Regulation of Growth and Salt Resistance in Cucumber Seedlings by Hydrogen-Rich Water

The secondary salinization of soil in facility agriculture is becoming increasingly serious. Cucumber is a moderately salt-sensitive crop, but because of the weak root system and poor resistance to salt stress in cucumber seedlings, it is sensitive to the accumulation of salt in facility soils. Hydrogen, a selective antioxidant and signal molecule, is nontoxic and harmless, and can repair damage in plants under stress. Therefore, the aim of the present paper was to understand the specific mechanism by which hydrogen-rich water (HRW) alleviated salt stress in cucumber seedlings ( Cucumis sativus L.). Our results showed that the addition of 50% saturation HRW significantly promoted seedling growth, development and photosynthetic efficiency. Pretreatment with HRW significantly alleviated salt stress symptoms, including the inhibition of fresh and dry weight, root length, lateral roots, and the root/shoot ratio of cucumber seedlings. Pretreatment with HRW increased the chlorophyll content, chlorophyll a/b ratio, and photochemical reaction efficiency, and reduced energy dissipation. These responses to HRW pretreatment were consistent with significant decreases in the superoxide anion, hydrogen peroxide, and malondialdehyde contents and the degree of lipid peroxidation, and increases in the activities of SOD, POD, CAT, APX and GR, and the contents of ASA and GSH in cucumber seedlings under salt stress. In addition, HRW pretreatment under salt stress inhibited the expression of the protein kinase ROP1 , which promoted the production of reactive oxygen, but upregulated the protein kinase LecRLK . The transcription factor TGA5 , which was involved in osmotic stress, ion stress and ROS clearance, and the expression of NHX1 and SOS2 , which were parts of the SOS signaling pathway. HRW enhanced the expression of genes that encoded antioxidant enzymes, including SOD , CAT and POD , and the expressions of GR and APX2 , which were key genes in the ASA-GSH cycle under salt stress. Taken together, these results suggested that HRW enhanced the active oxygen scavenging ability in cucumber seedlings, promoted the redox balance in cells, and reduced the degree of oxidative damage in plants under salt stress by reducing the content of active oxygen. Therefore, the application of HRW might be a promising strategy for improving salt stress tolerance in cucumber seedlings.