When the leaves are starting to turn brown and trees shed their leaves, everyone knows that autumn has come, school and university is starting. However, this well-known phenomenon has been fascinating scientists for a long time as it is not clear what is the true purpose of these colours.
In Sir David Attenborough’s BBC series, The private life of plants, he talked about that trees that "withdraw the valuable chlorophyll from their leaves. As the green pigment drains away, waste products that have accumulated over the year are revealed and the leaves change colour. In New England, day after day, whole hillsides of maples and aspens flush yellow, orange and red".
This short description simplifies an extremely complex subject.
Where do the colours come from in the first place?
The healthy looking green leaves’ colour comes from the well-known green chlorophyll pigments in chloroplasts, which plays the main role of photosynthesis. These mask the presence of carotenoids and flavonoids which are also present in the leaves. Xanthophylls, a subclass of carotenoids, are responsible for the yellow colour of the leaves. So when the chlorophyll production decreases in the autumn and the pigments start to degrade, the carotenoids become prevalent, hence the yellowness of the leaves.
Beta-carotene, one of the most common type of carotenoid, absorbs the green and blue lights strongly, reflects the red and yellow, causing the orange colouration. Although carotenoids also start degrading along with the chlorophyll, but at a much slower rate. The red and purple colours are due to anthocyanins, which are vacuolar flavonoids, and are only produced in the autumn. The intensity of red and purple colours varies at different chlorophyll concentrations, pH, presence of metal ions etc. Kevin Gould from University of Auckland titled his article eloquently ‘Nature’s Swiss Army Knife: The Diverse Protective Roles of Anthocyanins in Leaves’ in 2004 – describing the versatility of anthocyanins.
The question remains – why did evolution ‘come up with’ this mechanism and what are the trees adapting to?
The two broad explanations are:
(1) protection/reaction to environmental factors or
(2) induced by animal-plant interactions.
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