Abstract: The purpose of this paper is to investigate the effect of exogenous hydrogen（H2）on photosynthetic carbon assimilation and nitrogen metabolism of cucumber（Cucumis sativus L.）under low temperature. The‘Jinyou 35’cucumber seedlings subjected to low temperature stress（day/night temperature was 8 ℃/5 ℃）were presoaked with hydrogen-rich water（HRW，the donor of H2）or the deionized water（Control）for 8 h. The results illustrated that the chilling stress restrained the seedling growth，and decreased the pigment contents，net photosynthetic rate（Pn），stomatal conductance（Gs），transpiration rate（Tr），actual photochemical efficiency of PSⅡ（ΦPSΠ），maximum photochemical efficiency of PSII in darkness（Fv/Fm），antenna efficiency of PSⅡ（Fv′/Fm′），the activity of ribulose bisphosphate carboxylicase（RuBPCase）and root activity，while increased the intercellular CO2 concentration（Ci）and initial fluorescence（F0）. Further studies revealed that chilling stress induced enhancement of sucrose synthetase（SS）and sucrose phosphate synthetase（SPS）activities and the total sugar，sucrose content，but reduced the starch content. Meanwhile，the total nitrogen content，ammonia nitrogen content and the activities of reduced nitrate reductase（NR），glutaminase（GS），glutamate synthase（GOGAT）and glutamate dehydrogenase（GDH）increased in the early days of chilling stress but subsequently decreased under low temperature stress. The nitrate nitrogen content increased gradually. These change tendencies of HRW treatment were almost the same as those of control. In addition to low Ci and F0，most of the remaining indicators in HRW treatment were significantly higher than control，which indicated that exogenous H2 could enhance the tolerance of cucumber seedlings to lower temperature stress by increasing the activities of key photosynthetic enzymes，reducing photoinhibition and maintaining a high level of carbon and nitrogen metabolism，thus promoting the cucumber seedlings growth finally.

Key words: cucumber, low temperature stress, hydrogen-rich water, photosynthetic carbon assimilation, nitrogen metabolism