The following is the third in a series on fall leaf color in deciduous plants.
Carotenoid pigments are found abundantly in such vegetables as carrots and tomatoes. The carotenoids include lycopene and beta-carotene, known to be powerful antioxidants and cancer-fighting substances in humans. Another form of carotenoid found in senescing tree leaves is xanthophyll. Carotenoids are responsible for the yellow and orange colors of autumn leaves. The unmasking of the carotenoids accounts for the yellow fall leaf color of Ohio buckeye, yellow-poplar, sycamore, birches, hickories, ashes, and many other tree species.
Carotenoid pigments and chlorophyll are attached to membranes in intricate structures (organelles) called chloroplasts. Chloroplasts give leaves their green color. Carotenoid pigments assist chlorophyll in the capture of sunlight for photosynthesis. The light energy is converted to a form that charges energy-rich compounds needed to activate enzymatic reactions. These yellowish pigments are always present in leaves, but are not visible for most of the year because they are masked by larger amounts of green chlorophyll. As chlorophyll degrades in the fall, the carotenoid pigments degrade more slowly and persist, revealing their yellowish colors.
In pistachio trees undergoing summer leaf senescence in the Mediterranean, S. Munne-Bosch and J. Penuelas of the Science Faculty of the Autonomous University of Barcelona found that carotenoid substances actually increase during the early stages of senescence. Carotenoids are thought to provide both photo-protection and antioxidative protection to the photosynthetic apparatus. Carotenoids dampen damage, caused by high light intensity, to the susceptible photosynthetic apparatus of senescing leaves.
Information from "Why Tree Leaves Turn Color in Autumn" by Jeffrey O. Dawson, Professor of Tree Physiology, Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign. Go to http://web.extension.uiuc.edu/forestry/fall_colors.html for the full article.