Next to 'why is the sky blue' and 'where do babies come from,' 'why do leaves turn color in the fall' might be the most frequently asked question about nature. Every autumn, millions of Americans make a pilgrimage of sorts--not to religious shrines in the usual sense, but to express a deeply seated, perhaps evolutionary based sense of wonder at nature. From car windows, scenic overlooks, well trod trails and hotel balconies, they gaze at the display of leaf color in America's mountains and valleys.
According to Nancy Gray, a park ranger at Great Smoky Mountains National Park, more than 1.2 million people visited the 800-square-mile sylvan paradise in the southern Appalachians in October 2000, "mostly on weekends and in the last two weeks of the month" when leaf color peaks. "We have more than 100 species of native trees, most of which change color," Gray says. "It's...
Gray says the demographics of fall visitors also changes compared to summer tourists: she sees fewer kids. Oohing and ahhing over leaves is something adults like to do, without distractions from bored children whose idea of a weekend away centers on water slides, fast food, and noise.
Explaining How is Easy
Leaves turn color as perennial broad leaf trees and shrubs prepare for winter, when they're dormant. The change is part of a suite of responses, including declining cell division in the vascular cambium whose stem cells, or initials, generate secondary xylem and phloem, the stem's plumbing system. Like any organism, plants sense changes in the environment--in this case, increasing night length. But leaf color intensity is also affected by cool temperatures and moisture supply.
One of the ways plants protect themselves and conserve water in winter is to jettison tender leaves, whose high surface to volume ratio makes for good photosynthesis but renders leaves sensitive to freezing and desiccation. So, as night length increases in late summer and fall, leaves senesce. The chloroplast bound system of photosynthetic proteins and pigments disorganizes and degrades, leading to the eventual disappearance of green chlorophyll. In the final coup de grace, a specialized layer of cells called an abscission zone forms across the stem-like base of the leaf, or petiole. The zone acts like a weak link; in response to wind and gravity, the leaf breaks off and falls.
Look out over a fall forest and you'll see two basic mechanisms of color change. In oaks, hickories, and tulip trees, the receding green chlorophyll unmasks yellow and orange accessory pigments called carotenoids and xanthophylls, which normally function as antennae to funnel light energy to photosynthetic reaction centers, and to draw off excess energy that could damage the system. The pigments are lipid soluble and reside in degrading chloroplasts.
Other trees and shrubs, including dogwood and sugar maple, turn red and purple due to synthesis of water soluble pigments housed in another organelle, the vacuole. Plants make leaf anthocyanins, belonging to the flavonoid family of pigments, from sugar synthesized during bright autumn days.
Deciphering Why is Harder
Any biologist knows that how something happens is a much different question than why, which implies function. The yellow of senescing tulip tree leaves is a byproduct of the destruction of green chlorophyll--the carotenoid and xanthophyll pigments were there to begin with. But what about leaves that specifically synthesize anthocyanins in response to declining day length and cool nights? Pigment buildup requires the activity of a string of enzymes encoded by many genes. Logic says that all that activity must have a function, but the role leaf anthocyanins play at this time of the year has remained a mystery. Now, a study from Harvard University in Cambridge, Mass., and Florida International University in Miami may open the door to a better understanding of why dogwood leaves turn red.1 Coming as it does in the October issue of Plant Physiology, the journal responded to the article according to the season, decorating the front cover with a spray of colorful leaves.
A team consisting of graduate student Taylor Feild, David Lee, and Michele Holbrook wondered whether anthocyanins do anything for red-osier dogwood, a shrubby species related to the common woodland tree, Cornus florida. Lee, a professor of biological sciences at Florida International, has been interested in anthocyanins for a long time and decided to pursue the matter during a research leave in Holbrook's lab at Harvard.
The researchers took advantage of the fact that open grown red-osier dogwood leaves turn reddish purple in the fall, while those shaded by a canopy of overlying trees turn yellow, with little or no anthocyanin. Leaves of the red type accumulate anthocyanin in the upper layers of photosynthetic cells, called palisade (turn a red leaf over and it still looks green). In other words, the two kinds of leaves comprise a natural test of anthocyanin function.
When the experiments were done in mid-August to late September, as anthocyanin accumulated (but prior to actual chloroplast breakdown), both kinds of leaves had about 70 percent of the normal chlorophyll content seen during summer (July) and showed no discernible structural differences. However, when the researchers applied light to the top surface of the leaves, the photosynthetic cells below the palisade showed 50 percent greater levels of activity in red-turning leaves than in yellow. When bright light was turned off, photosynthesis in the red form recovered but that in the yellow type remained low, indicating permanent damage.
Since anthocyanins preferentially absorb blue light, the Harvard team then applied blue vs. red light to both leaves. Again, blue damaged the yellow form more than the red form. On the other hand, red light, which passes through the anthocyanin-enriched layer, reduced photosynthesis in both forms and recovery lagged equally.
Lee and co-workers think open grown red-osier leaves accumulate anthocyanins in advance of chloroplast senescence so the pigments will be in place to prevent oxidative damage brought on by light later on. Destruction of leaf photosystems liberates lots of free chlorophyll, which if excited, produces reactive oxygen species including free radicals and peroxide. By filtering out blue and green light, anthocyanins prevent chlorophyll excitation, thereby avoiding toxic reactive oxygen.
Why is that important? Species such as dogwood recycle much of the nitrogen and other constituents of leaves before they fall, importing them back into the stems and roots. The research team hypothesizes that reactive oxygen species could poison the recovery process. In other words, anthocyanins act as sunscreen to block light induced damage that would upset retrieval of nutrients important for next year's growth.
There's More to Learn
If anthocyanins are important in preventing photo-oxidative damage in the fall, why don't all deciduous leaves make them? The answer could be related to the fact that not all trees retrieve nutrients from senescing leaves to the same extent. Early succesional trees such as oaks don't do it very well. Holbrook, Lee, and Feild also wonder whether species that don't synthesize leaf anthocyanins rely on other mechanisms for protection. They speculate that "the importance of photoprotection during autumn senescence is likely related to both ecology and nitrogen economy of different species."1
Holbrook also admits that what happens in red-osier dogwood might not be true for other anthocyanin accumulating plants. "I wouldn't want to generalize yet," she says. "Many of the maples are different--anthocyanin comes on as the chloroplasts degenerate, not before." Lee agrees: "There's not a lot of literature in this area, so we have to be cautious." Still, ongoing studies seem to support their hypothesis. In a survey of 18 forest species, most produce anthocyanins even if a red color isn't visible to the unaided eye. And almost all of them show the same pigment distribution, with anthocyanin in the palisade layer. "It's a pretty widespread phenomenon," enthuses Lee. The work will be submitted for publication soon.
Lee and Holbrook haven't ruled out the possibility that anthocyanins play more than one role in senescing leaves. Lee says that anthocyanins directly scavenge free radicals in vitro and in vivo, citing recent work by Kevin Gould in New Zealand, so they may mop up any reactive oxygen that does manage to form. The real test will be determining whether anthocyanin production correlates with leaf tissue nutrient levels. Lee predicts lower levels of nitrogen in senescing red leaves compared to yellow ones.
While researchers worked on fall color biochemistry and function in Cambridge, millions streamed to the mountains to enjoy the end product of the process. Whether they were viewing sugar maples in Vermont or sourwoods in North Carolina, people may not know why leaves are so beautiful, but does it really matter? Everyone's in awe of nature's majesty nonetheless.
Barry A. Palevitz (firstname.lastname@example.org) is a contributing editor for The Scientist.
1. T.S. Feild et al., "Why leaves turn red in autumn. The role of anthocyanins in senescing leaves of red-osier dogwood," Plant Physiology, 127:566-74, October 2001.