Differential regulation of photosynthesis by various stress treatments in field grown soybean (Glycine max) plants

Soybean (Glycine max L.) is a vital crop valued for its protein and oil contents. Leaf physiological performance is highly affected by environmental stresses such as drought, nitrogen deficiency, and defoliation. This study aimed to investigate the impact of drought stress, nitrogen deficiency and defoliation on leaf photosynthetic efficiency and related metabolic components. Gas exchange parameters including photosynthetic rate (P), stomatal conductance (gs), transpiration rate (E), and internal CO₂ concentration (Ci) were measured alongside the contents of Rubisco protein, total soluble sugars, and starch in leaves. The data revealed that drought stress led to the greatest reduction in photosynthetic rate, possibly due non-stomatal limitations, mainly a significant decrease in Rubisco content. Nitrogen deficiency reduced photosynthesis but increased the transpiration rate and altered carbon metabolism, with a notable decrease in soluble sugar and starch contents. In contrast, defoliation caused a moderate decrease in photosynthetic rate with a minor reduction in Rubisco content but caused an accumulation of soluble sugars in leaves. Based on these results, it can be concluded that the reduction in photosynthetic efficiency in soybean leaves under these conditions was primarily due to non-stomatal and biochemical limitations rather than to stomatal limitation.