Gloss, Colour and Grip: Multifunctional Epidermal Cell Shapes in Bee- and Bird-Pollinated Flowers

Abstract

Flowers bear the function of filters supporting the attraction of pollinators as well as the deterrence of floral antagonists. The effect of epidermal cell shape on the visual display and tactile properties of flowers has been evaluated only recently. In this study we quantitatively measured epidermal cell shape, gloss and spectral reflectance of flowers pollinated by either bees or birds testing three hypotheses: The first two hypotheses imply that bee-pollinated flowers might benefit from rough surfaces on visually-active parts produced by conical epidermal cells, as they may enhance the colour signal of flowers as well as the grip on flowers for bees. In contrast, bird-pollinated flowers might benefit from flat surfaces produced by flat epidermal cells, by avoiding frequent visitation from non-pollinating bees due to a reduced colour signal, as birds do not rely on specific colour parameters while foraging. Moreover, flat petal surfaces in bird-pollinated flowers may hamper grip for bees that do not touch anthers and stigmas while consuming nectar and thus, are considered as nectar thieves. Beside this, the third hypothesis implies that those flower parts which are vulnerable to nectar robbing of bee- as well as bird-pollinated flowers benefit from flat epidermal cells, hampering grip for nectar robbing bees. Our comparative data show in fact that conical epidermal cells are restricted to visually-active parts of bee-pollinated flowers, whereas robbing-sensitive parts of bee-pollinated as well as the entire floral surface of bird-pollinated flowers possess on average flat epidermal cells. However, direct correlations between epidermal cell shape and colour parameters have not been found. Our results together with published experimental studies show that epidermal cell shape as a largely neglected flower trait might act as an important feature in pollinator attraction and avoidance of antagonists, and thus may contribute to the partitioning of flower-visitors.

The physics of pollinator attraction

“…Having a smooth, lustrous surface is an efficient way of being conspicuous and gloss is another optical effect, produced by physical means, which has been described in flowers of multiple species (Fig. 1c) (Parkin, 1928;Gaisterer et al, 1999;Vignolini et al, 2012a,c;Whitney et al, 2012;Papiorek et al, 2014). Petal gloss can be defined as the specular reflection of light from the surface of the petal and this effect is maximized when the petal surface is flat (Gaisterer et al, 1999;Whitney et al, 2012).…”

“…Is this optical trick part of the flower's arsenal to attract pollinators? Conical cells are present on the petal epidermal surface of the vast majority of angiosperms, and generally on the section that is directly exposed to pollinators (Kay et al, 1981;Papiorek et al, 2014). Wild-type A. majus with conical cells receive more pollinator visits and produce more fruits than mutants with flat cells (Glover & Martin, 1998;Comba et al, 2000;Dyer et al, 2007) and bumblebees can perceive the difference in hue generated by cell shape (Dyer et al, 2007).…”

Functional optics of glossy buttercup flowers

“…[40]), but is very rare among flowers. In flowers of the vast majority of plant species, specular reflections are absent, because of the presence of conical epidermal cells and/or epidermal microstructures [5,20] that scatter light into a wide angular space [19]. When directionally illuminated, buttercup flowers display a strong gloss in addition to an overall matte-yellow colour ( figures 1 and 2).…”

“…Structural coloration is exceedingly rare among flowers, because the interiors and surfaces of flowers are very irregularly shaped and ordered [19,20]. In those plant species with flowers that do have structures with periodicity of the order of the wavelength of light, structural coloration and iridescence can only be observed by applying very local and directional & 2017 The Author(s) Published by the Royal Society.…”

Reply to Lavi & Sapir (2015): floral colour and pollinator‐mediated selection in Oncocyclus irises (Iridaceae)

“…Hopkins & Rausher, ; Sobel & Streisfeld, ) or other surface effects (such as epidermal cell shape and structure, see e.g. Papiorek et al ., ). Integrating knowledge of insect vision, including the patterns of photoreceptor excitation in trichromatic bees (the pollinators of these irises), as well as the effect of background colour on visual discrimination capabilities of different floral colours by bees (Arnold et al ., ) and results from other recent studies would have been beneficial to the discussion of the adaptive value of floral colours as perceived by the pollinators.…”