Ozone Damage on Morning Glory Plant (Credit: Image courtesy of University of Massachusetts Amherst)
Plants take in the carbon dioxide they need for photosynthesis through microscopic breathing pores in the surface of leaves. But for each molecule of the gas gained, they lose hundreds of water molecules through these same openings. The pores can tighten to save water when CO2 is abundant, but scientists didn't know how that worked until now.
A team led by Julian Schroeder, professor of biology at the University of California, San Diego, has identified the protein sensors that control the response. Enzymes that react with CO2 cause cells surrounding the opening of the pores to close down they report in the journal Nature Cell Biology online December 13.
The research isn't really about whether high levels of CO2 might damage plants, but rather about how to harness this mechanism to genetically modify crops - so that's why it has been suggestive but not definitive for my purpose of trying to understand what is killing trees. However having listened to RPauli's lecture, I am wondering if CO2 levels are actually the root cause because they are rising at much higher rates than plants can adapt to by limiting the number of stomata in their leaves. So perhaps trees are dying because their leaves are losing water because they have too many stomata for the current level of CO2 in the atmosphere! That is certainly how they have looked the last two summers - wilted.
Here's another provocative study, which has this to say:
“As human activity continues to raise atmospheric carbon dioxide levels, a better understanding of how plants respond to carbon dioxide is becoming imperative,” said Julian Schroeder, a professor of biology at UCSD who directed the project. “Our results provide new insights into how an increased concentration of atmospheric carbon dioxide leads to changes within a plant cell that trigger the closing of the stomata—the breathing or gas exchange pores in the leaf surface.”
And, as usual, concludes we just don't know much:
“These molecular mechanisms are like fundamental parts of machinery,” explained Young. “It's hard to predict what an instrument will do, if you don't even know anything about the parts that it is made from.
Similarly, this tantalizing report also examines, tangentially, the relationship between CO2 and the behavior of stomata, as it affects the nitrogen and water cycles, where a co-author concludes:
"Our environment and quality of life depend on less uncertainty on this front."
On the other hand:
“When ozone enters the leaf through the stomatal pores, it damages the plants photosynthetic machinery and basically causes green leaves to lose their color, a process called chlorosis,” said Julian Schroeder, a professor of biological sciences at UC San Diego and one of the principal authors of the recent study. “Plants have a way to protect themselves and they do that by closing the stomatal pores when concentrations of ozone increase.”
While this protective mechanism minimizes the damage to plants, he adds, it also minimizes their ability to photosynthesize when ozone levels are high, because the stomatal pores are also the breathing holes in leaves through which carbon dioxide enters leaves. The result is diminished plant growth or at least less than one might expect given the rising levels of carbon dioxide.
Some scientists assessing the impacts of rising greenhouse gases had initially estimated that increased plant growth generated from extra carbon dioxide in the atmosphere could sequester much of the excess atmospheric carbon in plant material. But in a paper published last July in Nature, researchers from Britain’s Hadley Centre for Climate Prediction and Research concluded that the damage done to plants by increasing ozone pollution would actually reduce the ability of plants to soak up carbon from the atmosphere by 15 percent which corresponds to about 30 billion tons of carbon per year on a global scale---a dire prediction given that humans are already putting more carbon into the atmosphere than plants can soak up.
"Heat waves, droughts and fuel prices are just a few reasons for the current global food crisis that is making headlines around the world. Research by William Manning of the University of Massachusetts Amherst indicates that rising background levels of ozone in the atmosphere are a likely contributor to the problem, lowering the yield of important food crops, such as wheat and soybeans.'Plants are much more sensitive to ozone than people, and a slight increase in exposure can have a large impact on their productivity,' says Manning, a professor of plant, soil and insect sciences. 'The new ozone standard set by the U.S. EPA in March 2008 is based on protecting human health, and may not be strict enough to protect plants.'"