Leaf Secretory Structures in Asteraceae: A Synthesis of Their Diversity and Evolution

This study presents the first comparative analysis of the leaf secretory structures across Asteraceae. In this work, the leaf secretory structures of more than 500 species of 35 of the 40 tribes and 11 of the 13 subfamilies of Asteraceae are described and compared to evaluate their diversity at the tribe level and to identify evolutionary patterns. Leaf secretory structures are present in 28 of the 35 analyzed tribes and correspond to canals (recorded in 17 tribes), secretory cavities (1 tribe), hydathodes (19 tribes), laticifers (4 tribes) and glandular trichomes (24 tribes). Canals are mostly associated with vascular bundles and predominate in Asteroideae, while cavities were only present within Tageteae. Hydathodes occur in leaves without divisions and with well-developed teeth. Laticifers were observed only in the tribes of Cichorioideae. Seven glandular trichome morphotypes were differentiated by their cellular composition and shape. These observations together with the available information showed that secretory structures are found in 80% of the Asteraceae tribes. Four of the 40 tribes did not present any type of secretory structure. Our study reveals that almost all of the tribes possess one to three types of secretory structures, and are absent in some early-diverging clades. Character evolution analyses show that glandular trichomes are plesiomorphic in Asteraceae. This study found that secretory structures prevail in late-diverging lineages and were taxonomically informative at different levels. Our comparative study of the secretory structures in Asteraceae is essential for the standardization of its terminology and will provide a frame of reference for future studies.