Comparative Studies On The Root Growth Characteristics Of Apple Dwarfing And Standard Rootstocks

Root architecture varies according to rootstocks.Generally,dwarf rootstocks are poorly anchored due to their shallow rooting systems,whereas vigorous rootstocks show better root growth and deep rooting architecture.However,relationship between dwarfing effects and deep rooting was not unclear.It was also unknown that whether can breed the dwarfing rootstocks with deep-rooting trait.To analysis the root growth and development characteristics and explore the potential factors that influence the deep rooting trait of apple(Malus domestica Borkh.)rootstocks,we summarize a 6-year-trial study using minirhizotrons to investigate the relationships between fine root production,mortality and distribution in Red Fuji(RF)trees grafted on five rootstocks/inerstems i.e.RF/M9,RF/SH40,RF/Baleng Crab(BC),RF/M9/BC and RF/SH40/BC.Furthermore,root morphological traits(e.g.root angle and length)were analyzed by using the cuttings of M9,Baleng Crab and their hybrid progenies crossed from Baleng Crab × M9.The results were as followed:1.Root length density(RLD)-based fine root production and mortality was markedly lower in dwarfing RF/M9 and RF/SH40 than that of standard RF/BC;Insertion of M9 or SH40 interstem pieces led to an extensive reduction in fine root production and mortality in comparison to RF/BC;Fine root production and mortality varied between fruit bearing and non-bearing trees or years,two active fine-root production and mortality periods were monitored before bearing fruit,otherwise plants showed uni-modal growth after fruit-bearing.Root number density(RND),but not average root length(ARL),showed similar patterns to RLD,indicating RND,not ARL,was the predominant contributor to RLD.2.Approximately one half(50.07%)of fine roots in RF/M9 were scattered in 0-20 cm topsoil,indicating shallow rooting in M9,while in RF/BC,55.15%of fine roots were distributed in 100-150 cm deep soil,indicating a pattern of deep rooting.Although SH40 is classified as dwarfing rootstock,44.08%of fine roots in RF/SH40 was produced in the 0-20 cm soil depths but 37.38%was found in the 100-150 cm soil depths,showing a rooting pattern intermediate between RF/M9 and RF/BC.3.Root phenotypic analysis of shallow-rooted M9 and deep-rooted Baleng Crab(BC)showed that M9 had a smaller root growth angle(30.30°)than Baleng Crab(52.23°);IAA in roots of M9(48.87 ng/g FW)was significantly lower than that of Baleng Crab(229.47 ng/g FW);Fewer amyloplasts were also detected in columella cells in the root cap of M9 in comparison with Baleng Crab.4.qRT-PCR analysis indicated that genes of MdDR01(predominantly determine root growth angle)and MdPIN11(mainly controls root gravitropism)were significantly lower expressed in roots of M9 than in that of Baleng Crab,in which the relative expression of MdDRO1 asnd MdPIN11 were 3.20 and 8.17 folds of M9,respectively.This might indicate that root growth angle and root gravitropism were the major contributors to deep rooting in M.domestica,which were associated with the expression of MdDRO1 and MdPINll,respectively.5.Sequence analysis indicated that both promoter sequences of MdDRO1 and MdPIN11 exhibited significantly differences in M9 and Baleng Crab.Though it contained a 17 bp-insertion in the promoter region ofMdPIN11 both in M9 and Baleng Crab,it was heterozygous in M9,while Baleng Crab showed a type of homozygosiy.As for MdDRO1 promoter,the sequence of M9 was coordination with DRO1 reference that was obtained from GDR database,but there were three insertions(3 bp,42 bp and 54 bp,respertively)in the promoter region of MdDRO1 in Baleng Crab.And,the type was heterozygous.GUS-staining showed that the promoter activity was enhanced by the 99-bp insertion,suggesting that the higher MdDR0O expression in Baleng Crab might resulted from its higher promoter activity.6.Both root growth angle and average root length showed widely transgressive segregation in hybrid population cross by Baleng Crab × M9,and both of them exhibited normal distribution,with the broad-sense heritability(H2)was 0.96 and 0.89,respectively,suggesting a necessity to identify the genomic region of QTLs that control root growth angle and deep root length.