Effect of meteorological conditions on antioxidant flavonoids in Portuguese cultivars of white and red onions

Although onion (Allium cepa L.) bulbs are good sources of phenolic compounds, the levels of these secondary metabolites are highly variable, depending on the cultivar, production, metereological conditions and post-harvest practices. The aim of this study was to characterize the interannual variation of flavonoid content in two Portuguese landrace varieties of onion (‘Branca da Póvoa’, white, and ‘Vermelha da Póvoa’, red), grown in the Spring–Summer of 2004, 2005, 2006, 2007 and 2008. HPLC-DAD was used to determine flavonoid concentration.Quercetin 3,4′-diglucoside and quercetin 4′-monoglucoside were the main flavonols in both varieties. Additionally, six cyanidin derivatives were identified in the red variety. Total and individual flavonoids levels varied significantly among seasons, with higher levels in 2005, a very dry and hot season. The red onion variety did not accumulate detectable amounts of anthocyanins in 2007, the year with the lowest air and soil temperature and highest soil water content.