Format: ORAL

Casper J. van der Kooi

University of Groningen, The Netherlands

Flower colour is a key enabler for plants to attract animal pollinators. The colours of flowers are generated by two basic optical principles: wavelength-specific absorption of light by floral pigments and reflection/scattering of incident light by floral structures (e.g., cell walls, starch granules, epidermal surface). We study the evolution of flower colouration using optics, anatomical methods and behavioural experiments with insects. Sampling different Angiosperm groups that are characterised by independent transitions between pollinators with different visual systems, we found that the optical properties of flowers evolve to maximise visibility to pollinators in four complementary ways. First, thetypeof pigment, which determines a flowers overall hue, is tuned to the spectral sensitivity of pollinators. For example, reflection of ultraviolet, blue, yellow or red light is linked to whether the prime pollinator can perceive this colour. Second, theamountof pigment, which determines the spectral filtering of the scattered light, is optimised to yield high colour contrast. Low pigment amounts yield pale colours, intermediate amounts vivid colours, and high amounts dull colours. Third, theamount of scattering (brightness) of flowers is determined by the number and inhomogeneity of cell layers. Flower brightness increases upon a switch from diurnal to nocturnal pollination, and this is caused by modifications of both flower thickness and cell structure. Fourth, the epidermal surface determines the glossiness (specularity) of a flower. In specific taxa, such as sexually deceptive orchids and buttercups, very flat and specular surfaces create a brilliant visual effect. Behavioural experiments confirm that bees use surface gloss as a visual cue. In most species, however, cone-shaped epidermal cells annihilate surface gloss and so increase the absorption by pigments and enhance a flowers visibility. Together, our findings show that floral visual signals converge via different, complementary optical mechanisms to maximise visibility to their pollinators.