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