Foliar uptake, biotransformation, and impact of CuO nanoparticles in Lactuca sativa L. var. ramosa Hort.

Plant leaves can intercept and directly absorb nanoparticles (NPs) that deposit on their surface, which can lead severe phytotoxicity. However, there is a large blind spot when it comes to the fate and phytotoxicity of NPs after leaf exposure, even though foliar uptake is likely to occur. In this study, lettuce leaves (Lactuca sativa L. var. ramosa Hort.) were exposed to different concentrations of copper-oxide NPs (CuO-NPs, 0, 100, and 1000 mg L-1) for 5, 10, and 15 days. Foliar uptake, subcellular distribution, chemical forms, and impact of CuO-NPs on nutrient status, antioxidant systems, and lettuce growth were examined. Substantially elevated Cu levels were observed in lettuce leaves (up to 6350 mg kg-1), which was one magnitude greater than that in the roots (up to 525 mg kg-1). Cu translocation factors from leaves to roots ranged from 1.80 to 15.6%. The application of CuO-NPs severely inhibited lettuce growth and altered the nutrient status in plants (especially Mn, K, and Ca). Moreover, CuO-NPs increased H2O2 generation, malonaldehyde level (on the 5th and 10th day of exposure), and catalase activity (on the 15th day of exposure) in lettuce leaves. The Cu concentrations in subcellular fractions were ranked: cell wall ≈ organelles > soluble fraction in lettuce leaves, and organelles > cell wall > soluble fraction in lettuce roots. Undissolved Cu forms were predominant in lettuce, which may have helped to reduce the Cu's mobility and phytotoxicity in the plant. The findings of this study will be of great interest in areas with high levels of metal-NPs in the atmosphere.

Airborne foliar transfer of particular metals in Lactuca sativa L.: translocation, phytotoxicity, and bioaccessibility.

The uptake, translocation, and human bioaccessibility of metals originating from atmospheric fine particulate matters (PM) after foliar exposure is not well understood. Lettuce (Lactuca sativa L.) plants were exposed to micronic PbO, CuO, and CdO particulate matters (PMs) by the foliar pathway and mature plants (6 weeks old) were analyzed in terms of: (1) metal accumulation and localization on plant leaf surface, and metal translocation factor (TF) and global enrichment factor (GEF) in the plants; (2) shoot growth, plant dry weight (DW), net photosynthesis (Pn), stomatal conductance (Gs), and fatty acid ratio; (3) metal bioaccessibility in the plants and soil; and (4) the hazard quotient (HQ) associated with consumption of contaminated plants. Substantial levels of metals were observed in the directly exposed edible leaves and newly formed leaves of lettuce, highlighting both the possible metal transfers throughout the plant and the potential for human exposure after plant ingestion. No significant changes were observed in plant biomass after exposure to PbO, CuO, and CdO-PMs. The Gs and fatty acid ratio were increased in leaves after metal exposure. A dilution effect after foliar uptake was suggested which could alleviate metal phytotoxicity to some degree. However, plant shoot growth and Pn were inhibited when the plants are exposed to PbO, and necrosis enriched with Cd was observed on the leaf surface. Gastric bioaccessibility of plant leaves is ranked: Cd > Cu > Pb. Our results highlight a serious health risk of PbO, CuO, and CdO-PMs associated with consumption of vegetables exposed to these metals, even in newly formed leaves in the case of PbO and CdO exposure. Finally, the study highlights the fate and toxicity of metal rich-PMs, especially in the highly populated urban areas which are increasingly cultivated to promote local food.