Comparative efficacy of raw and HNO3-modified biochar derived from rice straw on vanadium transformation and its uptake by rice (Oryza sativa L.): Insights from photosynthesis, antioxidative response, and gene-expression profile

Low concentrations of vanadium (V) are essential for various plant species but it becomes toxic to plants, animals, and humans at high levels. A significant amount of V is currently being emitted into the atmosphere due to intensified industrial processing. Therefore, this study aimed at evaluating the effect of raw (BC) and HNO3modified biochar (OBC) derived from rice straw on growth, photosynthetic assimilation, relative chlorophyll content, SPAD index, ion leakage, enzyme activities, hydrogen peroxide (H2O2), bioavailability and V uptake by rice in a laboratory-scale experiment. Characterization of OBC and BC by FTIR (Fourier transform infrared spectroscopy), SEM (scan electron microscopy), BET (Brunauer-Emmett-Teller), elemental analysis, and z-potential revealed a substantial difference between both of them. The V-stress significantly reduced the rice plant growth, biomass yield, chlorophyll parameters, root length and surface area. Under V-stress conditions, root accumulated more V than shoots and OBC significantly improved the above-mentioned parameters, while, decreasing hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels in plants. The antioxidant function and gene expression levels induced by V-stress and OBC application further increased the expression profile of three genes (SOD, POD, and CAT) encoding antioxidant enzymes and one metal-tolerant conferring gene (OsFSD1). In summary, these results demonstrated the critical role of OBC in mitigating the detrimental effects of high V-stress on rice growth and enhancing plant defence against V-stress.