Foliar Spraying of NaHS Alleviates Cucumber Salt Stress by Maintaining N+/K+ Balance and Activating Salt Tolerance Signaling Pathways

Abstract

Hydrogen sulfide (H2S) is involved in the regulation of plant salt stress as a potential signaling molecule. This work investigated the effect of H2S on cucumber growth, photosynthesis, antioxidation, ion balance, and other salt tolerance pathways. The plant height, stem diameter, leaf area and photosynthesis of cucumber seedlings were significantly inhibited by 50 mmol·L−1 NaCl. Moreover, NaCl treatment induced superoxide anion (O2·−) and Na+ accumulation and affected the absorption of other mineral ions. On the contrary, exogenous spraying of 200 μmol·L−1 sodium hydrosulfide (NaHS) maintained the growth of cucumber seedlings, increased photosynthesis, enhanced the ascorbate–glutathione cycle (AsA–GSH), and promoted the absorption of mineral ions under salt stress. Meanwhile, NaHS upregulated SOS1, SOS2, SOS3, NHX1, and AKT1 genes to maintain Na+/K+ balance and increased the relative expression of MAPK3, MAPK4, MAPK6, and MAPK9 genes to enhance salt tolerance. These positive effects of H2S could be reversed by 150 mmol·L−1 propargylglycine (PAG, a specific inhibitor of H2S biosynthesis). These results indicated that H2S could mitigate salt damage in cucumber, mainly by improving photosynthesis, enhancing the AsA–GSH cycle, reducing the Na+/K+ ratio, and inducing the SOS pathway and MAPK pathway.