Foliar Uptake, Metabolism, and Phytotoxicity of Typical Citric Acid Esters in Lettuce ( <i>Lactuca sativa</i> L.) Plants

This study investigated the foliar uptake, metabolism, and phytotoxicity of two common citric acid esters (CAEs), acetyl tributyl citrate (ATBC) and acetyl triethyl citrate (ATEC), in lettuce (<i>Lactuca sativa</i> L.) seedlings. After 14 days of exposure, the accumulation of ATBC in leaves (350.30 μg kg^-1) exceeded that of ATEC (320.48 μg kg^-1). In contrast, ATEC exhibited a translocation efficiency higher than that of ATBC, attributable to its greater hydrophilicity and smaller molecular size. Metabolite profiling revealed shared metabolic pathways, including deacetylation, carboxylic-ester hydrolysis, and O-debutylation. Both CAEs significantly inhibited seedling growth and reduced the photosynthetic rate, with ATBC exerting a stronger inhibitory effect. The two compounds also directly bound to the antioxidant enzyme superoxide dismutase, with ATEC exhibiting a higher binding affinity compared to ATBC. These results elucidate the metabolic transformation and phytotoxic mechanisms of CAEs in an edible vegetable, providing valuable insights for the environmental risk assessment of such contaminants.