2.3. Ferric-reducing antioxidant power (FRAP) assay
The FRAP assay was carried out according to the procedure described in the literature with minor modifications (Benzie& Strain, 1996). Briefly, the FRAP reagent was prepared from sodium acetate buffer (300 mmol/L, pH 3.6), 10 mmol/L TPTZ solution (40 mmol/L HCl as solvent) and 20 mmol/L iron(III) chloride solution in a volume ratio of 10:1:1, respectively. The FRAP reagent was prepared freshly daily and warmed to 37 C in a water bath before use. One hundred microliters of the diluted sample was added to a portion of the FRAP reagent (3 mL). After 4 min, the absorbance of the reaction mixture was measured at 593 nm using a Shimadzu UV-2450 ultraviolet-visible spectrophotometer (Kyoto, Japan). The standard curve was constructed using FeSO4 solution, and the results were expressed as lmol Fe(II)/g wet weight of flowers.