Pakchoi (<i>Brassica rapa</i> L.) Modulates the Antioxidant System and Energy Metabolism in Response to 6PPD and 6PPD-Q

The widespread presence of the emerging contaminants <i>N</i>-(1,3-dimethylbutyl)-<i>N</i>'-phenyl-<i>p</i>-phenylenediamine (6PPD) and its derivative 6PPD-quinone (6PPD-Q), along with their associated environmental risks, has garnered significant public concern. To assess the food safety risk of 6PPD and its oxidized product 6PPD-Q in the environment, we explored the effects of different concentrations (0.1, 1, 10, and 100 μg/L) of 6PPD and 6PPD-Q on pakchoi (<i>Brassica rapa</i> L.). Preferential accumulation of 6PPD-Q was observed in pakchoi roots, reaching 159 ng/g under 100 μg/L exposure. Pakchoi exhibited severe physiological damage, as evidenced by significantly reduced biomass, elevated levels of reactive oxygen species (ROS), and increased malondialdehyde (MDA) content. Elevated photosynthetic pigment content and antioxidant enzyme activities indicate the activation of the antioxidant system in pakchoi. Flavonoid metabolites were upregulated to participate in the resistance to oxidative stress induced by 6PPD-Q. In contrast, 6PPD specifically caused cysteine-related protective mechanisms. Furthermore, the energy metabolism of pakchoi was significantly adjusted. These findings provide insights into the defensive mechanisms employed by pakchoi against 6PPD and 6PPD-Q, offering new perspectives for assessing their phytotoxicity and environmental risk.