The Physics of Fall Leaf Color
The following description appeared in the UT Agricultural Extension Service publication number SP 529 and was written by Dr. Wayne Clatterbuck, Assistant Professor of Forestry, Wildlife & Fisheries.
Several pigments in leaves are responsible for color: chlorophyll, carotene, xanthophyll and anthocyanins.
Chlorophyll is the pigment in chloroplasts of plants that reflects green light. Plants use the energy absorbed by chlorophyll in photosynthesis to produce food for plant growth and development. Chlorophyll is continually broken down during photosynthesis and being replenished by the plant.
Carotene and xanthophyll are pigments that reflect orange and yellow light respectively. Both are present in the chloroplasts, with chlorophyll enabling the plant to absorb a wider range of wavelenghts of light and thus capture more energy. These pigments are in such small quantities that they are masked by the more dominant chlorophyll during the growing season.
With the passing of summer, days become shorter. The phytochrome, the light-sensing mechanisms in leaves, recognizes the shorter day lengths. The shorter days and lower temperatures arrest chlorophyll production. Chlorophyll breaks down faster than it is replaced, allowing the yellow and orange pigments to be unmasked.
The molecules reflecting red wavelengths, anthocyanins, are water-soluble pigments that occur in the cell sap creating the red, pink, and purple hues. Not all trees produce anthocyanins. These pigments are not present during the summer, but their formation is encouraged during a succession of cool nights and sunny days. During these days when photosynthesis and chlorophyll production are decreasing, an abundance of sugars accumulates in the leaf. The cool nights promote a separation layer of cells in the petiole, where the leaf attaches to the twig, that prevents sugar from flowing out of the leaf, and also arrests the flow of nutrients into the leaf. The formation of anthocyanin requires bright light, a diminishing water supply and the accumulation of sugars trapped in the leaf.
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