Why Do Leaves Change Color In The Fall?

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By aaron

As we move into the autumn months, you might be wondering why leaves change color in the fall. What causes the leaves to change from green into the brilliant orange, red, and yellow colors? The answer has to do with leaf pigments.

There are 3 main types of pigment that give leaves their colors throughout the year:

-Chlorophyll
-Carotenoids
-Anthocyanins

Chlorophyll

Chlorophyll is the one many people have heard of, because it is what allows plants to perform photosynthesis, which is where plant get their energy. However, I'll add that chlorophyll is not just one pigment; it is a class of pigments. In any case, chlorophylls are green, which is where leaves get their green color.

Carotenoids

Carotenoids are orange, brown, and yellow. They are abundant in fruits and vegetables, which is how carrots, corn, bananas, and some other fruits and vegetables get their colors.

Anthocyanins

Anthocyanins are red. Like carotenoids, anthocyanins are found in fruits, which is where apples and a lot of berries get their various shades of red.

Together, these pigments mix and match to give different colors to each part of the plant to perform various functions. For example, the colors of flowers help attract pollinators like bees, while the bright colors of fruits attract animals that eat the fruits and help spread their seeds.

During the growing season, chlorophylls and carotenoids are abundant in leaves, where they play an important role in photsynthesis. Photosynthesis is the process plants use to absorb sunlight and turn it into energy for the plant, and this process works best with specific wavelengths of light. Some of these wavelengths are absorbed by chlorophyll, while others are absorbed by carotenoids. Together, these pigments absorb the optimum wavelengths of light needed for photosynthesis. During this time period, anthocyanins are present in fruits and vegetables, but they are rarely found in photosynthetic tissue like leaves.

However, in late summer and fall, the leaves gradually stop producing green chlorophylls and start producing red anthocyanins instead. So as chlorophyll production decreases and eventually stops, the green pigment fades away, allowing the orange carotenoids and red anthocyanins to show through, giving us the fall colors we all know and love.

Now, biochemically speaking, scientists have a good understanding of how this happens. As the days get shorter and sunlight decreases, photosynthesis slows down and sugar molecules in the leaf breakdown differently than they do during the growing season, resulting in the production of anthocyanins. That part is well understood.

What isn't well understood is *why* this happens. Why do leaves stop photosynthesizing and start producing anthocyanins during the fall? The process is so abundant in nature that there has to be some evolutionary advantage to it. But what? There are two proposed explanations for this.

One theory is that the anthocyanins act as a sort of "sunscreen" for the plant to actually block the light from being absorbed. There is evidence that this helps the plan more efficiently reabsorb nutrients from the leaf into the stem where it can be stored for winter.

Another theory is that the change in leaf color is to help avoid insects. By changing the color of the leaf, the insect's green camouflage becomes obsolete, causing the insect to stick out like a sore thumb to it's own predators. That allows the predators to eat the insects, preventing those insects from infesting the plant as a host during the hard winter months.

More research is needed to determine which theory is correct, or if there is even a different explanation that hasn't been thought of yet. So we don't know the "why" of fall colors, but the "how" is quite simple. During the growing season, leaves are green because the green chlorophylls mask the other pigments. As the days get shorter near the end of summer, chlorophyll production decreases, allowing the orange, yellow, and red colors of carotenoids and anthocyanins to show. #science #nature #biology #botany

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