Studies on Characteristic Variability and Pre-selection of Large Group Hybrid F₁Progenies in Apple

Cross-breeding is one of main ways for new variety selection in apple. However, it hasbrought the very difficulty for the directive breeding due to its complex genetic background,long juvenility of seedlings, as well as widely separation of characters. Therefore, applebreeders have continuously endeavored to seek to new approaches for accelerating thebreeding process, shortening the breeding cycle and improving the breeding efficiency. In thebreeding practice, the pre-selection and elimination of F₁seedlings is one of effective ways tosolve these problems on the basis of the breeding goals. To understanding the separation andvariability of main phenotypic characters in the progenies, and exploring the optimalpre-selection period, intensity and method are basic preconditions for the application ofpre-selection in the breeding practice, which is more important for large groupe hybrid F₁progenies for pre-selection.15traits were surveyed from six cross combinations （Qinguan×Fuji, Fuji×Qinguan,Fuji×Gala, Gala×Fuji, PinkLady×Fuji, Fuji×Pink Lady） of1-4years old F₁generations（with every combination more than one thousand seedlings） in the open field during threeconsecutive years. Based on the obtained data, the separation and variability trend of theseedlings were investigated using biostatistics and biomathematics methods. The pre-selectionmethods and models of large group hybrid F₁progenies were initially proposed. Concurrently,survey on incidence of marssonina mali in42apple （Malus domestica） cultivars grown in theopen field was conducted and their resistance to apple brown spot （M.mali） was evaluated inthe present study. In addition, the fruit evaluation index system was proposed by investigatingthe fruit quality traits from Qinguan×Fuji F₁progenies. This study will provide valuablereference for establishing an efficient apple breeding system in China.The main results were presented as follows:1. Early defoliation disease is one of the main objectives of apple breeding for diseaseresistance in China, among which apple brown spot （M. mali） is the main type of earlydefoliation disease. Survey on incidence of M. mali in42apple （Malus domestica） cultivarsgrown in the open field was conducted and their resistance to apple brown spot （M. mali） wasevaluated in the present study. Among the tested cultivars,14cultivars were found to beresistant, of which2were highly resistant to M. mali, while the rest28cultivars were susceptible,7of which were highly susceptible to M. mali. After M. mali inoculation, theactivities of SOD, PPO, POD and PAL and the content of lignin increased in different applecultivars. There was no significant difference in the activities of SOD and PPO betweenresistant and susceptible cultivars, whereas the activities of POD and PAL and content oflignin were higher in resistant cultivars than those of susceptible cultivars. These resultssuggested that POD and PAL activities, and lignin content of apple cultivars correlated withthe resistance to M. mali, which provided some references for parents selection in resistantbreeding and pre-selection of seedlings.2. The disease-resistant apple cultivars Qinguan, disease-susceptible cultivar Fuji and F₁progenies were assessed for brown blot disease resistance following natural infection in thefield during three consecutive years. The results indicated that there was a wide variation anda partial normal distribution on disease index of F₁hybrids. These further suggested thatbrown blot disease was a quantity trait which was controlled by major genes and regulated byminor genes. The rate of resistant plants was about49.1%, and the rate ofhigh-resistance（DI<5） plant was7.0%, while the rate of high-infection（DI>50） plant was14.6%, and the threshold of eliminated deeply-susceptible was14.6%. Resistance to BrownBlot Disease from reciprocal crosses were different. When Qinguan was used as female, F₁hybrids have higher resistance. The correlation coefficient of average disease index indifferent years was0.78-0.85, the resistance of a majority of plants showed stably. The lateAugust is the best time to identify resistance of plant during disease occurs.3. The15phenotypic traits of1-4-year old seedlings from six cross combinations （≥300for each combination）, including plant height, stem diameter, shoot length, shoot diameter,leaf area, leaf shape index, chlorophyll content, leaf petiole length, leaf petiole diameter,branch angle, internode length, spine, disease resistance （apple brown spot, alternaria blotch,powdery mildew） were evaluated in the present study. The results showed that there was awide range of variability and separation in most of all phenotypic traits in each combination.The variability in seedlings from Qinguan×Fuji and Fuji×Qinguan combinations weresequenced as powdery mildew> marssonina mali> alternaria blotch> plant height> leaf area>stem diameter> shoot length> spine> shoot diameter> leaf petiole length> internodelength> leaf petiole diameter> chlorophyll content> leaf shape index, in which the variationrate of the first nine traits were greater than21.4%. The8phenotypic traits including plantheight, stem diameter, leaf area, shoot length, shoot diameter, spine, disease index ofalternaria blotch and powdery mildew were normal distribution by frequency distributionhistograms drawing and K-S normality test, indicating that these traits exist great variation inthe seedlings, which belongs to the quantitative trait controlled by multiple genes, and can be used indexing factors for pre-selection in large group hybrid progenies.4. The8quantitative phenotypic traits （plant height, stem diameter, leaf area, shootlength, shoot diameter, spine, alternaria blotch and powdery mildew disease index） of100seedlings from Qinguan×Fuji and Fuji×Qin guan combinations, which meet normaldistribution, were further decomposed into a number of specific factors to constitute threesubset factors （cultivation, disease resistance and wildness） and the whole collection by usingfuzzy math-Weighted average mode. The score range from0.1258to0.7647, the seedlingscan be classified according to the composite score. In addition, the seedlings were selected20%and eliminated30%, which composite were>0.6score and <0.35, based on thebreeding objectives （high yield, high-quality, disease resistance）, and the numbers of thehybrid progenies. The technology system and dynamic mathematical models of apple largegroup hybrid F₁progenies pre-selection was initially established through the application offuzzy mathematics comprehensive evaluation method. The rationality would be furtherverified in the field.5. Survey on several hybrid combination indicated that the moderate pre-selection periodof apple hybrid F₁progenies is3-4years old seedlings and1-2years old seedlings can beused for investigating disease resistance. The pre-selection could be conducted, following bydisease resistance, to eliminate some wildness seedlings and then select good performedseedlings with related to cultivar habit, according to breeding objectives.6. The variability of fruit quality traits in360seedlings from Qinguan and Fujicombination were analyzed, and35seedlings with fine quality were evaluatedcomprehensively using gray relational grade analysis and analytic hierarchy process. The16fruit quality characteristics including fruit weight, coloring degree, fruit shape index, fruithardness, flavor etc, were used as an evaluation index system, and then established reasonablere-coefficient for each index through the analytic hierarchy process. The “reference variety”was determined according to breeding parents. The seedlings were scored according to thegray relational grade weighted score,5hybrids’（5F2-31,6B1-4,6A2-23,5H2-56,6B2-20）score were over than0.7, which performed better in comprehensive quality of this crosscombination and was determined as breeding objectives plants for further observation andevaluation. The rationality would be verified.