Why do leaves change colour in autumn?

By Becca Smithers

It is officially autumn! What a beautiful time of year! All around us leaves gradually change from green to orange, brown and red.

In this blog post we look at the science behind the falling leaves, do an experiment to see what colours are hiding in the green leaves, and explore the science behind the colours.

Winter is coming!

Why do the leaves fall in autumn and winter? Every year, without fail, thousands of leaves end up on the ground and trees stay still and silent until spring rolls around and new leaves come along.

CC-BY-SA  http://www.ForestWander.com

CC-BY-SA http://www.ForestWander.com

When trees lose leaves, they are essentially preparing for hibernation. Winter is a cold and difficult time for many organisms, including trees. The ground gets colder and harder in autumn meaning that water supplies can be restricted to the trees (despite rainy autumnal weather!) In winter the ground will be even harder and colder, so the trees must begin preparations early!

During late summer and early autumn a drop in a particular plant hormone, called auxin, causes a layer of cells to grow on the base of the stem of leaves where they join the trees. This layer, known as the abscission layer, restricts the food from getting to the leaves, it stops the transfer of carbohydrates and nutrients between the leaves and the branches.

This signals to the leaves that it is time to begin ageing and transferring nutrients to the tree as quickly as possible!

Not all trees will lose their leaves, evergreen trees can survive cold winters without shedding leaves; but deciduous trees, which are most of the trees we see around the UK, must lose their leaves in order to last through the winter.

Hang on a second! Don’t trees need leaves to make food? How else can they capture sunlight to photosynthesise and make those tasty sugars? The abscission layer restricts nutrient transfer, but the tree can still access the products of the leaves. Products of sugars and nutrients are made when the leaves break down and these are transferred to the tree. Once the abscission layer is complete the leaves fall from the tree so there isn’t much time to collect the nutrients from the leaves!

It is much more energy efficient for trees to lose their leaves in winter due to the shorter daylight hours. Leaves are energetically demanding and during winter they do not produce as many sugars through photosynthesis because of the shorter days and lack of sunlight. It is better for the tree to “shut down” over winter, lose their leaves and survive on their carbohydrate stores until spring comes around.

Autumnal Activity!

With a little bit of science, you can see the colours or pigments in a leaf at any time of year!

CC-BY science made simple

CC-BY science made simple

You will need:

  • 3 or 4 green leaves
  • Rubbing alcohol/ surgical spirit
  • A bowl of hot water
  • A glass or ceramic container
  • Cling film
  • Coffee filter paper
  • A pencil
  • Scissors
  • Sticky tape

What to do:

  • Tear up your leaves and place them in the container, and pour over just enough alcohol to cover them.

 

  • Place the cling film over the top of the container to stop the alcohol from evaporating.IMG_3176

 

  • Put the container in the bowl of hot water and leave (or leaf!) it for about an hour so that the alcohol is nice and green. The longer you leave it, the better the results!

    CC-BY science made simple

    CC-BY science made simple

 

  • Remove the cling film.

 

  • Tape a strip of the coffee filter paper to the pencil so that the pencil can rest on the top of the container and the paper can reach the alcohol. The paper should only just touch the alcohol.

 

  • Wait for the colours! This step can take a while, the best result in the science made simple office was left overnight.

 

CC-BY science made simple

CC-BY science made simple

 

The different colour pigments will separate out through this process, which is known as chromatography. You can tell which pigment is the heaviest as it will not go very far, and the lightest pigment will travel the furthest! Which ones can you see?

The results of science made simple‘s chromatography show the green chlorophyll and the yellow/orange carotenoid pigments.

You can try this with different types of leaves and see how their colours differ. Try it with different coloured leaves as well to get different patterns.

 

 

 

CC-BY-SA Chris Glass

CC-BY-SA Chris Glass

 

Pigments and Patterns

Where do these colours come from? Leaves get their colours from compounds known as pigments. As the leaves age in winter, these pigments are broken down in a particular order. We start with green, move to yellow and orange, then to red and finally brown. These colours are all made by different pigments.

The green in leaves come from a pigment called chlorophyll. This is the pigment that catches sunlight and is involved in photosynthesis, so we see green leaves during sunny spring and summer. Chlorophyll is constantly replenished in the leaves, but in autumn the abscission layer stops this from happening so chlorophyll levels decrease. Chlorophyll masks other pigments which are only seen when the chlorophyll breaks down in autumn.

Yellow and orange colours come from carotenoid pigments. There are many different types of carotenoid pigment so you see lots of different shades of yellow and orange depending on which pigment is present in the leaf.

The red colour of leaves come from a pigment called anthocyanin. Anthocyanin is not present in all leaves and only appears in certain situations. It can act as a sunscreen to the leaves as well as a nutrient sponge. It collects up all the sugars and nutrients not yet transferred to the tree and quickly sends them over before the abscission layer is complete and the leaf falls from the tree.

The final colour the leaves may show before they fall is brown. These are really old leaves! The pigments showing through here are tannins and they show the very final stage before the leaves fall.

Ageing, or senescence, happens at different rates at different parts of the tree, and some leaves may not all reach the same stage of senescence before they fall. This is why we see so many different colours of leaves in individual trees, and on the ground! As the days get shorter, trees lose more leaves until finally they are bare and ready for winter.

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Curriculum Links Why do leaves change colour?

Key Stage 1

Seasonal changes

  • observe changes across the 4 seasons.
  • observe and describe weather associated with the seasons and how day length varies.

Key Stage 3

Pure and impure substances

  • simple techniques for separating mixtures: filtration, evaporation, distillation and chromatography

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Posted in Activity, Biology, Chemistry