Dose-dependent response and biodegradation capacity of lettuce to chlortetracycline: hydroponic experiments

Abstract Antibiotics administered to livestock are excreted in large proportions into agricultural soils, where they can be taken up by crop plants. Among veterinary antibiotics, chlortetracycline (CTC) is particularly persistent and widely detected in agricultural environments. Despite growing concern over antibiotic residues in agriculture, little is known about the phytotoxic effects of CTC on leafy vegetables, including lettuce, or the mechanisms underlying their physiological response. In this study, the toxicological impact of CTC on lettuce seedlings was investigated through two sequential hydroponic experiments. In Experiment 1, seedlings were exposed to six concentrations of CTC (0, 0.1, 0.5, 1, 3, and 5 mg L⁻¹) for 21 days to evaluate dose-dependent growth and physiological responses. Concentrations above 0.5 mg L⁻¹ significantly reduced leaf number, shoot and root length, and fresh weight, accompanied by marked reductions in pigments. Notably, anthocyanin decreased by 88% at 5 mg L⁻¹ compared to control. Mineral uptake was also disrupted, with potassium and manganese contents reduced by 59.8% and 39.6%, respectively, at 5 mg L⁻¹. Based on the pronounced phytotoxicity observed at 5 mg L⁻¹, this concentration was selected for Experiment 2 to assess self-detoxification capacity. Seedlings were exposed to CTC for six days, followed by withdrawal until harvest. During exposure, malondialdehyde (MDA) levels and antioxidant enzyme activities increased significantly in both shoots and roots, indicating oxidative stress and activation of defense mechanisms. For instance, ascorbate peroxidase (APX) activity increased by 142.9% in shoots and 66.7% in roots relative to the control on Day 6. Following CTC withdrawal, residues and degradation product tetracycline of CTC remained detectable in plant tissues, while growth recovery was limited and oxidative stress persisted. Elevated MDA indicated ongoing lipid peroxidation, while antioxidant enzyme activities such as SOD and APX remained elevated but only partially effective, reflecting sustained yet insufficient defense responses. These findings provide new insights into CTC-induced phytotoxicity, accumulation, and degradation dynamics via hydroponic cultivation for lettuce. The results underscore potential risks for food safety and emphasize the importance of sustainable management strategies to mitigate antibiotic contamination in agroecosystems.