Developmentally plastic responses to pollinators by Lupinus perennis flowers and what they tell us about the pollination mechanism in the general lupine flower

Date of Completion

January 2005


Biology, Botany|Biology, Ecology|Biology, Plant Physiology




This dissertation explores the complex pollination ecology of the sundial lupine (Lupinus perennis; Fabaceae), a spring-flowering component of the well-drained, sandy soils of the pine barrens in the eastern United States and the oak savannah ecosystems of the upper Midwest. Its flowers lack nectar and reward pollinators instead with measured doses of pollen. Flowers communicate a depleted reward by changing the color of their banner petal, allowing pollinators to maximize foraging efficiency. The result is a larger floral display that attracts pollinators from a greater distance. Evidence presented here points to pollinator visit frequency as the trigger for color changes as compared to the more frequently cited pollen-pistil interactions. Pollinator visit frequency was also found to trigger accelerated stigma development and filament elongation. A new staining technique was developed to evaluate the prevailing view that lupine stigmas, while of the wet type, release secretions by some means other than cuticle rupture. In addition to revealing details of the acceleration in stigma development, this staining technique revealed that stigmatic secretions are released from the L. perennis stigma in the same manner as occurs in other wet stigmas, through breaks in the cuticle. The new data on developmental plasticity and stigma structure is integrated into a review of the mechanical operation of the lupine's piston-type flower. In contrast to the conventional view that the stamens in the lupine flower form the piston which extrudes pollen from within the keel petals, it is argued here that the pistil acts as the piston and the stamens merely reload the stigma. The conventional argument that the sterile ring of bristles surrounding the stigma is a palisade guarding against self-pollination is also challenged. Here, it is argued instead that these bristles serve to expand the surface area of the stigmatic pollen presentation mechanism. ^