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Why Leaves Change Color
A green leaf is green because of the presence of a group of pigments known as chlorophylls.
When they are abundant in the leaf's cells, as they are during the growing season, the chlorophylls' green color dominates and masks out the colors of any other pigments that may be present in the leaf. Thus the leaves of summer are characteristically green.
The chlorophylls have a vital function: they capture some of the sun's energy and utilize it in the manufacture of the plant's food- simple sugars which are produced from water and carbon dioxide. These sugars are the basis of the plant's nourishment - the sole source of the carbohydrates needed for growth and development.
In their food-manufacturing process, the chlorophylls themselves break down and thus are being continually "used up." During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.
But as autumn approaches, certain influences both inside and outside the plant cause the chlorophylls to be replaced at a slower rate than they are being used up. During this period, with the total supply of chlorophylls gradually dwindling, the "masking" effect slowly fades away. Then other pigments that have been present (along with the chlorophylls) in the cells all during the leaf's life begin to show through. These are the carotenoids; they give us colorations of yellow, brown, orange, and the many hues in between.
The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins. These pigments are not present in the leaf throughout the growing season as are the carotenoids. They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences - both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of a certain chemical (phosphate) in the leaf is reduced.
During the summer growing season, phosphate is at a high level. It has a vital role in the breakdown of the sugars manufactured by chlorophyll.
But in the fall, phosphate, along with the other chemicals and nutrients, moves out of the leaf into the stem of the plant. When this happens, the sugar-breakdown process changes, leading to the production of anthocyanin pigments. The brighter the light during this period, the greater the production of anthocyanins and the more brilliant the resulting color display we see. When the days of autumn are bright and cool, and the nights chilly but not freezing, the brightest colorations usually develop.
Anothocyanins temporarily color the edges of some of the very young leaves as they unfold from the buds in early spring. They also give the familiar color to such common fruits as cranberries, red apples, purple grapes, blueberries, cherries, strawberries, and plums.
In our autumn forests they show up vividly in the maples, oaks, sourwood, sweetgum, dogwood, tupelo, black gum, and persimmon. These same pigments often combine with the carotenoids' colors to give us the deeper orange, fiery reds, and bronzes typical of many hardwood species.
The carotenoids occur, along with the chlorophyll pigments, in tiny structures - called plastids - within the cells of leaves. Sometimes they are in such abundance in the leaf that they give a plant a yellow-green color, even during the summer; but usually we become aware of their presence for the first time in autumn, when the leaves begin to lose their chlorophyll.
Carotenoids are common in many living things, giving characteristic color to carrots, corn, canaries and daffodils, as well as egg yolks, rutabagas, buttercups and bananas. Their brilliant yellow and oranges tint the leaves of such hardwood species as hickories, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder.
Courtesy U.S. Department of Agriculture