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Water Use Efficiency (Agricultural Species) -- Summary
As the air's CO2 content rises, most plants exhibit increased rates of net photosynthesis and biomass production.  Moreover, on a per-unit-leaf-area basis, plants exposed to elevated CO2 concentrations are likely to lose less water via transpiration, as they tend to display lower stomatal conductances.  Hence, the amount of carbon gained per unit of water lost per unit leaf area - or water-use efficiency - should increase dramatically as the air's CO2 content rises; and in this review, we summarize some recent experimental results pertaining to this phenomenon in agricultural crops.

In the study of Serraj et al. (1999), soybeans grown at 700 ppm CO2 displayed 10 to 25% reductions in total water loss while simultaneously exhibiting increases in dry weight of as much as 33%.  Thus, elevated CO2 significantly increased the water-use efficiencies of the studied plants.  Likewise, Garcia et al. (1998) determined that spring wheat grown at 550 ppm CO2 exhibited a water-use efficiency that was about one-third greater than that exhibited by plants grown at 370 ppm CO2.  Similarly, Hakala et al. (1999) reported that twice-ambient CO2 concentrations increased the water-use efficiency of spring wheat by 70 to 100%, depending on experimental air temperature.  In addition, Hunsaker et al. (2000) reported CO2-induced increases in water-use efficiency for field-grown wheat that were 20 and 10% higher than those displayed by ambiently-grown wheat subjected to high and low soil nitrogen regimes, respectively.  Also, pea plants grown for two months in growth chambers receiving atmospheric CO2 concentrations of 700 ppm displayed an average water-use efficiency that was 27% greater than that exhibited by ambiently-grown control plants (Gavito et al., 2000).

In some cases, the water-use efficiency increases caused by atmospheric CO2 enrichment are spectacularly high.  De Luis et al. (1999), for example, demonstrated that alfalfa plants subjected to atmospheric CO2 concentrations of 700 ppm had water-use efficiencies that were 2.6 and 4.1 times greater than those displayed by control plants growing at 400 ppm CO2 under water-stressed and well-watered conditions, respectively.  Also, when grown at an atmospheric CO2 concentration of 700 ppm, a 2.7-fold increase in water-use efficiency was reported by Malmstrom and Field (1997) for oats infected with the barley yellow dwarf virus.

In addition to enhancing the water-use efficiencies of agricultural C3 crops, as reported in the preceding paragraphs, elevated CO2 also enhances the water-use efficiencies of crops possessing alternate carbon fixation pathways.  Maroco et al. (1999), for example, demonstrated that maize - a C4 crop - grown for 30 days at an atmospheric CO2 concentration of 1100 ppm exhibited an intrinsic water-use efficiency that was 225% higher than that of plants grown at 350 ppm CO2.  In addition, Conley et al. (2001) reported that a 200-ppm increase in the air's CO2 content boosted the water-use efficiency of field-grown sorghum by 9 and 19% under well-watered and water-stressed conditions, respectively.  Also, Zhu et al. (1999) reported that pineapple - a CAM plant - grown at 700 ppm CO2 exhibited water-use efficiencies that were always significantly greater than those displayed by control plants grown at 350 ppm CO2 over a range of growth temperatures.

In summary, it is clear that as the CO2 content of the air continues to rise, nearly all of earth's agricultural species will respond favorably by exhibiting increases in water-use efficiency.  It is thus likely that food and fiber production will increase on a worldwide basis, even in areas where productivity is severely restricted due to limited availability of soil moisture.  Therefore, one can expect global agricultural productivity to rise in tandem with future increases in the atmosphere's CO2 concentration.

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.

De Luis, J., Irigoyen, J.J. and Sanchez-Diaz, M.  1999.  Elevated CO2 enhances plant growth in droughted N2-fixing alfalfa without improving water stress.  Physiologia Plantarum 107: 84-89.

Garcia, R.L., Long, S.P., Wall, G.W., Osborne, C.P., Kimball, B.A., Nie, G.Y., Pinter Jr., P.J., LaMorte, R.L. and Wechsung, F.  1998.  Photosynthesis and conductance of spring-wheat leaves: field response to continuous free-air atmospheric CO2 enrichment.  Plant, Cell and Environment 21: 659-669.

Gavito, M.E., Curtis, P.S., Mikkelsen, T.N. and Jakobsen, I.  2000.  Atmospheric CO2 and mycorrhiza effects on biomass allocation and nutrient uptake of nodulated pea (Pisum sativum L.) plants.  Journal of Experimental Botany 52: 1931-1938.

Hakala, K. Helio, R., Tuhkanen, E. and Kaukoranta, T.  1999.  Photosynthesis and Rubisco kinetics in spring wheat and meadow fescue under conditions of simulated climate change with elevated CO2 and increased temperatures.  Agricultural and Food Science in Finland 8: 441-457.

Hunsaker, D.J., Kimball. B.A., Pinter, P.J., Jr., Wall, G.W., LaMorte, R.L., Adamsen, F.J., Leavitt, S.W., Thompson, T.L., Matthias, A.D. and Brooks, T.J.  2000.  CO2 enrichment and soil nitrogen effects on wheat evapotranspiration and water use efficiency.  Agricultural and Forest Meteorology 104: 85-105.

Malmstrom, C.M. and Field, C.B.  1997.  Virus-induced differences in the response of oat plants to elevated carbon dioxide.  Plant, Cell and Environment 20: 178-188.

Maroco, J.P., Edwards, G.E. and Ku, M.S.B.  1999.  Photosynthetic acclimation of maize to growth under elevated levels of carbon dioxide.  Planta 210: 115-125.

Serraj, R., Allen, L.H., Jr., Sinclair, T.R.  1999.  Soybean leaf growth and gas exchange response to drought under carbon dioxide enrichment.  Global Change Biology 5: 283-291.

Zhu, J., Goldstein, G. and Bartholomew, D.P.  1999.  Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature.  Plant, Cell and Environment 22: 999-1007.