Fluorescent nectar in non-flying mammal-pollinated plants – observations and considerations in some Asparagaceae

Background and aims – Fluorescence is the emission of light by a fluorophore that has absorbed light of shorter wavelengths. While the role of fluorescence in visual communication has been documented in some animals (budgerigars, gelatinous zooplankton), it is controversially discussed in plants. Floral nectar fluorescence has been mainly found in flowers pollinated by bees. It has been suggested as direct visual cue by which bees can evaluate the available quantity of nectar, thus being important for pollination and foraging efficiency. However, this function has been questioned, since fluorescence is said to be obscured by floral reflections due to low quantum efficiency. The aim of this study was to examine the nectar of plants pollinated by non-flying mammals, namely Eucomis regia , Massonia grandiflora , M. echinata , and M. pustulata (Asparagaceae) from South Africa. Material and methods – To detect possible fluorescence in flowers, the plants were illuminated in a darkened room under UV light and photographed with a camera equipped with a UV/IR cut filter (transmitting at 400–700 nm). Key results – Within the inflorescences, the nectar of all species showed blue to bluish fluorescence and UV absorption. Separated nectar also fluoresced. Conclusion – As fluorescence in flowers occurs not only in bee-pollinated plants but also in plants pollinated by wind, and by nocturnal or crepuscular pollinators (non-flying mammals, bats, moths) for which floral scent is an important attractant, floral fluorescence seems to have no adaptive value for the attraction of flower visitors. We discuss the potential role of fluorescence in flowers as just a by-product of compounds that might have other functions such as visual attraction by reflection (or UV absorbance), protection of genetic material in pollen from UV induced damage, or as a floral filter causing nectar to be bitter, repelling ineffective pollinators but not effective ones.