Cruising around on Instagram the other day, I found this picture that my friend (@elderlybastard) posted, and I was struck by the redness of this tree’s leaves. Look at how vibrant they are! Maybe it was her photography skills, but 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.)
The absorption spectrum of both chlorophylls A and B.
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.
Ok. 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! In the meantime, I’m going to go chill with Cannonball and Miles.
Why do leaves change color and turn red?
Carotenoid-to-chlorophyll energy transfer
Understanding Vegetation and Its Reflectance Properties
Winter Adaptations of Trees