A Chromatographic Survey of Anthocyanin Types in the Genus Rhododendron

Abstract

Anthocyanins are present in the red and purple coloured flowers of Rhododendrons. Flower colour varies from pale pink to deep scarlet and then through a range of mauves and violets. In many cases flower colour is characteristic of a particular subdivision (series) of the genus. For instance many members of Fortunei series have deep red flowers, while blue-violet flowers are characteristic of Lapponicum series. Deep blue flower colour does not occur in this genus so that complexes of anthocyanins with metal ions and polysaccharides as found in e.g. cornflowers (Bayer et al, 1966) are unlikely. A number of colourless or lightly coloured compounds, often flavonoids, have been described as co-pigments (reviewed Paech, 1955), modifying the colours of both complexed and free anthocyanins. The effect of these co-pigments and metal ions is also modified by the type of organic acid in the cell buffering system which may preferentially complex with the metal (Jurd & Asen, 1966). Malvidin-based anthocyanins cannot form metal complexes as they lack adjacent phenolic hydroxy groups, so that purple and violet flowers contain these as the free oxonium compounds and variations in colour must be due to variations in glycosylation pattern, concentration, or co-pigmentation. For instance, anthocyanins with two glycosylated phenolic groups are bluer than those with only one (reviewed Harborne, 1963, I965a). In mauve and purple flowers containing cyanidin-based anthocyanins, the possibility of complex formation exists and colour variation could be due to mixtures in various proportions of the bound and free forms, as well as factors mentioned above. In this paper only variations in anthocyanin composition are considered, with a passing reference to yellow flavonoid pigments. Anthocyanins from flowers from a wide range of Rhododendron species have been examined recently, largely by chromatographic methods (Harborne, 1962) but only a few of the pigments occurring in the genus have received rigorous chemical examination. A survey of anthocyanins described in the literature is presented in Table I, p. 414. A range of cyanidin glycosides have been identified from red flowers and a smaller number of malvidin