Metabolic adjustment via microbial agents and biochar synergy enhances mercury stress tolerance in Vigna radiata L.

Mercury (Hg) as an abiotic stressor poses significant challenges to plant growth. This study investigates the response of Vigna radiata L plant to three levels of Hg stress (0, 20, and 40 mg/L) using a hydroponic system, inoculated with microbial biostimulators. The two-factorial experiment focused on plant growth, total soluble sugars (TSS), and free amino acids. Results indicated a decrease in plant biomass with increasing stress severity. Under moderate stress (20 mg/L Hg), arbuscular mycorrhiza (AM) was effective in preserving biomass, while under severe stress (40 mg/L Hg), a combination of biochar, AM, and bacterial biostimulants (BAB) was most effective. The highest biomass was recorded by combined treatment of bacteria and AM (BaAM) although it dropped sharply under stress conditions. Except for BaAM, TSS content increased in all moderate stress treatments, particularly with AM, and was notably enhanced by BAB under severe stress. V. radiata plants generally showed an increase in Proline, Phenylalanine, Tyrosine, Tryptophan, Asparagine, Glycine, and Valin levels when exposed to Hg stress, whereas Aspartate decreased across all stress treatments. Higher Hg tolerance was linked to greater accumulation of TSS and specific free amino acids like Proline, Phenylalanine, and Asparagine. The interaction with microbial inoculation suggests that V. radiata plants adapt to Hg toxicity by altering their sugar and amino acid profiles, which serve as multifunctional molecules and precursors for stress resistance metabolites.