To determine how elevated CO2 may reduce water use by crops,
plant physiologist James Bunce measures water vapor conductance of barley
leaves grown at twice the current atmospheric CO2 concentration.
Understanding climate change on a
global scale means getting up close and personal with a single plant--or even
with a single cell in a plant.
"Nature has a way of rewarding those who take the time to look closely
at basic processes," says Steven J. Britz, an ARS plant physiologist at
Agency scientists around the country are examining how elevated atmospheric
CO2 and other greenhouse gases affect three essential biological
processes: respiration, or the exchange of oxygen for CO2; the use
of light in photosynthesis to remove CO2 from the air for plant
growth and reproduction; and water use.
Research to date both confirms some long-held beliefs about plant response
to elevated CO2 and adds to what we already know.
For example, elevated levels affect a plant's respiration. James A. Bunce,
an ARS plant physiologist at Beltsville, grew soybean plants in CO2
chambers at nearly double the current atmospheric level. Surprisingly, while
higher levels of CO2 increased plant growth, they lowered plant
respiration. "We expected the plants to have a higher rate of
respiration," says Bunce. "It's still a mystery how the rate of
respiration can be reduced without a negative impact on the plant."
Photosynthesis in wheat plants can be measured in field
chambers like this one being adjusted by plant physiologist Richard
Other studies show that changes in the
atmosphere affect how plants use water. Like scientists at other ARS
laboratories, Bunce and colleagues found that plant water use changes
dramatically when the plants grow in higher atmospheric CO2.
By studying the plant stomata--the pores on the leaf surface that regulate
water loss from the leaf--they found that at higher CO2 levels,
plants use less water to produce the same amount of growth. This response is
commonly seen in the growth chamber and greenhouse, but the overall reduction
in water use for crops grown in the field seems to be less than 5 percent, for
reasons that are not yet understood.
ARS soil scientist Bruce A. Kimball and colleagues at Phoenix, Arizona,
confirmed that plant photosynthesis is immediately stimulated when you double
the atmospheric CO2. He also showed it doesn't necessarily slow down
over time in crops such as wheat and cotton or fruit trees like oranges. In
experiments with sour orange trees, Citrus aurantium, physicist Sherwood
B. Idso observed sustained explosive growth over a 9-year period when the trees
grew outdoors under experimentally elevated CO2.
Physicist Sherwood Idso and soil scientist Bruce Kimball assess
fruit production on an orange tree growing in an open-top chamber with enriched
Scientists speculate that this level of
response to increased CO2 concentrations will lead to an overall net
increase in productivity in many ecosystems.
Other greenhouse gases can add to this effect. For example, ARS plant
physiologist Joseph E. Miller and co-workers at Raleigh, North Carolina, found
that the atmospheric concentration of ozone near ground level affected the
degree to which elevated atmospheric CO2 stimulated photosynthesis
in soybean leaves. Under today's CO2 concentrations, ozone can
suppress photosynthesis, but Miller's experiments showed that photosynthesis
and yield were increased more by elevated CO2 if plants were
stressed by ozone.
"This is one example of the complexities involved in understanding how
plants will respond to global environmental change," Miller says.
"Clearly, we have a lot to learn about how the different contributors to
climate change interact--and how those interactions will affect plant
The Free Air CO2 Enrichment project (FACE) in Arizona is helping
scientists from around the world to understand how plants respond to actual
field conditions representing those anticipated in the next 50 to 75 years.
Large amounts of CO2 are vented through upright pipes that maintain
a constant CO2 concentration of 550 parts per million in the
atmosphere around the plants.
"Our FACE project, begun in 1989, is the longest running of five now
providing researchers with information needed to assess impacts of global
change," says Kimball. "We have studied cotton and wheat, while the
other experiments concentrate on forage grasses, loblolly pine, chaparral, and
desert plants." In general, Kimball's work has shown that crop yields
increase as CO2 rises.