Study On Determining The Geographical Origin And Cultivation Methods Of Vegetable Based On Stable Isotope Technique

Vegetables are an essential part of a healthy diet,and promoting their quality and ensuring their authenticity is of great importance in meeting the growing material needs of modern people for high quality.Different origins and different cultivation methods of vegetables in the environment,fertilizers,pesticides and other differences in market prices,such as geographical indications,organic vegetables are relatively higher prices.Driven by profits,there are still businesses who take ordinary vegetables as organic,green or geographical indication vegetables,and it is difficult for consumers and even regulatory agencies to effectively identify their authenticity through packaging and traceability information alone.Stable isotope abundances(δ¹³C,δ¹⁵N,δ²H andδ¹⁸O)can objectively reflect the properties of agricultural products and the relation to their natural information in the growth chain（environment of origin,breeding methods,etc.）and are used to trace the origin of many agricultural products and to identify cultivation methods.Therefore,four stable isotopes(¹³C,¹⁵N,²H and ¹⁸O)were applied to trace the origin of vegetables in this study,and proposed characteristic indicators and thresholds for identifying organic vegetables based onδ¹⁵N,then the effect of pesticide residues on the abundance of stable isotopes in vegetables were investigated,which was an innovative try of stable isotope technique in the discrimination of vegetable origin and product authenticity.The study are as follows:（1）Determination of the geographic origin authenticity of vegetablesTheδ¹³C,δ¹⁵N,δ²H andδ¹⁸O values of five common vegetables（lettuce,garland chrysanthemum,pepper,tomato and cucumber）were analyzed by stable isotope mass spectrometer.The differences ofδ¹³C,δ¹⁵N,δ²H andδ¹⁸O between Shanghai local vegetables and other geographic origin vegetables were analyzed by one-way ANOVA,and principal component analysis（PCA）and partial least squares discriminant analysis（PLS-DA）were used to establish the discriminant models for Shanghai local vegetables.The results showed that theδ¹³C,δ¹⁵N,δ²H andδ¹⁸O values of Shanghai local vegetables were significantly different from those of other vegetables（P<0.05）.However,different stable isotopes play different roles in the discrimination of different vegetables,withδ¹⁵N andδ¹⁸O values for tomatoes,δ²H values for peppers,andδ¹⁵N values for garland chrysanthemum were more important.The optimal PLS-DA models could well distinguish the Shanghai lettuce,garland chrysanthemum and tomato with the accuracy rate of 100.00%,91.67%,85.71%respectively.The importance variables of PLS-DA wereδ¹⁵N,δ²H andδ¹⁸O values.This illustrated that stable isotope abundances can enable traceability discrimination of vegetable origins.（2）Discrimination of organic Brassica chinensis L cultivation methods and stable isotope thresholdsEA-IRMS was used to determine theδ¹³C,δ¹⁵N,δ²H andδ¹⁸O values of Brassica chinensis L,then a database ofδ¹³C,δ¹⁵N,δ²H andδ¹⁸O values of green,organic and conventional of Brassica chinensis L was built.The distribution characteristics of stable isotopes in Brassica chinensis L were analyzed by median value and multivariate statistical method.The results showed that theδ¹³C value of conventional Brassica chinensis L was higher than that of green and organic Brassica chinensis L,and was a significant difference with organic Brassica chinensis L（P<0.05）.And theδ¹⁵N value of organic Brassica chinensis L was higher than that of conventional and green,and the difference was significant（P<0.05）.Although theδ²H andδ¹⁸O values of Brassica chinensis L in three farming methods were different,the differences were not significant（P>0.05）.As a result,the most important stable isotope to distinguish farming methods of Brassica chinensis L was nitrogen stable isotope due to the difference between chemical and organic fertilisers,and ¹⁵N was suitable as a characteristic stable isotope for discriminating organic vegetables.The discrimination model of farming methods of Brassica chinensis L was established by PLS-DA with accuracy of organic Brassica chinensis L was 88.46%.According to 80%organic Brassica chinensis L could be identified,theδ¹⁵N value to distinguish organic Brassica chinensis L was seted at 6.8%.（3）Relationships between pesticide residues and stable isotopes in leafy vegetablesCommon leafy vegetables（cabbage,lettuce and spinach）were used as research objects,and the relationship between the residue of different types of insecticides and stable isotope ratio(δ¹³C,δ¹⁵N,δ²H andδ¹⁸O)were investigated by applying thiamethoxam（a neonicotinoid pesticides）and fenvalerate（a pyrethroid pesticides）.The results showed that there were differences in the degradation process of thiamethoxam and fenvalerate in spinach,cabbage and lettuce with time after two applications.The first degradation residues conformed to the primary reaction kinetic equation.The process of pesticide degradation and metabolism was also accompanied by the difference of stable isotope fractionation.The discrimination accuracy rate of three vegetables control groups was 100%based on the changes ofδ¹³C,δ¹⁵N,δ²H andδ¹⁸O value combined with PLS-DA.The discrimination accuracy rate of spinach after the first and second application of thiamethoxam,cabbage after the second application of thiamethoxam and after the first application of fenvalerate,lettuce after the first application of thiamethoxam were also100%.The accuracy rate of cabbage after the first application of thiamethoxam and lettuce after the first and secend application of fenvalerate should be further improved.The total accuracy rates of spinach and cabbage were 88.89%and 83.33%,respectively.Thus,the pesticide residues influenced the stable isotopes abundance in vegetables and showed some correlation.