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As the light levels begin to fall in early autumn, so the synthesis of chlorophyll by the plant also begins to fall and the autumn leaf begins to lose its greenness. The colours of autumn are due to a combination of new pigments made during the autumn, and pigments that were already present but which were concealed by the dark green chlorophyll.
The autumn colours of leaves are due to carotenoids and anthocyanins.
The anthocyanins only become evident once the corky layer has completely cut off the flow of sap from the leaf. The leaf continues for a while to make carbohydrate in the form of sugar (glucose). This becomes trapped in the leaf and, if the weather remains cool and bright, these sugars are transformed into anthocyanins. It is thought that the different colours of fallen leaves may reflect their living function. Thus brown colours are due to tannins and lignins, which are known to be protective against herbivorous predators. These also inhibit the breakdown of leaf litter after abscission, and so autumn leaf colour can be a good predictor of decomposition rates.
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