The Acid Test (P4P*)

A pink-flowered Hydrangea in the Royal Botanic Gardens, presumably responding to a shortage of aluminium: Photo Jaime Plaza

Most gardeners know that increasing the acidity of soil can change the colour of some hydrangea flowers from pink to blue. It doesn’t always work though, and that’s because there is more to it than acidity.

Yet at a cellular level, flower colour in some plants is controlled simply by the quantity of pigments and the pH (acidity) at which they are stored. Different plant species produce different kinds of pigments, with pinks and blues mostly due to a group called ‘anthocyanins’.

These anthocyanins are stored in storage compartments in the cells called vacuoles. They slosh around in an acidic solution inside these compartments. If the acidity of this solution changes – which can be caused by a genetic mutation effecting pH regulation – the colour can change.

For example, just recently (Science 5 Dec 08) variants of petunia were bred (by fiddling with genes in a laboratory) with a less acidic vacuole solution. Even though the pigments (the anthocyanins) remain in the same concentration, this change in acidity led to a change in colour – in this case from pink to a kind of blue-purple which is not found in nature.

But back to hydrangeas, and changes to acidity on a coarser scale. The flowers of most larger-leaved hydrangeas (e.g. Hydrangea macrophylla) are blue or pink, depending on whether aluminium is available in the soil. If yes, the flower is blue. If no, the flower is pink.

The intensity of the colour depends on both the amount of aluminium available in the soil, and the amount of pigment in the flower (if there is no pigment, the flower is white). Often gardeners adjust the acidity of the soil to change the colour of hydrangea flowers. This is because aluminium is more available to the plant in lower pH (i.e. more acidic, below ‘6.5’) soils. If there is plenty of aluminium in the soil anyway, acidity might not make any difference.

Typically a gardener will add lime to the soil to make it more alkaline, leading to less available aluminium, and all other things being equal, pink flowers. But if the soil is packed full or aluminium or naturally acidic you may kill the plant (e.g. from a shortage of available iron) before it produces pink flowers! If you want blue flowers, adding aluminium sulphate (sulfate) to the soil should do the trick.

Why aluminium and not just acidity like the petunia? In the case of the hydrangea, the aluminium binds directly with the anthocyanin to create a blue coloured compound. Inside the Petunia flower the anthocyanin is changing colour due to the direct effects of acidity, but that’s beyond what we can cover on a Saturday morning – and I’d have to do a lot more homework.

Not all hydrangeas respond to fiddling with the soil chemistry. For example the oak-leaved hydrangea – Hydrangea quercifolia – has white flowers, with a tinge of pink. Nothing you can do to these flowers in a garden setting will give them even the least flush of blue.

Maybe the vacuole in these petals is too well buffered, or maybe the pigments are of a kind that is not influenced by acidity. In any case, you won’t be able to change their colour with a hand full of lime or ammonium sulphate.

*This posting will also appear on the ABC Sydney website (possibly under 'gardening'), and is the gist of my radio interview with Simon Marnie on Saturday Morning sometime between 9-10 am on 702AM.