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

• Heavy metals adversely affect the yield of economic crops i.e. common beans. • Biological zinc nanoparticles (BZnNPs) and humic acid (HA) application significantly reduced the adverse effects of cadmium (Cd) stress in common bean plants. • The combined treatment of BZnNPs and HA improved various yield parameters in common beans compared to Cd-stressed plants. • The combined treatment stimulated the activity of antioxidant enzymes, helping the plants better cope with oxidative stress induced by Cd. • The combined application demonstrated a synergistic effect in mitigating Cd stress and improving plant performance compared to individual treatments. • Sustainable approaches (BZnNPs + HA) improve crop yield and alleviate heavy metal stress in agricultural soils. Cadmium (Cd +2 ) pollution is a significant environmental threat, negatively impacting plant growth and human health. This study investigates the potential of combining humic acid (HA) and biologically synthesized zinc nanoparticles (BZnNPs) to alleviate Cd² toxicity in Phaseolus vulgaris grown in artificially contaminated soil with Cd +2 . The BZnNPs were produced using Bacillus subtilis AA20, yielding spherical particles (35 nm) with a -26.9 mV charge and 88% antioxidant activity. The present study emphasized the beneficial function and effective role of HA and/or BZnNPs in mitigating abiotic stress related to P. vulgaris Cd² tolerance. Plants were subjected to three artificially contaminated Cd +2 levels (0, 1, and 1.5 mM), and four treatments were used, i.e., control (without HA and without BZnNPs treatment), HA (addition of 0.5 g kg −1 soil), BZnNPs (foliar spraying with 1.0 mM), and HA + BZnNPs. Cd +2 at 1.5 mM reduced plant height, plant dry matter weight, and photosynthetic efficiency by 47.1%, 59.9%, and 34.9%, respectively, while increasing malondialdehyde (MDA) by 212%, and electrolyte leakage (EL) by 62.2%, compared to control (0 mM Cd +2 ). The combined application of HA + BZnNPs significantly reduced Cd +2 mobility in soil and its accumulation in plant tissues, minimizing Cd +2 translocation and toxicity. This treatment at 1.5 mM Cd +2 enhanced antioxidant defense by 19.3%, 38.2%, and 22.2% for catalase, peroxidase, and superoxide dismutase, respectively, compared to control plants at 1.5 mM Cd +2 without HA and without BZnNPs treatment. The combined application of HA + BZnNPs at 1.5 mM Cd +2 also significantly enhanced the levels of total soluble sugars and free proline by 23.6%, and 10.8%, respectively compared to control plants at 1.5 mM Cd +2 without HA and without BZnNPs treatment. Additionally, HA + BZnNPs at 1.5 mM Cd +2 decreased electrolyte leakage (EL) and malondialdehyde (MDA) levels by 11.8%, and 11.1% compared to control plants at 1.5 mM Cd +2 without HA and without BZnNPs treatment by detoxifying reactive oxygen species, resulting in improved growth, higher pod yield, and better leaf health. The same application at 1.5 mM Cd +2 increased pod yield by 40.1%, chlorophyll relative content (SPAD) values by 22.7%, and relative water content by 20.3%, compared to control plants at 1.5 mM Cd +2 without HA and without BZnNPs treatment. The application of HA + BZnNPs at 1.5 mM Cd +2 reduced Cd +2 levels by 75.4%, 63.6%, and 78.3% in roots, leaves, and pods compared to control plants, while recovering the yield by 40 % at 1.5 mM Cd +2 without HA and without BZnNPs treatment. These findings demonstrated that HA and BZnNPs offer an eco-friendly strategy to mitigate Cd +2 toxicity in crops like common beans, promoting sustainable agriculture and food safety by reducing Cd +2 bioavailability, enhancing crop resilience, and improving productivity under metal stress.