中文摘要：

测定了人工模拟不同地下水埋深条件下幼龄胡杨、柽柳叶片的光合生理指标,通过逐步回归等方法分析2个物种在不同条件下净光合速率（Pn）与其生理生态因子的关系。结果表明：1潜地下水条件下,胡杨Pn显著高于无地下水条件,柽柳则差异不显著。2潜地下水条件下,胡杨Pn的主要影响因子依次为蒸腾速率（Tr）、气温（Ta）、大气水汽压亏缺（VPD）,其中Tr为主要决策因素,Ta为主要限制因素;柽柳主要受光合有效辐射（PAR）的影响。3无地下水条件下,胡杨Pn的主要影响因子依次为气孔导度（Gs）、胞间CO2浓度（Ci）、Tr,其中Gs为主要决策因素,Tr为限制因素;柽柳主要受Tr的影响。2个物种在水分充足条件下均通过降低水分利用效率（WUE）以增加水分消耗成本,维持高效碳同化水平,在无地下水利用时均通过增加WUE以维持碳同化能力,但柽柳调整能力高于胡杨,表明柽柳在水分利用方面倾向于灵活的策略以适应水分生境波动,胡杨则倾向保守。

英文摘要：

In this study,the portable LI- 6400 photosynthesis system was used to measure the photosynthetic traits of seedlings of Populus euphratica and Tamarix ramosissima under different groundwater depths. The relationships between net photosynthetic rate（ Pn） and environmental factors were analyzed by the stepwise regression analysis,path analysis and decision analysis. The results are as follows： 1 Pnof the seedlings of P. euphratica was significantly higher under high groundwater depth than that without groundwater availability,but there was no significant difference in that of T. ramosissima： 2 Under high groundwater depth,the daily variation of Pnof P. euphratica was mainly affected successively by transpiration rate（ Tr）,vapor pressure deficit（ VPD） and air temperature（ Ta）,among which VPD was the main decision factor,and Tawas the main restricting factor. In comparison,Pnof T. ramosissima was mainly affected by photosynthetically active radiation（ PAR）; 3 Under the conditions without groundwater availability,the daily variation of Pnof P. euphratica was mainly affected successively by stomatal conductance（ Gs）,intercellular CO2concentration（ Ci） and Tr,among which Gsand Ciwere the main decision factors,and Trwas the restricting factor. Differently,Pnof T. ramosissima was mainly affected by Tr. Our results suggested that both studied plant species decreased WUE under high groundwater depth,which was a strategy to increase carbon gain at the cost of high water use efficiency under favorable groundwater conditions. Under the conditions without groundwater availability,both species adjusted Gsto constrain water loss through evaporation.Under the conditions without groundwater availability,T. ramosissima was more flexible in water use strategy.