What makes a purple pigment blue? The answer could lead to the elusive blue rose says Kumi Yoshida of Nagoya University, Japan

Anthocyanins are to be thanked for beautiful flower colours. These sugar-containing flavonoids differ from other plant pigments - such as green chlorophylls, yellow and orange carotenoids and purple betalains - by exhibiting a wider variety of colours. When anthocyanins are found in petals, dissolved in the petal cells' vacuoles (large sacs that make up over 90 per cent of the cells' volume), they are responsible for an assortment of reds, purples and blues.

Heavenly blue anthocyanin gives the petals of blue Morning glory their colour

Many years of research have focused on the development of flower colour, particularly how blues are created. Numerous efforts have been undertaken to resolve two major mysteries: how so few anthocyanin chromophores can produce so many colours and what makes these colours, usually unstable above pH 4, survive inside living cells. 

An anthocyanin solution can change colour with pH in a similar fashion to litmus. For example, a flavilium cation with a red colour may form in strongly acidic conditions, an anhydrobase with a purple colour form at neutral pH, and an anhydrobase anion with a blue colour form in alkaline conditions. Therefore, from an organic chemistry perspective, it is very simple to say that flower colour comes about because anthocyanin chromophores - anthocyanidins - change their structure depending on the pH. Creating a blue-coloured flower involves understanding how to stabilise the blue anhydrobase anion form in a weakly acidic to pH neutral plant vacuole.

Plants use four main strategies to bloom blue flowers: generating a more oxidised chromophore; increasing the vacuolar pH; complexing the anthocyanins with metals; and stacking aromatic groups with the anthocyanidin chromophore, shifting where it absorbs in the visible spectrum and so changing its colour. Usually two or more of these phenomena occur concurrently, resulting in a beautiful blue petal.

The most important technique in producing blue flower colouration involves the metalloanthocyanins - complexes of anthocyanins, flavones and metal ions which are found in blue dayflowers, cornflowers and salvias. These pigments use the metal-complexation and aromatic stacking strategies to generate their colour. For example, cornflowers contain the cyanidin chromophore, which is the same as in the red rose. They create blue colouration by binding the chromophore to paramagnetic Fe³⁺. The blue dayflower, on the other hand, also resorts to the oxidised chromophore strategy. It uses a delphinidin chromophore, which has one more hydroxyl group than the cornflower's cyanidin. When its components are mixed with Mg²⁺ ions, they rapidly produce a blue supramolecule. 

Blue hydrangea sepals and blue poppy petals are also coloured due to metal complexation - with Al³⁺ and Fe³⁺, respectively. But in addition they use non-stoichiometric amounts of flavonols as co-pigments, which work by molecular stacking. Blue morning glory also exploits the stacking strategy but in combination with increasing the vacuolar pH. The petal anthocyanin contains three p-coumaroyl groups, and the aromatic parts of these groups stack intramolecularly and stabilise the blue colour. The pH of the blue cells increases to 7.7 at the opened flower-stage, which is an unusually high pH value for a plant. The same mechanisms are found in blue gentians, delphiniums, butterfly pea flowers and others. Almost all these petals use the oxidation strategy but the pH effect is not yet determined.

So could the true blue rose be on the horizon? Its pursuit has persisted for decades. Molecular breeding techniques have led to advanced flower colour chemistry so that you can now see roses and carnations with bluer hues. But they are not yet truly blue in colour. For the true blue rose to be developed, a multilateral strategy is necessary. Clarification of petal-bluing mechanisms is a crucial step that should open the door to this long-sought flower. 

Read more in the review 'Blue flower color development by anthocyanins: from chemical structure to cell physiology' in Natural Product Reports.