Atributos estruturais e histoquímicos de ductos secretores e cavidades: Uma abordagem funcional, ecológica e taxonômica em savanas amazônicas

Abstract

Amazonian savannas are open tropical phytophysiognomies, subject to high levels of solar radiation, precipitation, sandy soils with low availability of nutrients, and a prolonged period of drought. The anatomical attributes of its plants have been studied; however, information about the secretory system of these plants is scarce. In this sense, an intense investigation was carried out into the occurrence of possible secretory structures in 76 woody species belonging to 29 botanical families in two Amazonian savannas in the extreme north of the state of Mato Grosso. Then two families were selected as objects of study, being Calophyllaceae and Anacardiaceae. Calophyllaceae and Anacardiaceae are essential representatives of different phytophysiognomies of the Amazon and Cerrado. Both families are known for producing bioactive substances. In Calophyllaceae, these substances can be produced in specialized structures called secretory ducts and less frequently by cavities, while in Anacardiaceae, they can be produced mainly by ducts. This work aimed to characterize anatomical, histochemical, and ultrastructural aspects of secretory ducts and cavities in leaves of four species of Calophyllaceae, Kielmeyera rubriflora Cambess, Kielmeyera coriacea Mart. and Zucc, Calophyllum brasiliense Cambess and Caraipa densifolia Mart. and secretory ducts in three species of Anacardiaceae, Anacardium occidentale L., Anacardium giganteum W. Hancock ex Engl., and Tapirira obtusa (Benth.) J. D. Mitch. in two Amazonian savannas, to analyze functional, ecological, and taxonomic aspects. Samples of the botanical material were processed according to usual techniques for plant anatomy and ultrastructure. The results show that the type of secretory structure, distribution pattern, quantity, size, and shape of the lumen are different. These attributes can be used as diagnostic characters to delimit the species. In most species, secretion metabolites are similar and consist of phenolic substances, lipids, oleoresin, mucilage, polysaccharides, proteins, alkaloids, except in K. coriacea, which does not contain oleoresin. The production of these metabolites by the two secretory structures reveals possible plant-microbe, plantinsect, plant-plant, and plant-animal interactions. The exit of secretory activity products to the periplasmic space is mediated by the eccrine, granulocrine, and holocrine mechanisms. We associate that the soil properties, the high levels of solar radiation, temperature, the proliferation of microorganisms, herbivores, and the evolutionary history of the savannas are indicative of environmental filters that contributed the most for the species to invest in physical and chemical defense. In Anacardiaceae, A. occidentale invested more in the anatomical organization of the ducts. This study provides new and helpful information to understand the anatomy, functioning, and ecological and taxonomic role of ducts and cavities in Amazonian savanna species and represents the first work with secretory structures in Amazonian savannas.