| Pear(Pyrus L.)is considered to have originated in China,belonging to the Maloideae subfamily of the Rosaceae family.Pear is a kind of important temperate fruit,widely distributed throughout China,with the North China Plain as the main producing area.The fruit of pear is rich in nutrition,sweet,crisp and juicy,make it popular among the consumers,and has important social,ecological and economic values.As an important cash crop,fruit tree cultivation and related industries are one of the most important sources of income for China’s rural economy.In recent years,economy of China has been developing rapidly,while the fruit industry is in the need of making corresponding adjustment.The industrial structure of the pear industry is also gradually developing towards the characteristic agriculture of quality and environmental protection,and the improvement of the quality of pear germplasm resources is the foundation of the development of the pear industry.Fruit quality is the core of pear germplasm resources,which is affected by many aspects related to its own genetic characteristics,such as plant phenotypes,aroma components,metabolite composition,etc.These characteristics not only reflect the physiological function of the growth and development of the fruit tree,but also are closely related to the fruit quality,which directly determine the taste and nutrition of the fruit.At present,how to conduct a comprehensive and scientific evaluation of the pear germplasm resources,as well as the reasons for the formation of the flavor and nutrition of pear fruit,need to be further explored.In this study,18 pear germplasms collected in Tianjin were used as research materials,including the cultivars of Pyrus bretschneideri Rehd.,Pyrus ussuriensis Maxim.and Pyrus pyrifolia(Burm.f.)Nakai.16 fruit quality indicators,9 fruit mineral element indicators and 15 leaf phenotype indicators were determined and analyzed.The extraction and identification of the fruit volatile aroma compounds were carried out using HS-SPME coupled with GC-MS.The metabolite identification and KEGG metabolic pathway analysis were conducted utilizing a widely targeted UHPLC-QTOF-MS.A comprehensive evaluation model of pear fruit quality was established using factor analysis,and the reasons for the differences in fruit quality of pear germplasms were revealed from the aspects of fruit nutrition,leaf phenotypes,aroma components,and metabolomics,aimed to provide a theoretical foundation for the optimization of cultivar selection,also for the application of downstream industries of pear.The main results of this study are as follows:1)A comprehensive evaluation model of pear fruit quality based on factor analysis was established,and the characteristic indicators for the evaluation of pear fruit quality were selected.16 quality indicators representing the appearance characteristics and inner quality of the pear fruit were determined,and the coefficients of variation(CV)of each indicator in the 18 pear germplasms ranged from 3.11%to 314.44%.The principal factor comprehensive evaluation model was constructed by factor analysis,and the ranking of the pear germplasms according to the comprehensive scores was listed as follows:‘Mili’>‘Qiubai’>‘Zaosu’>‘Jieli’>‘Hongxiangsu’>‘Qingxiang’>‘Xuehua’>‘Yali’>‘Xinli No.7’>‘Daguo Yali’>‘Yaguang’>‘Huangguan’>‘Housi’>‘Hongxiao’>‘Suisho’>‘Yuluxiang’>‘Qiuyue’>‘Anli’.6 principal factors that determine the pear fruit quality were obtained from factor analysis,and simplified into 6 quality factors:solid-acid ratio,C~*,TSS,FSI,Vc and L~*.The simplified model was verified to be significantly correlated with the original model(R~2=0.95,p<0.01),and these 6 quality factors could be regarded as the characteristic indicators for the evaluation of pear fruit in Tianjin even North China.2)Fruit mineral nutrition and leaf phenotype were closely related to fruit quality.A multiple linear regression model(MLR)was established between the fruit mineral elements and the fruit quality factors,the leaf phenotypic traits and the fruit quality factors,respectively.It was found that there were significant or extremely significant correlations between some fruit mineral elements and the fruit quality factors(p<0.05 or p<0.01),such as K,B,Fe,Mn.Most of the leaf phenotypic traits showed remarkable or extremely remarkable linear relationship with the fruit quality factors(p<0.05 or p<0.01),such as C,P,K,C/N,leaf length,leaf dry weight,specific leaf area,chlorophyll-a content,and photochemical efficiency of PSⅡ,which may have different degrees of influence on the fruit quality.3)The extraction and identification of the fruit volatile aroma compounds from18 pear germplasms were carried out using HS-SPME coupled with GC-MS,and the differences in the fruit aroma components of different pear germplasms were illustrated from the profile of volatile aroma compounds.A total of 104 aroma compounds were identified in the fruit volatile aroma of the tested pear germplasms,including 12 alkanes,9 alkenes,10 alcohols,13 aldehydes,7 ketones,42 esters,and other 11 aroma compounds.From the perspective of aroma components,the aroma components detected in the tested pear germplasms were mainly fruit-flavored esters,followed by aldehydes and alkenes.Among them,ethyl heptanoate,ethyl palmitate,hexanal,(E)-2-hexenal,α-farnesene were detected in most pear germplasms.From the perspective of pear germplasms,the total content of volatile compounds of Pyrus ussuriensis cultivars was higher than that of Pyrus pyrifolia and Pyrus bretschneideri cultivars.In addition,the results of the loading diagram of principal components and the clustering heat map indicated that the pear germplasms of Pyrus pyrifolia showed more similar in the fruit aroma composition,while the pear germplasms of Pyrus bretschneideri and Pyrus ussuriensis showed more variable in the fruit aroma composition.4)The identification and quantification of the metabolites from 18 pear germplasms were conducted using widely targeted metabolomic analysis based on UHPLC-QTOF-MS.The correlations between the metabolites and phenotypic traits of fruit were explored,and the metabolic profiles were plotted,including the major metabolites that may determine the fruit quality,such as carbohydrate,organic acids,polyphenols,and amino acids.A total of 493 metabolites were identified,which contained numerous metabolites that may affect the flavor and taste of the fruit,consisting of 68 carbohydrates,47 organic acids,50 polyphenols,21 amino acids,20vitamins and other primary or secondary metabolites.Different pear germplasms have obvious differences in the metabolite composition,and the metabolite differences would directly affect the phenotypes,the metabolites such as sugars,organic acids,polyphenols,and amino acids have effects on the fruit phenotypic traits.The metabolites of carbohydrates and organic acids were clearly distinguished in the 3pear species of Pyrus bretschneideri,Pyrus ussuriensis and Pyrus pyrifolia,which may be the direct reason for the difference in fruit taste among the 3 pear species.The abundances of polyphenols in Pyrus ussuriensis cultivars were obviously higher than that in Pyrus pyrifolia and Pyrus bretschneideri cultivars,especially‘Anli’and‘Jieli’,which were rich in antioxidants such as proanthocyanidins,flavanols,flavanones and flavones.The abundances of amino acids in Pyrus pyrifolia cultivars were greater than that in Pyrus ussuriensis and Pyrus bretschneideri cultivars,and the abundances of amino acids such as L-phenylalanine,L-glutamine,L-glutamic acid and D-aspartic acid were at higher levels.5)Differentially expressed metabolites,including sugars,polyphenols and vitamins,which may affect the fruit taste and nutrition of different pear species,were selected using orthogonal partial least squares discriminant analysis(OPLS-DA)model,and the reasons for the differences in the fruit flavors of different pear species were explained from the perspective of the KEGG metabolic pathways.3 pear species fruits selected in this study have different metabolic profiles,and Pyrus ussuriensis were obviously distinguished from Pyrus bretschneideri and Pyrus pyrifolia,which may be the reason for the unique quality of Pyrus ussuriensis.The differential metabolites were screened by pairwise comparison based on the OPLS-DA model,and it was found that Pyrus ussuriensis had more differential metabolites up-regulated compared to Pyrus bretschneideri and Pyrus pyrifolia.The results of topology analysis and enrichment analysis of KEGG metabolic pathway showed that the pathway of phenylalanine metabolism was significantly different between Pyrus bretschneideri and Pyrus pyrifolia(p<0.05),and the related metabolites of this metabolic pathway may be the reason for the difference in the fruit quality between Pyrus bretschneideri and Pyrus pyrifolia.Based on variable importance in the projection(VIP),fold difference(FC)and two-tailed t-test,D-xylose,formononetin,proanthocyanidin A1,andβ-nicotinamide mononucleotide were finally selected as the crucial differential metabolites among Pyrus bretschneideri,Pyrus pyrifolia and Pyrus ussuriensis that may affect the fruit taste and nutrition. |
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