Photoinduced Back-and-Forth Transformation between Z - and E -Isomers of Cyetpyrafen: An Overlooked Pathway in Assessing the Associated Risks

Phototransformation is a critical pathway that mediates the fate of pesticides in plants. However, photoinduced back-and-forth transformation between Z- and E-isomers of pesticides has been largely unexplored when they are applied to plants, which may cause the active ingredient loss or unintended environmental/food contamination. Here, Z–E isomerization of insecticide Z-cyetpyrafen was identified in strawberry plants under field conditions. The maximum Z–E isomerization ratios were associated with the plant vertical spaces under foliar spray or root irrigation during 2 h–21 days of treatment: leaves (32.7% or 17.9%) > stems (22.5% or 9.6%) > roots (13.8% or 3.7%), indicating the extent of Z–E isomerization was dependent on light intensity. A shading experiment further confirmed that ultraviolet (UV) radiation (<370 nm) dominated the photoisomerization process. Leaf cuticle could accelerate Z–E photoisomerization of Z-cyetpyrafen by 21.4%. Photocatalytic direct energy transfer was proved as the main driving force for Z–E photoisomerization. Z- and E-cyetpyrafen exhibited similar target bioactivity (Tetranychus urticae) and nontarget toxicity (Daphnia magna and Apis mellifera L.). Thus, the development of their mixture instead of Z-cyetpyrafen would be more cost-effective. Dietary exposure risks for humans were underestimated by 5.6–21.6% when only Z-cyetpyrafen was considered, and E-cyetpyrafen was ignored. We proved that photoisomerization also occurred in other pesticides with C═C or C═N bonds, implying the risk assessment of individual isomers of such pesticides is critically needed.