What was done
The authors grew rice (Oryza sativa L. cv. Akita-Komachi) from hand-transplanting to harvest (May to September) under normal paddy culture near Shizukuishi, Iwate, Japan, within FACE rings maintained at either ambient or ambient + 200 ppm CO2 for 24 hours per day. Over this period they measured a number of micrometeorological parameters and plant characteristics that enabled them to calculate both the amount of water lost directly from the paddy-water surface and that lost by plant transpiration, which together with the plant biomass data they obtained at harvest enabled them to calculate total growing-season crop water use efficiency.

What was learned
Yoshimoto et al. report that "elevated CO2 reduced stomatal conductance by 13% in upper leaves and by 40% in lower leaves at the panicle initiation stage," but that the reduction declined thereafter. In addition, they observed that "stomata closed more in the elevated CO2 plot as vapour pressure deficit increased," i.e., during drier conditions. In more common terms, and averaged over the entire growing season, the Japanese researchers determined that the total water used by the crop was 268.7 mm in the ambient CO2 treatment and 246.7 mm in the elevated CO2 treatment. Combining this CO2-induced reduction in total evaporative water loss (8.2%) with the CO2-induced increase in total plant biomass that was observed (9.1%) thus indicated that season-long crop water use efficiency rose by about 19% in response to the ~54% increase in atmospheric CO2 concentration provided by the FACE apparatus.

What it means
As world population continues to grow, the increase in rice-crop water use efficiency provided by the concomitant increase in the atmosphere's CO2 concentration will be a great asset in helping to produce the extra food that will be needed to feed the planet's newcomers, while it simultaneously spares some of the precious water that will be needed to slake their thirst.