By Bill Collins
One of the most noticeable and enjoyable signs of autumn in the landscape is the coloring of leaves on trees and shrubs. The beautiful reds, oranges, yellows, golds, and browns of the leaves make it a special time of year. But what causes these changes?
The color in plants is due to a combination of pigments present in living plant cells or cell sap. Chlorophyll, anthocyanin, carotene, and zanthophyll are the most abundant pigments. The green color is due to the pigment, chlorophyll.
Chlorophyll is used continually in the making of starch and sugar in the leaf. Short days, in combination with cooler temperatures at night, reduce the leaf’s ability to replace the green chlorophyll. As the chlorophyll diminishes, the green slowly fades making the red-yellow pigments, xanthophyll or carotene, more visible. Surprisingly, these red-yellow pigments have been present in the leaves all summer long, but were hidden by the green chlorophyll. The most common of the yellow pigments is xanthophyll which produces the bright yellow and gold leaves of elm, ginkgo, hickory, tulip poplar, and birch. The result is an entire tree clothed in golden splendor and illuminating the landscape. Another pigment is carotene which produces red, yellow, or orange coloration. Carotene is also present in carrot roots.
Hues from scarlet to deep maroon are found in other trees and shrubs such as red maple, oak, sweet gum, dogwood, viburnum, and hydrangea. These red pigments, or anthocyanin, come about differently. Red and blue pigments are produced in cell sap and their formation is dependent on sun and cool temperatures. The purple leaf beech and plum produce these pigments in such great amounts that they mask the green chlorophyll and the leaves remain purple all summer. The side of the tree exposed to the most sun will have the most color. In addition to being found in the leaves, anthocyanin is present in fruits such as apples, cranberries, grapes, and peaches; in flowers like poppies and geraniums; and in the tracts of the red poinsettia. It also causes buds and twigs of many plants to be reddish or purple.
Anthocyanin is responsible for the beautiful scarlet or purplish hues in autumn foliage. Because these pigments form under the influence of the sun, a tree will often have the most brilliant color at the top and on the tips of its side branches, where the sun hits it most. The leaves that are more shaded on the inside of the canopy will be less vibrant or may even remain green. Sunlight alone will not cause them to color. Cool temperatures slow down the rate at which sugar accumulates, which encourages the formation of anthocyanin pigments causing the leaves in full sunlight to turn red. Whether a leaf turns read, purple, or deep maroon, is determined to some extent by the alkalinity or acidity of the cell sap.
The development of anthocyanin pigments is also influenced by genetic and environmental conditions. In plants where the appropriate genes are lacking, the pigments will not be produced. In plants which have the required genes, the amount of pigment which forms is often influenced by environmental conditions such as temperature, soil, water, and sun.
Changes in color are natural and begin well before the leaves fall and generally well in advance of a killing frost. They are intensified by periods of sunny, warm days and cool nights, which are usually common in the midwest. The brilliance of autumn color differs considerably from year to year, and often there are marked differences in the intensity of the color.
This fall make sure you enjoy the may gorgeous trees and shrubs which are splashed with color. A trip to Kingwood, a ride through the countryside, or a walk in your own neighborhood, is sure to delight everyone.