The following is a continuation of the series on fall leaf color in deciduous plants. This post is on tannins that cause brown and bronze colors in fall leaves.
Tannins cause the brown hues in leaves of some oaks and other trees in the autumn. The golden yellow or copper colors produced in some leaves, such as those of beech, result from the presence of tannins along with the yellow carotenoid pigments. Like the carotenoids, these compounds are always present, but only become visible as chlorophyll and carotenoids both disappear from leaves. They are common products of metabolism in trees, deposited in the cell sap inside the vacuole as well as in cell walls, and often accumulate in dead tissue. Considered waste products, tannins actually act as a defense mechanism in plants against pathogens, herbivores and hostile environmental conditions. Consumed in sufficient quantities, bitter-tasting tannins are toxic to susceptible herbivores. Oaks defoliated by gypsy moths often produce a secondary flush of leaves higher in protective tannins than the first set of leaves. Tannins discourage insect feeding on leaves and bark, and, in the case of the lip-puckering, unripe persimmon fruits, discourage fruit and seed consumption by animals until both the fruit and seed are ripe. Once ripened, fruits are sweet, attracting seed dispersing animals, and seeds have a fully developed seed coat to prevent seed destruction in passage through the digestive tracts of animals.
The leaves of green tea, a woody plant, are a source of beneficial human dietary tannins. Monomeric flavanols, the major components in green tea, are precursors of condensed tannins. Tannins from grapes and oak barrels used for aging play an important role in preventing oxidation in wine. Tannins in fruit juices, such as those found in pomegranate juice, provide antioxidant and other health benefits to humans. Perhaps research will also find evidence for a similar function in 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.