Evaluating standards of some fruit quantitative traits of pear genetic resources

Abstract: 【Objective】Among all the phenotypic traits of pear germplasm, the fruit traits are the most valuable phenotypic characteristics that can determine the quality of pears and identify pear accessions. Furthermore, they are essential for the classification of pear species. This study was to establish evaluating standards and provide reference cultivars for the evaluation of the fruit quantitative traits of pear genetic resources, which could facilitate the information exchange and pear cultivar introduction and provided theoretical basis for the standardization of pear germplasm resources description.【Methods】The data of nine fruit quantitative traits based on 570 pear accessions of 11 species and 114 bred cultivars planted in the National Germplasm Repository of Pear (Xingcheng, China) were analyzed by SPSS.19.0. The coefficient variation and skewness were calculated and the frequency distribution histogram was drawn, including a normal curve. The nine fruit quantitative traits were graded and the reference cultivars were selected.【Results】The coefficient variation of titratable acidity was the largest (128.43%); and that of soluble solids content was the smallest (13.67%). Therefore, the titratable acidity could better reflect the difference between varieties, and the genetic characteristics of the soluble solidscontent were more stable. All the nine fruit quantitative traits were skewed normal distribution with the highest skewness in titratable acidity (2.75) and the lowest in stalk length (0.13). The difference between the mean and median of the stalk thickness and flesh firmness were＜0.1, and the other traits were ≥0.1, indicating that the data of the stalk thickness and flesh firmness were relatively concentrated compared with the other seven traits. Based on the frequency distribution of the diversity, grade index and reference cultivars were given by statistical data for pear description. The classification the nine quantity characters of pear fruit was researched by isometric method. The stalk length and stalk thickness were divided into 5 grades, indicated by 1, 3, 5, 7 and 9, respectively and the distribution frequency was over 40%. The fruit core size was divided into 4 grades, indicated by 1, 3, 5 and 7, respectively and the distribution frequency was 70%. The stalk length had a certain effect on fruit setting rate, the longer stalk length would have stronger wind resistance. The stalk thickness had a certain effect on fruit enlargement, the thicker stalk thickness would be more conducive to the transportation of nutrients and the fruit would be easier to expand. The fruit core size was one of the important traits affecting the edible rate, when the fruit core was small, the edible rate was higher. Due to the significant differences of the fruit weight , fruit diameter, fruit length, flesh firmness, soluble solids content and titratable acidity among the Oriental crispy pear, P. ussuriensis (Ussurian pear) and European pear, grades were made separately. The coefficient variations of the fruit weight, fruit diameter and fruit length of European pear were 40.54%, 14.50% and 20.85% larger than those of the Oriental crispy pear, Ussurian pear. And the coefficient variations of the flesh firmness and soluble solids content of Ussurian pear were 26.46% and 10.51% larger than those of the European pear and Oriental crispy pear. The coefficient variation of titratable acidity of the Oriental crispy pear was 66.00% larger than that of the European pear and Ussurian pear. The average coefficient of variation of the European pear, Oriental crispy pear and Ussurian pear was 29.06% , 26.30% and 5.98% respectively. The average soluble solids content and titratable acidity of Ussurian pear were higher (14.24% and 0.70%), than those of the European pear (13.51% and 0.30%), and those of the Oriental crispy pear (12.28% and 0.28%) in order. Weight per fruit, fruit diameter and fruit length of Oriental crispy pear. The European pears were divided into 4 grades, indicated by 3, 5, 7 and 9, respectively, and the distribution frequency was over 40%. The Ussurian pears were divided into 3 grades, indicated by 3, 5, 7, respectively and the distribution frequency was over 60%. The flesh firmness was divided into 3 grades. The soluble solids content and titratable acidity were divided into 5 grades. Two reference cultivars were selected for each grade of the single character, except for the extreme characters.【Conclusion】The nine fruit quantitative traits of pear germplasm resources were skewed normal distribution, which might be the result of long-term human selection. The mean soluble solid content and titratable acidity of the soft pear (Ussurian pear and European pear) were higher than those the Oriental crispy pear. The nine fruit quantitative traits were classified by three different grading methods according to the characteristics of each trait. The grading index system of the fruit quantitative traits of pears was preliminarily built in this article which would provide reference for the evaluation, description and data standardization of pears.