Abstract: The photosynthetic characteristics and the relationship of net photosynthetic rate, water use efficiency with the environmental factors and stomatal conductance in four new poplar clones (3^#、6^#、8^#、9^#) were studied. The results showed that the peak value of the diurnal courses of the photosynthetic rate generally appeared around 10:00, and then reduced gradually. The diurnal courses of transpiration rate in different months were different. The peak value of the diurnal courses of stomatal conductance generally appeared at 7:00~9:00. The total mean net photosynthetic rate(μmol CO₂·m^-2·s^-1), transpiration rate (mmol H₂O·m^-2·s^-1) and water use efficiency (mmol CO₂·mol^-1 H₂O) had following order in June, July and August respectively: Clone 9^#（8.53）>6^#（7.21）>3^#（6.47）>8^#（4.98）; Clone 9^#（3.74）>3^#（2.76）>6^#（1.76）>8^#（1.47）; Clone 8^#（4.77）>6^#（4.35）>3^#（2.99）>9^#（2.40）. Clone 9^# had higher net photosynthetic rate and transpiration rate, with lower water use efficiency. On the contrary, clone 8^# had lower net photosynthetic rate and transpiration rate, with higher water use efficiency. Net photosynthetic rate and water use efficiency had a close relationship with the air temperature and relative humidity in 6^#, and had a close relationship with the photosynthetic active radiation and stomatal conductance in 3^#、8^# and 9^#.

Key words: photosynthetic rate, transpiration rate, photosynthetic active radiation, stomatal conductance, water use efficiency