Ever wondered why tree leaves change colour in autumn? Why do some plants have leaves in colours other than green?

The colour of a leaf results from an interaction of different pigments produced by the plant. The main pigment classes responsible for leaf colour are porphyrins, carotenoids, and flavonoids. The colour that we perceive depends on the amount and types of the pigments that are present. Chemical interactions within the plant, particularly in response to acidity (pH) also affect the leaf colour.

Porphyrins - The primary porphyrin in leaves is a green pigment called chlorophyll. There are different chemical forms of chlorophyll (e.g., chlorophyll a and chlorophyll b), which are responsible for carbohydrate synthesis within a plant. Chlorophyll is produced in response to sunlight. As the seasons change and the amount of sunlight decreases, less chlorophyll is produced, and the leaves appear less green. Chlorophyll is broken down into simpler compounds at a constant rate, so green leaf colour will gradually fade as chlorophyll production slows or stops.

Carotenoids - Examples of carotenoids found in leaves include lycopene, which is red, and xanthophyll, which is yellow. Light is not needed in order for a plant to produce carotenoids, therefore these pigments are always present in a living plant. Also, carotenoids decompose very slowly as compared to chlorophyll.

Flavonoids - Examples of flavonoids include flavone and flavol, which are yellow, and the anthocyanins, which may be red, blue, or purple, depending on pH.

Anthocyanins, such as cyanidin, provide a natural sunscreen for plants. Because the molecular structure of an anthocyanin includes a sugar, production of this class of pigments is dependent on the availability of carbohydrates within a plant. Anthocyanin colour changes with pH, so soil acidity affects leaf colour. Anthocyanin production also requires light, so sunny days are needed for the brightest fall colours!

Autumn Colour Change - When leaves appear green, it is because they contain an abundance of chlorophyll. Chlorophyll masks other pigment colours. Anthocyanins, in turn, mask carotenoids. As summer turns to autumn, decreasing light levels cause chlorophyll production to slow. However, the decomposition rate of chlorophyll remains constant, so the green colour will fade from the leaves. At the same time, anthocyanin production in leaves increases, in response to surging sugar concentrations. Leaves containing primarily anthocyanins will appear red. Leaves with good amounts of both anthocyanins and carotenoids will appear orange. Leaves with carotenoids but little or no anthocyanins will appear yellow. In the absence of these pigments, other plant chemicals also can affect leaf colour. An example includes tannins, which are responsible for the brownish colour of some oak leaves.

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