The screaming pinks, blazing oranges, neon reds and acid greens of many posters and signs owe their brightness to the way those materials are affected by light.
The secret to these bright colors is called fluorescence. A material, such as a pigment, fluoresces if it absorbs light of a certain wavelength and then, in turn, gives off light of a longer wavelength. For example, it might absorb ultraviolet light (black light), which is invisible to the human eye, and give off an eerie, greenish glow.
The parts of a four o'clock flower that appear yellow under white light (left) contain a special pigment. This pigment fluoresces, producing a greenish glow (right), as seen when other colors are filtered out.
|Fernando Gandía-Herrero, Josefa Escribano and Francisco García-Carmona|
Now, researchers 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, says a team of Spanish scientists. A few other types of flowers give off ultraviolet light.
These visibly glowing flowers owe their brightness to pigments called betaxanthins. The Spanish researchers found that blue light causes betaxanthins to glow yellowish-green. So, the parts of the flower that look yellow also emit green fluorescent light.
Four o'clocks also have a violet pigment called betanin in some places, the scientists found. Betanin works as an anti-fluorescent by absorbing 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 suggest. Attracting pollinators can't be the only explanation, though, because the effect appears to be weak. It's also possible that betaxanthins help protect the flowers from stress in their environment.—E. Sohn
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).