23 October 2017

Plant colours are not all about pigments

Kew Scientist Paula Rudall reflects on a long-term Cambridge-Kew collaboration on why structural colour in plants is so important in the natural world, including helping birds and bees to find food and pollinate flowers.

A colourful wildflower meadow

Photonic flashes in both plants and animals

It has long been known that some bright colours in animals – such as the multi-coloured flash of a butterfly wing or the dazzling iridescent glint of a bird feather – are the result of physical structure rather than chemical pigments. Such structures are termed 'photonic', meaning that they have optical properties. For example, the intense blue colour of some butterflies is produced by light reflecting off multiple outer layers of the insect’s wing. The primary functions of such colour features in animals are for signalling, mimicry and mating.

Plants can also have photonic colouring that attracts animals such as birds and bees, either to help them find and disperse seeds or distribute pollen between flowers. The colours of some brightly coloured attractive fruits and seeds are caused by a special helicoidal pattern of multi-layered thickening of the outer cell walls. This type of structure was recently described in the fruits of two flowering plants, the genus Pollia and genus Margaritaria, as part of a long-term collaboration between researchers at the University of Cambridge and Royal Botanic Gardens, Kew. The intense blue and green colours of the fruits can even be observed in preserved specimens from the Kew Herbarium, some dating back more than forty years. In these cases, the colour is not caused by pigment but rather by structure: light is reflected by the helicoid structure that exists in the cell walls of the growing fruit.

Jar of blue fruits in spirit
Pollia condensata, fruits collected in Ethiopia in 1974 and preserved in spirit in Kew's Herbarium. Despite their age, the fruits still display an intense blue colour that is caused by helicoidal thickenings in the outer cells walls ©RBG Kew.

Flowers with enhanced colour can attract more bees

In addition to fruits and seeds, Cambridge and Kew are also studying colour in flowers. Flowers of many species need to attract potential airborne pollinators such as insects and birds, and structure is important in achieving this goal. The structures of plants that create colour often interact with chemical signals such as pigments and scents to attract pollinators. Many flowers also provide a nectar reward, in a belt-and-braces approach to pollinator attraction. At a larger (macroscopic) level, flowers of some species such as the buttercup (Ranunculus repens) are shaped like a parabolic dish, focusing light and heat from the sun, thereby enticing insects into their transient embrace. Not only are petal surfaces attractive, but they can also be self-cleaning: in some cases, the surface is so smooth that drops of water roll off it. 

At the microscopic level, many interacting chemical and physical forces operate to attract pollinators to the flower. Petal colour is determined primarily by pigments, but the Cambridge-Kew collaboration has demonstrated several different types of structures that enhance colour even further. For example, flat outer cells of flower petals can result in a glossy surface that has a mirror-like effect, such as the widespread Mediterranean mirror orchid (Ophrys speculum) or buttercup. 

Many flowering plants, including the much-studied snapdragon (Antirrhinum majus), have conical cells on the petal surface that help to scatter reflected light. Parallel striations of the cuticle occur on the petals of a wide range of flowering plants, and can generate an iridescent signal. There can even be variation across the flower; for example, in Hibiscus trionum, the white region of the petal consists of conical-shaped cells whereas the red region has flat cells with parallel striations that make this region weakly iridescent. Experiments in the Cambridge lab have shown that bumblebees can use these enhanced optical effects and signals to identify food. 

Left is the flower of a mirror orchid; right is part of the flower under scanning electron microscope
Left: Flower of the Mediterranean mirror orchid (Ophrys speculum) ©R.M Bateman/RBG Kew. . Right: Scanning Electron Micrograph of part of flower showing the flat cells of the glossy speculum, which have a mirror-like effect.

Nature’s disorder

The pattern on the surface of flower petals is never quite as perfectly regular as a machine-generated structure because some degree of disorder is inevitable in nature. To what extent does this disorder affect the ability of flower petals to signal with colour and light? The latest chapter in this ongoing story has recently been published in the journal Nature (Moyroud et al., 2017). Remarkably, Kew and the University of Cambridge’s combined studies show that some disorder in structural colour is not harmful and can even be functional in many flowers, producing a halo effect that often appears blue to the human eye. Studies of insects in the Cambridge lab have shown that foraging bumblebees actually respond to this blue-halo effect, helping them to find food while also helping the plants to be pollinated. This research demonstrates that not only is the special structural colour of flowers, seeds and fruits important to both plants and animals, but that some disorder can be additionally advantageous in the complex interplay between physical forces and living species. 

Buttercup flower plus images of the flower under the scanning electron microscope
Buttercup (Ranunculus repens). Top left: Flower. Right: Scanning Electron Micrograph of entire petal showing the smooth surface. Bottom left: Detail of petal surface. © RBG Kew.

Further reading

Kay, Q.O.N, Daoud, H.S., Stirton C.H. (1981). Pigment distribution, light reflection and cell structure in petals. Botanical Journal of the Linnean Society 83: 57–84.

Whitney HM, Bennett KMV, Dorling M, Sandbach L, Prince D, Chittka L, Glover BJ. (2011). Why do so many petals have conical epidermal cells? Annals of Botany 108: 609–616.

Paula Rudall and collaborators have published an ongoing series of papers on structural colour:

Moyroud, E., Wenzel, T., Middleton, R., Rudall, P.J., Banks, H., Reed, A., Mellers, G., Killoran, P., Thomas, M.M., Steiner, U., Vignolini, S., Glover, B.J. (2017). Disorder in convergent floral nanostructures enhances signalling to bees. Nature DOI: 10.1038/nature24285. Available online

Vignolini, S., Rudall, P.J., Rowland, A.V., Reed, A., Moyroud, E., Faden, R.B., Baumberg, J.J., Glover, B.J., Steiner, U. (2012). Pointillist structural color in Pollia fruit. Proceedings of the National Academy of Sciences 109: 15712–15715. Available online

Vignolini, S., Davey, M.P., Bateman, R.M., Rudall, P.J., Moyroud, E., Tratt, J., Malmgren, S., Steiner, U., Glover, B.J. (2012). The mirror crack’d: both structure and pigment contribute to the intense blue colour of the labellum of Ophrys speculum. New Phytologist 196: 1038–1047. Available online 

Vignolini, S., Thomas, M.M., Kolle, M., Wenzel, T., Rowland, A., Rudall, P.J., Baumberg, J.J., Glover, B.J., Steiner, U. (2012). Directional scattering from the glossy flower of Ranunculus: how the buttercup lights up your chin. Journal of the Royal Society Interface 9: 1295–1301. Available online

Vignolini, S., Moyroud, E., Hingant, T., Banks, H., Rudall, P.J., Steiner, U., Glover, B.J. (2014). The flower of Hibiscus trionum is both visibly and measurably iridescent. New Phytologist 205: 97–101. DOI: 10.1111/nph.12958. Available online

Vignolini, S., Gregory, T., Kolle, M., Lethbridge, A., Moyroud, E., Steiner, U., Glover, B.J., Vukusic, P., Rudall, P.J. (2016). Structural colour from helicoidal cell-wall architecture in fruits of Margaritaria nobilisJournal of the Royal Society Interface 13: 20160645. DOI: 10.1098/rsif.2016.0645. Available online

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