Ozone Puts New Wrinkle
in CO₂ Yield Projection

Rising CO₂ stimulates growth and increases yield in a variety of food and fiber crops. But will yields surge enough to feed the world's population of 9 billion expected by 2050?

To find out, scientists have developed computer models for each of the major crops—mainly grains. "We use models to project the effects of both rising CO₂ and projected climate changes on world food supply," says Cynthia Rosenzweig. She heads the project at NASA's Goddard Institute for Space Studies at Columbia University in New York City.

New ARS research suggests that the models may often overestimate how much CO₂ enrichment will stimulate growth. The scientists are proposing that another gas be included in the models: the air pollutant ozone. This oxidizing agent damages plant tissue and decreases crop yield.

"But studies to measure the positive effects of CO₂ and the negative effects of ground-level ozone on crop yield have traditionally looked at each gas separately," says plant physiologist Joseph E. Miller. He is the head of ARS' Air Quality-Plant Growth and Development Unit in Raleigh, North Carolina.

Miller and colleagues Allen S. Heagle, Edwin L. Fiscus, and Fitzgerald L. Booker have been combining the two gases in various concentrations on several crops throughout the growing season. The results suggest that much of the CO₂ yield boost reported by other researchers may not be true increases but rather a case of CO₂ preventing losses caused by ozone—at least in ozone-sensitive crops.

"When ozone stress is low, increasing CO₂ concentration does not always stimulate plant growth a lot," says Heagle. Extra CO₂ by itself will cause some growth stimulation because the plants have more "food" for photosynthesis.

So far, the researchers have seen this interaction in field tests of soybeans, winter wheat, rice, and cotton and in greenhouse tests of snap bean and white clover—a forage crop. Each crop or variety responds to a greater or lesser degree, depending on its sensitivity to each gas, but the trend is always the same.

Soybeans are quite sensitive to ozone, as is cotton, Miller notes. And some varieties of wheat and rice also suffer damage and yield loss under high ozone.

Other researchers have reported a similar interaction between CO₂ and water stress. Some crops don't respond as much to high CO₂ when they get ample water throughout the growing season, says Heagle.

He explains that extra CO₂ partially closes the leaf pores, or stomates, through which plants exchange gases. This reduces the ozone that gets in and the water vapor that gets out. "So if plants are under ozone or water stress, you'll get a greater response to CO₂ enrichment. The more ozone stress, the more damage prevention and the greater the apparent growth stimulation."

"It doesn't seem to matter what ozone damage we measure—photosynthesis, foliar injury, or yield—all are prevented by CO_2," says Heagle. "The greater the ozone stress, the greater the amount of CO₂ that will be needed to prevent it, and it's not necessarily a linear relationship."

Why do some experiments show greater response to elevated CO₂ than others? "It may be due to unrecognized differences in water or ozone stress among different studies," says Miller.

"I think the CO₂ stimulation [the Raleigh researchers] are getting in low ozone is generally lower than what is reported in the literature," says Kimball.

This research is part of Global Change, an ARS National Program (#204) described on the World Wide Web at http://www.nps.ars.usda.gov/programs/nrsas.htm.