Structural colour and iridescence in plants: the poorly studied relations of pigment colour

Abstract

Background: Colour is a consequence of the optical properties of an object and the visual system of the animal perceiving it. Colour is produced through chemical and structural means, but structural colour has been relatively poorly studied in plants.

Scope: This Botanical Briefing describes the mechanisms by which structures can produce colour. In plants, as in animals, the most common mechanisms are multilayers and diffraction gratings. The functions of structural colour are then discussed. In animals, these colours act primarily as signals between members of the same species, although they can also play roles in camouflaging animals from their predators. In plants, multilayers are found predominantly in shade-plant leaves, suggesting a role either in photoprotection or in optimizing capture of photosynthetically active light. Diffraction gratings may be a surprisingly common feature of petals, and recent work has shown that they can be used by bees as cues to identify rewarding flowers.

Conclusions: Structural colour may be surprisingly frequent in the plant kingdom, playing important roles alongside pigment colour. Much remains to be discovered about its distribution, development and function.

Fig. 1.

Colour is a property of the light reflected by an object and the visual system of the animal observing it. If a flower reflects all wavelengths of light, it is perceived as white (top). If it absorbs all wavelengths then it appears black (centre). However, if it absorbs all wavelengths apart from one region of the spectrum, it has a colour. The flower shown in the bottom panel reflects red light. To the vertebrate eye, which has red-light receptors, the flower appears red. However, to the bee eye, which has no red-light receptors but whose green-light receptors are weakly stimulated by red light, the flower appears a dull green.

Fig. 2.

Structural colour and iridescence. (A) The intense blue colour of the Morpho butterfly is due to reflection of light by multilayers. (B) Multilayers generate iridescence by reflecting different wavelengths of light at different angles at each boundary between layers. (C) Diffraction gratings consist of ordered parallel grooves at particular frequencies, like the cuticular striations on this tulip petal. (D) An iridescent beetle (rose chafer, Cetonia aurata) visits an artichoke flower.

Fig. 3.

Plant iridescence. (A) The inner part of the Hibiscus trionum petal has an oily iridescence overlying red pigmentation. (B) Scanning electron microscopy of this region shows that the cells overlying the red pigment are covered with a diffraction grating made from cuticular striations, although the cells over the white region are smooth. (C) When petal diffraction gratings are replicated in transparent optical epoxy, light reflected from the epoxy is not white but shows a range of colours. (D) The iridescent labellum of Ophrys speculum is thought to mimic the wings of female pollinators. (E) Mentzelia lindleyi is iridescent as a result of diffraction gratings, but the iridescence is only detectable in the bee-visible UV region of the spectrum.