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Autumn Colour

Perhaps it is because the colours of autumn are so variable in the UK that we value them all the more when they appear. As levels of sunlight fall in autumn and the days become shorter, photosynthesis is no longer effective. For the tree, leaves that once provided the tree with energy are now becoming a liability, and so it allows the whole mechanism of photosynthesis to collapse.

Autumn Colour

As the green coloured Chlorophyll begins to break down, oranges and yellow carotenoid pigments (carotenes and xanthophylls) start to show through.

But to get brilliant reds and purples, however, you need a different set of molecules. These are very clever indeed and are called anthocyanins.

Autumn Colour

In autumn, anthocyanins only occur once the tree has cut off the flow of sap from the leaf. For a while the leaf continues to make carbohydrate in the form of sugar (glucose). This becomes trapped in the leaf and, and if the weather remains cool and bright, the sugars are transformed into anthocyanins. Anthocyanins have a dual character. If the sap is acidic, the anthocyanins turn red; if neutral to alkaline, they turn blue or purple. We may love the red colour of autumn leaves, but some scientists believe that they may have the opposite effect on insects. Research on apple trees found that those with red leaves were less affected by aphids looking for a winter food source, than those whose leaves were green. This reduced the chance of the trees contracting potentially fatal diseases such as fireblight.

If this is true, Anthocyanins are pretty useful things. In the summer they act as a sunscreen to protect chlorophyll androm damaging UV rays and in autumn they help ward off disease bearing predators. Either way they are responsible for a spectacular colour display.

Tree and shrubs with great autumn colour


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