The content of polyphenol was determined by a Folin phenol reagent according to the method of Liu et al.( Liu et al., 2019). Weigh accurately 0.1g of stem sample, placed in a 100mL Erlenmeyer flask, add 50mL of 70% ethanol, shake for 4 hours, filter, and the filtrate was dilute to 100mL. Accurately measure 1 mL of the extract, add 0.3 mL of FC reagent, and mix for 5 min. Add 1.5 mL of saturated sodium carbonate solution, add water to a volume of 10 mL, shake for 30 min in a 30℃ water-bath. Then, the absorbance was measured at 760 nm. The standard curve is made with gallic acid.