Internal secretory spaces in thickened underground systems of Asteraceae species

Secretory structures are present in many vascular plants and have an important ecological role as a plant defence mechanism against herbivors and pathogens. Internal secretory spaces of lipid substances are widespread in the Asteraceae. However, information about the occurrence of these structures in thickened underground systems is sparse, compared with what we know about aerial systems. The main objective of the present paper was to investigate the occurrence, formation and chemical nature of the secretory structures in six Asteraceae species belonging to the following tribes: Eupatorieae (Mikania cordifolia and M. sessilifolia), Mutisiae (Trixis nobilis), Plucheeae (Pterocaulon alopecuroides) and Vernonieae (Vernonia elegans and V. megapotamica). The samples were collected in areas of Cerrado ( tropical savanna) in the state of Sao Paulo, Brazil. The secretory structures found were cortical canals in roots ( T. nobilis, P. alopecuroides, V. elegans and V. megapotamica), cortical cavities in roots ( M. cordifolia, M. sessilifolia and P. alopecuroides), cavities in the secondary phloem of roots ( T. nobilis), cortical cavities in the xylopodium ( M. cordifolia, M. sessilifolia, P. alopecuroides and V. megapotamica) and in the underground stem ( T. nobilis), and canals in the secondary xylem in the xylopodium ( M. cordifolia and M. sessilifolia). Histochemical tests showed the presence of lipid substances in all structures.