Background and Aims
Many orchid flowers have glands called elaiophores and these reward pollinating insects with oil. In contrast to other reward-producing structures such as nectaries, the anatomy of the elaiophore and the process of oil secretion have not been extensively studied. In this paper, elaiophore structure is described for two members of Oncidiinae, Oncidium trulliferum Lindl. and Ornithophora radicans (Rchb.f.) Garay & Pabst.
Elaiophores of both species were examined using light microscopy, scanning electron microscopy and transmission electron microscopy.
Key Results and Conclusions
In flowers of Oncidium trulliferum and Ornithophora radicans, oil is secreted by morphologically distinct elaiophores associated with the labellar callus. However, in O. trulliferum, elaiophores also occur on the lateral lobes of the labellum. In both these species, the epithelial elaiophores are composed of a single layer of palisade-like epidermal cells and a distinct subepithelial layer. Secretory elaiophore cells may contain numerous, starchless plastids, mitochondria and smooth endoplasmic reticulum profiles. In O. trulliferum, the cytoplasm contains myelin-like figures but these are absent from O. radicans. In the former species, cavities occur in the cell wall and these presumably facilitate the passage of oil onto the elaiophore surface. In O. radicans, the accumulation of oil between the outer tangential wall and the cuticle causes the latter to become distended. Since it is probable that the full discharge of oil from the elaiophores of O. radicans occurs only when the cuticle is ruptured by a visiting insect, this may contribute towards pollinator specificity. The structure of the elaiophore in these species resembles both that found in previously investigated species of Oncidiinae and that of certain members of the Malpighiaceae.
Elaiophore; Oncidium trulliferum; Ornithophora radicans; Orchidaceae; oil secretion; pollination
Recently, there have been a lot of intense debates about the acceptance/rejection of paraphyletic groups in biological classification. On the one hand, evolutionary classification states that similarity and common descent are two criteria for biological classification and paraphyletic groups are natural units of biological classification. On the other hand, cladistic classification considers that common descent is the only criterion in biological classification and monophyly should be strictly adhered to. Holcoglossum is used herein as a case to illustrate this problem. Although Holcoglossum is a small orchid genus of less than 20 species, there is little consensus about its generic circumscription since it was established, which leads to confusion in taxonomic treatments in the Aerides-Vanda group. Based on the analyses of molecular and morphological evidence, our results suggest that the clade comprising Holcoglossum s.s., Ascolabium, Penkimia and Ascocentrum himalaicum is strongly supported as a monophyly, and that the three taxa are nested within different subclades of Holcoglossum s.s. Thus, it is reasonable to recognize a monophyletic circumscription of Holcoglossum, which is also well supported by some vegetative and floral characters. The Holcoglossum s.l. would facilitate a better understanding of pollinator-driven floral divergence and vegetative stasis than a paraphyletic and narrowly defined genus.
• Background and Aims In spite of the impressive species diversity in the Asteraceae and their widespread appeal to many generalist pollinators, floral-nectary ultrastructure in the family has rarely been investigated. To redress this, a study using Echinacea purpurea, a plant of horticultural and nutraceutical value, was undertaken. Nectar secretion of disc florets was compared with floral nectary ultrastructure taking into account nectar's potential impact upon the reproductive success of this outcrossing species.
• Methods Micropipette collections of nectar in conjunction with refractometry were used to determine the volume and nectar-sugar quantities of disc florets throughout their phenology, from commencement of its production to cessation of secretion. Light, scanning-electron and transmission-electron microscopy were utilized to examine morphology, anatomy and ultrastructure of nectaries of the disc florets.
• Key Results Florets were protandrous with nectar being secreted from anthesis until the third day of the pistillate phase. Nectar production per floret peaked on the first day of stigma receptivity, making the two innermost whorls of open florets most attractive to foraging visitors. Modified stomata were situated along the apical rim of the collar-like nectary, which surrounds the style base and sits on top of the inferior ovary. The floral nectary was supplied by phloem only, and both sieve elements and companion cells were found adjacent to the epidermis; the latter participated in the origin of some of the precursor cells that yielded these specialized cells of phloem. Companion cells possessed wall ingrowths (transfer cells). Lobed nuclei were a key feature of secretory parenchyma cells.
• Conclusions The abundance of mitochondria suggests an eccrine mechanism of secretion, although dictyosomal vesicles may contribute to a granulocrine process. Phloem sap evidently is the main contributor of nectar carbohydrates. From the sieve elements and companion cells, an apoplastic route via intercellular spaces and cell walls, leading to the pores of modified stomata, is available. A symplastic pathway, via plasmodesmata connecting sieve elements to companion, parenchyma and epidermal cells, is also feasible. Uncollected nectar was reabsorbed, and the direct innervation of the nectary by sieve tubes potentially serves a second important route for nectar-sugar reclamation. Microchannels in the outer cuticle may facilitate both secretion and reabsorption.
Echinacea purpurea; eccrine process; floral nectary; floret phenology; modified stomata; nectar reabsorption; nectar secretion; phloem; ultrastructure
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.
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.
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.
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.
Cell ultrastructure; cerrado vegetation; extrastomatic bodies; Fabaceae; floral nectary; Hymenaea stigonocarpa; nectar; plastids; secretion; starch hydrolysis; vacuole
• Background and Aims The two closely related subtribes Bifrenariinae Dressler and Maxillariinae Benth. are easily distinguished on morphological grounds. Recently, however, molecular techniques have supported the inclusion of Bifrenariinae within a more broadly defined Maxillariinae. The present paper describes the diverse labellar micromorphology found amongst representatives of Bifrenariinae (Bifrenaria Lindl., Rudolfiella Hoehne, Teuscheria Garay and Xylobium Lindl.) and compares it with that found in Maxillaria Pabst & Dungs and Mormolyca Fenzl (Maxillariinae).
• Methods The labella of 35 specimens representing 22 species of Bifrenariinae were examined by means of light microscopy and scanning electron microscopy and their micromorphology compared with that of Maxillaria sensu stricto and Mormolyca spp. The labellar epidermis of representatives of Bifrenaria, Xylobium and Mormolyca was tested for protein, starch and lipids in order to ascertain whether this tissue is involved in the rewarding of pollinators.
• Key Results and Conclusions The labella of Bifrenaria spp. and Mormolyca spp. are densely pubescent but those of Xylobium, Teuscheria and Rudolfiella are generally papillose. However, whereas the trichomes of Bifrenaria and Mormolyca are unicellular, those found in the other three genera are multicellular. Hitherto, no unicellular trichomes have been described for Maxillaria, although the labella of a number of species secrete a viscid substance or bear moniliform, pseudopollen-producing hairs. Moniliform hairs and secretory material also occur in certain species of Xylobium and Teuscheria and these genera, together with Maxillaria, are thought to be pollinated by stingless bees (Meliponini). Differences in the labellar micromorphology of Bifrenaria and Mormolyca are perhaps related to Euglossine- and/ or bumble bee-mediated pollination and pseudocopulation, respectively. Although Xylobium and Teuscheria share a number of labellar features with Maxillaria sensu stricto, this does not necessarily reflect taxonomic relationships but may be indicative of convergence in response to similar pollinator pressures.
Bifrenaria; Bifrenariinae; Maxillaria; Maxillariinae; Meliponini; papillae; pollination; pseudopollen; Rudolfiella; Teuscheria; trichomes; Xylobium
• Background and Aims The labellar ‘hairs’ of some Cymbidium spp. are said to be thin-walled and to contain ‘plasma’, oil and sugars and it has long been speculated that they may function as food-hairs. However, the present authors' preliminary studies showed that certain atypical papillae may have a different role and, by reflecting light, function as a speculum. The purpose of the paper is to test this hypothesis.
• Methods Light microscopy, scanning electron microscopy, transmission electron microscopy, histochemistry and ultraviolet photography were used to investigate the structure, food content and light-reflecting properties of these papillae.
• Key Results and Conclusions The labellum of Cymbidium lowianum (Rchb.f.) Rchb.f. is densely clothed with obconical to conical papillae with wide bases and pointed tips. However, on either side of the median axis of the lip occur silvery patches comprising papillae with truncated tips and it is thought that these reflect light and thereby attract insect pollinators. Similar patches are also found in Cymbidium devonianum Paxton, and in both species, they are set against a reddish background, which, since bees cannot perceive this colour, probably appears dark to the insect thus enhancing the visual impact of the light-reflecting patches. In Cymbidium tigrinum Parish ex Hook. and Cymbidium mastersii Griff. ex Lindl., however, the labellum is mainly white and no light-reflecting patches were observed. Instead, unlike C. lowianum and C. devonianum, these species are highly fragrant and the attraction of insects probably depends to a greater extent on olfactory cues. In C. lowianum both types of papillae contain protein, starch and lipid bodies but only protein is seemingly present at elevated concentrations. However, lipoidal material also occurs upon the surface of the labellum and it is possible that this may be gathered by insects as reported for C. iridifolium A. Cunn (syn. C. madidum Lindl.). The labellar papillae of C. lowianum, thus, have the potential to function as food-hairs, although direct evidence for this is lacking.
Cymbidium lowianum; Cymbidium devonianum; floral rewards; food-hairs; labellum; light absorption; light reflection; papillae; speculum
Background and Aims
Most neotropical Melastomataceae have bee-pollinated flowers with poricidal anthers. However, nectar rewards are known to be produced in about 80 species in eight genera from four different tribes. These nectar-producing species are pollinated by both vertebrates and invertebrates.
The floral morphology and anatomy of 14 species was studied in six genera of nectar-producing Melastomataceae (Blakea, Brachyotum, Charianthus, Huilaea, Meriania and Miconia). Anatomical methods included scanning electron microscopy, and serial sections of paraffin-embedded flowers.
All vertebrate-pollinated melastome flowers have petals that do not open completely at anthesis, thus forming a pseudo-tubular corolla, while closely related species that are bee pollinated have rotate or reflexed corollas. In most species, nectar secretion is related to stomatal or epidermal nectaries and not filament slits as previously reported. Moreover, the nectar is probably supplied by large vascular bundles near the release area. Blakea and Huilaea have nectary stomata located upon the dorsal anther connective appendages. Brachyotum also has nectary stomata on the anther connectives, but these are distributed lengthwise along most of the connective. Meriania may release nectar through the anther connective, but has additional nectary stomata on the inner walls of the hypanthium. Miconia has nectary stomata on the ovary apex. Charianthus nectaries were not found, but there is circumstantial evidence that nectar release occurs through the epidermis at the apex of the ovary and the lower portions of the inner wall of the hypanthium.
Nectar release in Melastomataceae is apparently related to nectary stomata and not filament slits. The presence of nectary stomata on stamens and on ovary apices in different lineages suggests that the acquisition of nectaries is a derived condition. Nectary location also supports a derived condition, because location is strongly consistent within each genus, but differs between genera.
Blakea; Brachyotum; Charianthus; Huilaea; Meriania; Melastomataceae; Miconia; nectaries; nectary stomata; pollination
Background and Aims
The occurrence of nectaries in fruits is restricted to a minority of plant families and consistent reports of their occurrence are not found associated with Fabaceae, mainly showing cellular details. The present study aims to describe the anatomical organization and ultrastructure of the pericarpial nectaries (PNs) in Erythrina speciosa, a bird-pollinated species, discussing functional aspects of these unusual structures.
Samples of floral buds, ovaries of flowers at anthesis and fruits at several developmental stages were fixed and processed by the usual methods for studies using light, and scanning and transmission electron microscopy. Nectar samples collected by filter paper wicks were subjected to chemical analysis using thin-layer chromatography.
The PNs are distributed in isolation on the exocarp. Each PN is represented by a single hyaline trichome that consists of a basal cell at epidermal level, stalk cell(s) and a small secretory multicellular head. The apical stalk cell shows inner periclinal and anticlinal walls impregnated by lipids and lignin and has dense cytoplasm with a prevalence of mitochondria and endoplasmic reticulum. The secretory cells show voluminous nuclei and dense cytoplasm, which predominantly has dictyosomes, rough endoplasmic reticulum, plastids, mitochondria and free ribosomes. At the secretory stage the periplasmic space is prominent and contains secretion residues. Tests for sugar indicate the presence of non-reducing sugars in the secretory cells. Nectar samples from PNs contained sucrose, glucose and fructose.
The secretory stage of these PNs extends until fruit maturation and evidence suggests that the energetic source of nectar production is based on pericarp photosynthesis. Patrolling ants were seen foraging on fruits during all stages of fruit development, which suggests that the PNs mediate a symbiotic relationship between ants and plant, similar to the common role of many extrafloral nectaries.
Ant–plant interactions; Erythrina speciosa; Fabaceae; Faboideae; fruit anatomy; nectar secretion; nectary; post-floral nectaries
As one of largest angiosperm families, orchids have long fascinated evolutionary biologists with their staggering diversity in floral design and display to promote outcrossing. Two of the most intriguing aspects of orchid pollination that promote cross-pollination are pollinarium reconfiguration (PR) and deceptive pollination. PR and generalized food deception employ virtually antagonistic methods of promoting cross-pollination: PR occurs through delayed pollination, involving the relatively long visitation periods that are typically observed for the pollinators of one flower or inflorescence; conversely, generalized food deception leads to reductions in the visitation periods of pollinators to one flower or inflorescence. Thus, it is logical to hypothesize that PR is unnecessary or PR happens soon in generalized food-deceptive orchids in the promotion of cross-pollination. Using Doritis pulcherrima as a model, the aim of this study was to understand the following: (1) the pollination and breeding system of D. pulcherrima; (2) the morphological interactions between orchids and their pollinators; and (3) whether PR is necessary in the promotion of cross-pollination in D. pulcherrima.
Our observations indicated that Doritis pulcherrima is pollinated almost exclusively by Amegilla nigritar (Hymenoptera: Apidae) and possesses pollinia that are deposited on the “occiputs” (cervical membranes) of these insects. All of evidences are indicated that D. pulcherrima is a generalized food-deceptive orchid. Our morphometric measurements of the flowers and pollinators show that the heights of the “occiputs” with un-oriented pollinaria were equal to the distances between stigmas and surfaces of the middle lobes, suggesting that pollinarium reconfiguration is not necessary in Doritis pulcherrima.
Our observation and analyses supported the hypothesis that pollinarium reconfiguration is unnecessary in generalized food-deceptive orchids, such as Doritis pulcherrima, for the promotion of cross-pollination. This conclusion was indirectly supported by the abundance of deceptive orchids that do not exhibit pollinarium reconfiguration. There are two mechanisms (i.e. clone-growing characteristics and a long flowering season) that promote fruit sets in the epiphytic food-deceptive orchids in tropical regions.
Aeridinae, Doritis pulcherrima; floral deception; Orchidaceae; pollinarium reconfiguration
Background and Aims
This study is an investigation into the floral development and anatomy of two genera of the small family Salvadoraceae, which belongs to the Brassicales in a clade with Batis and Koeberlinia. Salvadoraceae remains little known, despite its wide distribution in arid areas of the globe. Floral morphological data are scarce, and information on floral anatomy is limited to a single study, although morphological and anatomical characters are now used increasingly as a counterpart of molecular data. There remain a number of controversial morphological questions, such as the fusion of the petals, the number of carpels and the nature of the nectaries.
Floral anatomy and ontogeny were studied in two species of Salvadora and one species of Dobera. Only for S. persica could a full floral developmental sequence be done.
The floral development demonstrates that the ovary of Salvadoraceae is basically bicarpellate and pseudomonomerous with a single locule and parietal placenta. The ovary of Dobera resembles Azima tetracantha in the presence of a false apical septum. Evidence for a staminodial nature of the nectaries is not decisive. In Salvadora petals and stamens are lifted by a short hypanthium.
Salvadoraceae share several morphological and developmental synapomorphies with Batis (Bataceae) and possibly Koeberlinia (Koeberliniaceae), supporting their close relationship as indicated by molecular phylogeny.
Batis; Brassicales; Dobera; Emblingia; floral development; floral anatomy; Koeberlinia; phylogeny; Salvadora; Salvadoraceae; SEM
Background and Aims
Gross vegetative and floral morphology, as well as modern molecular techniques, indicate that Cryptocentrum Benth. and Sepalosaccus Schltr. are related to Maxillaria Ruiz & Pav. However, they differ from Maxillaria in their possession of floral spurs and, in this respect, are atypical of Maxillariinae. The labellar micromorphology of Maxillaria, unlike that of the other two genera, has been extensively studied. In the present report, the labellar micromorphology of Cryptocentrum and Sepalosaccus is compared with that of Maxillaria and, for the first time, the micromorphology of the floral spur as found in Maxillariinae is described.
Labella and dissected floral spurs of Cryptocentrum and Sepalosaccus were examined using light microscopy (LM) and scanning electron microscopy (SEM).
In each case, the labellum consists of a papillose mid-lobe (epichile), a cymbiform region (hypochile) and, proximally, a spur, which is pronounced in Cryptocentrum but short and blunt in Sepalosaccus. The inner epidermal surface of the spur of Cryptocentrum is glabrous or pubescent, and the bicellular hairs, where present, are unlike any hitherto described for Maxillariinae. Similar but unicellular hairs also occur in the floral spur of Sepalosaccus, whereas the glabrous epidermis lining the spur of C. peruvianum contains putative nectar pores.
The labellar micromorphology of Cryptocentrum and Sepalosaccus generally resembles that of Maxillaria. The floral spur of Cryptocentrum displays two types of organization in that the epidermal lining may be glabrous (possibly with nectar pores) or pubescent. This may have taxonomic significance and perhaps reflects physiological differences relating to nectar secretion. The trichomes found within the spurs of Cryptocentrum and Sepalosaccus more closely resemble the hairs of certain unrelated, nectariferous orchid taxa than those found in the largely nectarless genus Maxillaria, and this further supports the case for parallelism.
Labellum; Maxillariinae; micromorphology; nectar pore; nectary; spur; trichome
Background and Aims
Floral elaiophores, although widespread amongst orchids, have not previously been described for Maxillariinae sensu lato. Here, two claims that epithelial, floral elaiophores occur in the genus Rudolfiella Hoehne (Bifrenaria clade) are investigated. Presumed elaiophores were compared with those of Oncidiinae Benth. and the floral, resin-secreting tissues of Rhetinantha M.A. Blanco and Heterotaxis Lindl., both genera formerly assigned to Maxillaria Ruiz & Pav. (Maxillariinae sensu stricto).
Putative, floral elaiophore tissue of Rudolfiella picta (Schltr.) Hoehne and floral elaiophores of Oncidium ornithorhynchum H.B.K. were examined by means of light microscopy, histochemistry, scanning electron microscopy and transmission electron microscopy.
Key Results and Conclusions
Floral, epithelial elaiophores are present in Rudolfiella picta, indicating, for the first time, that oil secretion occurs amongst members of the Bifrenaria clade (Maxillariinae sensu lato). However, whereas the elaiophore of R. picta is borne upon the labellar callus, the elaiophores of O. ornithorhynchum occur on the lateral lobes of the labellum. In both species, the elaiophore comprises a single layer of palisade secretory cells and parenchymatous, subsecretory tissue. Cell wall cavities are absent from both and there is no evidence of cuticular distension in response to oil accumulation between the outer tangential wall and the overlying cuticle in R. picta. Distension of the cuticle, however, occurs in O. ornithorhynchum. Secretory cells of R. picta contain characteristic, spherical or oval plastids with abundant plastoglobuli and these more closely resemble plastids found in labellar, secretory cells of representatives of Rhetinantha (formerly Maxillaria acuminata Lindl. alliance) than elaiophore plastids of Oncidiinae. In Rhetinantha, such plastids are involved in the synthesis of resin-like material or wax. Despite these differences, the elaiophore anatomy of both R. picta (Bifrenaria clade) and O. ornithorhynchum (Oncidiinae) fundamentally resembles that of several representatives of Oncidiinae. These, in their possession of palisade secretory cells, in turn, resemble the floral elaiophores of certain members of Malpighiaceae, indicating that convergence has occurred here in response to similar pollination pressures.
Bifrenaria clade; elaiophore; floral oil; Heterotaxis; Maxillariinae; Oncidiinae; Oncidium ornithorhynchum; Rhetinantha; Rudolfiella picta; secretion
In protection mutualisms, one mutualist defends its partner against a natural enemy in exchange for a reward, usually food or shelter. For both partners, the costs and benefits of these interactions often vary considerably in space because the outcome (positive, negative or neutral) depends on the local abundance of at least three species: the protector, the beneficiary of protection and the beneficiary's natural enemy. In Gossypium thurberi (wild cotton), ants benefit nutritionally from the plant's extrafloral nectaries and guard plants from herbivores. Experimentally altering the availability of both ants and extrafloral nectar in three populations demonstrated that the mutualism is facultative, depending, in part, on the abundance of ants and the level of herbivore damage. The species composition of ants and a parasitic alga that clogs extrafloral nectaries were also implicated in altering the outcome of plant-ant interactions. Furthermore, experimental treatments that excluded ants (the putative selective agents) in combination with phenotypic selection analyses revealed that selection on extrafloral nectary traits was mediated by ants and, importantly, varied across populations. This work is some of the first to manipulate interactions experimentally across multiple sites and thereby document that geographically variable selection, mediated by a mutualist, can shape the evolution of plant traits.
The morphology and development of flowers and pseudanthia of Calycopeplus paucifolius are described in detail in the context of recent molecular phylogenies of the tribe Euphorbieae and a recent comparative developmental analysis of other taxa within this tribe. Calycopeplus resembles subtribes Neoguillauminiinae and Anthosteminae in some respects (dichasial formation of male flowers within male partial inflorescences, late formation of a constriction in male and female flowers and early formation of a female perianth), but resembles Dichostemma (subtribe Anthosteminae) in possessing only four male partial inflorescences. Calycopeplus and all other Euphorbieae possess only three carpels, except Dichostemma, which has four carpels per female flower. The studied species differs from the closely related Neoguillauminia cleopatra (subtribe Neoguillauminiinae) in that only four nectaries are formed, situated on the rim of the cuplike involucre (in Neoguillauminia 8–10 nectaries arise directly from the base of the pseudanthium). In contrast to all other studied Euphorbieae with trimerous gynoecia, the unpaired carpel of C. paucifolius is oriented in an upper/adaxial position (it lies in the lower/abaxial position in all other studied taxa). On the basis of these results we discuss possible pathways of cyathium evolution and the role of the cyathium as a possible key innovation within Euphorbieae.
‘Calycopeplus is as perfect an example of a connecting link as a morphologist may wish for.’ (Croizat 1937, p. 404)
Many flowering plants attract pollinators by offering a reward of floral nectar. Remarkably, the molecular events involved in the development of nectaries, the organs that produce nectar, as well as the synthesis and secretion of nectar itself, are poorly understood. Indeed, to date, no genes have been shown to directly affect the de novo production or quality of floral nectar. To address this gap in knowledge, the ATH1 Affymetrix® GeneChip array was used to systematically investigate the Arabidopsis nectary transcriptome to identify genes and pathways potentially involved in nectar production.
In this study, we identified a large number of genes differentially expressed between secretory lateral nectaries and non-secretory median nectary tissues, as well as between mature lateral nectaries (post-anthesis) and immature lateral nectaries (pre-anthesis). Expression within nectaries was also compared to thirteen non-nectary reference tissues, from which 270 genes were identified as being significantly upregulated in nectaries. The expression patterns of 14 nectary-enriched genes were also confirmed via RT PCR. Upon looking into functional groups of upregulated genes, pathways involved in gene regulation, carbohydrate metabolism, and lipid metabolism were particularly enriched in nectaries versus reference tissues.
A large number of genes preferentially expressed in nectaries, as well as between nectary types and developmental stages, were identified. Several hypotheses relating to mechanisms of nectar production and regulation thereof are proposed, and provide a starting point for reverse genetics approaches to determine molecular mechanisms underlying nectar synthesis and secretion.
Background and Aims
Until recently, there was no consensus regarding the phylogenetic relationships of the Neotropical orchid genera Scuticaria Lindl. and Dichaea Lindl. However, recent evidence derived from both gross morphological and molecular studies supports the inclusion of Scuticaria and Dichaea in sub-tribes Maxillariinae and Zygopetalinae, respectively. The present paper describes the labellar micromorphology of both genera and seeks to establish whether labellar characters support the assignment of Scuticaria and Dichaea to these sub-tribes.
The labella of four species of Scuticaria and 14 species of Dichaea were examined using light microscopy and scanning electron microscopy, and their micromorphology was compared with that of representative species of Maxillariinae sensu lato and Zygopetalinae (Huntleya clade).
Key Results and Conclusions
In most specimens of Scuticaria examined, the papillose labella bear uniseriate, multicellular, unbranched trichomes. However, in S. steelii (Lindl.) Lindl., branched hairs may also be present and some trichomes may fragment and form pseudopollen. Multicellular, leaf-like scales were also present in one species of Scuticaria. Similar, unbranched hairs are present in certain species of Maxillaria Ruiz & Pav. (Maxillariinae sensu stricto) and Chaubardia Rchb.f. (Huntleya clade). As yet, moniliform, pseudopollen-forming hairs have not been observed for Zygopetalinae, but their presence in Scuticaria steelii, Maxillaria and Heterotaxis Lindl. supports the placing of Scuticaria in Maxillariinae. As other genera are sampled, the presence of branched hairs, hitherto unknown for Maxillariinae sensu lato, may prove to be a useful character in taxonomy and phylogenetic studies. Euglossophily occurs in Dichaea, as well as Chondrorhyncha Lindl. and Pescatorea Rchb.f. (Huntleya clade), and all three genera tend to lack distinctive labellar features. Instead, lip micromorphology is relatively simple and glabrous or papillose. However, two of the Dichaea species examined bear unicellular, labellar trichomes very similar to those found in Bifrenaria Lindl. (pollinated by both euglossine bees and Bombus spp.), and this feature may have arisen by convergence in response to similar pollination pressures.
Bifrenaria; Bifrenaria clade; Chaubardia; Chondrorhyncha; Dichaea; Dichaeinae; Heterotaxis; Huntleya clade; Huntleyinae; labellum; Maxillaria; Maxillariinae; papillae; Pescatorea; scales; Scuticaria; trichomes; Zygopetalinae
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.
The ultrastructure of the nectary was investigated at different stages of development by analysis of a series of ultrathin (7 × 10−8 m) sections 7 × 10−7 m apart from one another. Proportions of the cells typical of the nectary were documented by 3D-reconstruction and morphometry.
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.
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.
Floral nectary; Digitalis purpurea; 3D-reconstruction; morphometry; fluid-filled intercellular space; phloem innervation; programmed cell death; nectariferous tissue
The relationship between species number and island area is a fundamental rule in ecology. However, the extent to which interactions with exotic species and how the structure of species interactions is related to island area remain unexplored. Here, I document the relationship between island area and (i) interactions with exotic species and (ii) network structure of species interactions in the context of mutualistic interactions between ants and extrafloral nectary-bearing plants on the oceanic Ogasawara (Bonin) Islands, Japan. Pooled data contained 122 interactions among 19 plant (including five exotic) and 23 ant (including 20 exotic) species. Of the observed interactions, 82.8 per cent involved at least one exotic species, ranging from 68.2 to 86.4 per cent among islands. The number of links including exotic species increased in proportion to island area, although the number of links excluding exotic species did not. These results indicate that the number of interactions with exotic species increased in proportion to island area. Connectance, or the proportion of interactions actually observed among all possible interactions, decreased with island area. Nestedness, an asymmetry index in the species interaction network, also decreased with island area. Therefore, island area affects both the number of interactions with exotic species and the network structure.
exotic species; island species–area relationships; mutualistic networks; native species; oceanic islands; plant–ant networks
Mutualistic networks involving plants and their pollinators or frugivores have been shown recently to exhibit a particular asymmetrical organization of interactions among species called nestedness: a core of reciprocal generalists accompanied by specialist species that interact almost exclusively with generalists. This structure contrasts with compartmentalized assemblage structures that have been verified in antagonistic food webs. Here we evaluated whether nestedness is a property of another type of mutualism—the interactions between ants and extrafloral nectary-bearing plants—and whether species richness may lead to differences in degree of nestedness among biological communities. We investigated network structure in four communities in Mexico. Nested patterns in ant–plant networks were very similar to those previously reported for pollination and frugivore systems, indicating that this form of asymmetry in specialization is a common feature of mutualisms between free-living species, but not always present in species-poor systems. Other ecological factors also appeared to contribute to the nested asymmetry in specialization, because some assemblages showed more extreme asymmetry than others even when species richness was held constant. Our results support a promising approach for the development of multispecies coevolutionary theory, leading to the idea that specialization may coevolve in different but simple ways in antagonistic and mutualistic assemblages.
asymmetric specialization; complex networks; extrafloral nectaries; geographic mosaic; nestedness
Chelonanthus alatus is a bat-pollinated, pioneer Gentianaceae
that clusters in patches where still-standing, dried-out stems are interspersed
among live individuals. Flowers bear circum-floral nectaries (CFNs) that are
attractive to ants, and seed dispersal is both barochorous and anemochorous.
Although, in this study, live individuals never sheltered ant colonies,
dried-out hollow stems - that can remain standing for 2 years - did. Workers
from species nesting in dried-out stems as well as from ground-nesting species
exploited the CFNs of live C. alatus individuals in the same
patches during the daytime, but were absent at night (when bat pollination
occurs) on 60.5% of the plants. By visiting the CFNs, the ants indirectly
protect the flowers - but not the plant foliage - from herbivorous insects. We
show that this protection is provided mostly by species nesting in dried-out
stems, predominantly Pseudomyrmex gracilis. That dried-out
stems remain standing for years and are regularly replaced results in an
opportunistic, but stable association where colonies are sheltered by one
generation of dead C. alatus while the live individuals nearby,
belonging to the next generation, provide them with nectar; in turn, the ants
protect their flowers from herbivores. We suggest that the investment in wood by
C. alatus individuals permitting still-standing, dried-out
stems to shelter ant colonies constitutes an extended phenotype because foraging
workers protect the flowers of live individuals in the same patch. Also, through
this process these dried-out stems indirectly favor the reproduction (and so the
fitness) of the next generation including both their own offspring and that of
their siblings, all adding up to a potential case of inclusive fitness in
Background and Aims
Myrmecophytes, or ant-plants, are characterized by their ability to shelter colonies of some ant species in hollow structures, or ant-domatia, that are often formed by hypertrophy of the internal tissue at specific locations (i.e. trunk, branches, thorns and leaf pouches). In Hirtella physophora (Chrysobalanaceae), the focal species of this study, the ant-domatia consist of leaf pouches formed when the leaf rolls over onto itself to create two spheres at the base of the blade.
The morphological and anatomical changes through which foliar ant-domatia developed from the laminas are studied for the first time by using fresh and fixed mature leaves from the same H. physophora individuals.
Ant-domatia were characterized by larger extra-floral nectaries, longer stomatal apertures and lower stomatal density. The anatomical structure of the domatia differed in the parenchymatous tissue where palisade and spongy parenchyma were indistinct; chloroplast density was lower and lignified sclerenchymal fibres were more numerous compared with the lamina. In addition, the domatia were thicker than the lamina, largely because the parenchymatous and epidermal cells were enlarged.
Herein, the morphological and anatomical changes that permit foliar ant-domatia to be defined as a specialized leaf structure are highlighted. Similarities as well as structural modifications in the foliar ant-domatia compared with the lamina are discussed from botanical, functional and mutualistic points of view. These results are also important to understanding the reciprocal evolutionary changes in traits and, thus, the coevolutionary processes occurring in insect–plant mutualisms.
Anatomy; ant–plant mutualism; Chrysobalanaceae; extra-floral nectaries; French Guiana; Hirtella physophora; secondary domatia
Background and Aims
Abolboda (Xyridaceae) belongs to the Poales, a predominantly wind-pollinated order whose phylogeny has been widely studied in recent years. The reproductive biology of Abolboda pulchella and A. poarchon was studied to determine the main pollination system of these species, providing the first experimental data on reproduction in the Xyridaceae.
A field study was conducted, including observations on the morphology and biology of the flowers, insect visits and pollinator behaviour. Experimental pollination treatments were performed to assess agamospermy, spontaneous self-pollination and self-compatibility. Pollination success was determined by pollen tube growth, and reproductive success was assessed by fruit- and seed-set.
Abolboda pulchella and A. poarchon were pollinated by Apidae, Megachilidae and Halictidae bees. The floral resources were pollen and nectar that was produced by stylar appendages, an uncommom nectary type for monocotyledons. The species were self-compatible, and pollen tube growth from self-pollen was similar to that of cross-pollen. However, herkogamy prevented spontaneous selfing, rendering the plants dependent on the pollinator's activity. There was no production of seeds by agamospermy.
Melittophily is the main pollination system of these two Abolboda species. Nectar production was first recorded here for Xyridaceae, and along with self-compatibility, herkogamy and bee pollination, is an informative characteristic that can be used in future phylogenetic analyses of the family as well as Poales.
Abolboda pulchella; Abolboda poarchon; bee pollination; herkogamy; nectar; Poales; reproduction; self-compatibility; stylar appendages; Xyridaceae
Bailospermum raziana Keshav. Et. Yog. Is a newly identified species belonging to the family Euphorbiaceae. It is a leafy shout monoecious, erect herb. The plant differentiates from the other species i.e Baliospermum montanum (Wild) Muell-Arg by the presence of extra floral nectaries found on the abaxial side all along the margins of the leaf lamina and long peduncled racemes. Histologically the root is differentiated in to periderm and stele. Groups of cortical fibres are randomly distributed among the inner layers of phelloderm. Tanniniferous content and crystals of calcium oxalate are found in the periderm. Simple starch grains are located in the stellar region. Vessel element occurring one at each end. Perforation plates are two in each element occurring one at each end. Phytochemical studies subjected the possibilities of the alkaloids, carbohydrates, flavonoids, triterpenes, tannins etc. The aqueous extract has shown mild purgative action (600 mg /kg body wt) in the animal models. Morphologically there are many similarities between B. raziana and B. montanum an important Ayurvedic herbal drug. In commerce both the drugs are being sold under the common name “Danti’. In the present investigation diagnostic characters of both species have been discussed briefly.
The leaves of Nepenthes pitcher plants are specialized pitfall traps which capture and digest arthropod prey. In many species, insects become trapped by ‘aquaplaning’ on the wet pitcher rim (peristome). Here we investigate the ecological implications of this capture mechanism in Nepenthes rafflesiana var. typica. We combine meteorological data and continuous field measurements of peristome wetness using electrical conductance with experimental assessments of the pitchers' capture efficiency. Our results demonstrate that pitchers can be highly effective traps with capture rates as high as 80% but completely ineffective at other times. These dramatic changes are due to the wetting condition of the peristome. Variation of peristome wetness and capture efficiency was perfectly synchronous, and caused by rain, condensation and nectar secreted from peristome nectaries. The presence of nectar on the peristome increased surface wetness mainly indirectly by its hygroscopic properties. Experiments confirmed that pitchers with removed peristome nectaries remained generally drier and captured prey less efficiently than untreated controls. This role of nectar in prey capture represents a novel function of plant nectar. We propose that the intermittent and unpredictable activation of Nepenthes pitcher traps facilitates ant recruitment and constitutes a strategy to maximize prey capture.
carnivorous plants; extrafloral nectar; leaf wetness; aquaplaning
Background and Aims
Intra-specific variation in nectar chemistry under natural conditions has been only rarely explored, yet it is an essential aspect of our understanding of how pollinator-mediated selection might act on nectar traits. This paper examines intra-specific variation in nectar sugar composition in field and glasshouse plants of the bumblebee-pollinated perennial herbs Aquilegia vulgaris subsp. vulgaris and Aquilegia pyrenaica subsp. cazorlensis (Ranunculaceae). The aims of the study are to assess the generality of extreme intra-plant variation in nectar sugar composition recently reported for other species in the field, and gaining insight on the possible mechanisms involved.
The proportions of glucose, fructose and sucrose in single-nectary nectar samples collected from field and glasshouse plants were determined using high performance liquid chromatography. A hierarchical variance partition was used to dissect total variance into components due to variation among plants, flowers within plants, and nectaries within flowers.
Nectar of the two species was mostly sucrose-dominated, but composition varied widely in the field, ranging from sucrose-only to fructose-dominated. Most intra-specific variance was due to differences among nectaries of the same flower, and flowers of the same plant. The high intra-plant variation in sugar composition exhibited by field plants vanished in the glasshouse, where nectar composition emerged as a remarkably constant feature across plants, flowers and nectaries.
In addition to corroborating the results of previous studies documenting extreme intra-plant variation in nectar sugar composition in the field, this study suggests that such variation may ultimately be caused by biotic factors operating on the nectar in the field but not in the glasshouse. Pollinator visitation and pollinator-borne yeasts are suggested as likely causal agents.
Abiotic environment; Aquilegia pyrenaica subsp. cazorlensis; Aquilegia vulgaris subsp. vulgaris; biotic factors; field conditions; glasshouse; Iberian Peninsula; inter- and intra-specific variation; nectar-sugar composition; nectary; variance components