Abstract: The water relations to annual growth in seedlings of Populus × euramericana(Dode)Guinier cv.Ⅰ-214,P.× euramericana(Dode)Guinier cv.Sacrau 79,P.deltoides Bartr.cv."Lux"(ex.Ⅰ-69/55),P.× euramaricana(Dode)Guinier cv."san Martino"(ex.Ⅰ-72/58),P.× nigra var.italica(M(u|¨)ench)Koehne × Cathayana Rehd,P.nigra var.italica(M(u|¨)ench) Koehne× simonii Carr.,P.simoii Carr × nigra L.,P.× Opera Hs(u|¨),P.× pupoloris Hs(u|¨) and P.simoii Carr.were studied respectively under the controlled conditions both in greenhouse and field in 1979 and 1980.Special attention was paid to determine their diurnal and seasonal changes of transpiration rates, water consumption per hectare by calculation,water loss of excised leaves, and the ratio of transpiration to photosynthetic rates. Besides,stomatal size,frequency of distribution,and control of stomatal apertures were studied.Based on the results in the contents mentioned above,the studied species and hybrids were classified into five groups according to the capacity of their drought tolerance and the concerned physiological charateristics in water relations.