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BIOPHOTONICS

Consistent disorder

Structural colour is the result of the interference of light from various structures, such as multilayered materials, crystalline inclusions and surface diffraction gratings. While most are based on regular periodic structures, in biological organisms, structural disorder inevitably exists. A question that remains is whether disorder is evolutionary or a result of biological developmental processes.

Credit: Macmillan Publishers Ltd

A study by Silvia Vignolini and co-workers now reveals that disordered striations on the surface of flower petals have evolved to generate a photonic signature that is attractive to insect pollinators (Nature 550, 469–474; 2017). Despite structural differences in terms of striation height, width and spacing, the photonic structures of the flowering plants examined have converged on a similar degree of disorder.

Vignolini and co-workers used differential conditioning to test the ability of bumblebees (Bombus terrestrial) to distinguish between disordered photonic structures fabricated via electron-beam lithography and those of natural flowers. The artificial structures had a similar scattering response to their natural counterparts. From the behavioural experiments, the researchers concluded that the floral nanostructures have converged on an optimized form that generates signals that are salient to insect pollinators. In particular, the researchers observed that the presence of the disordered striations contributed to the optical response of all the flowers in a similar way — a scattering response collected perpendicularly to the striations is enhanced in the ultraviolet–blue wavelength region of the spectrum, termed as the blue halo, on the petals that pollinators find attractive, and it is most intense between –25 °C and +25 °C.

“From the biological point of view, we discovered that all the flowers with ridges produced a similar blue halo, even though the specific details of ridge spacing, height and width were different in all the different flowers. We concluded that the blue halo had evolved independently many times in the plant kingdom, and that this convergent evolution suggested that it was very important in flower function,” Vignolini told Nature Photonics.

“Another finding that is general and more oriented to the optical community is that flowers are able to make, at room temperature and with biopolymers, simple one-dimensional structures that are capable of producing non-iridescent colourations. This will inspire scientists to produce non-fading coatings avoiding the use of toxic colourants,” Vignolini commented.

Based on both plant biology and optical materials science, the outcome of this study may provide a better understanding of the behaviour of bees and provide ideas for future optical device designs.

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Correspondence to Rachel Won.

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Won, R. Consistent disorder. Nature Photon 11, 747 (2017). https://doi.org/10.1038/s41566-017-0061-8

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