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

Juan Hao; Panpan Lou; Yidie Han; Lijun Zheng; Jiangjie Lu; Zhehao Chen; Jun Ni; Yanjun Yang; Maojun Xu

doi:10.3390/plants11060766

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

Plants (Basel). 2022 Mar 13;11(6):766. doi: 10.3390/plants11060766.

Authors

Juan Hao¹, Panpan Lou¹, Yidie Han¹, Lijun Zheng¹, Jiangjie Lu¹, Zhehao Chen¹, Jun Ni¹, Yanjun Yang¹, Maojun Xu^{1

2}

Affiliations

¹ Zhejiang Provincial Key Laboratory for Genetic Improvement and Quality Control of Medicinal Plants, Hangzhou Normal University, Hangzhou 311121, China.
² Key Laboratory of Hangzhou City for Quality and Safety of Agricultural Products, College of Life and Environmental Sciences, Hangzhou Normal University, Hangzhou 311121, China.

Abstract

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.

Keywords: UV-B; antioxidant activity; biosynthetic pathway; flavonoids; greenhouse; pakchoi.