Stomatal and non-stomatal limitations to photosynthesis of Populus euphratica Oliv. leaves in an extremely arid area of northwestern China

Abstract On the basis of successive measurements of leaf gas exchange during the main growing seasons of Populus euphratica Oliv. in 2013 and 2014, respectively, we analyzed the stomatal and non-stomatal limitations to photosynthesis under natural conditions in an extremely arid region of northwestern China. Our results showed that (1) the distribution patterns of net photosynthesis (Pn) and stomatal conductance (gs) were similar, both of which increased in the morning, peaked at around noon, and then decreased. This contrasted with the observed changes in sub-stomatal CO2 concentrations (Ci). (2) The phenomenon of midday depression of photosynthesis (MDP) was obvious from July to September during the two years. At the beginning of MDP, the stomatal limitation to photosynthesis (Ls) peaked, where its predominance was supported by Ci being at a minimum. Thereafter, Ls decreased and Ci/gs increased sharply, indicating that the non-stomatal limitation to photosynthesis predominated. (3) Both the Ls and relative stomatal limitation to photosynthesis (RLs) increased in the morning, and then decreased, whereas Ci/gs presented contrary changes. (4) The RLs values were greater than the Ls values, which was mainly due to the nonlinearity of the Pn/Ci curve, which often leads to large overestimations. (5) The Ls values in our study were much greater than those from other studies under natural conditions. The most probable reason was that the extremely high temperature and scarce water resource caused the stomata to close to reduce transpiration, resulting in the stomatal limitation to photosynthesis being more intense.