Extrafloral nectary phenotypic plasticity is damage- and resource-dependent in Vicia faba

Phenotypic plasticity enables many damaged plants to increase nectar secretion rates from extrafloral nectaries (EFNs), or in the case of broad bean, Vicia faba L. to produce additional EFNs, to attract natural enemies of herbivores. While plants benefit greatly from these defensive mutualisms, the costs of producing EFNs are largely unknown. We hypothesized that if EFN production is costly, then damaged plants with high resource levels would be able to produce more EFNs than plants that are resource-limited. Here, we show that this indirect inducible defence does follow this general pattern. Vicia faba enriched with 6 or 12 g of 14:14:14 NPK fertilizer increased EFN numbers after leaf damage by 46 and 60%, respectively, compared with nutrient-poor plants. Thus, EFN production is both damage- and resource-dependent. Analogous to direct defences, production of EFNs may limit the overall loss of leaf tissue when risk of herbivory increases.     

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.     

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.     

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.     

Spatiotemporal dynamics and plant-part preference patterns of the plant-visiting ants and the insect herbivores of sponge gourd plants

Field investigations revealed a synchrony between daily and seasonal activity patterns of two Raphidopalpa pests on the extrafloral nectary (EFN)-bearing sponge gourd, Luffa cylindrica (L.) plants. A similar pattern was revealed in six of the most abundant plant-visiting ant species. The spatial activity patterns of the herbivores R. foveicollis and R. intermedia revealed a high frequency on the leaves (except, newly unfolded leaves) and the corolla, but not on the bracts, bracteoles and the calyces of male/female buds and flowers. Young expanding leaves (leaf age class 2) had a significantly higher density of extrafloral nectaries (EFNs), as well as minimum leaf damage compared with the other leaf age classes. The density of EFNs was highest on the bracteoles and lowest on the leaves. In behavioural assays, ants demonstrated significantly greater preference for the 2-day old buds/newly opened leaves than the 2-day old intact unisexual flowers/corolla. Also, young, newly expanding leaves were preferred over young (unopened) and mature leaves. Thus, the higher density of EFNs at vulnerable plant sites, along with the visits of ants to such sites, may influence the spatiotemporal activity patterns of the insect herbivores. These results provide support for the optimal defense hypothesis since they show that younger tissues and reproductive tissues may be getting better protection by the ant species.     

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.     

No time for candy: passionfruit (Passiflora edulis) plants down-regulate damage-induced extra floral nectar production in response to light signals of competition

Plant fitness is often defined by the combined effects of herbivory and competition, and plants must strike a delicate balance between their ability to capture limiting resources and defend against herbivore attack. Many plants use indirect defenses, such as volatile compounds and extrafloral nectaries (EFN), to attract canopy arthropods that are natural enemies of herbivorous organisms. While recent evidence suggests that upon perception of low red to far-red (R:FR) ratios, which signal the proximity of competitors, plants down-regulate resource allocation to direct chemical defenses, it is unknown if a similar phytochrome-mediated response occurs for indirect defenses. We evaluated the interactive effects of R:FR ratio and simulated herbivory on nectar production by EFNs of passionfruit (Passiflora edulis f. flavicarpa). The activity of petiolar EFNs dramatically increased in response to simulated herbivory and hormonal treatment with methyl jasmonate (MeJA). Low R:FR ratios, which induced a classic “shade-avoidance” repertoire of increased stem elongation in P. edulis, strongly suppressed the EFN response triggered by simulated herbivory or MeJA application. Strikingly, the EFN response to wounding and light quality was localized to the branches that received the treatments. In vines like P. edulis, a local response would allow the plants to precisely adjust their light harvesting and defense phenotypes to the local conditions encountered by individual branches when foraging for resources in patchy canopies. Consistent with the emerging paradigm that phytochrome regulation of jasmonate signaling is a central modulator of adaptive phenotypic plasticity, our results demonstrate that light quality is a strong regulator of indirect defenses.     

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.

     

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.     

Interactive effects of soil fertility and herbivory on Brassica nigra

Soil nutrient availability may affect both the amount of damage that plants receive from herbivores and the ability of plants to recover from herbivory, but these two factors are rarely considered together. In the experiment reported here, I examined how soil fertility influenced both the degree of defoliation and compensation for herbivory for Brassica nigra plants damaged by Pieris rapae caterpillars. Realistic levels of defoliation were obtained by placing caterpillars on potted host plants early in the life cycle and allowing them to feed until just before pupation on the designated plant. Percent defoliation was more than twice as great at low soil fertility compared to high (48.2% and 21.0%, respectively), even though plants grown at high soil fertility lost a greater absolute amount of leaf area (38.2 cm² and 22.1 cm², respectively). At both low and high soil fertility, total seed number and mean mass per seed of damaged plants were equivalent to those of undamaged plants. Thus soil fertility did not influence plant compensation in terms of maternal fitness. However, the pathways used to achieve compensation in seed production were different at low and high soil fertility. At low soil fertility, relative leaf growth rates (area added per inital area per day) of damaged plants were drastically reduced over the second week of caterpillar feeding. Damaged plants recovered the leaf area lost to herbivory in the two weeks following insect removal by increasing leaf relative growth rates above the levels seen for undamaged plants, but the replacement of leaf tissue lost to herbivory came at the expense of stem biomass. At high soil fertility, relative leaf growth rates of damaged plants were similar to those of undamaged plants both over the second week of caterpillar feeding and following caterpillar removal, and stem biomass was not affected by herbivory. These results suggest that higher levels of soil nutrients increased the ability of plants to stay ahead of their herbivores as they were being eaten. Because damaged plants at high soil fertility were able to maintain leaf growth rates to a greater extent than damaged plants at low soil fertility, they did not fall as far behind undamaged plants over the period of insect feeding and did not have as much catching up to do after feeding ended to compensate for herbivory.     

Community‐wide impacts of early season herbivory on flower visitors on tall goldenrod

1. The flower visitor community consists not only of pollinators but also of non‐pollinators, such as florivores, thieves and predators that attack flower visitors. Although there is increasing evidence that early‐season foliar herbivory influences pollinator visitation through changes in floral traits, few studies have explored indirect effects of foliar herbivory on community structure of the flower visitors. We examined how early‐season foliar herbivory influences the flower visitor community established in late season. 2. We conducted an inoculation experiment using a lacebug (Corythucha marmorata), which is a predominantly herbivorous insect attacking leaves of tall goldenrod (Solidago altissima) in Japan. 3. Flower abundance significantly decreased when damaged by the lacebug. The numbers of pollinators, florivores and thieves were positively correlated with flower abundance, whereas predators were not. In response to flower abundance, florivores decreased on damaged plants. On the other hand, thieves increased on damaged plants, and pollinators and predators did not differ between damaged and undamaged plants. 4. When effects of flower abundance were excluded, foliar herbivory still influenced florivores negatively and thieves positively. This implies that factors besides flower abundance may have affected the numbers of florivores and thieves. 5. Community composition of flower visitors on damaged plants significantly differed from undamaged plants, although overall abundance, taxonomic richness and taxonomic evenness were unaffected by foliar herbivory in the early season. It is important to recognise that only evaluating species diversity and overall abundance may fail to detect the significant consequence of early‐season herbivory on the flower visitor community.     

Experimental manipulation of biotic and abiotic parameters changes the outcome of insect-plant interactions

Plants usually respond to environmental shifts with morpho-physiological adjustments, which trickles down to biotic interactions in the insect-plant system. We evaluate how Copaifera langsdorffii, a widespread tree species adapted to multiple ecosystems, responds to shifts in nutrient and water availability through experimental supplementation and how it affects its insect communities. We also evaluate how the presence of extrafloral nectaries (EFN) exudates modifies galling insect diversity and herbivory rates. Such experimental approach is scarce, as we simultaneously evaluate biotic and abiotic factors and cover the entire reproductive cycle of a species in natural environments, bringing important contributions to better understand how bottom-up factors drive species interactions and possibly community assembly. The experiment was set in an ironstone outcrop vegetation, where we deployed supplementation treatments in 80 plant individuals divided into the following field treatments: (T1 = Fertilizer, T2 = EFN simulation (Eppendorf tubes with a 20% sugar solution), T3 = fertilizer + EFN simulation, T4 = water spray, T5 = EFN control (microtube with water), and T6 = Control. We observed lower sclerophylly and greater leaf area in plant individuals that were supplemented with nutrients and water. Herbivory rates were lower and ant abundance was higher in plants with artificial EFNs available. While we did not observe variations in richness and abundance of galling insects, the patterns of co-occurrence of galls varied with the availability of resources (nesting space) in the plant. This study illustrates how variations in nutrient availability to plants modify interactions with insect communities. Ant-plant interactions can have a negative impact on general herbivory rates, however ants seem to have a more harmonious relationship with the galling insects.     

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.     

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.     

Decreased indirect defense in the invasive tree, Triadica sebifera

In the absence of coevolved natural enemies, plants are expected to experience selection away from costly herbivore defenses toward growth and reproduction [evolution of increased competitive ability hypothesis (EICA)], yet no one has demonstrated EICA for an indirect defense trait. Likewise, we have little understanding of how constitutive and induced levels of defense vary among native and invasive plant populations. We conducted a greenhouse experiment in the introduced range to test whether invasive populations have reduced constitutive and induced investment in an indirect defense trait, extrafloral nectar (EFN) production, compared to native populations of Chinese tallow tree, Triadica sebifera, through an experimental leaf damage treatment. Overall, native populations invested more in indirect defense: Native populations had a greater number (+16?%) and percentage of leaves producing EFN (35 vs. 28?%), produced more EFN (63?% greater volume), and produced more sugar (+33?%) compared to invasive populations, independent of damage treatment. Of these traits, number of leaves producing EFN and volume of EFN exhibited a trade-off between constitutive and induced investment but these did not depend on plant origin. Our results are the first to support the EICA hypothesis for an indirect defense trait. This suggests that tri-trophic interactions such as indirect defense are under similar selection as direct defense traits within introduced populations. Despite reduced investment in EFN production, invasive populations still retain the ability to produce EFN, which may enable invasive plants to defend against herbivores in the introduced range.     

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.     

A herbivory-induced increase in the proportion of floating seeds in an invasive plant

It is important to determine the factors prompting seed dispersal because for plant species seed dispersal is the only opportunity to disperse into a new habitat. Previous studies showed that the maternal stress, such as high density and low nutrient levels, induces the adaptive plastic increase of the dispersal ability in seed heteromorphic plants. In this study, we examined whether herbivory can change the relative proportion of dispersal-related seed heteromorphism (floating or non floating seeds) in an invasive weed Ambrosia artemisiifolia. Because A. artemisiifolia often distributes in the riparian habitat, floating seeds might contribute to the long distance dispersal by hydrochory. Floating ability and seed weight were compared between plants damaged by a specialist herbivore Ophraella communa and undamaged plants. The damaged plants produced lighter and more likely floating seeds than the undamaged plants. However, multi-regression analysis revealed that the probability of floating was affected by seed weight but was not affected by herbivore treatment (damaged vs. undamaged plants). These results suggest that the increased proportion of floating seeds was not a direct response to the herbivore signal but an indirect response through the herbivore's effect on the reduction of seed weight. Plants damaged by herbivores might not only decrease seed production and quality but also increase the dispersal ability. These responses in dispersal ability against the herbivores might contribute to the spread of invasive plants.     

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.     

Testing the optimal defense hypothesis in Stryphnodendron adstringens (Fabaceae,Mimosoideae) leaves: the role of structure,number, position and nectar composition of extrafloral nectaries

Stryphnodendron adstringens is a common Cerrado tree that possesses extrafloral nectaries (EFNs) on its leaves, which are located at the base and apex of the rachis and along the secondary veins. The position of EFNs and their nectar production can be affected by defense strategies because plant organs possess different values and herbivory vulnerability. Here we aimed to elucidate anatomy, histochemistry, nectar composition and EFN number on leaves of S. adstringens in the light of the optimal defense hypothesis. We found a convergence on anatomy and histochemical characterization because the three studied types of EFNs have epidermis, secretory parenchyma and vascular tissue, showing phenolic compounds and polysaccharides in the secretory parenchyma cells. The nectar contained glucose, fructose and sucrose, which attract ants of the Camponotus and Cephalotes genus. We found differences in the number of EFNs along the secondary veins and in the nectar composition between EFNs located at the base and apex of the rachis of the leaf. The number of EFNs on the secondary veins increases from the base to the apex, suggesting a strategy to induce ant patrolling over the entire leaf region. EFNs at the base secreted more nectar, which should be related to the protection of the leaf base, which is the part most vulnerable to herbivore attack and the most valued organ. We concluded that EFNs of S. adstringens are anti‐herbivore defenses whose pattern matches the predictions of the optimal defense hypothesis.     

Induction of extrafloral nectar depends on herbivore type in invasive and native Chinese tallow seedlings

Although induced defenses are widespread in nature, and a potentially important strategy used by invasive plants, it is unclear how induced defenses vary among populations and whether the intensity and duration of induced defenses depends on herbivore type. For invasive plants, low herbivore loads in their introduced ranges can lead to differences in herbivore defense compared to their native ranges, but we currently know little about how induced defenses vary among native and invasive populations. We conducted a greenhouse experiment to examine variation in one type of induced defense, extrafloral nectar (EFN) production, among native and invasive populations of Chinese tallow tree, Triadica sebifera. We experimentally manipulated herbivory from an exotic generalist scale insect, a native generalist caterpillar, both herbivores, or neither and then examined EFN production by Triadica. Damage from leaf-chewing caterpillars resulted in strongly induced EFN in both native and invasive populations while damage from phloem-feeding scales did not. Extrafloral nectar production and dissolved solute content peaked 4 days after caterpillar herbivory for both native and invasive populations. Number and proportion of leaves producing EFN, EFN volume and concentration of dissolved solutes were similar among native and invasive populations. These results suggest that selection for indirect defenses may be different than selection for other defenses in the introduced ranges of invasive plants, as constitutive and induced EFN production is retained in invasive populations.