By Amelia Sadat
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shades of autumn illustrations by Amelia SADAT Autumn is a time of transitions; hedgehogs begin hibernation, rabbits grow thicker fur, and a new group of humans leave home to start life at university. Perhaps most noticeable of all though is the changing colour of the leaves. The phenomenon of ‘leaf yellowing’ is a stunning spectacle that most of us have witnessed since childhood, but few of us know much about.
The Iroquois story of how leaves change colour is a satisfying (if gruesome) tale. They believed in a Great Bear who was killed by hunters in the heavens long ago. The blood from this event was thought to drip over the Earth’s forests, colouring them red every autumn. Luckily for the bear, this is likely not the case. So, how do leaves change their colour? To answer this question, it is useful to understand how a leaf obtains its characteristic greenness in the first place. A leaf is an instrumental organ for performing photosynthesis, the biological process that uses energy from sunlight to convert carbon found in the air to a form such as glucose. This requires a powerful light-harnessing pigment called chlorophyll, which makes the leaf green.
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So, how do leaves change their colour ? Additional pigments such as carotenoids (for orange colours – note the connection to carrots) and xanthophylls (for yellow colours) are also produced in varying ratios, making a unique leaf pigmentation pattern comparable to a human fingerprint. Having so many of these different pigments in each leaf dramatically increases the amount of light absorption able to occur. In spring and summer, carotenoid and xanthophyll pigments are often masked by the large quantities of green chlorophyll produced by the leaves. However, as the dimmer and colder months arrive, critical biological changes occur.
