FACTORS AFFECTING FLOWER COLOUR
Anthocyanins are generallyconsidered responsible for the colour of most pink and blue flowers. Structural differences that effect the colour of anthocyanins are methylation and hydroxylation. Methylation produced less blueness, whereas, increased hydroxylation results in a marked increase in blueness. Anthocyanins are coloured in fairly acid media but they are virtually colourless above pH4. Since the pHof most flowers is usually about 5.5, but within a range of 2.8 to 6.2, it is most improbable that anthocyanins per se could significantly contribute to the colour of most flowers. Only anthocyanins with an ortho-dihydroxyl group form stable coloured metal complexes above pH 3, but there is little spectral evidence that this occurs in nature. Copigmentation, the association of anthocyanins with other flavonoids and related compounds, also stabilizes colour in the pH range where anthocyanins alone are virtually colourless. In contrast to metal chelation, copigmentation occurs with all anthocyanins. In-vivo absorption spectra (Colour) of many flowers have been simulated with model systems of anthocyanins and copigments. Thus, copigmentation offers a more logical explanation of colour and the infinite variations in pink and blue flowers that exist in a pH range where anthocyanins alone are virtually colourless.
Asen, S. (1975). FACTORS AFFECTING FLOWER COLOUR. Acta Hortic. 41, 57-68