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

Changes of stomatal conductance ( g), net photosynthetic rates ( P) and water use efficiency (WUE) were investigated in Populus euphratica grown on sites with different groundwater depths (GDs) under two CO concentrations in the lower reaches of Tarim River, Xinjiang, northwestern China. P in P. euphratica only slightly decreased when the groundwater depth increased from 4.12 to 7.74 m below the ground surface. P values significantly increased in response to an elevated CO 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 CO concentration. The g values measured at 4.12 and 4.74 m both decreased indistinctively by only 3% due to CO enrichment; however, when GDs increased to 5.54 and 7.74 m, the g values decreased significantly by about 10%. It shows that the response of g in P. euphratica to elevated CO 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 CO enrichment in P. euphratica in arid desert areas. [ABSTRACT FROM AUTHOR]