Secretory ducts in Anacardiaceae leaves: Their ecological role in plant-environment interactions across three amazonian savanna species

Abstract

This study contributes significantly to the anatomical and biological understanding of secretory ducts in the leaves of three Anacardiaceae species in Amazonian savannas. The lack of prior data in this context motivated us to investigate the structural and histochemical features of these ducts in Anacardium occidentale L., Anacardium giganteum W. Hancock ex Engl., and Tapirira obtusa (Benth.) J. D. Mitch., to advance our knowledge of their development and potential ecological functions. The aim was to interpret and discuss functional, ecological, and taxonomic features of these species. Samples of botanical material were processed using conventional techniques of light, scanning, and transmission electron microscopy. The three species analyzed exhibit significant differences in the structure and distribution of the ducts, as well as in the physical characteristics of their secretions. These differences may potentially serve as diagnostic characters. Duct size and length increase through lateral coalescence and anastomosis. Phloem ducts primarily accumulate lipophilic substances, whereas the ducts located in the parenchyma surrounding the vascular tissues of the midvein, here termed the midvein parenchyma, specialize in the accumulation of hydrophilic substances. The epithelium and sheath can be uniseriate or multiseriate. Histochemical tests revealed that the metabolites protect leaves against desiccation, herbivore attacks, pathogens, physical injuries, solar radiation, and local seasonality. Epithelial cells are organelle-poor and serve as sites for synthesis and secretion accumulation. The secretion mechanism is eccrine, granulocrine, and holocrine. These novel insights represent a significant structural, functional, and ecological contribution to plants in tropical phytogeography formations.