Dahlias are very popular garden flowers; about 20,000 types have been bred, and display a wide range of shapes, sizes and splashy colors. These colors range from white and yellow to shapes of red, orange and pink. There also is a very rare variety: black.
About a dozen black dahlias exist, known by creative names such as “Arabian Knight,” “Chat Noir,” and, oddly, “Charles de Gaulle.”
Growers and plant scientists have long known how most dahlias got their colors. Metabolites called flavonoids accumulate in the plants, and these flavonoids provide specific hues:
• Anthocyanins produce red colors
• Flavones produce white colors
• Chalcones produce yellow colors
In addition to white, flavones play a very important role in stabilizing anthocyanins as they produce red colors. But until now, nobody knew how a black dahlia was created.
Heidi Halbwirth and her team at Vienna University of Technology in Austria found that the black coloring came from throwing off the balance of anthocyanins and flavones. In a study published in the November 23 BMC Plant Biology, the researcher found that the black flowers had much higher concentrations of anthocyanins (as much as 10 fold over bright red varieties), and much lower concentrations of flavones (by about the same ratio as anthocyanins). These concentrations were drastically different even from red varieties, which also are produced by anthocyanins.
This anthocyanin-heavy black dahlias got that way because of reduced activity of a key enzyme, FNS II (short for flavone synthase), that usually makes flavones. With low levels of these enzyme, lower concentrations of flavones can’t compete with anthocyanins for the building block molecules they share; hence, more anthocyanins and the black color. The group also found that once black dahlia’s petals get older, they tend to turn redder, pointing further to anthocyanin’s role in the flower’s blackness.
While anthocyanins and other flavonoids contain powerful antioxidant properties (the same flavonoids are found in berries and other brightly color flowers). These antioxidants can fend off prey and diseases, both in plants and animals that consume them. Using genetic engineering techniques, breeders could create plants that contain tailored amounts of flavonoids, which could help in nutrition (for edible plants) as well as in gardening and breeding flowers.
Source: BioMed Central
Thill J, Miosic S, Ahmed R, Schlangen K, Muster G, Stich K, & Halbwirth H (2012). ‘Le Rouge et le Noir’: A decline in flavone formation correlates with the rare color of black dahlia (Dahlia variabilis hort.) flowers. BMC plant biology, 12 (1) PMID: 23176321