How relevant are flavonoids as antioxidants in plants?

Flavonoids are a large family of plant secondary metabolites, principally recognized for their health-promoting properties in human diets. Most flavonoids outperform well-known antioxidants, such as ascorbate (vitamin C) and alpha-tocopherol (vitamin E), in in vitro antioxidant assays because of their strong capacity to donate electrons or hydrogen atoms. However, experimental evidence for an antioxidant function in plants is limited to a few individual flavonoids under very specific experimental and developmental conditions. As we discuss here, although flavonoids have been demonstrated to accumulate with oxidative stress during abiotic and biotic environmental assaults, a convincing spatio-temporal correlation with the flavonoid oxidation products is not yet available. Thereby, the widely accepted antioxidant function of flavonoids in plants is still a matter of debate.