Floral morphology and phenology of Sassafras tzumu (Lauraceae)

BACKGROUND: Sassafras has been considered to belong to trib. Laureae of Lauraceae and has been assumed to have unisexual flowers. However, recent molecular phylogenetic studies have consistently suggested that Sassafras does not belong to the trib. Laureae but to Cinnamomeae and that it is nested within Cinnamomum. A recent morphological study revealed that one of the Asian species, S. randaiense, possesses bisexual flowers that are plesiomorphic in the family Lauraceae. As reports on the flower structure of the second Asian species, S. tzumu, have been contradictory, we wanted to ascertain if it has bisexual flowers or not. If the flowers were bisexual, could earlier reports that they were unisexual have been based on dichogamous flowering? RESULTS: In this study, we investigated two populations of S. tzumu. We found that this species has determinate botryoid racemes, and possesses bisexual flowers. Among the three extant species, S. tzumu is more similar to its sister species S. randaiense but markedly different from the American S. albidum: the two Asian species possess bisexual flowers while the American species has unisexual flowers. The bisexual flower of S. tzumu is protogynous, and shows two phenological phases typical of Lauraceae: 1) in a flower, the pistil functions first, the stigma is fresh and white, stamens of the outer two whorls are spreading, anthers do not open, and the staminodes secrete nectar at this stage; 2) in the second phase, the stigma becomes brown, staminodes are withered, stamens of the third whorl stand up and surround the pistil, glands of the third whorl of stamens secrete nectar, and the anthers open and release pollen. CONCLUSIONS: The similarity of racemose inflorescences between Sassafras and some members of Laureae were caused by parallel evolution; the racemose inflorescence of ancestral Sassafras originated from the thyrsoid-cymose inflorescence in Cinnamomum. The Asian species S. tzumu and S. randaiense possess bisexual flowers with two phenological phases, the American S. albidum evolved unisexual flowers independently from other clades with unisexual flowers in the Lauraceae, i.e., the Laureae, Alseodaphnopsis in the Perseeae and the unisexual clade in the Ocotea complex of the Cinnamomeae.