Exogenous hydrogen sulfide improves salt stress tolerance of seedlings by regulating active oxygen metabolism.

Hydrogen sulfide (HS), as an endogenous gas signaling molecule, plays an important role in plant growth regulation and resistance to abiotic stress. This study aims to investigate the mechanism of exogenous HS on the growth and development of seedlings under salt stress and to determine the optimal concentration for foliar application. To investigate the regulatory effects of exogenous HS (donor sodium hydrosulfide, NaHS) at concentrations ranging from 0 to 1 mM on reactive oxygen species (ROS), antioxidant system, and osmoregulation in seedlings under 300 mM NaCl stress. The growth of seedlings was inhibited by salt stress, which resulted in a decrease in the leaf relative water content (LRWC), specific leaf area (SLA), and soluble sugar content in leaves, elevated activity levels of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT); and accumulated superoxide anion (O), proline, malondialdehyde (MDA), and soluble protein content in leaves; and increased L-cysteine desulfhydrase (LCD) activity and endogenous HS content. This indicated that a high level of ROS was produced in the leaves of  seedlings and seriously affected the growth and development of seedlings. The exogenous application of different concentrations of NaHS reduced the content of O , proline and MDA, increased the activity of antioxidant enzymes and the content of osmoregulators (soluble sugars and soluble proteins), while the LCD enzyme activity and the content of endogenous HS were further increased with the continuous application of exogenous HS. The inhibitory effects of salt stress on the growth rate of plant height and ground diameter, the LRWC, biomass, and SLA were effectively alleviated. A comprehensive analysis showed that the LRWC, POD, and proline could be used as the main indicators to evaluate the alleviating effect of exogenous HS on seedlings under salt stress. The optimal concentration of exogenous HS for seedlings under salt stress was 0.025 mM. This study provides an important theoretical foundation for understanding the salt tolerance mechanism of and for cultivating high-quality germplasm resources.