Adaptive floral traits that impact stigma pollen load and seed production in Penstemon digitalis (Plantaginaceae)

Penstemon digitalis is an early spring‐flowering species of the northwest Missouri prairie community, a period often characterized by low and unpredictable bee visitation. We conducted an experimental field study to evaluate how pollinator‐mediated pollen transfer and floral longevity interact to influence pollen acquisition and seed production. The study involved the tagging and bagging of plants and flowers to record flower phenology, flower longevity, pollen tube growth to evaluate stigma receptivity, stigma pollen loads, and single visit deposition experiments to flowers by bees to evaluate pollinator effectiveness. Our results demonstrated that flowers were not autonomous but required bee visits for successful reproduction with fertilization dependent on the flowers remaining viable for three days. Single visit deposition experiments performed using Anthophora terminalis , Bombus sp., Ceratina sp. and Hoplitis pilosifrons , resulted in the deposition of relatively few pollen grains per visit indicating that multiple visits during the two‐day period of stigma receptivity were likely necessary for successful reproduction. Pollen germinated on receptive stigmas on each of the two days of receptivity and flowers exhibited a fertilization interval of an estimated 48 hours, as evidenced by pollen tube growth through the style via fluorescence microscopy. Although pollen accumulated on stigmas over the two‐day period of receptivity, the pattern of seeds produced per flower remained essentially the same for stigmas pollinated on day 2 of anthesis as the combined days of stigma receptivity, day 2 plus day 3. These results suggest that prolonged stigma receptivity functions as a mechanism that safeguards against temporally variable pollinator visitation typical of early spring, more so than increasing per‐flower seed production.