The reproductive biology of Marathrum rubrum Novelo & C. T. Philbrick: morphological and anatomical changes during flower development

Background: Podostemaceae is the largest family of strictly aquatic angiosperms. Species of this family possess unusual morphological and reproductive characters, grow attached to rocks in rivers with strong currents, and have a life cycle tightly linked to seasonality. Due to its reduced floral structures, which characterizes most species, the analysis of developmental stages and events such as anthesis or stigma receptivity has proven difficult. Question: How floral morphology, floral development and stigma receptivity of different stages of flower development of Marathrum rubrum Novelo & C. T. Philbrick are related among them? Studied species: Marathrum rubrum Novelo & C. T. Philbrick. Study site and dates: Horcones River, Jalisco, Mexico. January, 2014 and 2015. Methods: We made an in situ and lab analyses of flower morphology to assess the relationships between flower morphology, stages of development, reproductive structures, and stigma receptivity, during different stages of flower development. Results: When flowers emerge from the spathella, both the ovules and pollen grains are fully developed, implying that sporogenesis and gametogenesis take place within the stem while the plant is still submerged. Conclusions: The morphological changes observed in the flowers of M. rubrum during the analyzed stages are related only to events associated with fertilization and fruit development. The time-course of morphological changes in the flowers, from anthesis to fruit formation, lasts 20 to 30 days. The detailed observation of the aerial reproductive biology of M. rubrum bears important implications to the genetic structure of populations, plant fitness and conservation biology in threatened environments.