The extrafloral nectaries of cowpea (Vigna unguiculata (L.) Walp): I. Morphology, anatomy and fine structure

Cowpea extrafloral nectar has potential to provide ecosystem services lost in agricultural intensification and support native parasitoids that suppress the wheat stem sawfly

The native parasitoids Bracon cephi (Gahan) and B. lissogaster Muesebeck (Hymenoptera: Braconidae) reduce populations of Cephus cinctus Norton (Hymenoptera: Cephidae), a native grassland species, and major wheat pest on the Northern Great Plains of North America. Non-host feeding adults of these braconids increase longevity, egg load, and egg volume when provisioned carbohydrate-rich diets. Nutrition from nectar can enhance the success of natural enemies in pest management programs. Cowpea, Vigna unguiculata (L.) Walpers, is a potential cover-crop that could add resilient features to the landscape and has extrafloral nectaries (EFN), easy-access nectar sources for beneficial insects. If more cowpea was grown on the Northern Great Plains, would B. cephi and B. lissogaster benefit from foraging on putatively beneficial EFN? We investigated cowpea inflorescence stalk extrafloral nectars (IS-EFN) and leaf stipel extrafloral nectars (LS-EFN) as potential food sources for these parasitoids. Females were caged on EFN sources on living cowpea plants to assess longevity. Egg load and volume were measured at 2, 5, and 10 days after placement. Bracon cephi survived 10 days on water, 38 days on IS-EFN; B. lissogaster 6 days on water, 28 days on IS-EFN. Bracon lissogaster maintained a constant egg load and volume across treatments while B. cephi produced 2.1-fold more eggs that were 1.6-fold larger on IS-EFN. Y-tube olfactometry indicated adult females were attracted to airstreams containing cowpea volatiles. These results demonstrate that non-native, warm-season cowpea benefits these native parasitoids and may improve conservation biocontrol of C. cinctus.

Development and differentiation of the extrafloral nectaries from flower buds in Vigna luteola (Leguminosae, Phaseolinae)

To study the ontogeny of the extrafloral nectaries present in the inflorescences of Vigna luteola (Jacq.) Benth (Leguminosae, Phaseolinae), the location, morphology, anatomy of the earliest stages, histology of the definitive structures and ultrastructure of the secretory stage were analyzed. The extrafloral nectaries at different developmental stages were examined with light microscopy and scanning electron microscopy. The secretory stage was also examined with transmission electron microscopy. The racemose inflorescence of V. luteola has six nodes. At each node, a short globose secondary axis bears two flowers and one to three extrafloral nectaries. Each extrafloral nectary originates from the abscission of a flower bud and is formed by two differentiated zones: a ring of epidermal cells surrounding a group of longitudinally enlarged papillose central cells, both with underlying secretory parenchyma. The primary secretory tissue consists of the central cells, while the ring contributes to secretion to a lesser degree. Secretion is granulocrine, by means of exocytotic vesicles and plasmalemma invaginations. Four developmental stages succeed; the third one being the secretory. The extrafloral nectaries activity period starts when the flowers of the same secondary axis open and ceases before fruit development.

Dynamic extrafloral nectar production: the timing of leaf damage affects the defensive response in Senna mexicana var. chapmanii (Fabaceae)

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PREMISE OF THE STUDY

Extrafloral nectar (EFN) mediates food for protection mutualisms between plants and defensive insects. Understanding sources of variation in EFN production is important because such variations may affect the number and identity of visitors and the effectiveness of plant defense. We investigated the influence of plant developmental stage, time of day, leaf age, and leaf damage on EFN production in Senna mexicana var. chapmanii. The observed patterns of variation in EFN production were compared with those predicted by optimal defense theory.•

METHODS

Greenhouse experiments with potted plants were conducted to determine how plant age, time of day, and leaf damage affected EFN production. A subsequent field study was conducted to determine how leaf damage, and the resulting increase in EFN production, affected ant visitation in S. chapmanii.•

KEY RESULTS

More nectar was produced at night and by older plants. Leaf damage resulted in increased EFN production, and the magnitude of the response was greater in plants damaged in the morning than those damaged at night. Damage to young leaves elicited a stronger defensive response than damage to older leaves, in line with optimal defense theory. Damage to the leaves of S. chapmanii also resulted in significantly higher ant visitation in the field.•

CONCLUSIONS

Extrafloral nectar is an inducible defense in S. chapmanii. Developmental variations in its production support the growth differentiation balance hypothesis, while within-plant variations and damage responses support optimal defense theory.

Ontogeny of the extrafloral nectaries of Vigna adenantha (Leguminosae, Phaseolae) and its relation with floral development

BACKGROUND

The inflorescences of the genus Vigna Savi have extrafloral nectaries (EFNs) among the flowers whose origin is still unknown. The disposition, anatomy and morphology, as well as the ontogeny of the extrafloral nectaries (EFNs) associated with the inflorescences of Vigna adenantha (G.F.W. Meyer) Maréchal, Mascherpa & Stainier (Leguminosae, Papilionoideae, Phaseolae) were studied. Besides, the ultrastructure of the secretory stage was described.

RESULTS

The inflorescence, a raceme, bears a brief globose secondary axis in each node with 2 flowers and 5-7 EFNs, which develop in acropetal direction. Each EFN originates from the abscission of a flower bud that interrupts its development, resulting in an elevated EFN. This secretory structure is formed by a ring of epidermal and parenchymatic cells surrounding a group of elongated central cells. The nectary is irrigated by phloem and xylem. Four developmental stages proceed; each one relates to a different embryological stage of the flowers in each secondary axis.

CONCLUSIONS

The first functional EFN of each secondary axis of the inflorescence reaches its maturity when both the pollen grains and the embryo sacs are completely developed and the flowers begin to open. The secretion is granulocrine. The following EFNs develop in the same way.

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.

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.

Ultrastructure and post-floral secretion of the pericarpial nectaries of Erythrina speciosa (Fabaceae)

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.

METHODS

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.

KEY RESULTS

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.

CONCLUSIONS

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.

Herbivore-induced, indirect plant defences

Indirect responses are defensive strategies by which plants attract natural enemies of their herbivores that act as plant defending agents. Such defences can be either constitutively expressed or induced by the combined action of mechanical damage and low- or high-molecular-weight elicitors from the attacking herbivore. Here, we focus on two induced indirect defences, namely the de novo production of volatiles and the secretion of extrafloral nectar, which both mediate interactions with organisms from higher trophic levels (i.e., parasitoids or carnivores). We give an overview on elicitors, early signals, and signal transduction resulting in a complex regulation of indirect defences and discuss effects of cross-talks between the signalling pathways (synergistic and antagonistic effects). In the light of recent findings, we review molecular and genetic aspects of the biosynthesis of herbivore-induced plant volatiles comprising terpenoids, aromatic compounds, and metabolites of fatty acids which act as infochemicals for animals and some of which even induce defence genes in neighbouring plants. Finally, ecological aspects of these two indirect defences such as their variability, specificity, evolution as well as their ecological relevance in nature are discussed.

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. [(14)C]Sucrose fed to leaves was translocated as such to nectaries, where it was partly inverted to [(14)C]glucose and [(14)C]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 (14)C-labelled nitrogenous solutes resulted in these substrates and their metabolic products appearing in fruit-phloem sap and adjacent inflorescence-stalk nectar. (14)C-labelled asparagine, valine and histidine transferred freely into phloem and appeared still largely as such in nectar. (14)C-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.