Effect of Flower Pollination on Fruit Set and Cropping in Apple

Abstract

Apple (Malus x domestica Borkh.), being self-incompatible, requires cross-pollination to set fruit regularly. Apple flowers contain ten ovules and the higher the number of fertilized ovules, the larger the fruit size. In commercial orchards, cross pollination is usually guaranteed by intercropping different cultivars and by introducing beehives in the orchard at flowering time. Despite evidence that pollen donors should be close for successful pollen transportation, apple growers often plant large monovarietal blocks with negative consequences on crop load and fruit size. The contribution of wind and bees to apple pollination was studied and compared with hand-pollination in two apple cultivars, 'Royal Gala' and 'Golden Delicious'. Self-pollination was also included as control. Each treatments was replicated in 120 cymes randomly assigned to plants in the central part of large monovarietal blocks (1.61 and 2.72 ha respectively). Fruit set and fruit weight increased moving from wind-pollination to wind + bee-pollination to hand-pollination as a consequence of the increase in the number of seeds per fruit. Fertilization of a single ovule was enough to set fruit after flowering, but the more seed, the larger the fruit. The increase in fruit weight per additional seed was 2.0 g for 'Royal Gala' and 4.6 g for 'Golden Delicious'. This relationship between seed count and fruit weight resulted in fruit set by wind-pollination being mainly in the lower size classes, while wind + bee-pollination and, largely, hand-pollination shifted the fruit size distribution towards larger size classes. This resulted, in turn, in a dramatic increase in grower income. No significant difference between treatments in fruit quality, as assessed by fruit firmness, soluble solids content, titratable acidity, dry matter and starch content, was observed after fruit storage. It is concluded that limiting pollination in the hope of reducing the labor of thinning is not a good strategy, because flowers set fruit with a minimal cross-pollination, even if only a single seed is produced; conversely, the low seed content of fruit due to insufficient pollination is responsible for great losses for grower in both total crop yield and income.