Chlortetracycline Disrupts Nutrient Homeostasis and Hormonal Balance in Rice Seedlings in a Dose-Dependent Manner

The widespread use of veterinary antibiotics in poultry and cattle production has resulted in their continuous release into agricultural environments, raising concerns about unintended effects on terrestrial ecosystems. Chlortetracycline (CTC), one of the most commonly applied veterinary antibiotics, is frequently detected in agricultural soils and water; however, its impact on nutrient and hormonal balance in crop plants remains poorly understood. In this study, rice (Oryza sativa L.) seedlings were exposed to increasing concentrations of CTC (0, 0.1, 1, and 5 mg L^-1) under controlled hydroponic conditions to evaluate phytotoxic responses, with particular emphasis on mineral nutrient status, phytohormone levels, and stress-related parameters. CTC exposure caused a clear, dose-dependent inhibition of seedling growth, reflected in significant reductions in root and shoot length and in biomass accumulation. Photosynthetic pigment contents (chlorophyll a and b) and maximum quantum efficiency of photosystem II (Fv/Fm) were markedly reduced, accompanied by altered expression of genes associated with chloroplast function. CTC predominantly accumulated in roots, with limited translocation to shoots, while trace amounts of tetracycline (TC) were detected, suggesting partial transformation within plant tissues. Oxidative stress was evidenced by elevated malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) levels and reduced relative water content, leading to enhanced activities of superoxide dismutase (SOD) and catalase (CAT), whereas ascorbate peroxidase (APX) and peroxidase (POD) activities were suppressed. In parallel, concentrations of essential mineral nutrients, including calcium (Ca), magnesium (Mg), manganese (Mn), zinc (Zn), and molybdenum (Mo), were significantly decreased in shoots. Phytohormone profiling revealed declines in gibberellins (GA₃) and indole-3-acetic acid (IAA), alongside increased levels of abscisic acid (ABA), salicylic acid (SA), and jasmonic acid (JA). Overall, these findings demonstrate that CTC disrupts nutrient and hormonal balance in rice seedlings and induces pronounced stress responses, underscoring potential risks of antibiotic contamination for crop growth in agricultural systems.