Systematic Significance of Pollen Morphology of the Genus Acmella Rich. (heliantheae: Asteraceae)

Pollen pattern plays an important role in discriminate and definite infraspecific as well as interspecific diversity for species recognition. In this study, we analyzed pollen morphology of the 86 accessions under nine currently recognized taxa of Acmella to estimate our current understanding of the phylogeny of this genus. Species relationship can be evaluated on the basis of pollen characteristics. The study shows the significance of palynological characteristics such as aperture type, spine length and nature, stature of basal cushion and sculpturing pattern in the delimitation of taxa in the interspecific levels. Pollen grains of these species are isopolar, zonocolporate pollen and are radially symmetric. Shape is more or less circular in polar view, with the colpi slightly intruding, from circular to slightly elliptical in equatorial view. But the number of colpus and ora varies. Acmella calva, A. uliginosa and A. uliginosa Var. pentamera have 3-zonocolporate pollen, A. ciliata, A. ghoshinis, A. radicans and A. tetralobata have 2, 3, 4 zonocolporate pollen and A. vazhachalensis and A. paniculata have 2, 3, 4, 5 zonocolporate pollen. Pollen shape varied from oblate spheroidal to prolate spheroidal. Exine is thick, 2.1–5.2 µm normally. Ornamentation is spinate; 1.16–5.46 µm long, spine apex shape varies in each species. Scanning electron microscopic studies showed differences in the spine length, spine structure and attachment. We used statistical analysis to explore variations in pollen grain size and shape among Acmella species, and to compare systematic groupings of Acmella based on pollen characters.