Surprising absence of association between flower surface microstructure and pollination system.

M Kraaij,C J Van Der Kooi,Z.-X Ren

doi:10.1111/plb.13071

Abstract

The epidermal cells of flowers come in different shapes and have different functions, but how they evolved remains largely unknown. Floral micro‐texture can provide tactile cues to insects, and increases in surface roughness by means of conical (papillose) epidermal cells may facilitate flower handling by landing insect pollinators. Whether flower microstructure correlates with pollination system remains unknown.Here, we investigate the floral epidermal microstructure in 29 (congeneric) species pairs with contrasting pollination system. We test whether flowers pollinated by bees and/or flies feature more structured, rougher surfaces than flowers pollinated by non‐landing moths or birds and flowers that self‐pollinate.In contrast with earlier studies, we find no correlation between epidermal microstructure and pollination system. The shape, cell height and roughness of floral epidermal cells varies among species, but is not correlated with pollinators at large. Intriguingly, however, we find that the upper (adaxial) flower surface that surrounds the reproductive organs and often constitutes the floral display is markedly more structured than the lower (abaxial) surface.We thus conclude that conical epidermal cells probably play a role in plant reproduction other than providing grip or tactile cues, such as increasing hydrophobicity or enhancing the visual signal.

Highlights

The variety in shape and structure of flower surfaces and their function in terms of interactions between plants and insects has intrigued scientists for decades (e.g. Kay et al 1981; Lee 2007; Whitney et al 2011; Papiorek et al 2014; van der Kooi et al 2014; Ojeda et al 2016)
In contrast with earlier studies, we find no correlation between epidermal microstructure and pollination system
Flower surface does not correlate with pollination system

Summary

Introduction

The variety in shape and structure of flower surfaces and their function in terms of interactions between plants and insects has intrigued scientists for decades (e.g. Kay et al 1981; Lee 2007; Whitney et al 2011; Papiorek et al 2014; van der Kooi et al 2014; Ojeda et al 2016). There are three types of epidermal cell shapes in flowers, distinguished by their shape. Flat cells are the least common type, but are characteristic of flowers in the buttercup genera Ranunculus and Ficaria (Ranunculaceae), contributing to their glossy appearance (Kay et al 1981; Vignolini et al 2012; van der Kooi et al 2017). The epidermal cells of leaves and stems are generally flat, why do flowers of so many species have cone-shaped epidermal cells?. Modelling studies suggested that under perpendicular illumination, conical epidermal cells could act as micro-lenses that focus incident light on the floral pigments (Gorton & Vogelmann 1996; Wilts et al 2018), resulting in reflected light being more strongly filtered by pigments and generating stronger flower coloration. Experimental studies with bees showed that presence or absence of cones does not change the flower’s salience as perceived by bees (Dyer et al 2007)

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