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 µmol·m -2 ·s -1 UV-B for 4 h or 2 µmol·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.