What was done
Well watered and fertilized plants of four carrot (Daucus carota var. sativus L.) cultivars (Cascade, Caro Choice, Oranza and Red Core Chantenay) were grown from seed in 15-cm-diameter plastic pots in a controlled environment facility for 30 days past emergence, whereupon leaf net photosynthetic rate (PN), stomatal conductance (Gs), and transpiration rate (E) were measured at 100-ppm intervals of short-term (5-minute) atmospheric CO2 enrichment yielding absolute CO2 concentrations (Ca) stretching from 50 to 1050 ppm.

What was learned
First of all, the authors report that "an increase in Ca from 50 to 350 ppm produced a 100-fold increase in PN and the value increased by 43% when Ca was elevated from 350 to 650 ppm," but that "only [a] 7% increase in PN was observed when Ca was increased from 650 to 1050 ppm."  Second, they say that "increasing Ca from 50 to 350 ppm increased Gs to a maximum and thereafter Gs declined by 17% when Ca was increased to 650 ppm," while "a three-fold increase in Ca from 350 to 1050 ppm decreased Gs by 53%."  Third, "E reached maximum values (0.9-1.1 mmol m^-2 s^-1) at 350 ppm followed by a decline to 0.40-0.60 mmol m^-2 s^-1 when Ca was increased to 1050 ppm."  Fourth, "water use efficiency increased linearly with Ca due to increases in PN in addition to the decline in E at high Ca," such that "increasing Ca from 350 to 650 ppm improved water use efficiency by 76%, whereas a three-fold increase in Ca from 350-1050 ppm also resulted in a three-fold increase in water use efficiency."

What it means
Kyei-Boahen et al. conclude that "future enrichment in the atmospheric CO2 may lead to adjustments in PN and Gs, which could improve carrot productivity and water utilization."  How right they are!