Lack of consistent flower formation is the underlying cause of biennial bearing. Flower formation in apple (Malus ×domestica Borkh.) has been associated with different factors, such as leaf area, shoot growth, bourse length, crop load, and seed number. However, it is unclear how these different factors interact to promote or inhibit flower formation. The effect of spur defoliation, fruit removal, and their interaction were evaluated on spur flower formation and bourse length in annual-bearing ‘Gala’ and the biennial-bearing ‘Honeycrisp’. Eight different combinations of spur defoliation and fruiting treatments were applied in three consecutive springs, 2013–15. Bourse shoot defoliation and fruiting treatments inhibited spur flower formation in both cultivars, but in different patterns from year to year. In addition, spur leaf defoliation did not affect flower formation in either cultivar. Furthermore, local defoliation and fruiting treatments did not affect bourse length. We propose that bourse leaves play a major role in both producing and transporting flower formation signals, but the effect depends on cultivar.
Mokhles A. Elsysy and Peter M. Hirst
Mokhles A. Elsysy, Michael V. Mickelbart and Peter M. Hirst
Uniform annual apple (Malus ×domestica) fruit production is highly dependent on consistent flower formation from year to year, as inconsistent flowering can lead to the biennial bearing observed in some high-value cultivars. The presence of fruit on a spur has been considered the main cause of the expression of biennial bearing and the inhibition of flower initiation, with a number of theories being introduced to explain the phenomenon. In the current experiment, individual spurs of annual bearing cultivars (Gala, Ruby Jon, and Pink Lady) and biennial bearing cultivars (Honeycrisp, Fuji, and Golden Delicious) were thinned to a single fruit or completely defruited at petal fall. Spurs were sampled at the end of the growing season. Effects of fruiting on spur characteristics such as spur and bourse leaf area, stomatal density, leaf gas exchange, and flower formation were determined. Across all cultivars, the presence of fruit on a spur did not affect spur characteristics or flower formation compared with nonfruiting spurs. Similarly, flowering was unaffected by those factors associated with greater spur carbohydrate status, such as bourse leaf area and assimilation rate. Cultivars with greater transpiration and stomatal conductance (g s) rates had lower rates of flower formation. Future studies should focus on xylem flow and expression of genes regulating flowering and plant growth regulators in annual and biennial bearing cultivars.