Why do the leaves change colour in Autumn?

Trees survive by the process of photosynthesis – probably the one bit of school science we remember. It’s the process where plants use energy from the sun to convert carbon dioxide and water into sugars. Key to this conversion is the chemical compound chlorophyll, held in the leaf cells, which requires warmth and light for its production. It’s also chlorophyll that makes leaves look green. As summer turns to autumn and light levels lessen, so too does the production of chlorophyll. This means that the green colour fades from the leaves, letting other chemical compounds come to the fore.

As the chlorophyll reduces then other chemical compounds become more visible. Carotenoids and flavonoids are the families of chemical compounds that we see as yellow and orange when the trees start to blaze because they fade from leaves more slowly than chlorophyll. Both sets produce yellow light waves while carotenoids – as you could guess from the name – are also responsible for red and orange hues. The carotenoids family includes beta-carotene, the orange colour in carrots, lutein, part of the yellow of egg yolks, and lycopene, which makes tomatoes look red.

The vivid red, purple, and magenta hues that we also see on autumn leaves are from anthocyanins, another compound in the flavanoid family. They are not present in leaves all year round, but rather are produced as light levels decrease. The exact purpose of anthocyanins isn’t known. Another mystery of nature that pleases our eyes but puzzles our minds!