Exogenous Melatonin Induces Salt Stress Tolerance in Cucumber by Promoting Plant Growth and Defense System

This study aims to investigate the regulatory effect of exogenous melatonin (MT) on the growth and development of cucumbers subjected to salt stress. Using the XinTaiMiCi material and indoor pot culture method, seven treatments were set up: control group (CK), T0 (salt treatment group, 150 mM S + 0 μM MT), T1 (150 mM S + 25 μM MT), T2 (150 mM S + 50 μM MT), T3 (150 mM S + 100 μM MT), T4 (150 mM S + 150 μM MT), and T5 (150 mM S + 200 μM MT). Changes in plant height, stem diameter, leaf area, relative chlorophyll content, antioxidant enzyme activity, reactive oxygen species content, and osmotic adjustment substance content in cucumber seeds and seedlings under different treatments were studied, and a correlation analysis of these indicators was conducted. Meanwhile, the expression of salt stress-related genes was detected in all seven treatment groups. The results showed that, compared to the CK, T0 significantly reduced the hypocotyl length, root length, hypocotyl diameter, root diameter, and fresh and dry weights of cucumber seeds; in the later stage of salt stress treatment, T0 significantly increased the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and malondialdehyde (MDA) and the content of soluble protein in seeds. Additionally, T0 significantly increased the plant height, root length, stem diameter, leaf area, and fresh and dry weights of cucumber seedlings per plant; in the later stage of salt stress treatment, T0 significantly increased the activities of SOD, POD, CAT, and MDA and the content of soluble protein and chlorophyll in leaves. Compared to T0, the application of 50 μmol·L^-1 MT under salt stress significantly increased the plant height, stem diameter, root length, leaf area, and fresh and dry weights of cucumber seedlings per plant; significantly increased the activities of SOD, POD, and CAT; decreased the MDA activity; and significantly increased the content of soluble protein and chlorophyll. Under salt stress conditions, the exogenous application of low-concentration melatonin increased the expression levels of salt stress response genes (such as CsSOS, CsNHX, CsHSF, and CsDREB) in cucumber. The germination rate (GR), germination potential (GP), germination index (GI), plant height (PH), root length (RL), leaf area index (LAI), fresh weight (FW), dry weight (DW), soluble protein (SP), relative chlorophyll content (SPAD), POD, CAT, and SOD of cucumber seedlings exhibited significant positive correlations, whereas they were negatively correlated with MDA content. In conclusion, the application of 50 μM MT can effectively alleviate the oxidative and osmotic stress caused by a high-salt environment in cucumber, promote cucumber growth, and improve salt tolerance.