Determination of flavonoid level variation in onion (Allium cepa L.) infected by Fusarium oxysporum using liquid chromatography–tandem mass spectrometry

In order to evaluate the flavonoid level variation in an onion ( Allium cepa L.) infected by Fusarium oxysporum , the bulbs of a healthy onion and of an infected one were analysed for flavonoids via high-performance liquid chromatography coupled with tandem mass spectrometry (HPLC–MS/MS). Among eleven flavonoids characterised, isorhamnetin 4′- O -galactoside ( 8 ) was identified in an onion for the first time. When the healthy bulb was inoculated with the fungus, the two quercetin derivatives ( 4 and 7 ) and the two isorhamnetin derivatives ( 5 and 9 ) underwent concentration changes typical for the defense materials against pathogens. The yellow granules that were accumulated on the abaxial epidermal cell layers after 8 days of inoculation were confirmed as quercetin ( 10 ) and isorhamnetin ( 11 ). It was deduced that they were produced from flavonoids 4 , 5 , 7 and 9 by hydrolysis enzyme of the fungus.