What was done
The authors grew well-watered and fertilized maize (Zea mays L. cv. Pioneer 3733) plants from seed to developmental stage R3 (milky ripe stage, 70 days after planting) in sunlit soil-plant-atmosphere research (SPAR) chambers maintained at either 370 (ambient) or 750 (elevated) ppm CO2 concentrations, while periodically measuring a number of plant physiological parameters.

What was learned
At saturating photosynthetically-active radiation (PAR, 2000 µmol m^-2 s^-1), rates of leaf net photosynthesis in the elevated CO2 chambers were only 4% greater than those in the ambient CO2 chambers. However, upper-canopy leaves grown at elevated CO2 exhibited reductions in excess of 50% in both leaf stomatal conductance and transpiration; and as a result of these changes, instantaneous leaf water use efficiency was more than doubled in the high-CO2 treatment. Because of shading within the canopy, however, and because the difference in stomatal conductance between ambient and elevated CO2 declines as PAR drops, the CO2-induced reduction in canopy evapotranspiration was only a little over 20%.

What it means
"Although several previous studies reported that CO2 enrichment enhanced the growth of maize under well watered and fertilized conditions," according to Kim et al., they report that in their study "there was little evidence of increased biomass accumulation." On the other hand, they say that the sizable CO2-induced reduction in canopy evapotranspiration "is comparable to findings from other studies of C4 plants." Consequently, although the maize plants of this study were somewhat anomalous in their failure to significantly boost their growth in response to atmospheric CO2 enrichment, their more characteristic transpiration response enabled them to produce their slightly enhanced biomass with a considerably smaller consumptive use of water than that of the plants growing in ambient air.