Pollen morphology of European bladderworts (Utricularia L., Lentibulariaceae)

The pollen morphology of the seven known European bladderworts: Utricularia australis, Utricularia bremii, Utricularia intermedia, Utricularia minor, Utricularia ochroleuca, Utricularia stygia and Utricularia vulgaris was studied. Their pollen grains, coming from different populations, were investigated using both light microscopy and scanning electron microscopy to give data on size, shape (P/E ratio), number of colpori and exine ornamentation: important diagnostic characteristics for Utricularia pollen. Within the investigated species, the pollen grains were usually medium sized (~30 μm), sub-isopolar, radially symmetric and zonocolporate. For the non-fruiting species Utricularia bremii, Utricularia stygia and Utricularia ochroleuca, the grains were often malformed, asymmetric or in the form of gigapollen or micropollen. A significant number of gigapollen grains were observed in Utricularia stygia while micropollen was observed in Utricularia ochroleuca. The shape of the normal grains was variable from suboblate to prolate spheroidal and they were (10)-11-18-(19)-zonocolporate. The prevalent ornamentations were psilate (on mesocolpi) and fossulate (on apocolpium) except for Utricularia bremii, which had a somewhat perforate ornamentation. The pollen of Utricularia stygia is described here for the first time. A pollen key, based on these micromorphological data, is presented for European Utricularia species.