The Photosynthesis of Populus euphratica Oliv. Is Not Limited by Drought Stress in the Hyper-Arid Zone of Northwest China

The Ejin Oasis is located in the lower reaches of the Heihe River Basin of northwestern China. It is one of the most arid regions in the world, and Populus euphratica Oliv. is the foundation species of the desert riparian forests there. The photosynthesis of P. euphratica is one of the first physiological processes that is most likely to be affected by the extremely arid climate conditions. The factors impacting photosynthesis can be divided into stomatal and non-stomatal limitations. In order to investigate whether the photosynthesis of P. euphratica was limited and, if so, whether this limitation was caused by drought stress in the P. euphratica Forest Reserve on the Ejin River, we analyzed stomatal, non-stomatal, and relative stomatal limitations (reflecting the relative importance of the stoma in controlling the processes of photosynthesis) of photosynthesis. The results show that, at the beginning of the midday depression of photosynthesis, the values of stomatal limitation of photosynthesis (Ls) peaked, with its predominance being supported by sub-stomatal CO2 concentrations (Ci) being at a minimum. Thereafter, Ls decreased and non-stomatal limitation (Ci/stomatal conductance (gs)) increased sharply, indicating that the non-stomatal limitation of photosynthesis was predominant. Both Ls and relative stomatal limitation of photosynthesis increased in the morning, and then decreased, whereas Ci/gs showed the opposite trend. We concluded that P. euphratica did not experience drought stress by analyzing leaf water potential, groundwater table, and the decoupling coefficient (a parameter characterizing the coupling degree between vegetation canopy and atmospheric water vapor flux); however, the Ls values of P. euphratica were much greater than those of other species. This was likely because P. euphratica has a relatively conservative water use strategy even when growing under favorable water conditions. Extremely high temperatures caused the closure of the stoma to reduce transpiration, resulting in more intense stomatal limitations of photosynthesis.