The threshold effect of Ce-Zn compound pollution on maize roots: implications for plant tolerance to combined metal pollution

Heavy metal pollution is becoming increasingly severe, and rare earth elements (REEs) play a crucial role in influencing the environmental impact of heavy metals. Mixed heavy metal pollution presents a significant challenge due to its complex effects. While many studies have focused on single heavy metal pollution, relatively few have examined the pollution caused by rare earth-heavy metal composites. This study investigated the toxic effects of Ce-Zn compound pollution on maize roots and used the integrated biomarker response (IBR) to assess the toxicity of different concentrations of Ce-Zn compound pollution on maize roots. The results showed that Ce-Zn compound pollution had a threshold effect: promoting maize growth at low concentrations but inhibiting growth at a threshold concentration of Ce 50 mg/kg-Zn 50 mg/kg. Ce-Zn compound pollution-induced oxidative stress in maize. When the pollution concentration exceeded the threshold, the H₂O₂ content in maize roots increased by 3.30 and 3.56 times, respectively, compared to the CK group, while the O₂⁻ content increased by 1.45 and 1.48 times, respectively. Cell structure damage was observed, and the levels of antioxidant enzymes—peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD)—in maize roots gradually increased. At low concentrations, the Ca²⁺ absorption rate in maize roots increased by 82.99 % compared to CK. By regulating the Ca²⁺ concentration, the absorption of H⁺ and other ions was affected, enhancing intracellular enzyme activity and improving the tolerance of maize roots to compound pollution. These findings enhance our understanding of the harmful effects of combined pollution from rare earth metals and heavy metals on plants.