Characterisation of flower colouration in 30 Rhododendron species via anthocyanin and flavonol identification and quantitative traits

Floral colour is a key reproductive character, often associated with environmental adaptation, and subject to human intervention. A large number of Rhododendron species differ widely in flower colour, providing a good model for flower colouration. The chromatic features and anthocyanin compositions of 30 species from seven subgenera were systematically analysed. The Royal Horticultural Society Colour Chart and CIE L*a*b* system were employed to describe and investigate flower colours. The UPLC-PDA/ESI-MSn system was used to identify and quantify anthocyanins in petal extracts. The flower colours of 30 Rhododendron species were categorised into four groups - red, purplish pink, purple and white. Seven anthocyanins were identified and quantified in petals: delphinidin, cyanidin and malvidin 3-O-arabinoside-5-O-glucosides, cyanidin 3,5-di-O-glucoside, 3-O-galactoside and 3-O-arabinoside, and delphinidin 3-O-glucoside. The red-flowered species mainly contained cyanidin monoglycosides and had much higher total anthocyanin content than purplish pink- and purple-flowered species. Purplish pink- and purple-flowered species had similar anthocyanin types and content. The chromatic differences were significant among groups, except the purplish pink and purple groups. Statistical analysis showed that Cy3Gal and Cy3Arb are characteristic for red-flowered species, and Mv3Arb5G and Dp3Arb5G play important roles in purple colouration; their contents were major components that greatly affected the chromatic parameters. In total, 21 flavonol derivates were identified. However, total flavonol content and co-pigmentation index showed no significant difference or correlation among/with colour groups, suggesting that flavonols might not play a major role in colouration. These results enhance our knowledge of the biochemical basis of flower colouration in Rhododendron species, and provide a foundation for genetic variation studies and aid in breeding cultivars with novel flower colours.