Blue flower coloration of Corydalis ambigua requires ferric ion and kaempferol glycoside

Corydalis ambigua (Japanese name, Ezoengosaku) flowers bloom with blue to purplish petals in early spring in Hokkaido prefecture. In this study, a mechanism for blue petal coloration by ferric ions and keampferol glycoside was elucidated. Blue petals and cell sap exhibited similar visible (Vis) spectra, with λmax at approximately 600 nm and circular dichroism (CD) with positive exciton-type Cotton effects in the Vis region. Analysis of the organic components of the petals confirmed cyanidin 3-O-sambubioside and kaempferol 3-O-sambubioside as the major flavonoids. Mg, Al, and Fe were detected in petals using atomic emission spectroscopy. Color, Vis absorption, and CD consistent with those of blue petals were reproduced by mixing cyanidin 3-O-sambubioside, kaempferol 3-O-sambubioside, and Fe3+ in a buffered aqueous solution at pH 6.5. Both Fe3+ and flavonol were essential for blue coloration.