Abstract: Irrigation with exogenous 5-aminolevulinic acid (ALA) significantly promoted leaf net photosynthetic rate(Pn)of melon(Cucumis melo L.‘Ximiya 1’)seedlings grown under low light irradiation,as well as the plants treated by 8 ℃ chilling stress.The leaves of the plants treated with 10 mg·L^-1 ALA solutions showed hi gher apparent quantum yield(AQY)，carboxylation eficiency(CE)，chlorophyll(especially chlorophyl b)content，dark respiration and bigger stomata aperture but lower respiration under light.Furthermore,after seedlings were transferred to 8℃ chilling for 4 hours and then recovered at 25-30℃ for 2 hours，the photosynthesis of the control leaves was only 12%-18% of that before chiling,while that in the ALA-pretreated leaf was 22%-38%.which would further recovered in 20 hours to 76%-101% of the control before chilling treatment.If the seedlings were chiled for 6 hours．the control plants all died while the ALA-pretreated ones only showed a few injury symptoms in the leaf margin．The analysis also showed that the soluble sugar content of leaves in ALA-retreated plants Was very significantly higher than that of control，which might be partly responsible for the higher chiling tolerance．The evidence presented here sugested that ALA treatment might promote low light tolerance and chilling tolerance of plants.

Key words: 5-Aminolevulinic acid( A), Chiling, Low light, Melon, Photosynthesis, Stress tolerance