Abstract: Two 2-year-old hybrid clones with high (H4) and low (L3) growth vigor of Populus deltoides and their paternal parent (F) were used as the experimental materials to illustrate their difference of diurnal changes of photosynthesis, light response curves and CO₂ response curves. The result showed that diurnal changes of the net photosynthetic rate of H4, L3 and F all displayed a single peak curve H4 started its efficient photosynthesis significantly earlier than L3 and F. The analysis of light response curves indicated that the maximum net photosynthetic rate (P_nmax), apparent quantum yield (AQY), light compensation point(LCP) and light saturation point (LSP) expect dark respiration rate (R_d) all exhibited significant differences over the clones. The P_nmax of H4 was largest, reaching to 21.47 μmol ·m^-2s^-1, and 52.8% higher than L3, and the LSP was also largest and 17.03% and 10.35% higher than L3 and F respectively, but the LCP was the lowest, only 15.09 μmol ·m^-2s^-1. The result of CO₂ response curves showed that there were no significant differences in the carboxylation efficiency, the CO₂ compensation point (CCP) and photorespiration rate except P_nmax. The carboxylation efficiency and CCP of H4 were slightly higher and lower than L3 and F respectively, which revealed the clone with high growth vigor had a little advantage in the utilization of CO₂. In a word, the clone with high growth vigor had a stronger ability to make use of low light and performed relatively efficient photosynthesis in strong light, only with a little advantage in the absorption and utilization of CO₂.

Key words: Populus deltoides, cross, growth vigor, photosynthetic characteristics