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Elevated CO2 Enhances Water Use Efficiency in Field-Grown Sorghum
Conley, M.M., Kimball, B.A., Brooks, T.J., Pinter Jr., P.J., Hunsaker, D.J., Wall, G.W., Adams, N.R., LaMorte, R.L., Matthias, A.D., Thompson, T.L., Leavitt, S.W., Ottman, M.J., Cousins, A.B. and Triggs, J.M.  2001.  CO2 enrichment increases water-use efficiency in sorghum.  New Phytologist 151: 407-412.

What was done
Sorghum (Sorghum bicolor) plants were grown for two consecutive seasons in a FACE experiment conducted near Maricopa, Arizona, USA, within 25-meter-diameter field plots.  Plants were fumigated with air containing 370 and 570 ppm CO2 and were further subjected to irrigation regimes resulting in adequate and much less than adequate levels of soil moisture.  In this paper, the authors report the interactive effects of elevated CO2 and soil moisture on water use efficiency in sorghum, which is an important agricultural C4 crop in many parts of the world.

What was learned
Averaged over the two years of experimentation, elevated CO2 reduced cumulative crop evapotranspiration by 10 and 4% under well-watered and water-stressed conditions, respectively.  When analyzed in conjunction with their previously reported grain yield results (see our Journal Review Elevated CO2 Increases Yield in Water-Stressed Sorghum), the authors determined that atmospheric CO2 enrichment enhanced plant water-use efficiency on a grain-yield basis by 9 and 19% in well-watered and water-stressed treatments, respectively.

What it means
As the atmospheric CO2 concentration continues to rise, it is likely that sorghum will require less water than it currently uses to produce equivalent, or even greater, grain yields.  In addition, with lower total water requirements for this crop, its production in areas that are subjected to drought and low levels of soil moisture is likely to increase.