Research Article |
Corresponding author: Petra Wester ( westerpetra3@gmail.com ) Academic editor: Renate Wesselingh
© 2024 Petra Wester, Patricia Brühn.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Wester P, Brühn P (2024) Fluorescent nectar in non-flying mammal-pollinated plants – observations and considerations in some Asparagaceae. Plant Ecology and Evolution 157(3): 327-335. https://doi.org/10.5091/plecevo.124295
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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.
Asparagaceae, fluorescence, nectar, UV absorption, non-flying mammals, pollination
Communication between animals as well as between plants and animals operates mainly via olfactory and visual channels (
Fluorescence is a phenomenon that occurs when light of shorter wavelength (e.g. UV, blue, blue-green) is absorbed by a fluorophore (e.g. a carotenoid) and subsequently some of the absorbed energy is emitted as light of longer wavelength (e.g. blue, green, yellow, or red fluorescence) (
Regarding flowers, different visual and olfactory floral signals attract animals that visit flowers to satisfy their needs (mostly food: mainly sugar-containing nectar, also pollen) and thereby transfer pollen (
Concerning flowers, fluorescence occurs in different floral organs such as petals (
Usually, floral nectar is a transparent liquid, but occasionally it is coloured and attracts attention by colouration or potentially gloss (
In the only study examining nectar of more than hundred plant species, fluorescence has been detected mainly in bee-pollinated species, but not in plants pollinated by butterflies, moths, or birds (
The aim of our study is to examine nectar for potential fluorescence of plants pollinated by non-flying mammals, namely four species of South African Asparagaceae, the Pineapple lily Eucomis regia subsp. regia (Fig.
Inflorescences of Eucomis regia (A, B), Massonia grandiflora (C, D), M. pustulata (E, F), and M. echinata (G–H) photographed with a camera being sensitive to UV, visible, and IR light, in combination with a UV-transmissible lens and a UV/IR cut filter (transmitting at 400–700 nm) under ambient room light (A, C, E, G) and the same inflorescences under UV illumination in a darkened room showing blue (B, D) to bluish green (F, H) fluorescent nectar. Note that the fluorescence appears only in the flowers containing nectar (in E. regia: nectar clearly visible only in the two middle lower flowers where nectar accumulates in the lower gaps between the filaments and the ovary and is not hidden by other flower parts, in M. grandiflora: nectar only in the left flowers of the inflorescence, in M. pustulata: nectar only in the right flowers of the inflorescence, in M. echinata: nectar only in the lower flowers of the inflorescence) and not in the Massonia flowers in which nectar was removed from the floral tubes. The light spots outside the floral tubes are nectar drops at the two tepal tips in the bottom left flower of the E. regia inflorescence (A, B) and a tepal coated with nectar on the left side of the M. pustulata inflorescence (C, D). Scale bars = 1 cm.
Flowering plants of three species of Massonia and one of Eucomis L’Hér., all Hyacintheae of the Asparagaceae subfamily Scilloideae (previously Hyacinthaceae subfam. Hyacinthoideae), that have been collected in South Africa (Table
Plant species | Locality |
Eucomis regia | Farm Fairfield, Western Cape, Overberg, 14 km NW of Napier; elevation 250 m, collected in September 2009 |
Massonia echinata | Oorlogskloof Nature Reserve, Northern Cape, Bokkeveld, south of Nieuwoudtville; elevation about 700 m, collected in September 2015 |
Massonia grandiflora | Near the Kliphuis campsite at the Pakhuis pass, Western Cape, northern Cederberg; elevation 740 m, collected in September 2017 |
Massonia pustulata | Napier, Western Cape, Overberg; elevation 250 m, collected in August 2014 |
To detect possible fluorescence in the flowers of the four species, inflorescences were photographed with a Lumix GH-1 camera (Panasonic, Osaka, Japan) without low-pass filter in front of the sensor (being sensitive additionally for UV and infrared light), mounted on a tripod, in combination with a UV-transmissible Ultra-Achromatic-Takumar 1 : 4.5/85 quartz glass lens (Pentax, Tokyo, Japan) and a UV/IR cut filter (transmitting at 400–700 nm, Baader, Mammendorf, Germany) in a darkened room under UV illumination. Pure UV illumination was achieved by means of a UV torch (UV 365 nm; U301, MTE, Shenzhen, China) in combination with a UV filter (transmitting at 320–380 nm, Baader) in front of the torch. Photographing with the UV/IR cut filter (transmitting in the visible light only) in the dark with UV illumination only allows merely fluorescence (part of the visible light) to be recorded. As reference, standard photos were taken with the same set-up, but under ambient room light (without UV illumination and UV filter). To detect possible ultraviolet patterns in M. grandiflora and M. echinata inflorescences, photos were taken in combination with the UV filter and under UV illumination. Brightness was adjusted using a white polytetrafluoroethylene (Teflon) disc, reflecting from 300 to 700 nm. Aperture was set manually and exposure time was set automatically.
In the Massonia species, nectar was completely removed in half of the flowers per inflorescence. The removed nectar was also examined separately.
In the inflorescences, the nectar of all tested species showed blue to bluish green fluorescence (E. regia: Fig.
Massonia inflorescences, photographed with a camera being sensitive to UV, visible, and IR light, in combination with a UV-transmissible lens and a UV filter (transmitting at 320–380 nm) under UV illumination, showing strong UV absorption (appearing black) of the nectar in flowers of M. grandiflora (A, note the glossy nectar only in the left flowers of the inflorescence) and M. echinata (B, note the glossy nectar only in the lower flowers of the inflorescence). Compare with Fig.
At the example of four Asparagaceae, this study shows that fluorescence of nectar occurs also in non-flying mammal-pollinated plants. We also demonstrate for M. grandiflora and M. echinata that fluorescence in nectar is accompanied by UV absorption (including that of pollen and filaments), occurring also in M. pustulata (not shown) and E. regia (
Although the mice and elephant-shrews that visit and pollinate the flowers of the four study species (
While fluorescence of E. regia pollen is not clearly noticeable, or, if existent, at most only weak (Fig.
Fluorescent pollen has been also found in bee-pollinated plant species with keel flowers (e.g. Crotalaria spectabilis Roth, Vigna umbellata (Thunb.) Ohwi & H.Ohashi, Fabaceae;
Fluorescence in flowers occurs also in plants pollinated by other animal groups. In bat-pollinated flowers, fluorescent nectar (Cheirostemon platanoides Bonpl., Malvaceae;
Thus, fluorescence in flowers seems to have no significance or adaptive value for increasing visibility or the attraction of flower visitors. This is consistent with the view that fluorescence is obscured by floral reflections due to its low quantum efficiency (
However, further observations and experiments are necessary to clearly evaluate whether flower visitors are able to perceive floral fluorescence and whether fluorescent nectar or other flower parts play a role in the attraction of flower visitors, independent of other floral cues such as colour, gloss, shape, or scent.
Regarding the examined plant species, further analyses should include a comparison with insect-pollinated conspecifics (nectar fluorescence), measurement of the absorption and emission spectra of the fluorescence as well as chemical analyses of nectar for (fluorescent) secondary compounds (e.g. phenolics). Furthermore, the latter should be tested by bioassays for biological functions (e.g. taste filter, antibiotic function) as an alternative for visual signalling.
We thank the gardeners of the research greenhouses (Anja Salaka, Andrea Wüster, and Jochen Stappmanns) of the Botanical Garden of the Heinrich-Heine-University, Düsseldorf (Germany) for their excellent care of the studied plants, Klaus Lunau (Heinrich-Heine-University, Düsseldorf) for the opportunity to use the photographic equipment as well as Cape Nature (Cape Town, South Africa), and the Northern Cape Department of Environment and Nature Conservation (Kimberley, South Africa) for the necessary permits.