Effects of Cadmium on the Accumulation and Phytotoxicity of Uranium in Radish (Raphanus sativus L.) Seedlings

Cadmium (Cd) is a major co-occurring, highly toxic heavy metal in uranium (U) tailings that poses synergistic risks to ecological and human health. This study aimed to investigate the effects of Cd on U accumulation and phytotoxicity in plants using radish (Raphanus sativus L.) as a model organism under hydroponic conditions. Treatments included U alone (25 &mu:M and 50 &mu:M), low-concentration Cd alone (10 &mu:M), and U + Cd co-treatments (U25 + Cd and U50 + Cd). Results revealed that exposure exerted minimal phytotoxicity, whereas U treatment induced severe root toxicity, characterized by cell death and an 11.9&ndash:63.8% reduction in root biomass compared to the control. Notably, U + Cd co-treatment exacerbated root cell death and biomass loss relative to U alone. Physiologically, elevated U concentrations significantly increased superoxide anion radical (O2&minus:) production rate, hydrogen peroxide (H2O2) content, and malondialdehyde (MDA)&mdash:a marker of oxidative damage&mdash:inducing cellular oxidative stress. Under U + Cd co-treatment, O2&minus: production, H2O2 content, and MDA levels in radish roots were all significantly higher than under U alone. Concurrently, activities of antioxidant enzymes (superoxide dismutase [SOD], catalase [CAT], and peroxidase [POD]) were lower in U + Cd-treated roots than in U-treated roots, further exacerbating oxidative damage. Regarding heavy metal accumulation, the content of U in radish under U + Cd treatment was significantly higher than that in the U treatment group. However, no significant differences were observed in the expression of uranium (U)-related transport genes (MCA1, MCA3, and ANN1) between the single U treatment and the U-Cd co-treatment. Notably, the inhibitory effect of NRAMP3&mdash:a gene associated with Cd transport&mdash:was weakened under the coexistence of U, indicating that U exacerbates toxicity by promoting Cd transport. This study shows that Cd appears to enhance the accumulation of U in radish roots and exacerbate the phytotoxicity of U.