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Premise of research. Tristyly is a floral polymorphism that is reliably reported from six flowering plant families. Populations of tristylous species often contain three floral morphs that differ reciprocally in stigma and anther height. Trimorphic incompatibility and a suite of ancillary pollen and stigma polymorphisms are commonly associated with sex organ trimorphism. Here, we investigate the tristylous syndrome of Pontederia subovata (Pontederiaceae), a little-known aquatic from southwest Brazil. Methodology. We conducted measurements of sex organ position, pollen size, pollen production, and stigmatic papillae length on the three floral morphs under glasshouse conditions and carried out a controlled pollination program to investigate whether P. subovata possesses a trimorphic incompatibility system and whether its expression varies among the floral morphs. Pivotal results. Pontederia subovata displays reciprocal stigma and anther heights and trimorphism in the size of pollen grains and the length of stigmatic papillae. Controlled pollinations provided evidence of trimorphic incompatibility: self- and intramorph crosses yielded significantly less seed than do intermorph crosses when pollen from anthers and stigmas of equivalent height were employed. However, trimorphic incompatibility was weakly expressed in the mid-styled morph when pollinations involved long-level anthers. The occurrence of stem-borne tubers is reported for the first time from Pontederiaceae. Conclusions. Our results confirm the presence of tristyly in P. subovata. The species has several features in common with other tristylous members of Pontederia, including striking variation in the strength of incompatibility among the floral morphs. We suggest that weak incompatibility in the mid-styled morph may provide reproductive assurance during colonization. Tuber formation in P. subovata probably evolved as an adaptation enabling persistence of genets during the marked dry season that characterizes the aquatic habitats in which the species occurs. [ABSTRACT FROM AUTHOR]