Comparative phytotoxicity of ZnO NPs, bulk ZnO, and ionic zinc onto the alfalfa plants symbiotically associated with Sinorhizobium meliloti in soil

ZnO nanoparticles (NPs) are reported as potentially phytotoxic in hydroponic and soil media. However, studies on ZnO NPs toxicity in a plant inoculated with bacterium in soil are limited. In this study, ZnO NPs, bulk ZnO, and ZnCl2 were exposed to the symbiotic alfalfa (Medicago sativa L.)–Sinorhizobium meliloti association at concentrations ranging from 0 to 750mg/kg soil. Plant growth, Zn bioaccumulation, dry biomass, leaf area, total protein, and catalase (CAT) activity were measured in 30day-old plants. Results showed 50% germination reduction by bulk ZnO at 500 and 750mg/kg and all ZnCl2 concentrations. ZnO NPs and ionic Zn reduced root and shoot biomass by 80% and 25%, respectively. Conversely, bulk ZnO at 750mg/kg increased shoot and root biomass by 225% and 10%, respectively, compared to control. At 500 and 750mg/kg, ZnCl2 reduced CAT activity in stems and leaves. Total leaf protein significantly decreased as external ZnCl2 concentration increased. STEM-EDX imaging revealed the presence of ZnO particles in the root, stem, leaf, and nodule tissues. ZnO NPs showed less toxicity compared to ZnCl2 and bulk ZnO found to be growth enhancing on measured traits. These findings are significant to reveal the toxicity effects of different Zn species (NPs, bulk, and ionic Zn) into environmentally important plant-bacterial system in soil. [Display omitted] •ZnO NPs, bulk ZnO and ZnCl2 differentially affected alfalfa–S. meliloti symbiosis.•ZnO NPs and ionic Zn reduced root and shoot biomass by 80% and 25%, respectively.•Ionic treatments showed a significant reduction in leaf protein content.•Both ZnO NP and bulk ZnO treatments altered CAT levels in roots, stems, and leaves.