Darwin and Fall Colors

In eastern France, one of the first plants to have red leaves is Virginia creeper (Parthenocissus quinquefolia). As the name suggests, the plant is originally from North America. Someone brought it from America and planted it in a European garden, and it spread widely from there, either by its vine branches, or by its seeds, which resemble grapes and are eaten by birds. Another plant species that has bright autumn leaf colors, including red, is another woody plant from North America: the sweetgum tree (Liquidambar styraciflua). This essay accompanies a YouTube video.

Leaves changing color in the autumn is part of a process known as senescence. It is part of a perennial plant’s way of getting ready for winter. It is a long, slow process. It must begin in late summer, before the temperatures even get chilly. The only way a leaf can “know” that autumn is coming is by measuring the length of the night, which gets longer every year starting on June 21 no matter what the temperature might be. Leaves use the pigment phytochrome to measure the length of the night.

Part of senescence is the formation of an abscission layer. This is essentially a layer of scar tissue at the base of the leaf that allows it to easily break off and blow away when senescence has finished. By the time this happens, the abscission layer has already healed up what would have been a gaping wound. As the abscission layer forms, the ability of the leaf to export molecules gradually declines. One of the reasons an old leaf is less valuable to a plant than a young leaf (as I will explain in a separate essay, with its own video) is that a young leaf does not have an abscission layer, and it is able to export sugar and other molecules rapidly to the rest of the plant. In an old leaf, there is some resistance to exporting photosynthetic products such as sugar. An old leaf produces less sugar, and in addition is less able to export that sugar to the rest of the plant.

Why do leaves turn yellow and red in the autumn? Let’s start with yellow. The yellow color is carotenoid pigments, which were present in the leaf throughout its life. In this photo, the non-water-soluble leaf pigments have been separated by a process of chromatography [ref]. You can see two green bands (chlorophyll a is emerald green, chlorophyll b is olive green) and yellow bands of carotenoids, including one faintly visible at the top. These pigments came from fresh young leaves of supermarket spinach.

Carotenoids are not directly photosynthetic, but they assist the chlorophyll in the process. When the plant recycles its chlorophyll—including the valuable magnesium atoms in the chlorophyll molecules—the yellow color that was there all along is revealed.

But why do leaves often turn red in the fall? The red color is anthocyanins. Of course, most of them do not. But in hundreds of woody plant species, autumn leaves turn red. The reason this happens is that a leaf in autumn has little photosynthesis, but it does have some, and it produces sugars, which are difficult to export. The leaf uses these sugars to manufacture anthocyanins. This especially occurs on crisply chilly but sunny autumn days.

It seems likely to me that the anthocyanins are the chance result of sugar buildup. That is, they do the plant no good, but just happen to be beautiful to us. But studies have shown that lots of woody plant species have independently evolved the ability to produce red leaves in the autumn. If something evolves over and over again, there is likely to be a good evolutionary reason for it. But what is that reason? It’s hard to say and impossible to prove.

One suggestion is that the red anthocyanins protect photosynthesis from ultraviolet radiation. When I first heard it, this explanation made no sense to me, since by the time a leaf turns red most of its useful life is past. But not quite all of it. Perhaps the red pigment protects the leaf cells from ultraviolet damage during the very important process of senescence itself.

Human plant breeders have selected many species that have bright red leaves even in the middle of the season. These plants do not grow as well as the ones with solid green leaves, but that does not matter. Their success depends not so much on growth and seed production as upon their ability to please human tastes.

We still do not know why most of the tree species that produce red leaves in the autumn are in northeastern North America and Asia. This photo shows bright autumn colors, due mostly to sugar maple (Acer saccharum) in New York.

In Europe, most native species just turn a dull yellow or maybe just degrade directly to brown. When American painters used bright red on their canvases, European painters thought they were just making it up, since there is nothing like it on the native landscapes of Europe.

If Darwin walked around his estate at Down, outside London, he would have seen red leaves in the autumn. The Down House website boasted about these colors earlier this year (the photos will probably be gone by the time you visit the site). But most of the plants with red leaves were Virginia creeper and sweetgum that were planted there.

Science is Alive and Well in Oklahoma, part one.

I posted the following on the Oklahoma Academy of Sciences blog a few weeks ago, but the rest of you might want to know about this too.

The Oklahoma Academy of Science held its 2014 Technical Meeting at Northeastern Oklahoma State University (Broken Arrow campus) on November 7, preceded by the Executive Council meeting on November 6. As president, I enjoyed watching and occasionally coordinating the good and enthusiastic work of so many students and faculty from around the state. Nobody had to be there. It was sheer enthusiasm for science that made the meeting a success.

I wanted to mention one paper that really got my attention. Lois Ablin, a chemist at Oral Roberts University, talked about advances in “green chemistry,” particularly in student organic chemistry laboratories. I took organic chemistry in 1976 and it has been downhill from there. Back then, we poured toxic chemicals all over the place (including benzene on our hands), and all of them ended up down the drain and probably out in the ocean (I was at UC Santa Barbara). Today, thankfully, we have many rules that preserve personal and environmental safety. One of the easiest ways to reduce the amount of waste produced by student labs is simply to use small-scale reactions. In my day we used whole flasks and beakers of toxic chemicals. But in green chemistry, the same reactions can be performed in small vials, heated in a microwave oven instead of over a burner or in a hot glove. It saves time, too: you can microwave a reaction for eight minutes with the same result that you would get with an hour-and-a-half reflux. Some universities have even gone so far as to carry out reactions on filter paper, rendering fume hoods unnecessary.

There were lots of student posters. This is an time for faculty to see the excellent work done by students at other universities. I barely had time to glance at them and take grainy photos. I got to stop and look at a poster from a student at Cameron University who had studied the stomach contents of a mammoth that had lived in what is now southern Oklahoma during the last ice age (in case you didn’t know there were mammoths here). The mammoth had eaten horsetails.

  

Bruce Carnes, from the University of Oklahoma Health Sciences Center, gave the luncheon presentation about the evolution of aging. What an interesting topic, especially for people who might have wondered what evolution has to do with medicine. For those who might have thought that aging is simply a problem that can be solved by some magic medical bullet, Bruce (who described himself as a disappointed optimist) had some bad news. Natural selection has indeed produced a human species that is guaranteed, in the absence of intrinsic and extrinsic accidents, to live for about 55 years, which is enough time not only for nearly all reproduction to be completed but for a person in tribal society to discharge their grand-parental duties as well. Fifty-five years, then, is our “warranty period.” After age 55, the body starts to break down in multiple ways. There’s no way to stop it, even though we try very hard to prolong our lives as much as possible. It makes more sense, Bruce indicated, to try to have a healthy old age rather than simply a long one. Once the “expiration date” has passed, a car or a person might keep running for a long time, but will require more and more intervention. Old age is not a problem to be solved but a process to be managed.

In the next entry, I will write about the symposium about science-related issues in the afternoon. It was one of the most exciting things the Oklahoma Academy of Sciences has ever done, I think.