The content of total flavonoids was determined according to the method of Quy et al. (Quy et al., 2014). 0.1 g of stem sample was placed in a 100 mL round bottom flask, add 50 mL of 75% ethanol and reflux at 80℃ for 2 h. After that, the extracting solution was filted, and, the filtrate was collected. The filtrate was dilute to 50 mL with 75% ethanol. Take 1 mL of the filtrate in a 10 mL volumetric flask, add 4 mL of 50% ethanol solution, add 0.3 mL of 5% sodium nitrite solution, shake well, place for 6 min. Next add 0.3 mL of 10% aluminum nitrate solution in it, shake well, then place for 6 min, add 4 mL of 1 mol/L sodium hydroxide solution was diluted to 50% ethanol, and shaken for 15 min. The absorbance at 510 nm was measured. The standard curve was made with rutin reference solution.

2.5 Determination of polysaccharide- zinc in Dendrobium nobile

A certain amount of polysaccharide samples was put into microwave digestion tube, and 1mL hydrofluoric acid was added to 5 mL concentrated nitric acid. Then put the tube in ventilation room for overnight. Fixed volume to 50 mL after complete digestion. Determination of zinc in each component by Flame Atomic Absorption Spectrometry(Altair et al., 2018).