PS 41-116
Floral nectar makes small female flowers attractive to fly and wasp pollinators as large male flowers
On flowering plants, floral traits strongly affect plant fitness through pollinator attraction. A widely accepted hypothesis is that larger flowers attract more pollinators. The majority of studies supporting the hypothesis has focused on flowers associated with bee pollinators. However, many plants depend their pollination on flies, wasps, and/or beetles. Eurya japonica is a sexually dimorphic dioecious shrub in East Asia, and its main pollinators are flies and wasps. Sexual dimorphism is one of polymorphisms within species, and polymorphic traits are suitable to examine the relationship between floral traits and pollinator attraction. To test the above hypothesis in fly and wasp pollination systems, we examine whether floral size of E. japonica differs between sexes, and whether floral size affects attractiveness to pollinators. We measured corolla size, volume and sugar content of floral nectar, and counted the number of fly and wasp pollinators. To reveal how these floral traits affect plant fitness through pollinator attraction, we examined the proportion of fruit set and matured seeds. Furthermore, we conducted field experiment to test the pollen limitation, which is a criterion for selection pressure on pollinator attraction, by artificial pollination.
Results/Conclusions
Female flowers were significantly smaller, but provided greater production of nectar with higher sugar contents than male flowers. The number of fly and wasp pollinators, and the proportion of fruit set and matured seeds were positively affected by nectar volume and sugar content, but were unaffected by corolla size. The number of pollinators visited to female flowers did not differ from male flowers. The proportion of fruit set of female flowers exposed to pollinators did not significantly differ from that of artificially pollinated flowers. However, the proportion of matured seeds in flowers exposed to pollinators was significantly lower than artificially pollinated flowers. Our results demonstrated that despite small size, female flowers attracted pollinators as same as male flowers, and that they suffered from pollen limitation. Greater and richer rewards of female flowers to pollinators may be adaptive under pollen limitation. Our results did not support the hypothesis that larger flowers attract more pollinators, but support an alternative hypothesis that flowers with greater rewards attract more pollinators. The pollen limitation, which would be common in many pollination systems, may promote greater rewards of females to compensate for small floral size.