Abstract: The authers researched the relations of photosynthetic rates with light intensity, CO₂ concentration and leaf age in P. deltoides (Ⅰ-69) and P. tomentosa by infared-CO₂-analysis method, and the daily and yearly progresses of photosynthetic rate of P. deltoides(Ⅰ-69) by improving dry weight method, as well as the yearly progresses of changes in chlorophyll content, specific leaf weight, single leaf area. It is shown that the light saturation point of P.deltoides(Ⅰ-69)is similar to that of P. tomentosa, but light compensation point is higher, CO₂ compensation point is lower than that of P. tomentosa. The higher net photosynthetic rate, more founction leaves and longer period of founction leaf of P. deltoides are the bases of more rapid growth. The two-peak curve of daily photosynthetic progress of P. deltoides even during the period of high temperature and moisture (July and August) indicates that P. deltoides is sensitive to water deficit. The yearly photosynthetic progress is also in two-peak curve, which appears in June to July and October respectivly. The lower photosynthetic rate of P. deltoides in August is related to its larger single leaf area, lower chlorophll content and smaller specific leaf weight. Otherwise, the lower mineral and nitrogenous nutration during quick growth period are the external causes for it.

Key words: Populus deltoides, P. tomentosa, Potosynthelic properties