Meristem size drives merism lability in the Hexandrum group, a highly variable lineage of Solanum (Solanaceae).

Floral merism, the number of organs per whorl, is a conserved trait in angiosperms and has significant implications for plant reproduction. Solanum (Solanaceae) presents typically pentamerous flowers, while some species can present meristic variations. S. hexandrum is a species with typical hexamerous flowers, which represents a fixed trait in some population, while its closely related species S. sublentum exhibit the typical pentamerous flower. Importantly, both species show meristic lability, indicating that floral organ number is flexible within the group. This study investigates the developmental basis for merism variation in these species by comparing the floral ontogeny across three populations of S. hexandrum and S. sublentum. We used scanning electron microscopy, Nano-CT analysis and floral anatomy to analyse the meristem size, floral development and floral vascularization to understand the pressures influencing this morphology and its impacts on floral architecture. We found that merism variations (in a range of tetramery to heptamery) is strongly correlated with floral meristem size. Populations with larger meristems exhibited supernumerary organs, while those with smaller meristems showed subnumerary organs or stable pentamery. The meristic changes were predominantly isomerous across the whorls. Flower ontogenesis also differed among populations and species, directly influencing the final flower form. Our result demonstrates that merism size and spatio-temporal developmental constraints are key factors shaping meristic lability in Solanum. These results improve the knowledge about floral diversification and meristic variation in the genus and can support future work aiming to study floral evolution and unusual morphologies in Solanum.