Cerium Induces Biphasic Responses in Brassica rapa L. through Modulated Photosynthesis, Oxidative Homeostasis, and Gene Expression.

Cerium (Ce), the most abundant of the rare Earth elements (REEs), is increasingly recognized as an environmental contaminant due to its growing applications in various industrial and agricultural sectors. This study investigates the physiological, biochemical, and molecular responses of Brassica rapa L. plants to varying concentrations of Ce exposure to elucidate its effects on plant growth, metabolism, and stress responses. Through chemical analytical, biochemical, and gene expression methods, we revealed a biphasic (hormetic) effect of Ce on B. rapa. Low-level Ce exposure (1µM) stimulated plant growth, evidenced by increased leaf area and fresh biomass. Conversely, elevated Ce concentrations (1mM and 10mM) induced significant photosynthetic dysfunction, characterized by diminished chlorophyll a and b content, impaired photosystem II (PSII) efficiency, and altered chlorophyll fluorescence. Ce exposure also modulated oxidative stress responses, exhibiting a hormetic pattern in reactive oxygen species (ROS) accumulation, alongside a general increase in proline. Secondary metabolism was selectively impacted, with higher Ce levels specifically promoting the accumulation of kaempferol derivatives. Mineral nutrient analysis revealed substantial Ce accumulation in leaves and a concomitant decrease in essential elements (Al, Se, Na). Gene expression analysis further elucidated that Ce exposure triggered differential expression of genes involved in carotenoid and flavonoid biosynthesis, chlorophyll metabolism, and ion transport. These comprehensive findings offer novel insights into the multifaceted physiological, biochemical, and molecular responses of B. rapa to Ce, underscoring both the potential ecological risks of Ce contamination and the intricate adaptive strategies employed by plants under REE stress.