Biochar potentially enhances maize tolerance to arsenic toxicity by improving physiological and biochemical responses to excessive arsenate

Metalloid pollution, including arsenic poisoning, is a serious environmental issue, plaguing plant productivity and quality of life worldwide. Biochar, a carbon-rich material, has been known to alleviate the negative effects of environmental pollutants on plants. However, the specific role of biochar in mitigating arsenic stress in maize remains relatively unexplored. Here, we elucidated the functions of biochar in improving maize growth under the elevated level of sodium arsenate (Na₂AsO₄, As^V). Maize plants were grown in pot-soils amended with two doses of biochar (2.5% (B1) and 5.0% (B2) biochar Kg⁻¹ of soil) for 5 days, followed by exposure to Na₂AsO₄ ('B1 + As^V'and 'B2 + As^V') for 9 days. Maize plants exposed to As^V only accumulated substantial amount of arsenic in both roots and leaves, triggering severe phytotoxic effects, including stunted growth, leaf-yellowing, chlorosis, reduced photosynthesis, and nutritional imbalance, when compared with control plants. Contrariwise, biochar addition improved the phenotype and growth of As^V-stressed maize plants by reducing root-to-leaf As^V translocation (by 46.56 and 57.46% in ‘B1 + As^V’ and ‘B2 + As^V’ plants), improving gas-exchange attributes, and elevating chlorophylls and mineral levels beyond As^V-stressed plants. Biochar pretreatment also substantially counteracted As^V-induced oxidative stress by lowering reactive oxygen species accumulation, lipoxygenase activity, malondialdehyde level, and electrolyte leakage. Less oxidative stress in ‘B1 + As^V’ and ‘B2 + As^V’ plants likely supported by a strong antioxidant system powered by biochar-mediated increased activities of superoxide dismutase (by 25.12 and 46.55%), catalase (51.78 and 82.82%), and glutathione S-transferase (61.48 and 153.83%), and improved flavonoid levels (41.48 and 75.37%, respectively). Furthermore, increased levels of soluble sugars and free amino acids also correlated with improved leaf relative water content, suggesting a better osmotic acclimatization mechanism in biochar-pretreated As^V-exposed plants. Overall, our findings provided mechanistic insight into how biochar facilitates maize’s active recovery from As^V-stress, implying that biochar application may be a viable technique for mitigating negative effects of arsenic in maize, and perhaps, in other important cereal crops. Graphical Abstract: [Figure not available: see fulltext.]