidase (POX, EC 1.11.1.7) activities. The decrease in dehydroascorbate reductase (DHAR, EC 1.8.5.1) and monodehydroascorbate reductase (MDHAR, EC 1.6.5.4) activities suggests that other mechanisms play a major role in the regeneration of reduced ascorbate. The changes in catalase (CAT, EC 1.11.1.6) activities, both in roots and in leaves, may be important in H2O2 homeostasis.     

Monodehydroascorbate Reductase in Spinach Chloroplasts and Its Participation in Regeneration of Ascorbate for Scavenging Hydrogen Peroxide

The primary reaction product of chloroplast ascorbate peroxidaseactivity was shown to be monodehydroascorbate radical (MDA).MDA reductase (EC 1.6.5.4 [EC] ) was localized in spinach chloroplaststroma. The MDA reductase activity of spinach chloroplasts,using NAD(P)H as electron donor, could account for the regenerationof ascorbate from MDA produced by ascorbate peroxidase activity.In the absence of MDA reductase, MDA disproportionated to ascorbate(AsA) and dehydroascorbate (DHA). The DHA was reduced to AsAby DHA reductase (EC 1.8.5.1 [EC] ) in chloroplasts. Both NADH andNADPH served as the electron donor of partially purified MDAreductase from spinach leaves. (Received September 24, 1983; Accepted January 23, 1984)     

Age-dependence of the antioxidative system in tobacco with enhanced glutathione reductase activity or senescence-induced production of cytokinins

Tobacco leaves of plants with enhanced glutathione reductase activity (GR46-27, Nicotiana tabacum L. cv. Samsun) or with autoregulated senescence-induced production of cytokinins (P_SAG12-IPT, N. tabacum L. cv. Wisconsin) were studied during the course of leaf development and senescence by measuring photosynthesis, chlorophyll and protein content, the antioxidants ascorbate, glutathione and α -tocopherol as well as the antioxidative enzymes ascorbate peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.6.4.2) and superoxide dismutase (SOD, EC 1.15.1.1). The photosynthetic rate, as well as the chlorophyll and protein content, dropped with increasing leaf age after having reached a maximum at the end of the exponential growth phase. The concentrations of the water-soluble antioxidants ascorbate and glutathione fell continuously with age, whereas the concentration of the lipophilic α -tocopherol increased. The activities of the antioxidative enzymes APX, GR and SOD reached their maximum at the beginning of leaf development, but were reduced in senescing leaves. The age-dependent course of the measured leaf parameters in GR46-27 leaves was similar to the one in wild-type leaves, with the exception of an overall enhanced GR activity. In contrast, in old leaves of P_SAG12-IPT plants, which possess a much higher life span, the chlorophyll and protein content, the photosynthetic rate, the antioxidant concentrations of ascorbate and glutathione as well as the activities of the antioxidative enzymes were higher than in wild-type leaves. The results show that the capacity of the antioxidative system to scavenge radicals is sufficiently balanced with the plant metabolism, and its decline with increasing age is not the cause, but a consequence of senescence and ageing in plants.     

Evidence for the Presence of the Ascorbate-Glutathione Cycle in Mitochondria and Peroxisomes of Pea Leaves

The presence of the enzymes of the ascorbate-glutathione cycle was investigated in mitochondria and peroxisomes purified from pea (Pisum sativum L.) leaves. All four enzymes, ascorbate peroxidase (APX; EC 1.11.1.11), monodehydroascorbate reductase (EC 1.6.5.4), dehydroascorbate reductase (EC 1.8.5.1), and glutathione reductase (EC 1.6.4.2), were present in mitochondria and peroxisomes, as well as in the antioxidants ascorbate and glutathione. The activity of the ascorbate-glutathione cycle enzymes was higher in mitochondria than in peroxisomes, except for APX, which was more active in peroxisomes than in mitochondria. Intact mitochondria and peroxisomes had no latent APX activity, and this remained in the membrane fraction after solubilization assays with 0.2 M KCl. Monodehydroascorbate reductase was highly latent in intact mitochondria and peroxisomes and was membrane-bound, suggesting that the electron acceptor and donor sites of this redox protein are not on the external side of the mitochondrial and peroxisomal membranes. Dehydroascorbate reductase was found mainly in the soluble peroxisomal and mitochondrial fractions. Glutathione reductase had a high latency in mitochondria and peroxisomes and was present in the soluble fractions of both organelles. In intact peroxisomes and mitochondria, the presence of reduced ascorbate and glutathione and the oxidized forms of ascorbate and glutathione were demonstrated by high-performance liquid chromatography analysis. The ascorbate-glutathione cycle of mitochondria and peroxisomes could represent an important antioxidant protection system against H2O2 generated in both plant organelles.     

Senescence-related changes in the antioxidant status of ginkgo and birch leaves during autumn yellowing

The antioxidant status of birch and ginkgo leaves during autumnal senescence was characterized by the activities of catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX) and superoxide dismutase (SOD). The contents of leaf H₂O₂ and ascorbate were used as indicators of oxidative stress. Degradation of chlorophyll (chl) during natural senescence was not accompanied either by an increase of H₂O₂ or by a decrease of reduced ascorbate. A transient decrease of reduced ascorbate in ginkgo and birch leaves in early senescence was accompanied by CAT inactivation. The activity of ionically-bound PODs was stimulated in late senescence in both species, when more than 30% of chl was degraded. Induction of MnSOD in both species and new isoforms of CuZnSOD in birch in late senescence was accompanied by the disappearance of other CuZnSOD isoforms in birch and FeSOD in ginkgo. The role of antioxidative enzymes in keeping ascorbate reduced and endogenous H₂O₂ at low levels in senescent leaves of deciduous trees was discussed.     

Antioxidative responses to different altitudes in leaves of alpine plant Polygonum viviparum in summer

Mountain environmental stresses result in increased formation of hydrogen peroxide (H₂O₂) and accumulation of malondialdehyde (MDA) in leaves of Polygonum viviparum. The activities of several antioxidative system enzymes such as superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7), glutathione reductase (GR, EC 1.6.4.2), dehydroascorbate reductase (DHAR, EC 1.8.5.1) and the contents of several non-enzymatic antioxidants such as reduced form of ascorbate (ASC), dehydroascorbate (DHA), reduced glutathione (GSH), and oxidized glutathione (GSSG) were investigated in leaves of P. viviparum, which were collected from three altitudes (2,200, 3,200, and 3,900 m) of Tianshan Mountain in China. The activities of these four antioxidative enzymes were accompanied by increases of H₂O₂ levels from 2,200 to 3,200 m. However, the activities of CAT and POD were decreased, whereas the activities of SOD and GR continually increased at 3,900 m. Analyses of isoforms of SOD, CAT, POD, and GR showed that the leaves of P. viviparum exposed different altitude conditions are capable of differentially altering the intensity. Additionally, two new isoforms of SOD were detected at 3900 m. A continual increase in the ASC, ASC to DHA ratio, GSH and GSH/[GSH + GSSG] ratio, and the activity of DHAR were observed in leaves of P. viviparum with the elevation of altitude. These results suggest that the higher contents of ASC, GSH as well as an increase in reduced redox state may be essential to antioxidation processes in the leaves of P. viviparum, whereas antioxidant enzymes system is a cofactor in the processes.     

Interactive effects of ozone and low UV‐B radiation on antioxidants in spruce (Picea abies) and pine (Pinus sylvestris) needles

To study the role of low UV‐B radiation in modulating the response of antioxidants to ozone, 4‐year‐old pine ( Pinus sylvestris L.) and spruce ( Picea abies L.) seedlings potted in natural soil, were exposed in phytochambers to fluctuating ozone concentrations between 9 and 113 nl 1⁻¹ according to field data recorded at Mt Wank (1175 m above sea level, Bavaria, Germany) and two‐times ambient O₃ levels. UV‐B radiation was either added at a biologically effective level of ca 1.2 kJ m⁻² day⁻¹ , which is close to that found in March at Mt Wank, or was excluded by filters (<0.08 kJ m⁻² day⁻¹). After one growth phase current‐year needles were collected and analysed for antioxidative enzyme activities (superoxide dismutase, SOD, EC 1.15.1.1; catalase, CAT, EC 1.11.1.6; guaiacol peroxidase, POD, EC 1.11.1.7) and soluble antioxidants (ascorbate, glutathione). CAT, POD, ascorbate and glutathione, but not SOD, were increased in needles of both species in response to twice ambient O₃ levels. UV‐B radiation in the presence of ambient O₃ caused an increase in total SOD activity in spruce but had no effects on antioxidants in pine. Twice ambient O₃ levels together with low UV‐B radiation counteracted the O₃‐induced increases in ascorbate and CAT in pine but not in spruce. Under these conditions spruce needles showed the highest antioxidative protection and revealed no indication of lipid peroxidation. Pine needles exposed to UV‐B and elevated O₃ levels showed elevated lipid peroxidation and a 5‐fold increase in dehydroascorbate, suggesting that this species was less protected and suffered higher oxidative stress than spruce.     

Role of glutathione in adaptation and signalling during chilling and cold acclimation in plants

To test the hypothesis that antioxidant systems flexibly adjust to short-term, diurnal fluctuations of ambient environmental conditions, ascorbate-related systems were studied over several day/night cycles in mature sun-acclimated leaves of field-grown beech trees ( Fagus sylvatica ). Light-dependent increases in the activities of ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate radical reductase (MDAR, EC 1.1.5.4) and glutathione reductase (GR, EC 1.6.4.2) were not observed. Lowest activities of APX and MDAR were found on hot, sunny days. A strong negative correlation occurred between APX activities and ambient temperatures suggesting that this enzyme was temperature- rather than light-regulated. In contrast to the enzymatic defences, ascorbate levels increased by about 30% under bright sunlight suggesting that protection from excess light is mediated via the adjustment of metabolites. Under these conditions the apparent electron transport rate exceeded the capacity for assimilation and the dehydroascorbate pool increased twofold. Since dehydroascorbate reductase activities were hardly detected, MDAR activities seemed to be the major enzyme to keep ascorbate in its reduced state. However, MDAR appeared to be insufficient to maintain the redox balance of the ascorbate pool under high light intensities in the field.     

Effect of nitrogen limitation on foliar antioxidants in relationship to other metabolic characteristics

The long-term effect of limiting soil nitrogen (N) availability on foliar antioxidants, thermal energy dissipation, photosynthetic and respiratory electron transport, and carbohydrates was investigated in Spinacia oleracea L. Starch, sucrose, and glucose accumulated in leaves of N-limited spinach at predawn, consistent with a downregulation of chloroplast processes by whole-plant sink limitation in response to a limited supply of N-based macromolecules throughout the plant. On a leaf-area or dry-weight basis, levels of chlorophyll, carotenoid pools, photosynthetic electron transport capacity, as well as activities for the predominantly chloroplast-localized antioxidant enzymes ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2) were much lower in N-limited versus N-replete plants. When expressed on a chlorophyll basis, foliar levels of all of these parameters were similar in N-replete versus N-limited plants. However, on a total-protein basis, antioxidant enzyme activities were higher in N-limited plants. Nitrogen-limited spinach showed higher levels of thermal energy dissipation and of zeaxanthin and antheraxanthin at midday, as well as slightly higher ascorbate contents relative to chlorophyll. These results indicate that strong, long-term N limitation led not only to alterations in the balance between different processes but also to an overall downregulation of light collection, photosynthetic electron transport capacity, and chloroplast-based antioxidant enzymes. This is further supported by the finding that glucose-feeding of excised leaves led to strong concomitant decreases in photosynthetic electron transport capacity and ascorbate peroxidase activity. On a leaf-area basis, neither superoxide dismutase (EC 1.15.1.1) activity nor dark repiration rates showed a treatment effect. This indicates that overall mitochondrial electron transport activity does not decrease under long-term N limitation and is consistent with localization of an important fraction of foliar superoxide dismutase in mitochondria. Received: 19 March 1999 / Accepted: 13 April 1999     

Effects of 2-month ozone exposure in spinach leaves on photosynthesis,antioxidant systems and lipid peroxidation

The photosynthesis response, antioxidant systems and lipid peroxidation were studied in leaves from spinach plants (Spinacia oleracea L.) in response to ozone fumigation, ambient air and charcoal filtered air treatments. The photosynthetic activity was tested through gas exchange and chlorophyll a fluorescence measurements. Ambient air and ozone fumigation caused a decrease in the photosynthetic rate (25% and 63%, respectively) mainly due to a reduced mesophyll activity, as evidenced by the increased intercellular CO₂ concentration. These data agree with a large reduction in the non-cyclic electron flow (7% and 16%), a lower capacity to reduce the quinone pool and a higher development of non-photochemical quenching upon high O₃ concentration. The results suggest that the oxidative stress produced, together with the stimulation of superoxide dismutase (SOD, EC 1.15.1.1) and ascorbate peroxidase (APX, EC 1.11.1.11) activities and the increase in lipid peroxidation (20% and 36%, respectively), generated an alteration of the membrane properties.     

Antioxidants in the apoplast and symplast of beech (Fagus sylvatica L.) leaves: seasonal variations and responses to changing ozone concentrations in air

Concentrations of the antioxidants ascorbate and glutathione were measured in the apoplast of beech (Fagus sylvatica L.) leaves and in leaf tissue. During early leaf development, reduced ascorbate (ASC) was almost absent from the apoplast, whereas levels of oxidized ascorbate (DHA) were high. Less than 20% of the apoplastic ascorbate was reduced. ASC increased towards midsummer, reaching top levels of about 4molm^?3 apoplast volume in July and August. Reduction increased to 60–75% in summer. Neither DHA reductase nor glutathione was detected in the apoplast of beech leaves. Levels of apoplastic ascorbate were compared with ambient concentrations of ozone in air. Statistical analysis indicated a significant interrelation between atmospheric ozone and apoplastic ascorbate. In midsummer of 1993, contents of DHA were increased in the apoplast when ozone concentrations were high. Apoplastic ASC was also positively correlated with ambient ozone concentrations, but with a delay of 3 to 7d. In leaf tissue, levels of ascorbate were between 17 and 21 μmolg^?1 FW in summer. Except for late April and November, more than 95% of the intracellular ascorbate was reduced. Glutathione contents were lowest during the summer. Oxidation was increased in spring and autumn, when apoplastic ascorbate was also largely oxidized. Usually, 80 to 90% of the glutathione was reduced. During the summer, intracellular concentrations of oxidized glutathione (GSSG) were increased, with a delay of about 1d following periods of high ambient ozone concentrations. The transitory accumulation of GSSG may be explained by slow enzymatic regeneration of glutathione.     

Antioxidant status of the potato tuber and Ca2+ deficiency as a physiological stress

The antioxidant status of potato ( Solanum tuberosum L.) tubers of two genotypes, cv. Désirée and clone 10337de40 was investigated in relation to susceptibility to internal rust spot (IRS), a Ca²⁺-related physiological disorder. Concentrations of total calcium within the perimedulla tissue of tubers, grown with a restricted (1 m M CaCl₂) Ca²⁺ supply, were similar in cv. Désirée (IRS resistant) and clone 10337de40 (IRS susceptible). A range of antioxidants was assayed in order to assess antioxidant status in both genotypes under the two Ca²⁺ treatments. Although no appreciable differences were detected between low Ca²⁺ and control treatments, certain antioxidants were present at significantly higher levels in the IRS resistant genotype, cv. Désirée. These included dehydroascorbate reductase (EC 1.8.5.1) activity (more than 100% higher), total glutathione content (ca 40% higher), glutathione reductase (EC 1.6.4.2) activity (almost 50% higher), peroxidase (EC 1.11.1.7) activity (ca 60% higher) and superoxide dismutase (EC 1.15.1.1) activity (almost 80% higher). There was no difference in ascorbate content, ascorbate free radical reductase activity (EC 1.6.5.4), α-tocopherol levels and catalase activity (EC 1.11.1.6) between the two genotypes. The possible relationship between resistance to IRS and a superior antioxidant status, found in cv. Désirée, is discussed.     

The oxidative stress caused by salinity in two barley cultivars is mitigated by elevated CO2

Changes in antioxidant metabolism because of the effect of salinity stress (0, 80, 160 or 240 m M NaCl) on protective enzyme activities under ambient (350 μmol mol⁻¹) and elevated (700 μmol mol⁻¹) CO₂ concentrations were investigated in two barley cultivars ( Hordeum vulgare L., cvs Alpha and Iranis). Electrolyte leakage, peroxidation, antioxidant enzyme activities [superoxide dismutase (SOD), EC 1.15.1.1; ascorbate peroxidase (APX), EC 1.11.1.11; catalase (CAT), EC 1.11.1.6; dehydroascorbate reductase (DHAR), EC 1.8.5.1; monodehydroascorbate reductase (MDHAR), EC 1.6.5.4; glutathione reductase (GR), EC 1.6.4.2] and their isoenzymatic profiles were determined. Under salinity and ambient CO₂, upregulation of antioxidant enzymes such as SOD, APX, CAT, DHAR and GR occurred. However, this upregulation was not enough to counteract all ROS formation as both ion leakage and lipid peroxidation came into play. The higher constitutive SOD and CAT activities together with a higher contribution of Cu,Zn-SOD 1 detected in Iranis might possibly contribute and make this cultivar more salt-tolerant than Alpha. Elevated CO₂ alone had no effect on the constitutive levels of antioxidant enzymes in Iranis, whereas in Alpha it induced an increase in SOD, CAT and MDHAR together with a decrease of DHAR and GR. Under combined conditions of elevated CO₂ and salinity the oxidative damage recorded was lower, above all in Alpha, together with a lower upregulation of the antioxidant system. So it can be concluded that elevated CO₂ mitigates the oxidative stress caused by salinity, involving lower ROS generation and a better maintenance of redox homeostasis as a consequence of higher assimilation rates and lower photorespiration, being the response dependent on the cultivar analysed.     

Effect of flooding on the activities of some enzymes of activated oxygen metabolism,the levels of antioxidants,and lipid peroxidation in senescing tobacco leaves

Effects of flooding on the activities of some enzymes of activated oxygen metabolism, the levels of antioxidants, and lipid peroxidation in senescing leaves of tobacco were investigated. As judged by the decrease in chlorophyll and protein levels, flooding accelerated the senescence of tobacco leaves. Total peroxide and the lipid peroxidation product, malondialdehyde, increased in both control and flooding-treated leaves with increasing duration of the experiment. Throughout the duration of the experiment, flooded leaves had higher levels of total peroxide and malondialdehyde than did control leaves. Flooding resulted in an increase in peroxidase and ascorbate peroxidase activities and a reduction of superoxide dismutase activity in the senescing leaves. Glycolate oxidase, catalase, and glutathione reductase activities were not affected by flooding. Flooding increased the levels of total ascorbate and dehydroascorbate. Total glutathione, reduced form glutathione, or oxidized glutathione levels in flooded leaves were lower than in control leaves during the first two days of the experiment, but were higher than in control leaves at the later stage of the experiment. Our work suggests that senescence of tobacco induced by flooding may be a consequence of lipid peroxidation possibly controlled by superoxide dismutase activity. Our results also suggest that increased rates of hydrogen peroxide in leaves of flooded plants could lead to increased capacities of the scavenging system of hydrogen peroxide.Abbreviations GSH reduced form glutathione - GSSG oxidized form glutathione - GSSG reductase glutathione reductase - MDA malondialdehyde - SOD superoxide dismutase     

Spread of oxidative damage and antioxidative response through cell layers of tobacco callus after UV-C treatment.

Tobacco (Nicotiana tabacum L. cv. Petit Havana) callus cultures were exposed to UV-C high dose pulse-treatment (254 nm, 50 kJ m(-2), 1 h-treatment). After 6, 24 and 48 h from the end of the treatment, calli were cut transversally in two layers and oxidative damage (malondialdehyde [MDA] and hydrogen peroxide), non-enzymatic (radical scavenging antioxidants [RSA] and polyamines) and enzymatic antioxidants (ascorbate peroxidase [APX, EC 1.11.1.11], glutathione reductase [GR, EC 1.6.4.2], catalase [CAT, EC 1.11.1.6] and guaiacol peroxidase [GPX, EC 1.11.1.7]) were evaluated. At each time-point data referred to UV-C treated calli were compared to data of untreated ones (control). Despite of a strong increase of H2O2 content, a slight cellular damage was observed in both upper and lower layers 24 and 48 h after UV-C treatment. An activation first of non-enzymatic antioxidants and then of enzymatic antioxidants was detected in UV-C treated calli. In particular, RSA and putrescine (PUT) accumulated 6 h after UV-C treatment while APX, GR and GPX enzyme activities increased 24 h after UV-C irradiation. Catalase activity did not change. UV-C-induced oxidative stress and antioxidative response were observed also in cell layers not directly exposed to UV irradiation, indicating that a stress signal was transmitted to the whole mass of callus.     

Response of the cultivated tomato and its wild salt-tolerant relative Lycopersicon pennellii to salt-dependent oxidative stress: The root antioxidative system

The response of the antioxidant system to salt stress was studied in the roots of the cultivated tomato Lycopersicon esculentum Mill. cv. M82 (Lem) and its wild salt-tolerant relative L. pennellii (Corr.) D'Arcy accession Atico (Lpa). Roots of control and salt (100 m M NaCl)-stressed plants were sampled at various times after commencement of salinization. A gradual increase in the membrane lipid peroxidation in salt-stressed root of Lem was accompanied with decreased activities of the antioxidant enzymes: superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APX; EC 1.11.1.11) and decreased contents of the antioxidants ascorbate and glutathione and their redox states. In contrast, increased activities of the SOD, CAT, APX, monodehydroascorbate reductase (MDHAR; EC 1.6.5.4), and increased contents of the reduced forms of ascorbate and glutathione and their redox states were found in salt-stressed roots of Lpa, in which the level of membrane lipid peroxidation remained unchanged. It seems that the better protection of Lpa roots from salt-induced oxidative damage results, at least partially, from the increased activity of their antioxidative system.     

Effect of abscisic acid on active oxygen species, antioxidative defence system and oxidative damage in leaves of maize seedlings.

Leaves of maize (Zea mays L.) seedlings were supplied with different concentrations of abscisic acid (ABA). Its effects on the levels of superoxide radical (O(2)(-)), hydrogen peroxide (H(2)O(2)) and the content of catalytic Fe, the activities of several antioxidative enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR), the contents of several non-enzymatic antioxidants such as ascorbate (ASC), reduced glutathione (GSH), alpha-tocopherol (alpha-TOC) and carotenoid (CAR), and the degrees of the oxidative damage to the membrane lipids and proteins were examined. Treatment with 10 and 100 microM ABA significantly increased the levels of O(2)(-) and H(2)O(2), followed by an increase in activities of SOD, CAT, APX and GR, and the contents of ASC, GSH, alpha-TOC and CAR in a dose- and time-dependent pattern in leaves of maize seedlings. An oxidative damage expressed as lipid peroxidation, protein oxidation, and plasma membrane leakage did not occur except for a slight increase with 100 microM ABA treatment for 24 h. Treatment with 1,000 microM ABA led to a more abundant generation of O(2)(-) and H(2)O(2) and a significant increase in the content of catalytic Fe, which is critical for H(2)O(2)-dependent hydroxyl radical production. The activities of these antioxidative enzymes and the contents of alpha-TOC and CAR were still maintained at a higher level, but no longer further enhanced when compared with the treatment of 100 microM ABA. The contents of ASC and GSH had no changes in leaves treated with 1,000 microM ABA. These results indicate that treatment with low concentrations of ABA (10 to 100 microM) induced an antioxidative defence response against oxidative damage, but a high concentration of ABA (1,000 microM) induced an excessive generation of AOS and led to an oxidative damage in plant cells.