Changes And Fractionation Characteristics Of Carbon Stable Isotopes In Korla Fragrant Pear During Growth Period

Xinjiang Korla fragrant pear is a characteristic agricultural product with strong regional characteristics,it is one of the excellent local pear varieties in China,it has been selected into the first batch of protection lists of the China-EU geographical indication agreement.In order to protect the Korla fragrant pear brand,it is particularly important to use food traceability technology to trace the origin of the pear.Stable isotope fingerprinting technology is an effective technique widely used in food origin traceability.Carbon stable isotope is one of the most commonly used stable isotopes,origin traceability using stable isotope technology is based on the natural fractionation effect of isotopes.At present,it is unclear the fractionation characteristics of Korla fragrant pear carbon stable isotopes during the growth period and the influence of ecological environment factors on the fractionation mechanism,it has become the bottleneck problem of the accuracy and popularization and application of this technology.If this scientific problem can be solved,the reliability and stability of the origin traceability of stable isotope technology will be improved,which has important theoretical significance in the traceability and confirmation of the origin of agricultural products and food.Taking Korla fragrant pears from Jiuquan City,Gansu Province as the control,Korla fragrant pears from different geographical sources and different growth periods were collected from Korla and Aksu,theirδ¹³C values were determined by isotope ratio mass spectrometer,the composition of sugars and organic acids was analyzed by means of high performance liquid chromatography.Combined with the analysis methods of variance,principal component analysis and redundancy analysis,the changes and regional differences of fruit quality during the growth period,the variation law and fractionation characteristics of carbon isotope during the growth period,and the effects of eco-environmental factors on carbon isotope composition and nutritional quality were studied,the main conclusions are as follows:（1）At 60-140 days after flowering,the fruit of Korla fragrant pear increases rapidly with the continuous maturity of the fruit,the single fruit weight,the longitudinal and transverse diameters increase rapidly,and the hardness gradually decreases;the contents of reducing sugar,glucose and fructose gradually increase,and the content of sorbitol,polysaccharide and the content of fructose:glucose:sorbitol decreased gradually from 1:1.5:7.5 at 60 days after flowering to 1:0.9:0.6 at 140days after flowering.During fruit development,the organic acid content decreased gradually.The main types of organic acids in fragrant pear were malic acid and tartaric acid,which accounted for 96%of the total acid content.The sugar-acid ratio was the largest at 140d after flowering,and the flavor was the best.Among the sampled orchards,the four orchards in Korla had the best quality,followed by the three orchards in Jiuquan,and the three orchards in Aksu were of poor quality.（2）The variation range of fruitδ¹³C value in 2020 is-27.79‰～-25.48‰,the variation range of fruitδ¹³C value in 2021 is-26.53‰～-25.44‰,and the variation range of leafδ¹³C value in 2021 is-26.87‰～-25.91‰,the variation range of carbon stable isotopes in 2021 is smaller than that in 2020.Theδ¹³C value gradually increased at the young fruit stage（60-90d after flowering）,and theδ¹³C value gradually decreased as the fruit matured.（3）The fractionation coefficient of the leaves of Korla fragrant pear is greater than that of the fruit,the fractionation coefficient of the fruit at the bottom of the tree is greater than that of the top,and the fractionation coefficient of the peel is greater than that of the pulp.The fruits and leaves,the fruits at the top and bottom of the tree,the pulp and the peel all have carbon isotope fractionation.After repeated elution with ultrapure water to remove soluble sugars from fragrant pear fruit,theδ¹³C value was-28.114‰,and after repeated elution with 70%ethanol to remove monosaccharides,oligosaccharides and alkaloids,theδ¹³C value was-26.453‰,and after repeated elution with acetone to remove ketones and alcohols,theδ¹³C value was-26.334‰.Theδ¹³C value of specific compounds such as glucose in the ripe fruit is-27.355‰,theδ¹³C value of fructose is-28.636‰,and theδ¹³C value of ribitol is-27.023‰.Theδ¹³C value of specific compounds is ranked as ribitol>glucose>fructose.（4）The relationship between carbon isotope fractionation characteristics and nutritional quality and ecological environment factors was explained through redundancy analysis.Altitude,daily accumulated temperature,and day-night temperature difference were the key factors affecting carbon isotope fractionation during fruit growth.Altitude and accumulated daily temperature have a negative impact on carbon isotope fractionation,and a positive impact of the temperature difference between day and night.Daily accumulated temperature was a key factor affecting the nutritional quality of fruit during the growth period.Daily accumulated temperature,relative humidity,temperature difference between day and night,and sunshine hours had a positive impact on reducing sugar content and a negative impact on total acid content.