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Sexual dimorphism of bouncing bet flowers

Sandra Davis, Dana Dudle, Jenna Nawrocki, Leah Freestone, Peter Konieczny, Michael Tobin, and Michael Britton

Department of Biology, University of Indianapolis
Department of Biology, DePauw University 

Davis et al 2014 pdf

Introduction

Bouncing bet (Saponaria officinalis) is a non-native herbaceous weed.  It grows in disturbed areas such as along roadsides, forest edges, and stream banks.  It spreads underground through rhizomes and produces dense clusters of f lowers.  The flowers show sexual dimorphism during the course of their development.  The flowers start as pale white males (staminate phase) and develop into pink females (pistillate phase).  

Research Questions
What is the extent of variation in the flower size, flower color, and nectar production of bouncing bet?
What is the effect of sun and shade on sexual dimorphism of bouncing bet flowers?
How do diurnal pollinators respond to staminate- and pistillate-phase flowers?
How do flower color, sun, and shade affect seed production of bouncing bet? 

Methods
In 2012, we set up an experimental garden next to the Manning Environmental Field Station in the DePauw Nature Park.  We collected 25 plants from wild populations; each individual plant was a different genotype.  We split each individual plant into 8 separate plants ("clones").  We planted 25 plants (one clone of each genotype) in 8 separate test plots within the experimental garden  We constructed wooden frames over each test plot to create sunny and shaded environments.  We used a spectrophotometer to measure anthocyanin concentrations within male and female flowers.  We measured petal length, petal width, corolla diameter, and corolla mass of male and female flowers.  We collected nectar from individual flowers and measured nectar volume and sucrose concentration of the nectar.  We observed and recorded pollinator visits to arrays of cut flowers.  

Results
Female-phase flowers had larger petals, greater mass, and are pinker in color than male-phase flowers.  Anthocynanin concentrations were higher in female-phase flowers exposed to sun than shade.  There was no difference in nectar volume or sugar content between female- and male-phase flowers.  Diurnal pollinators were more likely to visit male-phase flowers than pink female-phase flowers.  But diurnal pollinators did not discriminate between male-phase flowers and pale female-phase flowers. 

Discussion
Diurnal pollinators use color to discriminate among flowers.  This pollinator discrimination may lead to a reduction in seed production.  Why do flowers turn pink if the trait actually reduces seed production?  Perhaps the increase in pinkness of female-phase flowers may protect against photoinhibition in plant tissues.