Sustainable alleviation of cadmium toxicity in soybean through biochar and silicon oxide nanoparticles: insights into photosynthesis and physiological responses

Cadmium (Cd) toxicity in soil presents a major threat to global agricultural sustainability by adversely affecting plant growth and productivity. This study aimed to evaluate the synergistic effects of silicon dioxide nanoparticles (SiO₂NPs) and biochar nanoparticles (BC NPs) in mitigating Cd-induced stress in<i>Glycine max</i>(soybean). Soybean seeds were surface-sterilized and germinated in Petri plats, and then transferred to pots (five replicates per treatment). At 15 d after transplanting (DAT), Cd stress (150<i>µ</i>M CdCl₂) was applied via soil, followed by foliar applications of BC NPs (50 ppm) and SiO₂NPs (100 ppm) from 25 to 30 DAT, while control plants received distilled water. Cadmium stress significantly reduced growth parameters, photosynthetic efficiency, protein content, osmolyte accumulation, and uptake of essential nutrient, while increasing oxidative stress biomarkers such as hydrogen peroxide (H₂O₂) and malondialdehyde. In contrast, the application of BC NPs and SiO₂NPs, both individually and in combination, markedly improved plant growth, photosynthetic performance, protein content, soluble sugar, starch, sucrose, proline levels, and antioxidant enzyme activities. Moreover, NPs treatments enhanced the uptake of essential nutrients including iron (Fe), manganese (Mn), phosphorus (P), potassium (K), nitrogen (N) while reducing Cd accumulation in the leaves. Notably, the combined application of BC NPs and SiO₂NPs exhibited the most pronounced protective effects across all measured physiological, biochemical, and nutritional parameters.