| Melon(Cucumis melon L.)is popular for its unique flavor.The area of melon‘s protected cultivation is gradually increasing in China,and artificial pollination and growth regulators are usually used for proper fruit setting because of the closed environment with in the greenhouses.During recent years,artificial pollination has been unable to satisfy the production demands because of the rising labor costs caused by the reduction of agricultural labor force.A growth regulator 1-(2-chloro-4-pyridyl)-3-phenylurea(CPPU)is usually used to replace the artificial pollination,however,the improper use of CPPU usually leads to malformation and quality decline of melon fruits.In the previous studies,we have found that bee pollination can replace artificial pollination as a fruit setting method under the protected cultivation,but its mechanism to affect fruit development and volatile flavoring compounds remains unclear.In this study,we used melon cultivars ’Elizabeth’ as the material to study the different effects of honey pollination and CPPU treatment on the accumulation of volatile compunds during the development of melon fruit under the protected cultivation system.The results are as follows:1.During the development of melon fruit,183 kinds of volatile substances were detected,mainly including esters,aldehydes and alcohols.There were mainly aldehydes and alcohols detected before fruit coloring(20 DAA).The esters began to accumulate at fruit coloring stage(color break stage)(25 DAA),and reached the peak at 38 DAA when the fruit is ripened which becoming the main characteristic of mature melon fruits‘ flavor.The main aldehydes detected before fruit color break stage includes(trans,cis)-2,6-nonadienal,cis-2-nonenal,sun aldehyde,hexanal,benzaldehyde,and nonanal.The total content of aldehydes in the bee pollinated group was higher than the CPPU treated group,as also for(trans-cis)-2,6-nonadienal and cis-2-nonenal.1-octen-3-ol,benzyl alcohol,cis-3-nonen-1-ol,(trans,cis)-3,6-nonadienol,cis-6-nonenol,2-ethylhexanol were the main alcohols during the fruit development of melon,the relative content of 1-octen-3-alcohol in CPPU treatment group was much higher than the bee pollinated fruits.During fruit ripening stage(38 DAA),esters were the main volatiles for both bee pollinated and CPPU treated fruit.Acetate esters were the most important esters in ’Elizabeth’ cultivars,while isobutyl acetate,2-methyl butyl acetate,benzyl acetate,hexyl acetate,ethyl acetate and methyl acetate were the main esters.The total content of esters and phenyl acetate in melon fruit of bee pollination were higher than the CPPU treatment.Thus,it seems that bee pollination can enhance the flavor of melon fruit by increasing the relative content of volatiles,especially esters.2.In the ripening melon fruit,the total soluble solid,overall taste,sucrose content,and total soluble sugar content in the bee pollination treatment melon were higher compared with the CPPU treated fruits.During the development of melon fruit,the content of amino acid gradually increased,and reached at maximum level at the ripening stage.At 28 DAA,the contents of alanine,glycine,aspartic acid,threonine,serine,valine,leucine,isoleucine,phenylalanine,lysine,histidine and arginine in CPPU treated fruits were much higher than in the bee pollinated fruits.The contents of glutamic acid,alanine,glycine,threonine,serine and arginine in CPPU treated fruits were significantly higher compared with bee pollinated fruits at 38 DAA.Melon fruit under bee pollination has better taste and higher sugar content,but may be because of more volatiles,and amino acid content of melon fruit were low in bee pollinated fruits compared with CPPU treated fruits.3.The activity of ADH(alcohol dehydrogenase)and AAT(alcohol acyltransferase),which has close relationship with metabolism of volatiles has been studied during melon fruit‘s development.The results showed that the activity of ADH was highest at 20 DAA and decreased during the fruit development.The activity of ADH in the bee pollination fruits is higher than in the CPPU treated fruits.The activity of AAT in bee pollination and CPPU treated fruits both increased gradually during fruit development,and reached the peak at 32 DAA,but the activity in bee pollination fruits was higher than in CPPU treated fruits.Bee pollination is able to enhance the synthesis of volatile substances by promoting ADH and AAT enzyme activity,thereby enhancing the flavor of the melon fruits.4.We compared relative expression level of key genes related to volatiles metabolism with different fruit setting methods during development of melon fruit by qRT-PCR.The results showed that the relative expression of CmAAT increased during fruit development,the relative expression in bee pollination was higher than CPPU treated fruits.The relative expression of CmArAT1 increased at first,and then decreased during fruit development.Before 30 DAA,the relative expression of CmArAT1 in bee pollination was higher than CPPU treated fruits.The relative expression level of CmBCAT1 increased during fruit development,bee pollination group has more relative expression than CPPU treated group.Bee pollination seems to impart better flavor to melon fruits by increasing relative expression level of key genes(CmAAT,CmBCAT1)related to synthesis of esters.5.The trascriptome data showed that at 38 DAA,the number of differentially expressed genes in melon fruit between two fruit setting methods is the highest,reaching 613 genes.The GO enrichment analysis results showed that melon fruit is impacted by fruit setting methods in biological regulation,cell process,development process,metabolic process,responses to stress,cell structure,membrane composition,catalytic activity,transport activity and other processes.Through pathway analysis,we found that different fruit setting methods have an impact on the secondary metabolite synthesis,phenylpropionic acid synthesis,fatty acid metabolism,tyrosine metabolism,sulfur compounds metabolism,linoleic acid metabolism and other key biological processes. |
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