Color change in leaves
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During the autumn months, many deciduous trees experience a bright color change in their leaves before the leaves fall. This color change present in a number of trees is often referred to as fall foliage or autumn colors.
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[edit] Chlorophyll and the green 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.
Chlorophyll has a vital function: that of capturing solar rays and utilizing the resulting energy 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.
In late summer, the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the chlorophyll begins to decrease.
Often the veins will still be green after the tissues between them have almost completely changed color.
[edit] Pigments which contribute to other colors
[edit] Carotenoids
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 carotenoids and they provide colorations of yellow, brown, orange, and the many hues in between.
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. Usually, however, they become prominent 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 yellows and oranges tint the leaves of such hardwood species as hickories, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder.
[edit] Anthocyanins
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 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. When the days of autumn are bright and cool, and the nights are chilly but not freezing, the brightest colorations usually develop.
Anthocyanins 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, 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 create the deeper orange, fiery reds, and bronzes typical of many hardwood species.
[edit] A theory for the purpose of leaf fall
The conventional reasoning behind leaf fall is that leaves may be unable to survive winter (untrue: many plant leaves do), or that they may run short of water and minerals (untrue: even aquatic plants shed their leaves). Evergreens are misnamed, for they also shed their leaves albeit not in autumn. An innovative theory of plant excretion proposed by British bioscientist Brian J. Ford shows that the shedding of the leaves is a primary means of plant excretion.[1] Thus, the turning of leaves to a brown coloration and their falling en masse to the ground is no less than the plant's annual toilet break.
[edit] Tourism
Although some autumn coloration occurs wherever deciduous trees are found, the most brightly colored autumn foliage is found in three regions of the world: most of Canada and the United States; a small area of central South America; and Eastern Asia, including China, Korea, and Japan.
Eastern Canada and the New England region of the United States are famous around the world[citation needed] for the brilliance of their "fall foliage," and a seasonal tourist industry has grown up around the few weeks in autumn when the leaves are at their peak. Some television and web-based weather forecasts even report on the status of the fall foliage throughout the season as a service to tourists. Fall foliage tourists are often referred to as "leaf peepers". Fall foliage tours to the Rocky Mountain states, the northwestern United States and far western Canada are becoming more popular as well.
[edit] References
This article incorporates text from the USDA Forest Service, a public domain work of the United States Government.
