The spread of cross-pollination in a solid block of Granny Smith apples

Abstract

Numbers of fruits per tree were determined in a solid block of Granny Smith apple trees into which shoots of the pollen donor Abas had been placed at flowering time. Fruit numbers declined rapidly from 46 per tree nearest the donor to a background level of 0-2 fruits 12 m distant. This distance was taken as the limit of cross-pollination. Apart from a significant spread of setting with the prevailing wind, fruit numbers were related to distance from the pollen donor. This relationship was described by an equation involving the limited work-range of an individual bee (12m), the dilution of donor pollen with increasing distance from the source (0.67m-1) and a scaling factor determining the general level of setting. It was assumed that donor pollen density was so low that multiple pollination could be ignored. Seed counts per fruit averaged 1.4 on windfalls, 2.3 on a random sample of picked fruit, and 5.2 on trees nearest the pollen donors. Fruits from heavily cropping orchards nearby with established pollen donors averaged between four and six seeds per fruit.