Somatic mutations in apple commonly develop into viable bud sports that can be propagated clonally. When the apple bud sport has a desirable attribute such as improved color, size, shape, flavor, firmness, sweetness, or harvest timing, it has potential to be introduced as a new cultivar that growers utilize, and consumers enjoy. The genetic mutations and related mechanisms associated with early or delayed maturation (respectively resulting in early or late harvest date) in apple sports are not known despite their value to the industry. By acquiring knowledge about genetic mutations affecting harvest date and their respective molecular mechanisms, breeders can identify markers to conduct more informed crosses to select for early or late maturing apple lines. Potentially, apple lines with ‘designed’ harvest windows could be developed to optimize the deployment of field laborers for the apple harvest and improve use of cultural practices to influence harvest time. Comparison of physiology-based markers of fruit developmental stages for apple sports with different maturation dates has been done, but, to the author’s knowledge, not on trees for which crop load has been appropriately adjusted to eliminate its effect on developmental rate. In this study, crop load was precisely adjusted for the late maturing ‘Gala’ sport ‘Autumn Gala’, the early maturing ‘Fuji’ sport ‘September Wonder Fuji’, and the early maturing ‘Cripps Pink’ (‘Pink Lady®’) sport ‘Maslin Cripps Pink’, as well as for controls for each cultivar (i.e., those possessing standard harvest times). We found that in each comparison, fruit growth rate of the early variant was significantly greater early in fruit development, during the cell division phase. The early emergence of phenotypic differences in growth rate between the bud sport and the control lines suggests the physiological processes leading to an early or late harvest date may also emerge very early in fruit development. If so, the early or delayed maturation date is very likely not strictly a function of ripening-related processes, but rather is derived from a season-long shift in metabolic activity. Genomic analyses were also done to identify genetic differences between early and late apple sports. Collectively, hundreds of genetic variants were identified. A future transcriptomic analysis will reduce these variant lists to a smaller set of candidate mutations responsible for the altered maturity time. Our phenological studies reduced the developmental window for these transcriptomic investigations.
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