Zinc oxide (ZnO) nanoparticles elevated iron and copper contents and mitigated the bioavailability of lead and cadmium in different leafy greens

Advances in large hydroponic production of leafy greens, easy adoption in urban agriculture, and large leaf surface area of many leafy greens, greatly increase their exposure to heavy metals and nanoparticles. Cadmium (Cd) and lead (Pb) are two highly toxic heavy metals, which threaten the health of humans and livestock even at trace levels. These heavy metals may be taken up by plant roots through the protein transporters used for essential minerals such as iron (Fe2+) and copper (Cu2+). Previous studies have shown that some metallic nanoparticles affect the performance of protein transporters and modify the plant uptake of co-existing heavy metal ions. This study aims to understand the role of zinc oxide nanoparticles (ZnONPs) in the uptake pattern of Cd and Pb and two key micronutrients of iron and copper in edible tissues of three leafy green species including spinach (Spinaciae oleracea), parsley (Petroselinum sativum) and cilantro (Coriandrum sativum). Pre-grown plant seedlings in soil (containing Cu and Fe) were transplanted to a hydroponic system (1/4th Hoagland solution) for 7 days as a transition, and then were exposed to four treatments in deionized water (1.0 mg L-1 Cd2+ 100.0 mg L-1 Pb2+, 1.0 mg L-1 Cd2+ +100.0 mg L-1 Pb2+ + 100 mg L-1 ZnONPs, 100 mg L-1 ZnO-ENPs and a control with no chemical exposure) for additional two weeks. At termination, shoots were gently separated from the roots, and the concentrations of Pb, Cd, Fe, Zn, and Cu in all plant tissues were quantified by inductively coupled plasma-mass spectrometry (ICP-MS). The results revealed that ZnONPs mitigated the uptake of both heavy metals in roots. The translocation of heavy metals was similar in the edible tissues of three species. The response of three leafy greens to the co-exposure of heavy metals and ZnONPs was different in Cu and Fe accumulation in edible tissues. Fe concentration in edible tissues in the co-exposed plants was increased in spinach (+10%) and cilantro (+9%) but decreased in parsley (-8%) compared to controls, while the Cu level in edible tissues increased in all three species following the order of cilantro (+ 8%) > spinach (+ 4%) > parsley (+1.5%).