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.
Highlights
Plant-animal interactions include mutualistic as well as antagonistic relationships
A first survey of epidermal cell structure in angiosperms revealed that 79% of the investigated plant species show some form of papillate or conical epidermal cells [11]
The current study provides more differentiated results and demonstrates that the distribution of epidermal cell shape is largely explained by the effective pollinator as well as by their position on petals
Summary
Plant-animal interactions include mutualistic as well as antagonistic relationships. Animal pollination was traditionally regarded as mutualism including reciprocal benefits for both interaction partners. Flowers are interpreted as sensorial and/or morphological filters supporting the attraction of pollinators as well as the deterrence of floral antagonists such as herbivores, pollen and nectar robbers or thieves (reviewed in [1]) These interactions bear on different communication tasks, with colour as one of the most important floral features that structures the flower-visitor composition [2], [3]. The diversity of flower colours in angiosperms is mainly attributed to pigments deriving from different biochemical pathways, their combinations, variable concentrations as well as additional co-pigments, the prevalent pH in the vacuole, metal ions, pigment packaging and location within the tissue layers [4] To these factors, the petals’ epidermal cell structure affects the visual appearance of flowers [5], [6], [7], [8], [9]. The presence of conical epidermal cells in contrast to flat ones might alter colour impression for flower-visitors by enhancing light absorption by pigments in a yet unexplored manner [10]
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