Comparative Study on the Aerial and Subterranean Flower Development in Amphicarpaea edgeworthii Benth. (Leguminosae: Papilionoideae), an Amphicarpic Species

Amphicarpy is a rare phenomenon in angiosperms. There are few studies on the floral development of aerial and especially subterranean flowers of the amphicarpic species. In this study, we examine the floral ontogeny of Amphicarpaea edgeworthii, an amphicarpic legume, using SEM. Three types of flowers can be produced in the same individual of the species: the subterranean flowers, solitary without subtending leaves; the aerial cleistogamous flowers, generally solitary in leaf axils; and the aerial chasmogamous flowers, one or two subtended by a bract. There is little difference in terms of the organ initiation among the three flower morphs. The order of initiation among whorls is sepals, petals, carpel, outer-whorl stamens, and inner-whorl stamens. Within each whorl, no obvious unidirectional order is found among the floral organ components. The development of petals, two-whorl stamens, and a carpel in the aerial and subterranean flowers diverges at the mid- to late-development stage. The mature aerial chasmogamous flowers become papilionaceous in the late stage by divergent form changes among petals, showing the potential for outcrossing. There is little difference in the floral development and morphology between the aerial and subterranean cleistogamous flowers. In contrast to aerial chasmogamous flowers, the specialized characteristics of cleistogamous flowers include partial suppression of petals and stamens. The findings in this study show that the subterranean flower was not a morphologically specific flower type diverged from the aerial cleistogamous flower.