An important source of uncertainty in predictions about global warming is how plants will respond to increasing atmospheric carbon dioxide. Now biologists at the University of California, San Diego have made significant advances toward understanding the mechanism plants use to regulate their carbon dioxide intake.
The study, published in the May 1 early on-line edition of the journal Proceedings of the National Academy of Sciences, shows how the level of carbon dioxide in the atmosphere controls the opening and closing of leaf stomata-pores through which plants "breathe" in carbon dioxide. The researchers say that their findings provide important insights into the cellular and genetic mechanisms through which increasing carbon dioxide emissions will impact the world's vegetation. The study will be published May 9 in the print edition of PNAS.
"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."
One of the standard arguments against taking action to reduce emissions of carbon dioxide from the burning of fossil fuels is that the elevated carbon dioxide will stimulate plants to grow faster. The assumption is that plants will take up excess carbon dioxide to produce carbohydrates-their stored energy source.
However, studies have shown that, contrary to expectations, increased carbon dioxide does not accelerate plant growth. Previous research has also shown that the doubling of atmospheric carbon dioxide expected to occur this century can cause leaf stomata to close by 20 to 40 percent in diverse plant species, thus reducing carbon dioxide intake. Little was known about the molecular and genetic mechanisms controlling this response.
Schroeder and colleagues discovered that in the cells surrounding the leaf stomata calcium ion "spikes"-or rapid increases and then decreases in calcium ion concentrations within cells-changed in frequency according to atmospheric carbon dioxide levels. As the carbon dioxide concentration was increased, the rapid drum roll of calcium spikes within the cells changed to a slower beat. The cells responded by reducing the size of the pores in the leaf.
In the presence of low carbon dioxide, a quick drumbeat was induced, but the stomata opened, rather than closed. Therefore, high carbon dioxide seems to prime the calcium sensors in the leaf. Jared Young, an assistant professor of biology at Mills College, who completed the study while he was a graduate student working with Schroeder at UCSD, likened this priming to removing ear plugs from someone at a rock concert.
"With very good ear plugs, someone might be able to sleep through the concert, but without the earplugs one would respond to the changes in the rhythms of the music," said Young. "Similarly, our findings suggest that carbon dioxide primes the calcium sensors to respond to the calcium spikes in the cell. Since changes in calcium concentration are used in other communication processes within cells, the need for sensor priming makes certain that the stomata don't close for inappropriate reasons."
The researchers speculate that narrowing the stomata in response to increased carbon dioxide may have an advantage for the plant.
"Even if a plant closes its pores a little in response to increasing atmospheric carbon dioxide, it would still have access to carbon dioxide," said Schroeder. "On the other hand, less water would be escaping through the pores, so the response might help plants use water more efficiently. Each plant species shows a different carbon dioxide responsiveness, and understanding the underlying mechanisms may make it possible to engineer improved water use efficiency in some crop plants or trees that will be exposed to higher carbon dioxide levels in the future."
In the paper, the researchers also report the discovery of the first known plant with a genetic mutation that makes it strongly insensitive to increased levels of carbon dioxide, which will provide additional information about the mechanism of plants' response to carbon dioxide levels. However, the researchers caution that a number of factors in addition to future atmospheric carbon dioxide concentrations, such as temperature, precipitation and available nutrient levels, will need to be considered before it will be possible to thoughtfully predict plant behavior based on molecular mechanisms.
"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. Identify and characterize the parts, and you can figure out how they fit together to generate the structure of the machine, and from there you can figure out what it will do when you push button A or pull lever B."
Other UCSD contributors to the study were Samar Mehta, a graduate student in the Department of Physics, Maria Israelsson, a postdoctoral fellow in the Division of Biological Sciences and Jan Godoski, a graduate student in the Division of Biological Sciences. Erwin Grill, a biology professor at the Technical University of Munich also contributed to the study.
The study was supported by grants from the National Science Foundation, the National Institutes of Health and the Department of Energy.