A comparative study of the superior fertility of 'Smoothee Golden Delicious' apple

Apple (Malus domestica) has a gametophytically determined self-incompatibility system, which limits inbreeding. As a consequence, apple trees depend on cross-fertilization for fruit-set, which may be enhanced by honeybees (Apis mellifera). In this study honeybee pollination effectiveness and setting potential of two important apple cultivars, 'Smoothee Golden Delicious' and 'Topred Red Delicious', were investigated to shed light on the high fruit-sets and yields of 'Smoothee', as compared with 'Topred'. Analysis of floral features and honeybee behaviour revealed that the differences between the two cultivars in flower size and nectar traits did not affect honeybee visitation frequency, which was the same for both cultivars. Due to the wide basal gaps between the stamen filaments in 'Smoothee' and 'Topred' flowers, which enabled "sideworking", a low proportion of " topworkers" was found in both cultivars. Yet, more than double the number of "topworkers" was found in 'Smoothee', as compared to 'Topred', probably because of the wider diameter of the stamen filament spread of the 'Smoothee' flowers, facilitating nectar collection from the top and thus increasing pollination effectiveness. 'Smoothee' fruit-set was significantly higher than 'Topred' in all stages of flower development, in accordance with the assumption that 'Topred' has a shorter ovule longevity than 'Smoothee'. However, higher fruit-set of 'Smoothee' flowers pollinated before anthesis, at the "Balloon" stage, was also found. This implies that the high fruit-set and fertility of 'Smoothee' are also due to additional genetic features.