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ASK THE EXPERTS
Q: What determines the color a tree's leaves turn in the fall? A: There is a variety of pigments present in the leaf cells of deciduous trees that affect the apparent color of the leaf to the human eye. The most important are pigments that absorb light for photosynthesis. The predominate pigments are two types of chlorophyll which absorb both blue and red light and transmit wavelengths that appear green to our eye. In green leaves there are accessory pigments termed carotenoids that appear yellow-orange in color. Similar carotenoids give carrots their characteristic orange color. These pigments are always present in leaves during the growing season, but are masked by the predominate chlorophylls. In the fall, with decreasing day-length and temperature, chlorophylls degrade, leaving behind the more stable carotenoids that appear yellow and orange. Some trees will also produce other pigments anew such as the red water-soluble pigment called anthocyanin. These anthocyanin-producing species may develop brilliantly red leaves. This pigment production is enhanced during cold, sunny weather. As a rule, the most spectacular autumnal coloration develops during years when cool, clear weather prevails in fall, following summers with adequate rainfall.
Source: William G. Eickmeier -- Adrienne Spain E-mail your questions to asktheexperts@vanderbilt.edu, or via mail to "Ask the Experts" c/o Division of Public Affairs, 511 Kirkland Hall, Nashville, TN 37240
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