Occurrence and distribution of free flavonoid aglycones in plants

Flavonoids are widely present in plants as water-soluble glycosides but the lipophilic free aglycones are far less abundant. The 462 flavonoids reported so far to be present in the free state and their plant sources are listed. Evaluation of these data reveals a correlation in most cases between the occurrence of flavonoid aglycones, the presence of secretory structures and the production of other lipophilic plant products. Their accumulation in some plant organs and in certain taxa is discussed. Special attention is given to their occurrence in materials deposited externally on leaves and buds.