Science: Why Do Leaves Turn Red?

Society | November 1, 2002, Friday // 00:00

A leaf turning red in the fall makes for a much greater mystery than a leaf turning yellow does. The yellowing signals simply a dropping of veils because the yellow pigment has lain hidden in the leaf during its long, green summer. When summer ends and the green pigments break down, the yellow shines through. Reds, however, don't loll around all summer. Cell physiologists have found a world inside an autumn leaf that resembles the pandemonium on a sinking ship. Metabolic pathways start to fail. Compounds break apart. Doomed cells rush to salvage the valuables, especially nitrogen, by sending them off to safer tissues. So in this final crisis, why make a special effort to turn red? The most abundant evidence has revived a 19th-century notion that the red pigments called anthocyanins serve as a protective device for faltering photosynthetic chemistry. А leaf might make anthocyanins during a period of vulnerability, to shield the green chlorophyll pigments from sunburn. This hypothesis gets more influential suggesting that plants like humans use sun protection.

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