Hygroscopic awns and inflorescence architecture in a wind-pollinated Australian monocot: functional convergence with grasses.

Background And Aims: Spikelets are fundamental units of the inflorescence in many members of Poales, including the species-rich families Poaceae and Cyperaceae. Comparative studies of spikelet morphology have illuminated key aspects of floral evolution, function, and homology across these groups. However, spikelets of Restionaceae, a predominantly Australasian and South African family of Poales, remain poorly understood. Female inflorescences of some Restionaceae exhibit highly modified structures that have led to conflicting interpretations of the spikelet-flower boundary. This study clarifies the homology and functional significance of floral units in Restionaceae, with a focus on structural and developmental comparisons that highlight convergence with grasses.

Methods: We used scanning electron microscopy and high-resolution X-ray computed tomography to study reproductive morphology and ontogeny in Leptocarpus denmarkicus that is remarkable in possessing a grass-like awn. Ontogenetic series were used to trace the development of reproductive structures, focusing on bract arrangement, flower position, and awn formation.

Key Results: The female units represent compound spikes, each consisting of numerous reduced, single-flowered spikelets. Each spikelet bears two bracts, one of which subtends a flower. Ontogenetic observations and positional data support this interpretation and challenge previous assumptions about bracteole identity in Restionaceae. The awn is formed by the flower-subtending bract and exhibits hygroscopic twisting, which is reported here for the first time for Restionaceae. It likely facilitates seed dispersal and provides a strickling example of functional parallelism with grasses.

Conclusions: This study redefines the structure and developmental basis of inflorescences and identifies a novel seed dispersal mechanism in Restionaceae. The findings challenge long-standing assumptions on spikelet and bracteole identity and provide compelling evidence for evolutionary plasticity within Poales. The presence of hygroscopic awns in Restionaceae highlights parallel evolution of dispersal strategies with grasses, underscoring the broader significance of functional morphology in understanding inflorescence evolution across monocots.