The Flavonoids (1975)
Jeffrey B. Harborne, Tom J. Mabry, Helga Mabry

Physiology and functions of flavonoids, pp. 1016-1017
Jerry W. McClure

18.4.2.1 Flavonoids and visible colour

It is well known that a wide range of insects have visual systems which allow them to be particularly sensitive to flavone and flavonol glycosides absorbing near 350 nm and to the yellow flavonoids such as chalcones, aurones, 3-deoxyanthocyanins, or flavonols with extra hydroxyl or methoxyI substituents in the 6- or 8-position (Harborne, 1972). On the basis of colour vision and from field observations, it is also clear that bees generally prefer blue and yellow, butterflies pink or white, birds red, and moths white (Harborne, 1972). Most correlations between flower colour and the types of insects that visit them can be attributed, at least in part, to the flavonoids. Kevan (1972) made extensive field studies in the Canadian Arctic and found that white and yellow flowers are predominant. Here the high UV reflectance from the flowers gives them a greater luminance and they stand out from their backgrounds in both colour and brightness to the insect. Horovitz (1969) traced the relationship between flower colour and bees in blue or white flowering forms of the California annual Lupinus nanas. Reflectance of the flowers in the blue region (400 to 470 nm) was closely associated with high female outcrossing, while in the white flowers there was a very low incidence of outcrossing.

Some of the most complex and interesting relationships between insects and pollination are found in flowers of the Orchidaceae. Anthocyanins, with possibly other flavonoids, produce elaborate patterns that, to the insect, promises food, deceives it by producing structures with the form and colour of copulation partners, or are repulsive. Other patterns evoke the insects' anger, attack, and eventual pollination. Post-pollination responses in the orchids are often rapid and the flavonoid-determined pattern may be obscured in a matter of a few hours. It is likely that these changes discourage the pollinator from again visiting the flower and increase the likelihood that it will visit an unpollinated member of the same species (Van Der Pijl and Dodson, 1966).

Ultraviolet patterns in flowers have been examined by televising images through a visible-absorbing, ultraviolet-transmitting filter (Eisner et al., 1969). The images show that flowers pollinated by birds or bats generally lack UV nectar guides or other special markings. By contrast, flowers pollinated by insects have characteristic markings and are presumably distinctive. When Horovitz and Cohen (1972) photographed UV reflectance patterns of flowers of 26 eastern Mediterranean wild species and three cultivated species of crucifers, they found striking and complex patterns associated with the veins near the base of the petals. Patuletin (20) and quercetagetin (21) have been identified as the UV absorbing and reflecting compounds in the nectar guides of Rudbeckia hirta (Thompson et al, 1972). Bloom and Vickery (1973) found that pattern-partitioning in yellow flowered Mimulus luteus was due to mixtures of carotenoids and flavonoids. The carotenoids were uniformly present, while cyanidin 3-glucoside and quercetin 3-glucoside were localized within the red spots. The yellow flavonol herbacitrin (8-hydroxykaempferol 7-glucoside) and the anthocyanin were mutually exclusive.

 
20 Patuletin   21 Quercetagetin

Birds have powerful vision but a very poor sense of smell and most bird-flowers are odourless and pigmented with anthocyanin in shades ranging from orange to scarlet. Plants which have flowers with sufficient calorific rewards to attract birds consistently as pollinators must limit the availability of their nectar to effective pollinators. Red and orange are not conspicuous to insects, excepting a few butterflies, and to the insect these flowers usually blend into the background foliage. High levels of anthocyanins uniformly distributed throughout the tissues would also dampen the visibiity of the flavone and flavonol glycosides in the ultraviolet range. Thus, although birds have no intrinsic preference to red, this is the only colour that is at once inconspicuous to most insects and an excellent visual signal to birds (Raven, 1972; Grant and Grant, 1968).