Nectary Parenchyma Research Articles

Floral structure and nectary anatomy of Bertholletia excelsa (Lecythidaceae)

ABSTRACT The floral nectary of Bertholletia excelsa was studied through permanent, histological cross-sections. The flower showed two rings of androecial vascular bundles, the inner for the fertile stamens and the outer for the androecial hood. The androecial hood, originating from the abaxial side of the androecium, is a well-developed, curved, and highly unilateral structure that covers the fertile stamens and stigma. At the apex and on the inner surface of this structure, several finger-like appendages. Histological cross-sections of the androecial hood, from the distal part towards the centre of the flower, reveal that the secretory parenchyma originates in the medium of the inner-distal part of the hood. The secretory tissue is surrounded by vascular bundles that more distally supply the finger-like appendages of the androecial hood. The nectariferous parenchyma lacks internal vascular bundles and consists of small cells, starch-rich, with large vacuoles and a prominent nucleus. The nectariferous epidermis is striated. No stomata or glandular hairs were observed associated with the nectariferous epidermis. The nectary is located within the distal part of the androecial hood and can be classified as a structured, non-vascularized, androecial type. This specialization in the location of the nectar secreting tissue is linked with a complex pollination mechanism, allowing access to the reward only for large bees with long proboscis.

Anatomical, histochemical, and developmental approaches reveal the long-term functioning of the floral nectary in Tocoyena formosa (Rubiaceae).

Tocoyena formosa has a persistent floral nectary that continues producing nectar throughout flower and fruit development. This plant also presents an intriguing non-anthetic nectary derived from early-developing floral buds with premature abscised corolla. In this study, we characterize the structure, morphological changes, and functioning of T. formosafloralnectary at different developmental stages. We subdivided the nectary into four categories based on the floral and fruit development stage at which nectar production started: (i) non-anthetic nectary; (ii) anthetic nectary, which follows the regular floral development; (iii) pericarpial nectary, derived from pollinated flowers following fruit development; and (iv) post-anthetic nectary that results from non-pollinated flowers after anthesis. The nectary has a uniseriate epidermis with stomata, nectariferous parenchyma, and vascular bundles, with a predominating phloem at the periphery. The non-anthetic nectary presents immature tissues that release the exudate. The nectary progressively becomes more rigid as the flower and fruit develop. The main nectary changes during flower and fruit development comprised the thickening of the cuticle and epidermal cell walls, formation of cuticular epithelium, and an increase in the abundance of calcium oxalate crystals and phenolic cells near the vascular bundles. Projections of the outer periclinal walls toward the cuticle in the post-anthetic nectary suggest nectar reabsorption. The anatomical changes of the nectary allow it to function for an extended period throughout floral and fruit development. Hence, T. formosa nectary is a bivalent secretory structure that plays a crucial role in the reproductive and defensive interactions of this plant species.

Histochemical, metabolic and ultrastructural changes in leaf patelliform nectaries explain extrafloral nectar synthesis and secretion in Clerodendrum chinense.

Extrafloral nectaries are nectar-secreting structures present on vegetative parts of plants which provide indirect defences against herbivore attack. Extrafloral nectaries in Clerodendrum chinense are patelliform-shaped specialized trichomatous structures. However, a complete understanding of patelliform extrafloral nectaries in general, and of C. chinense in particular, has not yet been established to provide fundamental insight into the cellular physiological machinery involved in nectar biosynthesis and secretory processes. We studied temporal changes in the morphological, anatomical and ultrastructural features in the architectures of extrafloral nectaries. We also compared metabolite profiles of extrafloral nectar, nectary tissue, non-nectary tissue and phloem sap. Further, both in situ histolocalization and normal in vitro activities of enzymes related to sugar metabolism were examined. Four distinct tissue regions in the nectar gland were revealed from histochemical characterization, among which the middle nectariferous tissue was found to be the metabolically active region, while the intermediate layer was found to be lipid-rich. Ultrastructural study showed the presence of a large number of mitochondria along with starch-bearing chloroplasts in the nectariferous region. However, starch depletion was noted with progressive maturation of nectaries. Metabolite analysis revealed compositional differences among nectar, phloem sap, nectary and non-nectary tissue. Invertase activity was higher in secretory stages and localized in nectariferous tissue and adjacent region. Our study suggests extrafloral nectar secretion in C. chinense to be both eccrine and merocrine in nature. A distinct intermediate lipid-rich layer that separates the epidermis from nectary parenchyma was revealed, which possibly acts as a barrier to water flow in nectar. This study also revealed a distinction between nectar and phloem sap, and starch could act as a nectar precursor, as evidenced from enzymatic and ultrastructural studies. Thus, our findings on changing architecture of extrafloral nectaries with temporal secretion revealed a cell physiological process involved in nectar biosynthesis and secretion.

Morphoanatomy, histochemical and secretions process of extrafloral nectaries in Joannesia princeps Vell. (Euphorbiaceae)

Joannesia princeps presents two acropetiolar glands, with topography and morphology similar to extrafloral nectaries (EFNs). In the present study we described the anatomy and the ultrastructure of acropetiolar glands, and analyzed the histochemistry and chemical composition of secretion in order to (i) verify if these glands are nectaries and (ii) understand the complexity of secretion and the cell compartment involved with the production and exudation. The samples were processed according to the standard methodology for anatomical and ultrastructural studies. The gland has uniseriate epidermis composed by palisade secretory cells. The secretion produced by the secretory epidermis is accumulated in a subcuticular space, which causes distention of the cuticle until it be broken, allowing the exudation. Xylem and phloem vascularize the gland, with the phloem reaching the nectariferous parenchyma, even close to the secretory epidermis. The histochemical tests revealed polysaccharides, mucilage and protein in the secretion, whereas the exudate chemical analysis showed that glucose, fructose and sucrose are components of the nectar, being fructose the sugar predominant. The secretion is complex and different organelles are involved in this synthesis and secretion process. Secretory parenchyma cells presented a large peripheral nucleus, vacuole with granular material, conspicuous endoplasmic reticulum, numerous mitochondria, plastids, and plasmodesmata. During the secretion process, a granular material is accumulated into the vacuole, which merges with the plasma membrane of the secretory epidermis, releasing this material into the periplasmatic spaces. After crossing the cell wall of the epidermal cell, the secretion is accumulated in a subcuticular space, until ready to be released by the cuticle disruption. Based on this study, we can affirm that the glands of it are extrafloral nectaries.

How do fertile stamens and staminodes go differently in Kingdonia uniflora? Evidence from stamen numbers, ontogeny, and structure

Kingdonia uniflora is in Circaeasteraceae and has fertile stamens and staminodes which are both variable in number; however, the differentiating process of these two types of stamens and the nectariferous structure of staminodes are still unclear. Variation in number and ontogeny (including the degeneration stage of staminodes) of stamens in K. uniflora are investigated under SEM, TEM and LM. The results indicate that there are 63 floral formulas in 80 flowers counted, and the most frequent flower floral formula (4 flowers) is P5A°11A5G6. The correlation between the stamen number and the total floral organ number is high (0.61), while the highest correlation with the total number of floral organs is the number of carpels, and there is a weak correlation between the two types of stamens (-0.23). The stamen development period is divided into five stages, with differentiation of stamens occurring at stage 2. The anther wall formation of fertile stamens is of the basic type, and the simultaneous meiosis form tetrahedral tetrads, while mature pollens are 2-celled and tricolpate. Sporogenous tissue of staminodes fails to develop further at an earlier stage, without the formation of secondary sporogenous cells. The head of mature staminodes has nectary tissue consisting of the secretory epidermis (two types of secretory cells), nectary parenchyma (about 15 layers) and vascular tissue with vessel and sieve tube elements. There are well-developed microchannels in epidermal cell walls, and the cells related to secreting nectar are rich in mitochondria, endoplasmic reticula and small vesicles. In addition, microsporogenesis, male gametophyte development, the secretory function of staminodes, and other characteristics in K. uniflora are similar to those in Ranunculaceae. This study clarifies the differentiating stage of fertile stamens and staminodes and explores the new structure (nectary) that staminodes formed, and also, it enriched the secretion modes in Ranunculales.

Nectar guides and floral nectary in Lamium album L. subsp. album: structure and histochemistry in light, fluorescence, and electron microscopy

Lamium album is a native common plant growing in Eurasia. It is used in medicine and cosmetics and is highly valued in apiculture. The aim of the study was to investigate the structure of the floral nectary in three stages of flower development. Additionally, histochemical studies of the nectary and nectar guides present on the lower corolla lobe were carried out. No detailed analyses of nectary tissues in this species have been conducted to date. The present analyses were performed with the use of light, fluorescence, and scanning electron microscopy. The nectary gland in the flowers of Lamium album subsp. album formed an incomplete ring at the ovary base. The nectarostomata were arranged in clusters only in the adaxial epidermis of the anterior part of the nectary. During the secretory activity of the nectary (1st day of flowering), numerous small vacuoles and cells with large lobulate nuclei with surrounding plastid clusters were observed in the epidermis and glandular parenchyma cells. The vascular bundles contained xylem and phloem elements. Corolla wilting (3rd day of flowering) was accompanied by destructive changes in the nectary parenchyma, leading to the formation of empty spaces and appearance of cell remnants on the nectary surface. The histochemical analyses revealed the presence of starch and phenolic compounds as well as acidic and neutral lipids, which are characteristic of essential oils, in the nectary tissues. The nectar guides were composed of large yellow papillae containing phenolic compounds and acidic and neutral lipids, which were also present in glandular trichomes and abaxial parenchyma cells. The present study has demonstrated that the scent of Lamium album subsp. album flowers is produced with the involvement of essential oils contained in adaxial and abaxial epidermis cells, glandular trichomes, and nectary tissues.

Comparative Nectary Morphology across Cleomaceae (Brassicales).

Floral nectaries have evolved multiple times and rapidly diversified with the adaptive radiation of animal pollinators. As such, floral nectaries exhibit extraordinary variation in location, size, shape, and secretory mechanism. Despite the intricate ties to pollinator interactions, floral nectaries are often overlooked in morphological and developmental studies. As Cleomaceae exhibits substantial floral diversity, our objective was to describe and compare floral nectaries between and within genera. Floral nectary morphology was assessed through scanning electron microscopy and histology across three developmental stages of nine Cleomaceae species including representatives for seven genera. A modified fast green and safranin O staining protocol was used to yield vibrant sections without highly hazardous chemicals. Cleomaceae floral nectaries are most commonly receptacular, located between the perianth and stamens. The floral nectaries are supplied by vasculature, often contain nectary parenchyma, and have nectarostomata. Despite the shared location, components, and secretory mechanism, the floral nectaries display dramatic diversity in size and shape, ranging from adaxial protrusions or concavities to annular disks. Our data reveal substantive lability in form with both adaxial and annular floral nectaries interspersed across Cleomaceae. Floral nectaries contribute to the vast morphological diversity of Cleomaceae flowers and so are valuable for taxonomic descriptions. Though Cleomaceae floral nectaries are often derived from the receptacle and receptacular nectaries are common across flowering plants, the role of the receptacle in floral evolution and diversification is overlooked and warrants further exploration.

STRUCTURE AND HISTOCHEMISTRY OF THE FLORAL NECTARY OF <i>Bauhinia monandra</i> (FABACEAE)

The structure and histochemistry of the floral nectary of Bauhinia monandra Kurz (Fabaceae) were investigated. Besides possessing medicinal properties, this tree is also used in the recovery of degraded areas and urban arborization. Nectaries samples were obtained from newly bloomed flowers. The nectary was located on the tubular hypanthium. This tube was partially coated by a nectary epidermis, whose cells had secretory features such as a relatively large nucleus, a dense cytoplasm, and small vacuoles. Subjacent to the nectary epidermis, there was a nectary parenchyma with eight to fifteen layers of cells which also have secretory features. Both the nectary epidermis and nectary parenchyma possessed starch grains. Subjacent to the nectary parenchyma passed collateral to amphicribral concentric vascular bundles constituted by more phloem than xylem. Although these vascular bundles did not emit terminations directed to the nectary parenchyma, the arrangement of the latter about the former suggests the supply of nectar precursors by the vascularisation. In the basal region of the hypanthium tube occurred modified stomata which were probably the main route of nectar release; and tector trichomes, possibly involved in the nectar retention.

How can structure and nectar composition explain the secretory process in super productive nuptial nectaries of Mabea fistulifera Mart. (Euphorbiaceae)?

Mabea fistulifera, a species pollinated mainly by diurnal and nocturnal vertebrates, presents pendulous inflorescences with approximately 70 pairs of nuptial nectaries (NNs). These NNs exude voluminous nectar drops that defy gravity, remaining exposed at the inflorescence for more than a day. We aimed to investigate the NN secretory process and the unique nectar presentation of M. fistulifera. NNs and their exudate were collected at different secretory stages and submitted to structural studies and chemical analysis. The epidermis is devoid of stomata and constitutes the main site of synthesis for non-sugar metabolites found on nectar and nectar-coating film. Nectary parenchyma presents few small starch grains, and vascular strands are distributed until the nectary parenchyma cells close to the epidermis. Vascular tissues at the nectary parenchyma seem to provide sugar and water for the nectar. A film composed of lipids, alkaloids, and proteins covers the nectar drops. The film guarantees the nectar offering for several hours, as it minimizes water loss and prevents falls by gravitational action. The release of large nectar drops is intrinsically linked to the NN anatomical traits and the exudate composition. Low sugar concentration and predominance of hexoses in M. fistulifera nectar are essential for maintaining nectar exudation for many hours, which results in the visitation of a broad spectrum of pollinators.

Does the Floral Nectary in Dracocephalum moldavica L. Produce Nectar and Essential Oil? Structure and Histochemistry of the Nectary.

Dracocephalum moldavica is an aromatic plant with a lemon scent and versatile use. Its flowers produce large amounts of nectar, which is collected by bees and bumblebees. The aim of the study was to investigate the structure of the floral nectary in this melliferous plant, which has not been analysed to date. The analyses were carried out with the use of light, fluorescence, scanning electron, and transmission electron microscopy, as well as histochemical techniques. The four-lobed nectary with a diameter of 0.9-1.2 mm and a maximum height of 1.2 mm is located at the ovary base; one of its lobes is larger than the others and bears 20-30 nectarostomata and 8-9 glandular trichomes. The histochemical assays revealed the presence of essential oil and phenolic compounds in the nectary tissues and in glandular trichomes. The nectary tissues are supplied by xylem- and phloem-containing vascular bundles. The nectariferous parenchyma cells have numerous mitochondria, plastids, ribosomes, dictyosomes, ER profiles, vesicles, thin cell walls, and plasmodesmata. Starch grains are present only in the tissues of nectaries in floral buds. The study showed high metabolic activity of D. moldavica nectary glands, i.e., production of not only nectar but also essential oil, which may increase the attractiveness of the flowers to pollinators, inhibit the growth of fungal and bacterial pathogens, and limit pest foraging.

Detaling morphological traits of Trollius europeus L. flowers, nectary structure, and holocrine nectar secretion through combined light and electron microscopy

Trollius europeus L. flowers produce nectar available to various groups of insects. No anatomical studies of these floral nectaries have been conducted to date. This study presents the structure of nectaries at different levels of organisation and the characteristics of petaloids, nectary leaves (petals), and stamens, including their micromorphology. The analyses of the nectaries were carried out with the use of light, fluorescence, scanning electron, and transmission electron microscopy techniques. The nectary located in a small cavity on the nectary leaf consists of the epidermis, nectar-producing parenchyma, and subnectary parenchyma, which has three large vascular bundles containing phloem and xylem. Amyloplasts with starch granules are present only in the parenchyma surrounding the vascular bundles. The other cells of the nectary parenchyma contain only chromoplasts with large plastoglobules. Since there are no chloroplasts, sugars required for nectar production are assumed to originate from phloem sap. The numerous plasmodesmata in the cell walls indicate a symplastic route of pre-nectar. Nectar is secreted onto the nectary surface in the holocrine mode. After disorganisation of the cytoplasmic structure, the epidermal cell wall is disrupted and the cell contents along with the nectar are released into the depression on the nectary leaf. The secretion of nectar from cells is non-synchronous and lasts 4–5 days. The content of nectar proteins and lipids derived from the cytoplasm of epidermal cells increases the nutritional value of the secretion, which may be important for pollinators.

The diversity of elaborate petals in Isopyreae (Ranunculaceae): a special focus on nectary structure.

Elaborate petals are highly diverse in morphology, structure, and epidermal differentiation and play a key role in attracting pollinators. There have been few studies on the elaborate structure of petals in the tribe Isopyreae (Ranunculaceae). Seven genera in Isopyreae (Aquilegia, Semiaquilegia, Urophysa, Isopyrum, Paraquilegia, Dichocarpum, and Leptopyrum) have petals that vary in morphology, and two genera (Enemion and Thalictrum) have no petals. The petals of nine species belonged to 7 genera in the tribe were studied to reveal their nectary structure, epidermal micromorphology and ancestral traits. The petal nectaries of Isopyreae examined in this study were located at the tip of spurs (Aquilegia yabeana and A. rockii), or the bottom of shallow sacs (Semiaquilegia adoxoides, Urophysa henryi, Isopyrum manshuricum, and Paraquilegia microphylla), a cup-shaped structure (Dichocarpum fargesii) and a bilabiate structure (Leptopyrum fumarioides). The petal nectary of eight species in Isopyreae (except A. ecalcarata) was composed of secretory epidermis, nectary parenchyma, and vascular tissues, and some sieve tubes reached the secretory parenchyma cells. Among the eight species with nectaries examined in the present study, A. yabeana had the most developed nectaries, with 10-15 layers of secretory parenchyma cells. The epidermal cells of mature petals of the nine species were divided into 11 types. Among these 11 types, there were two types of secretory cells and two types of trichomes. Aquilegia yabeana and A. rockii had the highest number of cell types (eight types), and I. manshuricum and L. fumarioides had the lowest number of cell types (three types). Aquilegia ecalcarata had no secretory cells, and the papillose conical polygonal secretory cells of D. fargesii were different from those of the other seven species with nectaries. Trichomes were found only in Aquilegia, Semiaquilegia, Urophysa, and Paraquilegia. The ancestral mode of nectar presentation in Isopyreae was petals with hidden nectar (70.58%). The different modes of nectar presentation in petals may reflect adaptations to different pollinators in Isopyreae.

Extrafloral nectary development, characterization and secretion in Chamaecrista debilis and Chamaecrista desvauxii var. langsdorfii (Leguminosae, Caesalpinioideae)

Although the petiole glands in Chamaecrista are usually termed extrafloral nectaries (EFNs), additional studies on the morphoanatomy and chemical nature of the exudates are necessary to properly classify a plant secretory structure. The correct classification of such structures is also important in taxonomic and phylogenetic studies. Based on standard anatomical procedures for light microscopy our study aims to broaden our current understanding of the EFNs in Chamaecrista. We describe and compare the ontogenesis and the morphoanatomy of the petiole EFNs in C. debilis and C. desvauxii var. langsdorfii, species that belong to two distinct sections in Chamaecrista. The morphoanatomy along with the presence of sugars in exudates supports the classification of the petiole glands in both species as EFNs. The EFN development is rapid, and their secretory activity starts before the young leaves complete their expansion. The major difference among the EFNs of both species is their outer morphology which is useful on the taxonomy. Both EFNs are anatomically similar and composed of a secretory epidermis, a nectariferous parenchyma and vascular tissue more abundant in phloem in comparison to xylem.

Morphological and anatomical traits during development: Highlighting extrafloral nectaries in Passiflora organensis.

Passiflora organensis is a small herbaceous vine with characteristic morphological variations throughout its development. The plant bears button-shaped extrafloral nectaries exclusively in adult leaves. Extrafloral nectaries are structures that secrete nectar and play an important role in plant-animal interactions as a strategy for protecting plants against herbivory. In this work, we performed anatomical and ultrastructural studies to characterize P. organensis extrafloral nectaries during their secretory phase. We showed extrafloral nectaries in Passiflora organensis are composed of three distinct regions: nectary epidermis, nectariferous parenchyma, and subnectariferous parenchyma. Our data suggests that all nectary regions constitute a functional unit involved in nectar production and release. The high metabolic activity in the nectary cells is characterized by the juxtaposition of organelles such as mitochondria and plastids together plasmalemma. In addition, calcium oxalate crystals are frequently associated to the nectaries. An increasing concentration of calcium during leaf development and nectary differentiation was observed, corresponding to the calcium deposition as calcium oxalate crystals. This is the first description of extrafloral nectaries in Passiflora organensis that is a promising tropical model species for several studies. RESEARCH HIGHLIGHTS: The anatomical and ultrastructural characteristics and the presence of calcium oxalate crystals in the nectary tissue suggest novel strategies against herbivory in the genus Passiflora.

Structure of the nectary in Chaenomeles japonica (Thunb.) Lindl. ex Spach. in different stages of flowering with focus on nectar secretion.

Chaenomeles japonica is an attractive ornamental shrub flowering in spring. It is also a valuable source of nectar and pollen for entomofauna. The study was carried out to investigate the structure of hypanthial nectaries present in the flowers of this species with the use of light and scanning electron microscopy. Nectary tissues were examined in three stages of flowering, with special focus placed on changes occurring in the epidermisand nectariferous parnechyma. Long-styled flowers, which produce nectar abundantly, were selected for the study. The nectary parenchyma was shown to consist of multiple cell layers (up to 20). The epidermis was initially single-layered, but the number of layers gradually increased to 2-3 during the following days of flowering. The outer walls of epidermis cells were covered by a strongly undulating cuticle with massive striae. Cracks and perforations, which are probably nectar release sites, were visible between the striae. The presence of the secretion in the intercellular spaces between the parenchyma layer and the epidermis may indicate apoplastic nectar transport. The presence of stomata, as well as pores in the cuticle layer of the nectary epidermis, suggests that C. japonica nectar is secreted in two ways: (i) through the nectarostomata and (ii) ordinary epidermis cells with the involvement of the cuticle.

Morphoanatomy and ecology of the extrafloral nectaries in two species of Passiflora L. (Passifloraceae)

• The number of layers of the secretory epidermis varies according to the subgenera. • The secretor phase is evident with the distention of the cuticle. • The glucose occurrence in the exudates of the EFN in the leaves, several genera of ants and fungi associated to those structures allowed to identify those structures as extrafloral nectaries. The foliar glands of Passiflora are generally known as extrafloral nectaries, which, unlike floral nectaries, are not related to pollination. The present study describes and analyzes the foliar glands found in Passiflora ceratocarpa and Passiflora quadrangularis , in order to confirm their identity as extrafloral nectaries and discuss the taxonomic and ecological implications of these structures. The study material was collected in the metropolitan region of Belém (Brazil) and submitted to standard protocols for the analysis of plant anatomy by light and scanning microscopy, as well as histochemical testing. The extrafloral nectaries of the two species are anatomically very similar, including a multiseriate palisade epidermis, a nectariferous parenchyma consisting of voluminous, thin-walled cells, and predominantly phloematic vascularization. Their chemical composition is also highly similar, and includes sugars, and lipids. Five ant genera were found visiting the two Passiflora species, of which Camponotus was the most abundant, being found on both the study species.

The structure of nectaries in the genus Strombocactus (Cactaceae)

Background: Floral nectar is offered by the plant to its pollinators to promote cross-fertilization.Questions: Are floral nectaries morpho-anatomically similar among the taxa of Strombocactus ? What sugars do nectaries offer their pollinators?Studied species: Strombocactus disciformis subsp. disciformis , S. disciformis subsp. esperanzae and S. corregidorae, during 2016-2018.Methods: Bud flowers and flowers in anthesis from each taxon of Strombocactus , were processed to be observed in SEM; other specimens were embedded in Paraplast and sectioned to histochemical tests. The concentration of nectar was measured using a refractometer.Results: The nectaries are similar among the three studied taxa, located below the stamens and reach the upper part of the ovary forming a ring in the hypanthium. An almost flat epidermis constitutes the nectary in S. disciformis subsp. esperanzae , although in S. disciformis subsp. disciformis is slightly bulked and papillae in S . corregidorae, a nectariferous parenchyma composed of metabolically very active cells, and a subnectariferous parenchyma of larger cells, associated with vascular bundles. The nectar is secreted through nectarostomata. The concentration of nectar was 8.65 ± 3.98, 16.12 ± 4.48 and 22.09 ± 7.42 °Brix for S. disciformis subsp. esperanzae , S. corregidorae and S. disciformis subsp. disciformis , respectively.Conclusions: All taxa of Strombocactus have an annular-type nectary that secretes a low volume of nectar and together with pollen are offered as rewards. Based on other floral characteristics observed, such as the diurnal anthesis, the infundibuliform shape and the color of the flower, we propose that bees pollinate these species.

Structure and ultrastructure of floral nectaries of two Opuntia species (Cactaceae) in relation to their floral visitors

Opuntia is the most diverse and widespread genus in the cactus family. In spite of the strong relationship with biotic pollination, little is known about their floral nectaries. We present the first deep structural and ultrastructural description of the floral nectaries of two Opuntia species (Opuntia anacantha var. retrorsa; Opuntia elata var. elata), linking these attributes to their functioning. A characterization of their floral visitors is also made. Both nectaries are very similar and represent the hypanthial type, with the presence of surface undulations that favor nectar retention. In O. elata, the style dilatation forms a nectar chamber. Three tissues are distinguished in these nectaries: an epidermis consisting of a single layer of cells with a thin cuticle, a nectariferous parenchyma with numerous vascular bundles with xylem and phloem and, a subnectariferous parenchyma with numerous amyloplasts. Pre-nectar may come from the starch hydrolysis of these plastids as well as from other green tissues of the plant. The exudation occurs through stomata and cuticle microchannels. O. anacantha may be a melittophilous plant, while in O. elata the presence of a nectar chamber and the orange color of the perianth suggest an incipient ornithophily.

Structural associations between organelle membranes in nectary parenchyma cells.

The close association between membranes and organelles, and the intense chloroplast remodeling in parenchyma cells of extrafloral nectaries occurred only at the secretion time and suggest a relationship with the nectar secretion. Associations between membranes and organelles have been well documented in different tissues and cells of plants, but poorly explored in secretory cells. Here, we described the close physical juxtaposition between membranes and organelles, mainly with chloroplasts, in parenchyma cells of Citharexylum myrianthum (Verbenaeceae) extrafloral nectaries under transmission electron microscopy, using conventional and microwave fixation. At the time of nectar secretion, nectary parenchyma cells exhibit a multitude of different organelle and membrane associations as mitochondria-mitochondria, mitochondria-endoplasmic reticulum, mitochondria-chloroplast, chloroplast-nuclear envelope, mitochondria-nuclear envelope, chloroplast-plasmalemma, chloroplast-chloroplast, chloroplast-tonoplast, chloroplast-peroxisome, and mitochondria-peroxisome. These associations were visualized as amorphous electron-dense material, a network of dense fibrillar material and/or dense bridges. Chloroplasts exhibited protrusions variable in shape and extension, which bring them closer to each other and to plasmalemma, tonoplast, and nuclear envelope. Parenchyma cells in the pre- and post-secretory stages did not exhibit any association or juxtaposition of membranes and organelles, and chloroplast protrusions were absent. Chloroplasts had peripheral reticulum that was more developed in the secretory stage. We propose that such subcellular phenomena during the time of nectar secretion optimize the movement of signaling molecules and the exchange of metabolites. Our results open new avenues on the potential mechanisms of organelle contact in parenchyma nectary cells, and reveal new attributes of the secretory cells on the subcellular level.

Influence of genotype, floral stage, and water stress on floral nectar yield and composition of mānuka (Leptospermum scoparium).

Background and AimsFloral nectar can be variable in composition, influencing pollinator behaviour and the composition of honey derived from it. The non-peroxide antibacterial activity of mānuka (Leptospermum scoparium, Myrtaceae) honey results from the chemical conversion of the triose sugar dihydroxyacetone (DHA), after DHA accumulates for an unknown reason in the nectar. This study examined variation in nectar DHA, glucose, fructose and sucrose content with floral stage of development, between mānuka genotypes with differing flower morphology, and in response to water stress.MethodsSix mānuka genotypes were grown without nectar-feeding insects. Stages of flower development were defined, nectar was harvested and its composition was compared between stages and genotypes, and with floral morphology. Water stress was imposed and its effect on nectar composition was examined.Key ResultsNectar was present from soon after flower opening until the end of petal abscission, with the quantity of accumulated nectar sugars rising, then stabilizing or falling, indicating nectar secretion followed by reabsorption in some genotypes. The quantity of DHA, the ratio of DHA to other nectar sugars and the fructose to glucose ratio also varied with stage of development, indicating differences in rates of production and reabsorption between nectar components. Nectar composition and yield per flower also differed between genotypes, although neither was positively related to nectary area or stomatal density. Drying soil had no effect on nectar composition or yield, but variation in nectar yield was correlated with temperature prior to nectar sampling.ConclusionsMānuka nectar yield and composition are strongly influenced by plant genotype, flower age and the environment. There were clear stoichiometric relationships between glucose, fructose and sucrose per flower, but DHA per flower was only weakly correlated with the amount of other sugars, suggesting that accumulation of the triose sugar is indirectly coupled to secretion of the larger sugars by the nectary parenchyma.