Regulation of Return Bloom In Honeycrisp Apple: Fruit Position and Seed Number

The apple variety ‘Honeycrisp’ tends to be strongly biennial. More than seven fruit per cm² trunk cross sectional area, TCA, results in few flowers the next spring. While adjustment to four to seven fruit per TCA appears optimum, it is not sufficient to assure adequate return bloom for all trees. We hypothesize the king fruit (from the center flower of a fruit bud) has greater influence on return bloom than lateral fruit (from an outer flower) manifested either through growth rate or seed production of GA (gibberellic acid), which inhibits flower bud initiation (FBI) in that spur for the next season. Experiments were initiated in the spring of 2008 on two Michigan sites of ‘Honeycrisp’ on M-9 rootstock: a grower orchard in the southwest (SW) at 1m x 4m, and the west central Clarksville Horticultural Experiment Station (CHES) at 1.5m x 4.5m (1998). At full bloom, every flowering spur on entire trees were hand thinned to a single king flower or a single lateral flower. At fruit set (2-3 weeks later), crop load was adjusted to range between four to six fruit per TCA. From fruit set, 100 fruit per treatment were measured monthly (SW) and bi-weekly (CHES) to determine fruit growth rate.  At harvest, fruit size, size and weight distributions, seed number and bitter pit data were collected. Lateral fruit had more seeds per fruit than king fruit at both locations. Seed number ranged from 1-14, and averaged 7.1 for laterals and 6.4 for king fruit at SW, and 9.0 and 7.7, respectively, at CHES.  Lateral fruit were larger than king fruit. There was no significant difference in crop load. Season-end fruit weight was not greatly affected by seed number. Bitter pit was inversely related to seed number and positively related to fruit size. The results were surprising in that king fruit are usually larger and have more seeds that lateral fruit in most varieties.  In addition, the number of locules (3-7) and number of viable seeds (1-14) were more variable than found in other varieties. The number of seeds is important in relationship to return bloom because they produce GA, which is known to inhibit flower bud formation in apple. Flowering data in the spring of 2009 will relate return bloom to previous crop size, fruit growth rate and seed number.