The Alleviating Effect Of Exogenous ?-Aminobutyric Acid (GABA) On Alkaline Stress In Apple Seedlings And Its Physiological Mechanism

Apple production plays a significant role in the fruit industry in China.The Loess Plateau in the northwest China is the largest apple producing area,which is also the best apple producing area in China due to its sufficient sunshine,deep soil layer and large temperature difference between day and night.Whereas,the Loess Plateau is located in an arid and semi-arid region,where low rainfall and large evaporation lead to serious soil salinization,and the soil p H value of some orchards can even be as high as 8.5-9.0.In addition to drought stress,alkaline stress has become another essential factor to restrict apple industry in this area.Alkaline stress will inhibit the normal growth of apple seedlings,resulting in a tremendous decrease in apple yield and quality.In order to solve the problem of high p H in apple producing areas in Northwest China.Malus hupehensis var.pingyiensis,an apple rootstock,was used in this study.Firstly,the suitable concentration of exogenous GABA to alleviate the alkaline stress was decided by hydroponic culturing techniques.And then we further studied the effects of exogenous GABA on apple seedling under alkali stress,including biomass,relative electrolyte leakage,root structure,photosynthetic characteristics,antioxidant enzyme activities,organic acid and amino acid content.The results of this study will provided certain theoretical basis for improving apple alkali stress resistance research.The main results of this dissertation are as follows:1. After 15 days'alkaline stress treatment,the growth of apple seedlings was inhibited,but the application of exogenous GABA could alleviate this inhibition to some extent.By applying different concentrations of GABA,compared with that of the control group the dry weight,fresh weight and the REL of apple seedlings all changed to different degrees.Exogenous application could better alleviate the alkali stress of apple seedlings,by applying different concentrations of GABA,0.5m M GABA was the most suitable concentration to alleviate alkaline stress of apple seedlings.2. After 15 days of alkaline stress treatment,compared with the control group,the plant height,fresh weight and dry weight of apple seedlings decreased by 27.6%,47.1%and 37.4%,respectively.Exogenous GABA treatment alleviated the growth inhibition of alkaline stress on apple seedling,and the total fresh and dry weight increased by 48.4%and39.4%respectively when compared with the alkaline stress group.In addition,after alkaline stress treatment the root structure of apple seedlings changed,compared with control group the amplitude of total length,surface areas,average diameter,volume,number of root tips,and number of root forks decreased by 30.5%,18.8%,18.8%,27.3%,25.5%,and 26%,respectively.Compared the group with the alkaline stress treatment,application of exogenous GABA changed the root architecture of apple seedling,increased total length,surface areas,average diameter,volume,tips,and forks of roots by 25.1%,23.2%,26.2%,30.8%,29.1%,and 26%,respectively.The REL of leaves and roots increased significantly under alkaline stress,exogenous 0.5 m M GABA could alleviate these inhibitory effects and promote the growth and development of apple seedlings under alkaline stress.3. Exogenous GABA treatment could enhance the antioxidant system and photosynthetic system of apple seedlings under alkaline stress.After 15 days of alkaline stress,the content of H₂O₂ significantly increased,while exogenous GABA could reduce the content of H₂O₂ in apple seedling leaves.Under alkaline stress,the activities of superoxide dismutase,catalase and peroxidase in apple seedling leaves significantly decreased,and exogenous GABA treatment could enhance the activities of SOD,CAT and POD in apple seedling.In addition,under alkaline stress,the net photosynthetic rate and stomatal conductance decreased and intercellular carbon of seedlings dioxide increased,indicating alkaline stress also caused damage to the photosynthetic system of apple seedlings.However,exogenous 0.5 m M GABA could effectively alleviate these effects.4. Exogenous GABA increased the content of organic acids?malic acid,citric acid and succinic acid?in apple seedlings under alkaline stress.Compared with the control group,the content of malic acid and citric acid in leaves increased significantly under alkaline stress,but there was no significant difference in the contents of succinic acid.However,the contents of malic acid,citric acid and succinic acid increased significantly after exogenous of GABA under alkaline stress.At the same time,exogenous GABA raised the expression of Md GABA-T and Md SSADH2 and promoted the shunt activity of GABA under alkaline stress.Exogenous GABA could significantly increase the activities of MDH,CS,ICD and ACO,thus increasing the accumulation of malic acid,citric acid and succinate and improving the tolerance to alkaline stress of apple seedling.5. Exogenous GABA affected the accumulation of amino acids under alkaline stress.Compared with alkaline stress,exogenous GABA significantly increased the content of aspartic acid,phenylalanine,alanine,serine,lysine,threonine,proline and glycine,and significantly decreased the content of arginine,valine,tyrosine and leucine of apple seedling.There was no significant difference between isoleucine and histidine content.It is suggested that exogenous GABA could change the content of amino acids in apple seedlings under alkaline stress,so as to improve the tolerance to alkaline stress.The above results indicated that under the alkaline stress,exogenous GABA can improve the ability of photosynthetic system and antioxidant system of apple seedlings under alkaline stress,enhanced the accumulation of organic acid content,and influenced the accumulation of amino acid content,so as to alleviate alkaline stress and promote plant growth.