Root and foliar uptake, bidirectional translocation, subcellular distribution, and metabolism of tire wear particle-derived p-phenylenediamines and their quinones in Chinese cabbage and bok choy

Tire wear particles (TWPs) and TWP-derived compounds have caused global concern for their toxicity, but their impacts on vegetables, particularly through foliar absorption, remain largely unknown. We investigated the absorption, translocation, accumulation, and metabolism of TWP-derived p-phenylenediamines (PPDs) and PPD-quinones (PPDQs) in hydroponic and soil-cultured leafy vegetables by exposing them to TWPs through roots and leaves. PPDs and PPDQs can be absorbed by vegetables through both roots and leaves, with subsequent bidirectional translocation within vegetables. TWPs with smaller particle sizes and higher aging degrees can release more PPDs and PPDQs, resulting in increased absorptions by the vegetables. Leaves with higher wax secretion, lipid content, and trichome abundance exhibit greater uptake capabilities. PPDs and PPDQs absorbed by leaves can be released back into the environment through roots. The subcellular distribution and translocation of PPDs and PPDQs in vegetables are mainly controlled by their octanol-water partition coefficients with highly hydrophobic congeners primarily accumulating in cell walls and organelles. We screened the 6PPD and 6PPDQ metabolites in leafy vegetables and found that their metabolic processes are influenced by their exposure medium and tissue specificity. The results highlight the risks posed by vegetables absorbing, accumulating, and metabolizing airborne TWP-derived chemicals through their leaves.