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Chloris virgata, a naturally alkali-resistant halophyte, was chosen as the test material for our research. The seedlings of C. virgata were treated with varying short salt stress （1:9 molar ratio of NaCl to Na2SO4; pH 6.70-6.72; 40, 80 mM） and alkali stress （1:9 molar ratio of NaHCO3 to Na2CO3; pH 10.46-10.62; 40, 80 mM）. Firstly, the composition and content of organic acids in shoots was analyzed, the results indicated that there was not only a significant increase in total organic acids, but also obvious changes in different components of organic acids under the alkali stress. The increment of citrate was the largest and malate followed. However, none of the organic acids showed significant alteration in the content and component under salt stress. According to the characteristics of accumulation in organic acids, some enzymes in the related metabolism were selected, and determined in order to further explore the physiological mechanism of the alkali tolerance in the level of enzyme. The results showed that the activities of citrate synthase （CS, EC 4.1.3.7）, malate synthase （MS, EC 4.1.3.2）, NADP-isocitrate dehydrogenase （NADP-ICDH, EC 1.1.1.42） and isocitrate lyase （ICL, EC 4.1.3.1） increased significantly, but the activities of aconitase （EC 4.2.1.3） and phosphoenolpyruvate carboxylase （PEPCase, EC 4.1.1.31） had little change under alkali stress. Malate dehydrogenase （MDH, EC 1.1.1.37） activity decreased obviously under a stronger alkali stress （80 mM）. However, under salt stress, besides increased MS activity, there was no significant change for other enzymes. These results strongly demonstrated that the enzymatic regulation of organic acid metabolism may be the biochemical basis of the alkali tolerance for C. virgata. CS, MS and ICL might be the key enzymes which decide the alkali tolerance of C. virgata.In addition, seedings of C. virgata were subjected to different intensity short-term （3 days） alkali-stress （1:1 molar ratio of NaHCO3 to Na2CO3; pH 9.93–9.98; 50, 100,150 mM） and salt stress （1:1 molar ratio of NaCl to Na2SO4; pH 6.32–6.64; 50,100,150 mM）. The data of Photosynthesis parameters, chlorophyll fluorescence, photosynthesis chlorophyll content, activity of PEPCase and content of k+, Na+ indicated that different response were existed under alkali stress and salt stress. These results indicated that salt stress and alkali stress are two different stresses. Photosynthesis parameters, chlorophyll fluorescence, photosynthesis chlorophyll content, activity of PEPCase and content of k+ , Na+ indicated had visible change under short alkali stress. In contrast, there were a little or no significant effects in C. virgata under short term salt stress. The different results in C. virgata indicated that alkali stress with greater destructive effects, different from salt stress, is an important abiotic stress, and worthy of further study. Plant are imposed high pH under alkali stress, the key feature, different from salt stress, may disturbs root structure, induces increase in Na+ and decrease in K+ in leaves.The High leaf Na+ concentration probably reduces PN due to ion toxicity which disorderd photosynthetic apparatus, changed fluorescence and other parameters of C. virgata.
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