Effects Of γ-aminobutyric Acid On The Seed Germination And Seedlings Amino Acid Metabolism Of Melon Under Hypoxia Stress

In soilless culture, hypoxia stress come into appearance as a result of insufficient oxygen supply or root-cushion formation, which influences the normal growth and development of melon seedling. Now hypoxia stress has become a bottle neck of soilless culture application to large area production. Therefore, inproving the hypoxia tolerance of crops plays a vital role in soilless culture.γ-Aminobutyric acid (GABA) is a four-carbon non-protein amino acid conserved from bacteria to plants and vertebrates. In plant physiological metabolism process , GABA can regulate cytosolic pH, limit the accumulation of reactive oxygen intermediates under oxidative stress conditions, deter insect feeding by means of interferes with the normal development of insects, have the role of signal transduction and osmotic adjustment, and induce synthesis of ethylene.The effects of exogenous GABA on the seed germination and growth active oxygen metabolism after seed germination were studied under hypoxia stress.The results show that germination and radicle length, hypoxotyl length, activity of antioxidant enzymes after seed germination significant decreased, and content of MDA increased under hypoxia. Applying GABA could significantly increase the ermination rate, germination and radicle length, hypoxotyl length, activity of antioxidant enzymes after seed germination, and decrease the content of MDA. It indicted that GABA treatment of exogenous spraying to leaves can increased the activities of antioxidant enzyme under hypoxia stress, which played an important role in enhancing the resistance to hypoxia stress of melon seedlings through keeping low ROS contents.By the method of hydroponic culture, this paper studied the effects of exogenous GABA on the endogenous amino acids content, enzyme activity in GABA shunt, growth, photosynthesis, and chlorophyll fluorescence parameters of melon seedlings under hypoxia stress. Impact mechanism of growth , photosynthesis and path of amino acid metabolism under hypoxia were preliminarily elucidated. Soilless culture of plants in search of relief of hypoxia stress injury and effective measures to provide a theoretical basis.The results show that there is a significant increase the content of GABA, glutamic acid and activity of GAD under hypoxia stress. Applying GABA could significantly increase GABA content, decrease glutamic acid content and the activity of GAD under control and hypoxia. GABA and glutamic acid content and activity of GAD decreased under application of GABA and VGB simultaneously.Hypoxia stress induced a significant decrease the content ofα-ketoglutarate, the activity of GDH, GOGAT and GS increased. Applying GABA could significantly decreaseα-ketoglutarate content and the activity of GS, increase the activity of GDH, and increaseα-ketoglutarate content, the activity of GDH, and decrease activity of GOGAT and GS. The activity of GDH increased,α-ketoglutarate content and the activity of GOGAT and GS under application of GABA and VGB simultaneously under hypoxia stress.There is a significant increase content of pyruvate, alanine, aspartate and the activity of ALT and ASP under hypoxia. Applying GABA could significantly increase pyruvate, alanine, aspartate content, the activity of ALT and ASP decreased under control, and decrease pyruvate content, the activity of ALT and ASP, increase content of alanine and aspartate under hypoxia stress. Content of pyruvate and the activity of ALT and ASP increased, decrease alanine and aspartate content under application of GABA and VGB simultaneously under hypoxia stress.Hypoxia stress could increase Proline content, decrease Arginine, Histidine etal other 12 kinds of amino acid content of seedlings significantly. Applying GABA could significantly increase Proline, Arginine, Histidine etal other 13 kind of amino acid content of seedlings under control, Proline, Arginine, Serine etal other 11 kinds of amino acid content of seedlings, and decrease Histidine content under hypoxia stress. However, simultaneously applying GABA and VGB could significantly decrease the alleviation effect of amino acid content of GABA under hypoxia stress.Hypoxia stress induced inhibition of the growth of plants, and a significant decrease of photosynthetic pigment contents, resulting in the decrease of photosynthesis. Applying GABA could significantly promote seedlings growth and increase the photosynthetic pigment contents, net photosynthetic rate (Pn) , stomatal conductance (Gs), intercellular CO₂ concentration (Ci), carboxylation efficiency (CE), maximal photochemical efficiency of PSâ…¡(Fv/Fm), photochemical quenching (qP), apparent photosynthetic electron transfer rate (ETR), and quantum yield of PSâ…¡electron transport (ΦPSâ…¡), and decrease the stomatal limitation value (Ls), minimal fluorescence (Fo), and non-photochemical quenching (NPQ), under both hypoxic and normal conditions. The alleviation effect of GABA on photosynthetic characteristics was more obvious under hypoxia stress. However, simultaneously applying GABA and VGB could significantly decrease the alleviation effect of GABA under hypoxia stress.