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Why do leaves change color in autumn?

Each autumn tempts our eyes with the color of colors. Fever red, golden yellow and any amber of amber color dyed the trees, then the ground with a colorful carpet. Why do the leaves of trees pass through this dramatic change?

Let’s try to understand what is happening with the leaves of the trees during this transitional season from the warm summer to the winter frosts.

In spring and summer the leaves are green because they contain a pigment called chlorophyll. Every child knows it. Chlorophyll is contained in small disc-like structures, called chloroplasts, scattered in plant cells. Chlorophyll molecules absorb the red and blue portions of the light spectrum, but they reflect the green, which is why we see the leaves exactly in that color.

Chlorophyll is the basis of photosynthesis, which is a process that transforms light energy into a chemical. Photosynthesis proceeds like this: chlorophyll absorbs sunlight and the energy from this absorption is used to convert carbon dioxide and water into oxygen and carbohydrates. Carbohydrates serve to feed the plants, and oxygen is released into the atmosphere. Chlorophyll is relatively unstable and therefore, over time, decays into plant cells. To keep enough of it in its leaves, the plant constantly synthesizes it again. Normally, the synthesis process requires sunlight and high temperatures.

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Then we start to enjoy the colorful change in the colors of the trees. Chlorophyll has "overshadowed" the remaining pigments in the plant's leaves until then, but with its exhaustion reveals the real tale of autumn colors. Yellow-orange shades of autumn leaves are due to carotenoid pigments, and in bright yellow the leaves containing xanthophile are dyed.

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The leaves contain a number of pigments, but chlorophyll is predominant during the warm seasons, so in the spring and summer the trees are green leaves. At the end of the summer and early autumn in the plants begins to synthesize a hormone called the axin which causes the growth of a special layer of surface cells, called a "layer of the leaves". By reducing sunshine during this period of the year, plants respond to a dim daylight. In the end, after passing a certain limit, the production of axin is terminated. The sudden change in the amount of the hormone allows the base of each leaf handle to form a layer of cells that interrupt the supply of water and mineral salts to the leaf. In addition, carbohydrates from the leaves to the rest of the plant are also discontinued. When this happens, the leaves can no longer reproduce chlorophyll, and it gradually collapses.

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The red color of the leaves comes from different types of pigments, called anthocyanins. They owe their color and some fruits such as raspberries and capins, for example. Unlike carotenoids and xanthophylla, anthocyanins are not constant in the leaves, but are synthesized in autumn. They are formed by the reaction of the sugars left in the leaves and the molecules of the residual juices of the tree. The color produced by anthocyanins depends on the acidity of cellulite juices. If the medium is highly acidic, red shades are formed, with weak acidity the colors are in the blue-violet part of the spectrum.

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The question arises why plants lose their sugars to create pigments at a time when photosynthesis is reduced anyway? There is a few evidence that anthocyanins play the role of a "sunscreen" for the residual chlorophyll in the leaves. In the summer months, strong sunlight puts at risk plants that can quickly consume their chlorophyll without being able to catch up with the synthesis of new quantities. Pigmentation creates a shield that keeps chlorophyll for longer to allow the plant to extract more of its required substances. It is also thought that plants synthesize anthocyanins to protect themselves from insects. For example, much of the insects take the red color as a sign of high toxicity, and this prevents them from putting their eggs on the red-colored leaves.

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The palette of colors we enjoy in autumn also depends on the weather and humidity of the soil. Cold weather and clear days give more time to anthocyanins, but early drops of temperatures below zero lead to an early break in their synthesis process and a shorter time to enjoy the colors of autumn. Dry time helps to increase the concentration of sugars in the residual leaf juices and also increases the production of anthocyanins.

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As the autumn advances, the cells at the base of the leaves of the leaves are gone. The connections between them weaken and the leaves are detached. Some of the trees lose their crowns from the leaves very quickly, while others manage to keep their longer leaves, but their colors are no longer so bright. This is explained by low temperatures and diminished sunlight, which causes the degradation of residual pigments.

All photos are taken in the village of Yagodovo Bulgaria

© 2017 -Elenka Smilenova  All Rights Reserved

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