1 Department of Horticulture and Landscape Architecture, Purdue University, 625 Agriculture Mall Drive, West Lafayette, IN 47907; and Department of Pomology, College of Agriculture, Assiut University, Assiut 71515, Egypt
2 Department of Horticulture and Landscape Architecture, Purdue University, 625 Agriculture Mall Drive, West Lafayette, IN, 47907; and Department of Botany and Plant Pathology, Purdue University, 915 W. State Street, West Lafayette, IN 47907
3 Department of Horticulture and Landscape Architecture, Purdue University, 625 Agriculture Mall Drive, West Lafayette, IN 47907
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 (gs) 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.
We thank pomology lab members Andres Mayorga, Fatemeh Sheibani, Songwen Zhang, and Bofan Xie for their help in the different phases of the experiment. We also thank Cary Mitchell and Celina Gomez for their help with the gas exchange measurements.
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