Ozone Puts New Wrinkle in CO2 Yield Projection
Plant physiologist Joe Miller (left) and
plant pathologist Allen Heagle discuss
an experiment on the effects of elevated carbon dioxide and ozone on soybeans.
| Concentrations of carbon dioxide in
our air are on the rise. During this century, CO2 levels are
expected to double what they were in preindustrial times.
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
cropsmainly 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,
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
ozoneat 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
clovera 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
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
"It doesn't seem to matter what ozone damage we
measurephotosynthesis, foliar injury, or yieldall are prevented by
CO2," 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.
The Raleigh findings have CO2 researcher Bruce A. Kimball scratching
his head. Kimball, who heads ARS' Environmental and Plant Dynamics Research
Unit in Phoenix, Arizona, says that many of the studies reporting yield
increases from CO2 enrichment were probably performed at low ozone
"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.
Steven J. Britz, who heads the Climate Stress Laboratory in Beltsville,
Maryland, adds, "The Raleigh experiments are a good example that the field
of CO2 response is still very much in development."By
Judy McBride, Agricultural
Research Service Information Staff.
This research is part of Global Change, an ARS National Program (#204)
described on the World Wide Web at
Joseph E. Miller and Allen S. Heagle are in the USDA-ARS Air Quality-Plant Growth and Development Research Unit, 3908 Inwood Rd., Raleigh, NC 27603; phone (919) 515-3312, fax (919) 515-3593.
"Ozone Puts New Wrinkle in CO2 Yield Projection" was published in the April 2000 issue of Agricultural Research magazine.