Flowers change colour and back again to advertise their opening hours

Many living things, from chameleons to fish to squid, have the ability to change their colour. But flowers? Yes, over 450 species of flower have the ability to shapeshift, altering their colour and positions over the course of a day. The goal, as with many aspects of a flower's nature, is communication. The secondary palette tells pollinators that a particular flower has already been visited and not only needs no pollen but has little nectar to offer as a reward. The visitor's attentions (and the pollen it carries) are directed towards needier flowers.

The legume Desmodium setigerum is one of these colour-changers. Its small flowers, just a centimetre across, last for just a day and start off with a lilac hue. When pollinating bees land on the flower, their weight "trips" one of the petals and explosively reveals the flower's reproductive parts.

After these visits, the flowers' top petal falls down, obscuring the anthers and stamen, and the petals transform from lilac to white and turquoise. The whole process takes less than two hours. The move to turquoise happens naturally with age but visits from bees greatly speed up the process.

But this change works both ways. Pat Willmer from the University of St Andrews has found that D.setigenrum can reverse it transformation if it hasn't received enough pollen from its visitor. Like shopkeepers flipping their "CLOSED" signs to "OPEN", the flowers advertise themselves as back for business by once again shifting to a lilac colour. It gives them a second chance at being pollinated.

Willmer spent two months studying the flower in Uganda's Kibale National Park and noticed that some flowers managed to reverse their turquoise transformations, either returning to lilac or darkening to a stronger turquoise. In all cases, the drooping top petal lifted away to once again reveal the stigma.

The plants' stigmas revealed the reason behind these reversals. All flowers changed colour and closed down after bee visits, but those that had received little or no pollen were much more likely to reopen, while those that got their fair share stayed shut. Reopening increases their odds of actually getting a decent dusting of pollen, since bees virtually only ever visit lilac flowers.

Willmer believes that this ability to repeal a colour change is unique to flowers. How it does it is unclear, but as pollen tubes erupt from pollen grains, plants produce various hormones that could potentially launch the right biochemical changes. In another flower, Viola cornuta, the hormones ethylene and gibberellic acid probably switch on genes that produce purple pigments called anthocyanins, which change the flower's hue from white to purple. Without the growth of pollen tubes, low hormone levels could drive the opposite change.

Reference: Willmer, P., Stanley, D., Steijven, K., Matthews, I., & Nuttman, C. (2009). Bidirectional Flower Color and Shape Changes Allow a Second Opportunity for Pollination Current Biology, 19 (11), 919-923 DOI: 10.1016/j.cub.2009.03.070

More on flowers:

• Bumpy petals help bees get a grip on flowers
• Of flowers and pollinators - a case study of punctuated evolution
• Orchid lures in pollinating wasps with promise of fresh meat
• Extra genomes helped plants to survive extinction event that killed dinosaurs