Extractions and assays of ATP, ADP and AMP were conducted by applying the method of Ozogul et al. (2000) with a minor modification. Longan pericarp tissue (5 g from 10 fruit) was ground in liquid nitrogen to obtain a fine powder. The powder was then homogenized with 25 mL of 0.6 M perchloric acid. The homogenate was centrifuged at 16,000 × g for 10 min at 4◦C. The supernatant(10 mL) was quickly neutralized to pH 6.5–6.8 using 1 M KOH, and then diluted to 4 mL and passed through a 0.45 m filter. Contents of ATP, ADP and AMP were measured by a Waters 2695 analytical high performance liquid chromatography with DAD detector(HPLC, Waters Corporation, USA) using a Megres TMC 18 column(4.6 mm × 250 mm) and an ultraviolet detector at 254 nm. Mobile phase A consisted of 0.05 M dipotassium hydrogen phosphate and0.05 M potassium dihydrogen phosphate dissolved in deionized water and then was adjusted to pH 7.0 with 0.1 M KOH. Mobile
phase B was pure methanol. The elution was conducted by a linear gradient program with 85–100% A and 0–15% B for 35 min, with flow rate of 20 L s−1. Samples of 50 L were injected into the HPLC.ATP, ADP and AMP contents were calculated according to the external standard program. Energy charge was calculated by [ATP + 1/2 ADP]/[ATP + ADP + AMP]