By Sean Henahan, Access Excellence

WEST LAFAYETTE, Ind. Scientist Robert Joly, is much in demand when the leaves begin to change in Autumn. It is at this time that friends, relations and students turn to him, an associate professor of horticulture at Purdue University, to again explain why the leaves change color and die.

"There's something about the trees changing colors in the fall that just captivates people. And it's really a dynamic process," Joly says. "Even my wife asks me to explain it to her every year."

The changing of the fall foliage represents the interplay of three pigments found in most leaves: chlorophyll (green), carotenoids (yellow-orange) and anthocyanin (red).

Chorophyll dominates during spring and summer. Chlorophyll serves as a photoreceptor, or light receiver, for the tree. Chlorophyll helps plants produce energy through photosynthesis, the process plants use to convert sunlight into sugars and starches.

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 familiar yellow, orange, red, and purple colors of autumn - are unmasked.

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.

"Photosynthesis is not totally efficient. It's more efficient at capturing energy from the sun than anything scientists have developed, but it's not perfect. Carotenoids work in conjunction with chlorophyll by capturing sunlight and quickly transferring the energy they receive to chlorophyll. They assist by capturing different wavelengths of light, and also by guarding chlorophyll from receiving excessive solar energy," explains Joly.

Trees with brilliant displays of yellow in the autumn include aspen, beech, honeylocust, poplars, birch and ginkgo trees.

The red colors in leaves are created by anthocyanin, which is in a class of pigments known as flavonoids. Flavonoids are found in a variety of plants and in a variety of colors, and they are the substances that give flowers and fruits their hues. Flavonoids are thought to be important in attracting birds, insects and mammals to the plant for pollination. 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.

Clear days and cool nights produce the finest display of fall foliage for leaf peepers. The clear days allow photosynthesis to continue and allow the maximum production of anthocyanin. The cooler temperatures of autumn decrease the loss of nutrients, such as carbohydrates, through respiration and allow the pigments to accumulate. A mild or moderate drought will also increase the brilliance of the reds of autumn by stimulating anthocyanin production.

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. As the seasons slowly roll by almost imperceptibly day-by-day, the trees are able to perceive tiny changes in their environment.

"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."

As autumn approaches, the tree begins breaking down chlorophyll in the leaves and moving the sugars and starches into storage cells in the twigs. The nitrogen in the chlorophyll also is moved into storage cells. "Nitrogen is valuable to plants, and each chlorophyll molecule contains four atoms of it," Joly says. "That's a heavy biological investment for that tree. It doesn't let that nitrogen just fall to the ground with the leaves."

Once the leaf is drained of its chlorophyll, the tree begins a process that removes the leaf. During the growing season, trees release two hormones - ethylene, which causes the leaves to fall, and auxin, which counteracts ethylene. As winter approaches, ethylene levels rise as auxin levels drop. The ethylene acts upon a thin layer of cells near the base of the leaf called the abscission zone, triggering the production of enzymes that dissolve the polysaccharide "glue" that holds the cells together. This causes the cells in the abscission zone to separate and the leaf to drop to the ground.

"This is definitely not a passive process," Joly says. "The tree actively removes that leaf." Trees shed their leaves each year for a variety of reasons. Fragile leaves often are damaged by insects, diseases or weather. The thin leaves also are easily damaged by freezing. Replacing the leaves each spring allows the tree to start fresh each year with a set of undamaged light collectors.

In the spring the tree will use the nitrogen it took up from the leaves to make amino acids, which are the building blocks of proteins used to make new tissue, such as new leaves. Some of the nitrogen also will be used to create new chlorophyll, so the tree can begin the process over again.

"It's pretty slick, isn't it?" Joly says. "Some people view trees as objects that are practically lifeless," Joly says. "But they are living organisms that are exquisitely responsive to their environment."

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