BY AFSHAN NABI (MBG/III)
afshan.nabi@ug.bilkent.edu.tr
This column might have been more effective had it reached you a little earlier, when the trees around you were turning color, impossibly fast and yet incredibly slow. Shades of brilliant yellow, soft gold, blazing red, and mosaics of those in between might have made it more satisfying. This idea arrived a little late; I offer many apologies and hope that the memory of the magic of autumn is still fresh in your mind.
You probably have a dim idea, from school biology, just as I had, about why leaves change color: different non-green pigments are already present in leaves, but since green chlorophyll is present in great amounts, the other pigments become visible only when the chlorophyll is degraded. Since chlorophyll is degraded in autumn, when the temperature begins to drop and the days begin to shorten, we see the secret, marvelous shades of all manner of trees only in autumn.
I decided to research further, since that is what studying molecular biology does to you; some curious, speculative part of you demands that you find definitive, clear-cut answers to questions like why leaves change color. I discovered two impressively startling things: the first is that our education has been carefully crafted to hide the truth, and the second is that no one knows why leaves change color.
Let us start with The Second Startling Discovery, since that will provide us with some evidence to defend the first one. Scientists have discovered that all the colors of autumn are a result of two main classes of pigments: carotenoids, which provide the yellow–orange, and anthocyanins, which are responsible for the red–purple. Carotenoids. Anthocyanins. Such elegant names are fitting for these creators of magical, fiery colors. Furthermore, it is known that carotenoids are present in leaves throughout the year; they are merely masked by the green of chlorophyll. In autumn, because chlorophyll is broken down into colorless substances, wonderful yellow–oranges begin to appear. Okay, that much is consistent with what we learned in school. But in contrast, anthocyanins are not present throughout; they are created in autumn in some trees, shortly before the leaves fall.
Biologists have come up with several hypotheses about why trees would expend energy on producing red pigments in leaves that are about to be shed anyway. The most popular of these is the photoprotection hypothesis, which states that the red color acts as a “sunscreen” to protect the leaves from oxidative damage, while the trees harvest all possible nutrients from them. Such a sunscreen is needed more in autumn because the breakdown of chlorophyll prevents “self-shading,” which is particularly important since trees receive an increased amount of light due to the thinning canopy.
The second most popular hypothesis is the coevolution hypothesis. Some insects have a life cycle wherein they migrate to a tree in autumn, and lay eggs on its twigs. These eggs hatch in spring, and the insects then develop on the tree before migrating to a summer host. For the tree, it is optimal to host a lower number of insects: insects cause damage to the tree, and feed on its sap, especially in spring when the baby insects are newly hatched. And as if stealing from their host were not enough, these insects also carry disease-causing bacteria, and viruses that can infect trees. Experts believe that the red pigmentation signals that the tree has a lower nutritional content than one with green-pigmented leaves. Insects able to recognize this color difference thus migrate preferentially to green-leaved trees. This ensures that weak trees have a better chance of survival, while trees with sufficient nutrients can afford to save energy by not making red pigment and hosting insects instead. Such a scheme provides better survival odds for both the insects and the trees.
Many experiments have been performed to test the validity of each of these hypotheses; some experiments support one of them, while others reject both. Scientists continue to design and carry out ingenious experiments to resolve this mystery. But in the meantime, nobody knows why some leaves turn red.
This brings us to The First Startling Discovery: education systems keep us in the dark while attempting to enlighten us. It does not feel right that in schools the focus is only on what humans know, and no glimpses of the gaps in between are allowed to reach students. Interesting, incredibly complex, incompletely understood phenomena are reduced to small paragraphs, with no hint that there is an ocean of detail underneath the surface that completely escapes our current understanding. It is only at university that one begins to appreciate the great dark unknown that exists outside human understanding; ironically, this happens while one is being bombarded by what is already known. I wonder why this is so; is it because kids, who are more aware, more accepting of all manner of mysteries, need reassurance that the world we live in is actually within the scope of our understanding? Or is it to reassure adults, who are alarmed by the unknown?
But the silver lining in this cloud is that the real party begins once the gaps in human knowledge have been discovered. Then you can start wondering about the incredible phenomena that surround you; you can look at beauty and attempt to understand it. Maybe you can find the answer to why leaves turn red.