Molybdenum enhances soybean growth and seed quality by modulating osmoregulation, ion homeostasis, and antioxidant activity under saline–alkaline conditions

Soil salinization is a major global challenge that limits crop productivity by inhibiting plant growth, nutritional quality and decreasing yields. This study examined the effect of foliar application of molybdenum (Mo) on two soybean varieties, HN517 (salt-sensitive) and DS118 (salt-tolerant) under saline-alkali soil conditions. Mo was applied via foliar spraying at four concentrations 0 (control), 70, 140, and 210 g ha−1 referred as CK, MO1, MO2, and MO3, respectively. These treatments were employed to investigate the influence of Mo on soybean physiological growth parameters, antioxidant defense mechanisms, osmotic regulation, ionic homeostasis, and nutrient accumulation. Results showed that foliar application of Mo treatment MO3 significantly improved plant height, root biomass, and total biomass by 39.24%, 43.67%, and 56.01% in HN517, and by 36.95%, 28.98%, and 108.0% in DS118 variety of soybean. The activities of antioxidant enzymes including superoxide dismutase, catalase, peroxidase, and nitrate reductase, increased by 17.75%, 20.90%, 21.42%, and 13.23% in HN517, and by 24.63%, 27.01%, 18.75%, and 13.44% in DS118, respectively, compared with control. Simultaneously, the levels of hydrogen peroxide, superoxide anion, and malondialdehyde decreased by 26.45% and 35.02%, 23.11% and 22.93%, and 33.30% and 38.38% in HN517 and DS118, respectively, indicating a reduction in oxidative damage. Mo application also enhanced osmotic regulation by increasing proline, soluble protein, and soluble sugar contents by 21.47%, 18.75%, and 16.74% in HN517, and by 19.12%, 23.34%, and 19.69% in DS118. Furthermore, Mo enhanced nutrient uptake, including nitrogen, phosphorus, potassium, and Mo in both varieties. It also improved the K⁺/Na⁺ ratio, mitigating ion toxicity and enhancing stress tolerance. This improvement contributed to higher nutrient acquisition and antioxidant capacity contributed to higher protein and oil content, leading to increased crop yield under saline-alkali field conditions. These findings concluded that Mo application to the foliage improves soybean resilience, growth, yield, and nutritional quality of soybean under saline alkali field conditions.