Comparative pollen morphology of the Kirengeshoma (Hydrangeaceae)

Abstract The genus Kirengeshoma, in the family Hydrangeaceae, is considered by some botanists to be monotypic while others divide it into two species, K. palmata, native to Japan and China, and K. koreana, native to South Korea. We have conducted a comparative pollen morphological study of five populations of the two putative species to investigate this question. Pollen morphological characteristics were observed with scanning and transmission electron microscopy. The pollen characteristics of all five populations were similar. All populations dispersed pollen as the monad and the pollen grain shape were either spheroidal or sub-spheroidal and approximately 12 to 18 µm in diameter, falling in the small grain category. In all five populations, the pollen grains were iso-polar and radio-symmetrical, with tri-colporate apertures. Additionally, pollen surface sculpture patterns of all populations were semi-tectate and reticulate. As a result, the two putative species differed in that the pollen grains of K. palmata were smaller, and their endexine slightly thicker, than those of K. koreana. Despite these differences, we conclude that the similar pollen morphology of all the populations does not support separating them into two independent species. However, different species of angiosperms often have similar pollen morphology. Therefore, we suggest that the genus should be considered to consist of these two independent species unless further morphological or molecular evidence to the contrary is forthcoming.