Differential responses of hydrogen peroxide, lipid peroxidation and antioxidant enzymes in Azolla microphylla exposed to paraquat and nitric oxide

The present investigation was carried out to decipher the interplay between paraquat (PQ) and exogenously applied nitric oxide (NO) in Azolla microphylla. The addition of PQ (8 ??M) increased the activities of superoxide dismutase (SOD), catalase (CAT), guaiacol peroxidase (GPX), ascorbate peroxidase (APX) by 1.7, 2.7, 3.9 and 1.9 folds respectively than that control in the fronds of Azolla. The amount of H₂O₂ was also enhanced by 2.7 times in the PQ treated plants than that of control. The supplementation of sodium nitroprusside (SNP) from 8?C100 ??M along with PQ, suppressed the activities of antioxidative enzymes and the amount of H₂O₂ compared to PQ alone. The drop in the activity of antioxidative enzymes ?? SOD, GPX, CAT and APX was highest (39.9%, 48.4%, 41.6% and 41.3% respectively) on the supplementation of 100 ??M SNP with PQ treated fronds compared to PQ alone. The addition of NO scavengers along with NO donor in PQ treated fronds neutralized the effect of exogenously supplied NO. This indicates that NO can effectively protect Azolla against PQ toxicity by quenching reactive oxygen species. However, 200 ??M of SNP reversed the protective effect of lower concentration of NO donor against herbicide toxicity. Our study clearly suggests that (i) SNP released NO can work both as cytoprotective and cytotoxic in concentration dependent manner and (ii) involvement of NO in protecting Azolla against PQ toxicity.     

Changes in photosynthesis, antioxidant enzymes and lipid peroxidation in soybean seedlings exposed to UV-B radiation and/or Cd

Aims

With a high growth rate and biomass production, bamboos are frequently used for industrial applications and recently have proven to be useful for wastewater treatment. Bamboos are considered as Si accumulators and there is increasing evidence that silicon may alleviate abiotic stresses such as metal toxicity. The aim of this study was to investigate the extent of metal concentrations and possible correlations with Si concentrations in plants.

Methods

This study presents, for the first time, reference values for silicon (Si), copper (Cu) and zinc (Zn) concentrations in stems and leaves of various bamboo species grown under the natural pedo-climatic conditions of the island of Réunion (Indian Ocean).

Results

A broad range of silicon concentrations, from 0 (inferior to detection limit) to 183 mg g^?1 dry matter (DM), were found in stems and leaves. Mean leaf Cu and Zn concentrations were low, i.e. 5.1 mg kg^?1 DM and 15.7 mg kg^?1 DM, respectively. Silicon, Cu and Zn concentrations increased over the following gradient: stem base?<?stem tip?<?leaves. Significant differences in Si, Cu and Zn contents (except Zn in the stem) were noted between bamboo species, particularly between monopodial and sympodial bamboo species, which differ in their rhizome morphology. Sympodial bamboos accumulated more Si and Cu than monopodial bamboos, in both stems and leaves, whereas sympodial bamboos accumulated less Zn in leaves than monopodial bamboos.

Conclusions

The findings of this study suggest that a genotypic character may be responsible for Si, Cu and Zn accumulation in bamboo.     

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

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

Changes in antioxidant enzyme activities in detached leaves of cucumber exposed to chilling

We studied changes in biochemical and physiological status, level of oxidative damage, and antioxidant enzyme activities in detached leaves of cucumber plants (Cucumis sativus L. cv. Pyunggangnaebyungsamchuk) that were exposed to a low temperature (4°C). Chlorophyll fluorescence (F_v/F_m) declined during the chilling treatment, but was slowly restored after the tissues were returned to 25°C. Likewise, the fluorescence quenching coefficient and relative water content decreased during the stress period, but then increased during recovery. In contrast, we detected a significant rise in protein and hydrogen peroxide contents in the chilled leaves, as well as higher activities for superoxide dismutase, ascorbate peroxidase, peroxidase, and glutathione reductase. However, the level of catalase decreased not only during chilling but also after 24 h of recovery. These results indicate that exposure to low temperatures acts as an oxidative stress. Moreover, we propose that a regulating mechanism exists in the detached cucumber leaves and contains an antioxidant defense system that induces active oxygen species, thereby alleviating the effects of chilling stress within 12 h.     

Tocopherol content and enzymatic antioxidant activities in chloroplasts from NaCl-stressed tomato plants

Changes in tocopherol, chlorophyll and TBARS levels and the activities of antioxidant enzymes i.e., GSH-Px, GST, and SOD in chloroplasts of tomato plants subjected to moderate (50 mM) and severe (150 mM) NaCl stress were determined. Increase in tocopherol content around the second day under both stresses did not correlate with the chlorophyll degradation while such correlation was observed from the fifth day of severe stress. The activities of GSH-Px and GST as well as TBARS content showed NaCl-induced enhancement which was dose- and time-dependent. However, chloroplastic SOD was rather not involved in the response of tomato plants to NaCl stress. The obtained results suggest that under the moderate stress similarly as in the early phase of severe stress tocopherol functions as a typical antioxidant, while in the late phase of the latter it may be involved in senescence signaling pathway and enables the recovery and recycling of the compounds significant for a plant organism.     

Changes in the pattern of antioxidant enzymes in wheat exposed to water deficit and rewatering

Antioxidant defenses in two wheat cultivars differing in sensitivity to dehydration (YouJian (YJ-24) more sensitive than LongChun (LC-20) were analyzed during water deficit and rewatering. Resistant cultivar (LC-20) showed a higher relative water content than the sensitive cultivar (YJ-24) during the whole period of water withholding. In order to analyze the changes of antioxidant enzymes, native PAGE analysis of protein extract were performed. Wheat leaves had two isoforms of Mn-superoxide dismutase (SOD), two isoforms of Cu/Zn-SOD and one of Fe-SOD. Three catalase (CAT) isoforms were identified in the leaves of wheat. The activities of SOD and CAT isoforms were increased in two cultivars under water deficit. The intensities of SOD and CAT isoforms were slightly lower in LC-20 and increased continuously in YJ-24 after rewatering. Peroxidase (POD) isoforms were significantly increased during the whole dehydration-rehydration period. Three ascorbate peroxidase (APX) isoforms were present in gel. APX-1 and APX-3 were enhanced during water deficit and decreased during rewatering in LC-20. In YJ-24 only the activities of APX-2 were increased under water deficit. Seven isoforms of glutathione reductase (GR) were detected in the native gel. Activities of most of GR isoforms were higher in tolerant (LC-20) than in sensitive cultivar (YJ-24). Different isoforms of GR in two wheat cultivars behaved differently under water deficit and rewatering. These results collectively suggest that water deficit activates the SOD, CAT and ascorbate-glutathione cycle in wheat leaves. The response of enzyme isoforms to drought is not the same for all isoforms of antioxidant enzymes in two wheat cultivars.     

Effect of hydrogen peroxide on antioxidant enzyme activities in Saccharomyces cerevisiae is strain-specific

The effect of hydrogen peroxide on the survival and activity of antioxidant and associated enzymes in Saccharomyces cerevisiae has been studied. A difference found in the response of wild-type yeast strains treated with hydrogen peroxide was probably related to the different protective effects of antioxidant enzymes in these strains. Exposure of wild-type YPH250 cells to 0.25 mM H₂O₂ for 30 min increased activities of catalase and superoxide dismutase (SOD) by 3.4-and 2-fold, respectively. However, no activation of catalase in the EG103 strain, as well as of SOD in the YPH98 and EG103 wild strains was detected, which was in parallel to lower survival of these strains under oxidative stress. There is a strong positive correlation (R ² = 0.95) between activities of catalase and SOD in YPH250 cells treated with different concentrations of hydrogen peroxide. It is conceivable that catalase would protect SOD against inactivation caused by oxidative stress and vice versa. Finally, yeast cell treatment with hydrogen peroxide can lead to either a H₂O₂-induced increase in activities of antioxidant and associated enzymes or their decrease depending on the H₂O₂ concentration used or the yeast strain specificity. Published in Russion in Biokhimiya, 2006, Vol. 71, No. 9, pp. 1243–1252.     

Nitric oxide and hydrogen peroxide in tomato resistance. Nitric oxide modulates hydrogen peroxide level in o-hydroxyethylorutin-induced resistance to Botrytis cinerea in tomato.

Nitric oxide (NO) has been postulated to be required, together with reactive oxygen species (ROS), for activation of disease resistance reactions of plants to infection with a pathogen or elicitor treatment. However, biochemical mechanisms by which ROS and NO participate in these reactions are still under intensive study and controversial debate. We previously demonstrated that o-hydroxyethylorutin when applied on tomato leaves (Lycopersicon esculentum Mill. cv. "Perkoz") restricted Botrytis cinerea infection development. In this research we investigated ROS and NO generation in tomato plants treated with o-hydroxyethylorutin, non-treated and infected ones. The NO content was enhanced or decreased in the studied plants by supplying them with NO generator-SNP or scavenger-cPTIO. NO detection was carried out using diaminofluorescein diacetate (DAF-DA) in conjunction with confocal laser scanning microscopy. The influence of elevated and decreased levels of NO on B. cinerea infection development and ROS generation was studied. The elevated NO concentration in tomato leaves strongly decreased hydrogen peroxide concentration without affecting other studied ROS (superoxide anion and hydroxyl radical) levels. H2O2 concentrations in NO-supplied leaves were low regardless of further treatment of tomato leaves with o-hydroxyethylorutin or inoculation with B. cinerea. The low H2O2 concentration coincided with quick and severe infection development in NO-supplied leaves. As activities of enzymes generating (SOD EC 1.15.1.1)) and removing (APX EC 1.11.1.11, CAT EC 1.11.1.6) H2O2 were unchanged in the studied plants, the decrease in H2O2 concentration was probably due to a direct NO-H2O2 interaction.     

Effects of mercury and cadmium on the activities of antioxidative enzymes in the seedlings ofPhaseolus aureus

Phaseolus aureus Roxb. was exposed to HgCl₂ and Cd(NO₃)₂ either at the germination stage in concentration 0.5, 5 and 25 μM for 48 and 96 h, or at the seedling stage (5^th day of germination) in concentration 0.5, 5 and 20 μM for 6, 24 and 48 h. The germination and the growth of roots (germination stage treatment) were less in Hg than in Cd treatment. The seedlings (seedling stage treatment) were, however, more susceptible to Cd than Hg. Both root and leaf tissues of the plant treated at the germination stage showed enhanced lipid peroxidation and activities of the antioxidative enzymes (catalase, guaiacol peroxidase and ascorbate peroxidase), except the catalase in leaf in 25 μM Cd treatment. At seedling stage the content of malondialdehyde increased significantly only in the leaf tissue, during 6 h exposure. The activities of all the enzymes exhibited an increasing trend in both the tissue of the seedlings, particularly the leaf, at least after 24 and 48 h, except the catalase whose activity declined in response to Cd. Active involvement of the guaiacol and ascorbate peroxidases, rather than catalase, in scavenging cellular H₂O₂ was indicated. It was concluded that the two metals had little primary damaging effect on membranes.     

Exogenous nitric oxide promotes waterlogging tolerance as related to the activities of antioxidant enzymes in cucumber seedlings

We investigated the effects of exogenous sodium nitroprusside (SNP), a nitric oxide (NO) donor, on growth of cucumber (Cucumis sativus L., cv. Jinyou No.1) seedlings and antioxidant enzyme activities in cucumber leaves under waterlogging stress. The growth of cucumber seedlings was significantly inhibited when plants were exposed to waterlogging, whereas shoot spraying with SNP significantly alleviated the inhibition of growth from this type of stress: height, fresh and dry weights of the flooded plants increased obviously. Waterlogging also caused the activation of the antioxidant enzymes (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX)), the reduction of the chlorophyll content, and the accumulation of MDA and protein in leaves. It was found that SNP treatment further potentiated the antioxidant enzyme activities and maintained the chlorophyll and protein content during the entire water-logging period; however, it reduced the MDA content. Thus, NO protects plants from oxidative damage and promotes growth by activation of antioxidant enzymes in leaves in an extent sufficient for the alleviation of membrane injury. However, exogenous NO had no significant effects on cucumber seedlings growth and antioxidant enzyme activities under nonstress conditions.     

Combined effects of hypoxia and excess Mn2+ on oxidative stress and antioxidant enzymes in tomato seedlings

Effects of high level of Mn²⁺ on the changes in ROS generation, root cell viability, antioxidant enzyme activities, and related gene expression in tomato (Solanum lycopersicum L., cv. Zhongza 9) seedlings were studied under normoxic and hypoxia conditions. Mn²⁺ concentrations, ranged between 10 and 200 ??M, led to significantly higher activities of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APOD), glutathione reductase (GR), and also ascorbic acid (AsA) content in leaves and roots, improved root cell viability, and decreased O ₂ ^·? accumulation compared with the higher Mn²⁺ level under hypoxia stress, which indicated that low Mn²⁺ could eliminate the active oxygen and protect the membrane lipid from the hypoxia hurt. When the concentration of Mn²⁺ reached 400?C600 ??M under hypoxia stress, the activities of SOD, POD, APOD, and GR and AsA content were decreased remarkably. In contrast, the MDA content was increased at the higher Mn²⁺ concentration. A number of antioxidant-related genes showed high expression at the lower level of Mn²⁺. The expression levels of SOD, POD, CAT, APOD, and GR genes were 7.95, 5.27, 3.18, 5.54, and 8.81 times compared to control, respectively. These results illustrated that the appropriate amount of Mn²⁺ could alleviate the detrimental effects of hypoxia stress, but reversely, the high level of Mn²⁺ just aggravated the existing damage to the tomato seedlings.     

Enhanced antioxidant enzymes are associated with reduced hydrogen peroxide in barley roots under saline stress

Antioxidant enzymes are related to the resistance to various abiotic stresses including salinity. Barley is relatively tolerant to saline stress among crop plants, but little information is available on barley antioxidant enzymes under salinity stress. We investigated temporal and spatial responses of activities and isoform profiles of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), non-specific peroxidase (POX), and glutathione reductase (GR) to saline stress in barley seedlings treated with 200 mM NaCl for 0, 1, 2, 5 days, respectively. In the control plant, hydrogen peroxide content was about 2-fold higher in the root than in the shoot. Under saline stress, hydrogen peroxide content was decreased drastically by 70% at 2 d after NaCl treatment (DAT) in the root. In the leaf, however, the content was remained unchanged by 2 DAT and increased about 14 % at 5 DAT. In general, the activities of antioxidant enzymes were increased in the root and shoot under saline stress. But the increase was more significant and consistent in the root. The activities of SOD, CAT, APX, POX, and GR were increased significantly in the root within 1 DAT, and various elevated levels were maintained by 5 DAT. Among the antioxidant enzymes, CAT activity was increased the most drastically. The significant increase in the activities of SOD, CAT, APX, POX, and GR in the NaCl-stressed barley root was highly correlated with the increased expression of the constitutive isoforms as well as the induced ones. The hydrogen peroxide content in the root.     

Pattern of antioxidant enzyme activities and hydrogen peroxide content during developmental stages of rhizogenesis from hypocotyl explants of Mesembryanthemum crystallinum L.

Key message

H ₂ O ₂ is necessary to elicit rhizogenic action of auxin. Activities of specific catalase and manganese superoxide dismutase forms mark roots development.

Abstract

Hypocotyl explants of Mesembryanthemum crystallinum regenerated roots on medium containing 2,4-dichlorophenoxyacetic acid. Explants became competent to respond to the rhizogenic action of auxin on day 3 of culture, when hydrogen peroxide content in cultured tissue was the highest. l-Ascorbic acid added to the medium at 5 μM lowered the H₂O₂ level, inhibited rhizogenesis and induced non-regenerative callus, suggesting that certain level of H₂O₂ is required to promote root initiation. Coincident with the onset of rhizogenic determination, meristemoids formed at the periphery of the hypocotyl stele and the activity of the manganese form of superoxide dismutase, MnSOD-2 was induced. Once induced, MnSOD-2 activity was maintained through the post-determination phase of rooting, involving root growth. MnSOD-2 activity was not found in non-rhizogenic explants maintained in the presence of AA. Analyses of the maximum photochemical efficiency of photosystem II and the oxygen uptake rate revealed that the explants were metabolically arrested during the predetermination stage of rhizogenesis. Respiratory and photosynthetic rates were high during root elongation and maturation. Changes in catalase and peroxidase activities correlated with fluctuations of endogenous H₂O₂ content throughout rhizogenic culture. Expression of a specific CAT-2 form accompanied the post-determination stage of rooting and a high rate of carbohydrate metabolism during root growth. On the other hand, the occurrence of MnSOD-2 activity did not depend on the metabolic status of explants. The expression of MnSOD-2 activity throughout root development seems to relate it specifically to root metabolism and indicates it as a molecular marker of rhizogenesis in M. crystallinum.     

Changes in extreme high-temperature tolerance and activities of antioxidant enzymes of sacred lotus seeds

Sacred lotus (Nelumbo nucifera Gaertn. ‘Tielian’) seed is long-lived and extremely tolerant of high temperature. Water content of lotus and maize seeds was 0.103 and 0.129 g H₂O [g DW] ⁻¹, respectively. Water content, germination percentage and fresh weight of seedlings produced by surviving seeds gradually decreased with increasing treatment time at 100°C. Germination percentage of maize (Zea mays L. ‘Huangbaogu’) seeds was zero after they were treated at 100°Cfor 15 min and that of lotus seeds was 13.5% following the treatment at 100°C for 24 h. The time in which 50% of lotus and maize seeds were killed by 100°C was about 14.5 h and 6 min, respectively. With increasing treatment time at 100°C, relative electrolyte leakage of lotus axes increased significantly, and total chlorophyll content of lotus axes markedly decreased. When treatment time at 100°C was less than 12 h, subcellular structure of lotus hypocotyls remained fully intact. When treatment time at 100°C was more than 12 h, plasmolysis gradually occurred, endoplasmic reticulum became unclear, nuclei and nucleoli broke down, most of mitochondria swelled, lipid granules accumulated at the cell periphery, and organelles and plasmolemma collapsed. Malondialdehyde (MDA) content of lotus axes and cotyledons decreased during 0 −12 h of the treatment at 100°C and then increased. By contrast, the MDA content of maize embryos and endosperms increased during 5–10 min of the treatment at 100°C and then decreased slightly. For lotus seeds: (1) activities of superoxide dismutase (SOD) and glutathione reductase (GR) of axes and cotyledons and of catalase (CAT) of axes increased during the early phase of treatment at 100°C and then decreased; and (2) activities of ascorbate peroxidase (APX) and dehydroascorbate reductase (DHAR) of axes and cotyledons and of CAT of cotyledons gradually decreased with increasing treatment time at 100°C. For maize seeds: (1) activities of SOD and DHAR of embryos and endosperms and of GR of embryos increased during the early phase of the treatment at 100°C and then decreased; and (2) activities of APX and CAT of embryos and endosperms and of GR of endosperms rapidly decreased with increasing treatment time at 100°C. With decrease in seed germination, activities of SOD, APX, CAT, GR and DHAR of axes and cotyledons of lotus seeds decreased slowly, and those of embryos and endosperms of maize seeds decreased rapidly.     

Molecular evolution of hydrogen peroxide degrading enzymes

For efficient removal of intra- and/or extracellular hydrogen peroxide by dismutation to harmless dioxygen and water (2H(2)O(2) → O(2) + 2H(2)O), nature designed three metalloenzyme families that differ in oligomeric organization, monomer architecture as well as active site geometry and catalytic residues. Here we report on the updated reconstruction of the molecular phylogeny of these three gene families. Ubiquitous typical (monofunctional) heme catalases are found in all domains of life showing a high structural conservation. Their evolution was directed from large subunit towards small subunit proteins and further to fused proteins where the catalase fold was retained but lost its original functionality. Bifunctional catalase-peroxidases were at the origin of one of the two main heme peroxidase superfamilies (i.e. peroxidase-catalase superfamily) and constitute a protein family predominantly present among eubacteria and archaea, but two evolutionary branches are also found in the eukaryotic world. Non-heme manganese catalases are a relatively small protein family with very old roots only present among bacteria and archaea. Phylogenetic analyses of the three protein families reveal features typical (i) for the evolution of whole genomes as well as (ii) for specific evolutionary events including horizontal gene transfer, paralog formation and gene fusion. As catalases have reached a striking diversity among prokaryotic and eukaryotic pathogens, understanding their phylogenetic and molecular relationship and function will contribute to drug design for prevention of diseases of humans, animals and plants.     

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.     

Changes in activities of antioxidant enzymes and their gene expression during leaf development of sweetpotato

To understand the functions of antioxidant enzymes during leaf development in sweetpotato, we investigated the activities of several antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POX), ascorbate peroxidase (APX) and catalase (CAT). Significant increases were observed in the activities of SOD, POX and APX during the late stage of leaf development, whereas CAT activity increased during the early developmental stage. By RT-PCR analysis, various POX and APX genes showed differential expression patterns during leaf development. Four POX genes swpa3, swpa4, swpa6, swpb4 and one APX gene swAPX1 exhibited high levels of gene expression during the senescence stage of leaf development, but two POX genes, swpa1 and swpa7 were preferentially expressed at both the mature green and the late senescence stages of leaf development. These results indicate that hydrogen peroxide (H₂O₂)-related antioxidant enzymes are differentially regulated in the process of leaf development of sweetpotato.     

Changes in growth and activity of enzymes involved in nitrate reduction and ammonium assimilation in tomato seedlings in response to NaCl stress

BACKGROUND AND AIMS: In Tunisia, salt water is largely used for tomato irrigation. In this work, a study was made of the changes in the nitrate reduction and ammonium assimilation into amino acids in tomato seedlings under salinity in order to providee further insight into the salt effects on plant growth. Methods Ten-day-old tomatoes (Solanum lycopersicum) were subjected to 100 mm NaCl stress, and nitrogen metabolism in leaves and roots was studied. KEY RESULTS: The concentrations of Na+ and Cl- rapidly increased in the leaves and in the roots following exposure of tomato seedlings to NaCl stress. In contrast, the NO3- concentrations were lowered first in the roots and later in the leaves. From 5 to 10 d of treatment, salt ions provoked a decrease in the dry weight and an increase in the NH4+ concentrations in the leaves. Inhibition was observed in the leaves for the activities of nitrate reductase (NR, EC 1.6.6.1), ferredoxin-dependent glutamate synthase (Fd-GOGAT, EC 1.4.7.1) and deaminating glutamate dehydrogenase (NAD-GDH, EC 1.4.1.2). NaCl affected these enzyme activities less in the roots than in leaves. This was in accordance with the pronounced decrease of dry weight by salt in leaves compared with that in the roots. CONCLUSIONS: NaCl stress effects on growth, metabolite concentrations and enzyme activities depended on the duration of salt treatment and the plant tissue.