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Water Use Efficiency of Sorghum in a CO2-Enriched World of the Future
Triggs, J.M., Kimball, B.A., Pinter Jr., P.J., Wall, G.W., Conley, M.M., Brooks, T.J., LaMorte, R.L., Adam, N.R., Ottman, M.J., Matthaias, A.D., Leavitt, S.W. and Cerveny, R.S.  2004.  Free-air CO2 enrichment effects on the energy balance and evapotranspiration of sorghum.  Agricultural and Forest Meteorology 124: 63-79.

What was done
The authors grew sorghum (Sorghum bicolor (L.) Moench, a C4 grain crop) for two full seasons in control CO2 plots (about 370 ppm) and FACE plots (Control + 200 ppm) under both well-watered (Wet) and water-stressed (Dry, less than half the total water received by the Wet treatment via rainfall and irrigation) conditions near Maricopa, Arizona, USA, while assessing evapotranspiration (ET) on a continuous basis by means of micrometeorological measurements designed to allow the calculation of all of the other elements (net radiation, sensible heat flux, and soil surface heat flux) of the energy balance of the crop-soil interface with the atmosphere.  Simultaneously, final grain yields, which were used to calculate sorghum water use efficiency (WUE), were obtained by Ottman et al. (2001).

What was learned
"In the Wet treatments," in the words of Triggs et al., "a reduction in ET of about 19%, combined with only a slight increase in total biomass (+4%), resulted in a 28% increase in WUE in elevated CO2 conditions," while "in the Dry treatments, the relatively large increase in total biomass (+16% for both years) more than compensated for the approximate 5% increase in total ET, giving the FACE-Dry treatments an increase in WUE of 16% over both seasons."

What it means
Based on the results of their measurements, Triggs et al. conclude that "even if future climate change results in less water available for agriculture, higher atmospheric CO2 concentrations will still benefit C4 crops," while stating that "in regions with ample precipitation or irrigation, C3 crops with higher growth responses may be preferable."

Ottman, M.J., Kimball, B.A., Pinter Jr., P.J., Wall, G.W., Vanderlip, R.L., Leavitt, S.W., LaMorte, R.L., Matthias, A.D. and Brooks, T.J.  2001.  Elevated CO2 increases sorghum biomass under drought conditions.  New Phytologist 150: 261-273.

Reviewed 6 October 2004