If you live in northern Europe or North America/Canada then you will be familiar with the stunning seasonal colour changes of the native deciduous trees. Although it may look pretty to us, this movement from the usual green colouration to an often spectacular red and orange hue is actually just a by-product of the plants natural leaf dropping mechanism.

The chlorophyll pigment – as used for photosynthesis - is green, and the reason why the majority of leaves are coloured green is because leaves are packed full of the stuff. So how is it then that they are then able to change their colour?

Well, besides the highly specialised green chlorophyll pigment, there are two other important pigment groups that found within the leaf - carotenoids and anthocyanins. Carotenes are yellow coloured pigments while anthocyanins are red coloured pigments and along with the chlorophyll they all occur in differing ratios depending on the plant species, the variety, and sometimes the uniqueness of the individual plant.

Carotenoids and anthocyanins exist within the leaves for good reason because they are there to perform two important tasks. Firstly, they help by absorbing, and then transferring some of the light energy to drive the photosynthetic process. The second is to protect leaves from the damaging effects of UV light if they become over-exposed to high levels of sunlight. They do this by harmlessly dissipating excess light energy by – once again - absorbing it as heat. In the absence of carotenoids and anthocyanins, this excess light energy could easily destroy proteins, membranes, and other vital molecules within the leaf structure.

Leaf abscission layer - image credit http://botit.botany.wisc.edu/
During the growing season the abundance of chlorophyll pigments effectively masks these other two pigments in the majority of plants. However, as winter approaches, days will become progressively shorter and cooler, and this small yet crucial day by day change acts as a trigger for dormancy in deciduous plants. This environmental trigger begins the absorption of leaf nutrients and carbohydrates back into the stems, but it also starts an irreversible phase of leaf drop.

At the same time a membrane of specialised cells known as the abscission layer will begin to develop at the base of the leaf’s stem. As the membrane grows, it increasingly restricts the flow of sugars and water between the leaf and the rest of the tree. Incidentally, this change also helps to promote the breakdown of chlorophyll pigments for absorption back into the stems. As the building blocks for chlorophyll are absorbed back into the plant the carotenes and anthocyanins remain and it is their remaining red/orange/yellow pigmentation which gives autumn leaves their colour. The intensity of the colour will also depend on the concentration of remaining stored sugars still within the leaf.

When the abscission layer is completely formed, it is then dissolved causing the physical separation of the leaf from the tree.

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