The changing colours of the leaves is many people’s favourite part of autumn – the gorgeous hues are pretty much synonymous with the changing of the seasons. There is so much variation as well; many trees display a wide array of yellows, golds, oranges, reds and everything in-between. So, what causes this transformation to occur?
As the days get shorter and darker, there is less light intensity falling on the leaves of trees. This reduced light intensity initiates the breakdown of chlorophyll, a green pigment which is responsible for absorbing light to facilitate photosynthesis. As the chlorophyll begins to break down and be removed from the leaves, other pigments start to become visible. Funnily enough, it is these secondary pigments that cause the vibrant (and picturesque) autumnal colours. The main compounds which are revealed are xanthophyll, carotene and anthocyanins. Xanthophyll and carotene are responsible for the more orange shades, and any red colours are attributed to the anthocyanins.
At the same time as the chlorophyll is being broken down, a specialist layer of cells named ‘cork cells’ build up at the base of the leaf. These cells shut off the veins in the leaf from the rest of the tree, forming a leaf scar which protects the overall plant when the leaf is eventually severed.
The overall colours of the leaves are determined by a combination of environmental factors. Light intensity, temperature and the abundance of water all play roles. For example: the colder the temperature, the redder the leaves. This is due to the anthocyanins being more visible.
Currently there are two main theories as to why leaves change colour; photoprotection and coevolution. The idea behind the photoprotection theory is that as the chlorophyll breaks down and exposes more anthocyanins, they are then able to protect the leaves from damage caused by high light intensity at low temperatures. This helps to reduce photo-oxidation and photo-inhibition, therefore (theoretically) making the reabsorption of nutrients more efficient.
The coevolution theory states that the brighter red the leaves turn, the more toxic the tree is to certain types of insect. This would act as an indicator to the creatures affected, reducing the parasite load of the tree for the onset of winter. Equally, the vibrant colours could attract certain types of insect or bird. This said, neither of these theories has been fully proven, and neither of them provide a justifiable explanation as to how the colour change links to the inevitable dropping of the leaves.
So, to conclude: the fantastic autumnal colours of leaves are produced as chlorophyll breaks down, and no one fully understands why this occurs.
Image credit: http://www.mrwallpaper.com/wallpapers/Autumn-Leaves-Background-851×315.jpg