Trees produce another pigment group, the anthocyanins , primarily in autumn. These pigments give red and purple to such things as blueberries, cherries, red apples, concord grapes, and plums.
And autumn leaves. You can tell by the color of an autumn leaf what kind of pigment a tree specializes in. Oaks, dogwoods, black tupelo and some maples tend to turn red, brown or russet because they produce a lot of anthocyanins.
The precise timing of the color shift apparently is genetically controlled. Oaks, for example, are among the last trees to change color. Others, such as sourwood, flip the switch to fall colors as early as August. However, weather and soil moisture can affect the quality of fall color. A severe summer drought can delay fall color by a few weeks. A warm spell in autumn also tones down autumn colors. The most-dazzling displays of scarlet and crimson occur in autumns marked by warm, bright days and cool, crisp nights with temperatures above freezing.
Sun-lit autumn days stimulate leaves to produce sugars, and chill nights close the veins leading into and out of the leaf, locking in the sugars—which in turn lead to the production of anthocyanins and their crimsons and violets. Yellow and gold colors vary little from year to year, however, because leaves contain carotenoids at all times. Eventually, autumn leaves must fall.
By the end of summer, they may be damaged by insects, disease or general wear and tear and ready for renewal. They are equipped to self-destruct. When daylight hours are less and temperatures are cooler, photosynthesis slows down and there is less chlorophyll production.
This reduction reveals yellow or orange pigment called carotenoids and are usually hidden by the abundance of chlorophyll present in leaves during the growing season. Unlike chlorophyll and carotenoids, which are present in leaf cells throughout the growing season, anthocyanins are produced mainly in the fall.
These natural chemicals give color to familiar fruits such as cranberries, red apples, cherries, and plums. These complex compounds in leaf cells react with excess stored plant sugars and exposure to sunlight creating vivid pink, red, and purple leaves. A mixture of red anthocyanin pigment and beta carotene often results in the bright orange color seen in some leaves.
Why do leaves stay on the tree when the tree dies suddenly, but fall off the tree when a tree goes into fall? First, it is important to understand why leaves fall off the trees every year. The tree uses a lot of resources to manufacture each leaf, so it seems a waste to go through that same process every year. It turns out that, just like everything else in nature, there is a very good reason for this process.
Some believe that as the leaves start to receive less sunlight, the health of the leaf declines and it will become brittle and fall off. A good wind will easily knock off the colorful dying leaves.
It is not so simple. The trees actually do this on purpose. The shorter days, combined with cooler temperatures, trigger a chemical change in the tree. The leaf falls to the ground. We all know that the main purpose of a leaf is to gather sunlight and convert that sunlight to energy for the tree. That happens when the leaves are old or damaged. They can save all the good bits and when there is enough water, they can use them to grow brand new leaves.
The Conversation. Wednesday 8 May am. Share Facebook Twitter Mail Whatsapp. Imagine This. Thu 13 Jun Wed 5 Jun
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