Is nectar reabsorption restricted by the stalk cells of floral and extrafloral nectary trichomes?

Reabsorption is a phase of nectar dynamics that occurs concurrently with secretion; it has been described in floral nectaries that exude nectar through stomata or unicellular trichomes, but has not yet been recorded in extrafloral glands. Apparently, nectar reabsorption does not occur in multicellular secretory trichomes (MST) due to the presence of lipophilic impregnations – which resemble Casparian strips – in the anticlinal walls of the stalk cells. It has been assumed that these impregnations restrict solute movement within MST to occur unidirectionally and exclusively by the symplast, thereby preventing nectar reflux toward the underlying nectary tissues. We hypothesised that reabsorption is absent in nectaries possessing MST. The fluorochrome lucifer yellow (LYCH) was applied to standing nectar of two floral and extrafloral glands of distantly related species, and then emission spectra from nectary sections were systematically analysed using confocal microscopy. Passive uptake of LYCH via the stalk cells to the nectary tissues occurred in all MST examined. Moreover, we present evidence of nectar reabsorption in extrafloral nectaries, demonstrating that LYCH passed the stalk cells of MST, although it did not reach the deepest nectary tissues. Identical (control) experiments performed with neutral red (NR) demonstrated no uptake of this stain by actively secreting MST, whereas diffusion of NR did occur in plasmolysed MST of floral nectaries at the post‐secretory phase, indicating that nectar reabsorption by MST is governed by stalk cell physiology. Interestingly, non‐secretory trichomes failed to reabsorb nectar. The role of various nectary components is discussed in relation to the control of nectar reabsorption by secretory trichomes.     

Light response characteristics of a morphologically diverse group of southern hemisphere conifers as measured by chlorophyll fluorescence

Unlike northern hemisphere conifer families, the southern family, Podocarpaceae, produces a great variety of foliage forms ranging from functionally broad-, to needle-leaved. The production of broad photosynthetic surfaces in podocarps has been linked qualitatively to low-light-environments, and we undertook to assess the validity of this assumption by measuring the light response of a morphologically diverse group of podocarps. The light response, as apparent photochemical electron transport rate (ETR), was measured by modulated fluorescence in ten species of this family and six associated species (including five Cupressaceae and one functionally needle-leaved angiosperm) all grown under identical glasshouse conditions. In all species, ETR was found to increase as light intensity increased, reaching a peak value (ETR_max) at saturating quantum flux (PPFD_sat), and decreasing thereafter. ETR_max ranged from 217 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 1725 μmol photons · m⁻² · s⁻¹ in Actinostrobus acuminatus to an ETR of 60 μmol electrons · m⁻² · s⁻¹ at a PPFD_sat of 745 μmol electrons · m⁻² · s⁻¹ in Podocarpus dispermis. Good correlations were observed between ETR_max and both PPFD_sat and maximum assimilation rate measured by gas-exchange analysis. The effective quantum yield at light saturation remained constant in all species with an average value of 0.278 ± 0.0035 determined for all 16 species. Differences in the shapes of light response curves were related to differences in the response of non-photochemical quenching (q _n), with q _n saturating faster in species with low PPFD_sat. Amongst the species of Podocarpaceae, the log of average shoot width was well correlated with PPFD_sat, wider leaves saturating at lower light intensities. This suggests that broadly flattened shoots in the Podocarpaceae are an adaptation to low light intensity. Received: 15 April 1996 / Accepted: 30 September 1996     

Occurrence,structure, ontogeny and biology of nectaries inKigelia pinnata DC

The occurrence, morphology, ontogeny, structure and preliminary nectar analysis of floral and extrafloral nectaries are studied inKigelia pinnata of the Bignoniaceae. The extrafloral nectaries occur on foliage leaves, sepals and outer wall of the ovary, while the floral nectary is situated around the ovary base as an annular, massive, yellowish ring on the torus. The extrafloral nectaries originate from a single nectary initial. The floral nectary develops from a group of parenchymatous cells on the torus. The extrafloral nectaries are differentiated into multicellular foot, stalk and cupular or patelliform head. The floral nectary consists of parenchymatous tissue. The floral nectaries are supplied with phloem tissue. The secretion is copious in floral nectary. Function of the nectary, preliminary nectar analysis, and symbiotic relation between nectaries and animal visitors are discussed.     

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     

A macro- and micromorphological survey of floral and extrafloral nectaries in the epiphytic cactus Rhipsalis teres (Cactoideae: Rhipsalideae)

Floral and extrafloral nectaries in plants favor pollination and defense against herbivory. Despite their wide distribution in plants and differences in position, structure, and topography, their biological and systematic significance has been underutilized. This study investigated the macro- and micromorphology of floral and extrafloral nectaries in the epiphytic cactus Rhipsalis teres and reports unusual bristle-like structures (bracteoles) functioning as extrafloral nectaries in the cactus family. The floral nectary is disc-shaped embedded in the hypanthial floral cup with anomocytic stomata as secreting structures present on the epidermal nectarial tissue. Small multicellular bristle-like extrafloral nectar-secreting structures, homologues to bracts, were observed on the plants’ stems and function as bracteolar nectaries having a relatively long and continuous secretory activity throughout several stages of the reproductive structures. Both the floral and bracteolar nectaries are functional. It is possible that in the latter nectar discharge occurs though epidermal cells, which build up pressure inside as nectar accumulates, thereby ending with rupture of the cuticle to release the liquid. The nectar in both secreting structures is scentless and colorless, and the concentration from floral nectaries is slightly lower than that of the bracteolar nectaries, 70.6% and 76.4%, respectively. The relatively higher concentration in the latter might be correlated with exposure, relative humidity and water evaporation, leading to crystallization of sugars on the stem surface in a short period of time.     

How common is the ability of extrafloral nectaries to produce nectar droplets,to secrete nectar during the night and to store starch?

Evidence in favour of the ability of extrafloral nectaries (EFNs) to form nectar drop(let)s, secrete extrafloral nectar (EFNec) also during the night and store starch was compiled in order to refute controversial assertions. Not only were more than 150 reports of direct observations of EFNec drop(let)s found, but also 90 studies which suggest that EFNec secretion is copious enough to form drop(let)s automatically by forces of physics (surface tension strength), provided nectar accumulation is not interrupted by predatory animals. Twenty direct observations of nocturnal production of EFNec sufficiently proved that it is not always produced during the day. Additionally, numerous observations of the nocturnal activities of nectar consumers on EFNs indirectly indicated very common nocturnal secretion of EFNec. Although there is an early report of a starch‐containing EFN from 1881 (Trelease), few similar observations in other EFNs followed. Nevertheless, four studies have described the disappearance of stored starch during secretion and senescence of the EFNs. Referring back to an apparent relationship between the degradation of starch stored in a floral nectary and programmed cell death, at least in EFNs with transient storage of starch, a similar relationship cannot be excluded.     

Pollination-related characteristics in the mimosoid legumeAcacia terminalis (Leguminosae)

Intraspecific variation has been found for several pollination-related characteristics in two isolated populations of the self-incompatible treeAcacia terminalis: floral characteristics including colour and flowering time; style length; size and colour of extrafloral nectaries on the leaf petioles; chemical components of the extrafloral nectar; different taxa of bee pollinators; and frequency differences in bird pollinators. These differences possibly reflect the evolution of two different pollination syndromes within this species.     

The extrafloral nectaries of cowpea (Vigna unguiculata (L.) Walp.) II. Nectar composition,origin of nectar solutes,and nectary functioning

Nectar was collected from the extrafloral nectaries of leaf stipels and inflorescence stalks, and phloem sap from cryopunctured fruits of cowpea plants. Daily sugar losses as nectar were equivalent to only 0.1–2% of the plant's current net photosynthate, and were maximal in the fourth week after anthesis. Sucrose:glucose:fructose weight ratios of nectar varied from 1.5:1:1 to 0.5:1:1, whereas over 95% of phloem-sap sugar was sucrose. [¹⁴C]Sucrose fed to leaves was translocated as such to nectaries, where it was partly inverted to [¹⁴C]glucose and [¹⁴C]fructose prior to or during nectar secretion. Invertase (EC 3.2.1.26) activity was demonstrated for inflorescence-stalk nectar but not stipel nectar. The nectar invertase was largely associated with secretory cells that are extruded into the nectar during nectary functioning, and was active only after osmotic disruption of these cells upon dilution of the nectar. The nectar invertase functioned optimally (phloem-sap sucrose as substrate) at pH 5.5, with a starting sucrose concentration of 15% (w/v). Stipel nectar was much lower in amino compounds relative to sugars (0.08–0.17 mg g^-1 total sugar) than inflorescence nectar (22–30 mg g^-1) or phloem sap (81–162 mg g^-1). The two classes of nectar and phloem sap also differed noticeably in their complements of organic acids. Xylem feeding to leaves of a range of ¹⁴C-labelled nitrogenous solutes resulted in these substrates and their metabolic products appearing in fruit-phloem sap and adjacent inflorescence-stalk nectar. ¹⁴C-labelled asparagine, valine and histidine transferred freely into phloem and appeared still largely as such in nectar. ¹⁴C-labelled glycine, serine, arginine and aspartic acid showed limited direct access to phloem and nectar, although labelled metabolic products were transferred and secreted. The ureide allantoin was present in phloem, but absent from both types of nectar. Models of nectary functioning are proposed.     

Insect visitors to extrafloral nectaries of Byttneria aculeata (Sterculiaceae): relative importance and roles

Abstract. 1. Ants, parasitoids and flies are about equally frequent at foliar nectaries of Byttneria aculeata (Sterculiaceae) in lowland Costa Rica during the dry season, a pattern previously unreported but also observed at other plants in the area.
2. Species of Ectatomma, Crematogaster and Camponotus were the most frequent of twenty-four ant species on Byttneria, eight of which nested in the hollow stems. Ants spent most time at nectaries and little in patrolling.
3. Collections at nectaries yielded large numbers of species of parasitoid Hymenoptera with few individuals of each. Rearing studies of leaf-feeding herbivores yielded several species of parasitoids, including one species taken at a nectary and two others congeneric or closely related.
4. Flies appear to be nectar thieves, in the same sense as non-pollinating floral visitors, despite close association with Byttneria.
5. Ant-plants may be poor models for the larger number of species of less specialized plants with extrafloral nectaries. Plants which have only extrafloral nectaries may better suit the needs of parasitoids than ants, and plants such as Byttneria may benefit as much from parasitoids as from ants.     

Influence of elevated CO2 and ultraviolet-B radiation levels on floral nectar production: a nectary-morphological perspective

 Investigations of the effects of two global events – elevated CO₂ levels and enhanced ultraviolet-B (UV-B) radiation – on floral nectar production are reviewed from twelve dicotyledonous families. Furthermore, to allow comparisons between nectary morphology and nectar production in treated plants of these fifteen species, new data on floral nectary structure are provided for Malcolmia maritima (L.) R. Br. (Brassicaceae) and Scabiosa columbaria L. (Dipsacaceae). All but the last taxon possessed mesenchymatic floral nectaries with surface stomata. Few clear relationships existed between nectary morphology and various physiological responses to CO₂ or UV-B enrichment, indicating that species responded notwithstanding nectary structure itself. Overall, nectar-solute concentration was least affected by elevated CO₂ or UV-B radiation; consequently, changes in nectar volume were responsible for differences in nectar-sugar production per flower. Three species of Fabaceae experienced no change in floral nectar production upon exposure to elevated CO₂. To date, no study of enhanced UV-B radiation reported a consistent reduction in floral nectar production; three species of Brassicaceae responded differently, but various levels of ozone depletion were simulated. Experimentation with more taxa – including those possessing nectary types such as septal (gynopleural) nectaries (e.g. many monocotyledons) or aggregations of glandular trichomes – and expanding such physiological studies to species possessing extrafloral nectaries, are recommended. Received August 8, 2002; accepted November 23, 2002 Published online: June 2, 2003     

Structure and biology of nectaries in Tabebuia serratifolia Nichols (Bignoniaceae)

THOMAS, V. & DAVE, Y., 1992. Structure and biology of nectaries in Tabebuia serratifolia Nichols (Bignoniaceae) . Tabebuia has both floral and extrafloral nectaries, situated on the petiole, bract, calyx, around the ovary and on the pericarp. The floral nectary present around the ovary base is differentiated into epidermis, secretory zone and sub-secretory zone. It is supplied by phloem strands up to the secretory zone. A mature extrafloral nectary consists of a single large basal cell and a head comprising a layer of vertically arranged elongated cells. Starch, protein and lipid are present in the floral nectary. The major insect visitors to both types of nectaries are honey bees, houseflies and ants.     

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

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

Using the stable isotope marker 13C to study extrafloral nectar uptake by parasitoids under controlled conditions and in the field

Parasitic wasps are prominent natural enemies of crop pests. They usually feed on floral resources during the adult stage (nectar, pollen, or honeydew). Extrafloral nectar is an alternative source of sugar easily accessible to adult parasitoids. We developed an original method of nectar labelling based on the injection of labelled sugar solution into the plant stem in order to analyse the nectar uptake by parasitoids (cotton wick method). This method was used to artificially enrich extrafloral cornflower, Centaurea cyanus L. (Asteraceae), nectar with the stable isotope ¹³C. We analysed (1) the transfer of ¹³C from the sugar solution into extrafloral nectaries, (2) the uptake of labelled nectar by parasitoids under laboratory conditions, and (3) the ability of the method to discriminate, in an oilseed rape (Brassica napus L., Brassicaceae) field, between labelled parasitoids (i.e., those who have fed on labelled cornflowers located adjacent to the field) and unlabelled parasitoids to track parasitoid movements from the margin into the field. The extrafloral nectar of all test plants was ¹³C‐labelled. Most (66%) of the parasitoids were identified as marked after 96 h of exposure to labelled plants in the laboratory. We could also detect labelled parasitoids inside the field, but the detection rate was only 1%. The experiments clearly demonstrate that the cotton wick method is appropriate to label extrafloral nectar and parasitoids feeding on this labelled nectar. Further research is needed on the amount of labelled extrafloral nectar required to obtain a sufficient marker level to track parasitoid movements in the field.     

Anatomy and histochemistry of the nectaries of Rodriguezia venusta (Lindl.) Rchb. f. (Orchidaceae)

Orchidaceae is one of the largest angiosperm families. Although extensively studied, reports of anatomy of secretory structures of orchids are relatively scarce. Rodriguezia venusta is an epiphytic orchid occurring in Brazil and Peru that has floral and extrafloral nectaries. This study describes the structure and the histochemistry of these secretory structures. Floral and extrafloral nectary samples were obtained from R. venusta plants that were collected in a gallery forest in the State of Bahia, Brazil, and grown in a greenhouse. Theses samples were fixed and processed according to routine procedures in plant anatomy and histochemistry or for scanning electron microscopy. The extrafloral nectaries occur on the edge and sub-edge of young leaves and at the basal portion of bracts that subtend the floral buds. They are structurally very similar, being formed by a nectary parenchyma and a simple epidermis with stomata (“non-structured nectaries”). The floral nectary is inserted at the floral receptacle fused with the labellum base, between this structure and the two inferior connate sepals. This nectary consists of an epidermis with numerous specific nectar secreting trichomes, a subnectary and a nectary parenchyma abundantly supplied by vascular terminations. Its structure is complex and distinct from other floral nectaries described for Orchidaceae.     

Photosynthetic and biochemical characterization of in vitro-derived African violet (Saintpaulia ionantha H. Wendl) plants to ex vitro conditions

African violet (Saintpaulia ionantha H. Wendl) is one of the most easily and commonly tissue-cultured ornamental plants. Despite this, there are limited reports on photosynthetic capacity and its impact on the plant quality during acclimatization. Various growth, photosynthetic and biochemical parameters and activities of antioxidant enzymes and dehydrins of micropropagated plants were assessed under three light intensities (35, 70, and 100 µmol m^?2 s^?1 photosynthetic photon flux density – PPFD). Fresh and dry plant biomass, plant height, and leaf area were optimal with high irradiance (70–100 µmol m^?2 s^?1 PPFD). Chlorophyll and carotenoid contents and net photosynthesis were optimal in plants grown under 70 µmol m^?2 s^?1 PPFD. Stomatal resistance, malondialdehyde content, and F_v/F_m values were highest at low light irradiance (35 µmol m^?2 s^?1 PPFD). The activities of three antioxidant enzymes, superoxide dismutase, catalase, and glutathione peroxidase, increased as light irradiance increased, signaling that high light irradiance was an abiotic stress. The accumulation of 55, 33, and 25 kDa dehydrins was observed with all light treatments although the expression levels were highest at 35 µmol m^?2 s^?1 PPFD. Irradiance at 70 µmol m^?2 s^?1 PPFD was suitable for the acclimatization of African violet plants. Both low and high irradiance levels (35 and 100 µmol m^?2 s^?1 PPFD) induced the accumulation of antioxidants and dehydrins in plants which reveals enhanced stress levels and measures to counter it.     

GoNe encoding a class VIIIb AP2/ERF is required for both extrafloral and floral nectary development in Gossypium

The floral nectary, first recognized and described by Carl Linnaeus, is a remarkable organ that serves to provide carbohydrate-rich nectar to visiting pollinators in return for gamete transfer between flowers. Therefore, the nectary has indispensable biological significance in plant reproduction and even in evolution. Only two genes, CRC and STY, have been reported to regulate floral nectary development. However, it is still unknown what genes contribute to extrafloral nectary development. Here, we report that a nectary development gene in Gossypium (GoNe), annotated as an APETALA 2/ethylene-responsive factor (AP2/ERF), is responsible for the formation of both floral and extrafloral nectaries. GoNe plants that are silenced via virus-induced gene silencing technology and/or knocked out by Cas9 produce a nectariless phenotype. Point mutation and gene truncation simultaneously in duplicated genes Ne₁Ne₂ lead to impaired nectary development in tetraploid cotton. There is no difference in the expression of the CRC and STY genes between the nectary TM-1 and the nectariless MD90ne in cotton. Therefore, the GoNe gene responsible for the formation of floral and extrafloral nectaries may be independent of CRC and STY. A complex mechanism might exist that restricts the nectary to a specific position with different genetic factors. Characterization of these target genes regulating nectary production has provided insights into the development, evolution, and function of nectaries and insect-resistant breeding.     

Floral nectaries and nectar production in brown mustard (Brassica juncea) and white mustard (Sinapis alba) (Brassicaceae)

 The dynamics and abundance of nectar secretion as well as sugar productivity were studied in flowers of brown mustard (Brassica juncea) cv. Małopolska and white mustard (Sinapis alba) cv. Borowska. Moreover, floral nectaries were examined under LM and SEM. In both cultivars lateral and median pairs of nectaries secreted nectar. However, differences were found in morphology and activity of these pairs. The lateral nectaries produced more nectar than the median ones. Nectar secretion started at loose bud and peaked during anther dehiscence. Average amount of nectarsecreted by 100 flowers of cv. Małopolska and cv. Borowska were 119.9 mg and 134 mg. Mean concentration of nectar was 26.7% and 23.4%, respectively. One hundred flowers of cv. Małopolska and cv. Borowska secreted 28.4 mg and 24.9 mg of sugars in nectar. Estimated sugar productivity per 1 ha of crop was 65.5 kg and 71.2 kg, respectively. Received August 28, 2002; accepted November 2, 2002 Published online: June 2, 2003     

Why do ants shift their foraging from extrafloral nectar to aphid honeydew?

When aphids parasitize plants with extrafloral nectaries (EFNs) and aphid colony size is small, ants frequently use EFNs but hardly tend aphids. However, as the aphid colony size increases, ants stop using EFNs and strengthen their associations with aphids. Although the shift in ant behavior is important for determining the dynamics of the ant–plant–aphid interaction, it is not known why this shift occurs. Here, we test two hypotheses to explain the mechanism responsible for this behavioral shift: (1) Extrafloral nectar secretion changes in response to aphid herbivory, or (2) plants do not change extrafloral nectar secretion, but the total reward to ants from aphids will exceed that from EFNs above a certain aphid colony size. To judge which mechanism is plausible, we investigated secretion patterns of extrafloral nectar produced by plants with and without aphids, compared the amount of sugar supplied by EFNs and aphids, and examined whether extrafloral nectar or honeydew was more attractive to ants. Our results show that there was no inducible extrafloral secretion in response to aphid herbivory, but the sugar concentration in extrafloral nectar was higher than in honeydew, and more ant workers were attracted to an artificial extrafloral nectar solution than to an artificial aphid honeydew solution. These results indicate that extrafloral nectar is a more attractive reward than aphid honeydew per unit volume. However, even an aphid colony containing only two individuals can supply a greater reward to ants than EFNs. This suggests that the ant behavioral shift may be explained by the second hypothesis.