Abstract: The growth and photosynthetic characteristics of Populus deltoides Bartr.cv."Lux" (ex.Ⅰ-69/55) and P.×euramericana (Dode) Guinier cv.Ⅰ-214, which are clones of the tribe Algeiros;and P.×Opera Hsü and P.×popularis Hsü which are hybrids of P.Simonii,were studied under the controlled condition both in greenhouse and field during 1979—1981.Special attention was paid to determine their diurnal and seasonal changes of net photosynthetic rates and other related photosynthetic index.The light compensation points in the two clones of Algeiros were 2×10³ Lux,the light saturation points were 3—4×10⁴ Lux and the dark respiration rates were 2.87—3.75mg CO₂.dm^-2.hr^-1.While those in the two hybrids of P.simonii were respectively 3—4×10³ Lux,4—5×10⁴ Lux and 2.25—2.52mg CO₂.dm^-2.hr^-1.The diurnal changes of net photosynthetic rates of the studied four hybrids showed bimodal curve's patters from July to September,but monomodal patter in October.Their highest net photosyntheic rates appeared in July (15—19mg CO₂.dm^-2.hr^-1),the lower ones appeared in June,August and September (10—15mg CO₂.dm^-2.hr^-1),and the lowest in October (3—7mg CO₂.dm^-2,hr^-1).Under the similar environmental condition,the net photosynthetic rates of the two hybrids of P.simonii were higher than that of the clones of Algeiros.However,the growth rates and biomass of the latter were rather higher than the two hybrids of P.simonii,because of their larger total leaf area.Therefore,it is feasible to determine the growth potantial of Populus seedlings according to the magnitude of both net photosynthetic rate and total leaf area.