Chemistry of Autumn Leaf Colour

The color of a leaf results from an interaction of different pigments produced by the plant. The main pigment classes responsible for leaf color are porphyrins, carotenoids, and flavonoids. The color 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 color.

Pigment Class	Compound Type	Colors
Porphyrin	chlorophyll	green
Carotenoid	carotene and lycopene xanthophyll	yellow, orange, red yellow
Flavonoid	flavone flavonol anthocyanin	yellow yellow red, blue, purple, magenta

Chemistry of Plant Pigments
Let's take a closer look at the molecular structures and functions of the major plant pigments:
Porphyrins

All porphyrins have the following ring structure:

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 color will gradually fade as chlorophyll production slows or stops.

 Carotenoids
Carotenoids are terpenes made of isoprene subunits.

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
Flavonoids contain a diphenylpropene subunit.

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 color changes with pH, so soil acidity affects leaf color. Anthocyanin production also requires light, so sunny days are needed for the brightest fall colors!




Autumn Color Change
When leaves appear green, it is because they contain an abundance of chlorophyll. Chlorophyll masks other pigment colors. 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 color 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 color. An example includes tannins, which are responsible for the brownish color of some oak leaves.