Comparative account of nectary structure in Hexisea imbricata (Lindl.) Rchb.f. (Orchidaceae)

• Background and Aims Despite the number of orchid speciesthat are thought to be pollinated by hummingbirds, our knowledgeof the nectaries of these orchids is based solely on a singlespecies, Maxillaria coccinea (Jacq.) L.O. Williams ex Hodge.Nevertheless, it is predicted that such nectaries are likelyto be very diverse and the purpose of this paper is to comparethe nectary and the process of nectar secretion in Hexisea imbricata(Lindl.) Rchb.f. with that of Maxillaria coccinea so as to beginto characterize the nectaries of presumed ornithophilous Neotropicalorchids. • Methods Light microscopy, transmission electronmicroscopyand histochemistry were used to examine the histology and chemicalcomposition of nectary tissue and the process of nectar secretionin H. imbricata. • Key Results and Conclusions The nectary of H. imbricatahas a vascular supply, is bound by a single-layered epidermiswith few stomata and comprises two or three layers of subepidermalsecretory cells beneath which lie several layers of palisade-likeparenchymatous cells, some of which contain raphides or mucilage.The secretory cells are collenchymatous and their walls havenumerous pits with associated plasmodesmata. They contain thefull complement of organelles characteristic of secretory cellsas well as intravacuolar protein bodies but some of the secretoryepidermal cells, following secretion, collapse and their anticlinalwalls seem to fold. Nectar secretion is thought to be granulocrineand, following starch depletion, lipid droplets collect withinthe plastids. The nectar accumulates beneath the cuticle whichsubsequently forms swellings. Finally, nectar collects in thesaccate nectary spur formed by the fusion of the margins ofthe labellum and the base of the column-foot. Thus, althoughthe nectary of H. imbricata and M. coccinea have many featuresin common, they nevertheless display a number of important differences.     

Nectary Structure and Ultrastructure of Unisexual Flowers of Ecballium elaterium(L.) A. Rich. (Cucurbitaceae) and their Presumptive Pollinators

The structure and ultrastructure of the nectaries of the monoeciousspecies Ecballium elaterium were studied. Large differencesin size and structure of the nectaries were observed in thetwo genders of flowers, those of the staminate flowers beingmuch larger and more developed than those of the pistillateflowers. The latter do not secrete measurable amounts of nectar.In the nectariferous cells, especially of the staminate flowers,numerous plasmodesmata are present. The pre-nectar originatingin the phloem is stored in the plastids of the nectariferouscells primarily as starch grains. The nectar appears to be exudedfrom the nectary via modified stomata. Very small insects ofthe order Hemiptera were found to dwell inside the flowers ofthe two genders, but in different numbers; their number in thestaminate flowers was more than twice that in the pistillateflowers. These insects may take part in the process of pollination.Copyright 2001 Annals of Botany Company Ecballium elaterium, Cucurbitaceae, monoecious plant, nectaries, structure, ultrastructure, nectar secretion, stomata, Hemiptera insects     

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.     

Ultrastructure, Development and Secretion in the Nectary of Banana Flowers

The nectaries of Musa paradisiaca L. var. sapientum Kuntze werefound to secrete in addition to the sugar solution, a polysaccharidemucilage and a very electron dense, homogenous material whichwas apparently protein. The polysaccharide had already startedto appear outside the epithelial cells of the nectary at veryearly stages of nectary development. At somewhat later developmentalstages the very dense homogenous material appeared in the formof droplets between the plasmalemma and cell wall in massesin the nectary lumen. Nectar secretion started in flowers whenthe bract in the axil of which they occurred had just recoiled.The ER elements were dilated and formed vesicles and the Golgibodies were very active, at the stage of the nectar secretionand at stages preceding it, except at the stage just beforesecretion. In all stages of nectary development the dilatedER elements and most large Golgi vesicles contained fibrillarmaterial. It is suggested that both ER and the Golgi apparatusare involved in the secretion of the sugar solution and of thepolysaccharides. There was not enough evidence as to where inthe cell the very dense homogenous material is synthesized. A few developmental stages of the nectaries of the male flowersof the Dwarf Cavendish banana, which do not secrete nectar,were also studied. It was seen that at early stages of development,the ultra-structure of the nectary of this banana variety wassimilar to that of M. paradisiaca var. sapientum. However, theepithelial nectary cells of the Dwarf Cavendish banana disintegratedbefore maturation of the nectary. Musa paradisiaca L, banana, floral nectaries, ultrastructure     

The Effect of Light Quality on Shoot Extension Growth in Three Species of Grasses

The effect of light quality on the extension growth of vegetativeshoots and on the final size of their leaves was investigatedin plants of Lolium multiflorum, Sporobolus indicus and Paspalumdilatatum. Three experimental approaches were used, (a) redor far-red end-of-day irradiations of sunlight-grown plants,(b) different red/far-red ratios of white light in a growthroom and (c) sunlight enrichment with radiation of differentred/far-red ratios or with different amounts of far-red lightduring the photoperiod. Plants treated with end-of-day far-redor low red/far-red ratios throughout the photoperiod developedlonger leaves and, as a result, longer shoots. This effect wasmore marked in leaf sheaths than in blades. Tiller extensionand leaf sheath length increased with the amount of far-redadded to sunlight in a simple hyperbolic relationship. Theseresults show that vegetative grass shoots respond to light qualityin a way similar to internodes of dicotyledonous plants. Lolium multiflorum Lam., Sporobolus indicus (L.), Paspalum dilatatum (Poir.), leaf growth, tiller growth, photomorphogenesis     

The Effects of Plant Density on Shoot and Leaf Lamina Angles in Lolium multiflorum and Paspalum dilatatum

The effects of plant density on shoot and leaf lamina zenithangles (i e the angles with respect to the vertical) were investigatedin Lolium multiflorum and Paspalum dilatatum plants grown inpots either outdoors or in a glasshouse Tillers appeared abovethe ligule of their subtending leaf with a small zenith anglebut, in plants grown at low densities, gradually adopted a morehorizontal position In plants grown at high densities this gradualincrease in shoot zenith angle with age was strongly reducedThe average shoot zenith angle at a given time was lower (ie tillers were more erect) at high, compared to low plant densitiesShoot number per plant and average shoot zenith angle per plantwere directly related In P dilatatum plants the leaf lamina remained nearly horizontalat any plant density This observation indicates that the movementof the leaf lamina, pivoting at the point of attachment to theleaf sheath, compensated the movement of the shoot, pivotingat the base of the leaf sheath-tube Shoot angle responses to plant density would place leaf laminaeat different heights within the canopy, affecting their abilityto compete for light and their susceptibility to wind impactand herbivore grazing Foliage architecture, Lolium multiflorum Lam, Paspalum dilatatum Poir, plant density, neighbour effects, shoot angle, leaf angle     

A Histochemical Study of Nectaries of Hibiscus rosa-sinensis

Different histochemical and cytochemical methods were employedon nectaries of Hibiscus rosa-sinensis. Light microscopy revealedthe presence of oil and mucilage cells in the subglandular tissue.Electron microscopy showed intense activity of ATPase in thephloem subtending the nectary. When CaCl₂ or tannic acid areadded to the fixative, electron-dense globular deposits areencountered in close contact with the plasmalemma of the secretorycells. In this case the endoplasmic reticulum appears in alternatingelectron-dense areas. In young nectaries the application oftannic acid results in electron-opaque deposits at the cellplate of dividing cells. The prolonged incubation of nectariesin OsO₄ results in an obvious difference in staining betweennectary hairs and subglandular cells. Structures stained selectivelywith OsO₄ are the endoplasmic reticulum, nuclear envelope, plastids,and mitochondria. The cytochemical experiments support the viewthat in nectaries of Hibiscus rosa-sinensis, the pre-nectaroriginates from the phloem and it is symplastically carriedvia the plasmodesmata to the secretory cells of the hair fromwhere it is secreted. The principal element which is involvedboth in the pre-nectar transport and nectar secretion is theendoplasmic reticulum. Key words: Lipid staining, polysaccharides, tannic acid, calcium binding sites, ATPase activity, osmium impregnation     

The Extrafloral Nectaries of Sambucus nigra

The structure and ultrastructure of the extrafloral nectariesof Sambucus nigra L. were studied. These nectaries are stalk-likeand occur at the bases of the leaves and leaflets. The nectariferoustissue occurs at the top of the nectary and is continuous withthe single central vascular bundle. The nectariferous cellshave a dense cytoplasm and contain a well developed endoplasmicreticulum. With the commencement of nectar secretion disintegrationof the nectariferous cells takes place. This process startsat the summit of the nectary and proceeds downwards. The questionas to whether the process of secretion is holocrine or merocrineis discussed. Sambucus nigra, Extrafloral nectary, nectariferous cells, disintegration of cells     

Foliar Nectaries in Mahogany (Swietenia Jacq.)

The pinnately compound leaves of Swietenia spp. have numeroussmall nectaries on petiole, rachis, petiolules, and both surfacesof all leaflets. Nectaries are circular to elongate, eitherlevel with the epidermis or slightly raised or depressed. Thesecretory surface is usually smooth even under high SEM magnification.Each nectary has an outer zone of three or four cell layersof slightly elongate and densely cytoplasmic cells subtendedby two layers of rounded cells with thickened walls and lightlystaining cytoplasm. Proximity of nectaries to vasculature isvariable and seems to be only fortuitous. Meliaceae, Swietenia, mahogany, nectary anatomy, extrafloral nectaries, foliar nectaries     

Unraveling the secretion mechanism of the curious nectaries in Gentianaceae

Unusual nectaries were anatomically described as being usual traits for Gentianaceae. They are small, avascularized, and formed by 3 to 5 rosette cells with labyrinthine walls around one central cell. Such as nectaries have been reported for stems, petals, and sepals of different species of the family, however, there is no information on the mechanisms involved with the synthesis and release of secretion. Thus, this work aimed to unravel the mechanism of secretion and exudation of nectar for these curious nectaries using Calolisianthus speciosus as a model. Samples were processed according to standard methods for light and electron microscopy. Leaf and sepal nectaries were described, as were those of the apex of petals where ants were observed patrolling a darkened region. The enzymatic method was used for the detection of sugars, proteins, and amino acids in leaf and sepal exudates. The nectaries of petals of C. speciosus are similar to those of its leaves, sepals, and stem, although their activities are asynchronous. Polysaccharides were detected on the labyrinthine walls of rosette cells and protein in the opposite region of the cytoplasm. Labyrinthine walls increase the contact surface between rosette cells and the central cell, allowing for the transfer of secretion. After accumulation, the secretion is released to the subcuticular space of the central cell through disruption of the cuticle. The secretion and exudation of nectar were elucidated and involve distinct organelles.

     

The Effect of Herbivory on Temporal and Spatial Dynamics of Foliar Nectar Production in Cotton and Castor

The effects of feeding Spodoptera littoralis(Boisd.) (Lepidoptera:Noctuidae) larvae on the quantity and distribution of extrafloralnectar production by leaves of castor (Ricinus communis) andcotton (Gossypium herbaceum) were investigated. Following larvalfeeding, the total volume of nectar secreted by foliar nectariesincreased 2.5- and 12-fold, respectively. As HPLC-analysis showedno difference in sugar composition between extrafloral nectarfrom insect-damaged and control plants, it can be concludedthat the plants increased the secretion of carbohydrates inresponse to herbivory. In damaged castor leaves, the amountof sugar excreted through extrafloral nectaries representedapprox. 1     

Floral Nectar Secretion and Ploidy in Brassica rapa and B. napus (Brassicaceae). II. Quantified Variability of Nectary Structure and Function in Rapid-cycling Lines

Haploid, diploid and tetraploid lines ofBrassica rapaL. (syn.campestris),and allotetraploidB. napusL., were examined to determine theinfluence of ploidy on floral features, particularly nectarymorphology and anatomy, and to relate nectary structure to nectarproduction capacity. Except for haploids, all lines were rapid-cycling.Average flower dry weight, and petal length and width, werein the descending orderB. napus>B. rapa (4n) >2n>n.Pollen grains of 4nplants were larger than those of 2nplants;haploids lacked pollen. All lines developed nectaries. Typically, each flower producedtwo pairs of nectaries, of different types and nectar productioncapacity. Normally, each lateral gland was located above thebase of a short stamen, and together this pair yielded mostof a flower 's nectar carbohydrate. Each median nectary aroseat the outer junction of the bases of two adjacent long stamens.All lateral nectaries received a vascular supply of phloem alone,but median glands received reduced amounts of phloem, or lackedvasculature altogether. Most nectaries were solitary, but 14%of all flowers, and especially those of 2n B. rapa,had at leastone median and lateral gland connected. Obvious variation existed in nectary morphology between ploidylevels, between flowers of the same plant, and even within flowers.Ten forms of each nectary type were recognized. Plants producingthe most nectar carbohydrate had high frequencies of lateralnectaries which were symmetrical, unfurrowed swellings. TetraploidsofB. rapahad both the highest frequencies of furrowed lateralglands, and of isolated segments of nectarial tissue at thatposition. Even these separated nectarial outgrowths receivedphloem and produced a nectar droplet. At the median location,nectaries were commonly of two forms: peg- or fan-shaped. Lobeson median nectaries, up to four per nectary, were detected inalmost half of glands of 4nflowers examined; lobes were absentin haploids. Brassica rapa; Brassica napus; flower size; nectar production; nectary variability; petal size; ploidyphloem; pollen; rapeseed     

Effects of Flooding and Drought on the Anatomy of Paspalum dilatatum

Paspalum dilatatum occupies different topographic positionsin the Flooding Pampa, Argentina. Populations from differentpositions are subjected to various regimes of flooding and drought,both of which may occur in the same growing season. We investigatedthe constitutive and plastic anatomical traits of P. dilatatumpopulations from habitats with contrasting regimes of floodingand drought. Both events affected root and sheath anatomy, andthese effects were similar for clones from different topographicpositions. Flooding increased the aerenchymatous tissue in theroot cortex and the leaf sheaths and decreased the number ofroot hairs per unit of root length. Drought decreased the diameterof root metaxylem vessels, thus lowering the risk of embolismsand increasing water-flow resistance, and increased the numberof root hairs, thereby increasing water uptake ability. In additionto these plastic responses, all clones showed constitutive characteristicsthat may confer an ability to withstand sudden events of floodingor drought: a high proportion of aerenchyma, which may maintainaeration before plastic responses take place; sclerenchyma,which may prevent root and leaf sheath collapse by soil compaction;and a conspicuous endodermis, which may protect stelar tissuesfrom desiccation. Both constitutive and plastic anatomical characteristicsare likely to contribute to the ability of this species to occupywidely different topographic positions and to resist temporalvariations in water and oxygen availability. Copyright 2001Annals of Botany Company Flooding, drought, aerenchyma, vessels, roots, leaf sheaths, anatomy, Paspalum dilatatum Poir     

Development and Ultrastructure of Cucurbita pepo Nectaries of Male Flowers

The development of the nectary of the male flower ofCucurbitapepo L. was studied from 5d before to 2d after anthesis. Thenectary consists of parenchyma that stores starch in the presecretorystages, and epidermis. An hour before nectar secretion begins,the starch is hydrolyzed. The nectar exudes from the stomataand forms a continuous layer on the nectary surface. Duringanthesis the nectar may all be collected by pollinators or someor all of it may remain in the nectary and be successively resorbed.The nectary parenchyma stores material for synthesizing thesugar component of nectar and stores similar material againafter nectar resorption. It is also responsible for nectar productionand secretion. The epidermis is actively involved in the reabsorptionprocess. The resorption of nectar is a phenomenon that allowsthe plant to recover invested energy. Few observations on thisphenomenon have hitherto been published. Amyloplasts; Cucurbita pepo L.; courgette; nectaries; Nectar resorption; plastid; secretion; starch     

The Extrafloral Nectaries of Cotton: II. CYTOCHEMICAL LOCALIZATION OF ATPASE ACTIVITY AND CA2+ -BINDING SITES, AND SELECTIVE OSMIUM IMPREGNATION

Localization of ATPase activity in the extrafloral nectariesof cotton (Gossypium hirsutum) shows most pronounced granularstaining at the plasmalemma of the sieve tube companion cellsbelow the nectaries and at the plasmalemma of cells in the secretorypapillae, particularly in the apical cell. Staining was muchmore intense in mature, secreting nectaries than in young, non-secretingones. When CaCl₂ is included in the fixation and washing solutionswith the aim of localizing calcium binding sites, prominentamorphous electron dense globules are seen at the plasmalemmaof the cells in the papillae, but also associated with othermembranes. Both granular and amorphous deposits are seen whenstaining for Ca²⁺-ATPase activity. Selective osmium impregnationshows pronounced staining of the ER and nuclear envelope insecreting nectaries but not in young non-secreting ones. Key words: Adenosine triphosphatase, Gossypium hirsutum, Nectary, Secretion     

Effects of Fusicoccin and Abscisic Acid on Sugar and lon Transport from Plant Glands

Fusicoccin (FC) inhibited net excretion of Cl^– by theglands of the pitchers of the carnivorous plant Nepenthes hookeriana;of Na⁺ and Cl^– by the salt glands of the halophytes Limoniumvulgare and L. pectinatum and of K⁺ in the nectar of Acer platanoidesflowers. It had no effect on K⁺ elimination with nectar of Impatienswalleriana (extrafloral nectaries) and Abutilon striatum. Abscisicacid (ABA) stimulated net excretion of K⁺ and Cl^– in Nepenthesand of Na⁺ and Cl^– in Limonium but had no effects on K⁺in nectar. Thus, FC and ABA had opposing effects on ion excretionby the salt eliminating glands of Limonium and Nepenthes. Bothcompounds, however, had similar effects on sugar secretion ofnectary glands which was either inhibited or unaffected by FCand ABA. It is suggested that the effects of FC and ABA on ion excretionby gland cells could be reconciled with literature showing FC-stimulationand possible ABA-inhibition of proton pumps at the plasmalemmaof plant cells. Nepenthes hookeriana, Limonium vulgare, Limonium pectinatum, Acer platanoides, salt-glands, nectaries, excretion, fusicoccin, abscisic acid, proton pump     

Ultrastructure and Function of Floral Nectaries of Chamelaucium uncinatum (Myrtaceae)

Nectar is secreted for up to 11d after anthesis inChamelauciumuncinatum . The volume and sucrose concentration secreted variesbetween flowers, plants and days. The period of nectar secretioncoincides with the period of pollen presentation and stigmaticreceptivity suggesting nectar is part of an efficient reproductivestrategy inC. uncinatum . The nectary ofC. uncinatum consistsof the entire upper surface of the ovary and hypanthium. Theepidermis of the nectary is covered by a thickened cuticle whichis only broken at the sites of the numerous modified stomatawhich are scattered across its surface. It is suggested thatnectar is secreted onto the surface of the ovary via these modifiedstomata. The presence of extensive and well developed endoplasmicreticulum, mitochondria and Golgi bodies in the nectar secretingcells indicates that a granulocrine mechanism of secretion isoccurring inC. uncinatum . Chamelaucium uncinatum ; Geraldton Waxflower; floral nectaries; nectar production; modified stomata     

The role of alanine synthesis and nitrate-induced nitric oxide production during hypoxia stress in Cucurbita pepo nectaries

Floral nectar is a sugary solution produced by nectaries to attract and reward pollinators. Nectar metabolites, such as sugars, are synthesized within the nectary during secretion from both pre-stored and direct phloem-derived precursors. In addition to sugars, nectars contain nitrogenous compounds such as amino acids; however, little is known about the role(s) of nitrogen (N) compounds in nectary function. In this study, we investigated N metabolism in Cucurbita pepo (squash) floral nectaries in order to understand how various N-containing compounds are produced and determine the role of N metabolism in nectar secretion. The expression and activity of key enzymes involved in primary N assimilation, including nitrate reductase (NR) and alanine aminotransferase (AlaAT), were induced during secretion in C. pepo nectaries. Alanine (Ala) accumulated to about 35% of total amino acids in nectaries and nectar during peak secretion; however, alteration of vascular nitrate supply had no impact on Ala accumulation during secretion, suggesting that nectar(y) amino acids are produced by precursors other than nitrate. In addition, nitric oxide (NO) is produced from nitrate and nitrite, at least partially by NR, in nectaries and nectar. Hypoxia-related processes are induced in nectaries during secretion, including lactic acid and ethanolic fermentation. Finally, treatments that alter nitrate supply affect levels of hypoxic metabolites, nectar volume and nectar sugar composition. The induction of N metabolism in C. pepo nectaries thus plays an important role in the synthesis and secretion of nectar sugar.     

Monocots

Green nectaries have been frequently mentioned in the literature, leading to the assumption that photosynthesis of nectaries can supply the carbohydrates secreted in the nectar, especially when storage of starch is seen in the plastids in nectaries and this starch disappears during secretion. Photosynthesis in nectaries can also provide reduction equivalents for the nectar–redox cycle and energy for secretion. However, quantitative data on the photosynthetic capacity of nectaries are largely missing. Therefore, in the present study, the photosynthetic capacity of green nectaries from a range of plants was screened; 20 floral nectaries (including six septal nectaries) and six extrafloral nectaries were studied. For the screening, chlorophyll fluorescence parameters were measured as depending on photosynthetic photon flux density (PPFD). Parameters measured were basic ground fluorescence (F) and quantum yield (Y₀) of the dark adapted sample at 0 PPFD. From the light saturation curves saturating PPFD (PPFD_sat), quantum yield at saturation (Y_sat) and maximum apparent photosynthetic electron transport rates (ETR_max) were obtained. For comparison, leaves of the plants were also measured. In most cases, the performance of the nectaries was lower than that of the leaves. F was lower in 14 floral and four extrafloral nectaries (69% of total), ETR_max was lower in 18 floral and four extrafloral nectaries (85%), Y_sat was lower in 15 floral and three extrafloral nectaries (69%). In 18 floral and two extrafloral nectaries (77%) Y₀ was well below 0.8, indicating photoinhibition. In contrast, the range of ETR_max for green nectaries was 25–140 μmol m^?2 s^?1 and overlaps well with that of green tissues in general. The lower end of the range of rates of photosynthetic carbon dioxide (CO₂) uptake of sun leaves in the literature is 10 μmol CO₂ m^?2 s^?1. Taking this value for sun‐adapted green nectaries, i.e. having a PPFD_sat > 1000 μmol m^?2 s^?1, with an area of nectar tissue measured as 3–50 mm² per flower, sugar secretion related to photosynthetic CO₂ fixation in the green nectaries is estimated at approximately 0.2–3.0 μmol hexose units flower^?1 day^?1. This is compares well in order of magnitude with the range of secretion given in the literature and clearly suggests that photosynthetic activity of green nectaries can explain a significant part, if not all, of the sugar secreted. In some nectaries ETR did not saturate with PPFD. This could be attributable to spillover from photosystem II to photosystem I and cyclic photosynthetic electron transport. It is in agreement with observations in the literature and my preliminary findings that nectary plastids often lack grana thylakoids where photosytem II is located. Cyclic photophosphorylation could provide adenosine triphosphate (ATP) energy for the nectaries. This needs further investigation. © 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173 , 1–11.     

FOLIAR NECTARIES OF UNUSUAL STRUCTURE IN LEONARDOXA AFRICANA (LEGUMINOSAE), AN AFRICAN OBLIGATE MYRMECOPHYTE

Extrafloral nectaries of an African myrmecophyte, Leonardoxa africana (Baill.) Aubr., are disc-shaped, non-vascularized, embedded in the leaf mesophyll, and have a tiny neck extending to the abaxial leaf surface. These structural features differ from those found in other members of the Leguminosae, and represent an unusual type among flowering plants. These embedded foliar nectaries apparently have an energetic advantage over external nectaries and their presence supports the idea of independent and repeated evolution of nectaries in the legume family.