Effect of Fruiting and Biennial Bearing Potential on Spur Quality and Leaf Gas Exchange in Apple

Uniform annual apple (Malus xdomestica) 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.