Antioxidative response of ascorbate-glutathione pathway enzymes and metabolites to desiccation of recalcitrant Acer saccharinum seeds

Ascorbate–glutathione systems were studied during desiccation of recalcitrant seeds of the silver maple (Acer saccharinum L.). The desiccated seeds gradually lost their germination capacity and this was strongly correlated with an increase in electrolyte leakage from seeds. Simultaneously the increase of reactive oxygen species (ROS) (superoxide radical – O₂⁻ and hydrogen peroxide – H₂O₂) production was observed. The results indicate that remarkable changes in the concentrations and redox status of ascorbate and glutathione occur in embryo axes and cotyledons. After shedding, concentrations of ascorbic acid (ASA) and the reduced form of glutathione (GSH) are higher in embryo axes than in cotyledons and their redox status is high in both embryo parts. Cotyledons in freshly shed seeds are devoid of GSH. At the first stages of desiccation, up to a level of 43% of moisture content, ASA content in embryo axes and GSH content in cotyledons increased. Below this level of moisture content, the antioxidant contents as well as their redox status rapidly decreased. The enzymes of the ascorbate–glutathione pathway: ascorbate peroxidase (APX) (EC 1.11.1.11), monodehydroascorbate reductase (MR) (EC 1.6.5.4), dehydroascorbate reductase (DHAR) (EC 1.8.5.1) and glutathione reductase (GR) (EC 1.6.4.2) increased their activity during desiccation, but mainly in embryonic axes. The changes are probably required for counteracting the production of ROS during desiccation. The relationship between ascorbate and glutathione metabolism and their relevance during desiccation of recalcitrant Acer saccharinum seeds is discussed.     

Hydroperoxide metabolism in cyanobacteria

The enzymes involved in antioxidative activity and the cellular content of the antioxidants glutathione and ascorbate in the cyanobacteria Nostoc muscorum 7119 and Synechococcus 6311 have been examined for their roles in hydroperoxide removal. High activities of ascorbate peroxidase and catalase were found in vegetative cells of both species and in the heterocysts of N. muscorum. The affinity of ascorbate peroxidase for H2O2 was 15- to 25-fold higher than that of catalase. Increased activity of ascorbate peroxidase was observed in N. muscorum when H2O2 production was enhanced by photorespiration. Catalase activity was decreased in dilute cultures whereas ascorbate peroxidase activity increased. Ascorbate peroxidase activity also increased when the CO2 concentration was reduced. Ascorbate peroxidase appears to be a key enzyme in a cascade of reactions regenerating antioxidants. Dehydroascorbate reductase was found to regenerate ascorbate, and glutathione reductase recycled glutathione. In vegetative cells glutathione was present in high amounts (2-4 mM) whereas the ascorbate content was almost 100-fold lower (20-100 microM). Glutathione peroxidase was not detected in either cyanobacterium. It is concluded from the high activity of ascorbate peroxidase activity and the levels of antioxidants found that this enzyme can effectively remove low concentrations of peroxides. Catalase may remove H2O2 produced under photooxidative conditions where the peroxide concentration is higher.     

Accumulation of hydrogen peroxide and functioning of defense system in overwatered barley seedlings

The effects of overwatering (flooding) on the oxidative potential, the level of low-molecular-weight antioxidants, the content of stress proteins, and activities of antioxidant enzymes in green barley (Hordeum vulgare L.) seedlings were studied. Overwatering retarded barley seedling growth and induced hydrogen peroxide accumulation, a decrease in the total ascorbate content and an increase in the content of reduced glutathione (GSH), but it did not affect the content of oxidized glutathione (GSSG). After the cessation of stress factor action (post-stress period), the content of hydrogen peroxide declined to the initial level, the content of ascorbate reduced still stronger, whereas the content of GSH continued to rise. Under flooding conditions, activities of glutathione reductase (GR) and superoxide dismutase (SOD) increased. After the cessation of stress factor action, activities of these enzymes decreased but remained at rather high levels as compared with control. Activity of catalase (CAT) reduced during stress, whereas activity of ascorbate peroxidase (APX) was not essentially changed. In the post-stress period, CAT activity remained to be low; in contrast, APX activity increased. Barley seedling flooding induced the synthesis of stress proteins, HSP70 and dehydrins (DH). In the post-stress period, the content of stress proteins decreased; however, the content of DH in experimental leaves remained rather high. The results obtained indicate that barley defense system manifested a complex response to overwatering, which may be related to the oxygen shortage under stress conditions and sharp metabolism activation at re-aeration in the post-stress period.     

Short-term antioxidative responses of 15 microalgae exposed to excessive irradiance including ultraviolet radiation

Short-term photosensitivity and oxidative stress responses were compared for three groups of marine microalgae: Antarctic microalgae, temperate diatoms and temperate flagellates. In total, 15 low-light-acclimated species were exposed to simulated surface irradiance including ultraviolet radiation (SSI). Photosensitivity was assessed as the rate of recovery of Fv/Fm in the hours following SSI treatment. Before, during and after the SSI treatment, oxidative stress responses were assessed by following xanthophyll content and cycling, and activities of superoxide dismutase, ascorbate peroxidase and glutathione reductase, and glutathione redox status. When acclimated to low irradiance, antioxidant levels were not group specific. Superoxide dismutase activity was positively correlated with cell size, whereas in general, ascorbate peroxidase activity appeared to be lower and glutathione redox status appeared to be higher in the Antarctic than in the temperate species. After SSI exposure, the strong inhibition of PSII was followed by variable rates of recovery, although four species remained photosynthetically inactive. SSI tolerance appeared unrelated to geographic or taxonomic background, or to cell size. PSII recovery was enhanced in species with decreasing superoxide dismutase activity, glutathione redox status and increased xanthophyll cycle activity. We conclude that antioxidant responses are highly species specific and not related to the geographic or taxonomic background. Furthermore, xanthophyll cycling seems more important than antioxidants. Finally, it can be hypothesized that glutathione could function as a stress sensor and response regulator.     

Specific features of the ascorbate/glutathione cycle in cultured protoplasts

Protoplasts isolated from Nicotiana tabacum (L.) leaves were cultured for 6 days in liquid medium and some features of their antioxidant capacity were investigated. Ascorbate exported into the culture medium was oxidized non-enzymically, whereas important modifications of the enzymic scavenging activity were detected inside protoplasts. A new pool of isoenzymes, most of them with cytoplasmic characteristics, ensures the increased ascorbate peroxidase activity. The specific activity of other enzymes involved in the ascorbate/glutathione cycle, such as dehydroascorbate reductase and glutathione reductase, and of glutathione peroxidase were modified, resulting in cultured protoplasts with quantitative differences in antioxidant capacity compared to leaves. The hypothesis presented here suggests that the new scavenging system is related to differences in the compartment-specific accumulation of active oxygen species following protoplast isolation. Received: 8 December 1997 / Revision received: 27 March 1998 / Accepted: 10 April 1998     

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.     

Copper accumulation, synthesis of ascorbate and activation of ascorbate peroxidase in Enteromorpha compressa (L.) Grev. (Chlorophyta) from heavy metal-enriched environments in northern Chile

Enteromorpha compressa is the dominant species in coastal areas of northern Chile receiving copper mine wastes. Copper remains as the main heavy metal in these coastal waters and it is accumulated in E. compressa growing at the impacted sites. Algae from these sites showed higher levels of lipoperoxides than from non‐impacted sites, which suggests the occurrence of cellular damage resulting from oxidative stress. The strong activation of ascorbate peroxidase detected in this study probably occurs in order to buffer this oxidative stress. Unexpectedly, the activity of glutathione reductase, normally coupled to ascorbate peroxidase activity, was not affected by the chronic exposure to the mine wastes. Moreover, catalase, dehydroascorbate reductase and glutathione peroxidase, commonly reported to buffer oxidative stress in plants and algae, were not detected in E. compressa from any of the studied sites. Levels of total glutathione and phenolic compounds decreased in algae from mine‐impacted sites. In contrast, high levels of dehydroascorbate were found in algae from impacted sites, whereas ascorbate remained unchanged. Therefore, it is suggested that E. compressa tolerates a copper‐enriched environment, and the accompanying oxidative stress, through the accumulation of copper, activation of ascorbate peroxidase, synthesis of ascorbate (accumulated as dehydroascorbate) and consumption of glutathione and water‐soluble phenolic compounds.     

The Distribution and Cooperation of Antioxidant (Iso)enzymes and Antioxidants in Different Subcellular Compartments in Maize Leaves during Water Stress

The effects of mild water stress induced by polyethylene glycol (PEG) on the activities of antioxidant enzymes [superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR)] and their isoenzymes and the antioxidant content [ascorbate (ASC) and glutathione (GSH)] of different subcellular compartments were investigated in maize. For each subcellular compartment, the activities of almost all isoenzymes resolved on native PAGE increased after 4–12 h of exposure to water stress and declined after that, showing concomitant changes with the activities of their respective total enzymes and the antioxidant content. For each subcellular compartment, at least one isoform for the detected antioxidant enzymes was resolved, but different kinds of antioxidant isoenzymes in different subcellular compartments had different responses to water stress. The relative contribution of Fe–SOD in chloroplasts and Mn–SOD in mitochondria was higher than that in other subcellular compartments. However, in apoplasts the activities of Mn–SOD and Fe–SOD declined during the process of water stress, in contrast to those located in other subcellular compartments. The results from the activities of antioxidant (iso)enzymes demonstrated that all antioxidant enzymes in all subcellular compartments were mobilized in cooperation and responded synchronously under mild water stress, with the same trend of changes in their activity. This indicated their orchestrated effects in scavenging reactive oxygen species (ROS) in situ. Additionally, the results suggested that mitochondria and apoplasts, responding most actively, might be targets for improving plant performance under mild water stress.     

SEASONAL CHANGES IN THE FOLIAR ANTIOXIDANT SYSTEMS IN CYCLOBALANOPSIS HELFERIANA AND TERMINTHIA PANICULATA IN THE HOT-DRY VALLEY OF THE YUANJIANG RIVER, CHINA

 为探讨我国西南干热河谷这一严酷生境中植物抗氧化系统对多种胁迫因子的响应机制, 以该地区最干热的元江河谷萨王纳植被中光合能力有明显差异的两个优势种——常绿的毛枝青冈(Cyclobalanopsis helferiana)和干热季落叶的三叶漆(Terminthia paniculata)为材料, 研究了其抗氧化系统活性在高温雨季、干凉季和干热季的变化规律。结果表明: 从总体上看两树种抗氧化系统在干凉季活性最高, 然而, 两树种谷胱甘肽转移酶和谷胱甘肽过氧化物酶都在随后的干热季特异表达。两树种主要非酶抗氧化物质——抗坏血酸(ASC)和谷胱甘肽库容量与水-水循环起端酶超氧化物歧化酶(SOD)活性差异不大, 但光合速率低的三叶漆水-水循环和抗坏血酸-谷胱甘肽循环其它酶活性显著高于光合强的毛枝青冈。三叶漆抗氧化系统比毛枝青冈启动积极, 但后者有更持久的抗氧化能力。与其它逆境中植物相比, 两树种有更发达的抗氧化系统, 故能始终保持相对低的丙二醛含量。     

Oxidative stress and antioxidants in tomato (Solanum lycopersicum) plants subjected to boron toxicity

BACKGROUND AND AIMS: Boron (B) toxicity triggers the formation of reactive oxygen species in plant tissues. However, there is still a lack of knowledge as to how B toxicity affects the plant antioxidant defence system. It has been suggested that ascorbate could be important against B stress, although existing information is limited in this respect. The objective of this study was to analyse how ascorbate and some other components of the antioxidant network respond to B toxicity. METHODS: Two tomato (Solanum lycopersicum) cultivars ('Kosaco' and 'Josefina') were subjected to 0.05 (control), 0.5 and 2 mm B. The following were studied in leaves: dry weight; relative leaf growth rate; total and free B; H(2)O(2); malondialdehyde; ascorbate; glutathione; sugars; total non-enzymatic antioxidant activity, and the activity of superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, ascorbate oxidase and l-galactose dehydrogenase. KEY RESULTS: The B-toxicity treatments diminished growth and boosted the amount of B, malondialdehyde and H(2)O(2) in the leaves of the two cultivars, these trends being more pronounced in 'Josefina' than in 'Kosaco'. B toxicity increased ascorbate concentration in both cultivars and increased glutathione only in 'Kosaco'. Activities of antioxidant- and ascorbate-metabolizing enzymes were also induced. CONCLUSIONS: High B concentration in the culture medium provokes oxidative damage in tomato leaves and induces a general increase in antioxidant enzyme activity. In particular, B toxicity increased ascorbate pool size. It also increased the activity of l-galactose dehydrogenase, an enzyme involved in ascorbate biosynthesis, and the activity of enzymes of the Halliwell-Asada cycle. This work therefore provides a starting point towards a better understanding of the role of ascorbate in the plant response against B stress.     

Comparison of photosynthesis and antioxidant performance of several Citrus and Fortunella species (Rutaceae) under natural chilling stress

Citrus plants originate from southeastern Asia, in a large area with various climates characterized by a broad range of temperatures. Some species have been diversified in temperate climates, others in subtropical climates. Temperature is assumed to be a key factor in citrus species adaptation and diversification of basic cellular functions. In a field experiment, the tolerance of the three fundamental Citrus species C. medica L., C. reticulata Blanco and C. maxima (Burm.) Merr., and Fortunella japonica (Thunb.) Swing. to photooxidative stress caused by seasonal climatic changes was evaluated on adult trees by measuring net photosynthesis (Pnet), stomatal conductance (Gs), maximum photosynthesis (Pmax) and chlorophyll fluorescence (Fv/Fm). In addition, seasonal changes in oxidative status, antioxidant enzymes (superoxide dismutase, catalase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase) and antioxidant metabolites (ascorbate and glutathione) were monitored. Mandarin and pummelo appeared to be the most tolerant, showing the lowest down-regulation of photosynthetic parameters, and the lowest accumulation of oxidized compounds associated with efficiency of their antioxidant system. Kumquat showed intermediate behaviour, with a large diminution of photosynthetic parameters and marked accumulation of hydrogen peroxide, whereas the malondialdehyde content remained low, with a strong induction of glutathione synthesis. Finally, citron appeared to be the most sensitive genotype with a marked decrease in photosynthetic performance, the largest accumulation of oxidative parameters, insufficient induction of antioxidant enzymes and down-regulation of ascorbate and glutathione synthesis.     

Ectopic expression of Brassica rapa L. MDHAR increased tolerance to freezing stress by enhancing antioxidant systems of host plants

Ascorbate (vitamin C) plays an important role in detoxification of reactive oxygen species (ROS) in most living organisms. Monodehydroascorbate reductase (MDHAR; EC 1.6.5.4) is crucial to regeneration of the oxidized form of ascorbate (monodehydroascorbate) so that it can be recycled to maintain ROS scavenging ability. The MDHAR gene from Brassica rapa L. was cloned and introduced into Arabidopsis thaliana (L.) Heynh. to test the hypothesis that enhanced ROS scavenging activity of BrMDHAR alleviates freezing stress. BrMDHAR was expressed under the control of either the CaMV 35S promoter or stress inducible SWPA2 promoter. Ectopic expression of BrMDHAR led to the up-regulation of many antioxidant genes, including APX, DHAR, GR, SOD, GPX, and PRX Q, which are involved in ascorbate–glutathione cycle. And, transgenic plants showed improved stress tolerance against freezing with exhibiting higher levels of chlorophyll content and antioxidant molecules such as ascorbate and glutathione as well as alleviated redox status and malondialdehyde contents. These results suggested that ectopic expression of BrMDHAR conferred improved tolerance to freezing stress not only by simply recycling ascorbate, but also by inducing co-regulation of the ascorbate–glutathione cycle, which in turn enhances the antioxidant capacity of the host plants.     

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.     

Trichoderma harzianum enhances antioxidant defense of tomato seedlings and resistance to water deficit

Some plant-symbiotic strains of the genus Trichoderma colonize roots and induce profound changes in plant gene expression that lead to enhanced growth, especially under biotic and abiotic stresses. In this study, we tested the hypothesis that one of the protective mechanisms enhanced by T. harzianum T22 colonization is the antioxidant defense mechanism. Having established that strain T22 modulates the expression of the genes encoding antioxidant enzymes, the status of antioxidant defense of tomato seedlings in response to colonization by T22 and water deficit was investigated. Total ascorbate or glutathione levels were not affected by either stimuli, but under water deficit, antioxidant pools became more oxidized (lower ratios of reduced to oxidized forms), whereas colonized plants maintained redox state as high as or higher than unstressed and untreated plants. The enhanced redox state of colonized plants could be explained by their higher activity of ascorbate and glutathione-recycling enzymes, higher activity of superoxide dismutase, catalase, and ascorbate peroxidase, in both root and shoot throughout the experiment. Similar enzymes were induced in uncolonized plants in response to water-deficit stress but to a lower extent when compared with colonized plants. This orchestrated enhancement in activity of reactive oxygen species (ROS)-scavenging pathways in colonized plants in response to stress supports the hypothesis that enhanced resistance of colonized plants to water deficit is at least partly due to higher capacity to scavenge ROS and recycle oxidized ascorbate and glutathione, a mechanism that is expected to enhance tolerance to abiotic and biotic stresses.     

Time course of antioxidant responses of Capsicum annuum subjected to a progressive magnesium deficiency

The effect of magnesium (Mg²⁺)‐deficiency on the antioxidant responses of Capsicum annuum was investigated over a 60‐day period under controlled conditions. This Mg²⁺‐deficiency aimed to mimic the physiological conditions that plants may experience in the field. At each harvest time, five different leaf‐levels (L2 to L6) were distinguished. L2 and L6 correspond to the second and sixth youngest leaves, respectively. The following parameters were determined: Mg²⁺, chlorophyll and protein contents, total and redox pools of ascorbate and glutathione, and the activities of superoxide dismutase, ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase. Under Mg²⁺‐deficiency, leaf Mg²⁺ contents decreased over time in all leaf‐levels except in the second youngest leaves (L2), where they remained constant at about 0.25% (dry weight basis). Mg²⁺‐deficiency led to an increase in the antioxidant enzyme activities concomitant with an increase in the ascorbate and glutathione pools, whereas total chlorophyll and soluble protein contents decreased. The L2 leaves showed an increase in glutathione reductase activity and in the ascorbate redox state whereas no difference was observed for the other parameters. Superoxide dismutase activities increased in L5 leaves from day 15 and, afterwards, in L3 to L5 leaves, irrespective of Mg²⁺ content. At day 30, glutathione reductase activities increased in L2 to L4 leaves and dehydroascorbate reductase activities in L4 leaves. At day 45, we observed an increase in the ascorbate peroxidase activities in L3 to L5 leaves. At the same time, ascorbate and glutathione pools increased in intermediate leaves, whereas chlorophyll content decreased in L3 and L4 leaves, and protein content decreased in L4 leaves. Results suggest that pepper leaves enhance their defence capacities against oxidative stress by increasing ascorbate more than glutathione synthesis. However, cells showed higher regeneration rates for the glutathione redox state than for the ascorbate redox state.     

Riboflavin spraying impairs the antioxidant defense system but induces waterlogging tolerance in tobacco

Riboflavin, which causes plants to produce reactive oxygen species (ROS) when exposed to light, is an excellent photosensitizer for biocidal reactions. This study explores the possible protective role of riboflavin against waterlogging stress in tobacco plants. Tobacco seedlings (4 weeks old) were divided into four groups and pretreated with 0, 0.2, 0.5 or 1.0 mM riboflavin for 1 week, after which all groups were exposed to waterlogging stress for 7 days. We observed delayed leaf senescence and extended survival time, suggesting that riboflavin can confer increased waterlogging tolerance to plants as compared with the control (0 mM riboflavin). Enhanced stomatal closure was observed in the riboflavin-pretreated tobacco. We evaluated the levels of oxidative damage (H₂O₂ and lipid peroxidation), antioxidant enzyme (superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase) activity and antioxidant metabolites (including ascorbate and glutathione) in tobacco leaves that were pretreated with riboflavin. However, the results show that riboflavin pretreatment caused a decrease in chlorophyll content, antioxidant enzyme activity and redox values (AsA/DHA and GSH/GSSG), while causing a significant increase in lipid peroxidation, H₂O₂ accumulation and total ascorbate or glutathione content. In addition, the survival time and stomatal aperture of riboflavin-treated plants were significantly modified by exogenous application of GSH, well-known ROS scavenger. To explain the stomatal closure observed in tobacco plants, we propose a “damage avoidance” hypothesis based on riboflavin-mediated ROS toxicity. The protective function of the photosensitizer riboflavin may be highly significant for farming in frequently waterlogged areas.     

Oxidative stress is not related to the mode of action of herbicides that inhibit acetolactate synthase

Imazethapyr (IM) is an imidazolinone herbicide which inhibits the biosynthesis of branched chain amino acids, by blocking acetolactate synthase (ALS; EC 4.1.3.18), the first common enzyme of the pathway. To study new aspects of the mode of action of ALS-inhibiting herbicides, pea plants grown in hydroponic cultures were supplied with IM and were analysed with reference to the antioxidant system and oxidative markers. A slight lipid peroxidation was detected in leaves after IM treatment, but no changes were noted in electrolyte leakage or carbonyl content. The ascorbate pool of leaves was oxidized under IM treatment. The analysis of the antioxidant enzymes superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), catalase (CAT) and guaiacol peroxidase (GPX), showed that IM treatment only caused an enhancement of GPX activity in leaves. In roots, the herbicide caused a decrease in lipid peroxidation. The enhancement of the reduced glutathione content detected in IM-treated roots can be related to the detected increase of GR activity. The lack of more noticeable effects on antioxidant enzymatic activities could be explained by the inability of IM-treated plants to respond to oxidative stress with modifications in their protein synthesis. Our results suggest that oxidative stress is not related to the mode of action of ALS-inhibitors. The slight changes detected in the antioxidative status of treated plants are too secondary in time and intensity to be related to the lethality caused by ALS-inhibitors     

Effect of nitrogen deficiency on antioxidant status and Cd toxicity in rice seedlings

Effect of nitrogen (N) deficiency on antioxidant status and Cd toxicity in rice seedlings was investigated. N deficiency resulted in a reduction of shoot growth but not root growth. The contents of N-containing compounds such as nitrate, chlorophyll, and protein decreased in leaves of rice seedlings grown under N deficiency. Accumulation of abscisic acid and H₂O₂ in leaves was induced by N deficiency. The content of ascorbate and the activities of ascorbate peroxidase, glutathione reductase, and catalase in N-deficient leaves were lower than their respective control leaves. However, glutathione content was not affected and superoxide dismutase activity was increased by N deficiency. Cd toxicity in N-deficient seedlings was more pronounced than that in N-sufficient ones. Pretreatment with ascorbate or L-galactono-1,4-lactone, a biosynthetic precursor of ascorbate resulted in a reduction of Cd toxicity enhanced by N deficiency. N deficiency also resulted in an enhancement of Cd uptake in rice seedlings. The possible mechanism of Cd toxicity enhanced by N deficiency is discussed.