Abstract: In order to test the drought tolerant ability,the six poplar clones were treated with PEG(mW 6000)solution at three osmotic potential levels(-0.4 MPa, -1.0MPa,-1.6MPa),and their net photosyathesis rates(Pn)were siudied in growth culture chamber. The results show that under -1.6MPa stress Popelys simonii,P.poplaris 36 and 44 kept more photosynthesis product accumulation until the second day, however,that of P.nigra,P.euramericane 25/86 and 28/86 decreased to consumption level since the first day.And also there was the same trend in Pn decline under -1.0 MPa stress.According to this significant difference in Pn of different clones under medium and severe water stress,the concept of water potential conpensation point of net photosynthesis rate(Pn=0)is determined as a indicator evaluating drought tolerance.Thus, the six poplar clones can be divided into two types:The former group in which the water potnetial compensation point appears later,has a higherability to drought tolerance,and the latter group has a lower drought tolerance since the Water potential compensation point appears earlier.This will be significantly important for early identificantion and selection of genetic variation in drought tolerance for poplar clones.

Key words: Popular clones, Photosynthesis, Water potential compersation point, Water stress, Drought tolerance