Each autumn the soothing greens of summer explode into the brilliant colors of fall. Summer leaves are green because they contain the pigment chlorophyll. Autumn’s yellows and reds are produced in two distinct processes, but both are related to the fading of chlorophyll. Chlorophyll looks green because it reflects green light. It absorbs all other wavelengths of light. Chlorophyll’s ability to absorb color is more powerful than the ability of other pigments to reflect it. But in fact, leaves do contain other pigments. These include yellow and orange pigments called carotenoid (ker’o-ten-oid) pigments. At summer’s end, a plant stops producing chlorophyll and begins to die. The yellow carotenoid pigments are more stable than chlorophyll and they don’t break down as quickly, remaining in the aging leaf after the chlorophyll fades. With the chlorophyll gone, the carotenoid pigments get a chance to show off. They give leaves a vibrant yellow appearance in autumn.
A different process causes the passionate reds of maples and oaks. Their color comes from the brilliant red pigment anthocyanin (an’tho-si-e-nin). Unlike the carotenoid pigments, anthocyanin is not present in summer leaves. Leaves don’t make this pigment until after they lose their chlorophyll. When the leaves start to die, sugar is trapped within the leaves. The cycle of warm days and cool nights triggers a chemical reaction with the sugars, which produces anthocyanin and the blazing fall reds.
Whether a leaf turns yellow or red, its last color is a crisp brown caused by tannin. Tannin is produced by plants throughout the year. It is stored in leaves and in the bark of trees. Tannin gives some leaves a bitter taste, which discourages animals from eating them and so extends the life of the leaves.