Combined effects of ZnO NPs and Cd on sweet sorghum as influenced by an arbuscular mycorrhizal fungus.

Both metals and metal-based nanoparticles (NPs) can easily accumulate in soil, posing risks for plants and microbes. However, the interaction between NPs and toxic metals coexisting in soil is not yet well understood. Here, we studied the combined effects of ZnO NPs and Cd on sweet sorghum inoculated with or without the arbuscular mycorrhizal (AM) fungus Funneliformis caledonium. Plants were grown in soil amended with ZnO NPs (50, 250, and 500 mg/kg), alone or in combination with 5 mg/kg Cd. The two higher doses of ZnO NPs inhibited plant growth, leading to synergistic toxicity with Cd. However, at the lowest dose, ZnO NPs were non-phytotoxic, displaying antagonistic interactions with Cd on plant growth. When added with high doses of ZnO NPs, Cd significantly increased root Zn concentrations, but decreased shoot Zn concentrations at the low dose. Conversely, all doses of ZnO NPs significantly decreased shoot and root Cd concentrations. Furthermore, high doses of ZnO NPs generally inhibited soil enzyme activities, but Cd addition showed no significant or even stimulative effects, and mitigated the inhibitory effects of ZnO NPs. AM inoculation increased plant growth and P nutrition, and soil enzyme activities. When the low dose of ZnO NPs was added alone or in combination with Cd, AM inoculation decreased shoot Zn concentrations. Our results reveal complex interactions between ZnO NPs and Cd on plant growth and nutrition, plant Zn and Cd accumulation, and soil enzyme activities, while AM inoculation can help diminish the adverse effects induced by ZnO NPs and Cd.