Plant Diversity


Why do tree leaves turn color in the fall?

The carotenoids (pigments in the photosynthesizing cells)—responsible for the fall colors—are present in the leaves during the entire growing season. However, the colors are eclipsed by the green chlorophyll. Toward the end of summer, when chlorophyll production ceases, the other colors of the carotenoids (such as yellow, orange, red, or purple) become visible. Two factors are necessary in the production of red autumn leaves: Warm, bright, sunny days, during which the leaves manufacture sugar, which must be followed by cool nights with temperatures below 45°F (7°C). This weather combination traps the sugar and other materials in the leaves, thus resulting in the manufacture of red (anthocyanin). But a warm cloudy day restricts the formation of bright colors. With decreased sunlight, sugar production is decreased, and this small amount of sugar is transported back to the trunk and roots, where it has no color effect. (For more about plant pigments, see the chapter “Plant Structure, Function, and Use.”)


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