Ascorbic Acid Reduces the Phytotoxic Effects of Selenium on Rice (Oryza Sativa L.) by Up-Regulation of Antioxidative and Metal-Tolerance Mechanisms

Selenium (Se) in trace concentrations (<1mg/kg) is proved to be beneficial for many plant species, but is considered toxic at higher concentrations and adversely affects the potential growth and yield of plants. Hence, measures are needed to minimise the phyto-toxicity of Se on plants. Ascorbic acid (Asc) has vital roles as a redox buffer, a strong antioxidant, regulator of enzymes of photosynthesis, phytohormones, cell division and growth in plants. In the present study, rice, which is a sensitive to Se-stress, was grown under controlled laboratory conditions (30/25; 15/9h, light/ dark; light intensity: 350 umol m-2 s-1, RH: 65-70%) with selenium (as sodium selenate; 1, 2.5 and 5 ppm) in the absence or presence of 50 μM Asc for ten days in half-strength Hoagland solution. Se at 2.5 ppm concentration resulted in 22.6 and 30% inhibition of roots and shoots, respectively, which increased markedly at 5.0 ppm (47% for roots 60% for shoots). Addition of Asc to the rice plants treated with 2.5 and 5.0 ppm Se resulted in 36 and 21% improvement in root growth and 28 and 38% increase in shoot growth, respectively, over the plants growing without Se. The uptake of Se increased with rise in its exogenous concentration but it was not affected by Asc application. Asc-supplemented plants showed significant prevention of damage caused by Se to membranes, chlorophyll and PSII function. A significant reduction in oxidative stress, measured as malondialdehyde and hydrogen peroxide content, was observed in the presence of Asc in Se-stressed plants, which was found to be associated with improvement in activities of enzymatic (pertinently, ascorbate peroxidase, glutathione peroxidase) and non-enzymatic (ascorbic acid, reduced glutathione) antioxidants. Furthermore, Se-stressed plants supplemented with Asc showed significant up-regulation of metallothioneins, total thiols and glutathione-stransferase (GST) activity to reduce the toxic effects of Se.