Method of manipulating floral bud density affects fruit set responses in apple

Apple trees (Malus × domestica Borkh.) usually produce abundant flowers and fruit, and numbers must be reduced to achieve commercial targets of fruit size and quality. Profuse flowering and fruiting also creates high demand for a limited source of carbohydrates, which affects fruit set and development. Artificial spur extinction (ASE) uses hand-thinning of whole buds in late dormancy to reach targeted floral bud densities on every limb. ASE removes floral and vegetative sinks and potential leaf area. Consequently, ASE provides a useful tool to improve understanding of the physiological processes affecting fruit set. We investigated flowering, fruit set and yield of ‘Royal Gala’ trees treated with three thinning methods; ASE, flower cluster thinning (FCT) at pink bloom, and fruit thinning after final fruit set (Control). ASE and FCT were imposed at 2, 4 or 6 buds or flower clusters per cm² branch cross-sectional area (BCA). Final fruit thinning on all three thinning methods was imposed at two fruit per bud on 2 buds cm⁻² BCA and a single fruit per bud on 4 and 6 buds cm⁻² BCA. Consequently, although the density of fruit-bearing buds differed between 2 and 4 buds cm⁻² BCA, fruit density did not. Compared with the Control, removal of floral sinks before flowering in ASE and FCT increased fruit set within flower clusters by more than 50% and increased harvest mean fruit weight by 18–32 g fruit⁻¹. Compared with FCT, ASE resulted in marginally lower fruit set but did not alter components of yield and had little effect on fruit quality. Results imply that manipulation of floral bud density may have beneficially altered carbon availability within floral spurs during early-season development. In FCT, reducing the density of flower clusters may have increased carbon availability to remaining fruit buds, improving their fruit set and development. In ASE, removal and uniform spatial distribution of buds may have improved irradiance of fruiting spurs, thereby increasing photosynthate availability to developing fruit on that spur, improving fruit set and development compared with the Control, achieving results similar to FCT. Despite large differences in natural fruit set brought about by normal seasonal differences in environment, ASE produced precise fruit set outcomes. This contrasts starkly with current commercial thinning practices that rely heavily on chemical thinning, and supports the use of ASE as a precision alternative to existing commercial crop load management.