Abstract: The relationship between acidic-alkaline tolerance and organic acids secreted from roots was investigated in Ziyang Xiangcheng（ZYXC）（Citrus junos Sieb. ex Tanaka），trifoliate orange（TO）[Poncirus trifoliata（L.）Raf.] and Red tangerine（RT）（Citrus reticulata Blanco）. The seedlings of three rootstocks were cultured in the hydroponic solutions with different pH for 30 days. The growth index，root morphology，photosynthetic pigments content，and physiological parameters of superoxide dismutase （SOD），peroxidase（POD），catalase（CAT），malondialdehyde（MDA） and soluble protein were examined. The content of organic acids secreted from root system was analyzed by Liquid Chromatography（LC）. The highest biomass growth rate was observed in TO at pH 4. At pH 8，the biomass growth rate of RT was the highest and TO was the lowest. The photosynthetic pigments contents，root lengths and root surface areas of three rootstocks were significantly lower with pH 4 treatment. Under different pH conditions，the three rootstocks showed significant differences in activities of SOD and POD，as well as the contents of MDA and soluble protein. At lower pH，only oxalic and quinic acids were detected with the content of oxalic acid higher than that of quinic acid. Under higher pH condition，except those two acids，the malic acid was also detected. Overall，among three rootstocks，the RT released the highest level of organic acids followed by ZYXC，and then TO. At late stage of alkaline treatment，the highest level of malic acid was observed in ZYXC. The lowest level was in TO. This may relate to alkaline tolerance of the rootstock. In this case，the ZYXC and RT are more tolerant to alkalinity than that of TO. It can be suggested that the strong alkaline tolerance of ZYXC and RT may be related to the greater capacity of secreting malic acid from roots under alkaline condition. The strong acidic tolerance of TO would be associated with lower ability to release organic acids.

Key words: Ziyang Xiangcheng, Trifoliate orange, Red tangerine, pH, oxalic acid, quinic acid, malic acid