Zinc Oxide Nanoparticles Alleviated the Bioavailability of Cadmium and Lead and Changed the Uptake of Iron in Hydroponically Grown Lettuce (Lactuca sativa L. var. Longifolia)

Leafy vegetables are a rich source of iron and fibers for the human diet, which may become hazardous if exposed to heavy metal contamination. Cadmium (Cd) and lead (Pb) are two highly toxic metals even at trace concentrations. Engineered nanoparticles (ENPs) can alter the uptake of heavy metals and localization of essential minerals such as iron (Fe) through different mechanisms. The goal of this study was to understand the mutual effects of zinc oxide nanoparticles (ZnONPs) and coexisting heavy metals Pb2+ and Cd2+ on their uptake and accumulation as well as their effects on Fe concentrations in romaine lettuce (Lactuca sativa L. var. Longifolia) in a hydroponic system. At termination, shoots were gently separated from the roots, and the concentrations of Pb, Cd, Fe, and Zn in all plant tissues were quantified by inductively coupled plasma-mass spectrometry (ICP-MS). In addition, microbial density analysis in the growth media was performed for each treatment. The results indicated active interactions between ZnONPs and coexisting divalent heavy metals. ZnONPs significantly reduced the accumulation of Cd and Pb in roots by 49% and 81%, respectively. In shoots, Cd was reduced by 30%, and Pb elevated by 44%. Fe concentration in shoots was strongly affected by ZnONPs, and the total Zn in shoots was negatively correspond with the microbial population in the growth media. Exposure to ZnONPs alone increased the total Fe in shoots by 80% compared to controls, and the copresence of ZnONPs and heavy metals increased Fe concentration by about 77%. The results revealed the role of ENPs in governing the uptake and translocation of some essential elements and toxic heavy metals in plants.