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Abstract
Salinity stress is one of the major agricultural problems worldwide that cause oxidative stress to plants, leading to reduction of plant growth br and consequently crop productivity. br The present study was carried out to br investigate the effect of two forms of Se (1 ppm sodium selenite or yeast br selenium hydrolysate) on the tolerance level of cucumber seedling (25- days old) to oxidative stress (0.5 % H2O2) or salinity stress (2000 ppm NaCl) and to identify some biochemical parameters associated with Seinduced resistance in cucumber seedlings. The biochemical changes were monitored by determination of growth parameters i.e. dry weight, br electrolyte leakage percentage (EL %), lipid peroxidation (LPO), free proline and phenolic compounds concentration. Moreover, the activities of peroxidase (POD), ascorbate peroxidase (APX), catalase (CAT), superoxide dismutase (SOD) and phenylalanine ammonia-lyase (PAL) were determined along with a detection of peroxidase isoenzymes in an br attempt to elucidate the mode(s)of action of Se in alleviating the negative br effects of salt and oxidative stresses. br The results indicated that, salinity treatment of cucumber seedlings br in the presence or absence of inorganic Se or yeast Se caused a remarkable elevation in proline and phenolic compound concentrations. br Salinity treatment caused a high level of electrolyte leakage and lipid br peroxidation. While, Se treatment caused a significant reduction in the br percentage of electrolyte leakage (EL %) and malondialdehyde (MDA) br concentration in cucumber seedlings. br The specific activities of antioxidant defensive enzymes (POD, SOD and APX) and PAL were significantly increased by NaCl treatment in the absence of either inorganic Se or yeast Se. Both inorganic Se and yeast Se treatments caused a significant elevation in the specific activities of POD, CAT, SOD and APX in cucumber seedlings subjected to salt stress condition. While, inorganic Se or yeast Se caused a significant decrease in SOD activity in cucumber seedlings grown in non-salty condition. The treatment of inorganic Se not yeast Se caused a significant increase in the activities of (POD, CAT and PAL) in control cucumber seedlings. On the other hand, yeast selenium caused a significant increase in CAT activity in control cucumber seedlings. br Oxidative stress treatment caused a significant elevation in proline br and phenolic compound concentrations in cucumber seedlings grown in the presence or absence of inorganic Se or yeast Se. Hydrogen peroxide treatment caused a high level of electrolyte leakage and lipid peroxidation. While, inorganic Se treatment or yeast Se caused a significant reduction in the percentage of electrolyte leakage and MDA concentration of salt-stressed and H2O2-stressed cucumber seedlings. br The specific activities of antioxidant defensive enzymes and PAL activity br were significantly increased by H2O2 treatment in cucumber seedlings br grown in Se free medium. In addition, inorganic Se treatment caused a br significant increase in the activities of POD, CAT, SOD and PAL under br oxidative stress. Yeast selenium treatment caused a significant increase in the activities of (POD, CAT, APX and PAL) and a slight increase in the br activity of SOD in cucumber seedlings irrigated with 0.5 % H2O2. br Inorganic Se or yeast Se caused a significant decrease in SOD activity in br control cucumber seedlings. Inorganic Se caused a significant increase in br the activities of (POD,CAT and PAL) in control cucumber seedlings. br On the other hand,yeast selenium treatment caused a significant increase br in CAT activity and a slight decrease in APX activity in yeast control br cucumber seedlings. The isoenzyme pattern of POD demonstrated that br peroxidase was up-regulated by inorganic Se or yeast Se either in control or NaCl and H2O2-stressed seedlings. In the light of the preceding results, br it may be concluded that Se can reduce and overcome the oxidative injury br through reduction of lipid peroxidation, increasing of osmoregulatory compounds such as proline and increasing the antioxidant enzymatic and non-enzymatic systems in cucumber seedlings.