Effect of exogenous melatonin on seed germination and physiological characteristics of soybean seedlings under salt stress

Soil salinity represents a major global agricultural constraint that significantly limits soybean productivity. While melatonin (MT) has demonstrated potential as a priming agent to enhance abiotic stress tolerance, the optimal application concentration and specific physiological mechanisms underlying its alleviation of salt stress remain poorly defined. Consequently, this study investigated the effects of exogenous MT (0, 50, 100, 250, 500 µmol/L) on seed germination and seedling physiology in two soybean varieties subjected to 100 mmol/L NaCl stress. The 100 µmol/L MT treatment proved to be the most effective, significantly alleviating salt-induced inhibition of seed germination and seedling growth. It enhanced the activity of key antioxidant enzymes, including peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT), and elevated osmolyte content, while reducing oxidative stress markers (H₂O₂ and malondialdehyde, MDA). Crucially, hormonal profiling revealed that the mitigation of salt stress was associated with a fundamental shift in hormonal homeostasis. The optimal MT treatment orchestrated a pronounced decrease in the stress hormone abscisic acid (ABA) alongside a concurrent rise in key growth-promoting hormones (e.g., IAA, GA, JA), thereby redirecting the plant’s metabolism from stress containment back toward growth. Conversely, a high MT concentration (500µmol/L) exacerbated stress damage. Multivariate analyses confirmed positive correlations between germination/growth and antioxidant capacity/growth-promoting hormones, and negative correlations with oxidative stress/ABA. The variety DS-7 exhibited stronger salt tolerance than DS-4, which is attributed to its inherently higher capacity for antioxidant activity and osmotic adjustment. In conclusion, 100 µmol/L exogenous melatonin significantly enhances soybean salt tolerance by boosting antioxidant defense, regulating osmotic balance, and optimizing hormonal homeostasis. The superior inherent traits of the DS-7 variety further contribute to its resilience. These findings provide a foundation for utilizing melatonin to improve soybean establishment in saline soils.