Posters and signs often display designs in screaming pinks, blazing oranges, neon reds and acid greens. Many of them owe the brightness of those colors to the way light affects those materials.
The secret to these bright colors is called fluorescence (Flor-ESS-ents). A colorful material, such as a pigment, fluoresces if it absorbs light of a certain wavelength and later gives off light of a longer wavelength. For example, it might absorb ultraviolet light (black light), which is invisible to the human eye. Later, it may give off an eerie, greenish glow.
Now, a team of Spanish scientists have found that four-o’clocks, portulacas, and certain other flashy flowers glow, too. These are the first flowers that anyone has found that naturally glow within the range of light that people can see, the scientists report. A few other types of flowers give off ultraviolet light.
These visibly glowing flowers owe their brightness to pigments called betaxanthins (Bay-tuh-ZAN-thins). The Spanish researchers found that blue light causes these pigments to glow yellowish-green. So parts of the flower that look yellow also emit green fluorescent light.
Four o’clocks also have a violet pigment called betanin (BAY-tuh-nin) in some places, the scientists found. It works as an anti-fluorescent. By that they mean it absorbs most of the fluorescent light that the betaxanthins emit.
The pattern of fluorescence and non-fluorescence might help attract bees and other insects that pollinate the flowers, the scientists say. Attracting pollinators is not likely to be the only answer, though, because the effect appears weak. It’s also possible that betaxanthins help protect flowers from stress in their environment.
Milius, Susan. 2005. Day-Glo flowers: Some bright blooms naturally fluoresce. Science News 168(Sept. 17):180. Available at http://www.sciencenews.org/articles/20050917/fob3.asp .
You can learn more about fluorescence at en.wikipedia.org/wiki/Fluorescence (Wikipedia).