Diversity of flower structure and development correlates with inflorescence morphology in maples (Acer, Sapindaceae, Sapindales)

Abstract Background and Aims Acer, one of the largest genera of Sapindaceae, is well-known for its diverse inflorescence and flower morphologies. Structural diversity of maples makes this genus a wonderful model to explore morphogenetic interaction between inflorescence and flower construction. Methods We investigated the development and structure of inflorescences and flowers using scanning electron microscopy in ten species of Acer from nine (of 17) sections. Observations are interpreted in the framework of molecular phylogenies. Key results Inflorescences are represented by panicles, thyrsoids, botryoids and sciadioids. The evolutionary transition from thyrsoids or panicles to racemose inflorescences resulted in the loss of floral prophylls, another sequence of floral organ initiation and often the acquisition of a new flower groundplan. The main structural transformations are changes in the floral merism and the number of stamens caused by changes in the shape and size of the floral meristem. The order of sepal initiation, the position of the median sepal and flower orientation are largely related to mechanical pressures within the floral bud associated with variations in the inflorescence construction. The gynoecium position is also variable. In species with thyrsoids or panicles, the carpel position depends on the first sepal position or on the flower position within the inflorescence. In species with racemose inflorescences, the gynoecium is always inserted in the transverse plane. Conclusions The evolutionary transition from branched to racemose inflorescences was followed by changes in the geometry of floral meristem and ultimately resulted in considerable changes in flower construction and development. The presence of floral prophylls and their developmental dynamics is labile among species of Acer. Inflorescence construction represents not only an important taxonomic marker but also a key feature that probably defines the diversity of floral construction and development in Acer and Sapindaceae as a whole.