Shining a light on UV-fluorescent floral nectar after 50 years

Abstract

Nature is aglow with numerous captivating examples of UV-fluorescence in the animal kingdom. Despite a putative role as a visual signal, exploration of UV-fluorescence in plants and its role in plant-animal interactions is lagging in comparison. Almost 50 years ago, UV-fluorescence of floral nectar, a crucial reward for pollinators, was reported for 23 flowering plant species. Since this intriguing discovery, UV-fluorescent nectar has only seldom been addressed in the scientific literature and has not been scrutinized in a phylogenetic or ecological context. Here, we report the prevalence of vibrant UV-fluorescent floral nectar across the family Cleomaceae, including the first photographic documentation in vivo colour for flowering plants. Though Cleomaceae flowers are morphologically diverse varying in colour, nectary prominence, and nectar volume, UV-fluorescent floral nectar may be a ubiquitous characteristic of the family. Fluorescence spectra show that the identity and number of fluorescent compounds in floral nectar may differ among Cleomaceae species. As Cleomaceae pollinators range from insects to bats and birds, we suggest that the UV-fluorescent floral nectar not only functions as a visual cue for the diurnal pollinators but also for the nocturnal/crepuscular pollinators in low light settings.

Figure 1

UV-fluorescent floral nectar of Prunus species. (a) A spot of P. amygdalus nectar on filter paper under long- and short-wavelength UV radiation. From Thorp et al.. Reprinted with permission from AAAS. (b) A P. persica flower with nectar under UV radiation. From Radice and Galati. Reprinted with permission from SNCSC. n, nectar. Scale bar, 2.5 mm (b).

Figure 2

UV-fluorescent floral nectar of Cleome violacea and other Cleomaceae species. (a, b) Cleome violacea under white light (a) and UV-A radiation (b). (c) Close up of C. violacea flower under UV-A radiation. (d) Nectar of five Cleomaceae species and water in microcapillary tubes under white light (top) and UV-A radiation (bottom). Cv, Cleome violacea; Pd, Polanisia dodecandra; Th, Tarenaya houtteana; Sh, Sieruela hirta; Mg, Melidiscus giganteus. Scale bar, 1 cm (a, b).

Figure 3

UV-fluorescent floral nectar of Polanisia dodecandra and Tarenaya houtteana. (a, b) Polanisia dodecandra under white light (a) and UV-A radiation (b). (c, d) Tarenaya houtteana under white light (c) and UV-A radiation (d). Scale bars, 1 cm.

Figure 4

UV-fluorescent floral nectar of Sieruela hirta and Melidiscus giganteus. (a, b) Sieruela hirta under white light (a) and UV-A radiation (b). (c, d) Melidiscus giganteus under white light (c) and UV-A radiation (d). Arrowheads are pointing to the partially exposed nectar. Scale bars, 1 cm.

Figure 5

Fluorescence spectra for the floral nectar of five Cleomaceae species. Excitation spectra, dotted lines; emission spectra, solid lines. Colours correspond to the human perceived colours for the emission spectra maximum wavelengths. Cv, Cleome violacea; Pd, Polanisia dodecandra; Th, Tarenaya houtteana; Sh, Sieruela hirta; Mg, Melidiscus giganteus.

Figure 6

Phylogenetic distribution of UV-fluorescent and non-fluorescent floral nectar across flowering plants. A box is present for orders that have data for at least one species. The phylogeny is adapted from that of the Angiosperm Phylogeny Group and data is summarized from Thorp et al., Scogin, Roshchina et al., Nakanishi, Davis et al., and the present study.