Secretory ducts in Anacardiaceae revisited: Updated concepts and new findings based on histochemical evidence

Abstract

Secretory ducts in vegetative and reproductive organs of Anacardiaceae have long been documented. However, despite the numerous studies on the anatomy and ultrastructure of these ducts, our knowledge about a possible diversity of duct metabolism in a same plant is scarce. In this study, we aimed to revisit the structure, distribution and metabolic diversity of ducts in vegetative and reproductive organs of Anacardiaceae. The distribution and anatomy of the secretory ducts in the stem, leaves, flowers and fruits of four species of Anacardiaceae were studied and their secretions histochemically characterized. Differences in the composition of the secretion according to the distribution of the ducts were detected, and more than one type of duct can occur in the same organ. In vegetative organs all the phloem ducts produce a complex, mixed resin mostly composed of lipophilic compounds (resin sensu lato). Medullary ducts of Anacardium and Spondias produce only gum, while in Tapirira they have a mixed resin. In reproductive organs, the secretion has a similar composition to the vegetative organs in each species, except in the fruits of Anacardium, where the ducts produce only lipids (resin sensu stricto). The general morphology of the ducts and the production of resins are conservative not only in Anacardiaceae, but also in other Sapindalean families. Our findings demonstrated for the first time in Anacardiaceae the presence of two kinds of ducts, which produce different classes of compounds depending on their origin and distribution. Earlier descriptions referring Anacardiaceae gum-resin and/or latex ducts are in the current article denominate as resin ducts sensu lato.