Bees, flowers and UV

Ultraviolet light shining on flowers has various effects. In this review we assess functions of UV pigments and UV reflection patterns in flowers, including visual signalling by reflectance, fluorescence, and gloss, as well as protection against UV radiation. UV patterns originate from UV reflection and absorption in different floral parts and are visible to most pollinators, but invisible to humans. UV patterns can guide pollinators towards a floral reward, such as the centre-outward UV pattern, the so-called UV bull's eye. However, the diversity and complexity of floral colour patterns is much higher and may or may not include UV. For flower visitors, reflected UV light is merely a component of their colour vision rather than a UV signal processed separately. Yet, to humans it is a challenge to detect and represent UV reflectance in flowers. Advantages and limits of spectrophotometry, UV photography and false colour photography in bee view are discussed. Besides floral pigments causing absorption and fluorescence, flower signals can be produced by epidermal structures, i.e. smooth or conical epidermal cells, causing specular reflection (gloss) or refraction of light, and light-scattering structures causing reflection. Exposed nectar, pollen and stamens also display visual signals including UV. Finally, the absorption of UV light by pollen pigments protects the precious DNA inside the pollen grain from harmful UV radiation. UV-absorbing central parts on flowers also protect flower DNA by impeding the reflection of UV light from petals onto stamens and pollen. We briefly discuss how flower UV patterns may change in response to increasing global UV radiation, potentially influencing plant pollination.