Deciphering foliar uptake mechanisms of organophosphate esters in plant leaves: Molecular structure-cuticular wax interactions revealed by experimental evidence and machine learning.

The foliar uptake and inward penetration mechanisms of organophosphate esters (OPEs) in plant leaves from agricultural systems remain unclear. This study systematically investigated OPE foliar uptake on excised leaves from nine plant species (crops and ornamental woody plants). Aryl-OPEs exhibited rapid initial absorption (uptake rate constant of pseudo-first-order kinetics, k = 0.35-1.45 h⁻¹), whereas Cl-OPEs showed higher final accumulation (0.07-0.38 nmol per 25 cm²). Machine learning analysis using tree-based models (GBRT/RF) models under compound-grouped nested cross-validation highlighted that polymethyl-structure promoted initial partitioning of OPEs into cuticular wax but limited intra-leaf inward penetration. Additionally, P-OH groups (e.g., diester transformation products of OPEs) facilitated transport within leaves, while side-chain hydroxyl groups reduced uptake and inward penetration. Cuticular wax composition also modulated uptake: alkenes enhanced aryl-OPE absorption via π-electron-mediated dispersion interactions, whereas phytosterols and fatty acids exerted inhibitory effects on total accumulation and inward transfer, likely through polar hydrogen-bonding interactions. For inward penetration specifically, diterpenoids facilitated OPE transfer from the wax layer into inner tissues. These findings provide an integrated mechanistic framework for OPE foliar uptake, offering mechanistic insights into cuticular barrier function that are relevant to plant exposure in agricultural environments.