Integrated ultrastructural and morpho-anatomical analysis of stigma and style tissues in selected Alismataceae species (Alismatales).

Despite the marked diversity of carpel closure, stigmatic surfaces, and stylar configurations documented in Alismataceae, no ultrastructural analyses of these tissues have been conducted for the family. Here, we present the first integrated morphological, anatomical, and ultrastructural study of the stigma and style in four species representing the major clades of Alismataceae: Hydrocleys nymphoides, Alisma plantago-aquatica, Sagittaria montevidensis, and Echinodorus grandiflorus. Using light microscopy, SEM, and TEM, we found marked differences in both stigma and style organization among species. Three species exhibited papillose stigmas, whereas E. grandiflorus presented a non-papillose receptive surface. H. nymphoides showed an open style with a well-developed stylar canal, whereas the remaining species exhibited closed styles with reduced or absent transmitting tissue. These structural differences are associated with contrasting secretory patterns, organelle composition, and inferred pollen tube pathways, highlighting clear distinctions between open and closed stylar systems. Our results reveal substantial intrafamilial variation in reproductive structures and provide a framework for interpreting stigma and style characters within the phylogeny of Alismataceae. These findings also contribute to a broader understanding of gynoecium evolution in early-diverging monocots.