Antioxidative responses and proline level in leaves and roots of pea plants subjected to nickel stress

The activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), glutathione S-transferase (GST) as well as proline content were studied in leaves and roots of 14 day-old pea plants treated with NiSO₄ (10, 100, 200 μm) for 1, 3, 6 and 9 days. Exposure of pea plants to nickel (Ni) resulted in the decrease in CuZnSOD as well as total SOD activities in both leaves and roots. The activity of APX in leaves of plants treated with 100 and 200 μm Ni increased following the 3rd day after metal application, while in roots at the end of the experiment the activity of this enzyme was significantly reduced. In both organs CAT activity generally did not change in response to Ni treatment. The activity of GST in plants exposed to high concentrations of Ni increased, more markedly in roots. In both leaves and roots after Ni application accumulation of free proline was observed, but in the case of leaves concentration of this amino acid increased earlier and to a greater extent than in roots. The results indicate that stimulation of GST activity and accumulation of proline in the tissues rather than antioxidative enzymes are involved in response of pea plants to Ni stress.     

Effect of 28-homobrassinolide treatment on nickel toxicity in <Emphasis Type="Italic">Brassica juncea</Emphasis>

Plants of Brassica juncea L. cv. T-59 were supplied with 50 or 100 μM nickel (Ni₅₀, Ni₁₀₀) at 10 d after sowing (DAS), and sprayed with 28-homobrassinolide (HBR) at 20 DAS. The plants treated with Ni alone exhibited reduced growth, net photosynthetic rate, content of chlorophyll, and the activities of nitrate reductase (E.C.1.6.6.1) and carbonic anhydrase (E.C. 4.2.1.1) at observed 40 DAS, whereas, the contents of peroxidase (PER), catalase (CAT), and proline were increased. However, the spray of HBR partially neutralized the toxic effect of Ni on most of the parameters. Moreover, the treatment of HBR in association with either of the Ni concentration boosted the contents of PER and CAT in leaves and that of proline both in leaves and roots.     

Effects of Pb-EDTA and EDTA on oxidative stress reactions and mineral uptake in Phaseolus vulgaris

Sequestration of Pb by synthetic chelates has been reported to increase bioavailability, uptake, and translocation of this metal in plants. In this work the potential phytotoxic effects of Pb-EDTA were investigated in Phaseolus vulgaris L. cv. Limburgse vroege plants grown on hydroponics. Addition of 50 µ M Pb-EDTA to the nutrient solution caused a significant induction of syringaldazine peroxidase (SPOD; EC 1.11.1.7) in roots and primary leaves and guaiacol peroxidase (GPOD; EC 1.11.1.7) in leaves. Addition of 100 µ M Pb-EDTA further exacerbated ascorbate peroxidase (APOD; EC 1.11.1.11), GPOD, dehydroascorbate reductase (DHAR; EC 1.8.5.1), glutathione reductase (GR; EC 1.6.4.2) and malic enzyme (ME; EC 1.1.1.40) in roots and APOD and ME in primary leaves. Addition of 200 µ M Pb-EDTA also induced DHAR in leaves. This induction of peroxidases (SPOD, GPOD, APOD), enzymes of the ascorbate-glutathione cycle (DHAR, GR in roots) and of an NADP⁺ reducing enzyme in roots and primary leaves indicates that oxidative stress has been initiated. At 200 µ M Pb-EDTA, chlorophyll a and b content in leaves was significantly reduced while visible effects on root morphology and shoot length were observed, while no significant morphological effects were found in the leaves, confirming the sensitive character of the measured enzymes as plant stress indicators. Elevation of the Pb-EDTA concentration in the growth medium significantly reduced the content of Ca, Fe, Mn and Zn taken up by plants, probably due to ion leakage as a result of observed toxicity. Addition of up to 200 µ M EDTA increased chelation of divalent cations in nutrient solution resulting in reduced plant uptake of Zn, Cu, Fe and Mn. This did not result in phytotoxicity.     

Cadmium-induced oxidative damage and antioxidant responses in Brassica juncea plants

Indian mustard (Brassica juncea L. cv. Vitasso) plants exposed to 10, 30, 50 and 100 μM of Cd for 5 d in hydroponic culture were analysed with reference to the distribution of Cd²⁺, the accumulation of biomass and antioxidants and antioxidative enzymes in leaves. Cd induced a decrease in plant biomass. The maximum accumulation of Cd occurred in roots followed by stems and leaves. Cd induced a decrease in catalase (CAT) and guiacol peroxidase (GPX) activities but an increase in ascorbate peroxidase (APX) and monodehydroascorbate reductase (MDHAR) activities. Enhancement in dehydroascorbate reductase (DHAR) activity was also at 10 μM Cd. Glutathione reductase (GR) activity showed pronounced stimulation after all treatments, but glutathione S-transferase (GST) and glutathione peroxidase (GPOX) activities decreased. The effectiveness of ascorbate-glutathione cycle (AGC) was determined by the ratio of ascorbate to H₂O₂. This ratio decreased in the Cd-treated leaves which indicated that the cycle was disordered.     

Combined Effect of Salicylic Acid and Nitrogen Oxide Donor on Stress-Protective System of Wheat Plants under Drought Conditions

Presowing treatment of wheat (Triticum aestivum L.) seeds with 10 or 100 μM salicylic acid (SA) reduced the inhibition of 14-day-old plant growth under soil drought. The same effect was caused by the spraying of 7-day-old seedlings with 0.5 or 2 mM nitrogen oxide donor (sodium nitroprusside, SNP) before drought. The protective effect was enhanced by the combination of seed treatment with 10 μM SA and plant spraying with 0.5 mM SNP, while their combinations in higher concentrations caused weaker effects. SA treatment in both concentrations and 0.5 mM SNP under drought conditions increased the antioxidant enzyme activity (superoxide dismutase, catalase, and guaiacol peroxidase) in leaves. This effect was especially significant when 10 μM SA was combined with 0.5 mM SNP. Spraying with 2 mM SNP and its combination with seed presowing with 100 μM SA did not significantly change the antioxidant enzyme activity; however, the proline content in the leaves increased. It is concluded that the SA stress-protective action on plants can be modified with exogenous nitrogen oxide.     

Effect of Cadmium Ions on Antioxidant Defense System in Sunflower Cotyledons

Sunflower (Helianthus annuus L.) seeds were germinated and grown in the presence of 50, 100 and 200 μM CdCl₂. The lower concentration (50 μM) of Cd² ions produced slight decrease in reduced glutathione (GSH) content and overall increase (except superoxide dismutase) in antioxidant enzyme activities, and in H₂O₂ concentration. Chlorophyll content, lipid peroxidation and protein oxidation were not affected under 50 μM CdCl₂. GSH content was diminished under 100 and 200 μM CdCl₂, and except for superoxide dismutase, which activity remained unaltered, overall decreases in the antioxidant enzyme activities (catalase, ascorbate peroxidase, dehydroascorbate peroxidase, glutathione reductase) and in guaiacol peroxidase were observed. These Cd² concentrations caused a decrease in chlorophyll content as well as an increase in lipid peroxidation, protein oxidation and H₂O₂ concentration. All the observed effects were more evident with the highest concentration of cadmium chloride used. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nickel-induced Inhibition of Wheat Root Growth is Related to H2O2 Production, but not to Lipid Peroxidation

Effects of exogenous nickel (Ni: 10 and 200 μM) on growth, mitotic activity, Ni accumulation, H₂O₂ content and lipid peroxidation as well as the activities of various antioxidative enzymes, such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and glutathione peroxidase (GSH-Px) were investigated in wheat roots. A considerable Ni accumulation in the roots occurred at both the concentrations. Although Ni at 10 μM did not have any significant effect on root growth, it strongly inhibited the root growth at 200 μM. Mitotic activity in the root tips was not significantly affected by exposure of the seedlings to 10 μM Ni; however, it was almost completely inhibited at 200 μM treatment. Ni stress did not result in any significant changes in CAT and APX activities as well as lipid peroxidation. However, H₂O₂ concentration increased up to 82% over the control in the roots of seedlings exposed to 200 μM Ni. There was a significant decline in both SOD (50%) and GSH-Px (20–30%) activities in the roots when the seedlings were treated with 200 μM Ni. The results indicated that a strong inhibition of wheat root growth caused by Ni stress was not due to enhanced lipid peroxidation, but might be related to the accumulation of H₂O₂ in root tissue.     

Changes in Growth and Nitrogen Assimilation in Maize Plants Induced by NaCl and Growth Regulators

Experiments were conducted to determine the interactive effects of salinity and certain growth regulators on growth and nitrogen assimilation in maize (Zea mays L. cv. GS-2). 100 mM NaCl inhibited the biomass accumulation, chlorophyll and carotenoid contents in leaves, nitrate content and uptake and nitrate reductase activity. The application of kinetin, ascorbic acid and 10 and 50 μM abscisic acid in the first experiment and 50 and 100 μM abscisic acid in the second experiment induced a substantial increase in the above parameters, the effect was highest with abscisic acid in salinized as well as non-salinized plants. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of N and Ni supply on nitrogen metabolism and urease activity in rice (Oryza sativa L.)

Nickel is considered to be an essential micronutrient in plants because of its role in the metalloenzyme urease. In order to characterize the metabolic consequences of Ni deprivation, the significance of Ni supply for growth and N metabolism of rice plants grown with either NH4NO3 or urea as sole N source was evaluated. Growth of plants receiving NH4NO3 was not affected by the Ni status, and neither were the activities of arginase and glutamine synthetase. However, urease activity was not detectable in leaves of low-Ni plants, which in conjunction with arginase action, led to the accumulation of urea in plants grown with NH4NO3. Amino acid contents and mineral nutrient status (except Ni) were not affected by the Ni treatment.Urea-grown Ni-deprived plants showed reduced growth and accumulated large amounts of urea owing to the lack of urease activity. These plants were further characterized by low amino acid contents indicating impaired usage of the N supplied. They also exhibited reduced levels of the urea precursor arginine, which is merely attributed to the overall N economy in these plant. When urea-grown plants were supplied with 0.5 mmol m^-3 Ni in the nutrient solution, the dry weight and the amino acid N contents were increased at the expense of the urea contents, indicating efficient use of urea N in Ni-supplemented plants.A critical Ni concentration in the shoot regarding dry matter production of NH4NO3-grown plants could not be deduced, while 25 g Ni kg^-1 DW is certainly inadequate for urea-grown plants. This suggests that the Ni requirement strongly depends on the N source employed.Keywords: Amino acids, ornithine cycle, Ni supply, rice, urea, urease activity.      

Role of Jasmonic and Salicylic Acid on Enzymatic Changes in the Root of Two Alyssum inflatum Náyr. Populations Exposed to Nickel Toxicity

Phytohormones, including salicylic acid (SA) and jasmonic acid (JA) have the potential to ameliorate plant development and tolerance to deleterious effects of toxic metals like nickel (Ni). Therefore, the current study was carried out to evaluate SA and JA's interactive effect on the root antioxidative response of two Alyssum inflatum Nyár. populations against Ni-toxicity. Two A. inflatum species under different Ni concentrations (0, 100, 200, and 400 μM) were exposed to alone or combined levels of SA (0, 50, and 200 μM) and JA (0, 5, and 10 μM) treatments. Results showed that high Ni concentration (400 μM) reduced roots fresh weight in both populations than in control. However, external application of individual SA and JA or combined SA?+?JA in higher doses had ameliorated roots biomass by mitigating Ni-toxicity, especially in the NM population, in comparison to 400 μM Ni. Under Ni toxicity, SA and JA, especially their combination, induced high Ni accumulation in plants' roots. Moreover, the application of SA and JA alone, as well as combined SA?+?JA, was found to be effective in the scavenging of hydrogen peroxide by improving the activity of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase in both populations under Ni-toxicity. Overall, our results manifest that SA and JA's external use, especially combined SA?+?JA treatments, ameliorate root biomass and plant tolerance by restricting Ni translocation to the shoot, accumulating in roots, and enhancing antioxidant defense systems.

     

Changes in peroxidase and IAA oxidase activities during cell elongation in Phaseolus hypocotyls

Cytoplasmic and salt-extracted peroxidase and IAA oxidase activities were studied in Phaseolus vulgaris hypocotyls treated with gibberellic acid (GA, 200 μM), naphthyl acetic acid (NAA, 100 μM) and distilled water control (DW). Peroxidase activity was assayed with four hydrogen donors during the initial phase of hypocotyl elongation. Though peroxidase activity showed a decreasing trend with time in all the hydrogen donors studied; considerable variation with different hydrogen donors was observed. NAA had maximum peroxidase activity as compared to DW or GA treatment. The activity showed a clear inverse correlation with hypocotyl growth. IAA oxidase activity showed a similar trend with growth as peroxidase activity. A highly significant correlation was observed between peroxidase and IAA oxidase activities and high molecular weight xyloglucan content (P<0.001). Finally, the possible role of peroxidase and IAA oxidase activities in hypocotyl elongation growth is discussed.     

Application of Bacillus megaterium MCR-8 improved phytoextraction and stress alleviation of nickel in Vinca rosea

The current research was performed to evaluate the effect of Bacillus megaterium MCR-8 on mitigation of nickel (Ni) stress in Vinca rosea grown on Ni-contaminated soil (50, 100, and 200 mg Ni kg^?1 soil). The treated plants exhibited reduced growth, biomass, gas exchange capacity, and chlorophyll (Chl) content under Ni stress. The inoculated plants growing in Ni-contaminated media exhibited relatively higher growth, total soluble protein, and proline contents. Similarly, bacterial inoculation improved the activity of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) under Ni stress. The Ni stress alleviation in inoculated plants was attributed to the reduced level of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), enhanced synthesis of protein, proline, phenols, and flavonides in conjunction with improved activity of antioxidant enzymes. The growth-promoting characteristics of microbe such as 1-aminocyclopropane-1-carboxylate deaminase (ACCD) and phosphate solubilization activity, siderophore, and auxin production capability also improved the growth and stress mitigation in inoculated plants. Furthermore, the inoculated plants exhibited higher value for bioconcentration factor (BCF), translocation factor (TF), and resulted in higher loss of Ni content from soil. The current results exhibited the beneficial role of B. megaterium MCR-8 regarding stress alleviation and Ni phytoextraction by V. rosea.     

Response of antioxidant enzymes in rice (<Emphasis Type="Italic">Oryza sauva</Emphasis> L. cv. Dongjin) under mercury stress

We studied the effects of different concentrations of mercury (0.0 to 100 μM) on growth and photosynthetic efficiency in rice plants treated for 21 d. In addition, we investigated how this metal affected the malondialdehyde (MDA) content as well as the activity of five antioxidant enzymes — superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione reductase (GR), guaiacol peroxidase (POD), and catalase (CAT). Photosynthetic efficiency (Fμ/F_m) and seedling growth decreased as the concentration of Hg was increased in the growth media. Plants also responded to Hg-induced oxidative stress by changing the levels of their antioxidative enzymes. Enhanced lipid peroxidation was observed in both leaves and roots that had been exposed to oxidative stress, with leaves showing higher enzymatic activity. Both SOD and APX activities increased in treatments with up to 50 μM Hg, then decreased at higher concentrations. In the leaves, both CAT and POD activities increased gradually, with CAT levels decreasing at higher concentrations. In the roots, however, CAT activity remained unchanged while that of POD increased a bit more than did the control for concentrations of up to 10 μM Hg. At higher Hg levels, both CAT and POD activities decreased. GR activity increased in leaves exposed to no more than 0.25 μM Hg, then decreased gradually. In contrast, its activity was greatly inhibited in the roots. Based on these results, we suggest that when rice plants are exposed to different concentrations of mercury, their antioxidative enzymes become involved in defense mechanisms against the free radicals that are induced by this stress.     

Fructose-1,6-bisphosphate aldolase (class II) is the primary site of nickel toxicity in Escherichia coli

Nickel is toxic to all forms of life, but the mechanisms of cell damage are unknown. Indeed, environmentally relevant nickel levels (8 μM) inhibit wild-type Escherichia coli growth on glucose minimal medium. The same concentration of nickel also inhibits growth on fructose, but not succinate, lactate or glycerol; these results suggest that fructose-1,6-bisphosphate aldolase (FbaA) is a target of nickel toxicity. Cells stressed by 8 μM Ni(II) for 20 min lost 75% of their FbaA activity, demonstrating that FbaA is inactivated during nickel stress. Furthermore, overexpression of fbaA restored growth of an rcnA mutant in glucose minimal medium supplemented with 4 μM Ni(II), thus confirming that FbaA is a primary target of nickel toxicity. This class II aldolase has an active site zinc and a non-catalytic zinc nearby. Purified FbaA lost 80 % of its activity within 2 min when challenged with 8 μM Ni(II). Nickel-challenged FbaA lost 0.8 zinc and gained 0.8 nickel per inactivated monomer. FbaA mutants (D144A and E174A) affecting the non-catalytic zinc were resistant to nickel inhibition. These results define the primary site of nickel toxicity in E. coli as the class II aldolase FbaA through binding to the non-catalytic zinc site.     

The influence of calcium ions on nickel modulation of NMDA receptor currents

N-Methyl-D-aspartate (NMDA) receptors (NRs) are glutamate-gated channels critical for the functioning of the nervous system. They are assembled from two types of subunits, the essential GluN1 and at least one type of GluN2 (A, B, C, D) subunit. Nickel (Ni) modulates the NR current in a way dependent on the GluN2 subunit present. Besides voltage-dependent and voltage-independent inhibition, in GluN2B-containing channels Ni enhances channel activity. We have recently identified several domains of the channel involved in Ni interaction, but many aspects of this modulation remain elusive. The purpose of the present work is to investigate the role of calcium (Ca) in the effect of Ni on the NR current measured by voltage- and patch-clamp in RNA-injected Xenopus laevis oocytes or in transiently transfected mammalian HEK293 cells expressing GluN1/GluN2B recombinant receptors. In both expression systems, in the presence of a physiological concentration of Ca (1.8 mM), Ni increased the NR current with EC(50) in the μM range, but this potentiation was reduced by decreasing Ca concentration or when Ca was substituted with Ba. In injected oocytes, the effect of Ni in 0.3 mM external Ba was only inhibitory (IC(50) = 65 μM). Increasing the internal calcium buffering by EGTA and BAPTA application, as well as incubation with cytoskeleton perturbing agents, colchicine and cytochalasin-D, did not produce major modifications in the Ni effect. These observations indicate that Ni-mediated potentiation is not dependent on Ca influx and internal Ca concentration, but it is dependent on external Ca, which possibly interacts with the extracellular portion of the protein through a modulatory binding site.     

Salinity attenuates nickel-accumulating capacity of <Emphasis Type="Italic">Atropa belladonna</Emphasis> L. plants

Comparative analysis of growth and composition of Atropa belladonna L. plants was performed after separate and combined additions of NaCl and NiCl₂ to the nutrient medium. Plants were grown in water culture on modified Johnson solution for 8 weeks until the formation of the fifth leaf pair. Thereafter, NiCl₂ was introduced at final concentrations of 100 and 150 μM into the medium either separately or in combination with 100 mM NaCl. After completing the 7-day treatment with Ni ions, the plants' weight and the content of water and photosynthetic pigments were determined. The content of Ni, free polyamines (putrescine, spermidine, spermine), and atropine was determined in plant roots and leaves, whereas the content of Fe, proline, and malondialdehyde (MDA) was examined in leaves only. The distribution of Ni in various tissues was inspected using the dimethylglyoxime method. The presence of NiCl₂ in growth media diminished the increments in fresh weight of shoots and roots; lowered the content of water, pigments, and iron in leaves; and initiated chlorosis. The leaves of Ni-treated plants accumulated larger amounts of atropine, putrescine, proline, and MDA with respect to the control levels of these compounds. In contrast to the action of Ni alone, the combined application of NaCl and NiCl₂ was followed by the increased content of water and pigments in leaves. The presence of NaCl in the medium restricted the entry of Ni into roots and diminished the levels of MDA and proline in leaves. After growing the plants in the presence of 100 and 150 μM NiCl₂, nickel was located in the root outer cortex and the rhizoderm. In plants treated with 150 μM NiCl₂, nickel was also observed in tissues of the central cylinder, mostly in the pericycle, phloem, and xylem. In plants grown in the presence of 150 μM NiCl₂ and 100 mM NaCl, the decreased accumulation of nickel was noted in the tissues of the central cylinder in the root hair zone. Thus, the combined action of Ni and moderate salinity reduced nickel accumulation in roots and aboveground organs of A. belladonna plants. The reduced Ni content in plants mitigated the toxic effect of Ni present in the medium. This was manifested in stabilization of leaf water status, an increase in the content of photosynthetic pigments, and alleviation of oxidative stress, which was assessed from the content of low-molecular organic compounds exhibiting stress-protective and antioxidant action (proline, MDA, free polyamines, and atropine).     

Differential biochemical responses of wheat shoots and roots to nickel stress: antioxidative reactions and proline accumulation

Wheat (Triticum aestivum L. cv. ‘Zyta’) seedlings were treated with 10, 100 and 200 μM Ni. Tissue Ni accumulation, length, relative water content (RWC), proline and H₂O₂ concentrations as well as the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (POD) and glutathione S-transferase (GST) were studied in the shoots and roots after 6 days of Ni exposure. Treatment with Ni, except for its lowest concentration, resulted in a significant reduction in wheat growth. In comparison to the shoots, the roots showed greater inhibition of elongation, which corresponded with higher accumulation of Ni in these organs. Both shoots and roots responded to Ni application with a decrease in RWC and enhancement in proline concentration. Greater dehydration of the shoot tissue was accompanied by more intense accumulation of proline. Treatment of the wheat seedlings with the highest concentration of Ni led to about 60% increase in H₂O₂ concentration in both studied organs. Apart from CAT, constitutive activities of antioxidative enzymes were much higher in the roots than in the shoots. Exposure of the seedlings to Ni resulted in SOD activity decline, which was more marked in the roots. While the shoots showed a substantial decrease (up to 30%) in CAT activity, in the roots the activity of this enzyme remained unchanged. After Ni application APX, POD and GST activities increased several-fold in the shoots, whereas in the roots they were not significantly altered. The results suggest that differential antioxidative responses of the shoots and roots of wheat seedlings to Ni stress might be related to diverse constitutive levels of antioxidant enzyme activities in both organs.     

Interactive effects of cadmium and aluminum on growth and antioxidative enzymes in soybean

The effects of Al, Cd and pH on growth, photosynthesis, malondialdehyde (MDA) content, and some antioxidant enzyme activities of the two soybean cultivars with different Al tolerance were determined using a hydroponic culture. There were six treatments as follows: pH 6.5; pH 4.0; pH 6.5 + 1.0 μM Cd; pH 4.0 + 1.0 μM Cd; pH 4.0 + 150 μM Al; pH 4.0 + 1.0 μM Cd + 150 μM Al. The results showed that the low pH (4.0) and Al treatments caused marked reduction in the growth (root and shoot length and dry mass), chlorophyll content (SPAD value) and net photosynthetic rate. Higher malondialdehyde content, superoxide dismutase (SOD) and peroxidase (POD) activities were detected in the plants exposed to both Al and Cd than in those exposed to Al treatment alone. An expressive enhancement of SOD and POD was observed in the plants exposed to 150 μM Al in the comparison with the control plants, especially in Al-sensitive cv. Zhechun 2 which had also significantly higher Al and Cd content than Al tolerant cv. Liao-1. Cd addition increased Al content in the plants exposed to Al + Cd stress, and cv. Zhechun 2 had relatively lower Al content. The present research indicated that Al and Cd are synergistic in their effects on plant growth and some physiological traits.     

Exogenous melatonin improves growth and photosynthetic capacity of cucumber under salinity-induced stress

Melatonin mediates many physiological processes in animals and plants. To examine the potential roles of melatonin in salinity tolerance, we investigated the effects of exogenous melatonin on growth and antioxidant system in cucumber under 200 mM NaCl stress conditions. The results showed that the melatonin-treated plants significantly increased growth mass and antioxidant protection. Under salinity stress, the addition of melatonin effectively alleviated the decrease in the net photosynthetic rate, the maximum quantum efficiency of PSII, and the total chlorophyll content. Our data also suggested that melatonin and the resistance of plants exhibited a concentration effect. The application of 50–150 μM melatonin significantly improved the photosynthetic capacity. Additionally, the pretreatment with melatonin reduced the oxidative damage under salinity stress by scavenging directly H₂O₂ or enhancing activity of antioxidant enzymes (including superoxide dismutase, peroxidase, catalase, ascorbate peroxidase) and concentrations of antioxidants (ascorbic acid and glutathione). Therefore, the melatonin-treated plants could effectively enhance their salinity tolerance.     

Interactive effect of calcium and gibberellin on nickel tolerance in relation to antioxidant systems in <Emphasis Type="Italic">Triticum aestivum</Emphasis> L.

Nickel toxicity affects many metabolic facets of plants and induces anatomical and morphological changes resulting in reduced growth and productivity. To overcome the damaging effects of nickel (Ni) stress, different strategies of the application of nutrients with plant hormones are being adopted. The present experiment was carried out to assess the growth and physiological response of wheat plant (Triticum aestivum L.) cv. Samma to pre-sowing seed treatment with GA₃ alone as well as in combination with Ca²⁺ and/or Ni stress. The pre-sowing seed treatment of Ni decreased all the growth characteristics (plant height, root length, fresh, and dry weight) as well as chlorophyll (Chl) content and enzyme carbonic anhydrase (CA: E.C. 4.2.1.1) activity. However, an escalation was recorded in malondialdehyde content and electrolyte leakage in plants raised from seed soaked with Ni alone. Moreover, all the growth parameters and physiological attributes (Chl content, proline (Pro) content, CA, peroxidase (E.C.1.11.1.7), catalase (E.C. 1.11.1.6), superoxide dismutase (E.C. 1.15.1.1), ascorbate peroxidase (E.C. 1.11.1.11), and glutathione reductase (E.C. 1.6.4.2) were enhanced in the plants developed from the seeds soaked with the combination of GA₃ (10⁻⁶ M), Ca^2+, and Ni. The present study showed that pre-sowing seed treatment of GA₃ with Ca²⁺ was more capable in mitigation of adverse effect of Ni toxicity by improving the antioxidant system and Pro accumulation.