Evaluation of Salinity Tolerance Potentials of Two Contrasting Soybean Genotypes Based on Physiological and Biochemical Responses

Salinity stress is a major abiotic constraint limiting soybean (Glycine max L.) productivity in saline–alkali soils; however, the physiological and biochemical mechanisms underlying genotypic tolerance remain poorly understood. This study aimed to identify key traits that underpin salinity tolerance and can inform breeding and agronomic strategies to enhance soybean performance under saline conditions. Two contrasting soybean genotypes, YAKARTA and POCA, were exposed to 25, 50, 75, and 100 mM NaCl from the first to the fourth trifoliate stage (V1–V4) under controlled conditions for 30 days. YAKARTA maintained higher relative water content (75.51% vs. 66.97%), stomatal conductance (342 vs. 286 mmol H₂O m⁻² s⁻¹), proline (6.15 vs. 4.36 µmol g⁻¹ fresh weight), K⁺/Na⁺ ratio (61.8 vs. 32.2), and H₂O₂ (833.8 vs. 720.2 µmol g⁻¹ fresh weight) compared with POCA, whereas POCA exhibited elevated solute leakage (87.1% vs. 79.21%), malondialdehyde (122 vs. 112 µg g⁻¹), and ascorbic acid (334 vs. 293 µg g⁻¹), indicating greater sensitivity. At 100 mM NaCl, relative water content, stomatal conductance, K⁺/Na⁺ ratio, and H₂O₂ declined by 44.5%, 81.9%, 99.8%, and 49.5%, respectively, while proline, solute leakage, malondialdehyde, and ascorbic acid increased by 56-, 1.27-, 11.6-, and 1.68-fold, respectively. The contrasting physiological and biochemical responses between these genotypes highlight key traits, such as relative water content, stomatal conductance, proline accumulation, malondialdehyde content, and the K⁺/Na⁺ ratio, as promising potential markers associated with salinity tolerance in soybean. These findings provide a foundational understanding that can guide future research to validate these markers across a wider genetic pool and under field conditions.