After vortex, the sample was centrifuged at 2200 g for 15 min. Then 150 µL supernatant was transferred into a 1.5 mL tube, and 40 mL methoxyamination reagent was added. The sample was mixed well and shaken at 37◦C for 2 h before 70 µL MSTFA reagent and time standards (alkane mixtures) were added. The sample was shaken at 37◦C for another 30 min. Derivatization reaction of an empty reaction tube was also prepared as a control. Then the metabolites were analyzed using GC-TOF-MS (Agilent 7890A/5975C, CA, USA) according to the procedure of Lisec et al. (2006) and Shi et al.(2014c).