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.     

多叶斑叶兰花蜜特征和分泌规律及其对昆虫访花的影响

为探讨多叶斑叶兰(Goodyera foliosa)花蜜体积和成分的变异性,及其对传粉昆虫的影响,采用毛细管法、折光仪和高效液相-蒸发光法对一天内不同时间段单花花蜜体积、总糖浓度和可溶性糖成分及含量进行检测,用摄像机对其传粉昆虫访花行为进行观察。结果表明,单花花蜜体积在10:00达到最大[(7.19±2.29)μL, n=10],糖浓度在16:00达到最大[(25.85±1.83)%,n=10]。花蜜中的主要可溶性糖为果糖和蔗糖,果糖在上午10:00含量达到最高(78.310 mg/g);蔗糖在下午16:00含量达到最高(247.600 mg/g)。中华蜜蜂(Apis cerana)访花时间为每天9:00-15:00,访花高峰期在10:00-14:00;橘尾熊蜂(Bombus friseanus)访花时间为每天8:00-16:00,访花高峰期在10:00-12:00和16:00-18:00。因此,这两种传粉昆虫访花频率与多叶斑叶兰单花花蜜体积及糖浓度的变化有一定的相关性,它们更倾向于访问较高花蜜体积及糖浓度的花朵。多叶斑叶兰花蜜成分中蔗糖占优势,能有效吸引各种蜂类、蝶类和蚁类访花,且访问频率较高,中华蜜蜂和橘尾熊蜂能携带其花粉块,是主要的有效传粉昆虫。花形态和访花昆虫的体型大小的匹配,决定了是否成为有效传粉昆虫。     

Competition Between Hummingbirds and Insects for Nectar

Preliminary data are provided suggesting that hummingbirds competewith bees and hawkmoths for flower nectar while on southwardmigration through the Sierra Nevada of California during thesummer. Evidence is given that removal of hummingbirds enhancesforaging in bees and hawkmoths on plants shared with the birds.The birds were aggressive against moths but not usually againstbees. Competition between these taxa leads to temporal and spatialpartitioning of the resource that is apparently behaviorallymediated. It is speculated that for hummingbirds competitionwith insects is less predictable and more difficult to contendwith than is competition with other hummingbirds.     

Nectar biodiversity: a short review

 Nectaries differ in many aspects but a common feature is some kind of advantage for the plant conferred by foraging of consumers which may defend the plant from predators in the case of extrafloral nectaries, or be agents of pollination in the case of floral nectaries. This minireview is concerned mainly with floral nectaries and examines the following characteristics: position in flower; nectary structure; origin of carbohydrates, aminoacids and proteins; manner of exposure of nectar; site of nectar presentation; volume and production of nectar in time; sexual expression of flower and nectary morphology; nectar composition and floral sexual expression; variability of nectar composition; fate of nectar; energy cost of nectar production. The species of certain large families, such as Brassicaceae, Lamiaceae and Asteraceae, resemble each other in nectary organisation; other families, such as Cucurbitaceae and Ranunculaceae, have various types of organisation. A scheme is presented to illustrate factors influencing nectary and nectar biodiversity. Received July 23, 2002; accepted September 18, 2002 Published online: June 2, 2003     

Proximate Costs of Competition for Nectar

Food competition among coexisting nectarivorous birds is conspicuousand often intense, affecting patterns of flower choice, dailybehavior budgets, and timing of successful reproduction. Exploitativecompetition involves loss of accumulated nectar to other individualsthat visited a flower first. Preliminary data support the useof Poisson models of the frequencies of point-source visitationand overlap for determining the probabilities of actual competitiveevents. Nectar losses from monitored flowers can be estimatedin terms of time intervals between visits weighted by flower-specificnectar production and bird-specific nectar removal capabilities.Foraging time budgets then provide a meaningful common denominatorfor assessing the impacts of competitive nectar losses, becausecompensatory increases in foraging time are required to maintaina balanced energy budget. Flexibility in foraging time budgetsmade possible by high efficiency foraging and predictably lowcompetitive losses may be an important determinant of reproductivetiming and success in nectar feeding birds. Aggressive displacement of competitors and territorial defenseof flowers are common forms of interference competition in nectar-feedingbirds. Aggression has definable caloric costs that ultimatelymust relate to caloric gains. Defense of flowers increases theaggressor's exclusive use of nectar, increases the predictabilityof a nectar supply, and increases the average amount of nectarobtained per flower. Simple cost-benefit models of territorialitydefine conditions when net benefits of territoriality are greaterthan those of alternatives.     

Nectar resource use by Colias butterflies

Summary Nectar foraging preferences of Colias butterflies in two different mountain ecosystems are examined with respect to plant distribution, nectar quantity, carbohydrate (and amino acid) content of nectar, and visual pattern of the plants utilized and avoided. Colias, and apparently numerous other small, ectothermic, low-energy-demand pollinators, patronize plants producing relatively dilute nectars containing a high proportion of monosaccharide sugars and significant amounts of polar, nitrogen-rich amino acids. These plants also converge on a common target flower pattern in ultraviolet and human-visible light. High-energy demand, endothermic pollinators, by contrast, appear to require higher concentration nectars and/or higher proportions of di- and oligosaccharide sugars. These results are discussed in the light of water balance and energy budget demands of different pollinator classes. Questions are also raised concerning behavioral aspects of pollinator search for resources and the pertinence of these data to the concept of floral mimicry.     

Nectar Production in Louisiana Iris Hybrids

Nectar is an important attractant for pollinators, and a plant's success in sexual reproduction can be influenced by the amount and concentration of nectar produced by its flowers. We studied nectar production over flower lifetime in Iris fulva, Iris brevicaulis, and four classes of hybrids-reciprocal F1's and backcrosses-between these species. Iris fulva produced less concentrated nectar than did I. brevicaulis, whereas I. brevicaulis flowers had a shorter life span. Hybrids were not intermediate, but they had the high nectar concentration of I. brevicaulis combined with the long life span of I. fulva flowers. Nectar production and concentration declined after the first day in all classes, but flowers continued to produce nectar until they were completely wilted. Backcrosses did not show a shift in mean or increased variation for the characters that distinguished the parental species; backcrosses toward I. fulva retained the high nectar concentration of I. brevicaulis, and backcrosses toward I. brevicaulis did not have a reduced flower life span. Overall, F1 hybrid flowers produced the highest amounts of nectar and nectar sugar over their life spans. These results, together with previously obtained data on pollinator choice in mixed arrays of the same flower classes, show that F1 hybrids between these species do not suffer from reduced attractiveness to pollinators. F1 individuals produced more nectar and nectar sugar than did their parents, and thus, they are possibly even more attractive to pollinators that forage for nectar.     

Silencing NaTPI Expression Increases Nectar Germin,Nectarins, and Hydrogen Peroxide Levels and Inhibits Nectar Removal from Plants in Nature

Native flower visitors removed less nectar from trypsin proteinase inhibitor (TPI)-silenced Nicotiana attenuata plants (ir-pi) than from wild-type plants in four field seasons of releases, even when the nectar repellant, nicotine, was also silenced. Analysis of floral chemistry revealed no differences in the emission of the floral attractants benzylacetone and benzaldehyde or in the concentrations of nectar sugar and nicotine between wild-type and ir-pi flowers, suggesting that these two lines are equally able to attract insect visitors. TPI activity was found in all wild-type flower parts and was highest in anther heads, while TPI activity was not found in any parts of ir-pi flowers. The nectar of ir-pi flowers contained 3.6-fold more total proteins than the nectar of wild-type flowers. Proteomics analysis and hydrogen peroxide (H₂O₂) measurements revealed that ir-pi nectar contained more nectarins and nectar germin-like proteins and about 1.5-fold more H₂O₂ compared with wild-type nectar. Field experiments with wild-type flowers supplemented with a solution containing sugar and glucose oxidase demonstrated a causal association between the accumulation of H₂O₂ and the reduction in nectar removal. These results showed that silencing TPI expression increases the accumulation of nectar proteins and H₂O₂ levels, which in turn reduces nectar removal by native insect floral visitors. The effect of silencing TPIs on nectar protein accumulation suggests an endogenous regulatory function for TPIs in N. attenuata flowers. The repellency of H₂O₂ to floral visitors raises new questions about the qualities of nectar that make it attractive for pollinators.Floral nectar is an innovative feature of plants that is thought to have evolved as a reward for pollen-transporting floral visitors. Sugars (e.g. Glc, Fru, and Suc), amino acids, and lipids (Baker and Baker, 1982, 1986) provide nutritional rewards that are essential for many pollinators. But nectar is also known to contain other compounds, such as volatile organic compounds (VOCs), alkaloids, phenolics, and nonprotein amino acids (Baker, 1977, 1978; Raguso, 2004; Kessler and Baldwin, 2007), which do not increase the nutritional value of nectar. Nectar is also exploited as a food source by nectar robbers and nectar-infesting microorganisms, which do not provide mutualistic services to the plant and are known to directly reduce a plant''s fitness either by competing with pollinators or by infesting reproductive organs (Traveset et al., 1998; Irwin and Brody, 1999; Maloof and Inouye, 2000; Farkas et al., 2007). Therefore, flowers must solve the dilemma of repelling nectar thieves or florivores that provide no pollination services while simultaneously attracting fitness-enhancing pollinators.Most of the defensive compounds in nectar have been reported to act selectively (i.e. only on antagonists). For example, the floral nectar of Catalpa speciosa contains iridoid glycosides that fend off nectar robbers but not the plant''s specific pollinators (Stephenson, 1981). Similarly, the presence of phenols in the floral nectar of Aloe vryheidensis lowers its palatability to generalist floral visitors like sunbirds or honey bees while not affecting the attractiveness of the nectar to a specialist bird, the dark-capped bulbul (Johnson et al., 2006). In its native habitat, Nicotiana attenuata (Solanaceae) maximizes its maternal and paternal reproductive success while repelling herbivores, florivores, and nectar robbers by producing a sophisticated blend of both repellants (nicotine) and attractants (benzylacetone) in its nectar and floral head space (Kessler et al., 2008) as well as by changing its floral phenology in response to herbivore attack, so as to switch from the use of night-active hawkmoth pollinators, which oviposit herbivores on the plants they pollinate, to day-active hummingbird pollinators, which do not (Kessler et al., 2010). While this sophisticated use of chemical attractants and repellants is likely a common solution to the dilemma, very little is known about the function of most secondary metabolites found in nectar (Thornburg, 2007).Similar chemically mediated strategies are used to solve a similar problem when plants use a combination of direct and indirect defenses to protect their leaves from herbivore attack (Halitschke et al., 2008). In N. attenuata, attack by the specialist herbivore Manduca sexta elicits a remarkable array of direct and indirect defenses, most of which are elicited by the jasmonate signaling pathway in response to herbivore-specific elicitors (Baldwin, 2001; Kessler and Baldwin, 2002; Wu and Baldwin, 2009). These herbivory-elicited responses include the accumulation of toxins and digestibility reducers, which function as direct defenses, as well as the release of a complicated blend of VOCs (Gaquerel et al., 2009), which repel further oviposition by M. sexta moths and attract predacious bugs that feed on M. sexta eggs or larvae, thereby functioning as an indirect defense (Kessler and Baldwin, 2001).Once herbivores start feeding on N. attenuata leaves, they are frequently repelled by a suite of locally and systemically elicited direct defenses (Steppuhn et al., 2008). Trypsin protease inhibitors (TPIs) are an effective component of this inducible defensive system that reduces the performance of folivores by targeting their main proteolytic digestive enzymes and is strongly induced by herbivore attack (van Dam et al., 2000; Glawe et al., 2003; Zavala et al., 2004b, 2008; Horn et al., 2005). However, in N. attenuata, the biosynthesis of TPIs incurs substantial fitness costs (Zavala et al., 2004a); silencing the TPI gene in N. attenuata abolishes the plant''s capacity to produce TPIs and allows it to grow faster, flower earlier, and produce more seed capsules compared with TPI-producing genotypes (Zavala et al., 2004a). Similarly, restoring TPI production by transforming an ecotype of N. attenuata naturally deficient in TPI production (Wu et al., 2007) reduces lifetime seed production (Zavala et al., 2004a). TPIs are not only restricted to leaves but accumulates in reproductive organs, where they may protect these fitness-valuable tissues against attack from florivores and microbes. Atkinson et al. (1993) and Johnson et al. (2007) elegantly demonstrated that TPIs dramatically accumulate in Nicotiana alata stigmas to become the most abundant protein in these tissues. PIs have been reported to accumulate in Solanum americanum seeds, where they were shown to play an important role in seed development (Suk-Fong et al., 2006). These studies highlight that while it is clear that TPIs occur at high levels in reproductive organs, their role in floral function has not been thoroughly explored.As part of a research program to study the defensive functions of nicotine and TPIs against folivores, we planted N. attenuata plants that had been transformed with RNA interference constructs to silence their nicotine (ir-pmt), TPI (ir-pi), or both (ir-pmt/pi) in the plant''s native habitats in Utah during four field seasons. Serendipitously, we noticed that the amount of nectar removed by the native community of floral visitors from ir-pmt/pi plants did not differ from that removed from wild-type plants, although we had recently discovered that silencing nicotine alone (ir-pmt) consistently increased nectar removal (Kessler and Baldwin, 2007). These observations suggested that silencing TPIs alone might impede nectar removal by the native community of floral visitors. During two field seasons (2007 and 2009), we compared the amount of nectar removed from wild-type plants and from TPI-silenced plants (ir-pi) and found that, indeed, less nectar was consistently removed from ir-pi plants. To understand these observations, we compared the floral chemistry of wild-type and ir-pi plants, including floral volatiles, nectar sugar, nicotine, and proteomes. We found that silencing TPIs increased the accumulation of nectar proteins, especially the nectar germin-like proteins (GLPs) and nectarins, which are known to participate in the nectar redox cycle and generate hydrogen peroxide (H₂O₂; Carter and Thornburg, 2000). Consistent with data on nectar proteins, we also found significantly higher levels of H₂O₂ in the nectar of ir-pi plants compared with those of wild-type plants. To test whether the differences in the accumulation of H₂O₂ in the nectar of ir-pi and wild-type plants could explain the nectar removal observations in the field, we experimentally increased H₂O₂ in the nectar of wild-type plants to the levels found in ir-pi nectar using a mixture of Glc oxidase (GOX) and Glc and compared nectar removal by the native community of floral visitors.     

Nectar: generation, regulation and ecological functions

Nectar contains water, sugars and amino acids to attract pollinators and defenders and is protected from nectar robbers and microorganisms by secondary compounds and antimicrobial proteins. Floral and extrafloral nectar secretion can be induced by jasmonic acid, it is often adjusted to consumer identity and consumption rate and depends on invertase activity. Invertases are likely to play at least three roles: the uploading of sucrose from the phloem, carbohydrate mobilization during active secretion and the postsecretory adjustment of the sucrose:hexose ratio of nectar. However, it remains to be studied how plants produce and secrete non-carbohydrate components. More research is needed to understand how plants produce nectar, the most important mediator of their interactions with mutualistic animals.     

Nectar secretion inAbutilon: a new model

Summary Nectary trichomes ofAbutilon striatum secrete copious amounts of sucrose, fructose and glucose. The nectar emerges from transient pores in the cuticle overlying the trichome tip cells. Calculations of the required transmembrane fluxes, either across the tip cell plasmalemma or across the cell membrane of the whole trichome, give very high rates compared with those obtained from other situations in plants and, therefore, cast doubt on the possibility that nectar secretion inAbutilon is an eccrine process. Quantitative evaluation of the possibility of granulocrine secretion, by successive fusion of vesicles with the cell membrane, suggests that this is an even less probable mechanism of secretion. Rapid freezing followed by freeze-substitution or freeze-fracture replication reveals that an extensive secretory reticulum (SR) is present within the hair cells. As similar micrographs are obtained from conventional, chemical fixation it is argued that the secretory reticulum is a relatively stable endomembrane system. Freeze-fracture and freeze-substitution micrographs show that this internal membrane system is closely associated with the plasmalemma. Taken together with other structural information, as well as physiological data, it is concluded that prenectar is actively loaded into the secretory reticulum of all trichome cells. Increase in hydrostatic pressure within this compartment leads to the opening of sphincters which connect the cisternal space of the SR to the outside of the plasmalemma. Thus a pulse of nectar is forcibly expelled into an apoplastic compartment sealed to the outside by the impermeable cuticle and on the inside by the plasmalemma. As this apoplastic compartment is also sealed at the stalk cell, the only route for pressure release is via the transient pores which overlay the tip cell. Distension renders these patent so that, again, pulsed secretion is observed. This hypothesis overcomes the necessity for envisaging excessively high transmembrane fluxes or rates of vesicle fusion. It would imply the need for a continuing supply of prenectar to the hair cells accompanied by active loading into the SR. This loading process may well be supported by the hydrolysis of sucrose to glucose and fructose and is probably the site where ions and other low molecular weight solutes are filtered from the nectar.     

Nectar resource diversity organises flower-visitor community structure

Communities of nectar‐producing plants show high spatio‐temporal variation in the patterns of volume and concentration presentation. We illustrate a novel approach for quantifying nectar reward structures in complex communities, demonstrating that nectar resource diversity (defined as the variety of nectar volume–concentration combinations available) may be a fundamental factor organising nectarivore communities. In a series of diverse bee and entomophilous flower communities in Israel, our measure of nectar resource diversity alone explains the majority of variation in bee species richness, while other nectar variables (volume, concentration, energy value, and water content) have little predictive value per se. The new measure of nectar resource diversity is highly correlated with floral species richness and particularly with the species richness of annuals, yet it is additive in its effect on bee diversity. We conclude that relying solely upon measurements of mean nectar volume and mean nectar concentration overlooks a key characteristic of community‐level reward structure, nectar resource diversity, so that previous studies may have failed to identify an important determinant of flower‐visitor community structure.     

刺五加、短梗五加的花蜜分泌节律、花蜜成分及访花者多样性的比较研究

野外定位观测刺五加（Eleutherococcus senticosus）、短梗五加（E.sessiliflorus）的花蜜分泌节律、访花者的多样性,室内分析其花蜜的主要成分。结果表明,刺五加雄株的花杂在开花1－3（4)d分泌花蜜,雌株在开花5－7,6－8或7－9d分泌花蜜;短梗五加以及刺五加两性株的部分花杂,在开花后有两次分泌花蜜的过程：第1次与花药开裂散粉时间一致,第2次与柱头具可授性的时间一致。而且,刺五加和短梗五加都由动物帮助传粉,花蜜分泌的时间与多数访花者的访花时间一致,在一天之中,散出花粉的花朵分泌花蜜的时间早于接受花粉的花杂,这种时间差异应该是植物控制该 花者流向并导致传粉成功的关键。短梗五加与刺五加之间以及刺五加不同性别的植株之间,花蜜的成分及相对含量各有特点,但都以果糖和葡萄糖为主。在刺五加、短梗五加花朵上记录到的访花昆明分别为50余种和40余种,多数隶属于膜翅目、鳞翅目、鞘翅目和半翅目。其中膜翅目的胡蜂、马蜂、熊蜂,双翅目的食蚜蝇、寄蝇等是刺五加、短梗五加的常见访花者。     

Nectar sugars and subgeneric classification in Fritillaria

The analysis by PC and/or GLC of the nectar sugars of about 30 species of Fritillaria confirms the well-known special position within the genus of F. imperialis, with no detectable sucrose, and underlines the distinction (proposed in a recent revision of the genus) between the species of the series Meleagris, with about equal fructose and glucose, and the members of the other series of section Trichostylae Boiss., which show a distinct preponderance of fructose over glucose.     

Nectar intake and energy expenditure in a flower visiting bat

Summary In a coastal region of Venezuela the daily energy expenditure (DEE) and water turnover of the flower visiting bat Anoura caudifer was measured by using the doubly labeled water method. In flower visitors, this method allows independent measurement of energy intake and expenditure if the animals drink no additional water and if the nectar's energy content is known. An average DEE of 12.4 kcal/d and water exchange of 13.4 ml/d were found. Our data show a balanced energy budget when animals in the field imbibe nectar with a sugar concentration of 18–21%, which is roughly medial in the range of nectar concentrations of various bat flowers. The energy turnover of flower visiting bats is high compared with DEEs of other bat species, small mammals and birds; flower visiting bats seem to belong to those species having a fast spin of the life motor.     

Nectar feeding and body size in the ant Camponotus mus

Summary: Feeding behavior and worker polymorphism were studied in the ant Camponotus mus. Individual workers were conditioned to visit an arena in which an ad libitum food source of 60% (w/w) sucrose solution was offered. Individual feeding time, crop load, and intake rate were recorded. Worker head width and pronotum width were measured. Behavioral and morphometric variables were analyzed in relation to ant weight. C. mus workers of the laboratory nest have an elemental polymorphism with monophasic allometric growth. Worker size affected feeding dynamics. Load weight and intake rate were positively correlated with ant weight, whereas feeding time was independent of ant weight. Size related differences in intake rate could not be attributed to differences in pumping frequency, and could be attributed to differences in the volume ingested per pumping cycle.     

The Microflora of Frozen Passionfruit Nectar Base

A considerable and varied microflora is introduced into passionfruit nectar base under present methods of production. In spite of the great acidity of the nectar base (pH range: 2.8 to 3.2), the high sucrose concentration (approximately 50%), and storage at -20 C, remnants of the microflora persist for a year or longer. During storage, however, there is a steady and gradual decrease, until after about 18 months the microflora is near to extinction. Sample regression lines show straight-line slopes for this diminution in numbers.

A battery of nine media was used to grow a representative aerobic flora. Purified cultures of isolates were identified to genera. Yeasts were the most numerous organisms in all samples, followed by molds, bacteria, and streptomycetes. The bacteria were the first group to disappear during storage. No fecal streptococci or gram-negative bacilli were found in any samples.

     

Nectar dispersion patterns in three Australian plant species

Standing crops of floral nectar were measured from three species of plants: Dampiera stricta, Goodenia bellidifolia and Aotus ericoides. The amount of nectar in nearest neighbour blossoms in nine of the 10 samples were significantly correlated with one another suggesting that patches of high and low reward quality exist in these species. The patterns further suggest that the pollinators of these species employ arearestricted searching behaviour. The range of variability in standing crops for all three species was quite high. The nature of that variability suggests that A. ericoides is less variable in its rate of nectar production than are the other two species and, therefore, that it has the potential to be more tightly co-evolved with its pollinators.