The Autumnal Rainbow: Why Do Leaves Go Different Colours?

Aleksandra Nikoniuk

It’s here again; the most colourful season of the year. While walking down the street you might wonder why green leaves, which surround us in the summer, suddenly change their colours. Why do they go through all the hassle of displaying such a variety of shades and tones when it seems much easier to just turn brown and fall off the tree straight away?

Image Credit: Pexels

The answer is in the pigments that can be found in a leaf. You’ve probably heard of chlorophyll, a pigment present in all green plants, which is responsible for the production of energy. It absorbs solar rays and turns them into sugars, which allow plants to grow.

Large quantities of chlorophyll are present in many chloroplasts across a plant, which causes their green colour. However, there are other pigments as well, such as carotenoids. During spring and summer, the amount of chlorophyll is so great that it masks the effect of other pigments. When chlorophyll is not being replenished anymore, the amazing shades of yellow and orange are revealed.

Let me explain how it all works in a bit more detail:

During energy production chlorophyll is broken down. However, the veins of the leaf (which you can easily spot on its surface) provide essential nutrients for the chlorophyll to be synthesized. So during spring and summer when days are long and nutrients are readily available, chlorophyll production can continue.

As the days get shorter and nutrients more scarce those veins are closed off and as a result chlorophyll can’t be replenished anymore. This is when different coloration by other pigments in plant cells becomes visible.

Carotenoids are the second largest group of pigments (after chlorophyll) and are responsible for the yellow and orange colour of the leaves in autumn. You can also see them all year around in fruits and vegetables such as carrots or bananas.

The red colours of the autumnal leaves are caused by another kind of pigment called anthocyanins. Contrary to carotenoids they are not always present in the leaves and are produced only at a specific time of the year. The production of anthocyanins begins when the important nutrients such as phosphate, responsible for the breakdown of sugars produced by chlorophyll, stop reaching the leaves and concentrate in the stem of the plant instead.

That change in the sugar breakdown process triggers the production of anthocyanins. Interestingly, the weather influences the intensity of the pigment produced. The synthesis of red pigments begins in late summer and sunny weather during that period of time increases the intensity of the colour. Additionally, cool and bright autumn, with little rain and no frost greatly enhances the shades of anthocyanins.

Anthocyanins are synthesized in addition to existing pigments, so what could their function be? It has been suggested that the change of leaf colour could be a form of protection against parasites. There’s some evidence that the red colour of anthocyanins can scare off potential parasites, who would use that tree as a host, as it signals the presence of a toxic substance on that tree.

Unfortunately, we can only enjoy beautiful autumnal colours for a few weeks. Since the production of chlorophyll slowly ceases, an extra layer forms on the surface of the leaves and gradually limits the flow of nutrients.

By late autumn this prevents anything from being transported and a leaf falls off. So let’s enjoy the autumnal landscape while we still can.

#AleksandraNikoniuk