Biochar effects on drought tolerance in maize roots are linked to K+ concentration, Ca2+ efflux, and apoplastic pH

Biochar amendment is a management strategy to alleviate drought stress in plants. However, in-depth assessments are needed to elucidate how biochar amendment affects root growth by modulating various physiological and biochemical changes under drought stress. In this study, we investigated ion fluxes, metabolic levels, and physiological traits of maize roots in biochar-amended soil under drought stress using noninvasive micro-test technology, metabolomics profiling, and ratiometric fluorescence. The results revealed that the biochar treatment increased soil K+ supply and root sap K+ concentration, but decreased root Ca2+ efflux under moderate drought stress, compared to the no biochar treatment. Root apoplastic pH and abscisic acid content increased significantly in the biochar treatment under severe drought stress. Consequently, root osmolality and root malonaldehyde content decreased, whereas root water potential, root ascorbate peroxidase activity, and plant fresh weight increased in the biochar treatment under severe drought stress. In addition, the biochar treatment enhanced the accumulation of trehalose and 3-hydroxyanthranilic acid in response to moderate and severe drought stress while reducing the levels of uridine, cytidine, guanosine, L-tryptophan, and L-glutamine in maize roots. These results indicate that the biochar-mediated improvements in plant drought tolerance were associated with increased K+ concentration, less Ca2+ efflux, and an increase in apoplastic pH in maize roots.