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. A leaf with only a few weeks left to hang on its tree summons its faltering resources for a burst of bright-red-pigment making.
What a time to redecorate. 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? Does the red-making machine turn on by accident, or do the red pigments contribute something valuable? Why would passengers fleeing the Titanic stop to repaint their staterooms?
There are plenty of proposed explanations, says David Lee, a tropical botanist at Florida International University in Miami. He and other pigment researchers say that modern analytical techniques are enabling them to test these ideas in new ways–and finally get some answers. The most abundant evidence, he says, has revived a 19th-century notion that the red pigments called anthocyanins serve as a protective device for faltering photosynthetic chemistry.
RED START Ironically, Lee didn’t get interested in anthocyanins until a job took him to a place without fall. He grew up with the humdrum autumn colors of the relatively dry landscape of eastern Washington State. “There wasn’t much of an autumn show–a few trees in town turned red,” he says. However, in 1973, he left temperate seasons behind when he joined the faculty at the University of Malaya in Kuala Lumpur.
Some of the tropical trees there burst into astonishing reds, though not all at the same time or for the same reason as each other. …
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