Department of Environmental Science, University of California Riverside, CA 92521, United States; Graduate Program in Environmental Toxicology, University of California, Riverside, CA 92521, United States. Electronic address: [Email]
The use of recycled water for agricultural irrigation comes with the concern of exposure to crops by contaminants of emerging concerns (CECs). The concentration of CECs in plant tissues will depend on uptake, translocation and metabolism in plants. However, relatively little is known about plant metabolism of CECs, particularly under chronic exposure conditions. In this study, metabolism of the pharmaceutical diazepam was investigated in Arabidopsis thaliana cells and cucumber (Cucumis sativus) and radish (Raphanus sativus) seedlings grown in hydroponic solution following acute (7 d)/high concentration (1 mg L-1), and chronic (28 d)/low concentration (1 μg L-1) exposures. Liquid chromatography paired with mass spectrometry, 14C tracing, and enzyme extractions, were used to characterize the metabolic phases. The three major metabolites of diazepam - nordiazepam, temazepam and oxazepam - were detected as Phase I metabolites, with the longevity corresponding to that of human metabolism. Nordiazepam was the most prevalent metabolite at the end of the 5 d incubation in A. thaliana cells and 7 d, 28 d seedling cultivations. At the end of 7 d cultivation, non-extractable residues (Phase III) in radish and cucumber seedlings accounted for 14% and 33% of the added 14C-diazepam, respectively. By the end of 28 d incubation, the non-extractable radioactivity fraction further increased to 47% and 61%, indicating Phase III metabolism as an important destination for diazepam. Significant changes to glycosyltransferase activity were detected in both cucumber and radish seedlings exposed to diazepam. Findings of this study highlight the need to consider the formation of bioactive transformation intermediates and different phases of metabolism to achieve a comprehensive understanding of risks of CECs in agroecosystems.