Autumn Leaves

By Christine Johnson

This is the VOA Special English SCIENCE REPORT.

Plant scientists have learned a great deal about one of the mysteriesof autumn -- why green leaves change to bright red, yellow, or orange. Studies how that certain hormones and chemicals are involved in the process.

In the spring and early summer, three different hormones flow to all parts of the tree. They speed the growth of new green leaves. In the early autumn, the hormone abscissic [ab-SIZ-ik] acid and the chemical ethylene collect in the leaves. They begin the series of changes that take place several weeks before an old leaf falls off.

As a leaf prepares to die, it begins sending water and growth chemicals back into the tree. The chemicals include nitrogen, phosphorus, proteins, and carbohydrates.

These substances are stored safely inside the tree and its roots until they are needed again when new leaves come out in the spring. Aging leaves also send back chlorophyll. Chlorophyll is the green substance the tree uses to change sunlight, water and carbon dioxide into food energy.

As the green chlorophyll flows back into the tree, other colors that are in the leaf appear. These are the bright reds, yellows and oranges we see in autumn. The colors had always been in the leaves. But they could not be seen because there was much more green chlorophyll.

At the same time, a special group of cells at the bottom of the leaf begins to weaken. In a strong wind or heavy rain, the leaf breaks off at this weak place.

The aging and death of leaves is nature's way of saving water and growth chemicals from year to year. If the large flat leaves remained on the trees all the time, many of these substances would change into gases and vapors. They would rise up out of the leaves and be lost in the atmosphere.

Plant scientists note that the color of trees in autumn may not always be the same. In some years, the leaves may not be so bright. The difference seems to depend on the amount of rain or snow that falls during the rest of the year.

When the weather is extremely dry, many trees will not go through all the chemical steps necessary to produce bright colors. Scientists say the leaves will turn brown and fall off more quickly than usual, in an attempt to save as much water as possible.

This Science Report was written by Christine Johnson. SR