| China has a vast territory,and apple trees are mostly planted in the north area of China where apple trees are vulnerable to suffer from chilling stress and freezing stress.Late autumn and early frost in winter,low temperature in winter,and flowering frost affect the survival of apple seedling as well as the vegetative growth and reproductive growth of apple seedlings,and even affect the yield and quality of apple fruits.γ-Aminobutyric acid（GABA）is a kind of four-carbon nonprotein amino acid.It has been proved that GABA was involved in plant physiological metabolism,which helped to improve the stress tolerance of plants under environmental stress.To alleviate the loss of apple industry and provide theoretical basis for improving apple chilling tolerance and freezing tolerance,this experiment screened the suitable concentration of exogenous GABA,and analyzed the degree of chilling tolerance and freezing tolerance of six cultivars,including‘Qin Mi’,‘Honeycrisp’,‘Qin Cui’,‘Liquan Fuji’,‘Yuhua Fuji’,and‘Golden Delicious’.Furthermore,taking‘Qin Mi’and‘Honeycrisp’apple seedlings with differential freezing tolerance as experimental materials,the different responses to freezing stress in apple seedlings because of cultivars,virus infection,and domestication were analyzed,and the role of exogenous GABA on improving freezing tolerance of apple seedling were studied.It was to provide theoretical basis and data support for improving the chilling tolerance and freezing tolerance of apple seedlings.The results were as follows.（1）Under chilling stress and freezing stress,apple seedlings were wilting,the relative electric conductivity（REL）and malondialdehyde（MDA）content of apple seedlings increased,and the chlorophyll fluorescence（Fv/Fm）,catalase（CAT）and superoxide dismutase（SOD）activity of apple seedlings declined.After applying exogenous GABA with concentration of 0.5,5,10,20,35,50,75 and 100 m M,respectively,the degree of seedlings wilting alleviated,the REL and MDA content of leaves of apple seedlings decreased,and the Fv/Fm,CAT,and SOD activity of apple seedlings increased.During the eight exogenous GABA concentration gradient of this experiment,50 m M exogenous GABA had the best effect on improving the chilling tolerance and freezing tolerance of apple seedlings.（2）Under chilling stress,‘Qin Mi’exhibited the highest chilling tolerance,‘Qin Cui’and‘Honeycrisp’exhibited high chilling tolerance,‘Liquan Fuji’exhibited moderate chilling tolerance,and‘Yuhua Fuji’and‘Golden Delicious’exhibited low chilling tolerance via subordinate function values（SFV）analysis based on resistance index（RI）,and cluster analysis based on SFV.Under freezing stress,‘Qin Mi’and‘Qin Cui’exhibited the highest freezing tolerance,‘Honeycrisp’and‘Liquan Fuji’exhibited high freezing tolerance,‘Yuhua Fuji’exhibited moderate freezing tolerance,and‘Golden Delicious’exhibited low freezing tolerance via SFV analysis based on RI,and cluster analysis based on SFV.（3）The different freezing tolerance of apples cultivars resulted in the different response mechanisms of seedlings to freezing stress.Under freezing stress,compared with high freezing tolerance apple seedlings‘Honeycrisp’,the content of REL,MDA,hydrogen peroxide（H2O2）and superoxide anion(O2-.)of the highest freezing tolerance apple seedlings‘Qin Mi’declined by 43.3%,40.8%,15.1%,10.3%,respectively;the activity of CAT,SOD,peroxidase（POD）and ascorbate peroxidase（APX）increased by 39.9%,115.3%,48.1%,25.1%,respectively;and the content of proline（Pro）,GABA,soluble sugar,soluble protein and reduced glutathione（GSH）raised by 34.4%,5.8%,27.8%,17.5%,13.1%,respectively.（4）Under freezing stress,compared with‘Qin Mi’virus-carrying apple seedlings,the content of MDA,H2O2,and O2-.of‘Qin Mi’virus-free apple seedlings declined by 36.6%,50.9%,11.5%,respectively;the activity of CAT,POD,SOD,and APX increased by 13.7%,9.0%,26.8%,23.9%,respectively;and the content of Pro,GABA,GSH,glutamic acid（Glu）and soluble protein raised by 32.0%,2.1%,37.9%,21.2%,3.0%,respectively.（5）Under freezing stress,compared with‘Honeycrisp’non-domesticated apple seedlings,the content of REL,MDA,H2O2,and O2-.of‘Honeycrisp’domesticated apple seedlings declined by 81.7%,44.7%,27.8%,40.6%,respectively;the activity of CAT,POD,SOD,and APX increased by 98.8%,65.1%,182.8%,49.0%,respectively;and the content of Glu、Pro,GABA,GSH,soluble sugar and soluble protein raised by 43.8%,56.4%,6.6%,112.6%,77.7%,21.6%,respectively. |
