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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.