Exogenous proline promotes soybean seedling growth under salt stress by regulating antioxidant enzyme activities and Na + homeostasis

Soybean (Glycine max), an oilseed crop with moderate salt tolerance, is widely planted in salinized soils. However, soybean seedling growth is severely impaired by salt stress. Proline (Pro) functions as an osmoregulant, enhancing crop tolerance to abiotic stress. Yet, the concentration-dependent effects of exogenous Pro on mitigating salinity damage in soybean seedlings remain insufficiently understood. This study examined the protective effects of exogenous Pro on soybean seedlings and its growth-regulatory mechanisms under salt stress (150 mM NaCl), with Pro applied at 0, 0.1, 0.5, 2.0, 10, and 50 mM. Salt stress significantly inhibited seedling growth, whereas Pro application restored biomass accumulation and root development. Proline treatment also enhanced endogenous Pro content in roots and leaves, reduced H2O2 accumulation, and improved photosynthetic performance (increases in net photosynthetic rate, stomatal conductance, and transpiration rate and a decrease in intercellular CO2 concentration). Additionally, Pro application enhanced antioxidant enzyme activities and maintained ion homeostasis by promoting K+ and Ca2+ uptake while reducing Na+ accumulation, thereby alleviating Na+ toxicity. Among treatments, SP3 (150 mM NaCl + 2.0 mM Pro) exhibited the most pronounced protective effects. This study provides new insights into Pro-mediated salt tolerance mechanisms and offers practical strategies for improving soybean productivity in salinized ecosystems.