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CO2 Won’t Beat Your Wheat

By Robert C. Balling Jr., Ph.D.
Arizona State University

A current issue of New Phytologist is completely devoted to the effect of elevated atmospheric carbon dioxide on the global ecosystem. Not surprisingly, several of those articles deal exclusively with wheat growth rates; indeed, wheat is very popular with the scientific set, thanks to its responsiveness to weather and climate.

In the first study, a team of scientists from the Netherlands, Argentina, Denmark, and Germany built a sophisticated computer model of wheat-growth processes. They tested that model using wheat grown at CO2 levels of 410 parts per million (ppm) and 680 ppm. Among their many findings, Rodriguez and colleagues reported that "elevated CO2 promoted final biomass by 12 percent" and "photosynthesis was increased by 33 percent" when CO2 was higher. Radiation use efficiency, they found, increased by 39 percent when atmospheric carbon dioxide rose.

Scientists from Arizona, Germany, and Nebraska developed a numerical model of wheat growth that they compared with their model simulations to wheat grown in central Arizona with natural and elevated (550 ppm) atmospheric CO2 concentrations. Grossman-Clarke and colleagues noted from the outset that many previous studies had shown "an average increase in growth and yield of about 30 percent" when CO2 doubled. In discussing final grain biomass, they found that "This translates in a CO2 effect of 12 [percent] and 15 percent for the measurements and the simulations, respectively, under unlimited water supply and the higher values of 25 [percent] and 34 percent under water limitation." Furthermore, "The CO2 effect on grain mass is clearly higher under water limitation for both the measured and simulated results." They found that root biomass increased under elevated CO2, which gave the plants a competitive advantage in terms of water uptake.

And finally, a team of scientists from Washington and Arizona grew spring wheat over four different growing seasons in central Arizona in field plots with atmospheric CO2 concentrations maintained at natural (near 350 ppm) and 550 ppm. The levels of irrigation and nitrogen fertilizer varied. When the nitrogen levels were low, the grain quality and bread-making quality were negatively impacted by lower CO2. But they concluded that "with ample nitrogen fertilizer, the effects will be minor."

These three articles show us that with elevated atmospheric CO2 concentrations, a) wheat biomass will increase, b) wheat increases its overall growth efficiency, and c) with adequate fertilizer, future grain quality need not suffer.

Reference:

Grossman-Clarke, S., et al.,. 2001. Modelling a spring wheat crop under elevated CO2 and drought. New Phytologist, 150, 315–335.

Kimball, B.A., et al.,. 2001. Elevated CO2, drought and soil nitrogen effects on wheat grain quality. New Phytologist, 150, 295–303.

Rodriguez, D., et al., 2001. Modelling the response of wheat canopy assimilation to atmospheric CO2 concentrations. New Phytologist, 150, 337–346.