Influence of physical distance between cultivars on yield,outcrossing rate and selective fruit drop in avocado (Persea americana,Lauraceae)

Avocado shows protogynous dichogamy with two complementary cultivar types (A and B) that differ in their floral behaviour. Because of this peculiar flowering system, mixed plantings of cultivars of complementary flower type have been traditionally recommended to increase yield. However, the effect of planting complementary avocado cultivars on outcrossing rate and yield is a subject of controversy. In this work, we have studied the outcrossing rate with microsatellite markers under the growing conditions of Southern Spain. Outcrossing rate was determined at harvest on several ‘Hass’ trees situated in rows at different distances from a ‘Fuerte’ orchard for two consecutive years (2005 and 2006). Outcrossing rate ranged from 0.31 to 0.74 with an average of 0.47 showing a significant decrease from the first row (in proximity to the pollen source) to the rest of the rows. However, using data of 13 years no significant differences in yield have been recorded with increasing distance to the pollen donor trees. The paternity of abscised fruits was recorded weekly from June to the period of commercial harvest in March the following year. The results obtained indicate that the fruits that dropped during June were mostly derived from self‐fertilisation. However, a high proportion of those fruits were derived from flowers fertilised during the last weeks of the ‘Hass’ blooming season when the cultivar Fuerte presents either few or no flowers. Consequently, most of the late fertilised ‘Hass’ fruits were derived from self‐pollination and those small fruits compete for limited maternal resources with the earlier fertilised fruits derived from cross‐pollination. These results suggest that, under our growing conditions, fruit drop in avocado is rather determined by the fertilisation date and not by the embryo genetic composition.