Why do the leaves change colour in autumn?

22 Sep 2017

Image: SNEHIT/Shutterstock

Ever wonder why the leaves turn from a lush green to a golden amber? It’s all down to chemistry.

Today (22 September) marks the autumnal equinox, the official beginning of a new season.

Whether you’re an old-school admirer of the foliage, or a recent lover of pumpkin spice and Stranger Things, autumn is the best time of year for many a Halloween enthusiast.

As the chill in the air becomes more noticeable, we often cast our gaze towards the trees in the hope of spotting a visual manifestation of the seasonal change. We might forget, though, that there’s so much more science to the humble leaf than we realise.

Last year, MIT Technology Review suggested that the bionic leaf was “one step closer to reality”, thanks to new advances in artificial photosynthesis. With impressive results so far, this research could provide a sustainable alternative to fossil fuels.

Meanwhile, in March, Siliconrepublic.com reported on how a simple spinach leaf can be transformed into human heart tissue, potentially improving the lives of patients with cardiac conditions.

But why do some leaves change colour with the seasons?

Chromatic chemistry

As we may remember from our early nature studies, chlorophyll is the pigment responsible for the characteristic green colour of a leaf, absorbing sunlight (light energy) to turn carbon dioxide and water into glucose and oxygen (chemical energy).

However, as the temperatures drop and the sun disappears behind the clouds, the leaf is unable to produce as much of the molecule, hence the loss in colour. As chlorophyll fades, a type of carotenoid called xanthophylls takes its opportunity to shine and transforms the leaf into a golden yellow.

Carotenoids are also responsible for the orange shade that accompanies the changing of the seasons. A dominant form known as beta-carotene absorbs green and blue light, which in turn reflects red and yellow, creating a tangerine glow. These carotenoids also degrade with time, but they are a bit more durable and long-lasting than chlorophyll.

While all of the above are already present in the leaf, the arrival of autumn stimulates the production of anthocyanin, resulting in a reddish tint. With a higher sugar concentration, it is believed that anthocyanin could shield the leaves from excess light and keep them hanging on a little longer.

For a more detailed description, check out the colourful infographic from Compound Interest below.


Infographic: Compound Interest (CC BY-NC-ND 2.0). Click to enlarge.

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Shelly Madden was sub-editor of Silicon Republic