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Foliar applications of osmoprotectants could be a valuable tool to counteract the deleterious effects of salinity and protect plant cells from oxidative damage. Some of the known osmoprotectants used in this field are salicylic acid, proline, and/or glycine betaine but there is no available empirical research exists to figure out which of these chemicals is the most effective in reducing the adverse effects of salt stress. To compare the ameliorating effects of foliar application of these osmoprotectants on growth, some physiological and biochemical parameters, and productivity of cucumber plants under NaCl stress (0, 50, or 100 mM), a factorial experiment was carried out during 2015 and 2016 seasons at the Experimental Farm of Horticulture Department, Faculty of Agriculture, Ain Shams University, Qalubia Governorate, Egypt. Increasing NaCl levels reduced plant growth parameters, leaf relative water content, leaf membrane stability, chlorophyll contents, some macro-nutrients content and yield and the lowest values were recorded with the use of 100 mM of sodium chloride treatment. Foliar application of salicylic acid at 1 mM, proline at 10 mM, or glycine betaine at 10 mM to cucumber plants ameliorated the harmful effects of NaCl stress on the vegetative growth and yield through enhancing both leaf relative water content and leaf membrane stability. In addition, these treatments improved both chlorophyll contents, and nutrient content, altogether resulted in a significant reduction in sodium (Na+) and chloride (Cl−) accumulation toxicity. Moreover, the expression of cucumber peroxidase isozymes was analyzed in cucumber leaves 1, 3 and 7 days after the second application. Native polyacrylamide gel electrophoresis analysis indicated that exogenous applications led to the differential regulation of peroxidase enzyme. In conclusion, salicylic acid was the most effective in attenuating the negative effects of the salt stress and in up-regulating peroxidase enzyme expression followed by proline and by glycine betaine. Peroxidase enzyme appeared to function as one of the main molecular mechanisms underlying the resistance response of cucumber to salinity. [ABSTRACT FROM AUTHOR]

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