Identification of Flavonoid Pigments and Coloration Mechanisms in the Bluish-purple Flowers of Platycodon grandiflorus (Jacq.) A. DC. (Campanulaceae)

Abstract

We investigated bluish-purple Platycodon grandiflorus flowers for yet unidentified flavonoid-related compounds and their flower coloration mechanisms. We identified a new polyacylated anthocyanin, delphinidin 3-O-[6-O-(α-rhamnopyranosyl)-β-glucopyranoside]-7-O-[6-O-(4-O-(6-O-(4-O-(6-O-(4-O-(β-glucopyranosyl)-trans-caffeoyl)-β-glucopyranosyl)-trans-caffeoyl)-β-glucopyranosyl)-trans-caffeoyl)-β-glucopyranoside] (3) as the minor anthocyanin component along with four known anthocyanins, four known flavones, and chlorogenic acid. The major anthocyanin of the bluish-purple P. grandiflorus flowers is platyconin. While platyconin has two caffeic acids linked in series via glucose molecules at the 7-position of delphinidin, anthocyanin 3 has three caffeic acids linked in series via glucose molecules at the 7-position of delphinidin. To investigate the effects of the number of aromatic acyl groups in polyacylated anthocyanin on bluing and stability, the color and stability of anthocyanin 3 in diluted aqueous solution (5 × 10⁻⁵ M) at a weak acidic condition (pH 5.7, the same pH as the petal sap) were compared with those of platyconin. Results showed that even if the three caffeic acids were bound in series at the 7-position of delphinidin, there was no further bluing or stabilizing effects of anthocyanin 3 compared to platyconin, but a tendency toward weakening color intensity was observed. Investigation of the change in anthocyanin content at the flower developmental stages showed that delphinidin 3-rutinoside-7-glucoside, which is the deacylated form of platyconin, first accumulated up to sufficient amounts at the bud stage, followed by an increase in platyconin content. At the bud stage, the petals were not purple or violet, except for veins, despite the presence of a good amount of delphinidin 3-rutinoside-7-glucoside, indicating the delphinidin 3-rutinoside-7-glucoside was present in some colorless state.