Photosynthesis and water use efficiency of Populus euphratica in response to changing groundwater depth and CO₂ concentration

Changes of stomatal conductance (g _s), net photosynthetic rates (P _N) and water use efficiency (WUE) were investigated in Populus euphratica grown on sites with different groundwater depths (GDs) under two CO₂ concentrations in the lower reaches of Tarim River, Xinjiang, northwestern China. P _N in P. euphratica only slightly decreased when the groundwater depth increased from 4.12 to 7.74 m below the ground surface. P _N values significantly increased in response to an elevated CO₂ concentration at all GDs except at GD of 4.12 m for its good availability of groundwater. WUE values decreased with an initial increase in the groundwater depth, but increased when the groundwater depth reached 7.74 m especially under the elevated CO₂ concentration. The g _s values measured at 4.12 and 4.74 m both decreased indistinctively by only 3% due to CO₂ enrichment; however, when GDs increased to 5.54 and 7.74 m, the g _s values decreased significantly by about 10%. It shows that the response of g _s in P. euphratica to elevated CO₂ is weaker under lower groundwater depth (mild drought stress) but stronger under deeper groundwater depth (moderate drought stress). Results from this study suggest that groundwater depth could determine the response of photosynthesis to future CO₂ enrichment in P. euphratica in arid desert areas.