Anthocyanic vacuolar inclusions - their nature and significance in flower colouration

The petals of a number of flowers are shown to contain similar intensely coloured intravacuolar bodies referred to herein as anthocyanic vacuolar inclusions (AVIs). The AVIs in a blue-grey carnation and in purple lisianthus have been studied in detail. AVIs occur predominantly in the adaxial epidermal cells and their presence is shown to have a major influence on flower colour by enhancing both intensity and blueness. The latter effect is especially dramatic in the carnation where the normally pink pelargonidin pigments produce a blue-grey colouration. In lisianthus, the presence of large AVIs produces marked colour intensification in the inner zone of the petal by concentrating anthocyanins above levels that would be possible in vacuolar solution. Electron microscopy studies on lisianthus epidermal tissue failed to detect a membrane boundary in AVI bodies. AVIs isolated from lisianthus cells are shown to have a protein matrix. Bound to this matrix are four cyanidin and delphinidin acylated 3,5-diglycosides (three, new to lisianthus), which are relatively minor anthocyanins in whole petal extracts where acylated delphinidin triglycosides predominate. Flavonol glycosides were not bound. A high level of anthocyanin structural specificity in this association is thus implied. The specificity and effectiveness of this anthocyanin "trapping" is confirmed by the presence in the surrounding vacuolar solution of only delphinidin triglycosides, accompanied by the full range of flavonol glycosides. "Trapped" anthocyanins are shown to differ from solution anthocyanins only in that they lack a terminal rhamnose on the 3-linked galactose. The results of this study define for the first time the substantial effect AVIs have on flower colour, and provide insights into their nature and their specificity as vacuolar anthocyanin traps. (C) 2000 Elsevier Science Ltd. All rights reserved.