Effect of chilling on the diurnal rhythm of enzymes involved in protection against oxidative stress in a chilling-tolerant and a chilling-sensitive maize genotype

The effect of chilling on diurnal changes in activity of adenosine 5'-phosphosulfate sulfotransferase, glutathione reductase (EC 1.6.4.2) and glutathione transferase (EC 2.5.1.18) was analysed in the second leaf of Z 7, a chilling-tolerant, and Penjalinan, a chilling-sensitive maize (Zea mays L.) genotype. Nitrate reductase (EC 1.6.6.1) was measured for comparison. All enzyme activities examined changed with a typical diurnal rhythm in both genotypes cultivated at 25°C. Adenosine 5'-phosphosulfate sulfotransferase and nitrate reductase activity peaked during the light period, then decreased and reached lowest levels at the end of the dark period. Glutathione reductase activity increased in the dark and decreased during the light period. Maximum glutathione transferase activities were measured in the middle of the light period, minimal ones in the middle of the dark period. At 12°C these diurnal changes were eliminated in all enzymes examined of both genotypes.
The average adenosine 5'-phosphosulfate sulfotransferase and glutathione reductase activity were higher in the chilling-tolerant Z 7 than in the sensitive Penjanilan at 12°C in the light. Increased levels of both enzymes may contribute in establishing increased levels of cysteine and reduced glutathione in the chilling-tolerant Z 7. Indeed it has been shown before that the chilling-tolerant maize genotypes contain higher levels of both compounds at low temperatures than chilling-sensitive ones.     

Involvement of Protein Phosphorylation in Water Stress-induced Antioxidant Defense in Maize Leaves

Using pharmacological and biochemical approaches, the role of protein phosphorylation and the interrelationship between water stress-enhanced kinase activity, antioxidant enzyme activity, hydrogen peroxide (H2O2) accumulation and endogenous abscisic acid in maize (Zea mays L.) leaves were investigated. Water-stress upregulated the activities of total protein phosphorylation and Ca2+ -dependent protein kinase, and the upregulation was blocked in abscisic acid-deficient vp5 mutant. Furthermore, pretreatments with a nicotinamide adenine dinucleotide phosphate oxidase inhibitor and a scavenger of H2O2 significantly reduced the increased activities of total protein kinase and Ca2+-dependent protein kinase in maize leaves exposed to water stress. Pretreatments with different protein kinase inhibitors also reduced the water stress-induced H2O2 production and the water stress-enhanced activities of antioxidant enzymes such as superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase. The data suggest that protein phosphorylation and H2O2 generation are required for water stress-induced antioxidant defense in maize leaves and that crosstalk between protein phosphorylation and H2O2 generation may occur.     

Herbicide tolerance in maize is related to increased levels of glutathione and glutathione-associated enzymes

Treatment of 10 days old maize seedlings with metribuzin and pretilachlor near the recommended field-dose resulted in differential reductions in shoot fresh and dry weights during the following 16 days. Metribuzin showed great and consistent reductions, however, the reduction induced by pretilachlor, mostly nullified by the end of the experiment. Moreover, there were differential accumulations of lipid peroxides, carbonyl groups and H₂O₂ in maize leaves; metribuzin caused the greatest accumulation. Meanwhile, levels of thiol forms and reduced glutathione (GSH) were much more induced by pretilachlor than metribuzin; the contrary was true regarding oxidized glutathione (GSSG). The ratio of GSH/GSSG was highest following pretilachlor treatment and least by metribuzin. On the other hand, activities of glutathione-S-transferases (GSTs, EC 2.5.1.18), γ-glutamyl-cysteine synthetase (γ-GCS, EC 6.3.2.2), glutathione synthetase (GS, EC 6.3.2.3), glutathione peroxidase (GPX, EC 1.15.1.1) and glutathione reductase (GR, EC 1.6.4.2) were more enhanced in maize leaves by pretilachlor than metribuzin. These findings suggest the occurrence of an oxidative stress differentially induced in maize by the herbicides, a state that was most pronounced with metribuzin. Pretilachlor was concluded to be the least phytotoxic to maize, while metribuzin was the most, this differential tolerance seemed to be related to the induction of GSH and GSH-associated enzymes.     

Drought enhances maize chilling tolerance. II. Photosynthetic traits and protective mechanisms against oxidative stress

In the present research we studied the photosynthetic traits and protective mechanisms against oxidative stress in two maize ( Zea mays L.) genotypes differing in chilling sensitivity (Z7, tolerant and Penjalinan, sensitive) subjected to 5°C for 5 days, with or without pretreatment by drought. The drought pretreatment decreased the symptoms of chilling injury in Penjalinan plants estimated as necrotic leaf area and maximum quantum yield of photosystem II. Furthermore, drought pretreatment diminished the level of lipid peroxidation caused by chilling in Penjalinan plants. After one day of recovery from chilling the Z7 and drought-pretreated Penjalinan plants showed higher net photosynthesis rates than the non-drought-pretreated Penjalinan plants, thereby decreasing the probability of generating reactive oxygen species. The greater net photosynthesis was correlated with the greater NADP-malate dehydrogenase activity. No differences in either the de-epoxidation state of the xanthophyll cycle or the antioxidant enzyme activities were found among the chilled groups of plants. However, a drastic decrease in ascorbate content was observed in chilled Penjalinan plants without drought pretreatment. As we found an increase of H₂O₂ content after drought pretreatment, we suggest its involvement as a signal in the drought-enhanced chilling tolerance of maize.     

Oxidative damages of maize seedlings caused by exposure to a combination of potassium deficiency and salt stress

Soil salinity results in nutrient imbalances and potassium deficiency. However, very little work has been done on the damages of maize caused by exposure to a combination of potassium deficiency and salt-stress. Thus the influences of the combined stresses on the antioxidative defense system in maize seedlings were investigated. It was found that the growth of maize seedlings cultivated in a combination of potassium deficiency and salt stress was significantly inhibited, the compatible solute accumulation, permeability of plasma membrane, malondialdehyde as a degradation product of lipid peroxidation, reactive oxygen species such as superoxide radicals, hydrogen peroxide were higher than those of the individual potassium deficiency or salt-stress as expected. However, the antioxidative enzymes such as superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), and antioxidants such as ascorbic acid and glutathione were lower than those of the individual potassium deficiency or salt-stress. Taken together, salt stress impairs K nutrition of maize seedlings, K deficiency at the cellular level might be a contributory factor to salt-induced oxidative stress and related cell damage.     

Responses of reactive oxygen metabolism and quality in mango fruit to exogenous oxalic acid or salicylic acid under chilling temperature stress

After being immersed in water, oxalic acid (OA) or salicylic acid (SA) aqueous solutions, mango ( Mangifera indica L. cv. Zill) fruit were stored at 14°C or at 5°C with shelf life to determine the effects of exogenous OA or SA on reactive oxygen metabolism, quality and chilling injury (CI) of the fruit. Mango CI could be reduced by OA and SA treatments. Compared with that in control, accompanied with alleviated CI at shelf life, fruit treated with OA or SA had significantly higher reduction states of ascorbate and glutathione. Moreover, the treated fruit showed lower superoxide anion content, higher hydrogen peroxide content, lower lipoxygenase (EC 1.13.11.12) activity and higher activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), guaiacol peroxidase (EC 1.11.1.7), ascorbate peroxidase (EC 1.11.1.11) and glutathione reductase (EC 1.6.4.2). In addition, fruit firmness, total soluble solids and titratable acidity content were not obviously affected by OA and SA treatments. It was suggested that the effect of OA or SA on mango CI probably attributed to more reducing status of ascorbate and glutathione, less O₂⁻ accumulation and more H₂O₂ accumulation.     

Metabolic response to treatment with cold, paraquat, or 3-amino-1,2,4-triazole in leaves of winter wheat.

We treated leaves of winter wheat (Triticum aestivum L.) with cold, paraquat, or 3-amino-1,2,4-triazole and compared the responses. We assayed the activities of glucose-6-phosphate dehydrogenase, catalase, dehydroascorbate reductase and ascorbate free radical reductase and levels of hydrogen peroxide, glucose-6-phosphate, fructose-6-phosphate, ascorbate, dehydroascorbate, reduced and oxidized glutathione. With any of the three treatments, contents of cellular peroxides and hexose phosphates were raised. The content of ascorbate was lowered markedly by paraquat treatment, which produces active oxygen species, whereas such a decrease did not occur in other two treatments. When the plants were treated with 3-amino-1,2,4-triazole, which is a specific inhibitor of catalase, the content of oxidized glutathione increased severalfold. The glucose-6-phosphate dehydrogenase activity increased with all three treatments, but it decreased after glyphosate treatment, which does not stimulate the formation of peroxides. The activities of catalase and dehydroascorbate reductase were increased by the treatment of cold and paraquat, while 3-amino-1,2,4-triazole did not affect the dehydroascorbate reductase activity. The activity of ascorbate free radical reductase increased after treatment by paraquat only.     

Increasing the glutathione content in a chilling-sensitive maize genotype using safeners increased protection against chilling-induced injury.

With the aim of analyzing their protective function against chilling-induced injury, the pools of glutathione and its precursors, cysteine (Cys) and gamma-glutamyl-Cys, were increased in the chilling-sensitive maize (Zea mays) inbred line Penjalinan using a combination of two herbicide safeners. Compared with the controls, the greatest increase in the pool size of the three thiols was detected in the shoots and roots when both safeners were applied at a concentration of 5 microM. This combination increased the relative protection from chilling from 50% to 75%. It is interesting that this increase in the total glutathione (TG) level was accompanied by a rise in glutathione reductase (GR; EC 1.6.4.2) activity. When the most effective safener combination was applied simultaneously with increasing concentrations of buthionine sulfoximine, a specific inhibitor of glutathione synthesis, the total gamma-glutamyl-Cys and TG contents and GR activity were decreased to very low levels and relative protection was lowered from 75% to 44%. During chilling, the ratio of reduced to oxidized thiols first decreased independently of the treatments, but increased again to the initial value in safener-treated seedlings after 7 d at 5 degrees C. Taking all results together resulted in a linear relationship between TG and GR and a biphasic relationship between relative protection and GR or TG, thus demonstrating the relevance of the glutathione levels in protecting maize against chilling-induced injury.     

Role of abscissic acid in water stress-induced antioxidant defense in leaves of maize seedlings

Roles of abscisic acid (ABA) in water stress-induced oxidative stress were investigated in leaves of maize ( Zea mays L.) seedlings exposed to water stress induced by polyethylene glycol (PEG 6000). Treatment with PEG at -0.7 MPa for 12 and 24 h led to a reduction in leaf relative water content (RWC) by 7.8 and 14.1%, respectively. Duration of the osmotic treatments is considered as mild and moderate water stress. The mild water stress caused significant increases in the generation of superoxide radical ( O 2 - ) and hydrogen peroxide (H 2 O 2 ), the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) and the contents of ascorbate (ASC), reduced glutathione (GSH). The moderate water stress failed to further enhance the capacity of antioxidant defense systems, as compared to the mild water stress. The contents of catalytic Fe, which is critical for H 2 O 2 -dependent hydroxyl radical ( •OH) production, and the oxidized forms of ascorbate and glutathione pools, dehydroascorbate (DHA) and oxidized glutathione (GSSG), markedly increased, a significant oxidative damage to lipids and proteins took place under the moderate water stress. Pretreatment with ABA caused an obvious reduction in the content of catalytic Fe and significant increases in the activities of antioxidant enzymes and the contents of non-enzymatic antioxidants, and then significantly reduced the contents of DHA and GSSG and the degrees of oxidative damage in leaves exposed to the moderate water stress. Pretreatment with an ABA biosynthesis inhibitor, tungstate, significantly suppressed the accumulation of ABA induced by water stress, reduced the enhancement in the capacity of antioxidant defense systems, and resulted in an increase in catalytic Fe, DHA and GSSG, and oxidative damage in the water-stressed leaves. These effects were completely prevented by addition of ABA, which raised the internal ABA content. Our data indicate that ABA plays an important role in water stress-induced antioxidant defense against oxidative stress.     

The effect of different antioxidants on glutathione turnover in human cell lines and their interaction with hydrogen peroxide

BACKGROUND: Glutathione plays crucial roles in antioxidant defence and glutathione deficiency contributes to oxidative stress and may therefore play a key role in the pathogenesis of many diseases. The objectives of the present study were to evaluate the effects on glutathione turnover of thiol and non-thiol antioxidants in human cell cultures and if any of the antioxidant had a short-term cellular effect against different levels of hydrogen peroxide. METHODS: We have investigated the effect on the total glutathione amount in HeLa and hepatoma cell cultures of thiol antioxidants in comparison with non-thiol antioxidants, such as a copper chelator, Vitamin C, and a flavonoid. Furthermore, we have investigated the short-term (within 24h) interaction of the different antioxidants with hydrogen peroxide. RESULTS AND CONCLUSION: Lipoic acid and quercetin (Quer) were the two antioxidants that showed the highest stimulation of glutathione synthesis in cell cultures as judged by the total glutathione amount. However, no antioxidant protected against hydrogen peroxide present in concentrations that lowered cell protein. This finding may be attributed to the fact that it is necessary to incubate cell cultures with antioxidants or small doses of oxidants for a period before the cultures are exposed to hydrogen peroxide in order to enhance the antioxidant defence. The presence of Quer and Vitamin C lowered cell protein and total glutathione even in cell cultures containing hydrogen peroxide in concentrations that did not lower cell protein. This finding might be attributed to pro-oxidant properties and formation of excess reactive oxygen species in the presence of Quer and Vitamin C.     

Effect of salicylic acid potentiates cadmium-induced oxidative damage in <Emphasis Type="Italic">Oryza sativa</Emphasis> L. leaves

In the present investigation, we studied the possible potentiating effect of salicylic acid (SA) under Cd toxicity in Oryza sativa L. leaves. Cd treatments for 24 h reduced the shoot length, dry biomass and total chlorophyll content followed by high Cd accumulation in shoots. About 16 h presoaking with SA resulted in partial protection against Cd, as observed by minor changes in length, biomass and total chlorophyll. SA priming resulted in low Cd accumulation. Enhanced thiobarbituric acid reactive substances (TBARS), hydrogen peroxide (H₂O₂) and superoxide anion (O₂ ⁻) content were seen when Cd was applied alone, while under SA priming the extent of TBARS, H₂O₂ and O₂ ⁻ were significantly low, suggesting SA-regulated protection against oxidative stress. The antioxidant enzymes like Catalase (CAT), guaiacol peroxidase (GPx), glutathione reductase (GR) and superoxide dismutase (SOD) showed varied activities under Cd alone. CAT activity increased after Cd treatment, followed by a decline in GPX and GR activity. SOD also declined at the highest concentrations with an initial increase. Under SA-priming conditions, the efficiency of the antioxidant enzymes was significantly elevated. GPx and SOD activity showed significant increase in activity. The ascorbate activity increased after Cd treatment, followed by a decline in glutathione under SA-free condition. SA priming showed gradual increase in these non-enzymic antioxidants. Our results indicate that Cd-induced oxidative stress can be regulated by SA.     

Changes in antioxidant enzymes activity and oxidative stress by abscisic acid and salicylic acid in wheat genotypes

Abscisic acid (ABA) and salicylic acid (SA) were sprayed on leaves of wheat genotypes C 306 and Hira at 25 and 40 d after sowing under moderate water stress (−0.8 MPa) imposed by adding PEG-6000 in nutrient solution. ABA and SA increased the activities of superoxide dismutase, ascorbate peroxidase, glutathione reductase, and catalase in comparison to unsprayed control plants. Both ABA and SA treatments decreased the contents of hydrogen peroxide and thiobarbituric acid reactive substances, a measure of lipid peroxidation, compared to unsprayed plants. The beneficial effect of increase in antioxidant enzymes activity and decrease in oxidative stress was reflected in increase in chlorophyll and carotenoid contents, relative water content, membrane stability index, leaf area and total biomass over control plants. The lower concentrations of ABA (0.5 mM) and SA (1.0 mM) were generally more effective than higher concentrations.     

Antioxidant enzyme activities in embryologic and early larval stages of turbot

The antioxidant enzymes superoxide dismutase (SOD; EC 1.15.1.1), catalase (EC 1.11.1.6), selenium-dependent glutathione peroxidase (SeGPX; EC 1.11.1.9), glutathione reductase (EC 1.6.4.2) and DT-diaphorase (EC 1.6.99.2), plus total GPX activity (sum of SeGPX and Se-independent GPX activities), were studied in 13 500 g supernatants of embryos and 3-day and 11-day post-hatch larvae of turbot Scophthalmus maximus L. SOD activity decreased progressively during development from embryos to 11-day-old larvae, indicative of a decreased need to detoxify superoxide anion radical (O₂⁻). In contrast, catalase, SeGPX and glutathione reductase activities increased progressively from embryos to 11-day-old larvae, indicative of an increased need to metabolize hydrogen peroxide (H₂O₂) and organic peroxides. Consistent with the latter changes, levels of lipid peroxides (i.e. thiobarbituric acid reactive substances) increased 13-fold from embryos to 3-day-old larvae, whilst total peroxidizable lipid was indicated to decrease. Increases were seen for NADPH-dependent DT-diaphorase (after hatching) and total GPX (between 3 and 11 days post-hatch) activities, whilst no change was found in NADH-dependent DT-diaphorase activity. Overall, the results demonstrate a capacity for early life-stages of S. maximus to detoxify reactive oxygen species (O₂⁻ and H₂O₂) and other pro-oxidant compounds (organic peroxides, redox cycling chemicals). Furthermore, qualitative and quantitative antioxidant changes occur during hatching and development, possibly linked to such events as altered respiration rates (SOD changes) and tissue reorganization and development (catalase, SeGPX, lipid peroxidation).     

Induction of Protection against Paraquat-induced Oxidative Damage by Abscisic Acid in Maize Leaves is Mediated through Mitogen-activated Protein Kinase

Mitogen-activated protein kinase (MAPK) cascade has been shown to be important components in stress signal transduction pathway. In the present study, protection of maize seedlings ( Zea mays L.) against paraquat-generated oxidative toxicity by abscisic acid (ABA), its association with MAPK and ZmMPK5, a candidate for MAPK were investigated. Treatment of maize leaves with exogenous ABA led to significant decreases in the content of malondialdehyde, the percentage of ion leakage and the level of protein oxidation (in terms of carbonyl groups) under paraquat (PQ) stress. However, such decreases were blocked by the pretreatment with two MAPK kinase inhibitors PD98059 and U0126. The damage caused by PQ was further aggravated by inhibitors. Two inhibitors also suppressed the total 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 glutathione reductase (GR, EC 1.6.4.2). Besides, treatment with PQ stimulated the activation of a 46 kDa MAPK, which was identified as ZmMPK5 by in-gel kinase assay with immunoprecipitation. These results reveal that ABA-induced protection against PQ-generated oxidative damage is mediated through MAPK cascade in maize leaves, in which ZmMPK5, a candidate for MAPK, is demonstrated to be involved.     

Changes of antioxidants levels in two maize lines following atrazine treatments.

Growth and antioxidants levels of shoot of 10-d-old maize lines (Zea mays L. Hybrid 351 and Giza 2) differentially responded to atrazine treatment at the recommended field dose (RFD) during the following 20 d. Atrazine significantly reduced shoot fresh and dry weights but significantly accumulated H(2)O(2), lipid peroxides and carbonyl groups in Giza 2 during the whole experiment; an effect that prolonged with either elapse of time or increasing the herbicide dose. Meanwhile, ascorbic acid (AsA) and reduced glutathione (GSH) contents were significantly decreased along with significant inhibitions in activities of superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APX; EC 1.11.1.7), guaiacol peroxidase (GPX; EC 1.11.1.7), and glutathione-S-transferase (GST; EC 2.5.1.18). Similar responses were observed in Hybrid 351 only during the first 12 d, and seemed to be overcome thereafter. These findings indicate an induced oxidative stress in maize following atrazine treatments. Such state appeared to be counterbalanced in Hybrid 351 but continued in Giza 2 concluding Giza 2 as more susceptible to atrazine than Hybrid 351. Therefore, the differential susceptibility of Giza 2 to atrazine is related to deficiency in antioxidant levels.     

Metabolic Response to Treatment with Cold,Paraquat, or 3-Amino-1,2,4-triazole in Leaves of Winter Wheat

We treated leaves of winter wheat (Triticum aestivum L.) with cold, paraquat, or 3-amino-1,2,4-triazole and compared the responses. We assayed the activities of glucose-6-phosphate dehydrogenase, catalase, dehydroascorbate reductase and ascorbate free radical reductase and levels of hydrogen peroxide, glucose-6-phosphate, fructose-6-phosphate, ascorbate, dehydroascorbate, reduced and oxidized glutathione. With any of the three treatments, contents of cellular peroxides and hexose phosphates were raised. The content of ascorbate was lowered markedly by paraquat treatment, which produces active oxygen species, whereas such a decrease did not occur in other two treatments. When the plants were treated with 3-amino-1,2,4-triazole, which is a specific inhibitor of catalase, the content of oxidized glutathione increased severalfold. The glucose-6-phosphate dehydrogenase activity increased with all three treatments, but it decreased after glyphosate treatment, which does not stimulate the formation of peroxides. The activities of catalase and dehydroascorbate reductase were increased by the treatment of cold and paraquat, while 3-amino-1,2,4-triazole did not affect the dehydroascorbate reductase activity. The activity of ascorbate free radical reductase increased after treatment by paraquat only.     

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.     

Role of Abscisic Acid in Water Stress-induced Antioxidant Defense in Leaves of Maize Seedlings

Roles of abscisic acid (ABA) in water stress-induced oxidative stress were investigated in leaves of maize ( Zea mays L.) seedlings exposed to water stress induced by polyethylene glycol (PEG 6000). Treatment with PEG at &#109 0.7 MPa for 12 and 24 h led to a reduction in leaf relative water content (RWC) by 7.8 and 14.1%, respectively. Duration of the osmotic treatments is considered as mild and moderate water stress. The mild water stress caused significant increases in the generation of superoxide radical ( O 2 &#109 ) and hydrogen peroxide (H 2 O 2 ), the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione reductase (GR) and the contents of ascorbate (ASC), reduced glutathione (GSH). The moderate water stress failed to further enhance the capacity of antioxidant defense systems, as compared to the mild water stress. The contents of catalytic Fe, which is critical for H 2 O 2 -dependent hydroxyl radical ( &#148 OH) production, and the oxidized forms of ascorbate and glutathione pools, dehydroascorbate (DHA) and oxidized glutathione (GSSG), markedly increased, a significant oxidative damage to lipids and proteins took place under the moderate water stress. Pretreatment with ABA caused an obvious reduction in the content of catalytic Fe and significant increases in the activities of antioxidant enzymes and the contents of non-enzymatic antioxidants, and then significantly reduced the contents of DHA and GSSG and the degrees of oxidative damage in leaves exposed to the moderate water stress. Pretreatment with an ABA biosynthesis inhibitor, tungstate, significantly suppressed the accumulation of ABA induced by water stress, reduced the enhancement in the capacity of antioxidant defense systems, and resulted in an increase in catalytic Fe, DHA and GSSG, and oxidative damage in the water-stressed leaves. These effects were completely prevented by addition of ABA, which raised the internal ABA content. Our data indicate that ABA plays an important role in water stress-induced antioxidant defense against oxidative stress.     

Cadmium stimulates the accumulation of salicylic acid and its putative precursors in maize (Zea mays) plants

The effect of 10, 25 and 50 µ M Cd(NO₃)₂ on the salicylic acid (SA) metabolism was investigated in young maize seedlings ( Zea mays L., hybrid Norma). Cadmium (Cd) was translocated into the leaves and induced oxidative damage, as indicated by the reduced chlorophyll content, the decreased quantum efficiency of photosystem II and the enhanced malondialdehyde (MDA) content, especially after 7 days. The activity of glutathione reductase (EC 1.6.4.2) increased from the fourth day and that of guaiacol peroxidase (EC 1.11.1.7) after 7 days of Cd stress compared with the control leaves. These effects of Cd exhibited a correlation with the concentration. Under these conditions, Cd did not affect the MDA content or the antioxidant enzyme activities in the roots. After 7 days, Cd increased the levels of free and bound forms of benzoic acid (BA), O -coumaric acid ( O -hydroxy-cinnamic) ( O- HCA) and SA in the leaves, but in the roots, only the 50 µ M rate of Cd caused changes in the free O -HCA acid and bound BA content.     

The electron partitioning between the cytochrome and alternative respiratory pathways during chilling recovery in two cultivars of maize differing in chilling sensitivity

Chilling effects on respiration during the recovery period were studied in two maize (Zea mays L.) cultivars differing in their tolerance to chilling: Penjalinan, a chilling-sensitive cultivar, and Z7, a chilling-tolerant cultivar. Both cultivars were exposed to 5 degrees C for 5 d, after which measurements were taken at 25 degrees C. Chlorophyll fluorescence analysis in dark-adapted leaves showed less damage in cv Z7 than in cv Penjalinan during recovery from the chilling treatment. Studies of the electron partitioning between the cytochrome and the alternative respiratory pathways during chilling recovery using the oxygen isotope fractionation technique showed that, although total leaf respiration was not affected by the chilling treatment in either of the two cultivars, electron partitioning to the alternative pathway was significantly increased in the more stressed chilling-sensitive cv Penjalinan, suggesting that increased activity of the alternative pathway is not related to the plant tolerance to chilling. These results suggest a possible role of the alternative pathway in plants under stress rather than specifically contributing to plant resistance to chilling.