Ultraviolet-B Irradiation Increases Antioxidant Capacity of Pakchoi (Brassica rapa L.) by Inducing Flavonoid Biosynthesis

As an important abiotic stress factor, ultraviolet-B (UV-B) light can stimulate the accumulation of antioxidants in plants. In this study, the possibility of enhancing antioxidant capacity in pakchoi (Brassica rapa L.) by UV-B supplementation was assessed. Irradiation with 4 mu mol.M-2.s(-1) UV-B for 4 h or 2 mu mol center dot m(-2).s(-1) UV-B for 24 h significantly increased the 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging activity and total reductive capacity, as a result of inducing a greater accumulation of total polyphenols and flavonoids without affecting the plant biomass. A high performance liquid chromatography (HPLC) analysis showed that the concentrations of many flavonoids significantly increased in response to UV-B treatment. The activities of three enzymes involved in the early steps of flavonoid biosynthesis, namely phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H), and 4-coumarate: coenzyme A (CoA) ligase (4CL), were significantly increased after the corresponding UV-B treatment. Compared with the control, the expression levels of several flavonoid biosynthesis genes (namely BrPAL, BrC4H, Br4CL, BrCHS, BrF3H, BrF3'H, BrFLS, BrDFR, BrANS, and BrLDOX) were also significantly up-regulated in the UV-B treatment group. The results suggest that appropriate preharvest UV-B supplementation could improve the nutritional quality of greenhouse-grown pakchoi by promoting the accumulation of antioxidants.

Automated summary

This study assessed the possibility of enhancing the antioxidant capacity in pakchoi by UV-B supplementation. The results showed that UV-B treatment significantly increased the accumulation of antioxidants, including total polyphenols and flavonoids, without affecting the plant biomass. The study also found that UV-B treatment upregulated the activities of enzymes involved in flavonoid biosynthesis and the expression levels of flavonoid biosynthesis genes.