Climate Change and Basic Processes

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 Beltsville.

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 Garcia.

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 CO₂.

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 function."

The FACE Project

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.

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