[Effects of elevated CO₂ concentration on production and water use efficiency of spring wheat in semi-arid area.]

In the present study, the response of spring wheat production and water use efficiency (WUE) to the elevated CO₂ concentrations was investigated based on the open-top chamber (OTC) experiment platform in Dingxi, a typical semi-arid area. Three different CO₂ concentrations (390 μmol·mol^-1, 480 μmol·mol^-1 and 570 μmol·mol^-1) were involved. The results showed that the air temperature above plant canopy increased and the soil temperature at depth of 10 cm decreased by elevated CO₂. The increased CO₂ concentration substantially enhanced the total and each component biomass. The aboveground dry mass under the increased CO₂ concentrations (480 and 570 μmol·mol^-1) was increased by 20.6% and 41.5%, respectively, and the total dry mass was increased by 19.3% and 39.6%, respectively. The biomass enhacement was mainly due to the increases of dry mass of stems and leaves, which was strongly related to the material production capacity during the middle growth stage. The root/shoot ratio under the increased CO₂ concentrations (480 and 570 μmol·mol^-1) was decreased by 7.3% and 11.8%, respectively, indicating that the elevated CO₂ affected the dry matter accumulation of aboveground more than that of belowground. The yields of spring wheat under the increased CO₂ concentrations (480 and 570 μmol·mol^-1) were higher than that of the control by 8.9% and 19.9%, respectively, mainly due to the increase of grains per spike. The long-term effect of elevated CO₂ concentration on the photosynthesis of spring wheat was not obvious. The photosynthetic rate significantly increased, the transpiration rate decreased and the evapotranspiration reduced with the increases of CO₂ concentration. WUE at the leaf, population, and yield levels increased under elevated CO₂ concentration, with the increase range of WUE being the largest at the population level and the lowest at the yield level.