Effects of Cadmium on the Accumulation and Phytotoxicity of Uranium in Radish (<i>Raphanus sativus</i> 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 (<i>Raphanus sativus</i> L.) as a model organism under hydroponic conditions. Treatments included U alone (25 μM and 50 μM), low-concentration Cd alone (10 μM), 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 11.9–63.8% 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⁻) production rate, hydrogen peroxide (H₂O₂) content, and malondialdehyde (MDA)—a marker of oxidative damage—inducing cellular oxidative stress. Under U + Cd co-treatment, O₂⁻ 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 (<i>MCA1</i>, <i>MCA3</i>, and <i>ANN1</i>) between the single U treatment and the U-Cd co-treatment. Notably, the inhibitory effect of <i>NRAMP3</i>—a gene associated with Cd transport—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.