Synergistic interplay between ABA-generating bacteria and biochar in the reduction of heavy metal accumulation in radish, pakchoi, and tomato

Heavy metal (HM) contamination is an environmental concern that threatens the agricultural product safety and human health. To address this concern, we developed a novel strategy involving the synergistic application of Azospirillum brasilense, a growth-promoting rhizobacterium which produces abscisic acid (ABA), and biochar to minimize HM accumulation in the edible parts of vegetable crops. Compared to A. brasilense or biochar alone, the concentrations of Cd, Ni, Pb, and Zn in radish (Raphanus sativus L.), pakchoi (Brassica chinensis L.), and tomato (Lycopersicon esculentum L.) decreased by 18–63% and 14–56%, respectively. Additionally, the synergistic treatment led to a 14–63% decrease in the bioconcentration factor. The biomass of the edible parts of the three crops increased by 65–278% after synergistic treatment, surpassing the effects of single treatments. Furthermore, the synergistic application enhanced the SPAD values by 1–45% compared to single treatments. The MDA concentrations in stressed plants decreased by 16–39% with the bacteria-biochar co-treatment compared to single treatments. Co-treatment also resulted in increased soluble protein and sugar concentrations by 8–174%, and improvements in flavonoids, total phenols, ascorbic acid, and DPPH levels by 2–50%. Pearson correlation analysis and structural equation modeling revealed that the synergistic effect was attributed to the enhanced growth of A. brasilense facilitated by biochar and the improved availability of HMs in soils. Notably, although ABA concentrations were not as high as those achieved with A. brasilense alone, they were maintained at relatively high levels. Overall, the synergistic application of A. brasilense-biochar might have remarkable potential for reducing the accumulation of HMs while promoting growth and improving nutritional and antioxidant qualities in tuberous, leafy, and fruit crops.