Dual inoculation with Trichoderma viride and Ensifer meliloti improves antioxidant responses and growth of Medicago sativa under zinc and lead exposure

Heavy metal contamination is a major constraint on agricultural productivity, disrupting plant physiological processes, nutrient cycling, and symbiotic interactions. Zinc (Zn) and lead (Pb) are among the most widespread soil pollutants, yet their contrasting effects on plant–microbe systems and the potential of microbial mitigation remain insufficiently understood. Medicago sativa L. plants were exposed to Zn and Pb and inoculated with Trichoderma viride, Ensifer meliloti, or dual inoculation with both microorganisms. Oxidative stress markers, antioxidant enzyme activities, plant growth, nodulation, metal accumulation, and selected soil nutrient parameters were analysed. Both metals induced pronounced oxidative stress, reflected by increased malondialdehyde (MDA) accumulation and altered antioxidant enzyme activities. MDA reached 416 and 217 nmol g⁻ 1 FW in leaves and roots under Zn exposure, and 242 and 173 nmol g⁻ 1 FW under Pb exposure, respectively. Pb caused stronger and more persistent root-associated stress, whereas Zn predominantly affected shoots during active vegetative growth. Microbial inoculation reduced MDA levels by 55–80% and increased antioxidant enzyme activities by 2–fourfold. Dual inoculation with T. viride and E. meliloti consistently produced the strongest response. Plant fresh weight and nodulation were significantly enhanced under both metals. Zn was partially translocated to shoots, whereas Pb accumulated mainly in roots. Inoculation was also associated with changes in post-harvest soil soluble ion composition, including nitrogen- and phosphorus-related parameters. Dual inoculation with T. viride and E. meliloti effectively alleviated Zn- and Pb-induced stress, enhanced antioxidant responses, improved plant performance, and modified post-harvest soil soluble ion composition, highlighting its potential as a microbial strategy for improving plant resilience under metal stress conditions.