Floral morphology and morphogenesis in Sanguisorba (Rosaceae): flower diversification despite petal reduction and spatial constraints

摘要

Most Rosaceae flowers are pentamerous and have petals, but subtribe Sanguisorbinae have small tetramerous (or trimerous) flowers without petals, and their floral morphology and morphogenesis remain poorly known. We investigated the floral development of three Sanguisorba spp. using scanning electron microscopy to clarify the relationship between floral development and mature flowers, with emphasis on floral constraints affecting reduction of petals, diversity of androecial patterns and the development of the gynoecium and ovule, and to clarify the pollination mechanisms in the absence of petals. All species have botryoids, with numerous flowers initiated acropetally, and the maturation of flowers follows different directions. All flowers are tetramerous and sepals emerge pairwise. No petal primordia appear. Antesepalous stamens are initiated in a pairwise manner; only S. hakusanensis has a second whorl. The ovary appears inferior by development of a cystiform hypanthium. There is only one ovule with a single integument by reduction of the inner. The space occupied by the larger sepals forces the stamens to develop sequentially. Our data demonstrate that petals are basically absent and that there is a possible shift of pollination mechanisms in Sanguisorba among the mainly wind-pollinated Sanguisorbinae, as both anemophilous and entomophilous characters are found in this genus.