Pollination Quality, Seed Set, and Flower Traits in Polemonium Viscosum: Complementary Effects of Variation in Flower Scent and Size

Seed set in naturally pollinated flowers of the alpine skypilot, Polemonium viscosum , is influenced by two components of pollination quality: the number of outcross pollen grains received by a flower over its lifetime and the purity, or percentage of conspecific grains in the pollen mixture. Plants of this species have flowers that are either sweet or skunky in scent. Sweet flowers are larger on average than skunky ones. In populations frequented by a fly‐dominated assemblage of pollinators, skunky‐scented flowers tended to receive more outcross pollen than sweet‐scented flowers, regardless of their size. In contrast, in predominantly bumble bee‐pollinated populations, floral scent and size were both correlated with pollination quality. These results illustrate that advantages associated with flower phenotypes are habitat dependent. With bumble bee pollination, plants having large sweet‐scented flowers received purer pollen mixtures and more outcross grains than neighboring plants with smaller skunky‐scented flowers. When variation in pollination quality with flower size was accounted for statistically, floral scent differences remained correlated with differential outcross pollination. While both flower scent and size are secondary attractants, they affect independent components of pollination quality and have complementary effects on seed set.