Effects of Phenanthrene Soil Pollution on Cadmium Bioaccumulation and Metabolic Responses in Maize (<i>Zea mays</i> L.)

Co-contamination of cadmium (Cd) and polycyclic aromatic hydrocarbons (PAHs) in agricultural soils poses a critical threat to crops and food safety, but how PAHs affect Cd uptake and plant metabolism is still unclear. Maize (<i>Zea mays</i> L.) of the variety Hanyu 702 (HY702) was previously identified by our group asaccumulating Cd at low levels when grown in soil containing Cd and phenanthrene (Phe). These contaminants were used here as model pollutions, alone and in combination, to assess the accumulation, growth, physiological, and metabolic responses of HY702 seedlings. Four treatments were compared, including a control without pollution, single Phe pollution, single Cd pollution, and Cd and Phe combined pollution. The experiments followed a completely randomized design with three replicates per treatment. The results revealed that Cd accumulation in the plants was significantly reduced when Phe was present as well (9% reduction in roots and 44% in stems and leaves compared to Cd single pollution). The combined Cd-Phe pollution had no significant impact on the height or chlorophyll content of the maize plants but markedly reduced their malondialdehyde (MDA) content. In addition, it increased the proline content by 56% and antioxidant enzyme activity by 15% (peroxidase, POD), 24% (superoxide dismutase, SOD), and 57% (catalase, CAT) compared to the control treatment. Metabolomics analysis revealed that the coexistence of Phe and Cd activated four key metabolic pathways: (a) alanine, aspartate, and glutamate metabolism; (b) valine, leucine, and isoleucine biosynthesis; (c) aminoacyl-tRNA biosynthesis; and (d) histidine metabolism. This activation resulted in increased levels of six differential metabolites: L-asparagine, L-methionine, L-glutamate, (S)-2-acetyl-2-hydroxybutanoic acid, urocanic acid, and 2-isopropylmalic acid. These metabolites induced detoxification pathways and reduced Cd accumulation. The findings reported here offer new insights into how plants metabolically adapt to the combined pollution of Cd and PAHs and provide an important scientific basis for pollution control strategies.