Fall Colors, Falling Leaves, and Photosynthesis

Image from Science Daily

It’s fall! That means shorter days and cooler nights. And, in many parts of the United States, it means that trees are about to shed their leaves after a final colorful salute to summer.

Did you know that a tree’s leaves are orange and yellow — even in summer? Most leaves include the pigments of all three colors. It’s just that the pigment chlorophyll (the pigment that makes the leaves green) is a much stronger pigment than the others. It covers the yellow (xanthophyll) and orange (carotene) pigments that are natural to a tree’s leaves. But, come fall, a tree’s leaves produce less chlorophyll. Now the other colors can show through in brilliant explosions of color!

Another pigment (anthocyanin), which produces reds and purples, isn’t present all year long in most green leaves. It only shows up as the nights get cooler. In fall when trees are breaking down and reabsorbing important nutrients from their leaves, their photosynthetic tissues are especially unstable and vulnerable to too much light and other stresses. Yet trees need the energy from photosynthesis to drive the processes that allow them to recapture as many of those nutrients as possible. Just as this process begins, leaves start producing large amounts of anthocyanins near the leaf surface. Anthocyanin pigments protect the leaves’ dwindling ability to generate energy during this period.

But why do leaves fall off the tree? The roots and branches of most trees are tough enough to endure freezing temperatures, but leaves are more susceptible to damage when the water inside them solidifies and expands. This kind of damage to living tissues can expose a tree’s internal plumbing system to the open air, increasing the chances of infection and speeding up water loss. To ensure the tree’s survival, any part of the plant that is unable to live through the winter must be sealed off and shed before freezing temperatures set in. As the days get shorter and the amount of available sunlight decreases, the vessels that carry water to and from the leaves gradually close off. A “separation layer” of cells forms at the base of each leaf stem. When the separation layer is completely formed, the connection to the branch is weakened and the leaf soon falls off the tree. (Interestingly, evergreen trees do not lose their needles in the winter because they are covered with a heavy wax coating and contain fluids inside that resist freezing.)

Fall is a great time of year for learning about the process that gives life to trees and most other plants. As you know, this process is called photosynthesis, which literally means “putting together with light.” As winter nears, less sunlight and less water — elements essential to the process of photosynthesis — will be available to trees. That means less food for deciduous (leaf-shedding) trees! Soon the trees’ photosynthesis “factories” will shut down, the trees will recapture nutrients from their leaves, the leaves will drop to the ground, and the tree will rest until spring when water and light return and awaken the process.

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Biology Teacher

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