The inflorescence architecture in Nidularioid genera: Understanding the structure of congested inflorescences in Bromeliaceae

Abstract

In the subfamily Bromelioideae, the so-called Nidularioid genera share some morphological features, the most notable of which is the occurrence of congested inflorescences that accumulate different amounts of water. This study aims to describe and compare the inflorescence morphology in six Nidularioid genera and to estimate the “tank space” for water in each inflorescence. Inflorescences at different developmental stages were analyzed using light microscopy and described using a typology-based comparative approach. X-ray microtomography was used to obtain sections of young inflorescences, from which 3-dimensional images were built for exploring structural issues and estimating water impounding capacity. Inflorescences are simple racemes in N. johannis and heterothetic compound racemes in the other species. No adaxial prophylls were observed among Nidularioid genera, neither in floral axes nor in the inflorescence branches. The flowers are spirally arranged along the florescence axis although the polystichous condition is obscured by the dorsiventral compression of florescences. The development of Nidularioid compound inflorescences follows a bidirectional pattern since flower primordia differentiate acropetally and paraclade primordia develop basipetally. Bracts are used for water storage in species with a long scape, where such bracts are large and cover the entire paraclades. In species with a short floral scape, inflorescences remain submerged inside the vegetative phytotelmata, allowing these inflorescences to be always in contact with water. Despite the actual branching pattern is obscured because the inflorescence branches are closely condensed in the axils of water impounding bracts, all Nidularioid inflorescences constitute polytelic systems that follow the general pattern of Bromeliaceae. Although all Nidularioid inflorescences are associated with structures for water retention, they seem not to be synapomorphic. Thus, hypotheses of primary homology suggested by both the typological analysis of inflorescences and the survey of water-impounding structures are consistent with the non-monophyletic condition of the Nidularioid genera.