Autumn colour, Bollitree – geograph.org.uk – 1559556 (Photo credit: Wikipedia)
For most of the year, leaves are green. This is due to a green chemical in leaves called chlorophyll – the same chemical used in photosynthesis, the process by which plants use sunlight to make food. When the days begin to get shorter in the autumn there isn’t much sunlight around, so some trees shed their leaves in order to save energy over the winter. As the leaves prepare to fall off, they turn from their usual green colour to various shades of orange, red and yellow.
During the year, the green chlorophyll in the leaves continually breaks down and is replaced by new chlorophyll. There are also other pigments in the leaf which have different colours, such as yellow xanthophylls and orange beta-carotene; however, these are obscured by the colour of the chlorophylls, which are present in much greater numbers. When autumn comes, the veins which supply the leaf with fresh chlorophyll are blocked off by special cork cells in preparation for the leaf dying and falling off the tree. As the chlorophyll breaks down, the other pigments remain, causing the leaves to turn from their usual green colour to the colours of the other pigments.
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