Morphological and ultrastructural diversity of orbicules in Gentianaceae.

Minute granules of sporopollenin, called orbicules, can be observed on the innermost tangential and/or radial walls of secretory tapetum cells. Orbicules were investigated in 53 species of 34 Gentianaceae genera using light microscopy, scanning electron microscopy and transmission electron microscopy. This selection covered all different tribes and subtribes recognized in Gentianaceae (87 genera, +/-1650 species). Orbicules were found in 38 species (23 genera) distributed among the six tribes recognized in Gentianaceae. The orbicule typology is based on those described previously in Rubiaceae. Of the six orbicule types described previously, Type II orbicules are lacking. Type III orbicules are most common (17 species). Hockinia Gardner is the only representative with Type I orbicules. The number of representatives with orbicules belonging to the other orbicule types are equally distributed among the species studied: seven species possess Type IV orbicules, six species Type V and six species Type VI. The systematic usefulness of this typology is discussed in comparison with the latest systematic insights within the family, and palynological trends in Gentianaceae. Orbicule data have proven to be useful for evaluating tribal delimitation within Rubiaceae and Loganiaceae s.l.; however, they seem not to be useful for tribal delimitation in Gentianaceae. In the tribes Potalieae and Gentianeae orbicule data may be useful at subtribal level.