Interaction of preharvest UV-B and low temperature on antioxidant secondary plant metabolites in Brassica vegetables: A species-specific comparison of kale and pak choi

This study provides a direct comparison of UV-B radiation (0.5 kJ m^-2 d^-1) and low temperature (10/12 °C) on secondary metabolites and enzyme activities in kale and pak choi, assessing both carotenoids and phenolics and including Arabidopsis wild type and uvr8 mutant under identical conditions for mechanistic validation. UV-B induced rapid accumulation of lutein, β-carotene, chlorophyll a, chlorophyll b and kaempferol glycosides in kale, while in pak choi, long-term low temperature or UV-B treatments (3-5 days) triggered similar responses. Notably, combined stress triggered synergistic accumulation of specific phenolic compounds in both species. Low temperature increased antioxidant activity and UV-B enhanced the activities of phenylalanine ammonia-lyase and peroxidase in both species; however, the interactive effects differed between species. Arabidopsis validation demonstrated the regulatory role of the UVR8 photoreceptor in mediating antioxidant responses and secondary metabolism under UV-B and low temperature. Taken together, exposure to UV-B radiation and low temperature according to species-specific responses could be a biotechnological tool to optimize the accumulation of bioactive compounds in Brassica vegetables, especially effective for vertical farming approaches.