**2.1 Plant growth and treatment**

The experiment was started in the late of May 2010 in the Horticultural Experimental Station, Nanjing Agricultural University, Jiangsu Province China. Ten years old pear trees (*Pyrus pyrifolia* Nakai. 'Akemizu') were used, which were grew in the brown yellow soil with space of 4×5m. ALA solution in concentration of 0.5 mg·L-1 was sprayed to the leaves, with the distilled water as the control. The experimental trees were arranged in complete random design, with ten tree repeats for ALA treatment or control, respectively. The measurements of the gas exchange and chlorophyll fast fluorescence parameters were conducted one week after ALA spray at an interval of 2 hours from morning to dusk. And meanwhile, the leaf samples were taken and stored in liquid nitrogen for subsequent analysis.
