Pollen morphological traits analysis in the genus Hymenocrater Fisch. & C. A. Mey., (Lamiaceae)

Abstract

Hymenocrater Fisch. & C. A. Mey., a perennial genus of the Lamiaceae family, is mainly distributed in Iran. These aromatic plants produce several biologically important secondary metabolites. Although several investigations have been conducted on the genus, no inclusive palynological study is available for these species. In this evaluation, we studied pollen grain morphological traits in 10 Hymenocrater species, of which five are endemic to Iran. The pollen grains were acetolyzed according to the Erdtman method, and up to 25 pollen grains were examined per species using scanning electron microscopy and light microscopy. Our findings revealed that all species had the 6-zonocolpate monad pollen grains, except for a species that generate octocolpate pollen grains. Pollen size varied from large (in most species) to small. Pollen grains' shape differed as prolate, subprolate or spheroidal. The equatorial outlines were detected as truncate, obtuse convex or rarely circular. The outlines in ambitus were obtuse convex or circular. Moreover, most species exhibited a bireticulate exine surface sculpturing pattern and rarely reticulate. According to UPGMA clustering, these species were clustered into four groups; PCA biplot revealed that some species were characterized by the diagnostic palynological characteristics. However, the species clustering pattern did not agree with those which were detected by phylogenetic, anatomy and micromorphological data in the previous studies. Our findings indicated that, among the evaluated characteristics, the pollen grains shape and their ambitus were more variable than others and had a limited taxonomic value for the genus. However, other evaluated characteristics lack a taxonomic importance in species delamination or clustering due to their infraspecific variabilities or stability among the species.