Effect Of Rootstock On The Contents Of Citrulline And Other Metabolites In Watermelon Fruit

Use of grafting in watermelon(Citrullus lanatus L.)production is a common practice.China is the largest producer of watermelon and ranks first in its production,of which 40 % watermelon comes from grafted plants.Quality of watermelon fruit in terms of nutrients is largely influenced by rootstock.Green house used in this study,is under watermelon cultivation from last four years.In order to compare citrulline content among different cucurbits and to study the effect of rootstock on citrulline and other metabolites four experiments were performed.The main results were as follows:1.To find out the alternative source of citrulline.Citrulline contents were compared in fruits,young and old leaves,roots,and stems of selective cucurbits from 9 species and 7 genera.Of the 9 cucurbits crop maximum amount of citrulline was recorded in fruits(1.4 g per kilogram of fresh weight)followed by old leaves(0.85 g/kg.f.wt),young leaves(0.47 g/kg.f.wt),roots(0.43 g/kg.f.wt)and stem(0.39 g/kg.f.wt).Watermelon fruit accumulated highest amount of citrulline(1.4 g/kg.f.wt)compare to fruits of other cucurbits.Bitter gourd(Momordica charantia)and smooth luffa(Luffa aegyptiaca)had highest concentration of citrulline in young(0.47 g/kg.f.wt)and old leaves(0.85 g/kg.f.wt)compare to leaves of other cucurbits respectively.Among stems,calabash gourd(Lagenaria siceraria)was richest in citrulline content(0.39 g/kg.f.wt),while among the roots,bitter gourd had the maximum amount of citrulline(0.43 g/kg.f.wt).Results suggest that not only the fruits of Cucurbitaceae family but other plants parts of tested cucurbits can also be the potential source for citrulline.2.To understand the regulatory mechanism,parallel changes in the expression profiles of citrulline metabolic genes and citrulline content of watermelon were monitored during the development of no grafted watermelon(NGW)and self grafted watermelon(SGW),and watermelon grafted onto pumpkin(PGW),wild(WGW)and gourd(GGW)rootstocks.Results demonstrated that rootstock reduced the citrulline content and expressions of all the genes,suppression was more pronounced in GGW followed by WGW and PGW as compared to SGW and NGW.Significantly higher citrulline content in SGW and NGW was attributed to higher gene expressions of upstream genes and high down regulation of ASl2 and ARG.Our results provide the basis to understand the molecular mechanism of citrulline accumulation in different rootstocks.3.To study the effect of pumpkin rootstock on taste,flavor and health related metabolites,flesh samples from mature fruit of pumpkin-grafted watermelon(PGW)and self-rooted watermelon(SRW)were compared.Total contents of carbohydrates,lipids,vitamins,amino acids,alkaloids and organic acids increased by 17%,26%,60%,6%,8%,and 31% in PGW respectively,while contents of total phenols and flavonoids decreased by 24% and 11% respectively.PGW showed higher contents of taste-related compounds including sugars(sucrose,glucose,trehalose and glucose-6-phosphate)and organic acids(cis-aconitate,isocitrate,fumaric acid and malic acid),flavor related compounds(phenolics and amino acids)and health-related compounds(citrulline,glutathione and vitamins)as compared to SRW,which enhanced nutritional quality of watermelon.This study suggests that grafting onto pumpkin rootstock is useful to increase the contents of metabolites related to taste,flavor and health in the watermelon cropping field.4.To elucidate the effect of pumpkin rootstock on primary metabolites contents and underlying molecular process,metabolomics and transcriptome analysis were performed on both pumpkin-grafted watermelon and ungrafted watermelon at different developmental stages.In total,56 primary metabolites were identified with either high or low abundance between ungrafted and pumpkin-grafted watermelon.The results indicated that ornithine,arginine,lysine(amino acids),glucose,sucrose,glucosamine(sugars),malic acid,fumaric acid and succinic acid(organic acids)were among the dominant metabolites influencing fruit quality.Additionally,comparative RNA sequence analysis on grafted and ungrafted watermelon yielded 729,174,128 and 356 differentially expressed genes at 10,18,26 and 34 days after pollination respectively.Functional annotations of these genes indicated that grafting significantly altered the biological and metabolic processes related to fruit quality.Our comparative metabolomics and transcriptome analysis revealed that FBA2,FK,SuSy,SPS,IAI,AI and sugar transporter gene(SWT3b)played a central role in the accumulation of glucose and sucrose,whereas higher malic acid content was attributed to high down regulation of ALMT13 and ALMT8 in pumpkin-grafted watermelon.Changes in the ornithine,glutamine,alanine,tyrosine,valine,asparagine,phenylalanine,arginine and tryptophan contents were consistent with the transcript level of their metabolic genes such as NAOD,GS,AGT,TaT,aDH1,OGDH,aDC,4CL 1,PaL,CaT and two nitrate transporter genes(NRT1)in pumpkin-grafted watermelon.This study provides the theoratical basis for understanding the graft responsive changes in the metabolic profile and regulatory mechanism related to fruit quality.