Leaves in most trees contain some common chemicals:
1) Chlorophyll, which we know to be the only truly green pigment in nature,is extremely abundant in leaves and makes them green. Chlorophyll is the molecule which is responsible for converting light into usable energy in the tree.
2) Carotene, which is an orangy yellow colour. You will be most familiar with beta-carotene, which is responsible for the colour of carrots. Carotene acts as a supporting molecule for chlorophyll, and helps to absorb light frequencies that chlorophyll cannot and chemically stabilize the structures in the leaf (because being exposed to so much light can cause a rather reactive environment). Carotene will usually be found in a 1:3 ratio with chlorophyll, and because chlorophyll is so much more abundant than carotene, we only see the colour of chlorophyll. However, these carotene chemicals will still influence the colour of the leaf, and the darker a leaf is indicated a higher concentration of carotene.
3) Anthocyanins, the colours of which vary from vibrant reds to purples. The role of these chemicals is less clear, and it doesn’t have a role in photosynthesis. We do know that it is often produced in times of stress for a plant, so these chemicals could be produced in the leaves to protect from light damage (very likely, considering the nature of leaves). We also know that these anthocyanins are strong antioxidants, so their role in protecting the leaf seems quite likely.
Already, you have probably noticed these colours are all quite prominent in Autumn (or as our friends across the Atlantic Ocean call it, ‘Fall’), so now I’ll explain why leaves change into these colours before they fall.
Leaves start green, because they are extremely abundant in the green pigment chlorophyll. However, this molecule is extremely unstable, and although leaves contain carotenes and anthocyanins to help protect it, it still breaks down very quickly, and trees are constantly producing it. When the temperature drops, and day length (and so amount of light) shortens, trees stop producing more chlorophyll, and so the chlorophyll left in the leaves breaks down, and doesn’t get replaced. When the chlorophyll breaks down, it loses its green colour, and other colours of the leaves become visible.
At this point leaves of many trees become a beautiful and vibrant colour. A layer of cells stops sugars and sap leaving the leaves, and this causes the sugars to react with the sap to produce bright red anthocyanins. As I’ve said, the purpose for this isn’t very well understood, but it could be to protect the leaf and prevent and damage being made to the tree before the leaf is severed from the branch.
Not all trees turn red, many, such as an Oak tree will go straight from green to yellow. Leaves turn yellow because other pigments, such as chlorophyll and anthocyanins have all degraded and only carotene is left.
You could argue the last colour a leaf turns is brown. They turn brown because all the chemicals they contained are in different states of degradation, either by light or microbes, and so there is a large variety of different compounds in the leaf. This makes the overall colour brown. You can think of it a bit like what would happen if you mixed every colour together – you get brown.
A number of factors affect how bright and how quickly these colours are displayed. A perfect Autumn, in my opinion, would be one where all the leaves turned a bright red all at once. For this to happen you would need very cold temperatures (which will destroy the chlorophyll quicker, and possibly stimulate more anthocyanin production) and very bright autumn days, because anthocyanin production needs light.
Image courtesy of Stanley Zimny
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Leaves contain a number of chemicals, which have differnet colours. The most abundant is chlorophyll, which is green. However, chlorophyll is easily broken down by low temperatures. The other chemicals are more resistant, and these are anthocyanins (which are red) and carotenes which are yellow.