At the turn of the 20th century, most naturalists believed that nutrient recycling caused the autumn colors. Leaves, they said, were filled with several different pigments. The green pigment filled the leaves in summer, but come fall, that color drained¿leaving behind brilliant reds, yellows and oranges. This theory held sway for more than half a century, but in the 1970s, researchers found that one shade stood out from the crowd: red.
Researchers learned that during the autumn season certain tree species actually manufacture red pigment in their leaves. The pigment production uses valuable sugars that could help the tree survive the winter months. And so researchers couldn¿t figure out why a tree would sacrifice some of its much needed sugars for a dying leaf. "It¿s an enormous metabolic input into a leaf that¿s just about to fall off the tree," says Kevin Gould, a plant physiologist at the University of Auckland in New Zealand. "That¿s the reason it¿s aroused so much curiosity."
In the past months, two independent groups may have finally shed some light on the mystery. The red pigment, both groups claim, may act as a sunscreen. During the summer months, a leaf¿s photosynthetic tissues can handle most of the radiation they receive from the sun. But in autumn, the tree begins to break these tissues down in order to reabsorb their nutrients for the winter. During this process, explains William Hoch, a plant physiologist at the University of Wisconsin in Madison, sunlight can easily destroy the leaf¿s tissues. Red pigments, he suggests, stop light from destroying its tissue, while allowing just enough photosynthesis to drive the tree¿s delicate salvage operation.
"It¿s actually a pretty neat balancing act," he says. "The tree is trying to tear all this stuff down and at the same time keep [the leaf] working so it has enough energy to finish the job." Hoch¿s research, which appeared in the September issue of the journal Tree Physiology, shows that tree species native to sunny northern climates contain the most red pigments¿evidence, he says, that the pigment is protecting the dying leaves from sunlight.
"I think it¿s a really sound idea, and the proof is in the pudding," says David Lee, a plant physiologist at Florida International University and a member of the second group advocating the sunscreen hypothesis. Their experiment, published in this month¿s Plant Physiology, exposed leaves from a dogwood tree to simulated sunlight. The leaves with red pigment, the study found, were much more strongly protected than their green counterparts. Lee adds that the red pigment could actually be protecting the leaf in another way¿by absorbing dangerous molecules known as free radicals.
Free radicals need an extra electron for stability, and to get it, they steal from nearby molecules, which in turn steal electrons from their neighbors. The result is a dangerous domino effect that can destroy important cellular components, such as membranes or DNA. Free radicals are produced during photosynthesis, and the leaf is particularly susceptible to them while it¿s being disassembled. The red pigment, Lee says, acts as a powerful sponge, absorbing free radical molecules and further protecting the leaf¿s tissue from damage.
But not everyone is convinced that the crimson hue of autumn leaves comes with an SPF. Linda Chalker-Scott, a plant physiologist at the University of Washington in Seattle, believes instead that red pigments might protect a plant¿s water supply. Chalker-Scott¿s research shows that dry spells before the fall often trigger the early appearance of red pigments. "In Seattle, where we have summer droughts," she says, "trees and shrubs that haven¿t been irrigated turn red early." But, she adds, the leaves still fall at roughly the same time every year.
Red pigments, Chalker-Scott suggests, might actually protect a plant from drought by helping it retain water. The water-soluble pigment can reduce the loss of water through a leaf¿s surface and depress its freezing point. This natural antifreeze would be useful for safeguarding leaves on frosty autumn nights and might explain why some autumn foliage turns a vibrant red. Chalker-Scott had hoped to have more evidence for her theory this fall, but a spring arson attack on her lab by ecoterrorists stalled her research.
Still other theories claim that the red pigments exist as signals to either attract or repel animals. Research by Edmund Stiles of Rutgers University in New Jersey indicates that the red pigment of plants such as sumac is designed to attract animals to their berries. When the animals eat the berries, they unwittingly become carriers of the seeds and spread them to other locations. On the other hand, a theory proposed by the late William Hamilton, an evolutionary biologist, claims that the bright autumn colors might be designed to warn insects of dangerous toxins the trees produce¿effectively telling plant-eating bugs such as aphids to back off.
In the end, none of these theories can fully explain fall's reds. Trees lacking red pigments seem to manage fine without them. "It may be that they have other mechanisms to protect them," Chalker-Scott says, although no one knows for sure. Another problem is that the red pigments of autumn leaves appear in myriad other plants, fruits and flowers. In tropical forests, newly sprouted leaves bare the same rosy colors, and a host of plants native to New Zealand carry the tint year-round. "My gut feeling is that there is not a unified explanation, that there are many different functions for [red pigments]," Gould says. He believes, however, that new experiments will eventually settle the debate surrounding pigment production.
"That is the scientific account of the matter," Henry David Thoreau wrote in his essay "Autumnal Tints." "But I am more interested in the rosy cheek than I am to know what particular diet the maiden fed on."
This article was originally published on October 15, 2001.