Photosynthesis of Populus euphratica and its response to elevated CO2 concentration in an arid environment

The photosynthetic characterization of Populus euphratica and its response to the elevated carbon dioxide concentration (CO2]) were analyzed based on its net photosynthetic rate (Pn), stomatal conductance (gs), intercellular CO2 concentration (Ci), transpiration rate (Tr), and water use efficiency (WUE) at different groundwater depths measured by a portable gas exchange system (LI-6400) in the lower reaches of the Tarim River. The results showed that the elevation of CO2] decreased the gs, and increased the Pn, Ci and WUE of P. euphratica. However, the effects of the elevated CO2] on gs, Pn, Ci and WUE varied considerably with groundwater depth. The response of photosynthesis to rising CO2] was stronger at the greater groundwater depth (more than 6 m) than that at the shallower groundwater depth (less than 6 m). The critical groundwater depth required to maintain the normal survival of P. euphratica was less than 6 m. When the groundwater depth increased to more than 6 m, P. euphratica encountered moderate water stress, and the plant suffered severe water stress when the groundwater depth increased to more than 7 m.