Chemical speciation of ciprofloxacin in aqueous solution regulates its phytotoxicity and uptake by rice (Oryza sativa L.)

Highlights: • Ciprofloxacin (CIP) speciation affected its uptake and phytotoxicity by rice plants. • CIP exhibited a concentration-dependent uptake and toxicity effects to rice plants. • CIP adsorption and uptake by excised and living roots decreased with increasing pH. • The exacerbated phytotoxicity of CIP to rice plants occurred at a high solution pH. • Six products and two pathways were proposed for CIP metabolism in rice plants. Ciprofloxacin (CIP), a widely used fluoroquinolone antibiotic, is frequently detected in aqueous environments, and could be assimilated by vegetable plants to possess potential threats to human and animal health through food chains. However, plant uptake of CIP in different chemical speciation has still far from clear now. Thus, the toxicity and uptake of CIP by rice plants were investigated under different solution pH, owing to its contribution to different chemical speciation of CIP. Results display that high pH-driven changes of CIP from cation (CIP⁺) to anion (CIP⁻) decreased its adsorption and uptake by excised roots and intact plants, respectively. However, CIP concentrations in roots, stems and leaves all exhibited no significant differences with increasing solution pH. Moreover, six intermediates of CIP were detected and two possible transformation pathways were proposed in rice plants, including firstly oxidation and following consecutive cleavage of piperazine ring. After accumulated in plant tissues, CIP significantly inhibited the plant growth, decreased the photosynthetic pigments contents and enhanced the antioxidant enzyme activities in a concentration-dependent manner. Besides, high pH exacerbated the growth inhibition and changed the oxidative damage responses of rice plants to CIP. These findings indicate that the uptake and toxicity of CIP in rice plants were influenced by solution pH-driven changes of its chemical speciation.