Abstract: To investigate the effects of carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) combined with cadmium (Cd) stress on plant growth and physiology, we measured the changes of root growth, oxidative damage, activity of antioxidant enzymes, and contents of cadmium in roots of rice (Oryza sativa L.) seedlings treated with MWCNTs-COOH (0-12.0mg·L^-1) and Cd (10 μmol·L^-1) for three weeks. The results showed that: (1) Root length and fresh weight in MWCNTs-COOH treatment were lower than those of the control, and they increased first and then decreased over time. When the concentration of MWCNTs-COOH reached 12.0 mg·L^-1, root length and fresh weight were lower than those of the control, with a reduction of 9.3% and 15.2%, respectively. (2) Root length, fresh weight and dry weight of rice seedlings treated with the combination of MWCNTs-COOH and Cd were lower than those of the MWCNTs-COOH treatment alone. Under MWCNTs-COOH stress, superoxide radicals (O₂^-_·) accumulated significantly, and superoxide dismutase (SOD) and peroxidase (POD) activities of rice increased. SOD and POD activities were the highest under 3.0 and 6.0 mg·L^-1 MWCNTs-COOH treatments; (3) SOD and POD activities of rice roots under MWCNTs-COOH combined Cd stress were lower, while malondialdehyde (MDA) and carbonylation protein contents were significantly higher than those under Cd stress alone. (4) When rice was treated with combination of MWCNTs-COOH and Cd, the death of root tip cells accelerated. Most of the root cap cells treated with 10 μmol·L^-1 Cd+12.0 mg·L^-1 MWCNTs-COOH were stained with Evans Blue. (5) After treated with 1.5-6.0 mg·L^-1 MWCNTs-COOH and Cd, the cadmium contents in the roots of rice seedlings showed an increasing trend, and reached a maximum of 303.30 μg·g^-1 at the concentration of 6.0 mg·L^-1. Our results indicated that high concentrations of MWCNTs-COOH and its combination with Cd are toxic to rice roots.

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