Abstract: In this study,observation by Li-6400 system was used to analyze the light response of some physiological factors,such as net photosynthetic rate(Pn),transpiration rate(Tr),stomatal conductance(Gs),intercellular CO2(Ci),stomatal limitation value(Ls) and water use efficiency(WUE) of 3-years-old Eucommia ulmoides,which was planted in the rocky mountain of North China under different soil water stress.Soil water was engaged in four levels of about 75%(CK),55%(W1),40%(W2) and 30%(W3) of soil water holding capacity.The results showed that:(1) Light saturation point for W1,W2 and W3 of Eucommia ulmoides was 12.5%,44.8%,68.5% lower than CK respectively.The maximum net photosynthetic rate on sunny day reached crest under the condition of CK,which was 21.5%,47% and 69.7% higher than W1,W2,W3 respectively;(2) The transpiration rate(Tr) increased with the rise of photosynthetic active radiation(PAR).Compared with CK,Tr was 17%,52% and 93% lower under W1,W2 and W3;(3) Gs increased with the rise of PAR.Compared with CK,Gs decreased by 22.6%,67.9%,88.7% under W1,W2 and W3.Without water stress or with mild water stress,stomatal limitation was the main factor to influence photosynthesis.But under severe stress conditions,when the PAR came to 1000μmol· m-2·s-1,the main reason for impact of photosynthesis changed from stomatal limitation to non-stomatal limitation;(4) Water use efficiency(WUE) reached highest under W3 condition.There was little variation under CK,W1,W2.However,compared with W3,WUE decreased by 68.6%,69.5%,67.3%.

Key words: Eucommia ulmoides, Eucommia ulmoides, Water stress, Photosynthesis, Transpiration rate(Tr), Stomatal conductance(Gs), Stomatal limitation value(Ls), Water use efficiency(WUE)