Gas chromatography-mass spectrometry analysis
After derivatization, samples (1 μl) were injected randomly in splitless mode with a cold injection system (Gerstel, Mülheim a der Ruhr, Germany) into GC (Agilent GC6890, San Jose, CA, USA) and analysed as described previously (Watanabe et al., 2013). The numerical analyses of metabolome were based on the peak height values of the response values. Chromatograms were processed using high throughput data analysis method. These values were normalized by the sample fresh weight and ribitol (internal standard), using the cross-contribution-compensating multiple standard normalization algorithm (Rotaru & Sinclair, 2009).