Phylogenetic Relatedness Determines Heterospecific Pollen–Pistil Compatibility and Reproductive Outcome in the Apocarpous Species <i>Sagittaria trifolia</i> (Alismataceae)

Pollinator sharing among coflowering plants can reduce plant fitness through heterospecific pollen (HP) deposition. Compatible HP is expected to cause stronger reproductive interference, but the factors determining HP-pistil compatibility and its reproductive consequences remain poorly understood. Moreover, in some apocarpous taxa, the extragynoecial compitum, a basal pore formed by the incomplete fusion of carpel margins, can enhance reproductive assurance by facilitating conspecific pollen (CP) tube reallocation across free pistils. What remains unclear is how these taxa balance the trade-off between this benefit and the potential risk of reproductive interference from intercarpellary heterospecific pollen (HP) tube growth. In this study, widely distributed apocarpous <i>Sagittaria trifolia</i> with typical extragynoecial compitum was selected as a pollen recipient. Hand-pollination experiments were performed with 42 distributed and coflowering HP donors to assess pollen-pistil interaction and HP effects on seed quantity. Results showed that HP-pistil compatibility was independent of species origin (native/alien), pollen size, and aperture number of the HP donor. Instead, it was negatively correlated with phylogenetic distance: closely related HP exhibited higher compatibility with the pistil and caused greater reductions in seed set. Nevertheless, CP consistently sired over 78% of seeds whether applied simultaneously with or after HP. In the half-and-half pollination treatments with CP and compatible HP, CP sired significantly more than 50% of seeds. These indicated that CP advantage and intercarpellary CP tube growth collectively mitigate interspecific pollen interference. However, these compensatory mechanisms were less effective when faced with alien congeners. Our results provide a good basis for understanding the variation in HP-mediated fitness costs and shed light on how apocarpous lineages with extragynoecial compitum adaptively tolerate HP interference and maintain reproductive success.