Science Matters: Chemistry behind autumn beauty

Dramatic chemistry: There is a scientific reason for the beautiful gold and red tones of the leaves in autumn.
Dramatic chemistry: There is a scientific reason for the beautiful gold and red tones of the leaves in autumn.

I’ve always loved autumn in the New England. The days get shorter, the weather starts to get cooler.

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 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 the days are short and 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 over summer.

At the base of each leaf, where it’s 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 blocks the leaf off from the rest of the tree. 

When the leaf is blocked off from the rest of the plant, the chlorophyll  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.

Unfortunately, this colourful show won’t last for long. Soon enough these leaves, cut off from the rest of the tree, will dry up and fall. But come spring, the trees will start producing chlorophyll, greening up our world once more.