A group of plant compounds all of which contain a 2-phenylbenzopyran nucleus (see diagram). They include the *flavones
, and *flavonols
. Flavonoids usually occur in combination with sugars as glycosides. They have been isolated from bryophytes and vascular plants but not from algae, fungi, or bacteria. Certain types of flavonoid may be characteristic of a group of plants, the best example being the widespread occurrence of biflavonyls (dimers of the flavone apigenin) in the gymnosperms. Biflavonyls are scarce or absent among other plant groups. Flavonoids have been more often used than any other group of plant substances in chemotaxonomic studies. Flavonoids have traditionally been thought of as waste products and their functions largely remain obscure. Many act as flower pigments while some have been implicated in the control of IAA activity. For example, flavonols, such as quercitin, with two hydroxyls in the Β ring (forming a catechol group) inhibit IAA oxidase while flavonols with one hydroxyl (e.g. kaempferol) promote the enzyme's activity. Other flavonoids, especially certain of the isoflavonoids, serve as phytoalexins. An example is pisatin, which accumulates in pea (Pisum sativum) tissues in response to invasion by many fungal species. Certain isoflavones have oestrogen-like activity while others, e.g. rotenone, resemble the saponins and are extremely toxic to certain forms of animal life.
The flavonoid nucleus (2-phenylbenzopyran).