The following is a continuation of the series on fall leaf color in deciduous plants. This post is on Anthocyanins that cause pink, red, and purple colors in fall leaves.
Anthocyanin pigments are responsible for the pink, red, and purple leaves of sugar and red maple, sassafras, sumac, white and scarlet oak, and many other woody plants. They are formed in sap inside the vacuole, a storage compartment within plant cells, when sugars accumulate and combine with complex compounds called anthocyanidins. Anthocyanidins are a subclass of flavanoids, a group of antioxidant compounds found in plants including fruits and vegetables. The variety of pink to purple colors in leaves is due to many, slightly different compounds that can be formed. Their color is also influenced by cell pH. These pigments usually are red in tree species with acidic sap, and are purplish to blue in alkaline cell solution. Anthocyanins are not commonly present in leaves until they are produced during autumn coloration. A few trees, however, such as 'Crimson King' Norway maple produce reddish leaves throughout the growing season due to anthocyanins. Trees lacking the genes for production of anthocyanin develop yellow and brown shades of autumn color.
With the formation of the abscission layer and with higher viscosity of cell sap under cold conditions, the phloem tissues of a tree’s vascular system, the pathway for conduction of sugars out of leaves, become less efficient and are eventually severed where the leaf petiole joins the tree branch. However, the nonliving xylem vessels that transport water and nutrients from the roots upward, remain intact. This allows them to continue to carry water to the senescing leaves while sugars derived from continued photosynthesis and the conversion of stored starch to soluble sugars are trapped by the impaired phloem of the abscission layer and are available for anthocyanin production. Trees of the same species growing together often differ in color because of differences in amounts of soluble sugars in the leaves for anthocyanin production. These differences are caused by genetic and environmental factors. Leaves exposed to the sun, such as those on the outside branches of the tree crown, may continue photosynthesis and turn red while others in the shade may be yellow. A single tree may even have branches with different colored leaves due to differences in leaf shading. It is common to see sugar maples with reddish leaves only on exposed outer branches of the upper crown.
Fall weather conditions favoring formation of bright red autumn leaf color are warm sunny days followed by cool, but not freezing, nights. Rainy or cloudy days with their reduced sunlight near the time of peak coloration decrease the intensity of reddish autumn colors by limiting photosynthesis and the sugars available for anthocyanin production. There is an old wives' tale that claims rain washes the color out of leaves. It is not true, but the overcast conditions do reduce light intensity, and heavy rains and high winds can sweep the leaves off trees prematurely.
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.