Superoxide anion generation (O2−) rate was measured according to the method of Song et al. (2009), with a few modifications. Fresh peel tissues (2.0 g) from ten fingers withdrawn randomly from each treatment were homogenized with 10 mL of extract solution consisting of 1 mM EDTA, 0.3% Triton X-100 and 2% insoluble polyvinylpolypyrrolidone, and then centrifuged at 12,000 × g for 30 min at 4 °C. The extract solution (1 mL) was mixed with 1 mL of 1 mM hydroxylammonium chlroride, and then incubated for 30 min at 25 °C. One milliliter of the incubated solution was added to 1 mL of 17 mM 3-aminobenzenesulfonic acid (Sigma, USA) and 1 mL of 7 mM 1-naphthylamine (Sigma, USA) for further incubation for 20 min at 25 °C. The absorbance of the solution was monitored at 530 nm. A standard curve with NO2− was used to calculate the O2− production rate from the reaction equation of O2− with hydroxylamine. Superoxide anion generation rate was expressed on a fresh weight basis as nmol kg−1 s−1.