The absorption, translocation, and biotransformation behaviors of organophosphate esters (OPEs) and diesters (OPdEs) in a hydroponic system were investigated. The lateral root was found as the main accumulation and biotransformation place of OPEs and OPdEs in lettuce. The nontarget analysis using high-resolution mass spectrometry revealed five hydroxylated metabolites and five conjugating metabolites in the OPE exposure group, among which methylation, acetylation, and palmitoyl conjugating OPEs were reported as metabolites for the first time. Particularly, methylation on phosphate can be a significant process for plant metabolism, and methyl diphenyl phosphate (MDPP) accounted for the majority of metabolites. The translocation factor values of most identified OPE metabolites are negatively associated with their predicted logarithmic octanol-water partitioning coefficient (log Kow) values (0.75-2.45), indicating that hydrophilicity is a dominant factor in the translocation of OPE metabolites in lettuce. In contrast, palmitoyl conjugation may lead to an enhanced acropetal translocation and those with log Kow values < 0 may have limited translocation potential. Additionally, OPE diesters produced from the biotransformation of OPEs in lettuce showed a higher acropetal translocation potential than those exposed directly. These results further emphasize the necessity to consider biotransformation as an utmost important factor in the accumulation and acropetal translocation potential of OPEs in plants.
Keywords: high-resolution mass spectrometry; methylation metabolism; nontarget analysis; organophosphate flame retardants; plant transformation.