The coloration of autumn leaves is one of science’s perennial conundrums. Given the efforts of legions of scientists over decades of intellectual inquisition; one would think that the question “why leaves turn red?” would have an equally accepted and plausible answer. It doesn’t.
Some parts of the leaf coloration phenomenon are understood. It is known that leaves change color in the fall because the plant senses the colder temperatures and shuts down the production of chlorophyll, the green-colored, photosynthetic cells on which most (if not all) life depends. When the green disappears, other pigments are revealed, the color of the leaf depending on what pigments are present for that particular plant. The yellow color comes from carotenoid pigments (carotene and xanthophyll) and the red color from anthocyanin pigments. Ultimately, they all turn brown due to tannin, and fall off (white oak leaves among others hang on all winter).
A more scientific explanation is that deciduous trees have a layer of cells at the base of each leaf called the abscission. When temperatures get colder, signaling the onset of winter with shorter days with less sunlight, the tree starts to shutdown when the abscission cells grow a cork-like membrane that interrupts the flow of nutrients to the leaf. The production of chlorophyll declines and the green fades. Carotene, like chlorophyll, is a large molecule that is contained in the chloroplasts, the disc-shaped structures that are the photosynthetic factories of the plant. Carotene is an accessory absorber for chlorophyll, aiding it in absorbing energy from a slightly different spectrum. It is much more stable than chlorophyll so that it persists, resulting in the yellow leaves of such trees as hickories and birches.
The red color is another matter. The classic explanation (Peterson’s Field Guide to Eastern Forests for example) is that the anthocyanin is produced by plants that have high sugar content such as maple and sumac. When the abscission layer forms in the fall, the sugar is trapped in the leaf and is converted to anthocyanin. Thus, when you have a dry summer, little sugar is produced and the fall colors are subdued. In point of fact, however, quite the opposite is true, as a hot, parched summer is likely to yield more color. Recent research has demonstrated that anthocyanin production by different plant species is a complicated phenomenon and not just a matter of sugar.
Anthocyanin has been studied by scientists for several centuries. Known as “colored cell sap'” it is formed by the reaction between the sugar produced by the plant and proteins in the sap. It was named by the German botanist Ludwig Marquart in 1835, the Greek anthos meaning flower combined with kyanos meaning blue, as it is responsible for reds and blues of many plants, according to the acidity of the sap. Early research focused on the red and blue anthocyanin coloration of fruits and flowers, as the color was important in attracting seed dispersing and pollinating animals and insects to economically important agricultural products, like apples and flowers.
Scientists are now conducting experiments that will ultimately answer the question why leaves turn red, or, more broadly, why some leaves produce anthocyanin. Two recent discoveries are germane. One involves a phenomenon known as photoinhibition. Under bright light conditions, damage to photosynthetic plant tissues occurs when one part of the two part photosynthesis process is blocked, or inhibited. Anthocyanin has the property that it absorbs damaging light wavelengths that are outside the range of other leaf chemicals. The anthocyanin is thus one of several strategies that an individual plant may evolve to limit the damaging effects of photoinhibition.
The second research discovery is that anthocyanin is an antioxidant. Intense sunlight results in the production of reactive oxygen species and free radicals (molecules with a negative charge due to having one free, unpaired, electron), which react strongly with cell membranes, proteins, and DNA, the destruction of which can lead to the death of the cell. This is the same problem experienced by all living things subjected to free radicals. People take ascorbic acid and vitamin E since they are antioxidants; that is they react with the free radicals to neutralize them. Anthocyanin has four times the antioxidant capacity of these vitamins. This is the source of the general precept that a glass of red wine (containing the anthocyanin of the grape skin) a day is good for you. So the anthocyanin performs the same function for a leaf as vitamin E, only better.
Even with the demonstrated protective capacity of anthocyanin to reduce photoinhibition damage and to neutralize free radicals, it is not clear why a tree would produce this rather large molecule (with constituents that could be better invested in food storage for the winter) just before it sheds its leaves. There are a number of other theories that have been advanced to explain why this is so. One is that the anthocyanin is a catalyst that allows the plant to reabsorb nutrients such as nitrogen from the leaf before it falls, reinforcing the plant for its eventual emergence from the senescence of autumn to the refulgence of spring.
There is another school of thought concerning the function of anthocyanin in the life cycle of a plant. The biological evolutionary explanation is that the red color either acts to protect the leaf from being eaten by other animals or that it attracts selected animals to eat the leaf for propagation purposes, like the red and blue fruits. Red and orange coloration is used throughout nature as a means to ward off predators. The red eft and the monarch butterfly are good examples. There is evidence that some tropical trees have red tips to ward off predators until they mature, at which time the leaves turn green to maximize production. Conversely, there is some evidence that chimpanzees and monkeys in Uganda use the red coloration of leaf tips to locate the tenderest leaves.
So, why do leaves turn red? They turn red because that they contain anthocyanin. Why do leaves produce anthocyanin? We don’t know. We know that anthocyanin absorbs blue and green light and therefore appears red. We know that the green chlorophyll masks the red color until chlorophyll production stops. There are some theories about the nature of anthocyanin production, but, if it is so beneficial to a plant, why do only some plants have it? And why aren’t more leaves red all the time? I think the answer lies in the marvelous complexity of nature and the intricacies of evolution. Each plant and animal finds its own niche through trial and error. Chance mutations lead each organism down a circuitous path that leads ultimately to a unique place in the ecosystem. And that is the glory of nature. Which is why leaves turn red.