Abstract:To clear the main factor affecting photosynthesis and provide a theoretical basis for the restitution and restoration vegetation in transition zone,we determined gas exchange parameters of Alhagi sparsifolia and Tamarix ramosissima by the Li-6400 (LICOR,USA) under natural conditions from July to September,analyzed the relationship between photosynthesis and environmental factors,such as photosynthetically active radiation (PAR),air temperature (Ta),air CO2 concentration (Ca),air relative humidity (RH).The results show that:(1)The diurnal change pattern of Pn of A.sparsifolia is one-humped curve from July to September,while it could be expressed as a two-humped curve on July and September.The mean values of Pn were 7.08 μmol·m-2·s-1 and 5.54 μmol·m-2·s-1.(2)The curves of diurnal variation in Tr had one peak,the mean values of Tr were 5.46 mmol·m-2·s-1 and 4.40 mmol·m-2·s-1.They all exhibited an inverted bell-shaped variance in the diurnal change of Ci from July to September.(3)The diurnal change of WUE liked Pn,the mean value of WUE were 1.21 mmol·mol-1 and 0.97 mmol·mol-1,respectively.(4)Pn and PAR were positively correlated (P<0.01),negatively associated with RH (P<0.01) on partial correlation analysis;the diurnal changes of Pn in A.sparsifolia and T.ramosissima were 35.6%,42.4% respectively,which were caused by the diurnal variance of environmental factors on regression analysis;Each environmental factor has remarkable influence to Pn on path analysis,Ta>RH>PAR>Ca (A.sparsifolia) and PAR>Ca>Ta>RH (T.ramosissima).Importantly,the decision variable of A.sparsifolia was PAR,and RH was the limitation variable except for July;while PAR was only the decision variable on July and September,both RH and Ca were decision variables.These analyses indicated that A.sparsifolia had high photosynthetic rate,high transpiration rate and low water use efficiency in comparison with T.ramosissima,to average conditions observed in late from July to September.The decline of Pn in both species was mainly due to stomatal limitation on July,while August and September might be non-stomatal limitation.Again,Pn of species are mainly affected PAR and RH,and the influence of Ca on them is different in the whole growth period.In short,the results reflected the ecological adaptability of A.sparsifolia and T.ramosissima in the process of cooperative evolution with the environment.Horever,the WUE of T.ramosissima was obviously higher than that of A.sparsifolia,Presumably T.ramosissima had higher drought resistance capacity than that of A.sparsifolia.