Vienna, Nov 25 (IANS): Scientists have now decoded why some dahlias are black, a rarity, after analyzing as to why the plant displays varying hues, from white to yellow to red to purple.

The distinctive black-red colour is based on an increased accumulation of anthocyanins, a class of flavonoids that imparts a reddish hue to many fruits, vegetables grains and flowers, said researchers from the Vienna University of Technology, Austria.

Continuous dahlia breeding worldwide has led to the availability of 20,000 varieties - many of them showing red hues. However, black hued dahlia occur rarely, in comparison, the journal BMC Plant Biology reports.

Colour in dahlias is exclusively based on the accumulation of flavonoids, which includes anthocyanins, flavones and flavonols. It's known that red tones arise from anthocyanins, according to a Vienna statement.

Flavones and flavonoids are colourless, but they influence colouration by acting as co-pigments. The team from the Vienna used pigment, enzyme and gene expression analyses. They determined that the majority of black varieties have very low concentrations of flavones.

Heidi Halbwirth, from who led the study, emphasised that the black colour of dahlias is not due to increased activity of the anthocyanin, but rather is the result of the intermediates being converted into anthocyanins at the expense of formation of flavones.