Comparative study of the phytotoxicity of ZnO nanoparticles and Zn accumulation in nine crops grown in a calcareous soil and an acidic soil

Highlights: • Soil pH is crucial in Zn availability, Zn accumulation and phytotoxicity of ZnO NPs. • Plant species is a key factor in ZnO NP toxicity and in Zn accumulation in tissues. • Plant biomarker changes are not always associated with decreases in plant growth. • Cucumber and beet were the most sensitive species to ZnO NP in the calcareous soil. • Maize, wheat, and pea were the most tolerant crops to ZnO NP in the acidic soil. The increasing use of zinc oxide nanoparticles (ZnO NPs) in agriculture and consumer products has created the need to evaluate their impact on crops. Nine crops were investigated: wheat, maize, radish, bean, lettuce, tomato, pea, cucumber, and beet. The toxic effects of ZnO NPs on seed germination, plant growth, and biochemical parameters, including photosynthetic pigments, protein and malondialdehyde (MDA) content, reactive oxygen species (ROS), enzymes of the antioxidant defence system, as well as the Zn translocation in the plants were investigated using pots containing non-contaminated or ZnO NP-contaminated soil at concentrations of 20, 225, 450, and 900 mg Zn kg⁻¹. Two soils with different physicochemical properties, namely a calcareous soil and an acidic soil, were used. The total and available Zn in the soils were correlated with the Zn in the plants (roots and shoots) and the observed effects. In the calcareous soil, the available Zn was very low and the phytotoxicity was limited to a slight reduction in the biomass for wheat, cucumber, and beet at the highest concentration. Only beet showed an increase in photosynthetic pigments. The parameters related to oxidative stress were affected to different degrees depending on the crop, with the exceptions of maize, lettuce, pea, and beet. In the acidic soil, the available Zn was high, and the germination of bean, tomato, lettuce, and beet, and the growth of most of the crops were seriously affected. The calculated EC50 values (growth) in the acidic soil ranged from 110 to 520 mg Zn kg⁻¹. The photosynthetic pigments and most of the markers of oxidative stress were negatively affected in maize, wheat, bean, and pea. However, these changes were not always associated with a decrease in plant weight. In summary, soil pH and plant species are key factors affecting the Zn availability and phytotoxicity of ZnO NPs.