Floral morphology and phenology of Sassafras tzumu (Lauraceae)

Abstract

AbstractBackgroundSassafrashas 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 thatSassafrasdoes not belong to the trib. Laureae but to Cinnamomeae and that it is nested withinCinnamomum. 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?ResultsIn this study, we investigated two populations ofS. tzumu. We found that this species has determinate botryoid racemes, and possesses bisexual flowers. Among the three extant species,S. tzumuis more similar to its sister speciesS. randaiensebut markedly different from the AmericanS. albidum: the two Asian species possess bisexual flowers while the American species has unisexual flowers. The bisexual flower ofS. tzumuis 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.ConclusionsThe similarity of racemose inflorescences betweenSassafrasand some members of Laureae were caused by parallel evolution; the racemose inflorescence of ancestralSassafrasoriginated from the thyrsoid-cymose inflorescence inCinnamomum. The Asian speciesS. tzumuandS. randaiensepossess bisexual flowers with two phenological phases, the AmericanS. albidumevolved unisexual flowers independently from other clades with unisexual flowers in the Lauraceae, i.e., the Laureae,Alseodaphnopsisin the Perseeae and the unisexual clade in theOcoteacomplex of the Cinnamomeae.