Metabolomics-based Study Of The Differences In Chemical Composition Of Huanglongbing And Normal Navel Oranges

The navel orange belongs to the Rutaceae family and the citrus genus,which is one of the characteristic agricultural products of Jiangxi Province.It has been recognized and praised by customers and experts at home and abroad due to its high quality,high output and excellent taste.Navel oranges are rich in nutrients such as essential oils,alkaloids,flavonoids,carotenoids and other chemical components.They also have many kinds of physiological effects such as anti-tumor,antioxidant,antiinflammatory and so on.Huanglongbing(HLB),also know as greening disease,which is a plant disease caused by Candidatus Liberibacter asiaticus(Las),a phloem-limited bacterium.After citrus is infected with Huanglongbing,root rot,leaf yellowing,dwarf plants,and many dead branches will appear,and in more severe cases,it may cause the death of the plant.There are two types of the Huanlongbing fruits,green fruits and “red nose fruits”,whose sugar-acid ratio,shape,flavor,and taste can’t meet the demand of commercial sales.In this study,the volatile and non-volatile components of Huanglongbing navel oranges and normal ones were analyzed by chromatographic mass spectrometry combined with chemometrics.And the changes of organic compounds in the peel and pulp of the two kinds of navel oranges compared were compared.It has a certain theoretical guiding effect on the prevention and treatment of navel orange Huanglongbing and the reprocessing and utilization of the diseased fruit.The main findings are as follows:Chapter 1: The research progress in recent years on the chemical constituents,physiological activities of navel oranges,and the pathogenic mechanism and control of citrus Huanglongbing were described in detail.Navel oranges are rich in essential oils,alkaloids and other natural biologically active substances,which have a variety of physiological effects.But Huanglongbing has a great impact on their appearance,taste and nutrients.In addition,the non-targeted metabolomic techniques and chemometric analysis used in the project are discussed.Chapter 2: The volatile components of Huanglongbing and normal navel oranges were analyzed by headspace solid-phase microextraction-gas chromatography-triple quadrupole mass spectrometry(HS-SPME-GC-QQQ-MS)to explore the differences of volatile components in the peel and pulp of the two kinds of navel oranges.A total of92 volatile components were identified in the experiment,mainly included olefins(81.28-94.67%),aldehydes(2.04-12.91%)and alcohols(1.01-5.32%).In addition,using chemometric analysis method,12 and 14 differential compounds were respectively screened out in peel and pulp samples by OPLS-DA discriminant model.Among them,(E)-2-hexen-1-ol,carvone,citronellyl acetate and neryl acetate in the peel samples and trans-carvone,cis-carvone-5,6-oxide and cis-carvyl acetate in the pulp samples can be used as potential metabolic differences for identifying navel orange Huanglongbing disease.Chapter 3: The non-volatile components of Huanglongbing and normal navel oranges were analyzed by UPLC-QTOF-MS technology.86 and 95 non-volatile components were respectively identified in peel and pulp.According to the results of principal component analysis(PCA),it was found that the two kinds of navel oranges had a good degree of discrimination,and combined with the principles of VIP > 1.0,P value(Tukey test)< 0.001 and Fold Change value > 2 or < 0.5 in the OPLS-DA discriminant model,21 and 11 differential compounds were screened out from peel and pulp samples,and four of them were common differential compounds.According to the content analysis results of the differential compounds,the flavonoid glycosides in the navel oranges had a better enrichment effect after being infected with Huanglongbing.Chapter 4: In this chapter,the physicochemical properties,total phenol content,total flavonoids content and antioxidant activity indexes of Huanglongbing and normal navel oranges were determined.The results showed that the fruit weight,peel thickness and soluble solid content of navel oranges were decreased after infection with Huanglongbing,but the antioxidant activity was increased.Correlation analysis was performed between the above results and differential non-volatile components,and it was found that there was a significant positive correlation between phenols and flavonoids and their antioxidant activities,but several flavonoid-glycosides compounds in the differential compounds had a negative correlation with antioxidant activities.