MALVIDIN-3-O-GLUCOSIDE-5-O-(6-ACETYLGLUCOSIDE) AND ITS COLOR MANIFESTATION IN JOHNSONS-BLUE AND OTHER BLUE GERANIUMS

A study of the physicochemical factors contributing to flower colour i n three 'blue' geraniums, 'Johnson's Blue', G. pratense and G. sanguin eum, has led to an understanding of how such colour variants are produ ced in geraniums. All three contain the same new anthocyanin, pyranosi de-5-O-beta-D-[6-O-acetylglucopyranoside], the structure of which was established by 2D-NMR techniques. This anthocyanin is an isomer of the -acetylglucopyranoside]-5-O-beta-D-glucopyranoside reported recently from G. sylvaticum, but it was indistinguishable by HPLC from the pigm ent sourced from a locally available G. sylvaticum. It was accompanied in 'Johnson's Blue' and G. pratense with kaempferol and myricetin 3-O -glucosides and 3-O-sophorosides, but with only low levels of flavonol s in G. sanguineum. The pH values of pressed juice from these flowers were 5.4, 5.4 and 4.6, respectively. In in vitro colour reconstitution experiments, the petal spectra of 'Johnson's Blue' and G. pratense co uld only be produced from the anthocyanin at near vacuolar concentrati ons, in the presence of molar excesses of kaempferol-3-O-sophoroside ( the major copigment) and at a pH of 6.6 to 6.8. It is concluded that t he pH of epidermal cell vacuoles in these flowers is 1-1.4 units highe r than that of the pressed juice. In vitro colour reconstitution of th is type is proposed as a convenient method for determining the approxi mate pH of anthocyanin-containing epidermal cell vacuoles. The colours and colour stabilities of all three studied flowers can be adequately accounted for by the pigment structure and concentration, the flavono l type and ratio, and the epidermal pH. (C) 1997 Elsevier Science Ltd.