Purdue News

September 1995

Fall leaves show the colors they've carried all summer, expert says

The leaves aren't dying because they've been killed by frost. The changing of colors is an active process of the trees preparing for winter.

Robert Joly, professor of horticulture at Purdue University, says the first step to understanding the process is to know that tree leaves have three primary pigments:

• The green in the leaves from May to October is created by a pigment called chlorophyll. Chlorophyll gives leaves their green color because it usually is the most abundant pigment in the leaf. It helps plants produce energy through photosynthesis, the process plants use to convert sunlight into sugars and starches. Chlorophyll serves as a photoreceptor, or light receiver, for the tree.

• The yellow-orange colors in leaves in the autumn are caused by pigments known as carotenoids, and, as their name implies, they also give the characteristic color to carrots. These also play a role in photosynthesis. Trees with brilliant displays of yellow in the autumn include beech, honeylocust, poplars, birch and ginkgo trees.

• The red colors in leaves are created by anthocyanin. In some trees, such as the Norway maple, anthocyanins may be present in such high quantities that the trees have a red or purple leaf throughout the summer. Not all trees have anthocyanin, however. Ash and oak, for example, have no anthocyanin, and turn bright yellow in autumn. On the other hand, red maple, sugar maple, sumac, scarlet oak and winged euonymus all produce spectacular displays of red and crimson.

  By the time trees sport the bright colors of October, they are completing their preparation for winter, a process that began during the hot, hazy dog days of mid-August.

  "Trees are tuned in to a number of things, such as changes in the day length, light quality and temperature," Joly says. "The trees respond to their changing environment and transform these environmental changes into biological changes."

  In late summer, trees respond to the changing environment and stop the production of chlorophyll. The chlorophyll already present in the leaves begins breaking down into simpler sugars and starches, which are stored for the winter in the tree's twigs.

  The tree will use the stored nutrients to make more chlorophyll -- and green leaves -- come spring.

  As the chlorophyll is broken down, the other colors -- the familiar yellows, oranges, reds, and purples of autumn -- are unmasked.

  For the best color show, clear days and cool nights are needed. The clear days allow photosynthesis to continue and allow the maximum production of anthocyanin. The cooler temperatures of autumn decrease the loss of nutrients through respiration and allow the pigments to accumulate. A mild or moderate drought also will increase the brilliance of the reds of autumn by stimulating anthocyanin production.

Writer: Steve Tally, (765) 494-9809; home, (765) 463-4355; Internet, tally@ecn.purdue.edu

NOTE TO JOURNALISTS: A color photograph and a computer graphic showing the scientific processes that cause fall colors are available. Contact Steve Tally at (765) 494-2096.

To the Purdue News and Photos Page