Zinc oxide nanoparticles: a potential micronutrient fertilizer for horticultural crops with little toxicity

Whether zinc nanoparticles (NPs) are phytotoxic or beneficial to plants remains controversial because of limited research. In this study, we investigated the effect of various concentrations of zinc oxide (ZnO) NPs (0, 1, 5, 20, 100, and 1000 mg kg−1 of soil) on lettuce (Lactuca sativa L.) and carrot (Daucus carota subsp. sativus) plants. Plants of both crop species grown in ZnO NPs showed a significant increase in biomass compared with control plants (0 mg kg−1), except for lettuce plants grown in the highest concentration of NPs (1000 mg kg−1), indicating that ZnO NPs acted as a fertilizer. Additionally, control plants showed lower levels of chlorophyll and zinc, magnesium, and potassium than plants grown in ZnO NPs, except for the Mg content of carrot roots grown in 20 mg kg−1 ZnO NPs. Plants of both species grown in 5 and 20 mg kg−1 ZnO NPs showed the highest concentration of nitrogen. Negative effects of ZnO NPs were evident only in lettuce plants grown in 1000 mg kg−1 ZnO NPs, which is more likely an exaggerated level rather than an environmentally realistic concentration. Thus, our data suggest that ZnO NPs are a fertilizer for crop plants and would increase crop productivity when used carefully.