Study On ?-Aminobutyric Acid ?GABA? Accumulation And Regulation Mechanism In Longan ?Dimocarpus Longan Lour.? Fruit In Response To Cold Stress And CO₂ Stress

?-Aminobutyric acid?GABA?,a functional component,is widely distributed in plant,and expected to be widely used in the food industry,feed industry and pharmaceutical industry.However,GABA is difficult to be widely used due to low content,high extraction cost,and limited research on GABA accumulation and regulation mechanism in plant.Therefore,it is urgent and practical to study the accumulation mechanism of botanical GABA and to study GABA-physiological-funcation in response to abiotic stress druing post-harvest processing.In the first part of this dissertation,the impact of cold acclimation combined with controlled freezing-point storage on GABA metabolism and partial storage quality had been studied in longan?Dimocarpus longan Lour.?fruit.It was found that the treatment could accumulate GABA and maintain sensory quality in longan fruit.Additionally,GABA accumulation mechanism was clarified in longan fruit in reponse to cold stress.The advantages had finally been confirmed that cold acclimation combined with controlled freezing-point storage promote GABA accumulation in longan fruit.The second part focused on the impact of CO₂ stress and cold stress on GABA accumulation and regulation mechanism in longan fruit,using the only two metabolic pathway of botanical GABA accumulation?GABA shunt&Polyamine degradation pathway?as the breakthrough gaps,using the GABA,precursors and metabolites as the indicators,using the crucial enzymes,synthetases,catabolic enzymes and the related gene expression as the basises.The third part studies the impact of modified atmosphere packaging?MAP?on GABA metabolism and feasibility of MAP to accumulate GABA in longan fruit,using the crucial metabolites and crucial enzymes as the indicators,focusing on GABA shunt and gas compositions.The main results of this study are as follows:Firsty,the cold acclimation combined with controlled freezing-point storage significantly increased GABA concentration and maintained at a high level in longan fruit.The GABA concentration increased by 2.32 times compared with that of the untreated longan fruit,and was higher than that of the controlled freezing-point storage and cold storage under temperature of 4oC.GABA concentration was strongly and positively correlated with glutamate decarboxylase?GAD?activity,and has a generally opposite trend with?-aminobutyric acid transaminase?GABA-T?activity.The results revealed that the GABA accumulation was mainly caused by the increase of GAD activity and the decrease of GABA-T activity,which preliminarily explained the mechanism of GABA accumulation in longan fruit in response to cold stress.Additionally,the cold acclimation combined with controlled freezing-point storage were significantly better than the controlled freezing-point storage and cold storage under temperature of 4oC for maintaining soluble protein content,delaying softening,keeping color and the sensory quality in longan fruit.Consequently,the cold acclimation combined with controlled freezing-point storage had a significant advantage on both GABA accumulation and quality preservation in longan fruit.In addition,the controlled freezing-point storage could cause serious chilling injury to reduce the quality,grade,and economic value in longan fruit.Secondly,cold stress could promote GABA accumulation via GABA shunt that was a primary pathway for botanical GABA accumulation in longan fruit.High-level CO2 stress?10%CO₂?could strongly and transiently increase GABA concentration,and the level of CO₂ stress was consistent with GABA concentration in longan fruit.The GAD activity of the longan fruit under the high-level CO₂ stress was higher than that of the low-level CO₂stress,especially in the first 6 days of the treatment time.However,there was no significant difference in GAD activity between different level CO₂ stresses with the prolonging of treatment time.Additionally,GABA-T activity was no associated with CO₂ stress,suggesting that CO₂ concentration may not affect GABA-T activity in longan fruit.The results revealed that GABA accumulation in the GABA shunt of longan fruit was due to the increase in GAD activity to catalyze GABA synthesis,rather than reducing GABA-T activity to reduce GABA degradation.The moderate level of CO2 stress?5%CO2?was the best for maintaining the firmness in longan aril,color in longan pericarp,and sensory quality in longan fruit.However,high-level CO₂ stress may cause CO₂ injury for longan fruit,that causes accelerate the aril softening,pericarp browning,sensory quality decline,and economic value reducing.This study clarifies GABA accumulation and regulation mechanism via GABA shunt in response to high-level CO2 stress in longan fruit,and confirmed the feasibility of hydro-cooled precooling in iced water to accumulate GABA via GABA shunt after harvest.Thirdly,the contribution rate of polyamine degradation pathway to GABA accumulation was 20%²5%in longan fruit.Therefore,cold stress could still accumulate GABA due to enhanced capacity to accumulate GABA via GABA shunt,even if the cold stress could inhibit GABA accumulation via polyamine degradation pathway.The high-level CO₂ stress could decrease the activities of diamine oxidase?DAO?,polyamine oxidase?PAO?,aminoaldehyde dehydrogenase?AMADH?and the expression of their related genes,which cause the concentrations of putrescine?Put?,spermidine?Spd?and spermine?Spm?increased.As the treatment progressing,the activities and gene expressions of DAO,PAO and AMADH increased first and then decreased,and the concentrations of Put,Spd and Spm also increased first and then decreased.The high-level CO₂ stress could reduce the activities of DAO and AMADH that cause decrease the capacity for transforming Put and Spd into GABA.As the treatment progressing,the activities of DAO and AMADH increased first and then decreased that cause capacity of Put and Spd to be transformed into GABA was enhanced first and then decreased.The high-level CO₂ stress could inhibit the activity of DAO,PAO and AMADH,and reduce the expression of related genes,which cause reduces the capacity for accumulating GABA via polyamine degradation pathway.This study clarifies GABA accumulation and regulation mechanism via polyamine degradation pathway in response to high-level CO₂ stress,and determines the contribution rate of GABA pathway and polyamine degradation pathway to GABA accumulation in longan fruit.Finally,MAP could form a controlled atmosphere of high CO2 and low O2 by using0.03 mm polyethylene film,but it could not meet the needs of controlled atmosphere storage in longan fruit in terms of gas concentration or concentration stability.The maximum concentration of CO₂ was 5.4%and the minimum concentration of O₂ is 13.9%under treatment temperature of 4°C.GAD activity of both MAP treatment and air control treatment showed a decreasing trend in longan fruit,but the descending rate of MAP treatment was lower than that of the air control treatment.GABA-T activity of all the treatments was fluctuating during the whole treatment period,which may be related to the limitations of traditional measurement on GABA-T activity.MAP could retard the decrease of GABA concentration in longan aril and maintain the color of longan pericarp,but there was no significant effect on soluble protein concentration,firmness and color of longan aril.The study provides a new recommendation for the application of MAP in longan fruit,and provides a new idea to produce GABA-rich longan fruit.