Abstract: Spraying with exogenous 5-aminolevulinic acid（ALA）in high concentrations（600，900 and 1 200 mg · L-1）at the late of full blooming could significantly thin flowers of pear（Pyrus pyrifolia Nakai.‘Hosui’），and then promote fruit weight and quality at harvest. High concentrations of ALA treatment significantly reduced the stigma fertilizability of pear but increased the production rate of superoxide anion of floral organ，meanwhile，the activities of SOD and CAT were decreased greatly. In vitro，high concentrations of ALA inhibited pollen germination. Measurements of chlorophyll fluorescence showed that although the ALA solution was applied at blooming，leaf photosynthesis was obviously promoted in at least three months.

Key words: pear, 5-aminolevulinic acid, chemical thinning, fruit, quality, photosynthesis