Impact of zinc stress on antioxidant enzyme dynamics in soybean

A controlled pot experiment was conducted to evaluate the effects of graded concentrations of zinc sulphate (ZnSO₄·7H₂O) on the antioxidant defense system of soybean (Glycine max L.). Soil was amended with zinc at 0 (control), 250, 500, 750, 1000, and 1250 mg kg⁻№, and enzymatic responses were assessed at pre-flowering (30 days), peak-flowering (45 days), and post-flowering (60 days) stages under triplicate conditions. A significant enhancement in antioxidant enzyme activities was observed with increasing zinc concentration up to 1000 mg kg⁻№. Maximum activities of catalase (4.945, 5.467, 6.189 mM), peroxidase (2.328, 3.282, 3.778 mM), superoxide dismutase (2.65, 3.20, 4.89 U g⁻№), glutathione reductase (2.4568, 4.0234, 5.6712 mM), and ascorbate peroxidase (5.9636, 7.1428, 7.9636 mM) were recorded at pre-, peak-, and post-flowering stages, respectively, at 1000 mg kg⁻№. However, a further increase to 1250 mg kg⁻№ resulted in a decline in enzyme activities, indicating the onset of zinc-induced toxicity. The results highlight that moderate zinc enrichment stimulates the plant antioxidant machinery, enhancing tolerance against oxidative stress. Superoxide dismutase plays a pivotal role in dismutating superoxide radicals into H₂O₂, which is subsequently detoxified by catalase, peroxidase, and ascorbate peroxidase, thereby maintaining cellular redox homeostasis. Overall, the study demonstrates a dose-dependent dual role of zinc, acting as both a micronutrient and a potential toxicant at higher concentrations. These findings emphasize the need for careful management of zinc levels in agricultural soils to prevent toxicity while optimizing soybean productivity.