Foliar Magnesium Application Enhances Physiological Status and Grain Yield of Glycine max L. Through Modulation of Photosynthesis and Enzymatic Activities

Magnesium is an essential nutrient for crops such as Glycine max L., acting as a key regulator of photosynthetic processes. Mg deficiency compromises physiological performance and reduces the productivity of cultivated plants. This study evaluated the effects of foliar application of Mg, as magnesium sulfate (MgSO4), at 500, 700, and 900 g ha− 1 doses applied at the R2 (complete flowering) and R4 (pod formation) growth stages. Results: an increase in chlorophyll a (1011 µg mg− 1) and b (201 µg mg− 1) levels at the 500 g ha− 1 dose. The tannin content was higher at the 900 g ha− 1 (242 µg mg− 1) dosage over the course of 120 h. Chlorophyll-a fluorescence indicated greater photochemical efficiency and electron transport in photosystem II at the 700 g ha− 1 dose, contrasting with signs of energy overload at the 900 g ha− 1 dose. Increased activity of the antioxidant enzymes superoxide dismutase (SOD) and guaiacol peroxidase (GPOX) was associated with reduced levels of hydrogen peroxide (H2O2) and malondialdehyde (MDA), indicating lower lipid peroxidation. Conclusion: Magnesium supplementation resulted in higher grain yield compared to the control. Overall, foliar magnesium supplementation enhances soybean productivity by improving photosynthetic performance, antioxidant responses, and pigment accumulation. Magnesium application at the R4 (pod formation) stage increased carotenoid content in Glycine max. The 900 g ha− 1 dose increased electron dissipation (DI0/RC) after 72 h. The 500 g ha− 1 dose increased maximum fluorescence (Fm) and chlorophyll a content. Magnesium application increased ascorbate peroxidase (APX) activity. Magnesium application increased soybean grain yield.