The genus Sorbus s.l. consists of small to medium-sized trees primarily found in coastal areas, with three European hotspots of diversification: Fennoscandia, South-East Europe, and the British Isles. Taxonomic classification of Sorbus s.l. in Europe is still unresolved, and this study aims to address this issue by examining the pollen morphology of various Sorbus hybrids. Pollen morphology of 16 specimens from 13 species of Sorbus s.str., Aria and Hedlundia, some of them common, others are endemic and listed as threatened species, were studied. Measurements of the polar and equatorial axis of all pollen grains show that there are variations in pollen size and shape among different hybrids, with a correlation between ploidy levels and the equatorial diameter of the pollen. Surface structures of the pollen were observed using SEM. Principal Component Analysis (PCA) was performed to analyze the pollen characters, and the results show that it is possible to separate Aria from Sorbus and Hedlundia based on the length of the polar axis of the pollen grain, while the number of perforations is important for classification within Hedlundia. The study concludes that pollen morphology can be a useful tool for distinguishing between different Sorbus hybrids and can thus aid in understanding their past distributions.

Specimens were collected from various parts of Norway in 2019, either from native stands or from cultivated plants of known origin (Figure 1, Table1). Inflorescences were collected and dried of samples of Aria edulis (Wild.) M.J. Roemer, Aria obtusifolia (DC.) Sennikov & Kurtto, Aria rupicola (T.C.G. Rich) Sennikov & Kurtto, Hedlundia hybrida 1 (L.) Sennikov & Kurtto, Hedlundia hybrida 2 (L.) Sennikov & Kurtto, Hedlundia lancifolia (Hedl.) Sennikov & Kurtto, Hedlundia meinichii 1 (Lindeb. ex Hartman) Sennikov & Kurtto, Hedlundia meinichii 2 (Lindeb. ex Hartman) Sennikov & Kurtto, Hedlundia mougeotii (Soyer-Willemet & Godron) Sennikov & Kurtto, Hedlundia neglecta (Hedl.) Sennikov & Kurtto, Hedlundia sognensis (Hedl. ex Sennikov, Hjertson & Salvesen) Sennikov & Kurtto, Hedlundia subarranensis (Hyl. ex Sennikov, Hjertson & Salvesen) Sennikov & Kurtto, Hedlundia subpinnata 1 (Hedl.) Sennikov & Kurtto, Hedlundia subpinnata 2 (Hedl.) Sennikov & Kurtto, Hedlundia subsimilis (Hedl.) Sennikov & Kurtto, and Sorbus aucuparia L.

Anthers were treated with 10% KOH to remove cell contents, followed by acetolysis to remove cellulose (Fægri et al., 1989). The pollen was stained using fuchsine and mounted in glycerol. Using a light microscope (LM) the polar axis (P) and equatorial axis (E) of 100 pollen grains of each specimen were measured.