Biochar: A promising soil amendment to mitigate heavy metals toxicity in plants

Heavy metals (HMs) toxicity is serious abiotic stress that is significantly reducing crop productivity and posing a serious threat to human health, soil and environmental quality. Therefore, it is urgently needed to find appropriate measures to mitigate the adverse impacts of HMs on soil, plants, humans and the environment. Biochar (BC) has emerged as an excellent soil amendment to minimize the adverse impacts of HMs and to improve soil fertility and environmental quality. Biochar application decreases HMs uptake and their translocation to plant parts by forming complexes and precipitation. Biochar also has improved soil pH, soil fertility and soil cation exchange capacity (CEC) and it also increases adsorption of HMs thus reduces their mobility and subsequent availability to plants. BC application also maintains membrane stability and improves uptake of nutrients, osmolytes accumulation, antioxidant activities, and gene expression, therefore, improves the plant performance under HMs stress. Biochar application also improves the photosynthetic performance by increasing the synthesis of photosynthetic pigments, stomata conductance and increasing the water uptake by plants. Besides this, BC also scavenges ROS by increasing the antioxidant activities, gene expression, and accumulation of proline in HMs contaminated soils. This review highlights the role of BC to mitigate the HMs toxicity in plants. We have discussed the role of BC in the modification of soil properties to induce tolerance against HMs toxicity. Moreover, we have discussed various mechanisms mediated by BC at the plant level to induce tolerance against HMs. Additionally, we also identified research gaps that must be fulfilled in future research studies.