Azalea indica flower colour as related to the parameters pH, anthocyanins and flavonol co-pigments

Abstract

The influence of epidermal tissue pH on flower colour was studied in 104 cultivars of Azalea indica ( Rhododendron simsii Planch.). Based on H.C.C. visual colour number, cultivars were classified in 4 colour groups: purple; carmine red; red; white. A method for pH determination using microvolumes (30 μl) on a microelectrode has been perfected. Within each colour group, flower colour was determined within a mean pH environment of 3.04 ± 0.06. Epidermal pH was approximately 0.3 units lower than the pH of the expressed sap of entire petals. Purple colours occurred with co-pigmented malvidin or delphinidin glycosides. Carmine red and red colours were produced by the same cyanidin glycosides that in the case of carmine red were co-pigmented with flavonol glycosides. The lower the content of anthocyanins (cyanidin glycosides) and the higher that of flavonols, the bluer the red (higher H.C.C. number; bathochromic shift of about 20 nm). A carmine red flower colour (± 58 B) may also occur when delphinidin glycosides occur in the absence of flavonol glycosides (pH < 4). Prospects for obtaining a blue flower colour are, because of low epidermal pH, very small.