Mitigation of selenium toxicity in soybean by calcium oxide nanoparticles: comparison with bulk calcium and DFT-based...

Selenium (Se) stress poses a serious threat to soil health and sustainable crop production by disrupting plant morphophysiological processes. Although nanotechnology offers an eco-compatible strategy to mitigate metalloid stress, a comprehensive comparison of bulk calcium oxide (CaO) and calcium oxide nanoparticles (CaO-NPs) under Se stress remains unexplored. Hence, this study demonstrates that CaO-NPs outperform bulk CaO in mitigating Se toxicity in soybean (Glycine max L.), enhancing plant resilience in Se-affected soils. We tested CaO-NPs and bulk CaO (25, 50, and 100 mg kg^-1) under Se stress (7.75 ± 1.7 mg kg^-1). Results revealed that plants exposed to Se showed significant decreases in morphophysiological traits, and gas exchange attributes along with increased Se bioaccumulation in plant tissues. In contrast, CaO-NPs application (100 mg kg^-1) significantly improved plant biomass (39%), chlorophyll fluorescence efficiency (58%), and gas exchange attributes. The highest CaO-NPs dose elicited a pronounced Se-tolerance response, characterized by enhanced antioxidant enzyme activities, including superoxide dismutase (SOD (35%)), peroxidase (POD (34%)), catalase (CAT (32%)), and ascorbate peroxidase (APX (52%)), accompanied by reduced oxidative stress. Moreover, CaO-NPs modulated Se and Ca²⁺ accumulation in plant tissues, and qRT-PCR analysis revealed upregulation of key antioxidant defense genes (Cu/ZnSOD, GmPOD, GmCAT, and GmAPX). Supporting these experimental findings, density functional theory calculations confirmed the presence of stable Ca-Se interactions, indicating that Se immobilization as a key mitigation mechanism. Collectively, these results highlight CaO-NPs as a promising nanomaterial-based intervention for reducing Se phytotoxicity and enhancing crop resilience in Se-contaminated agroecosystems, with implications for food safety and sustainable agriculture.