Is floral iridescence a biologically relevant cue in plant–pollinator signaling?

Flowers allure potential pollinators by displaying attractive colour patterns, which are generally created by their petal arrangement and pigmentation. In addition to the pigmentary coloration, in certain cases reflecting surface structures may contribute to the flowers’ appearance. For instance, a flower petal surface with periodic striations, acting as a grating reflector, can create an angle-dependent coloured reflection, i.e. iridescence. A striking demonstration of this phenomenon is reported by Vignolini et al. (2014). They show that with specific illumination of a Hibiscus trionum L. flower, the proximal part of the petals, where the surface is regularly striated, displays an iridescent, bluish shine in addition to the deep-red pigmentary colour. Inspired by a previous study by (largely) the same team on floral iridescence across various plant species, entitled ‘Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators’ (Whitney et al. 2009), we investigated the surface reflections of numerous plant species and confirmed the widespread presence of striations that have diffractive properties in controlled conditions. However, the iridescent signal vanished under natural illumination, and therefore we concluded that in no case did surface reflections add a noticeable iridescent signal to the pigmentary coloration that could be utilized by an insect pollinator in a biologically relevant scenario (van der Kooi et al. 2014). Here, we substantiate our previous statement that floral iridescence acting as signaling cue to pollinators is presently untenable by presenting photographs of a number of different flowers with striated epidermal cells and a new set of angle-dependent reflectance measurements