Stamen dimorphism in Rhododendron ferrugineum (Ericaceae): Development andfunction

The function of stamen dimorphism in the breeding system of the alpine shru b Rhododendron ferrugineum was studied in two populations in the French Alp s. This species has pentameric flowers with two whorls of stamens: an inner whorl of Ave long stamens and an outer whorl of short stamens. We studied the development of stamens from buds to mature flowers (measurement of the filament, anther, and style lengths at five successive phenological stages) and compared the size and position of reproductive organs at maturity in c ontrol and partially emasculated flowers (removal of long-level stamens) to determine whether the presence of long-level stamens constitutes a constra int for the development of the short-level ones. Stamen dimorphism can be o bserved early in stamen development, from the bud stage of the year prior t o flowering. At this early stage, meiosis had already occurred. Emasculatio n of the long-level stamens induced the short-level ones to grow longer tha n in normal conditions. We also performed seven pollination treatments on t en randomly chosen individuals in each population, and the number of seeds following each treatment was recorded. Results from these treatments showed that R. ferrugineum produced spontaneous selfed seeds in the absence of po llinators. However, no seed was produced when short-level stamens were emas culated and pollinators excluded, suggesting that long-level stamens are no t responsible for selfing in the absence of pollinators and that reproducti ve assurance is promoted by short-level stamens.