Protective effects of exogenous L-proline on germination and early seedling development of soybean under osmotic stress

Abstract

Abstract

Salinity is a major abiotic stress limiting soybean germination and seedling establishment by impairing water uptake, metabolism, and early growth. Exogenous application of L-proline has emerged as a potential strategy to improve plant tolerance to saline conditions. Soybean seeds (GH 5933 IPRO) were treated with four concentrations of L-proline (0, 0.04, 0.08, and 0.12 g kg⁻¹ seed) and subjected to four osmotic potentials (0.0, −0.3, −0.6, and −0.9 MPa) induced by NaCl. Germination, seed vigor, seedling growth, and dry matter accumulation were evaluated in a completely randomized factorial design. Salinity significantly reduced seed vigor from approximately 92% under non-saline conditions to about 45% at −0.9 MPa, while the percentage of normal seedlings declined from approximately 95% to 47%. Application of 0.12 g kg⁻¹ L-proline increased seed vigor under severe stress to approximately 74% and improved the percentage of normal seedlings to about 75%, compared with untreated seeds. The proportion of hard and dead seeds under severe salinity decreased from approximately 50% in untreated seeds to about 17% following L-proline treatment. Root length increased from 3.6 to 4.5 cm, whereas shoot length increased from 1.9 to 2.4 cm with increasing L-proline concentration. Dry matter accumulation also increased, reaching approximately 0.31 g seedling⁻¹ at the highest L-proline dose under non-saline conditions, while biomass losses under saline stress were partially alleviated. These findings demonstrate that exogenous L-proline enhances osmotic adjustment, improves germination and seedling vigor, stimulates early growth, and reduces seed mortality under salt stress, highlighting its potential as a sustainable strategy for improving soybean establishment in saline environments.