Flowers use ‘blue halo’ optical trick to attract bees, say researchers
Flowers might seem like one of life’s simple pleasures, but it turns out there might be more to them than meets the eye.
Researchers have discovered that certain species of flowering plants boast tiny ridges on their petals that, thanks to variations in their height or spacing, scatter light to cast a blueish hue over the blooms.
While the effect is not always visible to humans, it can be spotted by bees – suggesting the optical effect might help to attract the pollinators.
“The exciting thing is that it is a new optical trick – we didn’t know that flowers could use disorder to generate a specific colour, and that is quite clever,” said Professor Beverley Glover, co-author of the research from the University of Cambridge.
The discovery has its roots in previous work from Glover’s team which revealed that tiny ridges on the petals of certain plants could diffract, or bend, light, giving them an iridescent sheen.
Writing in the journal Nature, the team reveal how they sought to explore the effect further to see if it occurred in other species with similar big, flat petals.
To do so they examined the petals of 12 species from across the flowering plant family tree, ranging from golden bartonia (Mentzelia lindleyi) to the dark tulip known as “Queen of the Night”, using a variety of microscopy techniques. For the 11 species for which fresh flowers were available, the team also looked at how they scatter and diffract light.
The results reveal that the architecture of the nanostructures differed from species to species, with some showing variations in the spacings of their ridges while others showed variations in their heights.
The researchers let bumblebees lose on ‘artificial flowers’. Photograph: Edwige Moyroud/AP
Despite these differences, all of the flowers were found to have a weak iridescent sheen. But the team stumbled upon an additional quirk: the ridges also scattered blue and ultraviolet light. The upshot is that all the flowers appeared to have a “blue halo” – an effect that can be seen by the human eye for flowers with dark pigments.
Further work using computer models revealed that the blue halo depends not on the specific architecture of the tiny ridges, but on their level of disorder – in other words the degree of variation in their heights or spacings.
To explore the impact of the phenomenon, the team constructed a series of small squares with different coloured pigments and surface textures – while one type had a smooth surface, another had ridges that gave rise to an iridescent sheen, and a third type had ridges that produced a blue halo.
The researchers then let bumblebees lose on these “artificial flowers”, with each bee in turn presented with two types of black square, one of which was coupled with a sweet solution and the other with a bitter-tasting one.
The results show that the bees were able to learn which square to visit for a tasty treat even though the squares were rearranged after each drink, suggesting bees can see the optical effects generated by the surface textures – and can spot the blue halo even without the presence of an iridescent sheen.
The bees even managed the feat when the “flowers” were made with a yellow pigment – a situation in which the blue halo is very tricky for humans to spot. What’s more, bees were about a third quicker at travelling between squares with a blue halo than smooth squares, suggesting the optical trick improved their efficiency at foraging, putting it on a par with speeds for blue-pigmented flowers.
The researchers say that presence of tiny ridges on the petals of species from across the family tree of flowering plants suggests these nanostructures evolved independently, possibly as a way of attracting pollinators by tapping into the ability of bees to see blue and UV light.
“I think it is just another way to make ‘blue’ – it is quite hard to make blue with chemical pigments, so maybe lots of flowers have come up with this other way,” said Glover. “Even though it is not really visible to my eye, it is a trick of the light that works pretty well for the bees, because they can see those flowers really well.”
Original post from The Guardian
Publication date: Oct-18-2017