Superoxide anion radical scavenging activity was analyzed by the method described previously (Duan et al., 2007), with a slight modification. Fresh peel tissue (2.0 g) from ten fingers withdrawn randomly from each treatment was mixed with 20 mL methanol, then centrifuged at 10,000 × g for 15 min and finally, the supernatant was collected. The reaction mixture contained 0.5 mL of 0.05 M phosphate buffer (pH 7.0), 0.3 mL of 50 μM riboflavin, 0.3 mL of 20 mM methionine, and 0.3 mL of 0.51 mM nitro blue tetrazolium, prior to addition of 1 mL of sample solution. The reaction was started by illuminating the reaction mixture with a fluorescent lamp. After 20 min of incubation, the absorbance was measured at 560 nm with a spectrophotometer. The reaction mixture without sample was used as the control. The superoxide anion radical scavenging activity (%) was calculated as (1 − absorbance of sample/absorbance of control) × 100.