Japanese pear (Pyrus pyrifolia Nakai), a member of the genus Pyrus (Rosaceae, Pyrinae) is cultivated throughout Japan. Pear fruit is commercially important, and Japanese pear production is estimated to be more than 70 billion yen per year, which ranks fourth among fruit-tree crops after citrus, apple, and grape production. Pear breeding in Japan started in the early 20th century (Machida, 1979), with fruit quality improvement being the most important objective of breeding programs. Important fruit quality parameters include texture, sweetness, and acidity. Improvements in fruit texture are particularly important because most traditional cultivars present a gritty texture, hence the name “sand pear.” ‘Kosui’, ‘Hosui’, and ‘Akizuki’, released in 1959, 1972, and 1998, respectively, are cultivars with fine fruit texture that are currently leading the Japanese pear market, with 70% of all Japanese pear cultivating areas in Japan dedicated to their production (Saito, 2016).
Because of the recent consumer demand for superior fruit quality, sweetness has also become an important target when breeding new cultivars (Nishio et al., 2018; Saito, 2016). Generally, sweetness is highly correlated not only with total sugar content (TSC) but also with the sugar component. The TSC of Asian pear fruit reportedly varies according to the cultivar (Abe et al., 1995; Kajiura et al., 1979a; Moriguchi et al., 1992). Furthermore, significant parent–offspring correlations or regression coefficients were obtained for the soluble solid content (SSC) (Abe et al., 1995, Machida and Kozaki, 1976). These results indicated that genetically high TSC cultivars are desirable as cross-parents for breeding cultivars with high TSC. Mature pear fruit mainly contains the following four sugars: sucrose (SUC), fructose (FRU), glucose (GLU), and sorbitol (SOR) (Kajiura et al., 1979a; Yamaki et al., 1976). These sugars have different levels of sweetness; if SUC is rated as 1, then FRU is rated as 1.50 to 1.75, GLU is rated as 0.70 to 0.80, and SOR is rated as 0.55 to 0.70 (Doty, 1976; Pangborn, 1963). In addition, the perception of sweetness of SUC is greater than that of FRU (Kaneshi, 1982). In Rosaceae, SOR is translocated from the sink to source tissues; then, it is converted to FRU and GLU by SOR dehydrogenase (Yamaki and Moriguchi, 1989). SUC is synthesized from SUC 6-phosphate by SUC-phosphate synthase (SPS) and is converted to GLU and FRU by SUC invertase (Yamaki, 2010). In addition, the reversible conversion of SUC and uridine diphosphate to uridine diphosphate-GLU and FRU is catalyzed by SUC synthase. Cultivars of Asian pear also exhibit large variations in sugar composition (Kajiura et al., 1979a; Moriguchi et al., 1992), whereas those of apple (Hecke et al., 2006; Wu et al., 2007) and peach (Byrne et al., 1991; Moriguchi et al., 1990), which also belong to Rosaceae, show fewer variations in sugar composition than that of pear. Therefore, it may be possible to breed high-sweetness cultivars with not only high sugar content but also high fructose and sucrose contents.
To efficiently breed cultivars with these traits, molecular markers associated with sugar accumulation and sugar metabolism are needed. Previously, we and our colleagues reported a number of quantitative trait loci (QTL) associated with individual sugar content and TSC by analyzing an F1 population derived from a cross between Japanese pear ‘Akizuki’ and breeding selection 373-55 (Nishio et al., 2018). Although this information will contribute to breeding pear cultivars with high sweetness, further genetic studies are required to obtain desirable high-sweetness genotypes more efficiently. Therefore, it is important for breeders to obtain precise genetic information from the phenotypic value, because the sugar content and composition are controlled by QTL that respond to environmental conditions such as location, year, tree, and fruit.
An analysis of variance (ANOVA) has been performed to estimate the contribution of genetic and environmental variance components to the determination of fruit traits of sweet cherry (Hansche and Beres, 1966; Hansche and Brooks, 1965), Japanese pear fruit (Kozaki, 1975,1976; Machida and Nishio et al., 2011), Japanese persimmon (Yamada et al., 1993, 2002), grape (Sato et al., 2000), citrus (Hamada et al., 2016; Nonaka et al., 2012), and chestnut (Nishio et al., 2014). These estimates provide information regarding the optimal yearly repetition, tree or vine replication, and number of fruits.
The objectives of this study were to evaluate the genotypic differences in TSC and sugar composition among leading commercial cultivars or new promising cultivars that are used as cross-parents for Japanese pear breeding in Japan and to obtain accurate estimates of environmental variance components and broad-sense heritability (hB2) of TSC, SUC, FRU, GLU, and SOR to breed Japanese pear cultivars with high sweetness. Such estimates will provide optimum measurement repetitions for phenotypic selection and further genomic analyses to develop molecular markers associated with TSC and sugar composition.
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