Revisiting the floral structure and ontogeny of Trapa natans L. (Lythraceae)

Abstract

As a result of recent phylogenetic researches, water chestnut (Trapa, former Trapaceae) was found nested in Lythraceae. This work aims to review the existing publications on floral morphology and ontogeny of Trapa and to revisit these topics with the scanning electron microscopy. Although previously described as tetramerous, flowers of Trapa are disymmetric with two dimerous whorls of sepals, four petals, four stamens in two dimerous whorls, and a semi-inferior ovary composed of two carpels. Sepals initiate first and are successively followed by stamens, petals and carpels. Flowers are borne in the axils of vegetative leaves, and no terminal flower is present in the rosette. Before and after anthesis, flowers develop underwater. As the nearest relatives of Trapa are the tropical genera Lagerstroemia, Sonneratia and Duabanga, floral morphology of the water chestnut is unique for its subclade. A lower flower merism, reduced inflorescences and some peculiarities of flowering biology of Trapa evolved as a part of a complex syndrome of aquatic plants with floating rosettes of leaves.