Science Matters: The colours of autumn

I’ve always loved autumn in the New England. The days get shorter, the weather starts to get cooler. I can justify wearing my funky boots and jackets again. And the trees in the region put on the most glorious displays of colour.

But it’s not just the changing colours of the leaves that are beautiful. The chemistry behind how and why this happens is pretty beautiful too.

Even in primary school we are taught that leaves are green because they contain chlorophyll. Chlorophyll is the molecule which allows plants to absorb energy from sunlight, and without it, plants can’t photosynthesise.

But right now, as the days get shorter and cooler, trees around the region are losing the chlorophyll from their leaves.

Although plants need chlorophyll to make their food, producing chlorophyll takes a lot of energy. So plants need to decide whether or not it is worthwhile expending that energy.

In the spring and summer, when the days are long and there is plenty of sunlight, it is absolutely worth the effort of making chlorophyll. During these parts of the year, the green leaves of plants act as food factories, harvesting and storing energy from the sun. But during the winter, when sunlight can be scarce, some plants decide that it really isn’t worth investing energy to make chlorophyll.

Instead, many plants rely on the food stores that they have built up, and shut down the food factories that are their leaves.

At the base of each leaf, where its stem attaches to the tree, there is a special type of plant cell. As the weather cools, these cells begin to multiply, until they form a layer that effectively blocks the leaf off from the rest of the tree. 

When the leaf is blocked off from the rest of the plant, the chlorophyll in the leaf starts to break down. And as the chlorophyll disappears, so does the green colour of the leaves.

You might expect that as chlorophyll disappears from leaves, they would just turn brown. But instead, we see such a lovely range of yellows, oranges and reds.

This is because chlorophyll is just one of the pigments that is found in leaves. There are two other pigments found in leaves - carotene, which is a yellowish colour, and anthocyanin, which is more of a red colour. These pigments are always present in the leaves, but when chlorophyll is also present it overwhelms them.

It’s only when the chlorophyll breaks down that we are able to see these other pigments, and are treated to the spectacle of trees covered in yellow, orange and red leaves.

Dr McMillan is the course co-ordinator for the Diploma in Science and Bachelor of Scientific Studies at the University of New England.