Identification and evaluation of the progenies of spur-type apple (Malus domestica Borkh.)

Abstract: 【Objective】Spur-type is one of the excellent agronomic traits in fruit trees, particularly in ap-ple, peach, and Chinese date. The spur-type apple has been widely cultivated in apple industry in the world. However, there are only a few spur-type apples developed by hybridization breeding The aim of this study was to seek for rapid and effective identification of spur-type hybrid individuals from the progenies generated from the hybridization between spur type apple varieties to improve the breeding efficiency.【Methods】Eight characters including length of shoot (LS), length of internode (LI), total number of sprouts (TNS), mean number of shoots (MNS), percentage of spurs (PSU), shoot base thick-ness (SBT), percentage of sprouts (PSP), and conicity (CI) of 658 apple hybrid seedlings (generate from the reciprocal cross between Malus domestica‘Starkrimson’and M. domestica‘Miyazaki Spur’)were determined. The trees investigated were three- years old after budding on Malus×robusta (CarriŠre) Re-hder. Firstly, correlation analysis was used to clarify the correlation between the investigated morphological characters, correlation and principal component analysis of these characters were then performed to simplify the number of morphological evaluation indicators. The principal components of  twenty-three fruit-bearing individual plants were calculated based on the eigenvalues and corresponding eigenvectors, and the threshold for determining the short shoot type was found. Phenotypic investiga-tion was carried out on these individual seedlings of the trees to verify the determination results of prin-cipal component analysis when the trees were ten -years old after budding.【Results】The distributions of values of the LI, SBT and PSP conformed to the normal distribution, but those of the LS, TNS and MNS tended to be more likely χ 2 distribution, thus the values of normally distributed characters could be divided into three grades by (X-0.524 6S) and (X+0.524 6S). i.e. X±0.524 6S were set in the indexes of germination rate, branch length and branch base thickness to divide each index into three grades: low, medium and high, which could be applied to the analysis and evaluation of shoot morphological characters. The correlation coefficient matrix of the eight related branch morphological characters was calculated, and it was found that there was a certain correlation among all indexes, and it was not possi-ble to accurately evaluate the characteristics of the branches by directly using the above single index.Therefore, multiple indexes should be used to comprehensively evaluate the hybrid offspring of spur type in apple. According to principal component analysis, the information constituted by the five factors of the eight morphological indexes of branches accounted for 89.16% of the total information, which ba- sically reflected the main information of branches. These factors were defined as length factor, spatial density factor, node length factor, node length factor and thickness factor of branches respectively. The results also showed that the composite principal component score value of each individual plant could determine whether a seedling was spur-type or not. When the score value was positive, it was mostly spur- type in the field. And when it appeared negative value, it is mostly non-spur type in the field. The principal component analysis of shoots and buds of spur-type apple could be used to evaluate the mor-phological type of individual seedlings, with coincidence rate of 86.9%.【Conclusion】The PSP, LS and SBT of the branches of spur-type seedlings were evaluated by probability grading method in this study. The information overlapped between the eight morphological characteristics of the branches of apple seedlings, so the characteristics of the branches could not be evaluated directly or accurately by a single index. Therefore, by using factor analysis, the eight morphological indexes could be integrated into five factors, and then the individual plant types could be identified according to the scores of principal com- ponent analysis, with the coincidence rate of more than 85%.