Abstract:Based on data of net photosynthetic rate (Pn), stomatal conductance (gs), intercellular CO2 concentration (Ci), transpiration rate (Tr), water use efficiency (WUE) measured by a portable gas exchange system (LI-6400) in Populus euphratica olive grown at different groundwater depths in the lower reaches of the Tarim River, the photosynthetic characterization of P. euphratica and its response to the elevated CO2 concentration, \[CO2\], and temperature were analyzed. Results showed that the elevation of \[CO2\] decreased gs, increased Pn, Ci, WUE in P. euphratica, but the magnitude of the effect of elevated \[CO2\] on gs, Pn, Ci, WUE varied considerably with groundwater depth; the response of photosynthesis to rising \[CO2\] was stronger at the deeper groundwater depth than at the shallower groundwater depth. High temperature led to non-uniform stomatal closure and photoinhibition, which ultimately decreased the photosynthesis of P. euphratica. The magnitude of decrease resulting from higher temperature was stronger at the deeper (>6m) than at the shallower groundwater depth.(<6m). Our study indicates that groundwater depth is the critical factor controlling the response of photosynthesis of P. euphratica to elevated \[CO2\] and temperature, and that 6m is the threshold groundwater depth for the normal growth of P. euphratica. When the groundwater depth increased to more than 6m, P. euphratica would encounter moderate water stress, and subject to severe water stress when the groundwater depth is at >7m.