Pollen morphology of Lippia L. sect. Rhodolippia Schauer (Verbenaceae)

Lippia is the second largest genus in the family Verbenaceae, comprising about 100 species widely distributed in the Neotropics and some representatives in Africa. The genus is considered poorly delimited taxonomically because many species can only be distinguished by minor morphological differences, resulting in a large number of described taxa. This study aimed to examine and describe the pollen characters of Lippia sect. Rhodolippia and identify important pollen attributes for the delimitation of species occurring in Brazil. Multivariate analysis was used to investigate similarities between species and find diagnostic characters for taxonomic identification. Pollen grains of 16 species were acetolyzed, measured, described, illustrated using a light microscope (LM) and submitted to statistical treatments. For a more detailed examination of pollen surfaces and apertures, non-acetolyzed pollen grains were observed and micrographed by scanning electron microscopy (SEM). The results showed that species belonging to the section Rhodolippia have pollen grains that are prolate spheroidal or oblate spheroidal, 3-colporate (3-4-colporate in L. felippei) or 3-colpororate, mesoaperture present or absent, endoaperture lalongate, with ends bifurcated, tapered, rounded, H-shaped or truncated. The exine ornamentation is psilate-perforate in most species. Principal component analysis and hierarchical clustering identified two groups: 3-colporate and 3-colpororate pollen grains. Palynological analysis evidenced the eurypollinic character of the genus and resulted in the first description of mesoaperture and the type of ends of the endoaperture. These findings can provide taxonomic and phylogenetic support for future studies on Lippia.