Color-Changing Process in Deciduous Leaves
As summer changes into autumn in the temperate zones, the long hot days change into shorter cooler days. During autumn, the decreasing amount of sunlight causes changes in deciduous plants, most noticeably in the color of their leaves. Most leaves are green because of the presence of chlorophyll, which is a green pigment (chemical that gives a substance color) needed for the food-making process called photosynthesis. Chlorophyll is produced in response to sunlight and warm temperatures. During the growing seasons of spring and summer, chlorophyll is continually being produced and broken down, and leaves appear green. In autumn, with less sunlight and cooler temperatures, chlorophyll production slows down and eventually stops. In time all the chlorophyll is destroyed.
As old chlorophyll breaks down, other pigments, which have always been present in the leaf but have been masked or hidden by the more abundant green chlorophyll, can now be seen. These pigments include xanthophyll, which produces yellow colors, and carotene, which produces yellow-orange colors. Xanthophyll and carotene do not break down as fast as chlorophyll does, so as the amount of chlorophyll decreases or disappears, these previously masked pigments are visible for a while. But eventually all of the pigments break down, and the leaves turn brown from the presence of tannin, a brown pigment. Most red pigment in autumn leaves is due to anthocyanin, a red plant pigment formed as a result of cool nighttime temperatures and bright sunny days.
As the daylight decreases, most deciduous plants start growing a layer of cells across the stem beneath the petiole of each leaf. This layer is called the abscission layer, and the leaves are actually "cut" from the stem by this layer of cells.
To determine the effect of sunlight on the color of leaves.
- deciduous tree or bush with large, dark
- green leaves
- 3-by-5-inch (7.5-by-12.5-cm) index card
- paper hole punch
- 4 paper clips
- transparent tape
- With adult approval, select 4 or more leaves of equal size on the plant that will be used in this experiment. The leaves should all receive equal amounts of sunlight and remain on the plant during the experiment.
- Fold the index card in half two times, placing the long sides together for each fold.
- Unfold the card and cut along each of the three folds, forming four strips.
- Fold one of the strips in half, placing the short ends together.
- Using the paper hole punch, punch two holes in the folded strip, cutting through both layers. Then slip this strip around one of the selected leaves. Secure the ends of the strip with a paper clip.
- Cover the holes in the paper strip on the top and underside of the leaf with transparent tape.
- Repeat steps 4 through 6, using the three remaining paper strips.
- Remove the paper strips after 7 or more days and observe the color of the leaves
In areas covered by the paper strip, the leaves change from dark green to pale green to yellow. The areas not covered by the strip, as well as those beneath the holes cut in the card, do not noticeably change color.