Although most people find the warm hues of October foliage soothing, the colors have some biologists seeing red. That¿s because recent research on autumn leaves has fueled a debate about why trees turn crimson. The research, done by two independent collaborations, indicates that red pigments are a sunscreen, which protects leaves while the tree reabsorbs their nutrients. But a host of competing theories make very different claims¿that the tint is a bug repellent, an animal attractor or a form of frost protection. The truth behind fall¿s splendor, it turns out, may be more complex than any single hypothesis can explain.
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.