Morphology and ultrastructure of orbicules in the subfamily Ixoroideae (Rubiaceae)

Orbicules were studied in 43 species belonging to 32 genera of the five tribes of the Ixoroideae (Rubiaceae). The orbicules were investigated with scanning electron microscopy (SEM), light microscopy (LM), and transmission electron microscopy (TEM). Orbicules are present in all species investigated of the tribes Pavetteae, Octotropideae and Coffeeae. In the tribes Gardenieae and Aulacocalyceae orbicules were found in some species, while they were absent in others. 15 species out of 11 genera lack orbicules. Three orbicule types (III, V, and VI) could be distinguished, mainly on the basis of general morphological and ultrastructural variations. Orbicules that belong to type III (0.50-1.29µm) have perforations in their wall, a regular or irregular shape and two or three electron transparent cores. This orbicule type, exclusively found in all Pavetteae species investigated, can be divided into two subtypes. Orbicules of subtype IIIa are present in all genera related to Ixora, and orbicules of subtype IIIb in those genera related to Tarenna. Orbicules of type V (0.97-1.86µm) are present in Himalrandia tetrasperma (Aulacocalyceae), and in all Octotropideae genera investigated, except Feretia. Complexes of more than three individual orbicules characterize this type. They are irregularly shaped and have many perforations as well as sporopollenin granules on the orbicule wall. In all species investigated of tribe Coffeeae, type VI orbicules (0.56-1.60µm) are present. These orbicules are characterized by their embedded position towards the tapetal membrane, their aggregated form and by the presence of perforations as well as sporopollenin granules on their orbicule wall. In the tribe Gardenieae different types of orbicule were found (V, VI and orbicules that cannot be classified in our typology). Our results suggest that orbicule characters in the Ixoroideae may be systematically useful on tribal level.