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Hiroshi Iwanami, Shigeki Moriya, Nobuhiro Kotoda, Sae Takahashi and Kazuyuki Abe

Progenies from 38 unbalanced crosses using 20 apple (Malus ×domestica Borkh.) cultivars/selections as parents were evaluated for changes in flesh firmness after harvest in two seasons to determine the mechanism of inheritance of fruit softening. The change in firmness was fitted by linear regression, and the softening rate (N·d−1) expressed as the regression coefficient was used as the phenotypic value of softening after harvest. Fruit were stored under 20 °C and 85% relative humidity after harvest for up to 40 days. The softening rates in the progeny populations were distributed continuously around the softening rates of parents, despite a distinct segregation in the degree of mealiness at 30 days of storage. The narrow-sense heritability of the softening rate was estimated by parent-offspring regression, and the estimate was high (h2 = 0.93). Because the softening rate can be influenced by mealiness, an undesirable trait in the apple industry, the progenies were divided into individuals with and without mealiness, and the breeding values of the parents were estimated based on the softening rate of the nonmealy progeny. The softening rate of the nonmealy progeny was analyzed using a mixed linear model and the restricted maximum likelihood method, with general combining ability (GCA) as parental effects and specific combining ability (SCA) as parental interaction effects. The variance of GCA was significant, but the variance of SCA was small and nonsignificant. The narrow-sense heritability of the softening rate in the nonmealy progeny was estimated by sib analysis, and the estimate was moderately high (h2 = 0.55). A significant correlation was observed between the phenotypic value and the breeding value (twice the GCA effects) in nonmealy parents, but the phenotypic value did not significantly correlate with the breeding value in mealy parents. Therefore, contribution of a mealy parent to the softening rate of nonmealy progenies cannot be predicted by its phenotypic value.

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Hiroshi Iwanami, Shigeki Moriya, Nobuhiro Kotoda, Sae Takahashi and Kazuyuki Abe

Changes in flesh firmness and mealiness during storage were investigated in 24 apple [Malus ×sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] cultivars and selections (genotypes) up to 40 days after harvest under 20 ± 2 °C and 85% ± 5% relative humidity storage conditions. Flesh firmness was measured using a penetrometer, while mealiness was quantified by measuring the degree of cell separation in tissue induced by shaking discs of tissue in a sucrose solution. According to the relationship between the change in firmness and mealiness, the genotypes can be divided into four groups: those that did not soften and remained hard and nonmealy during storage; those that softened without mealiness; those that softened with slight mealiness; and those that softened with mealiness. Firmness decreased below 30 N in fruit that softened with mealiness, and the minimum firmness during storage was correlated with the degree of mealiness at 30 days of storage. The rate of softening was the highest in fruit that softened with mealiness. Therefore, it was concluded that, by measuring the firmness and changes in firmness that take place during storage, the genotypes resulting in softening with mealiness and those that result in softening without mealiness could be identified.

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Hiroshi Iwanami, Shigeki Moriya, Nobuhiro Kotoda and Kazuyuki Abe

Changes in turgor and flesh firmness during storage at 20 °C were investigated using 27 apple (Malus ×domestica Borkh.) cultivars for 2 years. Flesh firmness was measured using a penetrometer, and turgor was determined using a thermocouple psychrometer. Firmness and turgor of fruit decreased during storage. The cultivars with little softening during storage had low rates of reduction in turgor. The softening rates in mealy cultivars were high, but there were cultivars with low rates of turgor reduction. When the rates of reduction in turgor after harvest were low, the mealy cultivars of the fruit tended to develop severe mealiness during storage. Therefore, a low rate of reduction in turgor could contribute to cultivars with both good shelf life and severe mealiness. The reduction rates of turgor in progeny cultivars were nearly identical to the mean reduction rates of turgor of their parents. This suggests that a cultivar with a low reduction rate of turgor, although it can be mealy, has the potential to produce a progeny with a low reduction rate of turgor.

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Hiroshi Iwanami, Shigeki Moriya, Nobuhiro Kotoda, Sae Takahashi and Kazuyuki Abe

To compare changes in fruit quality during cold storage with those during shelf life conditions, flesh firmness and titratable acidity (TA) were measured during storage in 20 apple (Malus ×domestica Borkh.) cultivars. Fruit of each cultivar were divided into two groups and stored in chambers controlled at 20 ± 2 °C and 85 ± 5% relative humidity (RH) (shelf life conditions) or 0.5 ± 0.3 °C and 95 ± 5% RH (cold storage). Five of the stored fruit were removed for measurements at 5- or 10-d intervals for 40 d and at 1-month intervals until 10 months after harvest at 20 °C and 0.5 °C, respectively. Data for firmness and TA were subjected to a linear regression and a nonlinear regression, respectively. Moreover, to determine the advantages of 0.5 °C storage over 20 °C storage for retaining firmness and TA, the effect of storage type on extending the storage period was introduced as a parameter. The estimate of the effect of storage type showed that firmness and TA could be retained 8.9 and 3.7 times, respectively, longer at 0.5 °C than 20 °C, independently of the cultivar. Therefore, firmness and TA after cold storage could be predicted by the change in firmness and TA during shelf life conditions. Moreover, cultivar differences regarding quality change under cold storage could be determined in a short period after harvest because the cultivar differences under shelf life conditions were detected within 1 month after harvest.

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Nobuhiro Kotoda, Hiroshi Iwanami, Sae Takahashi and Kazuyuki Abe

Because fruit trees such as apple (Malus ×domestica Borkh.) flower and set fruit only after an extended juvenile phase lasting several years, efficient breeding of fruit trees is limited. We previously suggested that MdTFL1 (Malus ×domestica TFL1) functions analogously to TERMINAL FLOWER 1 (TFL1) and that MdTFL1 is involved in the maintenance of the juvenile/vegetative phase in apple. To clarify the function of MdTFL1 in apple, we produced transgenic `Orin' apple trees expressing MdTFL1 antisense RNA. One of them flowered only 8 months after the transfer to the greenhouse, whereas the nontransformed control plants have not flowered in nearly 6 years. As expected, the expression of endogenous MdTFL1 was suppressed in the transgenic lines that showed precocious flowering. In addition, the expression level of the transgene was correlated with the reduction of the juvenile phase. These findings confirm that MdTFL1 functions like TFL1 and that MdTFL1 maintains the juvenile and vegetative phase in apple. Flower organs of the transgenic apple trees were normal in appearance, and a precocious flowering transgenic line set fruit and seeds. Interestingly, some flowers of the transgenic apple trees developed without undergoing dormancy. The expression of MdTFL1 in apple may affect flower development as well as flower induction.

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Takaya Moriguchi, Kazuyuki Abe, Tetsuro Sanada and Shohei Yamaki

Soluble sugar content and activities of the sucrose-metabolizing enzymes sucrose synthase (SS) (EC 2.4.1.13), sucrose-phosphate synthase (SPS) (EC 2.4.1.14), and acid invertase (EC 2.4.1.26) were analyzed in the pericarp of fruit from pear cultivars that differed in their potential to accumulate sucrose to identify key enzymes involved in sucrose accumulation in Asian pears. The Japanese pear `Chojuro' [Pyrus pyrifolia (Burro. f.) Nakai] was characterized as a high-sucrose-accumulating type based on the analysis of mature fruit, while the Chinese pear `Yali' (P. bretschneideri Rehd.) was a low-sucrose-accumulating type throughout all developmental stages. The activity of SS and SPS in `Chojuro' increased during maturation concomitant with sucrose accumulation, whereas the activity of these enzymes in `Yali' did not increase during maturation. The activity of SS and SPS in the former were seven and four times, respectively, higher than those in the latter at the mature stage. Further, among 23 pear cultivars, SS activity was closely correlated with sucrose content, while SPS activity was weakly correlated. Soluble acid invertase activity in `Chojuro' and `Yali' decreased with fruit maturation, but the relationships between soluble invertase activity and sucrose content were not significant. The results indicate that SS and SPS are important determinants of sucrose accumulation in Asian pear fruit and that a decrease of soluble acid invertase activity is not absolutely required for sucrose accumulation.

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Kentaro Kitahara, Shogo Matsumoto, Toshiya Yamamoto, Junichi Soejima, Tetsuya Kimura, Hiromitsu Komatsu and Kazuyuki Abe

As the parents of the some of the apple cultivars were unknown and others were uncertain, we investigated the parent-offspring relationships of eight apple cultivars by S-RNase analysis and SSR markers. The paternal parent of `Hida' was identified as `Golden Delicious', not the previously mentioned `Orin'. It was indicated that `Ryoka No Kisetsu' and `Korin' showing identical SSR genotype are likely sports of `Fuji'. `Fuji', rather than `Toko', seemed to be a maternal parent of `Kotoku', but was not a paternal parent of `Orei', `Starking Delicious', `Nero 26', `Empire', or `Aori 3'. Previously mentioned `Mutsu', `Indo', and `Shin Indo' were excluded as paternal parents of `Hokuto'. `Tsugaru' and `Jonathan' and were identified as the respective paternal parents of three cultivars described as having unknown paternal parents, i.e., `Aika No Kaori', `Yoko', and `Tsugaru'.

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Nobuhiro Kotoda, Masato Wada, Sadao Komori, Shin-ichiro Kidou, Kazuyuki Abe, Tetsuo Masuda and Junichi Soejima

Two apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] homologous fragments of FLO/LFY and SQUA/AP1 (AFL and MdAP1, respectively) were analyzed to determine the relationship between floral bud formation and floral gene expression in `Jonathan' apple. The AFL gene was expressed in reproductive and vegetative organs. By contrast, the MdAP1 gene, identified as MdMADS5, which is classified into the AP1 group, was expressed specifically in sepals concurrent with sepal formation. Based on these results, AFL may be involved in floral induction to a greater degree than MdAP1 since AFL transcription increased ≈2 months earlier than MdAP1. Characterization of AFL and MdAP1 should advance the understanding of the processes of floral initiation and flower development in woody plants, especially in fruit trees like apple.

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Kentaro Kitahara, Shogo Matsumoto, Toshiya Yamamoto, Junichi Soejima, Tetsuya Kimura, Hiromitsu Komatsu and Kazuyuki Abe

We examined the genetic diversity and relatedness among apple (Malus ×domestica Borkh.) cultivars in Japan. The 42 apple cultivars, including major cultivars in Japan, were divided into five groups based on SSR genotypes. Most economically important cultivars belong in three groups: Fuji-Delicious, Golden Delicious, and Jonathan groups, and their genetic backgrounds seemed to be narrow. We also investigated the parent-offspring relationships of nine apple cultivars. `Jonathan', `Fuji', and `Rero 11' were identified as the respective paternal parents of three cultivars described as having unknown paternal parents (i.e., `Akagi', `Ambitious', and `Hokuto'). `Starking Delicious', `Senshu', and `Golden Delicious', rather than `Ralls Janet', `Hatsuaki', and `Indo', seemed to be the paternal parents of `Kinsei', `Kiou', and `Mellow', respectively. `Carolina Red June' was excluded as a paternal parent of `Ranzan'. Both attributed parents of `Scarlet' (`Akane' and `Starking Delicious') were excluded, and it was suggested that `Fuji' was used as either a maternal or a paternal parent of `Scarlet'. `Jonathan' rather than `McIntosh' seems to be a maternal parent of `Yukari'.

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Chikako Honda, Hideo Bessho, Mari Murai, Hiroshi Iwanami, Shigeki Moriya, Kazuyuki Abe, Masato Wada, Yuki Moriya-Tanaka, Hiroko Hayama and Miho Tatsuki

The objective of this study was to investigate the effects of temperature treatments on anthocyanin accumulation and ethylene production in the fruit of early- and medium-maturing cultivars that were harvested early during fruit ripening. We first investigated the effects of various temperature treatments on anthocyanin accumulation in detached apples of ‘Tsugaru’, ‘Tsugaru Hime’, ‘Akane’ and ‘Akibae’ using an incubator. Three years of experiments demonstrated that at harvest, the lower-temperature treatments induced anthocyanin accumulation in ‘Tsugaru’, ‘Tsugaru Hime’, and ‘Akibae’ fruits, whereas the increases in anthocyanin accumulation under the 25 °C treatment were similar to those under the 15 and 20 °C treatments in ‘Akane’ fruit. The rate of ethylene production did not increase substantially during the temperature treatments in any of the four cultivars, except after the treatments of ‘Tsugaru’ fruit at harvest. The inhibition of ethylene action by the application of 1-methylcyclopropene (1-MCP) to detached fruits at harvest suppressed anthocyanin development under 15 and 20 °C temperature treatments in ‘Tsugaru’, ‘Tsugaru Hime’, and ‘Akibae’, but not in ‘Akane’. In the second experiment, we investigated changes in the anthocyanin concentration in attached fruit of ‘Misuzu Tsugaru’ under different temperature conditions in a greenhouse. At harvest, the anthocyanin concentration in fruit under the hotter climatic condition (29 °C 12 hours/19 °C 12 hours) was lower than that under the control condition (25 °C 12 hours/15 °C 12 hours). During the last week before harvest, anthocyanin development proceeded rapidly in fruit skin not only under the control condition, but also under the hotter climatic condition. The rapid accumulation of anthocyanin in the fruit skin of ‘Misuzu Tsugaru’ at harvest under a relatively high temperature (25 °C) condition was confirmed by the experiment using an incubator. At harvest, the maximum level of ethylene production in fruits sampled from trees grown under the hotter climatic condition was 9-fold higher than that in fruits from trees grown under the control condition. These results indicate that the comparison of pigmentation potential after the 15 or 25 °C treatments using detached fruit was effective for estimating anthocyanin accumulation in fruit skins under hotter climatic conditions in early- and medium-maturing cultivars that were harvested early and that a hotter climatic condition during ripening increased ethylene production in apple fruit after harvest.