Eco-friendly zinc nanoparticles and humic acid combination: An integrative application to mitigate cadmium pollution in common bean (Phaseolus vulgaris)

Cadmium (Cd) pollution is a significant environmental threat, that negatively impacts plant growth and human health. This study investigates the combined effect of humic acid (HA) and biologically synthesized zinc nanoparticles (BZnNPs) in mitigating Cd toxicity in Phaseolus vulgaris grown in artificially contaminated soil. BZnNPs were produced using Bacillus subtilis AA20, resulting in spherical nanoparticles (35 nm) with a -26.9 mV charge and 88 % antioxidant activity. Plants were exposed to three Cd concentrations (0, 1.0, and 1.5 mM), and treated with either HA (0.5 g kg⁻¹ soil), BZnNPs (1.0 mM foliar spray), or their combination. At 1.5 mM Cd, plant height, dry biomass, and photosynthetic efficiency declined by 47.1 %, 59.9 %, and 34.9 %, respectively, while malondialdehyde (MDA), electrolyte leakage (EL) increased by 212 %, and 62.2 %, compared to untreated controls. However, the combined HA + BZnNPs treatment significantly reduced Cd mobility in soil and its accumulation in roots, leaves, and pods by 75.4 %, 63.6 %, and 78.3 %, respectively. This treatment also enhanced antioxidant enzyme activity (catalase, peroxidase, and superoxide dismutase) by 19.3 %, 38.2 %, and 22.2 %, respectively, while boosting total soluble sugars and proline by 23.6 %, and 10.8 %. Moreover, HA + BZnNPs treatment decreased MDA and EL levels by 11.1 and 11.8 %, respectively, leading to better oxidative stress management. It also improved chlorophyll content (SPAD) by 22.7 %, relative water content by 20.3 %, and pod yield by 40.1 %, ultimately restoring yield by 40 % under severe Cd stress. These findings highlight HA and BZnNPs as a promising, eco-friendly strategy to reduce Cd bioavailability, enhance stress resilience, and improve growth and productivity in common bean, supporting sustainable agriculture and food safety.