Floral development and the formation of functionally unisexual flowers in Xanthoceras sorbifolium (Sapindaceae), a morphologically andromonoecious tree endemic to northern China

Abstract

KEY MESSAGE: Xanthoceras sorbifolium is apparently andromonoecious but exhibits a cryptically monoecious breeding system. Sexual differentiation in male and functional female flowers occurs 2 weeks before flowering. Individual trees of Xanthoceras sorbifolium bear male and morphologically bisexual flowers but functionally female flowers, and show labile sex expression. Investigations of the floral development are of significant value for understanding the breeding system and elucidating the systematic relationships. We studied floral development of male and bisexual flowers using scanning electron and light microscopy. The early stages of floral development were essentially the same, and all floral organ primordia were initiated in the two types of flowers of X. sorbifolium. Later, the stamens in bisexual flowers and the gynoecium in male flowers were aborted. We divided floral development into nine stages. Morphological differences between male and bisexual flowers appeared first at stage 8, when the style elongated obviously and the stigma papillae began to expand in the bisexual flowers but not in the male flowers. Ovule development was arrested shortly after formation of megaspore mother cells or during meiosis in the male flowers, whereas anther development was aberrant in the bisexual flowers. Morphologically bisexual flowers of X. sorbifolium do not have male function and are functionally female. Comparison of floral developmental characters did not support the separation of the Chinese monotypic genus Xanthoceras from the family Sapindaceae.