Effect of Selenium on Ascorbate–Glutathione Metabolism During PEG-induced Water Deficit in Trifolium repens L.

To elucidate the effect of selenium (Se) on the ascorbate–glutathione (ASC–GSH) cycle under drought stress, the activities of antioxidant enzymes and the levels of molecules involved in ASC–GSH metabolism were studied in Trifolium repens seedlings subjected to polyethylene glycol (PEG)-induced water deficit alone or combined with 5 μM Na2SeO4. Compared to the control, H2O2, thiobarbituric acid reactive substances (TBARS), ascorbate (ASC), dehydroascorbate (DHA), and glutathione disulfide (GSSG) contents increased, whereas a constant content of glutathione (GSH) and decreases in ASC/DHA and GSH/GSSG ratios were observed in the presence of PEG. The activities of ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) were upregulated, except for monodehydroascorbate reductase (MDHAR) activity during PEG-induced water deficit. Se application decreased the contents of H2O2, TBARS, DHA, and GSSG, increased the levels of GSH and ASC, and inhibited the decreases of ASC/DHA and GSH/GSSG ratios. Although it did not affect APX activity significantly, Se addition improved the activities of MDHAR, DHAR, and GR. Furthermore, GR activity showed the highest increase followed by that of DHAR and MDHAR in decreasing order. These data indicated that fluctuations in ASC–GSH metabolism resulting from Se may have a positive effect on drought stress mitigation, and the regulation in the ASC–GSH cycle can be attributed mainly to GR and DHAR in PEG + Se-treated T. repens seedlings.