| With the new poplar clones as the experimental materials, photosynthetic rate (Pn), transpiration rate (Tr), stotnatal conductance (Gs) and intercellular CO2 concentration (Ci) were measured using LI-6400 portable photosynthesis system. At the same time photosynthetically active radiation (PAR), air temperature (T) and relative humidity (RH) were recorded during the growth season in 2003.The diurnal courses of Pn and Tr were drawed with the mean values every hour; the daily mean Pn and the daily mean Tr were ordered; ANOVA analysis, correlation analysis and stepwise regression and curve fit were performed; and the main water parameters (Osmotic potential at water saturation (?)sat, Osmotic potential at rurgor loss point (?)tlp etc.) in Pressure-Volume (PV) curves and the drought resistance index (DI) values were used to evaluate the drought resistance of the tested clones.The diurnal courses of Pn and Tr generally showed a pattern of single peak and had a close relationship with PAR in the eight clone trees. The relationships of Pn and Tr with the environmental factors could be discribed with the optimal models including PAR. The daily mean net photosynthetic rates (u molCO2 m-2S-1) had following order in June: clone 8 (13.86)>6 (13.79)>4 (12.86)>2 (12.09)>3 (9.71)>5 (8.10)>7 (8.00)>1 (7.83) and in August: clone 4 (7.11) > 1(6.05) >3(4.83) >5(4.65) >6(2.98) >7(2.92) >8(2.68) >2(2.66). The daily mean transpiration rates (mmolH2O m-2s-1) could be ordered as follows: clone4(3.78) >5(3.24) > 2(3.14) >3(2.76) >8(2.69) >6(2.65) >7(2.29) > 1(1.75) in June and clone 1 (4.66)>4 (3.94) >5 (3.00)>3 (2.89)>6 (l.74)>8 (1.41)>7 (1.30)>2 (1.26) in August. The daily mean net photosynthetic rates were significantly different of the eight poplar clones, while the daily mean transpiration rates of them only had significant difference in August.The correlation coefficients between Pn and Gs, Pn and Ci, Tr and Gs, and Tr and Ci indicated that Pn and Tr were generally affected by Gs in August. The WUE had following order: clone 7> 1 >2>3>4>5 in June, and clone 7>2>4>5>3>1 in August.ψtlp of clone 3 and 1 were lowest (-2.0219MPa, -1.9434MPa) in eight clones and they can maintain normal osmotic potential under the water stress. The higher difference of ψsat and ψtlp of clone 3 and 1 indicated they had stronger osmotic regulation capacity, clone 4 could tolerate the drought environment because it had the lowest RWCtlp and ROWCtlp. DI values of eight poplar clones had the following order: clone 1>7>4>8>3>2>5>6. The eight clones were classified into tree groups by the cluster analysis: the first group with strongest drought resistance was clone 1, 7, 4, 8, 2; the second with average drought resistance was clone 3, 6; and the third group with the lowest drought resistance was clone 5.
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