Floral development of monoecious Pseuduvaria trimera (Annonaceae) and comparative morphology and structure of its stamens and indehiscent staminodes

摘要: Premise of research. Comparative development of staminate and pistillate flowers of Pseuduvaria trimera (Annonaceae) is firstly investigated to understand its sexual differentiation and developmental processes of flowers. Comparisons of morphological and anatomical features between mature stamens and staminodes are also described, in order to clarify the mode of anther dehiscence and understand the mechanism underlying the sterility of the staminodes. Methodology. Flowers at different developmental stages and mature stamens and staminodes were examined with SEM and light microscopy. Pivotal results. Flowers have complex whorled phyllotaxis as in other Annonaceae, with double positions in the outmost whorl of reproductive organs and variable stamen and carpel organ numbers per whorl. The stomium region of each theca is composed of the septum and stomium. The septum and stomium cells of stamens could be degenerated developmentally, facilitating anther opening, whereas those of staminodes maintain intact, forming indehiscent anthers. Conclusions. The developmental data suggest that staminate flower is unisexual from inception, while pistillate flower is morphologically hermaphroditic. The morphologically hermaphroditic flowers are nevertheless functionally pistillate as the anthers are indehiscent, and the structural andromonoecy of P. trimera actually functions as monoecy. The mechanism underlying the sterility of the indehiscent staminodes and delayed dehiscent outer stamens might be effective in promoting xenogamy. The occurrence of staminode is closely correlated with the acquisition and evolution of functional unisexual flowers in the genus Pseuduvaria.