| The salt-alkaline stress widespreads in the facility agriculture which result to decreasecrop yield and quality, and also restricts the development of crop production. Therefore, it isnecessary to study mechanisms of crop resistance and find a way to increase crop resistanceon salt-alkaline stress. Γ-aminobutyric acid (GABA) is a kind of bioactive non-protein aminoacid substance, which act as a signal substance to affect the plant growth and developmentindirectly under adversity stress. But it is still unclear about how exogenous GABA adjust andimprove the crops resistance and the mechanism.This research tried to find the response of different varieties of melon seedlings tosalt-alkaline stress by hydroponic, and screen out the appropriate varieties of melon whichcould grown in saline soil. And it also studied effects of different concentration exogenousGABA proporation on the salt-alkaline tolerance of melon and tried to find the optimalconcentration of exogenous GABA spraying on the leaf which could efficiently alleviate thesalt-alkaline stress.The effects of foliar spraied exogenous GABA on the biomass, chlorophyllcontent, photosynthetic parameters, chlorophyll fluorescence parameters and the chloroplastultrastructure of two varieties of melon seedling with significant difference of stress tolerancewere detected.The main results of this study are as follows:1. Under salt-alkaline stress conditions, the20different characteristics of varieties ofmelon seedling growth were inhibited. The shoot fresh weight, root fresh weight, root length,leaf area and chlorophyll content decreased in different degrees. Through the salt damagecoefficient by cluster analysis of melon varieties, this research found the comprehensiveevaluation of the resistance to salt-alkaline stress.‘Lingtian No.1’,‘Haimi No.4’,‘Baixiangmi’,‘Shantian No.1’ and ‘Jinhui No.1’ were salt-alkaline tolerance cultivars;‘Yipin Tianxia No.108’,‘Lingtian No.4’ and ‘Yipin Tianxia No.208’ were salt-alkalinesensitive cultivars;‘Qianyu No.1’,‘Shantian No.3’,‘Xuelian’,‘Wutaishan No.3’,‘Yulu’,‘Baiyuxiang’,‘Lingtian No.3’,‘Guifeixuemi’,‘Ruixuexinzaomi’,‘Zhimami’,‘Jingtian No.208’,‘Tianjiao’ were moderate salt-alkaline tolerance cultivars.2. Under salt-alkaline stress conditions, the seedling growth of the salt-alkaline tolerancecultivar ‘Jinhui No.1’ and the salt-alkaline sensitive cultivar ‘Yipin Tianxia No.208’ wasinhibited. The shoot fresh weight, root fresh weight, leaf area and chlorophyll content decreased significantly; the taproot length, root length, root surface area, root volume, rootaverage diameter, and root tips were also significantly reduced. The influence of differentconcentrations (0,25,50,75mmol·L-1) of exogenous GABA for seedling growth wasdifferent. When the foliar spraying exogenous GABA concentration between25and50mmol·L-1, it had a certain relief for the growth of melon seedling under salt-alkaline stress.The50mmol·L-1GABA had the best performance, while the75mmol·L-1GABA does notplay an effective role in mitigation.3. Under salt-alkaline stress conditions, the seedling biomass of salt-alkaline tolerancecultivar ‘Jinhui No.1’ and salt-alkaline sensitive cultivar ‘Yipin Tianxia No.208’ wasinhibited. The dry weight and chlorophyll content decreased significantly, and thephotosynthetic system was inhibited. The seedling leaf net photosynthetic rate, stomatalconductance, intercellular CO2concentration and transpiration rate decreasedin differentdegrees, and the stomatal limitation value increased. The Chlorophyll fluorescence in plantsleaves, such as the maximum photochemical efficiency, photochemical quenching coefficient,photosynthetic electron transport rate and PSⅡ photosynthetic electron transport quantumefficiency significantly reduced, but the non-photochemical quenching significantly increased.The chloroplast structure was damaged, therefore, the morphology changed andthe granumlamella was fractured or even disappeared. Foliar spraying with50mmol·L-1exogenousGABA significantly increased the melon seedling biomass, chlorophyll content,photosynthetic capacity and chlorophyll fluorescence activity under salt-alkaline stress, andmaintained the stability of the structure of the chloroplast membrane system, and alsoalleviated the injury from the salt-alkaline stress on plant chloroplasts. Under the condition ofnormal nutrient solution cultivation, the treatment of foliar spraying exogenous GABA had nosignificant difference in improving the two varieties of melon seedling photosyntheticsystem.While under salt-alkaline stress conditions, the relieve effect of the photosyntheticsystem was better for the salt-alkaline sensitive cultivar ‘Yipin Tianxia No.208’ than ‘JinhuiNo.1’. |
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