| Thin skinned melon(Cucumis melo var.makuwa Makino)is popular as a fresh and sweet fruit with aromatic flavor and abundant nutrition content.With improved agricultural production conditions and scientific research,melon production has rapidly developed in China.Continuous cropping has impeded the agricultural production,resulting in severe soil-borne diseases,and decreased yield and quality,thus restricting the sustainable development of melon industry.In practical production,grafting has been widely used to solve the above-mentioned problems.Lots of practice experience and researches indicated that melon grafting could overcome some continuous cropping obstacles,and grafting melon onto a suitable rootstock could make melon plants grow vigorously,improve the water and fertilizer absorption,enhance resistance to diseases and tolerance to high or low temperature and salt stress,and increase the melon yield.In this study,the screened superior ‘Xiangzhen NO.1’ pumpkin was taken as rootstock and ‘Guangmi NO.1’ thin-skinned melon as scion to investigate the physiological changes and proteomics in the growth and development process of grafted and self-rooted melon.The main results were as follows:1.A total of eight different rootstock varieties were screened for rootstock materials of ‘Guangmi NO.1’ thin-skinned melon.The disease resistance and compatibility of grafted rootstocks were identified,and the impacts of grafting of different rootstock varieties on plant growth,and yield and quality of melons were compared.8 rootstock cultivars were divided into three categories by subordinate function analysis and clustering analysis: ‘Xiangzhen NO.1’,which was the best rootstock varieties;SG-1 and BX-1,which were not suitable;the other five rootstock varieties,which were good rootstock varieties.Through comprehensive evaluation,‘Xiangzhen NO.1’ pumpkin rootstock was found to have strong wilt resistance and favorable compatibility,resulting in high survival rate,high seedling quality and remarkable melon yield increase after grafting while showing no adverse impact on melon quality.Therefore,it could be an ideal rootstock for ‘Guangmi NO.1’ thin-skinned melon.2.The changes of growth dynamics and physiological characteristics in the thin-skinned melon(Guangmi No.1)grafted onto pumpkin(Xiangzhen No.1)rootstock was studied after planting and the self-rooted melon was used as a control.The results indicated that internode length,number of leaves and leaf area were lower in grafted melon than self-rooted melon in 10 days after planting,presenting slower growth.After 30 days of plating,however,the growth vigor of the grafted melon plants was evidently enhanced,and internode length,stem diameter,number of leaves and leaf area were significantly higher than those in self-rooted melon.Soluble protein(SP)content,proline(Pro)content,superoxide dismutase(SOD)activity,peroxidase(POD)activity,catalase(CAD)activity,catalase(CAT)activity and lignin content in leaves were significantly higher in grafted melon plants than the self-rooted melon at different growth stages,while its malondialdehyde(MDA)content was significantly lower than that in self-rooted melon.Grafting had prominently improved the resistance of the melon plants to fusarium wilt,powdery mildew and gummy stem blight.Obviously,the growth advantage of grafted melon was mainly concentrated at the middle and later stages.In comparison with the self-rooted melon plants,the grafted melon plants had stronger antioxidant enzyme activity,accumulated more osmotic adjustment substances,and had more stabilized cytomembrane and higher lignin content,so as to have more superior growth performance,stronger disease-resistance and higher yield.3.Investigation on the changes of photosynthetic characteristics and chlorophyll fluorescence parameters showed that grafting significantly elevated leaf chlorophyll contents of melon at different growth stages.At the 10 th day after planting,net photosynlhetic rate(Pn)in grafted melon was significantly lower than that in self-rooted melon,but since the 20 th day after planting,Pn in grafted melon was always higher than that in self-rooted melon,and the differences were significant at the 40 th and 50 th days after planting.(Ci),(Gs)and(Tr)in grafted melon were higher than those in self-rooted melon at the 30 th day.At the 10 th day after planting,photochemical efficiency of PS II(Fv/Fm),actual PSII photosynthetic efficiency(ΦPSII)and electron transfer rate(ETR)in grafted melon were lower than those in self-rooted melon;but at the 30 th day after planting,Fv/Fm,ETR,q P and ΦPSII in grafted melon were significantly higher than those in self-rooted melon.In comparison with self-rooted melon,grafted melon had weaker photosynthetic capacity than self-rooted melon and grew slowly at the early growth stage,but the PSII reaction center in grafted melon was more stable at the middle and late growth stages,more light energy was absorbed,electron transfer rate and light energy conversion efficiency were higher,and photosynthetic capacity was stronger,thus more carbohydrates and production energy were synthesized,which boosted the growth of grafted melon.4.The protein expression differences in leaves of grafted and self-rooted melon at the same growing stage were compared using tandem mass tag(TMT)labeling quantification.The results showed that there were totally 5,150 proteins with quantitative information,and 2,187 differential proteins were identified,with 1,141 at seedling stage,716 at flowering stage and 330 at fruit ripening stage,respectively.Based on bioinformatics analysis,the low net photosynthetic rate and slow growth of grafted melon at seedling stage were related to down-regulated expression of photosynthesis-related proteins,repressed electron transfer and photochemical efficiency.Grafting superiority of melon might be associated with more energy generated in the processes such as photosynthesis and carbon metabolism at middle and late growth stages,and grafted melon might improve the disease resistance and stress tolerance by improving antioxidant defense capability and phenylpropanoid biosynthesis(lignin and flavonoids).Protein synthesis and degradation and post-transcriptional control played significant roles in the whole growth and development process of grafted melon.Taken together,we speculated that the possible mechanism of grafted melon having superior performances in growth,disease resistance,stress tolerance and yield increase advantage includes the following aspects: phenylpropanoid biosynthesis,chlorophyll metabolism,protein synthesis and degradation and post-transcriptional control exert great effects on growth improvement of grafted melon during the whole growth and development process;with higher antioxidant enzyme activity,more osmotic adjustment substances and stronger membrane lipid peroxidation resistance,the disease resistance and stress resistance of grafted melon are improved;more energy is generated by stronger photosynthesis and more stable carbon metabolism,which facilitate the superior growth performance at the middle and late growth stages.The TMT data were confirmed by the assays of physiological indicators,parallel reaction monitoring(PRM)protein levels of 8 differential proteins as well as the corresponding gene transcription levels.The results of this study provided a theoretical basis for application of grafting cultivation technique in melon production. |
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