The consequences of clone size for paternal and maternal success in domestic apple (Malus X Domestica)

Clonal growth in plants can increase pollen and ovule production per genet. However, paternal and maternal reproductive success may not increase because within-clone pollination (geitonogamy) can reduce pollen export to adjacent clones (pollen discounting) and pollen import to the central ramets (pollen limitation). The relationship between clone size and mating success was investigated using clones of Malus X domestica at four orchards (blocks of 1-5 rows of trees). For each block, maternal function was measured as fruit and seed set in all rows and paternal function as siting rate estimated from isozyme profiles in the first row of the adjacent block. Expected relations between reproductive success and clone size were generated from simulations and data on pollen dispersal in this species. Siting rate per clone averaged 70% and did not increase significantly with block size, consistent with simulations of pollen dispersal under pollen discounting. Simulations also indicated that the ratio of compatible to incompatible pollen received by a tree should decline with increased block size and from the periphery to the center of blocks. However, female function was not significantly reduced among block sizes or within blocks. The results suggest that paternal function may be more sensitive to the effects of clonality than female function.