About 5 g samples obtained from 9 cm from the asparagus tip until 13 cm from the tip were homogenized in 20 mL of 80% acetone with a tissue homogenizer (Polytron PT-MR2100; Kinematica
AG) at a moderate speed for 30 s, and centrifuged (10,000 × g,15 min) (3-30K; Sigma). Absorbance (A) was measured at 645 and 663 nm using an UV–Vis recording spectrophotometer (UV-2550;
SHIMADZU)