A major quality problem with apples in the marketplace is fruit softening (Johnston et al., 2002a). The degree or rate of softening after harvest depends on the cultivar (Iwanami et al., 2004). Although ‘Fuji’ and ‘Honeycrisp’ maintain their firmness long after harvest and have good storage capability (Tong et al., 1999; Yoshida et al., 1998), the fruit of ‘Red Delicious’ and ‘Golden Delicious’ become mealy (Hampson and Kemp, 2003) and soften rapidly when harvested late. Softening has not been sufficiently evaluated among commercial cultivars, and, at times, it is not considered as a trait for selection in apple breeding, although a cultivar in which the fruit hardly softens is advantageous to the apple market and industry. Crosses between commercial cultivars with good eating quality can produce progenies with varying rates and degrees of softening. To produce a new cultivar with good storage and shelf life potential, it is useful to determine genotypic differences in and mechanism of inheritance of fruit softening.
Resemblance among relatives is a basic genetic phenomenon, and the degree of resemblance determines heritability, which in turn facilitates the choice of a breeding method for use in genetic improvement (Falconer and Mackay, 1996). The estimation of the degree of resemblance for fruit softening after harvest, however, has received little attention in apple breeding because there is no effective phenotypic value or index for fruit softening upon which to compare relatives. Fruit softening is generally expressed as the decrease in firmness after storage. Apples soften by 25% to 50% of their firmness at harvest, and the final firmness depends on the cultivars (Johnston et al., 2002a). Furthermore, firmness does not always continue to decrease during storage (Iwanami et al., 2004), and the degree and period of decrease vary widely among cultivars. Johnston et al. (2001, 2002b) proposed a nonlinear regression model to fit the change in firmness during storage and to compare cultivars on degree and rate of softening based on a parameter (e.g., the rate of change in firmness) in the model. However, the model can estimate the parameter only when a rapid reduction in firmness is observed after harvest and it requires that a large number of fruit samples be used to estimate the parameter. The model, therefore, does not lend itself to breeding situations because apple seedlings do not always bear a sufficient number of fruit for storage tests or display rapid reduction in fruit firmness after harvest.
To evaluate differences in the softening of apple genotypes, Iwanami et al. (2004) proposed a linear regression model to fit the change in firmness during storage. The regression coefficient measures the softening rate and can be estimated from a limited number of harvested fruit in breeding situations. Iwanami et al. (2005a) also indicated that, although the estimate of the parameter was influenced by environmental factors such as year, tree, and sampling date, the magnitudes of the influence were relatively small, and the estimate was considered to be stable against environmental conditions. The linear regression coefficient, therefore, can be a useful index for comparing relatives as the phenotypic value of softening after harvest.
Changes in firmness after storage can also be influenced by mealiness (Iwanami et al., 2005b). A high degree of mealiness occurred in 7 of 24 cultivars tested and the softening rates of the mealy cultivars were all high, with about the same values (Iwanami et al., 2005b). These findings indicate that if a cultivar in which mealiness occurs produces progeny in which mealiness does not occur, the softening rate of the progeny cannot be predicted from the softening rate of the parent cultivar because the softening rate (phenotypic value) of the parent is affected by mealiness, which is not inherited by the progeny. To determine the contribution of a mealy parent to the softening rate of its nonmealy progeny, the estimation of the breeding value of the mealy parent is necessary.
Narrow-sense heritability or combining ability has been estimated to improve breeding strategies and to evaluate the potential of cultivars for use as parents in fruit breeding programs for several quantitative traits in blueberry (Vaccinium L.; Finn and Luby, 1992), grape (Vitis L.; Firoozabady and Olmo, 1987), Japanese quince [Chaenomeles japonica (Thunb.) Lindl. ex Spach] (Rumpunen and Kviklys, 2003), kiwifruit [Actinidia deliciosa (A. Chev.) C.F. Liang & A.R. Ferguson] (Daoyu et al., 2002), peach [Prunus persica (L.) Batsch] (de Souza et al., 1998; Hansche et al., 1972), and sweet cherry [Prunus avium (L.) L.] (Hansche et al., 1966). However, genetic parameters have not been determined for storage potential. Other than the estimation of the heritability of several storage disorders in apple by Volz et al. (2001), genetic parameters have rarely been determined for fruit storage potential. The objectives of the current study were to estimate the heritability and the breeding value for apple fruit softening to facilitate the selection of new cultivars with good storage potential in apple breeding programs.
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