Heterostyly and pollen dimorphism of Menyanthes trifoliata

Menyanthes trifoliata L., a heterostyly plant, is an endangered species in Korea. Floral morphology has been published for M. trifoliata, but few works have concentrated on pollen dimorphism differences accompanied by floral dimorphism. Here, we conducted a comparative morphological study of two morphs to investigate a dimorphism system with Korean populations. Pollen morphological characteristics were observed with a light microscope and a scanning electron microscope. For the pin type, pollen size is 36.01 ± 2.01 µm, whereas for the thrum type it is 41.28 ± 2.58 µm in terms of the equatorial diameter. The two morphs have a small apocolpium field at 5.62 ± 0.30 for the pin type and 6.24 ± 0.70 for the thrum type. The configuration of the aperture was tricolpate with a striate ridge in the two morphs. However, they have different pollen shapes and sizes, stigma shapes, and apocolpium sizes. M. trifoliata only has different pollen sizes and shapes between European populations and Korean populations. Nevertheless, Korean populations also show pollen dimorphism correlated with their floral dimorphism.