Effect Of Exogenous Γ-aminobutyric Acid On Nitrogen Metabolism And Photosynthesis Of Melon Seedlings Under Ca（NO₃）₂ Stress

Salinization of greenhouse soil seriously affects the yield and quality of horticultural crops and Ca（NO₃）₂ stress is one of the main greenhouse soil salinization. Previous studies showed that exogenous gamma-aminobutyric acid（GABA） can enhance the tolerance of muskmelon to Ca（NO₃）₂ stress by mitigating the metabolism of endogenesis polyamines and GABA,butthe exogenous GABAon nitrogen metabolism, photosynthesis and mineral elements absorption of vegetalble plants under Ca（NO₃）₂stress is not yet clear, and there is still lack of in-depth study.In this study, the salt tolerant variety ’Jinhui NO.1’ and the salt sensitive variety ’Yipintianxia 208’ were chosenas the experiment materials, the effects of spraying exogenous GABA on nitrogen metabolism, photosynthesis and mineral nutrients absorption of the two kinds of melon seedlings grown under Ca（NO₃）₂condition were investigated. It aimed to investigate the relationship between regulation of exogenous GABA on nitrogen metabolism, photosynthesis and mineral elements absorption of melon seedling and improving its salt tolerance, and to provide a scientific basis for the mechanism of exogenous GABA regulation onplant resistance under salt stress. The main results were as follows:1. Ca（NO₃）₂ stress seriouslyaffect the nitrogen metabolism balance ofmelon seedlings for salt-tolerant variety ’Jinhui NO.1’and salt-sensitive variety ’Yipintianxia 208’ melon seedlings grown under Ca（NO₃）₂ stress, the contentsof NH4+-N,soluble protein and free amino acid increased and the activitiesof glutamate dehydrogenase（GDH）, glutamic oxaloacetic transaminase（GOT） and glutamic pyruvic transaminase（GPT） also increased. Whilethe contents of NO3--N, the activities of nitrate reductase（NR）,glutamine synthetase（GS） and glutamate synthase（GOGAT）decreased.Spraying of exogenous GABA treatment significantly increased the activitiesof GS, GOGAT and NR, reduced the activities of GDH, GOT and GPT and improved assimilation of NH4+-N utilization of plant under Ca（NO₃）₂ stress. Meanwhile, exogenous GABA increased the accumulation of soluble protein and free amino acids which effectively lead to alleviate the nitrogen metabolism disorder caused by Ca（NO₃）₂ stress and enhanced the adaptability of melon seedlings to Ca（NO₃）₂ adversity.2.By measuring the 18 kinds of amino acids such as aspartic acid of the two varieties of melon seedlingson thethird dayof processing, the results show that:18 kinds of amino acids and total amino acids contents in leaves and roots of the two varieties melon seedlings with different resistance to Ca（NO₃）₂ stress were treated with Ca（NO₃）₂ stress and exogenous GABAspraying, wereall changed in order to better adapt to the changes of the environment. However, because of the different salt resistance ability ofthe two melon seedling varieties, there were some same changes of the amino acids contents, and there also exist certain differences between them. Specific performance as follows: aspartic acid, glutamine, glycine, proline and arginine contentswere changed significantlyof the two varieties melon seedlingsunder Ca（NO₃）₂ stress, cv. ’Jinhui No.1’ relieve stressthrough coordinatingthe relative accumulation of all kinds of amino acids in the leaves, while cv. ’Yipintianxia 208’tend to accumulatinga large number of amino acids in roots to relieve stress; Spraying exogenous GABA would further promotecv. ’Jinhui No.1’coordinating and accumulating all kinds of amino acids to relieve the effect of Ca（NO₃）₂ stress, but the total amino acid contents of cv. ’Yipintianxia 208’ were slightly reduced.Exogenous GABA improved the acclamation ofamino acid in cv. ’Jinhui No.1’ to increase the tolerance to Ca（NO₃）₂ stress, whilereduced the accumulation ofamino acid in cv. ’Yipintianxia 208’. And the effect on leaves of ’Yipintianxia 208’is greater than that of cv.’Jinhui No.1’; but the effect on roots of cv. ’Jinhui No.1’ is greater thanthat of cv.’Yipintianxia 208’.3. Ca（NO₃）₂ stresssignificantly inhibited the photosynthesis of the two varieties of melon seedlings and damaged the activity of PSⅡ, reduced the net photosynthetic rate. With the processing time increased, the restrictionof photosynthesis from the non-stomatal limitation translate to the stomatal limitation, and the photosynthesis of the salt sensitive variety cv. ’Yipintianxia 208’ was easier inhibited than the salt tolerant varietycv. ’Jinhui No.1’ under Ca（NO₃）₂ stress. The exogenous GABA significantly improved the photosynthetic indicators, and relieved the inhibition of photosynthesisand promoted thephotosynthesis of the two melon seedlings leaves and enhanced the plant’s ability to adapt to Ca（NO₃）₂stress.4. Ca（NO₃）₂stress significantly decreased the N, K element contents of the two melon seedling varieties and P contents of leaves, promoted the P accumulation of roots, and the N contents in rootsof salt sensitive varietiescv. ’Yipintianxia 208’decreased first and then increased, which means that Ca（NO₃）₂stress made N and P accumulation in roots to promote root growth to relieve stress; K element contents of the two melon seedlingswere significantly reduced under Ca（NO₃）₂stress. After spraying exogenous GABA, the N, P and K contentsof leaves and roots of the two varieties of melon seedlings were significantly increased, which suggested that GABA promoted the absorption and transportation of the three elements, promoted the distribution balance of elements and ions in leaves and roots, alleviated the Ca（NO₃）₂stress effect, enhanced the plant’s ability to adapt to stress, provided possibility of crop yield under the condition of adversity. In summary, Ca（NO₃）₂ stress disturbed nitrogen metabolism of the melon seedlings, inhibited the photosynthesis, and adverse impacted the absorption, transportation and distribution of mineral elements, thus disrupted melon seedling growth and metabolic processes.Exogenous GABA could relieve nitrogen metabolism disorder caused by the Ca（NO₃）₂ stress, promote the coordination and accumulation of all kinds of amino acids, and promote the photosynthesis, increase the absorption and distribution of mineral elements,which result inrelieve inhibitory effect of melon seedlings caused by Ca（NO₃）₂stress, and there wasdifferentmitigation effectsdue to thedifferent variety characteristics.