Anatomy and ultrastructure of the floral nectary in Peganum harmala L. (Nitrariaceae)

Anatomy and ultrastructure of the floral nectary of Peganum harmala L. were studied using light and transmission electron microscopy. The floral nectary was visible as a glabrous, regularly five‐lobed circular disc encircling the base of the ovary. Anatomically, it comprised a single layered epidermis and 15–20 layers of small, subepidermal secretory cells overlying several layers of large, ground parenchyma cells. The floral nectary was supplied by phloem and both sieve tubes and companion cells were found adjacent to the ground parenchyma. Based on our ultrastructural observations, plastids of secretory cells during the early stages of development were rich in starch grains and/or osmiophilic plastoglobuli, but these disappeared as nectar secretion progressed. The nectar appeared to exude through the modified stomata along symplastic and apoplastic routes. The abundant plastids and mitochondria suggest an eccrine mechanism of nectar secretion in P. harmala.     

Floral nectar production and carbohydrate composition and the structure of receptacular nectaries in the invasive plant <Emphasis Type="Italic">Bunias orientalis</Emphasis> L. (Brassicaceae)

The data relating to the nectaries and nectar secretion in invasive Brassicacean taxa are scarce. In the present paper, the nectar production and nectar carbohydrate composition as well as the morphology, anatomy and ultrastructure of the floral nectaries in Bunias orientalis were investigated. Nectary glands were examined using light, fluorescence, scanning electron and transmission electron microscopy. The quantities of nectar produced by flowers and total sugar mass in nectar were relatively low. Total nectar carbohydrate production per 10 flowers averaged 0.3 mg. Nectar contained exclusively glucose (G) and fructose (F) with overall G/F ratio greater than 1. The flowers of B. orientalis have four nectaries placed at the base of the ovary. The nectarium is intermediate between two nectary types: the lateral and median nectary type (lateral and median glands stay separated) and the annular nectary type (both nectaries are united into one). Both pairs of glands represent photosynthetic type and consist of epidermis and glandular tissue. However, they differ in their shape, size, secretory activity, dimensions of epidermal and parenchyma cells, thickness of secretory parenchyma, phloem supply, presence of modified stomata and cuticle ornamentation. The cells of nectaries contain dense cytoplasm, plastids with starch grains and numerous mitochondria. Companion cells of phloem lack cell wall ingrowths. The ultrastructure of secretory cells indicates an eccrine mechanism of secretion. Nectar is exuded throughout modified stomata.     

Ultrastructural evidence for a dual function of the phloem and programmed cell death in the floral nectary of Digitalis purpurea

BACKGROUND AND AIMS: The floral nectary of Digitalis purpurea is a transitory organ with stomatal exudation of nectar. In this type of nectary, the nectar is thought to be transported to the exterior via intercellular ducts that traverse the nectariferous tissue. The latter is also traversed by a ramified system of phloem strands from which prenectar sugar is most probably unloaded. The aims of this study were to provide some of the basic information needed to evaluate the possible mechanism involved in nectar secretion and to discover the fate of the nectary. METHODS: The ultrastructure of the nectary was investigated at different stages of development by analysis of a series of ultrathin (7 x 10(-8) m) sections 7 x 10(-7) m apart from one another. Proportions of the cells typical of the nectary were documented by 3D-reconstruction and morphometry. KEY RESULTS: The phloem consisted of variably shaped sieve elements and companion cells which, as a rule, were more voluminous than the sieve elements. Direct contact between the phloem strands and intercellular ducts was observed. In contrast to the phloem, which remained structurally intact beyond the secretory phase, the nectariferous tissue exhibited degenerative changes reminiscent of programmed cell death (PCD), which started as early as the onset of secretion and progressed in a cascade-like fashion until final cell death occurred in the exhausted nectary. Hallmarks of PCD were: increased vacuolation; increase in electron opacity of individual cells; progressive incorporation of plasmatic components into the vacuole reminiscent of autophagy; degradation of plastids starting with hydrolysis of starch; deformation of the nucleus and gradual disappearance of chromatin; loss of tonoplast integrity and subsequent autolysis of the rest of cellular debris. Degeneration of the cells occurred against a background of increasing cell size. CONCLUSIONS: The cytological and anatomical evidence presented here, and calculations of the solute fluxes necessary for accumulation of starch and for the production of nectar support the view that: (a) in the foxgloves' nectary, apoplastic phloem unloading dominates, at least during exudation of nectar; (b) the obsolete nectary may be dismantled by PCD; and (c) at least the products of late nectary degradation are loaded via the apoplast into the unchanged phloem and exported to sinks elsewhere in the plant for reallocation.     

Micromorphology and ultrastructure of the floral nectaries of Polemonium caeruleum L. (Polemoniaceae)

The Polemoniaceae family forms flowers diverse in the terms of pollination methods and nectar types. The micromorphology of the nectary surface and the tissue structures as well as the ultrastructure of the cells of the floral nectaries in Polemonium caeruleum L. were examined using light, scanning and transmission electron microscopy. A bowl-shaped nectary, detached from the ovary, grows at its base. Its contour shows folds with depressions in the places where the stamens grow, forming five-lobed disc (synapomorphic character). Nectar is secreted through modified anomocytic stomata, which are formed in the epidermis covering the tip and the lateral wall of the projection located between the staminal filaments. The undulate nectary consists of a single-layered epidermis and three to nine layers of parenchymal cells. The cells of the nectary contain a dense cytoplasm, numerous plastids with an osmophilic stroma and starch grains, well-developed endoplasmic reticulum, as well as a large number of mitochondria interacting with the Golgi bodies. The ultrastructure of nectary cells indicates the granulocrine secretion mechanism and diversified transport of nectar.     

Anatomy and ultrastructure of the floral nectary of Tontelea micrantha (Celastraceae: Salacioideae)

Among several native species of the Brazilian cerrado, a shrub, Tontelea micrantha, is exploited by traditional communities for the valuable oil extracted from its seeds, which has anti‐inflammatory properties. There have been no studies on the anatomy of its flower, and so the aim of this study is to describe the anatomy and ultrastructure of its floral nectary. Flower buds and flowers in anthesis were collected, fixed and processed for light and electron microscopy. The discoid floral nectary is composed of epidermis and a secretory parenchyma. Secretory cells are rich in plastids with starch grains and mitochondria. The nectar, sucrose dominant, is just sufficient to form a thin film on the nectary. The secretory cells show starch and oil droplets; however, during nectar production there is no evidence of hydrolysis of starch and some lipid reserves remain unchanged. Our results suggest a reduction in the amount of oil in the secretory cells during the secretory phase but this does not appear to imply a release of oil as a nectar component. In addition to maintaining part of the reserves, the lower frequency of organelles involved in nectar synthesis reinforces the hypothesis that phloem sap is the origin of nectar sugars. The tiny nectar film, released through modified stomata, is attractive to small insects such as flies. Considering the importance and intensity of use of T. micrantha in the Brazilian cerrado, we think that these data about its floral nectary can help to better explain its reproductive biology with positive impacts on its management and conservation.     

Nectaries and male-biased nectar production in protandrous flowers of a perennial umbellifer <Emphasis Type="Italic">Angelica sylvestris</Emphasis> L. (Apiaceae)

Nectar is the most common floral pollinator reward. In dichogamous species, floral nectar production rates can differ between sexual phases. We studied the structure of nectaries located on the stylopodium and nectar production in protandrous umbellifer Angelica sylvestris. Our study species produced nectar in both floral sexual phases. Nectar sugar concentration was low (on average 22 ± 11 %, mean ± SD) and the nectar hexose rich and composed of sucrose, glucose, fructose and a small amount of amino acids, including β-alanine, a non-protein amino acid. Although nectar composition and sugar concentration varied little between floral sexual phases, nectar production showed a threefold reduction during the stigma receptive period. This is in contrast to other studies of Apiaceae that have reported female-biased nectar production, but in the direction predicted by plant sexual selection theory, suggesting that in pollen-unlimited species, floral rewards mainly enhance male reproductive success. The structure of the nectary was similar at the two sexual stages investigated, and composed of a secretory epidermis and several layers of nectariferous and subsecretory parenchyma. The nectary cells were small, had large nuclei, numerous small vacuoles and dense, intensely staining cytoplasm with abundant endoplasmic reticulum, mitochondria and secretory vesicles. They contained abundant resin-like material that may potentially act as defence against microbes. Starch was rarely observed in the nectary cells, occurring predominantly at the female stage and mainly in guard and parenchyma cells in close proximity to stomata, and in subsecretory parenchyma. The main route of nectar release in A. sylvestris seems to be via modified stomata.     

地椒花蜜腺发育的解剖学观察

通过显微和亚显微观察对地椒花蜜腺的发育进行了研究。地椒花蜜腺位于子房基部的花盘上,属于盘状蜜腺,新鲜时呈绿色。蜜腺由分泌表皮和泌蜜组织组成,分泌表皮为一层细胞,表皮细胞角质膜较厚,表皮上分布着大量的气孔器,气孔器突出于表面;泌蜜组织细胞多层。花盘中央有维管束通向子房,在维管束和泌蜜组织之间有４￣５层大型的薄壁细胞。蜜腺由花盘的表皮及其内侧相邻的细胞发育而来,在发育过程中,其细胞中的液泡和贮藏的淀粉     

Floral nectary structure and nectar composition in Eccremocarpus scaber (Bignoniaceae), a hummingbird-pollinated plant of central Chile

Surface features, anatomy, and ultrastructure of the floral nectary of Eccremocarpus scaber (Bignoniaceae), pollinated predominantly by the largest-known hummingbird (Patagona gigas gigas), were studied together with nectar sugar content and secretion rate. The annular disk nectary comprises epidermis, secretory and ground parenchyma with intercellular spaces, and branched vascular bundles terminating in the secretory parenchyma where only phloem is found. Amyloplasts and vacuoles increase in size throughout development, the latter becoming sites of organelle degradation. Transferlike cells in nectary phloem and P-proteinlike fibrillar material in phloem parenchyma were observed. Flowers produced around 32 μl of nectar (mostly after anthesis) with 11 mg of sugar composed of fructose, glucose, sucrose, and maltose in a ratio of 0.34:0.32:0.17:0.17. Morphological studies as well as the presence of maltose and glucose in nectar suggest storage of the originally phloem-derived sugars as starch with its subsequent hydrolysis. The low sucrose/hexose ratio (0.25) and high nectary secretion force (nectar per flower biomass) observed places E. scaber close to large-bodied bat-pollinated plants. A hypothesis based on nectar origin and nectar secretion is advanced to explain pollinator-correlated variation in sucrose/hexose ratio.     

Anatomical and ultrastructural changes of the floral nectary ofPisum sativum L. during flower development

Summary The floral nectary ofPisum sativum L. is situated on the receptacle at the base of the gynoecium. The gland receives phloem alone which departed the vascular bundles supplying the staminal column. Throughout the nectary, only the companion cells of the phloem exhibited wall ingrowths typical of transfer cells. Modified stomata on the nectary surface served as exits for nectar, but stomatal pores developed well before the commencement of secretion. Furthermore, stomatal pores on the nectary usually closed by occlusion, not by guard-cell movements. Pore occlusion was detected most frequently in post-secretory and secretory glands, and less commonly in pre-secretory nectaries. A quantitative stereological study revealed few changes in nectary fine structure between buds, flowers secreting nectar, and post-secretory flowers. Dissolution of abundant starch grains in plastids of subepidermal secretory cells when secretion commenced suggests that starch is a precursor of nectar carbohydrate production. Throughout nectary development, mitochondria were consistently the most plentiful organelle in both epidermal and subepidermal cells, and in addition to the relative paucity of dictyosomes, endoplasmic reticulum, and their associated vesicles, the evidence suggests that floral nectar secretion inP. sativum is an energy-requiring (eccrine) process, rather that granulocrine.Abbreviations ER endoplasmic reticulum - GA glutaraldehyde - SEM scanning electron microscopy     

THE FLORAL AND EXTRA-FLORAL NECTARIES OF PASSIFLORA. II. THE EXTRA-FLORAL NECTARY

Extra-floral nectaries of nine species of Passiflora were studied with light and electron microscopy prior to and during secretion. There is no evidence of ER or Golgi participation in the secretion of nectar. The vascular tissue supplying the nectary is characterized by companion and phloem parenchyma cells which are usually larger than the sieve elements, a configuration similar to that found in leaf minor veins. In the petiolar nectaries, large masses of membrane-bound protein are commonly found in these cells. This protein is absent in laminar nectaries.     

The floral nectary of Hymenaea stigonocarpa (Fabaceae, Caesalpinioideae): structural aspects during floral development

BACKGROUND AND AIMS: 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. METHODS: 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. KEY RESULTS: 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. CONCLUSIONS: 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.     

Structure of the receptacular nectary and circadian metabolism of starch in the ant‐guarded plant Ipomoea cairica (Convolvulaceae)

Nectaries occur widely in Convolvulaceae. These structures remain little studied despite their possible importance in plant–animal interactions. In this paper, we sought to describe the structure and ultrastructure of the receptacular nectaries (RNs) of Ipomoea cairica, together with the dynamics of nectar secretion. Samples of floral buds, flowers at anthesis and immature fruits were collected, fixed and processed using routine methods for light, scanning and transmission electron microscopy. Circadian starch dynamics were determined through starch measurements on nectary sections. The secretion samples were subjected to thin layer chromatography. RNs of I. cairica were cryptic, having patches of nectar‐secreting trichomes, subglandular parenchyma cells and thick‐walled cells delimiting the nectary aperture. The glandular trichomes were peltate type and had typical ultrastructural features related to nectar secretion. The nectar is composed of sucrose, fructose and glucose. Nectar secretion was observed in young floral buds and continued as the flower developed, lasting until the fruit matured. The starch content of the subglandular tissue showed circadian variation, increasing during the day and decreasing at night. The plastids were distinct in different portions of the nectary. The continuous day–night secretory pattern of the RNs of I. cairica is associated with pre‐nectar source circadian changes in which the starch acts as a buffer, ensuring uninterrupted nectar secretion. This circadian variation may be present in other extrafloral nectaries and be responsible for full daytime secretion. We conclude that sampling time is relevant in ultrastructural studies of dynamic extranuptial nectaries that undergo various changes throughout the day.     

Morphology of nectaries and biology of nectar production in the distylous species Fagopyrum esculentum

Background and Aims

The mechanisms of floral nectar production in buckwheat (Fagopyrum esculentum, Polygonaceae), a distylous pseudo-cereal, have received relatively little attention, prompting an investigation of the factors that regulate this process. The aim was to perform a refined study of the structures that secrete nectar and of the internal and external parameters influencing nectar volumes and sugar concentrations.

Methods

In order to control environmental parameters, plants were cultivated in growth rooms under controlled conditions. The structure of nectaries was studied based on histological sections from flowers and flower buds. Nectar was extracted using glass micropipettes and the sugar concentration was measured with a hand refractometer. Sugar concentration in the phloem sap was measured using the anthrone method. To test the influence of photosynthesis on nectar production, different light and defoliation treatments were applied.

Key Results

Unicellular trichomes were located in the epidermis at the ventral part of eight nectary glands situated on the flower receptacle alternately with stamens. Vascular bundles consisting of both phloem and xylem were identified at the boundary between a multilayered nectary parenchyma and a sub-nectary parenchyma with chloroplasts. A higher volume of nectar in thrum morphs was observed. No other difference was found in morphology or in sugar supply to inflorescences between morphs. Nectar secretion was strongly influenced by plant age and inflorescence position. Nectar volumes were higher in the upper inflorescences and during the flowering peak. Light had a dual role, (1) acting directly on reproductive structures to trigger flower opening, which conditions nectar secretion, and (2) stimulating photosynthetic activity, which regulates nectar accumulation in open flowers.

Conclusions

In buckwheat, nectar is secreted by trichomes and probably proceeds, at least in part, from phloem sap. Nectar secretion is strongly influenced by floral morph type, plant age, inflorescence position and light.Key words: Buckwheat, distyly, Fagopyrum esculentum, inflorescence position, morph comparisons, nectary histology, nectar sugar concentration, nectar volume, light intensity, organ biomass, phloem sap, plant age     

Four o'clock pollination biology: nectaries,nectar and flower visitors in Nyctaginaceae from southern South America

Floral nectary structure and nectar sugar composition were investigated in relation to other floral traits and flower visitors in contrasting species of Nyctaginaceae from southern South America, representing four tribes (Bougainvilleeae, Colignonieae, Nyctagineae, Pisoneae). Our comparative data will aid in the understanding of plant–pollinator interactions and in the development of hypotheses on the origin of floral and reproductive characters in this family. The nectaries are located on the inner side of the staminal tube. The nectariferous tissue is composed of an epidermis and three to ten layers of secretory parenchymal cells, supplied indirectly by the filament vascular bundles. Stomata appear to be associated with nectar secretion. For the first time in Nyctaginaceae, nectary ultrastructure is described in Boerhavia diffusa var. leiocarpa. Nectary parenchyma cells are densely cytoplasmic and contain numerous starch grains. Plasmodesmata connect the nectariferous cells. Flowers of Nyctaginaceae secrete a small volume of nectar of variable concentration (10–47%). Nectar is dominated by hexoses, but Mirabilis jalapa showed a balanced proportion of sucrose and hexoses. Hymenoptera are the most common visitors for most species; nocturnal Lepidoptera are the most common visitors for M. jalapa and Bougainvillea stipitata. We found relatively low variation in the nectary characteristics of Nyctaginaceae compared with broad variation in flower structure, shape, colour and nectar traits. © 2013 The Linnean Society of London     

荆条花蜜腺发育解剖学研究

荆条（Ｖｉｔｅｘ ｃｈｉｎｅｎｓｉｓ Ｍｉｌｌ．）花蜜腺属于淀粉型子房蜜腺,呈圆筒状环绕于子房的基部。蜜腺外观上无特殊结构,表面有。由分泌表皮和泌蜜组织组成,包括分泌表皮、气孔器、泌蜜薄壁组织和维管束。密腺和子房壁起源相同。花蕾膨大期,泌蜜组织细胞中产生大液泡;露冠期,泌蜜组织中形成维管束;花蕾初放期,分泌表皮细胞分化形成气孔器,无气孔下室,淀粉粒的积累在此期达到高峰;盛花期,蜜腺中已无淀粉粒,密     

Nectary and gender‐biased nectar production in dichogamous Chamaenerion angustifolium (L.) Scop. (Onagraceae)

In dichogamous plants, nectar characteristics (i.e. nectar amount and its composition) can differ between sexual phases. In the present study, we investigated the structural organization of the floral nectary, nectar production and carbohydrate composition in the protandrous Chamaenerion angustifolium (L.) Scop. (Onagraceae). The receptacular nectary consisted of an epidermis with numerous nectarostomata, several layers of photosynthetic secretory parenchyma, and subsecretory parenchyma. Nectariferous tissue was not directly vascularized and starch grains were rarely observed in the secretory cells, occurring exclusively in the guard cells of modified stomata. The nectar was released via nectarostomata. The floral nectar was hexose rich (32.8/39.1/28.1% glucose/fructose/sucrose) and the total concentration was constant throughout the anthesis (47% on average). However, contrasting patterns in nectar amount and carbohydrate composition between the floral sexual phases were observed. On average, female‐phased flowers produced 1.4‐fold more nectar than male‐phased flowers, and although the nectar was sucrose rich during the male phase, it was hexose rich during the female phase, suggesting sucrose hydrolysis.     

Nectaries and reproductive biology of Croton sarcopetalus (Euphorbiaceae)

Flower morphology, nectary structure, nectar chemical composition, breeding system, floral visitors and pollination were analysed in Croton sarcopetalus , a diclinous-monoecious shrub from Argentina. Male flowers have five receptacular nectaries, with no special vascular bundles, that consist of a uniserial epidermis with stomata subtended by a secretory parenchyma. Female flowers bear two different types of nectaries: inner (IN) and outer (ON) floral nectaries. IN, five in all, are structurally similar to the nectaries of male flowers. The five ON are vascularized, stalked, and composed of secretory, column-shaped epidermal cells without stomata subtended by secretory and ground parenchyma. In addition, ON act as post-floral nectaries secreting nectar during fruit ripening. Extrafloral nectaries (EFN) are located on petioles, stipules and leaf margins. Petiolar EFN are patelliform, stalked and anatomically similar to the ON of the female flower. Nectar sampled from all nectary types is hexose dominant, except for the ON of the female flower at the post-floral stage that is sucrose dominant. The species is self-compatible, but geitonogamous fertilization is rarely possible because male and female flowers are not usually open at the same time in the same individual, i.e. there is temporal dioecism. Flowers are visited by 22 insect species, wasps being the most important group of pollinators. No significant differences were found in fruit and seed set between natural and hand pollinated flowers. This pattern indicates that fruit production in this species is not pollen/pollinator limited and is mediated by a wide array of pollinators.     

On the mechanisms of nectar secretion: revisited

Background and Scope

Models of nectar formation and exudation in multilayered nectaries with modified stomata or permeable cuticle are evaluated. In the current symplasmic model the pre-nectar moves from terminal phloem through the symplasm into the apoplasm (cell walls and intercellular spaces) with nectar formation by either granulocrine or eccrine secretion and its diffusion outwards. It is concluded, however, that no secretory granules are actually produced by the endoplasmic reticulum, and that secretory Golgi vesicles are not involved in the transport of nectar sugar. Therefore, the concept of granulocrine secretion of nectar should be discarded. The specific function of the endomembrane system in nectary cells remains unknown. According to the apoplasmic model, the pre-nectar moves from the terminal phloem in the apoplasm and, on the way, is transformed from phloem sap into nectar. However, viewed ultrastructurally, the unloading (terminal) phloem of nectaries appears to be less active than that of the leaf minor veins, and is therefore not actively involved in the secretion of pre-nectar components into the apoplasm. This invalidates the apoplasmic model. Neither model provides an explanation for the origin of the driving force for nectar discharge.

Proposal

A new model is proposed in which nectar moves by a pressure-driven mass flow in the nectary apoplasm while pre-nectar sugars diffuse from the sieve tubes through the symplasm to the secretory cells, where nectar is formed and sugars cross the plasma membrane by active transport (‘eccrine secretion’). The pressure originates as the result of water influx in the apoplasm from the symplasm along the sugar concentration gradient. It follows from this model that there can be no combinations of apoplasmic and symplasmic pre-nectar movements. The mass-flow mechanism of nectar exudation appears to be universal and applicable to all nectaries irrespective of their type, morphology and anatomy, presence or absence of modified stomata, and their own vascular system.     

鹅掌柴花蜜腺的发育解剖学研究

对鹅掌柴(Scheffler octophylla Harms．)花蜜腺的发育进行解剖结构观察。鹅掌柴花盘蜜腺位于下位子房上方环绕花柱基部。蜜腺由分泌表皮、产蜜组织组成,心皮维管束与其相邻并发出一些伸入蜜腺基部的短分枝。蜜腺起源于心皮原基基部外侧的几层细胞。鹅掌柴花蜜腺为淀粉型蜜腺,淀粉粒为许多微小颗粒聚集成的复粒。原蜜汁由蜜腺基部维管束的筛管提供,达产蜜组织细胞和表皮细胞后以淀粉粒的形式贮藏。泌出的蜜汁一部分来自淀粉粒的降解,一部分来自泌蜜期输入的原蜜汁。表皮和产蜜组织细胞均具泌蜜功能。泌出的蜜汁大部分通过气孔排出,还有部分由角质层渗出。     

Nectary Structure, Nectar Secretion Patterns and Nectar Composition in Two Helleborus Species

Abstract: The morphological and cytological characteristics of nectaries of Helleborus foetidus and H. bocconei during the secretory period are reported. The nectaries are derived from modified petals and secrete nectar continuously for about 20 days; they consist of a single layered epidermis, nectar-producing parenchyma and photosynthesizing parenchyma. Nectar secretion is holocrine and the nectar is released by rupture of the wall and cuticle of each epidermal cell. The nectaries of the two species differ in number and external morphology. In H. foetidus, secretion begins before anthesis and secretion rate decreases with nectary age. In H. bocconei it begins on the day of anthesis and proceeds at a constant rate. The nectar has a high sugar content, mainly sucrose, and also contains lipids and proteins.