Pollen morphology of the tribe Sorbarieae (Rosaceae)

The pollen morphology of the tribe Sorbarieae (Adenostoma, Chamaebatiaria, orbaria, and Spiraeanthus) and two related genera Gillenia and Lyonothamnus was investigated by light, scanning electron, and transmission electron microscopy. Sorbarieae pollen was monad, tri-colporate, small to medium in size (P = 10.2-40.0 μm, E = 10.8-32.4 μm), and oblate to prolate in shape (PIE = 0.74-1.88). The sexine ornamentation was striate, but four different types could be divided: striate-psilate, striate-plicate, striate-microechinate, and striate-perforate. The pollen wall stratification typically consisted of unbranched columellae and a continuous endexine. As an additional palynological characteristic, orbicules (small sporopollenin granules) were observed in all taxa and thus a possible synapomorphic character of Sorbarieae as a whole. The pollen characteristics are useful to recognize certain taxa. Gillenia is the only genus with both a pore flap on the aperture and the perforated sexine. Lyonothamnus has a significantly thick exine and relatively larger pollen grains, which is compared to that of Sorbarieae. Some quantitative characteristics (e.g., pollen and colpus size, exine thickness, and ridge width of striae) identified using principal components analysis (PCA) may have diagnostic importance among the taxa in the tribe.