Gras exchange parameters of three new poplar clones (JK-7，JK-10，and JK-11) and the correspond ing environmental factors were measured in order to explore the relationships of net photosynthetic rate (Pn)

Transpiration rate(Tr)

water use efficiency(WUE)，and the environmental factors such as photosyn-thetically active radiation (PAR)，air temperature(Ta)，vapor pressure deficit (VPD)，and relative humidity (RH}.Results showed that the three clones were different in response to the same environmental factors.The diurnal courses of P and T

displayed single peak curves in JK-7 and JK-11 and a double peak curve

in JK-10

while the diurnal courses of WUE displayed double-peak curves in all clones. There were significant differences in P and T

among these clones(P<0.05)

with the highest in JK-10 and lowest in JK-7

and they were significantly higher in JK-10 than in JK-7.There was no significant difference in WUE among these clones. The Pn and Tr

had close correlations with stomatal conductance(Gk)and PAR in the three clone s

while they had close correlations with RH in JK-7 and Tuain JK-10. There was significant negative correlation between WUE and Gk in the clones.