A Stem Cell Is A Part Of A Flower The Chemistry of Fall Colours

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The Chemistry of Fall Colours

Autumn is a second spring when every leaf is a flower ~ Albert Camus

Nature’s most spectacular paintwork graces the trees as the days shorten and the summer heat subsides. This transformation is created through an incredibly limited palette of chemicals that create countless autumnal hues.

Gold and yellow are not formed; Instead, they are opened as the leaves begin to close for the winter. Chlorophyll, the green color that gives our world its color, is a volatile compound. An active plant needs constant replacement of chlorophyll, because paradoxically it is destroyed by sunlight. Chlorophyll is assisted in its quest for light harvesting by a group of accessory compounds such as carotenoids, carotene (orange) and xanthophyll (yellow). Both chlorophyll and carotenoids are physically attached to plant cell membranes and cannot move around the plant. Carotenoids give color to things like: corn, carrots, daffodils, buttercups, and bananas.

The onset of autumn signals the plant to stop making new chlorophyll and conserve its energy. As the chlorophyll fades, the carotenoids survive, due to their chemical strength, and the green mask is shed. As a result, yellows and oranges are produced simply by removing the green pigment from the leaves.

What causes autumn to begin?

The main sign of autumn is the shortening of the days, but the cold nights also bring changes in the tree. Summer drought can delay fall as plants try to produce a little more sugar before winter. A corky membrane grows between the branches and leaf stems of the plant to protect itself from potential damage caused by frozen leaves, conserve water and minerals, excrete waste products, and prevent disease. Once the day length is shortened enough, the plant begins to gradually reduce its chlorophyll production and leaf veins gradually close. As abscission is complete and the connective tissue is closed, the leaf is ready to fall. The sealing membrane not only stops the inward flow of nutrients, but also blocks the outward flow of sugars, trapping them in the leaf. Block sugars in the leaves of some trees, such as maples, react to produce reddish pigments, called anthocyanins. Anthocyanin pigments give color to: cranberries, red apples, grapes, blueberries, cherries, strawberries and plums. The color produced by anthocyanins is sensitive to the pH of the cell sap. If the juice is too acidic, the pigments give a bright red color; If the juice is less acidic, its color is more purple.

Since nature hates waste, it is not known why these plants would waste the sugar in their leaves in this way. Some scientists believe it helps plants keep their leaves longer because anthocyanins lower the freezing point of the leaves. Others believe that anthocyanins, once absorbed into the soil from composted fallen leaves, help prevent certain other plant species from growing on the fallen leaves, thereby reducing competition for nutrients in the soil around the plant.

As you know, different trees can change different colors in autumn. Oaks become brown or red or a mixture of the two. Bronze Medal to Hickory; Aspen, golden; A mix of dogwood purple and red. Maples vary slightly depending on the species. Red Maple will be red in color. Sugar maple will turn orange-ish/red. Black maple will turn bright yellow. Elm leaves, on the other hand, simply fall without changing color before falling.

The final brown color of dead leaves comes from waste left in the leaves when the veins close, especially from oxidized tannins. Tannin is a bitter plant compound that binds to proteins and other organic compounds.

What determines how colorful autumn will be?

The clearest colors can usually be seen on the leaves of trees after very warm sunny days in autumn, which make the nights cool, but not freezing. This is because a lot of sugar will be produced in the leaves but it is trapped due to vein occlusion. This increases the production of anthocyanins. Anthocyanins develop best where the soil is acid; Nitrate is rare and light is abundant. Thus the lightly bathed tips of maple leaves and the sunny sides of apples are the reddest. Leaves that contain mainly anthocyanins will appear red. Leaves with good amounts of both anthocyanins and carotenoids appear orange. Leaves with carotenoids but little or no anthocyanins will appear yellow. Some leaves contain tannins that oxidize, which is responsible for the brown color of some oak leaves.

It is suggested that autumn comes in waves. First wave, yellow dominant. Willow, poplar, birch and some maple. Second wave, orange. Silver Maples and White Oaks. Finally, a third wave of red from trees like maples. The best autumn colors are between the second and third waves

Autumn bests you with this, its silent appeal to sympathize with its decay. ~Robert Browning Hamilton

Why do we have autumn colors?

The evolutionary function of autumn colors is less clear. As described above some autumn colors are not intentionally produced but appear as the green fades, others are actively produced by the tree during this period. There are two current theories to help explain why trees can benefit from autumn colors: photoprotection and coevolution. Photoprotection may be a method in which fall color chemicals help the plant maximize nutrients that can be reabsorbed before the plant leaves. Research has found that anthocyanins ‘protect’ leaves from the damaging effects of light at low temperatures, which would otherwise impede the reabsorption of nutrients such as nitrogen from leaves to branches. Coevolution suggests that colors warn insects that attach themselves or their eggs to plants in winter. Colors clearly indicate the degree of chemical protection against insects. Branches with the brightest or most intense colors deter most insects. Insects benefit by finding the least protected plants in which to lay their eggs. Plants with red leaves reduce the number of parasites they carry. This theory is based on a broader branch of evolutionary ‘signaling theory’, which states that plants must defend themselves in this way, otherwise they would not expend so much effort and expense to do so. There is some evidence that aphids avoid plants with red leaves.

Autumn pictures.

The most useful autumn photography tool is a polarizing filter. It works in two important ways; Firstly to boost the blue of the sky and increase the contrast with autumn colors and secondly to make the colors more vibrant by reducing the reflection of light from the leaves. Also, watch out for woodlands in autumn fog, be careful to take multiple exposures as the fog can fool the meter. The effect of the mist is enhanced by sunlight casting wonderful ‘crepuscular’ rays through the misty forests.

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