Effects Of Alkali Stress On Fruit Quality And The Mechanism In 'Gala' Apple

Apple is one of the largest fruit tree species cultivated in China.The Northwest Loess Plateau is the best producing area for apple in China.However,at present,the soil alkalization continues to increase under the influence of multiple factors in this area,and the soil pH is mostly above 8.0,which has become an important factor affecting the orchard fertilizer utilization and fruit quality.Researching on the effects of alkalized soil on apple traits and the mechanism of alkali stress affecting fruit flavor and coloring would provide a theoretical basis for the analysis of the eugenics mechanism of the dominant apple producing areas in Northwest China.It also would be of great significance to the cultivation of high-quality and high-yield apples,soil improvement,and the sustainable development of the apple industry in Northwest China.In this study,8-year-old'Gala'?M26 rootstock?apple trees were used to simulate the soil pH of the producing areas by artificially pouring alkaline nutrient solution?Na₂CO₃:NaHCO₃=1:1,pH=9.0?.Under the conditions,the appearance quality and inner quality of fruits were measured to analyze the effects of alkali stress on the fruit quality of'Gala'apple,and further analysis on the changes of content,relative enzyme activity and relative gene expression of sugar and anthocyanin in fruits under alkali stress was performed.The aim was to investigate the metabolism of sugar and titratable acid and coloring in'Gala'fruits under alkali stress.All results were as follow:1.Alkaline stress inhibited the photosynthesis of'Gala'leaves,resulting in a significant reduction of chlorophyll content.The single fruit weight,vertical and horizontal diameter of fruits decreased,and the yield per plant also decreased.There was no significant change in fruit firmness and fruit shape index,and Vc content increased.Alkali stress also resulted in a significant decrease in titratable acid content and an increase in the ratio of soluble solid and titratable acid,thus improving fruit flavor.The fruits under alkali stress ripened ahead of those of the control.In general,alkali stress improved the fruit quality of'Gala'apple.2.Alkali stress affected the fruit flavor of'Gala'apple,and changed the contents of sugar and titratable acid.Malic acid content decreased significantly in the fruits under alkaline stress.The fructose content did not change,the glucose content decreased significantly,the sucrose content increased significantly,and the total sugar content decreased.However,the fruit sweetness increased due to the substantial increase in the sucrose content,and the ratio of sweetness and titratable acid also increased.The expression of sugar metabolism and transport related genes,MdSPS and MdSUT,was significantly up-regulated and the expression of MdAI was decreased.Changes in the amount of gene expression and related enzyme activities resulted in the large changes in the proportion of sugar components in'Gala'apples.3.Alkali stress forced significant changes in the coloring of'Gala'fruits,with increased redness and its area.Measured color value showed a significant decrease in L value,with a significant increase in a value and b value.The content of anthocyanin in the peel of'Gala'fruits under alkaline stress increased significantly.The reason for this was that the activities of the key enzymes on the anthocyanin synthesis pathway,phenylalanine ammonia-lyase?PAL?and UDP-glucose-flavonoid-3-O-glucosyltransferase?UFGT?,were significantly increased by the alkali stress,and the expressions of the key enzyme genes,MdPAL and MdUFGT1,which related to the synthesize of anthocyanin,were significantly up-regulated.These changes were consistent with the corresponding changes in the enzyme activity.The expression of key transcription factor MdMYB10 was also significantly up-regulated.