Distinct Metabolic Fates of 6PPD and 6PPD-Q in Rice under Root and Foliar Exposure Pathways

Highly toxic tire-derived N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone derivative (6PPD-Q) threaten aquatic ecosystems, but their accumulation and metabolic fate in rice paddies remain unclear. This study investigated their uptake and biotransformation in rice under root and foliar exposure. Both compounds were absorbed by roots and leaves, with bioconcentration factors (log BCF) reaching 1.69 and 1.62 in roots and 0.74 and 0.53 in shoots for 6PPD and 6PPD-Q, respectively. Root exposure triggered Phase I oxidation (dealkylation, hydroxylation), while foliar exposure promoted Phase II conjugation. Molecular docking, enzyme activity, and gene expression supported these pathways, with enzyme levels increasing (cytochrome P450 monooxygenase by 12.5%-20.7%, glutathione S-transferase by 16.0%-59.0%). Metabolomics indicated a systemic response to root exposure, involving root metabolic inhibition and leaf-level compensatory adjustment. These findings clarify the biotransformation of 6PPD and 6PPD-Q in rice and emphasize the food-chain risks associated with multiroute exposure.