Each 0.5 g of leaf material was homogenized in 3 mL 25 mM HEPES buffer (pH 7.8) containing 0.2 mM EDTA and 2% (w/v) PVP. The homogenate was centrifuged for 20 min at 12,000 × g, and the supernatant was used for enzyme analysis. All operations were performed at a temperature range of 0–4 ◦C.