Surprising blue halo
Some flowers produce a blue halo on their petals to attract pollinators
Acquired and controlled through evolution, a degree of “disorder” in the nanometric structure of the petals of many flowers allows for more efficient pollination.
Researchers have discovered that some flowers have developed an additional strategy to encourage insects to forage on them. An ultraviolet halo effectively attracts pollinators.
Image: Ursinia speciosa, like other flower species, produces a blue halo on its petals to attract pollinators. © Edwige Moyroud
To attract insect pollinators, many flowers from different species have developed a means that is both remarkable and effective: a kind of “blue halo” that is invisible to the human eye—except when the flower has dark pigmentation—but helps insects to locate them. This phenomenon, described in an article published on 18 October 2017 in the prestigious journal Nature, is produced by fine ridges on the petals that scatter light. These nanometre-scale grooves exhibit a certain degree of “disorder,” meaning that their size and arrangement vary within a single flower and from one flower to another. The researchers who conducted the study nevertheless found that all of them displayed a similar level of “disorder” and produced halos with colours ranging from ultraviolet to blue.
A form of disorder “mastered” by evolution
To determine the effect of these halos on insects, the researchers created artificial surfaces, some producing “blue halos” and others not, and studied the behaviour of bumblebees. The latter detected surfaces with a blue halo more easily than those without.
According to the researchers, the “disordered” nanometre-scale structures of petals likely evolved differently among flowering plant species but, through convergent evolution, ultimately led to the same ability to produce halos visible to pollinators.
| “We had always considered that the disorder observed on petal surfaces was merely accidental—that flowers could not do any better,” said the study’s lead author, Beverley Glover of the Department of Botany at the University of Cambridge (United Kingdom), in a statement. “Discovering that the disorder itself is responsible for the important optical signal that allows bumblebees to find flowers more efficiently was a real surprise,” he adds. “The disorder we observe in the nanostructure of petals appears to have been shaped by evolution and to have resulted in improved communication between flowers and bees.” |
Striations observed under an electron microscope on a petal of Ursinia speciosa. Credit: Tobias Wenzel |
A phenomenon that is not essential to pollination
Previous studies had shown that many pollinator species are attracted to blue-violet colours. However, “many plants do not have the genetic or biochemical capacity” to produce blue pigments, notes another author, Silvia Vignolini of the Department of Chemistry at the University of Cambridge. Their ability to develop a blue luminous halo “is another way of producing signals that attract insects,” but it is not essential for pollination.
Source: https://www.nature.com/articles/nature24285 and https://www.sciencesetavenir.fr
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