A model for representing the relationships among crop load, timing of thinning, flower bud formation, and fruit weight in apples

The thinning of flowers or fruit is an essential practice for apple (Malus x domestica Borkh.) growers to achieve consistent production, and as many fruits as possible must grow to marketable size without biennial bearing to attain maximum economic value. Since almost no information is available to help determine when and how much to thin trees in each cultivar to obtain fruit of a certain size every year, we developed a theoretical model using 'Fuji' to explain the relationships among the timing of thinning, crop load, fruit weight, and bloom return. The rate of flower-bud formation in the current year could be explained by a regression model in which the timing of thinning, crop load, and rate of flower-bud formation in the previous year were used as variables. When trees are managed at the same timing and level of thinning every year avoiding biennial bearing, the rate of flower-bud formation is theoretically determined to be a certain value. The fruit weight in the current year could be explained by a regression model in which the timing of thinning, crop load, rate of flower-bud formation, and shoot length in the current year and the previous year were used as variables. Using the two regression models, we revealed that when 'Fuji' trees were managed so that the length of shoots was 30 cm, the fruit weight of the trees would be about 270 g in the case that the crop load was three fruits per cm2 of trunk cross-sectional area (TCA), whereas it would be 180 g in the case that the crop load was six fruits per cm2 TCA. When the trees were managed so that the length of the shoots was 40 cm, the fruit weight of the trees would be 50 g higher than that in trees with shoots 30 cm in length. On the other hand, when thinning was performed at 15 days after bloom, the fruit weight of the trees would be only 10-20 g higher than that in trees thinned at 30 days after bloom.