Summary: Calcium oxalate (CaOx) crystals occur as intravacuolar deposits in most angiosperm species. Different functions have been attributed to these crystals, some of which are very speculative, until now. Calcium regulation and homeostasis seem to be the most widespread function of CaOx crystals. Being rich in calcium, these crystals constitute a reserve of calcium for plants. However, despite being bioavailable, this reserve is functional in just a few situations due to the low mobility of calcium for phloem translocation. Therefore, CaOx crystals as a calcium reserve is a paradox because in most cases the reserve cannot be used. However, in most plants, these crystals occur in organs or tissues that will be discarded, which allows the elimination of excess calcium. This suggests that CaOx crystals have a functional role in excess calcium excretion. There is some evidence that, for calcium, this excretory function is relevant for plants since they lack an excretory system dedicated to discarding solid wastes, such as calcium salts.

Background In plants, the products of secretory activity leave the protoplast and cross the plasma membrane by means of transporters, fusion with membranous vesicles or, less commonly, as result of disintegration of the cell. These mechanisms do not address an intriguing question: How do secretory products cross the cell wall? Furthermore, how do these substances reach the external surface of the plant body? Such diverse substances as oils, polysaccharides or nectar are forced to cross the cell wall and, in fact, do so. How are chemical materials that are repelled by the cell wall or that are sufficiently viscous to not cross passively released from plant cells?

Considering that few studies on nectary anatomy and ultrastructure are available for chiropterophilous flowers and the importance of Hymenaea stigonocarpa in natural ‘cerrado’ communities, the present study sought to analyse the structure and cellular modifications that take place within its nectaries during the different stages of floral development, with special emphasis on plastid dynamics. For the structural and ultrastructural studies the nectary was processed as per usual techniques and studied under light, scanning and transmission electron microscopy. Histochemical tests were employed to identify the main metabolites on nectary tissue and secretion samples. The floral nectary consists of the inner epidermis of the hypanthium and vascularized parenchyma. Some evidence indicates that the nectar release occurs via the stomata. The high populations of mitochondria, and their juxtaposition with amyloplasts, seem to be related to energy needs for starch hydrolysis. Among the alterations observed during the secretory phase, the reduction in the plastid stromatic density and starch grain size are highlighted. When the secretory stage begins, the plastid envelope disappears and a new membrane is formed, enclosing this region and giving rise to new vacuoles. After the secretory stage, cellular structures named ‘extrastomatic bodies’ were observed and seem to be related to the nectar resorption. Starch hydrolysis contributes to nectar formation, in addition to the photosynthates derived directly from the phloem. In these nectaries, the secretion is an energy-requiring process. During the secretion stage, some plastids show starch grain hydrolysis and membrane rupture, and it was observed that the region previously occupied by this organelle continued to be reasonably well defined, and gave rise to new vacuoles. The extrastomatic bodies appear to be related to the resorption of uncollected nectar.

Reports concerning colleters in Fabaceae have been scarce, mainly in the Caesalpinioideae subfamily. The present work reports the occurrence, structure, and functional aspects of the colleters of Copaifera langsdorffii . Shoot apices and developing leaves were fixed and processed for examination by light and electron microscopy. Secretion samples were studied to determine their chemical nature and physical properties. The colleters are clavate and occur on the adaxial face of the stipules, petiole and rachis. The secretory stage of the colleters occurs during the leaf expansion, after which these structures turn brown and senesce. The secretion is composed of highly hygroscopic acidic polysaccharides and lipids. The colleters are composed of cells with thin walls, large nuclei, and dense cytoplasm with dictyosomes, mitochondria, plastids and the endoplasmic reticulum. Analyses of the secretion, placement, and functional aspects of the colleters present in C. langsdorffii indicate that these structures help protect young leaves from desiccation.