A ballistic pollen dispersal system influences pollination success and fruit-set pattern in pollinator-excluded environments for the endangered species Synaphea stenoloba (Proteaceae)

The critically endangered Synaphea stenoloba (Proteaceae) has numerous scentless flowers clustered in dense inflorescences and deploys a ballistic pollen ejection mechanism to release pollen. We examined the hypothesis that active pollen ejection and flowering patterns within an inflorescence influence the reproductive success (i.e. fruit formation) of individual flowers within or among inflorescences of S. stenoloba in a pollinator-excluded environment. Our results showed that: (1) no pollen grains were observed deposited on the stigma of their own flower after the pollen ejection system was manually activated, indicating self-pollination within an individual flower is improbable in S. stenoloba; (2) fruit set in the indoor open pollination treatment and the inflorescence-closed pollination treatment indicated that S. stenoloba is self-compatible and pollen ejection can potentially result in inter-floral pollination success; (3) fruit set in the inflorescence-closed pollination treatment was significantly lower than that of indoor open pollination, indicating within- and between-flower pollination events in an inflorescence are most likely limited, with pollination between inflorescences providing the highest reproductive opportunity; and (4) analysis of the spatial distribution of cumulative fruit set on inflorescences showed that pollen could reach any flower within an inflorescence and there was no functional limitation on seed set among flowers located at various positions within the inflorescence. These data suggest that the pollen ejection mechanism in S. stenoloba can enhance inter-plant pollination in pollinator-excluded environments and may suggest adaptation to pollinator scarcity attributable to habitat disturbance or competition for pollinators in a diverse flora.