Effect Of Γ-aminobutyric Acid On The Polyamine And EndogenousΓ-aminobutyric Acid Metabolism Of Muskmelon Under Ca（NO₃）₂ Stress

In recent years, the yield and quality of the fruits and vegetables affected by the deepen of the secondary salinization of soil in protected agriculture. γ-aminobutyric acid(GABA) as a new type of exogenous substance could effectively relieve the damage in response to biotic and abiotic stress. In this study, ‘Jinhui No.1’ which is tolerance to Ca(NO3)2 stress, ‘Yipintianxia 208’ which is sensitive to Ca(NO3)2 stress were choose as experiment materials. The muskmelon seedlings grown in hydroponic experiments were performed to select the optimal GABA concentration which can relieve the inhibition of muskmelon growth under Ca(NO3)2 stress by spraying different concentration of GABA; and were investigated the effects of exogenous GABA application on polyamine and endogenous GABA metabolism in muskmelon seedlings under Ca(NO3)2 stress. The results will provide theoretical basis for exogenous GABA alleviated salt damage for plant and enhancing salt-tolerance in plant.The main results were presented as follows:(1) The appropriate concentration of GABA was 50 mmol/L to alleviate the injury caused by Ca(NO3)2 stress. The leaf area,fresh and dry weight,root volume,root surface area, total root length, tips of root and the content of soluble protein significantly reduced under Ca(NO3)2 stress. The contents of photosynthetic pigments, proline and MDA were significantly increased under Ca(NO3)2 stress. Ca(NO3)2 stress inhibited the growth of muskmelon seedlings. Under Ca(NO3)2 stress, the contents of photosynthetic pigments, proline and MDA were decreased early then increased with increasing concentration of exogenous GABA and were minimum level at 50 mmol/L GABA; The content of soluble protein was increased early and declined later by spraying GABA and up to the maximum at 50 mmol/L GABA; spraying 50 mmol/L GABA can alleviate the inhibition of Ca(NO3)2 stress for the leaf area,fresh and dry weight,root volume,root surface area, total root length and tips of root. In addition, after Ca(NO3)2 stress spraying GABA has better effect than pre-spraying GABA.(2) Exogenous GABA involved in regulation on polyamine metabolism under Ca(NO3) 2 stress in muskmelon seedlings. Ca(NO3)2 stress significantly reduced the levels of conjugated spermidine(Spd), spermine(Spm) and bround Spd in leaves of ‘Yipintianxia 208’, and bround Spd and Spm in leaves of ‘Jinhui No.1’; reduced the levels of conjugated Spm and bround Spd in roots of ‘Yipintianxia 208’ and conjugated Spm in roots of ‘Jinhui No.1’, and significantly increased other form and type of polyamines in two cultivar. At the same time, Ca(NO3)2 stresssignificantly enhanced the activities of arginine decarboxylase(ADC), ornithine decarboxylase(ODC), S-adenosylmethionine decarboxylase(SAMDC), polyamine oxidase(PAO), and diamine oxidase(DAO). Application of exogenous GABA under Ca(NO3)2 stress conditions further increasing the activities of ADC, ODC, SAMDC, PAO and DAO and improving the capacity for polyamine biosynthesis to increasing the contents of PAs; converting free Put to insoluble bound PA form. In ‘Jinhui No.1’ seedlings, free Put converted to bround PAs rapidly, whereas free Put converted to conjugated PAs in ‘Yipintianxia 208’ seedlings.(3) There was a potential positive feedback mechanism in plant in order to enchanced the muskmelon seedlings of patience and different organs have differences performance. Ca(NO3)2 stress significantly enhanced glutamate decarboxylase(GAD) activity and promoted endogenous GABA levels accumulation, and suppressed GABA transaminase(GABA-T) activity in leaves and roots of two cultivars. Exogenous GABA application effectively reduced GAD activity and GABA concentration in roots, while GAD activity and GABA concentration further increased in leaves. In addition, GABA-T activity increased to promote the degradation of GABA in leaves and roots.