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Agriculture (Species - Sorghum: Water Relations) -- Summary
How will atmospheric CO2 enrichment impact the water relations of Sorghum bicolor (L.) Moench, one of the world's most important C4 crops, and what influence will that impact have on its agricultural potential in a future high-CO2 world?

In investigating this important question, Conley et al. (2001) grew sorghum plants for two consecutive years in a FACE experiment conducted near Phoenix, Arizona, USA, where the plants were fumigated with air containing either 370 or 570 ppm CO2 and where the crop was further subjected to irrigation regimes resulting in either adequate or much-less-than-adequate levels of soil moisture. Averaged over the two years of experimentation, the extra CO2 reduced cumulative crop evapotranspiration by 10% and 4% under the well-watered and water-stressed conditions, respectively; and when considered in conjunction with the concomitant CO2-induced increases in grain yield reported by Ottman et al. (2001), i.e., +4% and +16% in the wet and dry treatments, respectively, the researchers determined that the added CO2 increased sorghum water-use efficiency (grain yield per applied water) by 9% and 19% in the well-watered and water-stressed treatments, respectively.

In light of these findings, it would appear that as the air's CO2 content continues to rise, sorghum will likely require less water than it does currently to produce equivalent, or even greater, grain yields; and in areas subject to drought and low levels of soil moisture, the lower total water requirement of sorghum growing in CO2-enriched air should further enhance its agricultural potential. This is basically the same conclusion as that reached by Triggs et al. (2004), who in their report of a similar study conducted at the same site stated that "even if future climate change results in less water available for agriculture, higher atmospheric CO2 concentrations will still benefit C4 crops."

Building upon these findings, Grant et al. (2004) adjusted the crop growth and water relations model ecosys to represent sorghum and ran it for two full growing seasons (1 May 1998 to 31 Oct 1999) under both wet and dry irrigation schedules at two atmospheric CO2 concentrations (approximately 368 and 561 ppm) using hourly meteorological data measured in a field south of Phoenix, Arizona, USA, after which its simulated energy balances and water relations, which were verified by measurements of energy flux and water potential, were used to infer the effects of free-air atmospheric CO2 enrichment, i.e., FACE, on various plant parameters and processes.

The twelve researchers report that "model results, corroborated by field measurements, showed that elevated CO2 raised canopy water potential and lowered latent heat fluxes under high irrigation [both of which responses are beneficial] and delayed water stress under low irrigation [which is also beneficial]," or as they describe it in another place in their report, the elevated CO2 "reduced transpiration and hence improved water status of sorghum [and] lowered the vulnerability of sorghum CO2 fixation to soil or atmospheric water deficits, even when irrigation was high." Also, in applying their reality-tuned model to a scenario where the air's CO2 content is 50% higher and air temperature is 3°C warmer, they calculated that sorghum yields would rise by about 13%, and that "current high sorghum yields could be achieved with ~120 mm or ~20% less irrigation water if these rises in temperature and CO2 were to occur."

In light of these several observations, it would appear that rising atmospheric CO2 concentrations, even in the face of rising air temperatures, should be good for sorghum and the people who grow this important C4 crop, because of (1) the higher grain yields that can be produced under these conditions and (2) the smaller amounts of water needed to produce them.

References
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.

Grant, R.F., Kimball, B.A., Wall, G.W., Triggs, J.M., Brooks, T.J., Pinter Jr., P.J., Conley, M.M., Ottman, M.J., Lamorte, R.L., Leavitt, S.W., Thompson, T.L. and Matthias, A.D. 2004. Modeling elevated carbon dioxide effects on water relations, water use, and growth of irrigated sorghum. Agronomy Journal 96: 1693-1705.

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.

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.

Last updated 1 March 2006