| Early-fruiting walnuts(Juglans regia L.)have widely popularized in production.But the biological characteristics of early-fruiting lead to premature tree aging,susceptible to pests and diseases,which have become a bottleneck for the development of early-fruiting walnut industry.It is well known that the use of suitable walnut rootstocks can compensate for the defects of the scion,in complementary with strengthening the cultivation and management practices of the cultivars.Focusing the above problems,the first batch of interspecific hybrids has been selected by our research group with primary selection,re-selection and election of interspecific hybridization.In this study,four interspecific hybrid varieties were used as test materials,and their botanical characteristics were observed by years and the phenotypic markers were screened,with the aim to provide the foundation for the identification of interspecific hybrid varieties.Then,the growth characteristics of four interspecific hybrid varieties were analyzed to elucidate the growth advantage in compared with seedlings.On the basis of the above,the effects of different rootstock varieties on growth and photosynthetic characteristics were studied,which provided the theoretical basis for the selection and utilization of rootstock varieties.In addition,our research group has established the softwood cutting propagation system of walnut.Although the molecular mechanisms controlling adventitious root formations have been studied for walnut,the epigenetic regulation mechanisms are not clear.So,the mechanism of DNA methylation was also investigated to provide epigenetic basis for the asexual breeding and promoting adventitious root development.The main results were as follows:(1)Based on the DUS phenotypic traits of Juglans,19 phenotypic traits were screened out as key marker traits between four interspecific varieties,and the key list was compiled.Thirty-nine botanical characteristics of four hybrid varieties(J.hindsii × J.regia var.Zhongningqiang,J.hindsii × J.regia var.Zhongningsheng,J.hindsii × J.regia var.Zhongningqi,J.major × J.regia var.Zhongningyi)were systematically observed and recorded.Phylogenetic characteristics of leaf,bud and branch can be used as the key markers of interspecific hybrid varieties through the specificity,consistency and stability test,combined with the standard pictures of botanical characteristics.(2)Through analysis of tree growth characteristics and growth process,the growth potential of four interspecific hybrid varieties showed obvious heterosis and the DBH and volume growth were faster than their parents,which provided the theoretical basis for the utilization of urban greening tree species and fast-growing hardwood timber species.The super-dominant superiority of tree growth characteristics and branching characteristics of four hybrid varieties were 9.79%~50.00%.Based on the stem analysis,‘Zhongningqi' had the lowest variation coefficient of the annual ring width and early-wood and late-wood density,resulting in the highest uniformity of annual-ring density.The fast growing period of DBH and tree height of ‘Zhongningqi' were 2~8a.The total growth of timber volume had not reached the quantitative maturity during 23 a.The total DBH and timber volume increment of ‘Zhongningqi' was respectively 1.53 and 1.22 times,1.77 and 1.41 times higher than J.hindsii and J.major.Tree heights of the three species had no significant difference.(3)Grafting the same cultivars to different rootstocks,it was proved that the interspecific hybrids could be used as walnut rootstock.Firstly,the survival rates of the four hybrids combinations showed no significant difference in the grafting affinity than that of ‘Shangsong-14/‘Ningyou'.Secondly,the growth indexes of biomass and growth of four hybrid combinations were significantly or extremely significantly higher than those of ‘Shangsong-14/‘Ningyou'.Thirdly,hybrid varieties scionstock combinations had significant higher photosynthetic performance than the homo-grafted combination.The maximum net photosynthetic rate and apparent quantum yield of ‘Shangsong-14/‘Zhongningqi' were 21.93 ?mol·m-2s-1 and 0.062 mol·mol-1 respectively,which were significantly higher than those indexes of other combinations and increased by 17.90% and 16.98% than ‘Shangsong-14/‘Ningyou'.However,the light compensation point of ‘Shangsong-14/‘Zhongningqi' was reduced by 35.33% than ‘Shangsong-14'/‘Ningyou'.The light saturation point of ‘Shangsong-14'/‘Zhongninsheng' was the highest,increased by 17.27% than ‘Shangsong-14/‘Ningyou'.(4)MethyRAD-seq analysis showed that the methylation level and pattern of interspecific hybrids were changed after the treatment of rejuvenation,which affected the expression of auxin and adventitious root develop-related genes,and consequently caused the difference in rooting ability.The results illuminated the mechanism of DNA methylation in adventitious roots development.In genome level,total methylation level was significantly reduced after the treatment of rejuvenation,and methylation sites were not evenly distributed.Introns and gene body had the high degree of methylation.On the contrary,start and terminate sites of genes transcription had lower methylation level.In total 833 differentially methylated genes were selected.GO function enrichment analysis and KEGG metabolic pathway analysis showed that different methylated genes were mainly focus on biosynthesis of amino acid,carbon metabolism,phenylpropanoid biosynthesis,cell cycle,plant hormone signal transduction,etc.Among these pathways,the DNA methylation level and gene expression of NAC,WRKY,ARF and other genes or transcription factors were significantly changed,which suggested that these differences may contribute to the adventitious root development of walnuts. |
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