| This study is based on the climatic characteristics of Shenyang during early spring which temperature warms up rapidly and dramatically,but the soil temperature lags comparing with air temperature.In order to understand the characteristics of GABA metabolism and the physiological effect of GABA in apple roots in responding to sub-low temperature,MalusBaccata Borkh.seedlings were used in this study,combinating with the application of y-aminobutyric acid(GABA)and its metabolic pathway inhibitor(vigabatrin,VGB),the activities of key enzymes in GABA metabolism and TCA pathway,the contents of intermediate productions,the contents of osmotic adjustment substances,the contents of reactive oxygen species and the activities of antioxidant enzymes were measured under sub-low temperature of root-zone.The main results are as follows:1.The changes of reactive oxygen species and membrane lipid peroxidation of Malus Baccata Borkh.roots under sub-low temperature:O2-generation rate,H2O2 and MDA contents of Malus Baccata Borkh.roots were significantly higher than those of the control under sub-low temperature.It was suggested that sub-low temperature of root-zone caused oxidative stress and lipid peroxidation in the roots.Compared with the control,GABA application significantly reduced the rate of O2-generation,H2O2 and MDA contents.VGB treatment increased H2O2 and MDA contents.The results showed that the exogenous GABA can effectively relieve oxidative stress caused by sub-low temperature.2.The characteristics of GABA metabolism in responding to sub-low temperature:The GABA content presented an upward tendency under sub-low temperature,which was higher than that of the control.The activity of GAD changed in line with the variation tendency of GABA content.Compared with the control,GABA-T activity significantly reduced in the 0-48 h.and then rose to the control level in the 4-6 d.The changing trend of SSADH activity was similar with that of GABA-T,and SSADH activity was lower than that of the control during the temperature treatment.Sub-low temperature of root-zone promoted the conversion of Glu to GABA in roots,and inhibited the activities of GABA-T and SSADH,resulting in the accumulation of endogenous GABA.Exogenous GABA treatment promoted the accumulation of endogenous GABA and SA in roots,which were significantly higher than that of the control and the sub-low temperature treatment.The content of Glu was much higher than the control and the sub-low temperature treatment in the 4-6 d.Compared with the treatment of sub-low temperature,the activity of GAD reduced in GABA treatment.However,the activities of GABA-T and SSADH were enhanced.VGB treatment significantly inhibited the GABA-T activity.However,the activity of GAD in VGB treatment was significantly lower than that of the sub-low temperature treatment only in 48 h and 4 d,so did SSADH activity in 48 h.The GABA content in roots was significantly lower than that of sub-low temperature treatment from 0 h to 48 h.and then increased gradually,and at the stage of 4-6 d was higher than that of the sub-low temperature treatment,but lower than the GABA treatment.3.The changes of GABA on TCA cycle under sub-low temperature of root zone:The contents of citric acid,a-ketoglutarate,fumaric acid and malic acid in roots showed the first peak at 12 h,and then decreased,and began to rise at 48 h,in which citric acid continued to rise,other three organic acids declined after reached the second peak at 4 d.a-KGDH activity was significantly inhibited from 0 h to 24 h,and then increased and was significantly higher than the control;SDH activity was significantly lower than the control.In the GABA treatment,the contents of citric acid and a-ketoglutaric acid decreased further,but the contents of fumaric acid and malic acid increased.The activity of ?-KGDH showed an opposite trend to that of sub-low temperature treatment.SDH activity had a decreasing trend,but was significantly higher than that of sub-low temperature from 0 h to 48 h.In the treatment of VGB,the content of citric acid was higher than that of sub-low temperature only at 48 h and 4 d.The content of a-ketoglutarate was similar to that of sub-low temperature treatment from 48 h to 6 d,and the content of fumaric acid was significantly higher than that of sub-low temperature.The activities of a-KGDH and SDH were significantly higher than that of sub-low temperature from 0 h to 4 d,and then significantly decreased.4.The effects of GABA on the osmotic substances under sub-low temperature:The contents of soluble protein and proline(Pro)in the roots increased under sub-low temperature,but the GABA treatment promoted the accumulation of those two substances.VGB treatment also promoted the accumulation of soluble protein and Pro in 0-24 h.and then decreased.which were lower than thatl of sub-low temperature after 48 h.The content of soluble sugar was increasing in roots in the whole process.The contents glucose,fructose,sucrose,and sorbitol increased too,which increased by 2.4,1.8 and 0.5 times,respectively.In GABA treatment,the content of soluble sugar was relatively stable from 0 h to 4 d,and then significantly increased at the 6th day.Compared with the sub-low temperature treatment,the contents of those four kinds of sugars decreased in the 0-4 d,and then increased at the 6th day except for sucrose.In VGB treatment,the content of soluble sugar was higher than that of sub-low temperature treatment in the 24 h-6 d;and only the content of sucrose was lower than that of sub-low temperature treatment during the whole process.5.The effects of GABA on the activities of antioxidant enzymes in sub-low temperature:the activity of superoxide dismutase(SOD)was inhibited in the 48 h-6 d under sub-low temperature,and the activity of peroxidase(POD)was significantly lower than that of the control.CAT activity was significantly higher than that of the control after 48 h.The activity of ascorbate peroxidase(APX)was significantly higher than that of the control at 0-48 h.Compared with sub-low temperature treatment,SOD and CAT activities increased in GABA treatment,POD activity decreased significantly at the early stage(12 h-48 h),APX activity was significantly increased from 4 d to 6 d.In VGB treatment,SOD slightly changed.The activity of CAT significantly increased at the early stage(12 h-24 h),and then the activity of POD significantly decreased,the activity of APX significantly decreased after 12 h. |
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