Pollen morphology and its systematic and ecological significance in Rheum (Polygonaceae) from China

Pollen morphology of 40 species of Rheum , belonging to eight sections, was investigated under LM and SEM. Four new exine patterns were found in the species: a) microechinate-foveolate, b) rugulate, c) verrucate-perforate, and d) verrucate-rugulate ornamentation. In addition, two patterns, the Rheum-type pollens with microechinate-perforate and fine-reticulate, as previously described, were also confirmed in the present study. Based on above study the evolution trends of pollen morphology in the taxa involved were discussed phylogenetically as below. As microechinate-perforate exine pattern existed commonly, the pattern is, therefore, regarded as the most primitive among all the six types. The fine-reticulate type was thought as a derivative type, deriving from the basic microechinate-foveolate-perforate pattern, and followed by the rugulate and verrucate-perforate ornamentation. The verrucate-rugulate ornamentation should be the most advanced. More than one pollen type often exist in most of the sections in Rheum. The pollen morphology of Rheum was strongly correlated with its geographical and ecological distribution. Three medicinally important species R. officinale, R. palmatum and R. tanguticum can be palynologically distinguished by their ornamentations.