The Chemistry of Fall Color
During the summer, it is chlorophyll that gives leaves their green color. This complex chemical, which is essential in the photosynthetic production of food (sugars), is continually being manufactured and broken down at approximately equal rates.
As fall approaches, the steadily decreasing length of day and cooler temperatures interact to biologically trigger the formation of a corky layer of cells called the abcission layer across the base of the leaf petiole. This formation gradually decreases the supply of water and minerals to the leaf; reduces the manufacture of chlorophyll; and traps sugars in the leaf.
When chlorophyll is reduced, a pigment known as anthocyanin becomes prominent. This pigment is responsible for the red and purple coloration of autumn leaves and is built from the sugars trapped in the leaf. Bright, sunny, fall days increase the photosynthesis of sugars, therefore, increasing the production of anthocyanins. This condition gives the most brilliant red colorations in the fall.
If no anthocyanins are produced, the leaf will turn yellow or orange due to the presence of the pigments known as carotene and xanthophyll. These pigments were present in the leaf all summer, butr had been masked by chlorophyll until now. Since carotene and xanthophyll are not built from sugars, the yellow fall coloration is not as dependent on weather conditions, but rather on the absence of other pigments.
An attitional substance which affects foliage color is tannin. Most prevalent in oaks, walnuts, and hickories, tannin turns leaves brown or dark red.
Leaves begin to change color along their outer margins first. These outer edges are the last to get nutrients and water, and as the nutrient supply is decreased, these cells are the first to lose their chlorophyll. The midrib stays green the longest since it gets nutrients first and easiest, and here the chlorophyll can remain active longer.
Leaf coloration may vary within a species due to genetic differences in pigment production. Environmental conditions, such as soil acidity, can also cause variation in leaf color. Leaves of Red Maples growing in very acidic soils turn brilliant red, while trees in neutral or alkaline soil may have yellow leaves.
The best fall foliage occurs when there is a dry, late summer to start formation of the abcission layer to trap sugar in the leaf. Then, to prevent leaves from falling too soon, rain is needed in early fall. An alternation of heavy rain and bright sunshine along with the gradual dropping of temperatures gives the most brilliant colors.
The presence of a large number of plants in North America having brilliant fall foliage colors is more unusual when it is noted that the only other places in the world with a similar abundance of foliage colorations are northern China, Korea, and Japan.
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