Increased ZnO nanoparticle toxicity to wheat upon co-exposure to phenanthrene

Polycyclic aromatic hydrocarbons and zinc oxide nanoparticles are ubiquitous pollutants in the environment. However, little information is available about their toxicity interaction in food crops. In this study, seed germination and hydroponic experiments were conducted to assess the impact of ZnO (NPs and bulk at 250, 500 and 1000 mg L^-1) individual and combined with phenanthrene (1 mg L^-1) on wheat growth for 15 days. Under ZnO (NPs and bulk) alone and combined with phenanthrene exposure, dose-dependent toxicity in some indexes (germination rate, biomass, shoot height, root length) was observed. Both ZnO NPs and bulk inhibited plant growth at high concentrations, but no significant difference was observed between them (P > 0.05). The chlorophyll concentration of wheat leaves decreased by 0.43-0.60 fold when the levels of ZnO NPs and bulk treated were elevated. There was a negative correlation between ZnO (NPs and bulk) and total chlorophyll. Hill reaction activity also exhibited the same tendency. Through transmission electron microscopy, ZnO NPs were found in wheat seedling root apoplast and symplasm at 1000 mg L^-1 with or without phenanthrene. High doses (500 and 1000 mg L^-1) of ZnO (NPs and bulk) caused more DNA damage to wheat seedling root cells, and ZnO NPs induced stronger genotoxicity than bulk ones to wheat root cells. Superoxide dismutase (SOD) and catalase (CAT) activities of wheat seedling roots decreased at 1000 mg L^-1 ZnO (NPs and bulk), especially in the co-exposure treatments. Hence, ZnO (NPs and bulk) combined with phenanthrene cause more damage to wheat seedling roots, and even destroy the antioxidant system. Our findings are helpful for not only assessing the individual and combined toxicity between phenanthrene and ZnO (NPs and bulk), but also for understanding the different response of plants to individual and combined pollution.