Orbicules in south American Balanophoraceae: Analysis of its possible role and its taxonomic value in the family

Abstract

Balanophoraceae are root parasites, lacking both chlorophyll and typical cormophytic organization. There have been few studies of pollen dispersal mechanisms in Balanophoraceae, and no studies on structures accompanying pollen, such as orbicules. Anthers were analyzed in five South American taxa of Balanophoraceae (Langsdorffia hypogaea, Lophophytum pyramidale, L. mirabile. subsp. bolivianum, Ombrophytum subterraneum and Helosis cayennensis), in order to: (1) determine the presence of orbicules, characterize them and relate their presence with the tapetum type, characteristics of the anther wall and dehiscence, and the pollination mode; (2) and analyze its possible taxonomic value in relation to the phylogeny of the family Balanophoraceae. In three genera (Lophophytum, Langsdorffia and Ombrophytum), the anthers were found to be dithecous, tetrasporangiate with longitudinal dehiscence. In these taxa the endothecium is present and is responsible for dehiscing the anthers. In Helosis the pollen sacs are welded in a synandrium with apical groves and lacking an endothecium. In all species, a secretory tapetum is responsible for nourishing the pollen grains during their formation. The presence of orbicules is variable among the species studied: it is found in Lophophytum and Langsdorffia, and is absent in Ombrophytum and Helosis. In Lophophytum the orbicules appear as small and spheroidal corpuscles with smooth surfaces. In Langsdorffia hypogaea the orbicules are conspicuous spherical, ellipsoid to ovoid forms. This study shows that there is no relationship between layers present in mature anthers, dehiscence type, pollination mode and orbicules presence. We report for the first time the presence of orbicules in the holoparasitic family Balanophoraceae belonging to the order Santalales, in which there is scarce information available regarding the presence of orbicules. Given the intergeneric variations and the consistency at generic level observed in the taxa here analyzed, orbicules may have predictive value and potential for systematically oriented research questions.