Modulation of ascorbate-glutathione cycle by selenate and sulphate treatments in the seedlings of two rice (Oryza sativa L.) cultivars

The trace element Selenium (Se) has a dual role in the growth and metabolism of plants. Low concentration of selenium (2 ?M selenate) promotes growth and counteracts the detrimental effects of abiotic stress as opposed to higher levels (?10 ?M) where it acts as a pro-oxidant. We focused on both individual and interactive influence of selenate and sulphate on thiol metabolism in seedlings of rice cultivars, satabdi and khitish. Inhibition of ascorbate contents by about 17% on an average, in the test seedlings treated with Se correlated with increased activities of ascorbate peroxidase and ascorbate oxidase in the cultivars. The glutathione levels also increased significantly, on an average by about 102% in roots and 74% in shoots of cv. satabdi compared to a rise, by about 49% in roots and 56% in shoots of cv. khitish. The elevated level of glutathione coincided with the stimulatory influence of Se on its regulatory enzymes. Concomitantly the levels of ?-tocopherol and phytochelatins were also induced in both the test cultivars. Increase in ?-tocopherol activity reached a maximum by about 47% in roots and 80% in shoots of cv. satabdi whereas it increased by about 36% in roots and about 64% in shoots of cv. khitish. Substantive increase in the levels of PC4 followed by PC2 and PC3 was also noted. The effects were found to be less conspicuous in shoots than in roots. Rice seedlings exposed to combined Se and 10mM sulphate treatments showed improved growth and development as a result of better thiol metabolism due to amelioration of the adverse effects caused by selenium alone on all the parameters tested.