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. Unlike the carotenoids, these pigments are not present in the leaf throughout the growing season, but are actively produced towards the end of summer. The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins. Carotenoids are the dominant pigment in coloration of about 15–30% of tree species. 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. Usually, however, they become prominent for the first time in autumn, when the leaves begin to lose their chlorophyll.Ĭarotenoids are common in many living things, giving characteristic color to carrots, corn, canaries, and daffodils, as well as egg yolks, rutabagas, buttercups, and bananas. Sometimes, they are in such abundance in the leaf that they give a plant a yellow-green color, even during the summer. The carotenoids occur, along with the chlorophyll pigments, in tiny structures called plastids, within the cells of leaves. These are carotenoids and they provide colorations of yellow, brown, orange, and the many hues in between. Then other pigments present (along with the chlorophylls) in the leaf's cells begin to show through. During this period, with the total supply of chlorophylls gradually dwindling, the "masking" effect slowly fades away. 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. Carotenoids Ĭarotenoids are present in the leaves throughout the year, but their orange-yellow colors are usually masked by green chlorophyll. Pigments that contribute to other colors Autumn coloration at the Kalevanpuisto park in Pori, Finland. Īs the chlorophylls degrade, the hidden pigments of yellow xanthophylls and orange beta-carotene are revealed. Ĭhlorophylls degrade into colorless tetrapyrroles known as nonfluorescent chlorophyll catabolites. An important enzyme in the breakdown of the apoprotein is FtsH6, which belongs to the FtsH family of proteases. This is believed to destabilize the complex, at which point breakdown of the apoprotein occurs. Research suggests that the beginning of chlorophyll degradation is catalyzed by chlorophyll b reductase, which reduces chlorophyll b to 7‑hydroxymethyl chlorophyll a, which is then reduced to chlorophyll a. Chlorophyll degradation is thought to occur first. In the autumn, this complex is broken down. Often, the veins are still green after the tissues between them have almost completely changed color.Ĭhlorophyll is located in the thylakoid membrane of the chloroplast and it is composed of an apoprotein along with several ligands, the most important of which are chlorophylls a and b. During this time, the amount of chlorophyll in the leaf begins to decrease. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. In late summer, with daylight hours shortening and temperatures cooling, 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. During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green. In their food-manufacturing process, the chlorophylls break down, thus are continually "used up". These sugars are the basis of the plant's nourishment – the sole source of the carbohydrates needed for growth and development. Ĭhlorophyll has a vital function: it captures solar rays and uses the resulting energy in the manufacture of the plant's food – simple sugars which are produced from water and carbon dioxide. Thus, the leaves of summer are characteristically green. When abundant in the leaf's cells, as during the growing season, the chlorophyll's green color dominates and masks out the colors of any other pigments that may be present in the leaf. Cross-section of a leaf showing color changesĪ green leaf is green because of the presence of a pigment known as chlorophyll, which is inside an organelle called a chloroplast.
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