Flavonoids are widely distributed plant pigments. They are water soluble and commonly occur in vacuoles, membrane-enclosed structures within cells which also store water and nutrients.
Interestingly, light absorption by other photoreceptive plant pigments, such as phytochrome and flavins, induces synthesis of flavonoids in many species. Anthocyanins are the most common class of flavonoids and they are commonly orange, red, or blue in color. Anthocyanins are present in flowers, fruits, and vegetables. Roses, wine, apples, and cherries owe their red color to anthocyanins. In the autumn, the leaves of many temperate zone trees, such as red maple (Acer rubrum), change color due to synthesis of anthocyanins and destruction of chlorophylls.
Chemists have identified more than 3,000 naturally occurring flavonoids. Flavonoids are placed into 12 different classes, the best known of which are the anthocyanins, flavonols, and flavones. All flavonoids have 15 carbon atoms and consist of two 6-carbon rings connected to one another by a carbon ring which contains an oxygen atom. Most naturally occurring flavonoids are bound to one or more sugar molecules. Small changes in a flavonoid's structure can cause large changes in its color.
Flavonoids often occur in fruits, where they attract animals which eat the fruits and disperse the seeds. They also occur in flowers, where they attract insect pollinators. Many flavones and flavonols absorb radiation most strongly in the ultraviolet (UV) region and form special UV patterns on flowers which are visible to bees but not humans. Bees use these patterns, called nectar guides, to find the flower's nectar which they consume in recompense for pollinating the flower. UV-absorbing flavones and flavonols are also present in the leaves of many species, where they protect plants by screening out harmful ultraviolet radiation from the Sun.
Science EncyclopediaScience & Philosophy: Planck mass to PositPlant Pigment - Absorption Of Radiation, Chlorophylls, Carotenoids, Flavonoids, Phytochrome, Additional Plant Pigments