Conspicuous extra-floral nectaries are inducible in Vicia faba

Mutualistic interactions are dynamic associations that vary depending on the costs and benefits to each of the interacting parties. Phenotypic plasticity in mutualistic interactions allows organisms to produce rewards to attract mutualists when the benefits of their presence outweigh the costs of producing the rewards. In ant–plant defensive mutualisms, defences are indirect as plants produce extra‐floral nectaries (EFN) to attract predatory ants to deter herbivores. Here we demonstrate that in broad bean, Vicia faba, the overall number of EFNs on a plant increases dramatically following leaf damage. In two damage treatments, removal of: (1) one‐third of one leaf in a single leaf pair or (2) one‐third of both halves of a single leaf pair, resulted in a 59 and 106% increase in the number of EFNs on the plants, respectively, over 1 week. We suggest that the increased production of visually conspicuous EFNs is an adaptive inducible response, to attract predatory arthropods when risk of herbivory increases.     

Herbivory-induced overcompensation and resource-dependent production of extrafloral nectaries in Luffa cylindrica (Cucurbitaceae)

Extrafloral nectaries (EFNs) are nectar secretory structures involved in the indirect defense of plants. In the sponge gourd (Luffa cylindrica), EFNs commonly occur on the lower surface of leaf blades and stipules and remain functional until leaf senescence. To test the hypothesis that the development of EFNs is influenced by herbivore damage and resource availability, we grew Luffa cylindrica under different concentrations of Hoagland's nutrient solution (nutrient-poor conditions: 10%, 50%; and control condition: 100%) and two herbivory treatments (damaged and undamaged leaves). We collected ten leaves from treated plants to quantify leaf area and EFN density. Overall, leaf area increased and EFN decreased in damaged plants, but this significantly depended on nutritional status. In undamaged plants, EFN density tended to remain constant, whereas foliar area increased with nutrient input. Under herbivory, foliar area increased at 10% but decreased at 50 and 100% of nutrients in relation to undamaged plants, whereas EFN density tended to increase with nutrient availability to exceed undamaged plants under control concentrations. Plants under nutrient-poor conditions subjected to herbivory exhibited an increased foliar area, characterizing a compensatory mechanism. Our results suggest that herbivore-induced indirect defense is a damage- and resource-dependent response in Luffa cylindrica. These findings contribute to understanding the factors that modulate indirect defenses and plant-herbivore-environment interactions.     

Diversity and evolution of a trait mediating ant–plant interactions: insights from extrafloral nectaries in Senna (Leguminosae)

Background and Aims

Plants display a wide range of traits that allow them to use animals for vital tasks. To attract and reward aggressive ants that protect developing leaves and flowers from consumers, many plants bear extrafloral nectaries (EFNs). EFNs are exceptionally diverse in morphology and locations on a plant. In this study the evolution of EFN diversity is explored by focusing on the legume genus Senna, in which EFNs underwent remarkable morphological diversification and occur in over 80 % of the approx. 350 species.

Methods

EFN diversity in location, morphology and plant ontogeny was characterized in wild and cultivated plants, using scanning electron microscopy and microtome sectioning. From these data EFN evolution was reconstructed in a phylogenetic framework comprising 83 Senna species.

Key Results

Two distinct kinds of EFNs exist in two unrelated clades within Senna. ‘Individualized’ EFNs (iEFNs), located on the compound leaves and sometimes at the base of pedicels, display a conspicuous, gland-like nectary structure, are highly diverse in shape and characterize the species-rich EFN clade. Previously overlooked ‘non-individualized’ EFNs (non-iEFNs) embedded within stipules, bracts, and sepals are cryptic and may represent a new synapomorphy for clade II. Leaves bear EFNs consistently throughout plant ontogeny. In one species, however, early seedlings develop iEFNs between the first pair of leaflets, but later leaves produce them at the leaf base. This ontogenetic shift reflects our inferred diversification history of iEFN location: ancestral leaves bore EFNs between the first pair of leaflets, while leaves derived from them bore EFNs either between multiple pairs of leaflets or at the leaf base.

Conclusions

EFNs are more diverse than previously thought. EFN-bearing plant parts provide different opportunities for EFN presentation (i.e. location) and individualization (i.e. morphology), with implications for EFN morphological evolution, EFN–ant protective mutualisms and the evolutionary role of EFNs in plant diversification.     

Extrafloral nectary-bearing plant Mallotus japonicus uses different types of extrafloral nectaries to establish effective defense by ants

Extrafloral nectary (EFN)-bearing plants attract ants to gain protection against herbivores. Some EFN-bearing plants possess different types of EFNs, which might have different effects on ants on the plants. Mallotus japonicus (Thunb.) Muell. Arg. (Euphorbiaceae) bears two types of EFNs, including a pair of large EFNs at the leaf base and many small EFNs along the leaf edge. This study aimed to determine the different roles of the two types of EFNs in biotic defense by ants. A field experiment was conducted to investigate the effect of leaf damage on EFN production and on the distribution pattern of ants. After leaf damage, the number of leaf edge EFNs increased in the leaves first-produced. The number of ants on the leaves also increased, and the foraging area of ants extended from the leaf base to the leaf tip. An EFN-covering field experiment revealed that leaf edge EFNs had a greater effect than leaf base EFNs on ant dispersal on leaves. The extended foraging area of ants resulted in an increase of encounter or attack rate against an experimentally placed herbivore, Spodoptera litura. These results suggest that M. japonicus plants control the foraging area of ants on their leaves using different types of EFNs in response to leaf damage, thus achieving a very effective biotic defense against herbivores by ants.

     

Extrafloral nectaries in aspen (Populus tremuloides): heritable genetic variation and herbivore-induced expression

BACKGROUND AND AIMS: A wide variety of plants produce extrafloral nectaries (EFNs) that are visited by predatory arthropods. But very few studies have investigated the relationship between plant genetic variation and EFNs. The presence of foliar EFNs is highly variable among different aspen (Populus tremuloides) genotypes and the EFNs are visited by parasitic wasps and predatory flies. The aim here was to determine the heritability of EFNs among aspen genotypes and age classes, possible trade-offs between direct and indirect defences, EFN induction following herbivory, and the relationship between EFNs and predatory insects. METHODS: EFN density was quantified among aspen genotypes in Wisconsin on trees of different ages and broad-sense heritability from common garden trees was calculated. EFNs were also quantified in natural aspen stands in Utah. From the common garden trees foliar defensive chemical levels were quantified to evaluate their relationship with EFN density. A defoliation experiment was performed to determine if EFNs can be induced in response to herbivory. Finally, predatory arthropod abundance among aspen trees was quantified to determine the relationship between arthropod abundance and EFNs. KEY RESULTS: Broad-sense heritability for expression (0.74-0.82) and induction (0.85) of EFNs was high. One-year-old trees had 20% greater EFN density than 4-year-old trees and more than 50% greater EFN density than > or =10-year-old trees. No trade-offs were found between foliar chemical concentrations and EFN density. Predatory fly abundance varied among aspen genotypes, but predatory arthropod abundance and average EFN density were not related. CONCLUSIONS: Aspen extrafloral nectaries are strongly genetically determined and have the potential to respond rapidly to evolutionary forces. The pattern of EFN expression among different age classes of trees appears to follow predictions of optimal defence theory. The relationship between EFNs and predators likely varies in relation to multiple temporal and environmental factors.     

Concentration and retention of chlorophyll around the extrafloral nectary of Mallotus japonicus

Plants need to allocate some of their limited resources for defense against herbivores as well as for growth and reproduction. However, the priority of resource allocation within plants has not been investigated. We hypothesized that plants with extrafloral nectaries (EFNs) invest more chlorophyll around their EFNs—to support a high rate of carbon fixation there—than in other leaf parts of young leaves. Additionally, this chlorophyll may remain around EFNs rather than in the other leaf parts. We used Mallotus japonicus plants to investigate the chlorophyll content at leaf centers and edges and around EFNs at four stages of leaf development: middle‐expanded young leaves, fully expanded mature leaves, senior leaves, and leaves prior to abscission. These four stages of development were located at the third, fifth, eighth, and eleventh leaf positions from the apex, respectively. The results revealed that the chlorophyll content around the EFN side of the third‐position leaves was higher than that at the leaf center or edge. Although the chlorophyll content in the fifth‐position leaves did not differ between those at the leaf edge and around EFNs, the chlorophyll content around EFNs in the eighth‐position leaves was higher than that at the leaf centre and edge. The volume of EF nectar was positively correlated with the chlorophyll content around EFN during the leaf stage, but it was not correlated with the chlorophyll content in the leaf center and edge, except in fifth‐position leaves. These findings suggest that M. japonicus plants facilitate and maintain secretion of EF nectar in their young and old leaves, respectively, through the concentration and retention of chlorophyll around EFNs.     

Decreasing water availability across the globe improves the effectiveness of protective ant–plant mutualisms: a meta‐analysis

Abiotic conditions can increase the costs of services and/or the benefits of rewards provided by mutualistic partners. Consequently, in some situations, the outcome of mutualisms can move from beneficial to detrimental for at least one partner. In the case of protective mutualisms between ant bodyguards and plants bearing extrafloral nectaries (EFNs), plants from arid environments face a trade‐off between EFN production and maintenance and water and carbon economy. This trade‐off may increase EFN costs and decrease their value as a defensive strategy to plants in such environments. Despite this, the presence of EFNs is an ubiquitous trait in plants from arid environments, suggesting that they provide greater benefits to plants in these environments to compensate for their higher costs. We used a meta‐analysis to investigate if such benefits do increase with decreasing water availability and the possible underlying causes (such as ant behaviour or ant diversity). As predicted, ant effect on EFN plants performance increased as mean annual precipitation decreased. We also found that the frequency of dominant ants on EFN plants increased in drier areas. Due to the more aggressive behaviour of dominant ants, we suggest that they represent an important factor shaping the adaptive value of EFNs to plants in arid environments.     

The function of extrafloral nectaries in the pollination and reproduction of Sambucus javanica

Sambucus javanica is a perennial herb with extrafloral nectaries (EFNs) on its inflorescences. To explore the ecological functions of EFNs, a factorial combination experiment of ant (access or exclusion) and EFNs (with or without) at the plant level was created in two populations. The role of EFNs in the attraction of ants and flying pollinators, the defensive role of ants against foliar herbivores, the effects of ants on pollinator visitation and the effects of ant–pollinator interactions on fruit production in one or both populations were assessed. Ants were common on the ant-access plants with EFNs, but absent from the ant-access plants without EFNs. Foliar herbivory was independent of ant and EFN treatments and their interactions. The visitation frequency of flying pollinators (honeybees and syrphid flies) and fruit set were significantly higher for plants with EFNs than plants without EFNs, but were not affected by ant treatments or their interactions with EFN treatments. These results suggest that EFNs in S. javanica attracted both ants and flying pollinators, but ants did not present a defensive role against herbivores, did not deter flying pollinators from visiting inflorescences and had no effects on fruit production. In addition, ants were not significant pollen vectors.     

Defensive effects of extrafloral nectaries in quaking aspen differ with scale

The effects of plant defenses on herbivory can differ among spatial scales. This may be particularly common with indirect defenses, such as extrafloral nectaries (EFNs), that attract predatory arthropods and are dependent on predator distribution, abundance, and behavior. We tested the defensive effects of EFNs in quaking aspen (Populus tremuloides Michx.) against damage by a specialist herbivore, the aspen leaf miner (Phyllocnistis populiella Cham.), at the scale of individual leaves and entire ramets (i.e., stems). Experiments excluding crawling arthropods revealed that the effects of aspen EFNs differed at the leaf and ramet scales. Crawling predators caused similar reductions in the percent leaf area mined on individual leaves with and without EFNs. However, the extent to which crawling predators increased leaf miner mortality and, consequently, reduced mining damage increased with EFN expression at the ramet scale. Thus, aspen EFNs provided a diffuse defense, reducing damage to leaves across a ramet regardless of leaf-scale EFN expression. We detected lower leaf miner damage and survival unassociated with crawling predators on EFN-bearing leaves, suggesting that direct defenses (e.g., chemical defenses) were stronger on leaves with than without EFNs. Greater direct defenses on EFN-bearing leaves may reduce the probability of losing these leaves and thus weakening ramet-scale EFN defense. Aspen growth was not related to EFN expression or the presence of crawling predators over the course of a single season. Different effects of aspen EFNs at the leaf and ramet scales suggest that future studies may benefit from examining indirect defenses simultaneously at multiple scales.     

Preliminary Investigation of Extrafloral Nectaried Plants the Tropical Rainforests in Xishuangbanna,China

In order to understand the distribution pattern of the extrafloral nectaried plants in the tropical rainforest in Xishuangbanna, Yunnan , China, we investigated seven forest communities ( three primary forests and four secondary forests at different ages) in Xishuangbanna Nature Reserve . Fifty-two species belonging to 36 genera and 23 families accounting for 12.3% of 424 angiosperm species in the seven communities were found to bear extrafloral nectaries ( EFNs ). Among which, eight genera and 37 species were the first time to be recorded as plants to bear EFNs. The study indicated, (1)The EFN species mainly occurred in subclasses Dilleniidae, Rosidae and Asteridae of the Magnoliopsida , and the most common EFNs were flattened glands situated on leaf blades ; (2) The proportions of EFN species within the seven communities ranged from 9.5% ( Vatica guangxiensis forest ) to 18 .5% ( Macaranga denticulata forest ), and the EFN species appeared to be more abundant in the secondary forests than those in the primary forests; (3) EFN species were disproportionally distributed among different growth forms, with the abundance being: tree > shrub > liana > herb , and no epiphytic and parasitic plants were found to bear EFNs.     

Extrafloral Nectaries in Cipadessa (Meliaceae)

This is the first report of an extrafloral nectary (EFN) fromAsian Meliaceae and from subfamily Melioideae. The pinnatelycompound leaf of Cipadessa baccifera has 25–35 small,ellipsoidal EFNs abaxially on the rachis, with occasional EFNson leaflets. EFNs secrete nectar until leaf maturity, then graduallywither. Each convex, ellipsoidal EFN is parenchymatous, withouta palisade epidermis, a delimiting nectary sheath, or any vascularaffiliation. This EFN differs markedly from the typical ‘Flachnektarien’EFN described earlier from neotropical Swietenia species. Cipadessa baccifera (Roth.) Miq., extrafloral nectary, Meliaceae, nectary anatomy     

Quantity over quality: light intensity,but not red/far‐red ratio,affects extrafloral nectar production in Senna mexicana var. chapmanii

Extrafloral nectar (EFN) mediates food‐for‐protection mutualisms between plants and insects and provides plants with a form of indirect defense against herbivory. Understanding sources of variation in EFN production is important because such variations affect the number and identity of insect visitors and the effectiveness of plant defense. Light represents a potentially crucial tool for regulating resource allocation to defense, as it not only contributes energy but may help plants to anticipate future conditions. Low red/far‐red (R/FR) light ratios can act as a signal of the proximity of competing plants. Exposure to such light ratios has been shown to promote competitive behaviors that coincide with reduced resource allocation to direct chemical defenses. Little is known, however, about how such informational light signals might affect indirect defenses such as EFN, and the interactions that they mediate. Through controlled glasshouse experiments, we investigated the effects of light intensity, and R/FR light ratios, on EFN production in Senna mexicana var. chapmanii. Plants in light‐limited conditions produced significantly less EFN, and leaf damage elicited increased EFN production regardless of light conditions. Ratios of R/FR light, however, did not appear to affect EFN production in either damaged or undamaged plants. Understanding the effects of light on indirect defenses is of particular importance for plants in the threatened pine rockland habitats of south Florida, where light conditions are changing in predictable ways following extensive fragmentation and subsequent mismanagement. Around 27% of species in these habitats produce EFN and may rely on insect communities for defense.     

Disturbance Winners or Losers? Plants Bearing Extrafloral Nectaries in Brazilian Caatinga

Plants bearing extrafloral nectaries (EFNs) often have traits typical of pioneer species, and may be expected to proliferate in disturbed habitats. However, a negative effect of disturbance on visitation by attendant ants could prevent EFN‐bearing plants from acting as disturbance winners. Here, we address the effects of chronic anthropogenic disturbance on the abundance of EFN‐bearing plants and their interactions with attendant ants in Caatinga vegetation of northeastern Brazil. We recorded the abundance of EFN‐bearing plants, proportion of plants visited by ants and composition of ant attendant species at 24 sites varying in levels of disturbance. EFN‐bearing plants as a whole did not behave as a disturbance winner group. The responses of the 13 species to increasing disturbance were highly variable, with three species declining in abundance (loser species). The richness of ant species attending EFNs did not vary with disturbance, but species composition did. The overall proportion of EFN‐bearing plants attended by ants per 5‐min period was not affected by disturbance. However, for the three loser species, attendance decreased from about 50 percent with low and moderate disturbance to half that with very high disturbance. We hypothesize that disturbed sites are more stressful for loser species compared with other EFN‐bearing plants, with physiological stress resulting in lower production of EFN secretions and reduced attraction of ants. This would make such species double losers, with physiological stress at disturbed sites not only directly influencing their performance but also indirectly affecting it through the disruption of a key mutualism.     

Distribution and structural aspects of extrafloral nectaries in Cedrela fissilis (Meliaceae)

The occurrence of extrafloral nectaries (EFNs) in Meliaceae has been reported for some genera, but little anatomical data are available. Therefore, to determine the distribution and structural aspects of EFNs, Cedrela fissilis Vell. leaves in different stages of development were collected, fixed, and processed for light and scanning electron microscopy. On the petiole, rachis and petiolule, EFNs were found to be arranged predominantly towards the abaxial surface, while their occurrence in leaflet blades was restricted to the abaxial surface of the major veins, noticeably on the midrib. Basal leaflets displayed few EFNs; however, we observed an increase towards the leaf's apex. The leaf can contain more than 300 inconspicuous EFNs, which show secretory activity throughout the leaf's life. Two EFN morphotypes were visible: flattened or elevated, both circular or slightly elliptical and similar in origin and tissue composition. The secretory tissue is embedded in the rachis cortex or in the major veins of the leaf blade and EFNs are not vascularized. The EFN secretory pole shows a uniseriate epidermis with compactly arranged cells and a thin cuticle; stomata and trichomes are absent. The observation of ant visits at these structures reinforces the assumption that EFNs mediate ant–plant interactions and play a protective role against herbivores throughout the life of a leaf.     

Phylogenetic and Experimental Tests of Interactions among Mutualistic Plant Defense Traits in Viburnum (Adoxaceae)

Abstract Plant traits that mediate mutualistic interactions are widespread, yet few studies have linked their macroevolutionary patterns with the ecological interactions they mediate. Here we merged phylogenetic and experimental approaches to investigate the evolution of two common mutualistic plant traits, extrafloral nectaries (EFNs) and leaf domatia. By using the flowering plant clade Viburnum, we tested whether macroevolutionary patterns support adaptive hypotheses and conducted field surveys and manipulative experiments to examine whether ecological interactions are concordant with evolutionary predictions. Phylogenetic reconstructions suggested that EFN-bearing species are monophyletic, whereas the evolution of domatia correlated with leaf production strategy (deciduous or evergreen) and climate. Domatia were also more common in the EFN clade, suggesting that the two traits may jointly mediate ecological interactions. This result was further investigated in a common-garden survey, where plants with domatia and EFNs on the leaf blade had more mutualistic mites than plants with other trait combinations, and in manipulative field experiments, where the traits additively increased mutualist abundance. Taken together, our results suggest that mutualistic traits in Viburnum are not ecologically independent, as they work in concert to attract and retain mutualists, and their long-term evolution may be influenced by complex interactions among multiple traits, mutualists, and geography.     

Anti-herbivory defense of two <Emphasis Type="Italic">Vicia</Emphasis> species with and without extrafloral nectaries

The effects of direct and indirect defenses differ among plant species, and the variation in the mode of plant defenses might reflect physiological and/or ecological constraints of each mode of defense related to the growth and reproduction of individual plant species. To evaluate the advantages and disadvantages of indirect ant-mediated defense via extrafloral nectaries (EFNs), we compared the herbivory pressure, leaf chemicals, vegetative growth, and reproduction between two species of vetches, Vicia sativa var. angustifolia (Reichard) Wahlenb (Leguminosae) with EFNs and V. hirsuta (L.) SF Gray without EFNs (or with very small EFNs). Indirect ant defense of V. sativa was not consistently reliable because of the low constancy of ant attraction. In addition, V. sativa was more vulnerable to attack by herbivores than V. hirsuta. The estimated total amount of sugars secreted by EFNs of V. sativa corresponded to 0.5% of total leaf biomass, and 0.07% of total plant biomass, indicating a low investment to the production of extrafloral nectar. Vicia sativa plants grew more rapidly than V. hirsuta plants during the reproductive stage. Therefore, we consider that V. sativa adopts the ant defense via EFNs in spite of its low reliability because the indirect ant defense supported by EFNs requires only low investment, allowing the plants to attain rapid growth in the early spring.     

Temporal consistency and individual specialization in resource use by green turtles in successive life stages

For many insect herbivores, maternal host selection is a critical determinant of offspring survival; however, maternal fitness is also affected by adult resources such as food availability. Consequently, adult resources may promote oviposition in sub-optimal locations when measured in terms of offspring performance. We tested whether oviposition site preference is primarily shaped by proximity to adult food resources or offspring performance in the aspen leaf miner (Phyllocnistis populiella). Quaking aspen (Populus tremuloides) produce extrafloral nectaries (EFNs) on a subset of their leaves. EFN expression on leaves is associated with decreased P. populiella damage and larval performance; however, P. populiella adults feed from EFNs. We reduced extrafloral nectar availability on entire aspen ramets and excluded crawling predators in a full factorial experiment at two sites in interior Alaska, USA. Patterns of egg deposition by P. populiella appeared to be primarily affected by offspring survival rather than adult resource availability. While oviposition was unaffected by nectar availability, adult moths laid fewer eggs on leaves with than without EFNs. By avoiding leaves with EFNs, moths increased offspring survival. Both moths and predators distinguished between leaves with and without EFNs even when nectar and visual cues were obscured, and therefore may respond to chemical cues associated with EFN expression.     

Adaptations to biotic and abiotic stress: Macaranga-ant plants optimize investment in biotic defence

Obligate ant plants (myrmecophytes) in the genus Macaranga produce energy- and nutrient-rich food bodies (FBs) to nourish mutualistic ants which live inside the plants. These defend their host against biotic stress caused by herbivores and pathogens. Facultative, 'myrmecophilic' interactions are based on the provision of FBs and/or extrafloral nectar (EFN) to defending insects that are attracted from the vicinity. FB production by the myrmecophyte, M. triloba, was limited by soil nutrient content under field conditions and was regulated according to the presence or absence of an ant colony. However, increased FB production promoted growth of the ant colonies living in the plants. Ant colony size is an important defensive trait and is negatively correlated to a plant's leaf damage. Similar regulatory patterns occurred in the EFN production of the myrmecophilic M. tanarius. Nectar accumulation resulting from the absence of consumers strongly decreased nectar flow, which increased again when consumers had access to the plant. EFN flow could be induced via the octadecanoid pathway. Leaf damage increased levels of endogenous jasmonic acid (JA), and both leaf damage and exogenous JA application increased EFN flow. Higher numbers of nectary visiting insects and lower numbers of herbivores were present on JA-treated plants. In the long run, this decreased leaf damage significantly. Ant food production is controlled by different regulatory mechanisms which ensure that costs are only incurred when counterbalanced by defensive effects of mutualistic insects.     

Within-plant distribution of phenolic glycosides and extrafloral nectaries in trembling aspen (Populus tremuloides; Salicaceae)

Expression of foliar secondary compounds and extrafloral nectaries (EFNs) within the same leaves may be incompatible if secondary compounds repel beneficial insects that might otherwise be attracted to EFNs. This study examined the within-plant distributions of phenolic glycosides and EFNs in trembling aspen, Populus tremuloides, and their relationships to herbivore damage. Populus tremuloides expresses extrafloral nectaries (EFNs) on a subset of its leaves. We studied short and tall naturally occurring aspen ramets across multiple sites in interior Alaska. Contrary to our expectations, foliar phenolic glycoside concentrations were approximately 30% greater on leaves bearing EFNs than on leaves without EFNs. The mean concentration of foliar phenolic glycosides in short ramets was nine times that in tall ramets. Phenolic glycoside concentration was negatively related to leaf mining damage by Phyllocnistis populiella (Lepidoptera; Gracilliadae) at concentrations greater than 27 mg/g, whereas the presence of EFNs was unrelated to mining damage. The positive association of chemical defensive compounds and EFNs in leaves suggests that, for species with variation in EFN expression, negative correlations between herbivory and EFN expression may arise indirectly from associated effects of other, correlated types of defense.