Contributions of iridescence to floral patterning

Abstract

The Hibiscus trionum flower is distinctly patterned, with white petals each with a patch of red pigment at the base, producing a ‘bulls-eye’ pattern on the whole flower. The red pigmented patches are also iridescent, due to the presence of a series of overlying cuticular striations that act as a diffraction grating. We have previously reported that scanning electron microscopy revealed a sharply defined difference between the surface structure overlying the pigmented patch and that over the rest of the petal, with the diffraction grating only present over the pigmented region. Here we show that differences in petal surface structure overlie differences in pigment color in three other species, in a range of different patterns. Floral patterns have previously been shown to be advantageous in pollinator attraction, and we discuss whether emphasis of pigment patterns by structural color may increase floral recognition by pollinators.

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Acknowledgements

We would like to thank Bjørn Rørslett for his kind contribution of the ultraviolet image of Lathyrus pratensis, and Lars Chittka for helpful comments on the manuscript. Heather M. Whitney is in receipt of a Lloyd's of London Tercentenary foundation fellowship. Ruben Alvarez-Fernandez is in receipt of a fellowship from the Gobierno del Principado de Asturias (Spain), funded by Plan de Ciencia, Tecnologia e Innovacion (PCTI) of Asturias 2006–2009. This work was funded by Natural Environment Research Council grant NE/C000552/1, Engineering and Physical Sciences Research Council grant EP/D040884/1, the European RTN-6 Network Patterns, the Cambridge University Research Exchange, and German Academic Exchange Service DAAD.

Figures and Tables

Figure 1 Photographic floral images (both in visible and UV light) and corresponding scanning electron micrographs. (A and B) Hibiscus trionum flower showing ‘bulls-eye’ pattern. (B) SEM of Hibiscus trionum petal image showing the boundary region (B, scale bar = 50 µm) between the pigmented (striated) region and the white (conical) on the petal. (C) Ophrys apifera flower. (D–F) Nolana paradoxa flowers. (E) striated center white region (E, scale bar = 20 µm). (F) The outer blue region with conical cells (F, scale bar = 100 µm). (G–I) Tulipa humilis petal. (H) The dark inner center with no cuticular striations (H, scale bar = 10 µm). (I) the rest of the petal which has ordered striations (I, scale bar = 5 µm). (J–L) Lathyrus pratensis. (J) Lathyrus pratensis flowers in visible light. (K) Lathyrus pratensis flowers showing UV reflection. The nectar guides at the base of the flag petal are indicated with arrows. (L) SEM of flag showing unstriated region corresponding to nectar guides (base of flag indicted by arrow) (L, scale bar = 100 µm).

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