Infographic on the chemistry of dahlia flower colours. The colours of dahlias are a result of anthocyanin-, chalcone- and aurone-derived pigments. Colourless sflavones also interact with and stabilisse anthocyanin pigments. Chalcones and aurones lead to yellow and orange dahlias, while anthocyanins play a part in orange, pink, red and black dahlias. Blue dahlias aren't seen because dahlias lack an enzyme for making delphinidin, the anthocyanin which leads to blue flowers.
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Dahlias: the jewels of the late summer garden. Shades of red, yellow, orange, pink, with their petals forming intricate geometric structures. But, like roses and many other flowers, the dahlia spectrum is missing one colour: blue. So why are blue blooms so rare in nature?

Let’s start with what causes the dahlia colours we do see. These are due to a complex interplay of chemically related pigments: chalcones, aurones, and anthocyanins. These are all interlinked members of the huge range of compounds produced in plants. Reactions of chalcones can lead to both aurones and anthocyanins.

Compounds made from chalcones and aurones play a big part in yellow and orange dahlia flowers. Aurone derivatives give a more intense yellow colour than their chalcone cousins. In the orange flowers, another family of pigments can also contribute: the anthocyanins. Various sugar molecules can attach to the anthocyanins pelargonidin and cyanidin, with the resulting compounds contributing to oranges, reds and pinks.

Other, colourless compounds can also play a part: flavones. Though they themselves aren’t coloured, they can interact with and stabilise anthocyanin pigments. These pigments would otherwise be unstable to oxidation or other chemical reactions within the plant.

With these various pigments hanging out in their petals, you’d think blue flowers wouldn’t be too big a challenge. But a huge genetic roadblock stands in their way. Dahlias lack the gene for an enzyme called flavonoid 3′,5′-hydroxylase (F3′5′H for short). F3’5’H carries out a key step along the route to an anthocyanin called delphinidin – the key pigment for blue flowers. Without it, dahlias can’t get to these blue hues.

It’s not just dahlias that suffer from an inability to create blue flowers. Roses, chrysanthemums, and many other ornamental plants also lack the enzyme to produce delphinidin. So are you doomed to never see a blue rose or blue dahlia, save through the magic of Photoshop?

Well, naturally, yes, but researchers have found ways to interfere with pigment production in these flowers in an attempt to turn them blue. I say attempt because it’s not actually as simple as just making the plants produce delphinidin. That part is easy enough, relatively speaking, as the gene for the F3’5’H can be inserted through genetic engineering.

A Japanese company succeeded in doing so with roses in 2007, creating a ‘blue’ variety that accumulated delphinidin in the rose petals. The ability to produce and accumulate delphinidin could also be passed on to the roses bred from these roses – so far, so good. The reason that I’ve put ‘blue’ in inverted commas back there is because – well, take a look. It’s the mauve-est blue rose I’ve ever seen.

Turns out, the secret to blue flowers isn’t just getting them to produce delphinidin. Other factors can affect the colour of delphinidin even if it’s present: the flavone copigments we mentioned earlier, the acidity of the petal cells in which delphinidin builds up, and even the metal ions present in the petals can all have an influence. So, while these mauve roses are an impressive feat of genetic engineering, true blue roses (and dahlias) remain elusive.

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