Calcium Enhances the Effectiveness of Melatonin in Improving Nutritional Properties of Soybean Sprouts and Germination Under Salt and Cadmium Stress

Salinity and cadmium exposure to agrarian land lowers crop yield and imposes toxicity in the food chain, ultimately affecting sustainable agriculture. Melatonin (Mel) and calcium (Ca) have been reported as potent regulators of plant growth and stress resistance. Based on this scenario, this study investigated the sole and combined effects of Mel and Ca on improving the antioxidant properties, mineral content, germination of sprout, and stress tolerance of soybean seedlings under salt and cadmium (Cd) stress. Optimal doses of 20 µM Mel and 1 mM Ca were identified to enhance sprout quality and seed germination. Treatments with Mel > 20 µM inhibited germination, while the combination of Mel (20 µM) and Ca (1 mM) significantly improved germination, mineral content (Ca, P, K), and antioxidant properties, including DPPH(2,2-Diphenyl-1-picrylhydrazyl) activity, polyphenols, flavonoids, and superoxide dismutase (SOD) activity. However, melatonin > 50 µM could completely cease the sprouting, whereas a Ca concentration of up to 10 mM was observed to be normal in sprouting. Additionally, this combination reduced malondialdehyde (MDA) levels and enhanced the proline, indicating decreased oxidative stress in soybean seedlings under stress conditions. Among various treatments tested, the Mel-Ca combination was most effective in enhancing sprout biomass, antioxidant activity, and seed viability under Salt+Cd stress. These findings underscore the synergistic role of Ca in optimizing melatonin pretreatment for stress mitigation in soybean seeds and also address the precaution for a possible negative impact of melatonin effects.