Autumn is now well under way, with the nights drawing in and the weather turning colder. Still, autumn can be the most beautiful season of all with the landscape changing colour from green to a whole spectrum of oranges, reds and yellows. Here at the Garden, it is feeling very autumnal, with trees all around beginning to transform and brighten the landscape with their multi-coloured leaves. But why do leaves change colour and fall in autumn?
How do leaves change colour?
Within the leaves of trees and most plants is a green pigment called chlorophyll, which uses sunlight, carbon dioxide and water to produce energy in a process called photosynthesis. The energy produced is in the form of sugars and is the tree’s sole source of carbohydrates needed for growth and development. Therefore, it is really important for the tree to continually produce and replace chlorophyll during the growing season so that the leaves stay green and are able to absorb light.
However, during autumn as daylight hours shorten and temperatures get colder, deciduous trees (that lose their leaves for winter) start to shut down photosynthesis, and destroy the chlorophyll in their leaves to recycle the molecules and nutrients in other parts of the tree. As chlorophyll is broken down, the other pigments in the leaves (previously masked by the dominant green of chlorophyll) are revealed.
Carotenoids are the pigments responsible for the yellow-orange colours of autumn’s leaves. They act as accessory pigments for light absorption and energy dissipation in photosynthesis and are actually present in leaves all year round, but only become visible after the breakdown of chlorophyll in the autumn. Anthocyanins on the other hand are red and purple compounds that are newly generated in autumn, after about half of the chlorophyll has been lost. The exact mixture of these different compounds varies between species, consequently so does the degree of yellow, red, gold or brown in the leaves, creating a spectrum of colour that is a spectacle to behold!
Why do leaves change colour?
It is easy to think that the beautiful colours of autumn leaves are just a happy consequence of trees shedding their leaves in readiness for winter. However, it is now thought that there is an evolutionary advantage for the trees to produce red leaves. One hypothesis is that the anthocyanin acts as a sunscreen to shield leaf tissues against the harmful effects of light at low temperatures, allowing leaves to retain their function for longer and enabling more efficient reabsorption of nutrients from the leaf. Alternatively, the coevolution hypothesis suggests that red colouration could be a warning signal to reduce insect attack. The insects use leaf colour as an indicator of the quality of the tree, and therefore are less likely to colonise trees with red leaves as they want to find the most suitable host. This benefits the tree by reducing their insect load, and therefore red leaves in autumn could be an adaptation to reduce insect-induced fitness costs.
What triggers the colour-changing process?
The colour-changing process of leaves is controlled by a combination of day length and night temperature. As the day length shortens and temperatures decrease in autumn, the amount of sugar produced by photosynthesis dwindles, and the hormones in the plant trigger the leaf shedding process. Weather also has a great effect on this process, and on the depth of colour produced in the leaves. There is great concern that climate change is beginning to alter the timings and physiological processes of leaf colouration, as the process depends greatly on temperature and weather.
Falling leaves
When new leaves grow out from a branch, a layer of cells develops at the base of each stork, called the abscission layer. During the growing season, the hormone auxin is produced which prevents deterioration of the leaf and the onset of abscission (detachment of the leaf). During autumn, auxin production in leaves decreases and ethylene is produced, which triggers the elongation of cells within the abscission layer. Enzymes are also produced that break down cell walls in the abscission layer. This creates fractures in the leaf stalk, allowing the leaf to detach and fall from the tree. The process of abscission is vital for the tree to survive winter as it allows the tree to salvage and recycle nutrients in the leaves before the freezing temperatures of winter kill them. Shedding leaves also allows the tree to preserve moisture in their branches and trunk, and leafless trees are better able to tolerate winter storms as it allows wind to blow through the branches more easily, putting less strain on the tree.
So the production autumn colours is actually more complicated than what it first appears, with much activity and many different processes going on in the background. The next time you take a lovely autumnal walk underneath the colourful tree canopy, take a moment to appreciate the yellow-orange carotenoid pigments that have been hiding quietly in the leaf all summer, waiting for autumn to reveal their true colours!
References
- Archetti, M., Döring, T.F., Hagen, S.B., et al. (2008) Unravelling the evolution of autumn colours: an interdisciplinary approach, Cell Press, 24(3), Available: https://www.sciencedirect.com/science/article/pii/S0169534709000214
- Clennett, C. (2017) Why do leaves change colour in autumn, Royal Botanic Gardens Kew, Available: https://www.kew.org/read-and-watch/why-do-leaves-change-colour
- Keating, H. (2020) Why do leaves change colour and fall off in autumn?, Woodland Trust, Available: https://www.woodlandtrust.org.uk/blog/2020/10/why-autumn-leaves-change-colour/