Growth of Water-Stressed Maize and Sorghum Plants.

Reference
Allen Jr., L.H., Kakani, V.G., Vu, J.C.V. and Boote, K.J. 2011. Elevated CO2 increases water use efficiency by sustaining photosynthesis of water-limited maize and sorghum. Journal of Plant Physiology 168: 1909-1918.
Background
The authors write that "plants of the C4 photosynthetic pathway have a CO2-concentrating mechanism that overcomes limitations of low atmospheric CO2"and which thereby provides them with "a near-saturating photosynthetic capability at current atmospheric CO2." In this circumstance, as they continue, "a rise in atmospheric CO2 will theoretically have a limited direct impact on C4 photosynthesis." Nevertheless, they note that "a number of C4 crop plants express a positive response to elevated growth CO2, although to a smaller extent compared to C3 plants," citing the analyses of Kimball (1993) and Poorter et al. (1996).
What was done
Maize (Zea mays L. cv. Saturn Yellow) and grain sorghum (Sorghum bicolor L. cv. DeKalb 28E) seeds were sown in pots and grown for 39 days in sunlit controlled-environment chambers at 360 and 720 ppm CO2 concentrations, while throughout this period canopy net photosynthesis and evapotranspiration were measured and summarized daily from 08:00 to 17:00 hours, with irrigation being withheld from matched pairs of treatments starting 26 days after sowing, and with biomass determinations being made at 34 and 39 days after sowing for maize and grain sorghum, respectively.
What was learned
The four researchers report that their data indicated that for both maize and grain sorghum, there was a "maintenance of relatively high canopy photosynthetic rates in the face of decreased transpiration rates [that] resulted in enhanced water use efficiency when these plants were grown at elevated CO2 of 720 ppm, but not at 360 ppm." And as a result, they demonstrated that "both plants maintained growth better at double-ambient CO2 than their counterparts at ambient CO2 in the presence of drought," such that reductions in total above-ground biomass due to drought were 42% for maize and 36% for sorghum at ambient CO2, but only 18% for maize and 14% for sorghum at double-ambient CO2.
What it means
In discussing their several findings, Allen et al. write in their paper's last paragraph that they "agree with Leakey (2009) that drought stress in C4 crop plants can be ameliorated at elevated CO2 as a result of lower stomatal conductance and sustained intercellular CO2." And as a result of this experimentally-observed fact, they conclude that "management of irrigation water in a future high CO2 world could potentially increase overall C4 crop yields (in water-limited areas) by allocating limited water supplies [so as to be able] to irrigate larger crop areas at sub-optimal [as opposed to conventional optimal] plant-available soil water levels, at least during vegetative stages of growth," thereby obtaining greater total yields with the same allocation of available water.
References
Kimball, B.A. 1993. Effects of elevated CO2 and climate variables on plants. Journal of Soil and Water Conservation 48: 9-14.
Leakey, A.D.B. 2009. Rising atmospheric carbon dioxide concentration and the future of C4 crops for food and fuel. Proceedings of the Royal Society B 276: 2333-2343.
Poorter, H., Roumet, C. and Campbell, B.D. 1996. Interspecific variation in the growth response of plants to elevated CO2: a search for functional types. In: Korner, C. and Bazzaz, F.A. (Eds.). Carbon Dioxide, Populations, and Communities. Academic Press, New York, New York, USA, pl 375-412.

....