Day-Glo Flowers: Some bright blooms naturally fluoresce

Flowers such as four-o’clocks and portulacas invented the fluorescent look long before the psychedelic 1960s with its black light posters, new research shows.

AREA OF GLOW. A lab-grown four-o’clock flower under white light (left) looks yellow where a betaxanthin pigment fluoresces, as revealed by a filtered image (right), and no other pigment absorbs that light. Gandía-Herrero, J. Escribano and F. García-Carmona

These species rank as the first flowers found to fluoresce naturally in people’s visible-light range, says Fernando Gandía-Herrero of the University of Murcia in Spain. Some other plant pigments emit fluorescent light in the ultraviolet range.

The yellowish pigments that give the visible glow are called betaxanthins. These pigments show up in four-o’clocks (Mirabilis jalapa), portulacas, and certain other flashy flowers related to carnations, cacti, and bougainvilleas. Previous research on the pigments missed their capacity to fluoresce, says Gandía-Herrero.

A material fluoresces after light or some other form of energy excites its electrons enough to push them to a higher energy state. As the electrons fall back to their original state, they release energy as light at a wavelength different from that of the incoming light.

Don’t confuse such fluorescence with phosphorescence, warns Gandía-Herrero. When the original zap of exciting energy stops, a fluorescent material ceases to emit light. In contrast, a phosphorescent material keeps on glowing.

In three recent papers, Gandía-Herrero and his colleagues report that blue light excites betaxanthin pigments to glow yellowish green. In the Sept. 15 Nature, the researchers report that four-o’clocks have both fluorescent and antifluorescent pigments. The parts of the flower that look yellow are also emitting green fluorescent light. Other parts of the flower contain both betaxanthin and a violet pigment, betanin, which absorbs most of the fluorescence emissions. The combination of these pigments might create patterns that attract pollinators, Gandía-Herrero speculates.

The finding that some flowers fluoresce is “important because that means that there is another potential color cue to consider in plant-pollinator relationships,” says animal-vision specialist Adrian Dyer of La Trobe University in Bundoora, Australia. However, he speculates, “fluorescence is likely to be a weak color cue compared to the visual signals reflected by pigments in full daylight.”

Peter Kevan of the University of Guelph in Ontario, who has studied bees’ color perception, agrees that fluorescence would have only a weak effect on pollinators. He notes that fluorescence isn’t new to pollination studies. In 1975, Robbin Thorp of the University of California, Davis found that nectar fluoresces in some bee-pollinated flowers.

Gandía-Herrero suggests another possible benefit to flowers from betaxanthins: protection from destructive bursts of free radicals triggered by environmental stresses. The pigments readily mop up such damaging chemicals, he says.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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