Abstract: In order to clarify the optimal application period and appropriate concentration of hydrogen sulfide (H₂S), we sprayed H₂S donor sodium hydrosulfide (NaHS) solution (0, 25, 50, 100, 200 and 400 μmol·L^-1) at seedling, jointing, heading, flowering and filling stages of soil potted naked oats to examine the effects of H₂S on the content of osmotic adjustment substance and the antioxidant activity in leaves under salt-alkali mixed stress (NaCl, Na₂SO₄, NaHCO₃, Na₂CO₃ molar ratio is 12∶8∶9∶1). The results showed that spraying period, NaHS concentration, and their interaction significantly promoted or inhibited the content of osmotic adjustment substance and the antioxidant activity. Compared with non-spraying NaHS, spraying 50 and 200 μmol·L^-1 NaHS at the seedling stage significantly increased the scavenging activity of ABTS·⁺ and the antioxidant activity of FRAP; and spraying 400 μmol·L^-1 NaHS performed best in increasing the total phenol content, scavenging activity of O₂^-_· and total reducing activity. Spraying 50, 100, and 400 μmol·L^-1 NaHS at the jointing stage significantly increased the content of soluble protein, flavonoids, and proline by 30.43%, 31.44%, and 258.59%, respectively. Spraying 50, 100, and 400 μmol·L^-1 NaHS at the heading stage significantly increased the content of proanthocyanidins, soluble sugars and free amino acids by 150.57%, 42.71%, and 197.93%, respectively. Spraying 100 μmol·L^-1 NaHS at the flowering stage significantly increased the scavenging activity of  DPPH· by 93.98%, with the greatest magnitude of enhancement. Spraying 400 and 25 μmol·L^-1 NaHS at the grouting period had the strongest effect on increasing the K⁺ content and K⁺/Na⁺. The interaction between spraying period and NaHS concentration had no significant or negative effect on organic acid content and the scavenging activity of ·OH in leaves. Using membership function analysis to comprehensively evaluate the effects of spraying NaHS at different periods on the osmotic adjustment and antioxidant activity of naked oat leaves showed that the comprehensive evaluation value of spraying 400 μmol·L^-1 NaHS at the jointing and heading stages was significantly higher than that of other concentrations at other periods. These results indicated that exogenous H₂S can alleviate the saltalkali stress of naked oat by improving osmotic regulation and antioxidant activity, with the best performance of spraying 400 μmol·L^-1 NaHS at the jointing-heading stage.

Key words: hydrogen sulfide, naked oat, saline-alkali stress, osmotic adjustment substance, antioxidant activity.