The Science Guys
Science Guys > November 2003
Why do leaves change color in the Fall?
If sunsets were rare, people would travel thousands of miles to see them. Every autumn, our eastern hardwood forests are dressed in colors that are commonplace to us, but which few around the world experience. Folks travel long distances to see the autumn colors that we often take for granted.
An object’s colors are usually determined by how chemical compounds absorb visible light. White light from the Sun consists of the colors of a rainbow. The dye in your red shirt absorbs blue or violet light, leaving mainly red light which is reflected back to your eye. Leaf color is due to light absorption by three classes of organic (carbon-containing) compounds. The green is due to chlorophylls, ringed compounds containing the metal magnesium. Autumnal yellows are due to carotenoids. Reds and purples come from dye molecules with ringed structures, called anthocyanins.
Why do some plants produce these pigments? Chlorophyll absorbs light energy, and by an ingenious transfer of that energy within the molecules of the light harvesting complex, the plant makes sugars for fuel (the process called photosynthesis). This transfer of light energy is fast; parts of the process occur in trillionths of a second! Chlorophyll doesn’t use green light well, so green light is reflected from the leaves, giving them a green color. The yellow carotenoids are always there, but they’re masked by chlorophyll and other pigments. When plants shut down photosynthesis in the fall, carotenoids remain and the leaves look yellow. However, some trees produce anthocyanins as the leaves are preparing to drop for the fall, producing the vibrant reds and purples seen in sumac and red maple.
Why do leaves expend the energy to produce these colored pigments? As the efficiency of photosynthesis declines, carotenoid molecules soak up the overload of energy from the intense light, protecting the chlorophylls and light-harvesting complexes in the plant cells. Thus the leaves efficiently harvest light longer than they might otherwise. Anthocyanins help plants in several ways. They soak up excess damaging radiant energy by absorbing light wavelengths other plant pigments can’t. They’re also acid-resistant molecules that shield the delicate chlorophyll beneath. Anthocyanins also prevent the formation of some reactive chemicals like superoxides that form as a consequence of an organism’s metabolism. It’s helpful to limit the production of these reactive compounds, because they can damage important biological molecules. Anthocyanins do this well-they’re superb anti-oxidants, better than vitamin E-and they keep reactive oxygen species from damaging the photosynthetic reaction centers.
This is still an exciting area of research, with unanswered questions. Why do plants expend the energy to synthesize anthocyanins just when the plant is ready to shut down for the year? And if there is a big advantage conferred by having these pigments, why don’t more plants produce them? The annual cycle of color change is ancient, but new science is being discovered every year!