Why Leaves Change Color
|Every autumn we enjoy the spectacular beauty of the fall colors. The mixture of red, purple, orange, and yellow result from chemical processes that take place in the tree as the seasons change from summer to winter. Four leaf pigments are responsible for leaf color and its changes in the autumn: chlorophylls, carotenoids, tannins, and anthocyanins.|
Chlorophyll is located in organelles called chloroplasts in the cytoplasm of plant cells and gives leaves and other plant parts their green color. Chlorophyll absorbs the sun's radiant energy and is necessary for photosynthesis, the chemical reaction in which carbon dioxide and water are transformed to sugars which the tree uses for food. During the growing season, chlorophyll is continually being produced and destroyed and leaves appear green.
As days get shorter and temperatures cooler
in the autumn, the leaves stop their food- making process. Nitrogen
and phosphorus are slowly withdrawn from the leaves to be stored in twigs
and branches during the dormant winter period. The loss of these
nutrients together with the reduced exposure of leaves to sunlight as days
get shorter, gradually stops the production of chlorophyll. The green
color of the leaves fade and other pigments present in the leaf are then
unmasked and show their colors. The timing of chlorophyll loss varies
among different species,
Carotenoid pigments are responsible for the yellow and orange colors in leaves and also appear in such plants as corn, carrots, daffodils, rutabagas, buttercups, and bananas. Carotenoids are also located in the chloroplasts and assist chlorophyll in the capture of sunlight for photosynthesis. Caratenoids are always present in the leaves, but are not visible for most of the year because of greater amounts of chlorophyll present. The yellowish colors become unmasked as the chlorophyll degrades. Carotenoids are responsible for the yellow color of Norway maple, Ohio buckeye, yellow poplar, sycamore, birches, hickories, ashes, aspen, and many other trees. Carotenoids are also responsible for the yellowing of leaves at any time during the year if there is a deficiency in nutrients or diseas that reduces normal chlorphyll production.
Anthocyanins are responsible for the pink and purple leaves of sugar and red maple, sassafras, sumac, white and scarlet oak, winged euonymus, dogwood, sourwood, some oaks, and many other woody plants. They also give color to cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. Formed in the cell sap inside the vacuole, they are water soluble and appear in the water liquid of leaf cells. Anthocyanin pigments are formed when sugars combine with complex compounds called anthocyanidins. There can be great variety in the pink to purple colors and this is influenced mainly by cell pH. Anthocyanins are usually red in acid solution and may become purplish to blue if cell sap pH is increased. Anthocyanins are usually not present until they are produced in the autumn.
Tannins are responsible for the brown hues in the leaves of some oaks and other trees. The golden yellow in some leaves such as beech are a result of tannins being present along with the yellow carotenoid pigments. These compounds are always present in the leaves, but only become visible as chlorophyll ad carotenoids disappear from leaves. Tannins are bitter substances responsible for the color and flavor of tea. They are common waste products of tree metabolism, deposited in the cell sap inside the vacuole as well as in cell walls. They often accumulate in dead tissue.
How does weather affect
The amount and intensity of autumn color is related to weather conditions that occur before and during the time the chlorophyll in the leaves is diminishing. Temperature and moisture are the main influences.
A succession of warm, sunny days and cool (below 45 F), but not freezing nights brings about the most spectacular color displays.. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions will favor anthocyanin production and brilliant red autumn color. Early frost will weaken the brilliant colors by killing or severely injuring the leaves before the pigments reach their maximum development. Rainy and/or overcast days tend to decrease the intensity of autumn colors due to reduced light intensity limiting photosynthesis and the sugars available for anthocyanin production.
The amount of moisture in the soil also
affects autumn colors. Soil moisture varies greatly from year to
year. A late spring, or a severe summer drought can delay the onset
of autumn color by a few weeks. A warm period during fall will also
lower the intensity of autumn colors. A warm, wet spring, normal
summer rainfall, and warm sunny fall days with cool nights produce the
most brilliant summer colors.
Why do leaves fall?
Shorter days and cooler temperatures signal leaf senescence in which an increase in the enzymes that promote the breakdown of cells occurs. The veins that carry fluids into and out of the leaf gradually close off as a layer of cells (abscission layer) forms at the base of each leaf petiole where it is attached to the twig. These clogged veins trap sugars in the leaf and promote production of anthocyanins. Once the separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall.
What happens to fallen
Leaves that fall decompose and restock the soil with nutrients and make up part of the spongy humus layer of the forest floor that absorbs and holds rainfall. Fallen leaves also become food for numerous soil organisms.
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