Abstract: The net photosynthetic rates ( Pn) of young pear ( Pyrus pyrifolia Nakai) trees were determined under field condition with a CIRAS-1 portable photosynthesis system in the late of May and the early of October, respectively. The results showed that Pn of most leaves in the middle of shoots were rather high and
stable, while that in the base or top were low. Among 19 cultivars, the highest Pn, 13.13 μmol·m^{- 2} ·s^{- 1} , was found in‘Zhongli 1’, and the lowest in‘Akitsuge’, only 8.08 μmol·m^{- 2} ·s^{- 1} , and the others were the medium between them. The analysis of factors that affected leaf photosynthesis showed that leaf Pn was not dependent on shoot length directly, but on their growth situation. Pn of the leaves on a growing long shoot were significantly higher than that on a terminated middle or short shoot. The Pn when the upper surface of leaves was irradiated were significantly higher than that when the lower surface of leaves was irradiated. The Pn of young healthy leaves about 50-day-age on autumn shoots were significantly higher than that of older leaves about 150-day-age on sp ring shoots. The Pn determined in the morning were higher than that in the afternoon. The theoretical calculation showed that light compensation point of leaf photosynthesis of pear was 59.40μmol ·m^{- 2} ·s^{- 1} , apparent quantum efficiency 0.038, dark respiration 2.25μmol·m^{- 2} ·s^{- 1} , CO₂ compensation point 42.17μmol·mol^{- 1} , the lowest leaf temperature 13.77℃, the optimum leaf temperature 26.86℃, the highest leaf temperature 39.96℃, the optimum atmospheric relative humidity 26.50% , and the highest 84.23%.

Key words: Pear, Photosynthesis, Cultivar, Intrinsic factor, Environmental factor