Effects of selenium on damage of heavy metals in germination, growth and antioxidant activities ofcucumber (Cucumis sativus L.) seedling

Selenium is a beneficial element with antioxidant and antivirus function and it can increase plant growthand tolerance to environmental stress.Inthis study, the effects of selenium in stress conditions caused by heavy metals cadmium, nickel and lead in cucumber (Cucumis sativus L. cv Super Dominus) was investigated.The research was conducted using a completely randomized design with factorial arrangement in three replications for two germination tests .The first experiment was carried out to determine the optimal effectiveness of selenium and heavy metal toxicity threshold levels, Which contains levels of selenium (0, 4, 6, 8, 10 mgL-1) from sodium selenate, cadmium (0, 10, 15, 20, 25 µM) from cadmium chloride, nickel (0, 50, 100, 150, 200 µM) from nickel chloride and lead (0, 20, 40, 60, 100 µM) from lead chloride sources. The second experiment were consisted of two levels of selenium (0, 8 mgL-1) and toxicity threshold levels of heavy metals, including cadmium (0, 25 µM), nickel (0, 200 µM) and lead (0, 100 µM) treatments.The results showed that the germination index was significantly reduced as presence of toxic concentrations of heavy metals in the medium of cucumber seed germination. However, the application of selenium was improves the rate and percentage of germination, increase the length of shoot and root and seedling wet and dry weight in stress condition.Selenium was significantly increased cell membrane stability and decrease malondialdehyde under stress condition.Selenium improved antioxidant activity and increase activity of catalase, ascorbate peroxidase and guaiacol peroxidase enzyms in heavy metal toxicity condition .In general, the results of this study showed that the optimum concentration of selenium was reduces symptoms of toxicity due to the presence of heavy metals incucumber seedgermination mediumas  improving germination, growth and antioxidant activity.