Chlorophyll Breaking Down
It’s as if a magic brush painted the northern New England landscape with every conceivable shade of vibrant red, orange and yellow this past week. The major player in this phenomenon is chlorophyll, the pigment that gives leaves their green coloration during spring and summer. Chlorophyll is able to absorb from sunlight the energy that is used in transforming carbon dioxide and water to carbohydrates, such as sugars and starch, inside cell-like structures called chloroplasts, a process referred to as photosynthesis. But in the fall, because of changes in the length of daylight and changes in temperature, the leaves stop their food-making process. Chlorophyll breaks down and the green color of leaves disappears, revealing colors that have been masked by the chlorophyll all summer (as well as reds manufactured in the fall). Imagine a world without chlorophyll, where the bright golds, purples, yellows, oranges and reds of autumn leaves would be the natural colors seen in spring, summer and fall.
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How Does Weather Affect Foliage ?
Peak foliage has arrived in northern New England and will soon be evident further south. It’s fairly common knowledge that there are three main pigments that affect fall leaf colors: chlorophyll (green), carotenoids (yellow) and anthocyanins (red). Both chlorophyll and carotenoids are present in leaves throughout the growing season. Most anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells.
The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. Temperature and moisture are the main influences.
A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring 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 – lots of sugar and light – spur production of the brilliant anthocyanin pigments, which produce reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.
The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring or a severe summer drought can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors. (U.S. Forest Service, USDA)
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Abscission Layers Forming
As the days grow shorter and the nights longer, cells near where a leaf’s stem joins a tree’s branch start to divide rapidly. This is the start of the creation of the corky layer of cells known as the abcission layer.
The annual growth of a tree ends with the formation of the abcission layer. This layer prevents the transport of materials such as carbohydrates from the leaf to the branch and it blocks the flow of minerals from the roots into the leaves. Chlorophyll, critical to the process of photosynthesis, breaks down with exposure to light and is replaced continually by the leaves during the summer. When the abcission layer forms, this is no longer possible. The chlorophyll slowly breaks down and disappears, revealing the underlying xanthophylls (yellow pigments) and carotenoids (orange pigments) that the chlorophyll was masking. These pigments, in addition to the red pigments (anthocyanins) that are manufactured from sugars trapped in the leaf, provide us with our brilliant foliage.
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Ghosts of August
Flowering plants have a variety of ways to obtain food. Most have chlorophyll and thus are capable of photosynthesizing their own nutrients. A majority of these plants (90%) are also associated with mycorrhizal fungi – fungi which attach to the roots of other plants, often trees, with which most have a symbiotic relationship (both benefit). The plant receives minerals and water from the fungi, and the fungi feed on carbohydrates and other nutrients the plant produces.
Flowering plants with no chlorophyll cannot make their own food and must rely completely on other organisms for their nutrients. Some of these parasitic plants get their nutrients directly from the roots of another plant (Beechdrops) and others (Indian Pipe and Pinesap) receive food indirectly from fungi which get their nutrients from a photosynthetic plant. In these situations, the mycorrhizal relationship between the non-photosynthetic plant and the fungi is not mutualistic, as only the chlorophyll-lacking plant benefits. (Photo: Indian Pipe, Monotropa uniflora (one flower per stalk) and (insert) Pinesap, Monotropa hypopitys (many flowers per stalk).
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