Why do leaves change color?

The common thinking concerning how trees prepare for winter is all wrong, maintains Robert Joly, associate professor of horticulture, Purdue University. In the usual explanation, the leaves fade, turning yellow in October. Eventually, they die, falling off the trees. Hence, the season often is called "fall."

Biologically speaking, though, the season should be called "shed." The leaves don't just fall off by happenstance. The tree intentionally drops them to prepare for winter. Moreover, the changing of leaves' colors is an active process, not just a fading away.

The first step to understanding the process is to know that leaves have three main pigments: * Their green in spring and summer is created by a pigment called chlorophyll that helps plants produce energy through photosynthesis, the process they use to convert sunlight into sugars and starches. Chlorophyll serves as a photoreceptor, or light receiver, for the tree. As temperatures begin to drop in the late summer, trees prepare for winter by stopping the production of chlorophyll. The chlorophyll already present in the leaves begins breaking down into simpler compounds, which are stored for the winter in the tree's twigs. As the chlorophyll is broken down, other colors - the yellow, orange, red, and purple colors of autumn - are unmasked. * The yellow-orange colors are caused by pigments known as carotenoids that also play a role in photosynthesis. "Carotenoids work in conjuction with chlorophyll by capturing sunlight and quickly transferring the energy they received to chlorophyll. They assist by capturing different wavelenghts of light, and also by guarding chlorophyll from receiving excessive solar...