Ozone inhibits the ability of plants to open the microscopic pores on their leaves and breathe.
By John Roach
Wednesday, December 8, 1999
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Ozone inhibits the ability of plants to open the microscopic pores on their leaves and breathe. |
Ozone, a major smog constituent, inhibits the ability of plants to open the microscopic pores on their leaves and breathe, according to research published Tuesday in Proceedings of the National Academy of Sciences.
"Our work has shown, for the first time, that, rather than causing the pores or stomates on a plant's leaves to close, as was generally assumed, ozone actually inhibits stomatal opening by directly affecting the 'guard cells' that control the opening", said Gro Torsethaugen, a researcher at Penn State University.
The research could eventually lead to genetically engineered or selectively bred crops that offset the $3 billion in agricultural losses caused by ozone pollution in the United States each year.
"When we understand the mechanisms of how ozone impacts plants we might be able to genetically engineer plants that are more tolerant to ozone", she said.
The researchers already knew that ozone negatively impacts the process of photosynthesis by reducing the amount of carbon dioxide they are able to process and release as oxygen.
Torsethaugen and her colleagues conducted experiments on fava bean plants and discovered that ozone directly affected the opening of plant pores. To learn more, they looked at guard cells and flow of potassium.
 Plants take in the carbon dioxide they need through their stomates. |
As potassium builds up in a guard cell, water also comes into the cell via osmosis and the guard cells surrounding the stomote, or opening, swell. This swelling causes the pores to open, she said.
When exposed to ozone, less potassium flowed into the guard cells and thus inhibited the pores from opening.
In their paper, the researchers note that their findings may have particular relevance during drought.
"Stomatal closure during a period of drought may be less readily reversed in ozone-exposed plants. This may be particularly relevant because the highest ozone concentrations are sometimes associated with times of drought", the authors write.
The researchers also note that ozone inhibition of stomatal opening could retard stomatal reopening after the so-called midday depression because of the high solar irradiation associated with ozone generating smog.
"During the middle of the day when the temperature and light intensities can be very high, some plants do reduce their stomatal aperture to conserve water", said Torsethaugen.
Ozone from Seattle, Wash., and other urban centers in the state is drifting into Mount Rainier National Park and affecting the air quality there, according to a recent U.S. Geological Survey study.
According to Dr. David Peterson of the USGS Forest and Rangeland Ecosystem Science Center, the air around Mount Ranier contains higher concentrations of ozone, a major component of air pollution, than nearby urban centers. The study provides more evidence that pollution from outside sources does affect protected areas such as natural parks.
The study was published in the scientific journal "Atmospheric Environment".
Peterson and his students monitored ozone at Mount Rainier National Park from 1993 to 1997 and quantified the spatial distribution of this pollutant throughout western Washington in 1996.
The park consistently had the highest average weekly levels of tropospheric ozone measured anywhere in the state. According to the scientists, ozone concentrations tend to increase at higher altitudes, partly because of passive dispersion from the stratosphere, but largely due to the transport of pollutants by prevailing winds inland from urban sources, such as the Seattle metropolitan area.
Ozone is a natural component of the Earth’s atmosphere, but its effects vary depending on where and in what concentration it occurs. High above the Earth’s surface, in the stratosphere, a protective layer of ozone screens the Earth from biologically harmful frequencies of ultraviolet radiation. The stratospheric ozone layer is essential to the existence of many forms of life.
However in the lower part of the atmosphere, called the troposphere, human-produced ozone can be a dangerous pollutant. The colorless gas is formed from byproducts released during the burning of fossil fuels, and can be toxic to both plants and animals, including humans, even at fairly low concentrations.
"It’s well-documented that both periodic episodes of high ozone exposure and chronic moderate ozone exposure can be harmful to plants", said Peterson. "We know this from other regions of North America including national parks such as Sequoia and Great Smoky Mountains".
Ozone is usually caused by industrial pollutants and exhaust from automobiles. Emissions tend to be concentrated in urban areas, however the common assumption that ozone pollution is strictly an urban problem is proving to be false, Peterson said.
According to the study, a climber at Paradise, a common destination in the park that is 60 miles from Seattle, is subject to twice the monthly mean ozone concentration as Lake Sammamish, which is near sea level and less than 10 miles east of Seattle.
Mount Rainier National Park Resource Manager Barbara Samora said the ozone study "is just one more indication of how difficult it is to protect our national parks. It isn’t a simple matter of telling people to stay on footpaths or to sensibly locate campgrounds and parking lots. How do you manage a threat that is produced 60 miles from the park and transported here by winds?"
"It is time to consider the potential for damage to park ecosystems and even potential health hazards to park visitors", Peterson said. "We’ve long known that parks cannot be managed as islands, separate from their surroundings. These findings about tropospheric ozone concentrations lend additional support to that position".
For more information, contact David Peterson, University of Washington, (206)543-1587, e-mail: wild@u.washington.edu.