The effects of nectar level and flower development on pollen carry-over in inflorescences of fireweed (Epilobium angustifolium) (Onagraceae)

Abstract

Nectar in flowers of Epilobium angustifolium was manipulated experimentally to provide enriched versus depleted reward levels to bumblebee pollinators. Bumblebees visited significantly more flowers on inflorescences with high rewards than low ones and stayed longer on individual flowers with high rewards. The greatest pollen deposition occurred in older female flowers having enriched nectar levels. In older female flowers the stigma lobes coil backward, reducing the distance between their receptive surfaces and the nectary site. Younger female flowers, regardless of reward level, only occasionally received more than a few pollen grains per bee visit. Although higher nectar levels are likely to promote pollen receipt over the course of a flower's lifetime, because of changes in stigma configuration with flower development, the number of pollen grains transferred to the stigma at any given time depends on the flower's age as well as its reward status. In this self-compatible plant species, most of the increase in pollen receipt resulting from high nectar production is likely to lead to self-matings since (i) plants are clonal and bees usually fly to nearest neighbor ramets after rewarding visits, (ii) bees visit more flowers on enriched inflorescences than on depleted ones, and (iii) pollen carry-over declines exponentially over the course of an inflorescence visit.