Inaccuracy of sexual organ position and spatial variation of styles in monomorphic enantiostylous flowers of Senna rugosa (Fabaceae-Caesalpinioideae)

Abstract

Differences in the arrangement of floral whorls in enantiostylous species may affect the levels of reciprocity between the sexual organs of floral morphs and the functionality of the enantiostyly. In this study, we aimed to investigate enantiostyly in Senna rugosa, evaluating the variation in the position of sexual organs. Using different spatial measures, we investigated the level of herkogamy and used the inaccuracy index to describe the level of reciprocity between stigmas and anthers of different floral types and within the same floral type. We investigated the proportion of floral types within individuals of the population based on the position of the curved petal and the stigma. Stigmas of left-styled flowers are more likely to capture inter-morph pollen, whereas stigmas of right-styled flowers may receive greater proportions of intra-morph pollen. Horizontal distance was most responsible for the non-overlap of the sexual organs, whereas height was the measure that contributed most to their proximity. Individuals showed a similar proportion of floral types. However, from a morpho-functional perspective, the proportion of flowers differed, with a lower proportion of functionally left-styled flowers. Both floral types have functionally right-, left-, and central-styled flowers. Our study is the first report of a possible division of reproductive functions in pollen uptake between opposite floral types, a mechanism that may have evolved to provide reproductive assurance, and the second report of the presence of functionally central styles within the subtribe Cassiinae, which may be related to an increase in the pollen-capturing surface area on the pollinator body.