Abstract：The effects of magnesium stress on photosynthetic character of cucumber were studied in greenhouse and under soilless condition.The results showed that the contents of chlorophyll a,chlorophyll b were inecreased,and carotenoids content decreased under Mg-excessive,the contents of chlorophyll a,chlorophyll b and carotenoids were extremly significantly decreased under Mg- deficiency.Net photosynthesis rate（Pn）,stomata conductance（Gs）and transpiration rate（Tr）changed in one peak curve under Mg-deficiency，Pn and Tr changed in double peak curve under Mg-excessive.Ci under Mg-excessive and Mg-deficiency,Gs under Mg- excessive changed in inverted parabolic curve in daytime.Light saturation point，CO2 saturation point,apparent quantum yield and carboxylation efficiency under Mg-excessive and Mg-optimum are higher than Mg-deficiency.The main limiting factor of photosynthesis of cucumber seedling is non-stomata factors under Mg-deficiency.They were both stomata and non-stomata factors under Mg-excessive.
