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.     

Characteristics of floral nectaries and nectar in two species of Crataegus (Rosaceae)

 The structure of floral nectaries of Crataegus coccinea and C. crus-galli was examined using light and scanning electron microscopy. The radial length of the floral nectary, measured from longitudinal sections of flowers, was 30% larger in C. crus-galli than in C. coccinea. For both Crataegus species the glandular tissue thickness was similar – approx. 400 μm. Also, the number of stomata per mm² of nectary surface in C. crus-galli was much higher (by 43%) than for C. coccinea. Stomata were situated in deep hollows. For both taxa the period of nectar secretion was 4 days. The mean quantity of total sugar in nectar per 10 flowers of C. crus-galli and C. coccinea was 3.87 mg and 0.33 mg, respectively. Received August 28, 2002; accepted December 17, 2002 Published online: June 2, 2003     

Variability of nectar production and composition in Linaria vulgaris (L.) Mill. (Scrophulariaceae)

 We studied nectar characteristics during the long flowering period (late June to end of November) in two populations of Linaria vulgaris (L.) Mill. spontaneously growing in the Botanical Gardens of Siena University (Tuscany, central Italy). The two populations were close to each other but they differed in blooming period. Plants of population 1 sprouted in May and flowered from the end of June to the end of September. Population 2 sprouted at the end of August and flowered from September to the end of November. Differences in nectar production and composition were found between and within populations. Flowers of population 1 produced a very small amount of nectar (not collectable) that remained on the nectary surface. The quantity of nectar increased in late September, when each flower produced 2–3 μl of nectar that flowed into the spur. Total sugar concentration was 175.8 mg/ml in young flowers. Flowers of population 2 produced 5–8 μl of nectar with a total sugar concentration of 200.9 mg/ml in the young stage. In bagged senescent flowers nectar volume decreased in both populations and nectar sugar concentration decreased down to 11.6 mg/ml in population 2 and increased up to 289.6 mg/ml in population 1. For both populations, the decrease in nectar volume in bagged flowers may have been due to water loss by evaporation. In population 2, the decrease in sugar concentration may have been due to nectar reabsorption that was never observed in population 1. Nectar variability is discussed in relation to insect visits and seed set. Received August 14, 2002; accepted December 17, 2002 Published online: June 2, 2003     

Interpopulation variation in nectar production in Aconitum columbianum (Ranunculaceae)

Summary In Aconitum columbianum there are extreme interpopulation differences in rates of nectar secretion per flower. Since nectar sugar concentration varies little among populations, increased nectar secretion results in a greater mass of sugar per flower for pollinator attraction. These differences in the amount of reward offered per flower account at least in part for observed higher levels of pollinator activity in populations with high nectar production. Nectar production is correlated also with nectary depth, i.e., flowers in populations with deep nectaries have higher rates of nectar secretion than those with shallow nectaries. Nectary depth differences adapt populations to different pollinator-types. Populations with deeper nectaries are adapted to pollination by bumblebees with longer tongues and more specialized foraging behaviors. In conclusion, there are basic differences in pollination ecology among geographical races of a. columbianum, which are indicated by correlated interpopulution differences in (1) nectar production, (2) level of pollinator activity, (3) nectar depth, and (4) pollinator-type.     

Nectar secretion pattern of the dish-shaped flower,Cayratia japonica (Vitaceae), and nectar utilization patterns by insect visitors

We studied the relationship between the diurnal nectar secretion pattern of flowers of Cayratia japonica and insect visiting patterns to these flowers. Flower morphology of C. japonica changed greatly for about 12 hours after flower-opening and the maximum duration of nectar secretion was 2 days. The nectar volume peaked at 11∶00 and 15∶00, and declined at night and at 13∶00 regardless of time elapsed after flower-opening. The nectar volume at the two peaks was, on average, 0.25 μl on bagged inflorescences and 0.1μl on unbagged inflorescences (both, sugar concentration=60%). The flower secreted nectar compensatory when the nectar was removed. This means that insects consume more nectar than the difference of nectar volume between bagged and unbagged flowers. Apis cerana is a primary visitor of this flower, and was the only species for which we confirmed pollen on the body, among many species of flower visiting insects to this flower. Apis cerana visited intensively at the two peaks of nectar secretion. Visits of the other insects were rather constant or intensive only when there was no nectar secretion. Thus flowers of C. japonica with morphologically unprotected nectaries may increase likelihood that their nectar is used by certain pollinators, by controlling the nectar secretion time in day. In this study the pattern of nectar secretion allowed A. cerana maximum harvest of nectar.     

Nectar-carbohydrate production and composition vary in relation to nectary anatomy and location within individual flowers of several species of Brassicaceae

Nectar-carbohydrate production and composition were investigated by high-performance liquid chromatography and enzymology in nine species from five tribes of the Brassicaceae. In six species (Arabidopsis thaliana (L.) Heynh., Brassica napus L., B. rapa L., Lobularia maritima (L.) Desv., Raphanus sativus L., Sinapis arvensis L.) that produced nectar from both lateral nectaries (associated with the short stamens) and median nectaries (outside the long stamens), on average 95% of the total nectar carbohydrate was collected from the lateral ones. Nectar from these glands possessed a higher glucose/fructose ratio (usually 1.0–1.2) than that from the median nectaries (0.2–0.9) within the same flower. Comparatively little sucrose was detected in any nectar samples except from Matthiola bicornus (Sibth. et Sm.) DC., which possessed lateral nectaries only and produced a sucrose-dominant exudate. The anatomy of the nectarial tissue in nectar-secreting flowers of six species, Hesperis matronalis L., L. maritima, M. bicornus, R. sativus, S. arvensis, and Sisymbrium loeselii L., was studied by light and scanning-electron microscopy. Phloem alone supplied the nectaries. However, in accordance with their overall nectar-carbohydrate production, the lateral glands received relatively rich quantities of phloem that penetrated far into the glandular tissue, whereas median glands were supplied with phloem that often barely innervated them. All nectarial tissue possessed modified stomata (with the exception of the median glands of S. loeselii, which did not produce nectar); further evidence was gathered to indicate that these structures do not regulate nectar flow by guard-cell movements. The numbers of modified stomata per gland showed no relation to nectar-carbohydrate production. Taken together, the data on nectar biochemistry and nectary anatomy indicate the existence of two distinct nectary types in those Brassicacean species that possess both lateral and median nectaries, regardless of whether nectarial tissue is united around the entire receptacle or not. It is proposed that the term “nectarium” be used to represent collectively the multiple nectaries that can be found in individual flowers. Received: 21 July 1997 / Accepted: 19 September 1997     

Secretion and composition of nectar and the structure of perigonal nectaries in <Emphasis Type="Italic">Fritillaria meleagris</Emphasis> L. (Liliaceae)

The structure of perigonal nectaries, nectar production and carbohydrate composition were compared at various stages in the lifespan of the flower of Fritillaria meleagris L. The six nectaries each occupied a groove that is located 2–4 mm above the tepal base. The average nectary measured 11.0 mm long and 1.0–1.2 mm wide. The structure of nectaries situated on both inner and outer tepal whorls was identical, and at anthesis they were equally accessible to potential pollinators. However, secretion from nectaries associated with inner tepals tended to exceed that produced by nectaries located on the outer tepals. On average, regardless of flower stage, one flower secreted 10.87 ± 12.98 mg of nectar (mean and SD; N = 182). The nectar concentration ranged between 3 and 75%, with average concentration of sugars exceeding 50%. Both nectar production and concentration were dependent on the stage of anthesis, with the highest scores being recorded during full anthesis (21.75 ± 16.08 mg; 70.5%, mass and concentration, respectively) and the lowest at the end of anthesis (1.32 ± 2.69 mg; 16.9%, mass and concentration, respectively). A decline in both mass of nectar secreted and nectar concentration during the final stage of anthesis indicates nectar resorption. Nectar was composed of sucrose, glucose and fructose in approx. equal quantities, and its composition did not change significantly during subsequent stages of flowering. The nectaries comprised a single-layered secretory epidermis and several layers of subepidermal parenchyma. The nectariferous cells did not accumulate starch during any of the investigated stages. The nectary was supplied with one large and several smaller vascular bundles comprising xylem and phloem. Transport of assimilates and nectar secretion by protoplasts of secretory cells (and probably also nectar resorption) were facilitated by cell wall ingrowths present on the tangential walls of epidermal cells and subepidermal parenchyma. Epidermal cells lacked stomata. Nectar passed across the cell wall and through the cuticle which was clearly perforated with pores.     

A novel indirect defence in Brassicaceae: Structure and function of extrafloral nectaries in Brassica juncea

While nectaries are commonly found in flowers, some plants also form extrafloral nectaries on stems or leaves. For the first time in the family Brassicaceae, here we report extrafloral nectaries in Brassica juncea. The extrafloral nectar (EFN) was secreted from previously amorphic sites on stems, flowering stalks and leaf axils from the onset of flowering until silique formation. Transverse sections at the point of nectar secretion revealed a pocket‐like structure whose opening was surrounded by modified stomatal guard cells. The EFN droplets were viscous and up to 50% of the total weight was sugars, 97% of which was sucrose in the five varieties of B. juncea examined. Threonine, glutamine, arginine and glutamate were the most abundant amino acids. EFN droplets also contained glucosinolates, mainly gluconapin and sinigrin. Nectar secretion was increased when the plants were damaged by chewing above‐ and belowground herbivores and sap‐sucking aphids. Parasitoids of each herbivore species were tested for their preference, of which three parasitoids preferred EFN and sucrose solutions over water. Moreover, the survival and fecundity of parasitoids were positively affected by feeding on EFN. We conclude that EFN production in B. juncea may contribute to the indirect defence of this plant species.     

Floral rewards and pollination in Cytiseae (Fabaceae)

 Cytiseae have been reported to be mostly nectar-lacking, yet some taxa secrete nectar from extrastaminal nectaries. We studied the pollination biology of four shrubby species of Cytiseae (Cytisophyllum sessilifolium (L.) Lang, Spartium junceum L., Genista radiata (L.) Scop., Genista cilentina Valsecchi) which differ for ecology, distribution and population size. All species resulted obliged xenogamous, insect visits being necessary for successful pollination. Bumblebees and solitary bees are the principal pollinators, but also many beetles, some hover-flies, and few bugs visit flowers. Pollinator specificity is low, and this may be the reason of the scarce seed set compared to the number of ovules. Pollen is the main reward, but traces of glucose were detected in all species, at the base of vexillum or on the reproductive column. Nectar production is irregular in time, and apparently unpredictable. We suppose that nectar may play a role in attracting pollinators determining their right position for a successful pollination. Received August 12, 2002; accepted November 25, 2002 Published online: June 2, 2003     

芝麻菜花蜜腺的结构和发育研究

通过解剖镜观察、石蜡切片和薄切片等方法,对芝麻菜的花蜜腺的位置、形态、结构、发育过程及泌蜜前后组织化学变化进行了研究。芝麻菜花蜜腺4枚,分成两对,其中一对侧蜜腺较大,棱柱状,分别着生在外轮2个短雄蕊基部内侧的花托上,结构上由表皮、产蜜组织和维管组织构成;另一对中蜜腺较小,近棒状,分别着生在内轮4个长雄蕊外侧的花托上,结构上仅由表皮和产蜜组织构成。二者表皮细胞外都具角质层,且蜜腺产蜜组织细胞中只含少量的多糖物质。两类蜜腺的蜜汁均由变态气孔泌出体外。无论侧蜜腺还是中蜜腺,蜜腺原基皆是在雌、雄蕊已分化后,由花托相应位置表皮下的1～2层细胞分裂形成的。在蜜腺发育中,产蜜组织细胞在泌蜜前后不具明显的液泡变化。     

Floral longevity and nectar secretion of Platanthera chlorantha (Custer) Rchb. (Orchidaceae)

Flowering and nectar secretion were studied in Platanthera chlorantha in two years. Nectar was secreted and accumulated in this orchid's spur, originating from part of the labellum. The nectary spur was, on average, 32 mm long. It produced 6.86 micro l nectar in 1999 and 7.84 micro l in 2000. The number of flowers per inflorescence and the volume of nectar secreted per flower were not correlated. Nectar secretion and flower longevity differed depending on pollination and flower position in the inflorescence. Among pairs of pollinated and unpollinated flowers there was no difference in the volume of nectar produced; however, the life span of pollinated flowers was shorter than that of unpollinated ones. Within an inflorescence, the lowest-positioned flowers had the largest nectar production and the longest life compared with flowers positioned higher up.     

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     

Nectar resorption in the spur of Platanthera chlorantha Custer (Rchb.) Orchidaceae – structural and microautoradiographic study

 In the flowers of Platanthera chlorantha nectar is secreted and accumulated in a spur. Previous studies of this species revealed that after the period of secretion and cessation, rapid nectar resorption occurs. The aim of this study was the observation of nectar resorption on the structural level using tritium labelled sucrose. For this purpose, during the peak of nectar secretion, 10 μl of nectar accumulated in the spur was replaced with the same volume of labelled sucrose (10 μCi). Small fragments of spurs were sampled between 12–36 h of incubation, at the resorption phase. Afterwards, they were fixed and embedded in epoxy resin. Material for the microautoradiographic study was prepared with the dipping technique. The cells of secretory epidermis and unicellular secretory papillae had dense, strong stained cytoplasm, a large nucleus and small vacuoles. A characteristic feature of these cells was the presence of numerous starchless plastids, mitochondria and ER profiles. Many vesicles occurred in the close vicinity of the cell wall. In the cuticle covering cell walls no pores or cracks were observed. Presence of [³H] sucrose was detected mainly in the walls of the nectary cells, which would indicate an apoplastic route of resorbed nectar. Received August 3, 2002; accepted November 10, 2002 Published online: June 2, 2003     

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.     

Inter- and intra-floral heterogeneity of nectar production in Helleborus foetidus L. (Ranunculaceae)

Three southern Spanish populations of Helleborusfoetidus L. (Ranunculaceae) were sampled for nectar content in the absence of nectarivorous flower visitors. Nectar volume was measured in individual nectaries of flowers at the same stage in the anthesis cycle. Total nectar content per flower was extremely variable between plants within populations and between flowers within plants, but much less so between populations. Average sugar content per flower was roughly similar in the three populations sampled. High variances in nectar abundance occurred also among nectaries within the same flower. Heterogeneity in pollinator rewards thus occurs simultaneously at several spatial scales, namely between nectaries, between flowers and between plants. This implies a strong component of uncertainty to foraging pollinators, which may therefore tend to avoid Helleborus flowers.     

黄杨花蜜腺的形态解剖学研究

黄杨花单性,雌雄同株,雄花花蜜腺４枚,乳头状,着生于退化雌蕊子房顶部;雌花花蜜腺３枚,短柱状,位于３枚花柱之间。雌、雄花蜜腺均由分泌表皮、产蜜组织和维管束构成,在发育过程中产蜜组织细胞的液泡都发生有规律的变化。雌花蜜腺大,属非淀粉型蜜腺,泌蜜量大,蜜汁含糖分多,维管束中仅含韧皮部;雄花蜜腺小,属淀粉型蜜腺,泌蜜量小．蜜汁含糖量小,维管束由木质部和韧皮部构成。     

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     

FLORAL BIOLOGY OF NEPENTHES GRACILIS (NEPENTHACEAE) IN SUMATRA

Nepenthes gracilis, a dioecious carnivorous plant, has inconspicuous flowers lacking petals. Nectaries distributed on the upper surface of four sepals secrete dilute nectar (3%–12% sugar concentration) at night, but the nectar immediately disappears during the day by evaporation in the sunny environment of Sumatra. Male flowers have a higher nectar production rate but lower sugar concentration of nectar than female flowers. Flowers of both sexes were visited by pyralid moths at night and by calliphorid flies in the evening. Pollen was found attached on these insects visiting Nepenthes flowers. The pattern of nectar production of sepals is regarded as attracting nocturnal flying insects and avoiding ants, while the pitchers attract ants by nectar secreted on the pitcher rim.     

Structure,ultrastructure and evolution of floral nectaries in the twinflower tribe Linnaeeae and related taxa (Caprifoliaceae)

Linnaeeae is a small tribe of Caprifoliaceae consisting of six genera and c. 20 species. In Linnaeeae, floral nectaries are located on the corolla‐filament‐tube and nectar is produced from unicellular glandular hairs. We studied 23 taxa using scanning electron microscopy (SEM), light microscopy (LM) and transmission electron microscopy (TEM) and found two distinct nectary morphologies, zonate and gibbous types, and two distinct types of glandular hair, clavate and smooth base types. Plesiomorphic characters associated with the nectary and identified in the tribe include hypocrateriform corollas, dichogamous flowers, zonate nectaries, wet papillate stigmas, vestigial nectary disc and smooth pollen grains. Apomorphic characters include bilabiate corollas, homogamous flowers, bulging nectaries, dry papillate stigmas and echinulate pollen grains. The nectary structure is similar in Vesalea and Linnaea and differs from the rest of the tribe, in accordance with recent phylogenetic results. Nectar secretion is typically granulocrine with subcuticular accumulation of nectar, which we compared with the secretion in multicellular hairs of Adoxa moschatellina. The cuticle on the hair becomes detached from the cell wall and large subcuticular spaces filled with nectar are formed. Nectar is probably released in areas with a thin cuticle. In Zabelia, the smooth basal part of the hair could help to build up the hydrostatic pressure.     

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