A visit to another new reserve to me last weekend, Essex Wildlife Trust’s Warley place, set me thinking. Why do so many flowers of the borage family, the Boraginaceae change colour from pink to blue as they age? I noticed it in lungwort, Pulmonaria officinalis, at Warley place but have seen it in lots of other members of the plant family in disparate corners of the world; from Pseudomertensia in the Himalayas, to Echium both here and in Cyprus.
The colours of many flowers, including these, are caused by anthocyanin pigments produced expressly for the purpose of attracting insect pollinators. In the case of Pulmonaria, this means mainly solitary bees and bumblebees. It seems that the flowers change colour as they age to signal to potential pollinators that they may be running short of nectar, though the colour change is totally dependent on flower age and not triggered by the number of insect visits, pollination or fertilisation. Pollinators prefer the red flowers but keeping the older, now blue, flowers makes the display of flowers larger and easier to spot from a distance. Changing flower colour is a very efficient way of maximising the usefulness of each insect visit as, once the pollinators arrive, they will preferentially choose the red flowers with their nectar reward, fertile stigma and pollen ready to be transferred (Oberrath & Böhning-Gaese, 1999).
So, the reason for the colour change seems clear. What is less clear is how it is achieved. Anthocyanins are located in the fluid filled vacuoles which occupy much of the space inside most plant cells and their colour is known to change in response to acidity or alkalinity. Anthocyanins are red at lower, more acid pH vales and blue at higher, alkaline ones. In fact, the change is so obvious that you can make a simple pH indicator solution from red cabbage juice. It seems that the pH of the cell vacuoles must increase as the petals age and that this produces slight changes in the structure of the anthocyanin pigments, which in turn affects how these interact with light. It’s not surprising, this being the case, that the veins in the corolla are the last to change colour.
However the Boraginaceae are not the only plant family to change flower colour with age. The corolla (petal tube) of the common garden shrub in the honeysuckle family, Weigela japonica var. sinica, changes from white to red and the inner nectar guides change from yellow to purple over the four days the flower is open. In this case it seems that retaining the older flowers also offers a measure of protection to pollen tubes growing down the style to fertilise ovules (Zhang et al., 2012). Maybe the best known of all colour-changers is Lantana camara, a native of tropical Central and South America in the verbena family, often grown elsewhere as an ornamental. The flowers usually start off yellow and then become more orange and red as they age but generations of breeding have led to many colour variations.
Most people will also be familiar with the colour change of Morning glory, Ipomea purpurea, another Central American native naturalised all over the world, this time in the Convolvulaceae or bindweed family. It does exactly what it says on the can – beautiful large blue flowers which open first thing in the morning but fade to pink and wither by the middle of the day. Insect pollinators have only a brief chance to claim the nectar reward, signalled so boldly.
One thing all these plant families have in common is that the petals are fused along most of their length to form a tube – the corolla. Maybe this allows the plant to synchronise colour change better than if the petals were separate?
One plant family with more separate petals where there is colour change is the Primulaceae, though this time it is only the inner ‘eye’ which changes colour, from yellow to red in the case of Himalayan Rock jasmine, Androsace delavayi (below). Here the colour change is a signal that pollination has taken place as well, presumably, as that the flower is no longer producing a nectar reward.
Oberrath, R. & Böhning-Gaese, K. (1999) Floral Color Change and the Attraction of Insect Pollinators in Lungwort (Pulmonaria collina). Oecologia, 121, pp. 383-391.
Zhang, Y.W. et al. (2012) Temporal pattern of floral color change and time retention of post-change flowers in Weigela japonica var. sinica (Caprifoliaceae). Journal of Systematics and Evolution, 50,pp. 519-526