Structural similarity versus secretion composition in colleters of congeneric species of Prepusa (Gentianaceae)

Although the Atlantic Forest is considered a humid environment, plants that grow on rock outcrops present adaptive strategies, which correlate with tolerance to desiccation. Colleters are secretory structures that protect meristems and developing organs from dehydration and attack by herbivores or pathogens. Colleters are common in Gentianaceae; however, there is little research from an anatomical perspective. This study evaluated three Prepusa species (P. connata, P. hookeriana, and P. viridiflora) that are endemic to rock outcrops associated with the Atlantic Forest to investigate the distribution, anatomy, and histochemical composition of secretion of the their colleter throughout leaf growth and maturation. Samples of young, mature, and senescent leaf bases were collected, fixed, and processed following routine techniques in anatomical and micromorphological studies. In the three species, the colleters are located at the base of the leaves, on the adaxial surface. They are non-vascularized and have a short peduncle and a multicellular secretory portion coated by the cuticle. They develop asynchronously, as they are active in young leaves, and cease their activity in fully expanded leaves, but they are not deciduous. They have intercellular spaces, in which polysaccharide and protein secretions accumulate. Moreover, lipids were observed only in P. hookeriana and P. connata. Phenolic compounds are common and occur during colleter senescence, which is characterized by total cell collapse. Even though this study examined congeneric species from similar environments, the results confirm alterations in secretion composition and structural and chemical changes throughout leaf growth and maturation.