Foliar Exposure of Deuterium Stable Isotope-Labeled Nanoplastics to Lettuce: Quantitative Determination of Foliar Uptake, Transport, and Trophic Transfer in a Terrestrial Food Chain

Nanoplastics (NPs) are widely detected in the atmosphere and are likely to be deposited on plant leaves. However, our understanding of their foliar uptake, translocation, and trophic transfer profiles is limited due to a lack of quantitative analytical tools to effectively probe mechanisms of action. Here, using synthesized deuterium (²H) stable isotope-labeled polystyrene nanoplastics (²H-PSNPs), the foliar accumulation and translocation of NPs in lettuce and the dynamics of NP transfer along a lettuce-snail terrestrial food chain were investigated. Raman imaging and scanning electron microscopy demonstrated that foliar-applied NPs aggregated on the leaf surface, entered the mesophyll tissue via the stomatal pathway, and eventually translocated to root tissues. Quantitative analysis showed that increasing levels of foliar exposure to ²H-PSNPs (0.1, 1, and 5 mg/L in spray solutions, equivalent to receiving 0.15, 1.5, and 7.5 μg/d of NPs per plant) enhanced NP accumulation in leaves, with concentrations ranging from 0.73 to 15.6 μg/g (dw), but only limited translocation (<5%) to roots. After feeding on 5 mg/L ²H-PSNP-contaminated lettuce leaves for 14 days, snails accumulated NPs at 0.33 to 10.7 μg/kg (dw), with an overall kinetic trophic transfer factor of 0.45, demonstrating trophic dilution in this food chain. The reduced ingestion rate of 3.18 mg/g/day in exposed snails compared to 6.43 mg/g/day can be attributed to the accumulation of ²H-PSNPs and elevated levels of chemical defense metabolites in the lettuce leaves, which decreased the palatability for snails and disrupted their digestive function. This study provides critical quantitative information on the characteristics of airborne NP bioaccumulation and the associated risks to terrestrial food chains.