Ever wondered what exactly causes the leaves to shift from green to yellow, orange and red in fall? Turns out, it all comes down to the naturally occurring pigments and chemical transformations within the leaves.
The key pigment is chlorophyll. This molecule turns sunlight into energy for the plant in the process called photosynthesis, University of Wisconsin-Madison chemistry professor emeritus Bassam Shakhashiri told The Daily Cardinal.
“The leaves are green because they are giant factories for chlorophyll. Chlorophyll is a green pigment. Other chemicals are there also, but they are masked. They’re hidden by the immense presence of chlorophyll,” Shakhashiri said.
Chlorophyll absorbs blue and red light from the sun and reflects green. This reflected light is what we see, causing the leaves to appear green. During the summer, the leaves are rich in chlorophyll as it is necessary to perform photosynthesis.
However, the amount of chlorophyll present in the leaves is not constant throughout the year. In the fall, everything starts to change, and chlorophyll is destroyed as the trees prepare for winter.
“When the temperature drops down more, the chlorophyll factories begin to shut down,” Shakhashiri said.
This is when the other chemical pigments that were previously masked begin to shine through. Carotenoids, one of these pigments, appear yellow due to their absorption of blue and blue-green light. The carotenoids are destroyed at a slower rate than the chlorophyll, turning the leaves more yellow as chlorophyll content decreases.
But what about the red and orange leaves we see?
These are caused by other pigments called anthocyanins. Some trees create anthocyanins in the late summer and fall in order to prevent damage caused by the slowing of photosynthesis.
As chlorophyll production decreases, the leaf can turn less and less of the sunlight it receives into energy, and any extra absorbed sunlight only causes trouble. Anthocyanins act as a shield against sunlight from entering the leaf, preventing damage and also reflecting red light.
So, if a leaf has carotenoids and anthocyanins present, it will reflect both yellow and red light, appearing orange. Red leaves occur when the anthocyanin pigment is the only one left.
“When these other chemicals begin to be observed, it gives us dazzling views and a very, very impressive panorama of colors,” said Shakhashiri.
The vivid display of colors in the trees in the fall is an illustration of the idea that everything we interact with can ultimately be reduced to the actions of molecules, Shakhashiri said.
“It prompts an appreciation for the complexity of the beautiful, chemical world that we live in,” said Shakhashiri.
