Plants light trick to look blue to bees

18 October 2017
Posted by Chris Smith.

Plants use a trick of the light to make themselves look blue to bees, UK scientists have found.

Unlike humans, which dress up with the aim of attracting a mate, plants need to pretty themselves up to pull in pollinators. Their aim is to swap genes with other members of their species so that they can successfully set seeds.

Many have developed attractive blooms laced with a sugary treat intended to appeal to insects like bees. But the quandary for Cambridge plant scientist Beverley Glover and her colleagues was that insects, like bees, see best at the blue end of the spectrum. And yet blue is a very costly colour for many plants to make, which is why blue flowers tend to be rare.

Then the team noticed, when they looked at several unrelated plant species, that some plant petals have an iridescent effect, like a reflection from the surface of a CD, when light falls on them. "And then we realised that they also scatter blue light off the petal surfaces to produce a blue halo around the flowers," says Glover. "And when we investigated we found that cells on the surfaces of the petals have a finely wrinkled surface. The wrinkles are the same size as the waves of blue light, which is why the blue colour gets scattered off the petal surface."

Wondering whether the observation was a one-off in the couple of species they'd looked at, the Cambridge team then worked their way through the diverse collection of plant species at the Cambridge University Botanic Gardens, and also looked at specimens from Kew. Many of the flowers they studied were also endowed with cells in their petals capable of scattering blue light in this way. But although the end result was always the same - the appearance of a subtle blue halo around the flower - the individual shapes and structures of the cells responsible were different.

"That told us that all these different plant species had independently evolved this trait, because they were all doing it slightly differently."

The question is why. Glover and her colleagues suspected that, because blue is hard for a flower to make, perhaps they were using this trick of the light from their petals to make their flower look blue to a bee and thereby boost their pollination prospects.

"So then we did the experiment," she explains. They made mock flowers with surfaces etched at the nanoscale in an identical pattern to the wrinkles in the petal surfaces so that they produced the same blue halo seen around real blooms. These fake flowers were charged either with a sweet sugary treat, or a bitter quinine-laced nectar. Bees unleashed on the phony flowers learned to avoid the ones laced with quinine, or to target only the sugar-loaded examples, proving that they could clearly see the blue effect. They were also much more time efficient at visiting flowers when they were marked up with the blue halo effect.

"We've not yet proved in the wild, with real flowers, that producing this blue halo makes the flowers more attractive to bees," acknowledges Glover, "but we might be able to set up some experiments with mutant plants that either do or do not make the halo effect so we can test that in future."

All the same, on the basis of the paper the team have published this week in Nature, it looks like plants ingeniously forego the cost of producing blue flowers by using a trick of the light to bling themselves up for bees...

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