Palinologia e taxonomia de espécies de Praxelis Cass. (subtribo Praxelinae, Eupatorieae-Asteraceae) ocorrentes no Brasil

Abstract

Asteraceae is the second largest family of flowering plants with ca. 25.000 species distributed around the world. Praxelis is the second largest genus in number of species of Praxelinae subtribe, is characterized mainly by presenting cylindrical/campanulate heads, conical receptacle and glabrous, cypselae with asymmetrical carpopodium and pappus with numerous bristles and capillary. In Brazil, the predominant species in the Cerrado. The objective of this study was analyzed the palynology and taxonomy of species Praxelis Cass. occurring in Brazil. The plant material used was obtained from herbarium specimens deposited in Brazilian herbaria. The pollen grains were acetolysed, measured, described and illustrated using LM. For further details of surface and aperture, pollen grains not acetolysed were analyzed in SEM, and subsequently electron micrograph. The floral parts were placed on the same metal supports covered with carbon double-sided tape, previously numbered. For analysis in TEM, pollen grains were stained with 0,125% cacodylate buffered OsO4, focused on agar pads, and were put in Epon-Araldite resin. The leaves, the heads, the stalk trichomes and the number of bracts were involucral relevant attributes to characterize the species, particularly the sheath size and consistency. Pollen grains were characterized as the form, size, type of opening, the polarity and the formation of exine. They have been described as: small to medium isopolar, the oblate spheroidal subprolate, 3(4)-colporated with ornamented margin endoaperture with or without constriction, with the presence of cavea, and prickly the sexine is always greater than nexin. Therefore, the pollen analysis and taxonomic were used to segregate the species Praxelis, but only the external morphological attributes were more informative to differentiate the species of this genus.