Floral developmental and anatomical evidence clarify the causes for the intrinsic floral morphological variation in Cyperoideae (Cyperaceae): some case studies

Abstract

Abstract Cyperaceae is a widespread family composed of two subfamilies, Mapanioideae and Cyperoideae. As opposed to the dimerous Mapanioideae spicoid inflorescences, Cyperoideae flowers are known for their typical trimerous Bauplan, however, morphological variation is reported in each floral whorl. This wide range of variation is challenging, mainly regarding taxonomic descriptions and accessing morphological homologies to understand Cyperoideae evolutionary relationships. Therefore, it is compelling to investigate floral morphology variation in the subfamily, particularly in species belonging to the most diverse genera, Carex, Cyperus and Rhynchospora, as potential models to explore the main floral variations found among Cyperoideae. Thus, we aimed to investigate the floral anatomy and development of species belonging to these genera to clarify following questions: 1) the ontogenetic sequence of the Rhynchospora perianth, 2) floral sexuality within Cyperus spikelets, 3) the increase on the number of stamens in Carex male flowers and 4) the pistil morphological variation in Cyperoideae flowers. We reported a distinct ontogenetic sequence for the perianth formation in Rhynchospora, highlighting a different developmental pattern for the Cyperoideae perianth. The presence of bisexual and unisexual flowers within the same spikelet in Cyperus was confirmed and needs to be considered in further taxonomic descriptions. The increase on the number of stamens in Carex male flowers is caused by the fusion of primordia, corroborated by the presence of several vascular bundles supplying a single filament. Additionally, the presence of male flowers with a dome-shape primordium and a plexus of receptacular bundles were related to features commonly found in monocots inflorescences. This raises questions whether Carex male flowers could be interpreted as reduced inflorescences. In general, all species exhibited restrictions on the development of abaxial structures, mainly due to the mechanical constraints applied by the glume, which may be influencing the position of the dimerous dorsiventrally flattened pistil in Cyperoideae flowers.