Why do some tree leaves turn red?

By September 17th, 2013 at 11:43 am | Comments (4)

SciStarter has a whole round-up of tree-related projects for you this season. Branch out into citizen science!

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Walking around my neighborhood the other day, I was casually observing the local flora when I was struck by the redness of one particular set of leaves. While the tree pictured is not the exact one I spied upon, look at how vibrant these colors are! I began to wonder why this tree turned red while the others around it stayed orange and yellow. To begin, we must learn about why autumn leaves deviate from their greener shades in the first place.

As you probably already know, the color that most plants have is derived from chlorophyll, the yellow-green pigment found in chloroplasts responsible for allowing photosynthesis to take place. If you’ve forgotten how this process works, Crash Course Biology has a great video for this. While there are multiple forms of chlorophyll, it is generally true that most reflect green light, causing for plants to appear the way they do. (This raises the even better question of why aren’t plants black, but that deserves its own post.)

So, what happens to the chlorophyll as we approach the cooler months? When the temperature drops, deciduous plants slow the production of chlorophyll in preparation for the dormant period they will undergo during the winter. The plants will then be able to conserve energy by halting all photosynthetic processes during the lack of available sunlight. As this happens, orange and yellow carotenoids present in the leaves are exposed. These are pigments that are normally produced in leaves that help to absorb additional energy from the sun that is passed along to the chlorophyll and also to prevent auto-oxidation (basically the wear down of cells due to free radicals) from occurring. In addition to all of this, the plant begins to produce a cell wall between the stem and the leaf called an abscission layer. This will eventually cause for the leaf to be completely separated from the plant, allowing for it to fall to the ground.

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Absorption spectrum of chlorophyll A & B

Okay. We’ve covered green, orange, and yellow, but what produced the scarlet beauty found above and why doesn’t it occur in all trees? The answer is anthocyanins. If you’ve ever eaten a blueberry, raspberry, pomegranate, or any other fruit that can stain your hands and clothes, you’re probably already familiar with these little molecules. These pigments are similar to the carotenoids mentioned above but serve a different purpose. In cases during the late summer when plants are beginning to slow their photosynthetic processes but there is still plenty of sunlight abound, the leaves can actually be harmed by receiving too much high-intensity light in the region of Photosystem II (photoinhibition). In order to prevent this damage, the plant begins to synthesize anthocyanins to permeate through the leaves’ surfaces. Because of its red color, the pigment absorbs a large amount of the high energy visible and ultraviolet photons striking the plant, basically acting as a “plant sunscreen.” (Check out how you can even build your own anthocyanin-based solar cell!) Additionally, anthocyanins are good indicators of plant stressors including freezing temperatures and low nutrient levels.

Next time you see a particularly red tree, make sure to think about its environment! Does it receive an abundance of light? Has it been particularly cold? Feel free to comment with links to your own pictures of vibrant trees and plants!

Just like leaves, citizen science also happens to grow on trees! Don’t believe us? Check out our tree projects round-up!

References:

Why do leaves change color and turn red?

Carotenoid-to-chlorophyll energy transfer

Understanding Vegetation and Its Reflectance Properties

Winter Adaptations of Trees

Photo: Public Domain Pictures, Wikipedia


This was a guest post by Joe Diaz, a science educator and enthusiast. Follow @RealJoeDiaz. View the original post.

  • Curious George

    I have a red tree of my own in the back yard but it’s leaves are red all summer long. They’re supposed to be red because of the type if tree it is, but I’ve wondered if the anthocyanins practically act as sunscreen then how does this tree preform the procedure of photosynthesis . Is the pigment of the tree simply red or is there more to do with it?

  • Arvind Suresh

    Excellent question! Made me wonder and do a little digging around myself. Turns out plans with red leaves still do have chloroplasts to do photosynthesis but the anthocyanins dominate. In your case the tree maybe of a particular variety that does not need much sunshine and so the anthocyanins act to be protective? One thing to do would be to look at the leaves that get very little sunlight and see if they have patches of green or a green/grey tint to it. In any case this seems to be an active area of research! See here http://www.nytimes.com/2013/07/23/science/how-does-a-plant-with-red-leaves-support-itself-without-green-chlorophyll.html

  • Curious George

    Thanks

  • Swati Bhatt

    Nice and amazing article for leaves turn red. very informative post thanks for sharing Government Job