Bright scarlet, orange, gold, and even a splash of purple- these are the colours with which a part of the world ablazes to tell us that autumn has arrived. These leaves also fall down to the ground, covering it so that it crackles with every footstep we take. Wow, autumn! The colours are indeed beautiful. Everyone must have had a thought: Why do trees suddenly turn from green to these fiery colours? Why do different trees change colours differently? Well, for trees, this transformation is a process for survival; they do this in order to live through the cold and harsh weather.
When autumn arrives and the temperature begins to drop, deciduous trees will dispose of their leaves before winter. This cold weather will be a hazard to the leaves of many broadleaf trees and perennials. There isn’t much moisture and sunlight, so it is more difficult for plants to photosynthesise and gain energy. Therefore, instead of keeping all of the leaves, some will drop to minimise injuries and save energy. Also, we will see the change in colour before leaf abscission (the dropping of leaves).
The transformation of green to red, orange, or yellow leaves is simply due to the
degradation of chlorophyll. No, it is not because chlorophyll changes colour! There are three main pigments inside a leaf.
- Chlorophyll: gives leaves its green colour. Absorbs light for photosynthesis and the
production of glucose.
- Carotenoid: made up of xanthophylls (the yellow pigments) and carotenes (the orange
pigments). They are found in many fruits and vegetables such as corn, carrots, and
bananas.
- Anthocyanin: the red and purple pigments in plants.
Less sunlight in the autumn will cause chlorophyll to break down into smaller compounds to store in the tree’s twigs during the winter. As the leaves change colour, important nutrients like nitrogen are taken out of the leaf before it falls off the tree. The reduction of chlorophyll will reveal the yellow and orange pigments from carotenoid that were pre-existing with the chlorophyll but concealed by the overpowering green pigment before. This makes the leaves yellow!
Now, what about red and purple leaves? Anthocyanin will turn the yellowing leaves red. At this moment, trees will disconnect from their leaves. Sugars will then be trapped in the leaves and consequently form anthocyanin; this will act as the red pigment and protect the leaves from being eaten.
However, leaves aren’t just red or yellow. They paint a myriad of colours: burgundy,
vermillion, marigold, crimson, copper... the list does not end! This is because each tree has a different proportion of these three pigments. Anthocyanin and chlorophyll create brown
colours; anthocyanins and carotenoids create orange colours. How much chlorophyll is left in the leaf plays a significant role in this transformation. Additionally, the environment around
these plants also affects the colour. Whether there is frost, drought, cloudy, or warm weather, our autumn will just be full of breathtaking hues!
Bibliography:
https://www.zmescience.com/feature-post/natural-sciences/biology-reference/plants-
fungi/autumn-leaves-color-432432/
https://www.fs.usda.gov/visit/fall-colors/science-of-fall-
colors#:~:text=As%20night%20length%20increases%20in,unmasked%20and%20show%20their%20colors.
https://askabiologist.asu.edu/questions/why-do-leaves-change-color
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