N-acyl homoserine lactone production by bacteria within the sponge Suberites domuncula (Olivi, 1792) (Porifera, Demospongiae)

Many bacteria live in close association with sponges. Within these consortia, molecules of communication such as quorum-sensing and hormone-like molecules may occur in order to regulate the partnership. Of particular interest, bacterial N-acyl-l-homoserine lactones (AHLs) were screened in supernatants from Suberites domuncula-associated bacteria using an E. coli bioluminescent reporter system. These sponge-associated bacteria were beforehand isolated on several media supplemented or not with a sponge extract to attempt to isolate sponge-specific bacteria. Out of 81 AHL-producing bacteria, three strains requiring sponge extract to grow were selected for AHL characterization. The in vitro produced AHLs, that is, in bacterial culture supernatants, were identified as N-(3-butanoyl)-l-homoserine lactone and N-(3-oxododecanoyl)-l-homoserine lactone and quantified using LC–ESI–MS/MS. The in vivo production of AHLs by sponge-associated bacteria has also been demonstrated in a healthy host for the first time: N-(3-oxododecanoyl)-l-homoserine lactone, N-(3-hexanoyl)-l-homoserine lactone, and N-(3-heptanoyl)-l-homoserine lactone. This AHL production in sponges may suggest a potential role of these molecules between sponge-associated bacteria and/or between sponge-associated bacteria and the sponge.     

Screening for induced herbivore resistance in Swedish intertidal seaweeds

Terrestrial plants have long been known to induce resistance towards herbivores in response to direct grazing, and strong evidence of inter-plant information transfer through volatile signals has been reported recently. Still, little is known about information exchange in aquatic plant–herbivore interactions. In this study, 12 Swedish seaweed species were exposed either to direct grazing by a generalist crustacean herbivore (Idotea granulosa), or to waterborne signals produced by actively feeding herbivores for 1 week. In order to test for the presence of induced chemical resistance in the different seaweed species, the dried and homogenized seaweed tissues were incorporated into an agar matrix, and herbivores were allowed to choose between the resulting control and induced artificial diets in two different two-choice feeding trials. The herbivores were actively feeding from all seaweed species in the induction experiments, although the amount of seaweed tissue consumed differed significantly between species. A chemically based induced herbivore resistance was found in response to direct grazing in four of the tested seaweed species (two red, one brown, and one green seaweed species). Furthermore, four seaweeds (one red, two brown, and one green seaweed species) induced resistance towards further grazing in response to waterborne chemical signals. Several seaweed species responded differently when exposed to different herbivore-related cues, indicating that both cues and responses can be highly specific. The results show that herbivore-induced resistance is present in 7 of 12 of the tested Swedish seaweed species, but that different signals (i.e., direct grazing and waterborne cues) elicit complex responses in the seaweeds.     

Do plant density, nutrient availability, and herbivore grazing interact to affect phlorotannin plasticity in the brown seaweed Ascophyllum nodosum

Plants have different strategies to cope with herbivory, including induction of chemical defences and compensatory growth. The most favourable strategy for an individual plant may depend on the density at which the plants are growing and on the availability of nutrients, but this has not been tested previously for marine plant–herbivore interactions. We investigated the separate and interactive effects of plant density, nutrient availability, and herbivore grazing on the phlorotannin (polyphenolic) production in the brown seaweed Ascophyllum nodosum. Seaweed plants grown at low or high densities were exposed either to nutrient enrichment, herbivorous littorinid gastropods (Littorina obtusata), or a combination of nutrients and herbivores in an outdoor mesocosm experiment for 2 weeks. Seaweeds grown at a low density tended to have higher tissue nitrogen content compared to plants grown at a high density when exposed to elevated nutrient levels, indicating that there was a density dependent competition for nitrogen. Herbivore grazing induced a higher phlorotannin content in plants grown under ambient, but not enriched, nutrient levels, indicting either that phlorotannin plasticity is more costly when nutrients are abundant or that plants responded to herbivory by compensatory growth. However, there were no significant interactive or main effects of plant density on the seaweed phlorotannin content. The results indicate that plants in both high and low densities induce chemical defence, and that eutrophication may have indirect effects on marine plant–herbivore interactions through alterations of plant chemical defence allocation.     

Sources of variation in herbivore preference: among-individual and past diet effects on amphipod host choice

Understanding which factors affect the feeding preferences of herbivores is essential for predicting the effects of herbivores on plant assemblages and the evolution of plant–herbivore interactions. Most studies of marine herbivory have focussed on the plant traits that determine preferences (especially secondary metabolites), while few studies have considered how preferences may vary among individual herbivores due to genetic or environmental sources of variation. Such intraspecific variation is essential for evolutionary change in preference behaviour and may alter the outcome of plant–herbivore interactions. In an abundant marine herbivore, we determined the relative importance of among-individual and environmental effects on preferences for three host algae of varying quality. Repeated preference assays were conducted with the amphipod Peramphithoe parmerong and three of its brown algal hosts: Sargassum linearifolium, S. vestitum and Padina crassa. We found no evidence that preference varied among individuals, thus constraining the ability of natural selection to promote increased specialisation on high-quality S. linearifolium. Most of the variation in preference occurred within individuals, with amphipod preferences strongly influenced by past diet. The increased tendency for amphipods to select alternate hosts to that on which they had been recently feeding indicates that amphipods are actively seeking mixed diets. Such a feeding strategy provides an explanation for the persistence of this herbivore on hosts in the field that support poor growth and survival if consumed alone. The effects of past diet indicate that herbivore preferences are a function of herbivore history in addition to plant traits and are likely to vary with the availability of algae in space and time.     

Polymeric microspheres with <Emphasis Type="Italic">N</Emphasis>-methyl-<Emphasis Type="SmallCaps">d</Emphasis>-glucamine ligands for boron removal from water solution by adsorption–membrane filtration process

Polymeric microspheres with N-methyl-d-glucamine (NMDG) ligands have been tested in the adsorption–membrane filtration process for boron removal from aqueous solutions. The chelating resins were synthesized by reacting NMDG with the vinylbenzyl chloride–styrene–1,4-divinylbenzene (VBC/S/DVB) copolymer at the reflux temperature and in the microwave reactor. VBC/S/DVB spheres with a gel structure that contained 6 wt% DVB were obtained by membrane emulsification followed by suspension polymerization. By selecting the optimal emulsification and polymerization parameters, it was possible to obtain 25-μm-diameter particles with a narrow size distribution. Resins obtained by microwave modification showed the higher boron adsorption capacity.     

Different phagostimulants in potato foliage for Manduca sexta and Leptinotarsa decemlineata

Summary. In the past decades, several studies have focused on the identification of feeding stimulants for specialists that feed on solanaceous plants, especially potato (Solanum tuberosum). In the 1950's, a phagostimulant was isolated from potato and tomato for Manduca sexta and characterised as a glycoside. It was suggested that the phagostimulant for M. sexta and Leptinotarsa decemlineata is identical. We tested whether these insects indeed share the same recognition factor. Previous bioassays for L. decemlineata larvae and adults were found to be unsatisfactory, so a new assay system was developed, using starch and wheat flour-based wafers as a neutral substrate. An aqueous extract of potato foliage that was highly active as a stimulant was fractionated by reverse phase medium pressure liquid chromatography (MPLC). Both insects were stimulated by a MPLC fraction that eluted with 25% methanol in water. Further separation of this and the following fraction (35% methanol, stimulatory for the beetle only) by semi-preparative high performance liquid chromatography (HPLC), using gradients of water and acetonitrile, resulted in the isolation of two distinct fractions that stimulated feeding by L. decemlineata. None of the HPLC fractions was active for M. sexta. However, fractions of the flash chromatography with less polarity (45–75% methanol) had a stimulatory effect only on M. sexta. Thus, the two insects do not use a common feeding stimulant, and for both at least two compounds of different polarity are active. Received 3 July 2000; accepted 24 October 2000     

Variation in composition of predator-attracting allelochemicals emitted by herbivore-infested plants: Relative influence of plant and herbivore

Summary During foraging, natural enemies of herbivores may employ volatile allelochemicals that originate from an interaction of the herbivore and its host plant. The composition of allelochemical blends emitted by herbivore-infested plants is known to be affected by both the herbivore and the plant. Our chemical data add new evidence to the recent notion that the plants are more important than the herbivore in affecting the composition of the volatile blends. Blends emitted by apple leaves infested with spider mites of 2 different species,T. urticae andP. ulmi, differed less in composition (principally quantitative differences for some compounds) than blends emitted by leaves of two apple cultivars infested by the same spider-mite species,T. urticae (many quantitative and a few qualitative differences). Comparison between three plant species — apple, cucumber and Lima bean — reveals even larger differences between volatile blends emitted upon spider-mite damage (many quantitative differences and several qualitative differences).     

Systemically-induced response of cabbage plants against a specialist herbivore, Pieris brassicae

Summary. Plant responses to herbivory might directly affect the herbivore (“direct” defences) or might benefit the plant by promoting the effectiveness of natural antagonists of the herbivores (“indirect” defences). Brussels sprouts attacked by Pieris brassicae larvae release volatiles that attract a natural antagonist of the herbivores, the parasitoid Cotesia glomerata, to the damaged plant. In a previous study, we observed that feeding by caterpillars on the lower leaves of the plant triggers the systemic release of volatiles detectable by the parasitoids from upper leaves of the same plant.?The role of these systemically induced volatiles as indirect defence and the dynamics of their emission were investigated in wind-tunnel dual choice tests with C. glomerata. The systemically induced emission of volatiles varied depending on leaf age and on plant age. Systemic induction affected parasitoid effectiveness, as induced plants could be more easily located by parasitoids than non-induced ones.?The role of the systemic induction as a direct defence was investigated through behavioural and feeding tests with P. brassicae. In dual choice assays, 1st instar larvae preferred to feed and fed more on systemically induced than on non-induced leaves. In single choice assays, the leaf area consumed by caterpillars was larger on systemically induced leaves than on non-induced control leaves. However, caterpillars fed on systemically induced leaves attained the same weight as those feeding on non-induced controls. In addition, P. brassicae pupae whose larvae were fed on systemically induced leaves had longer developmental times than those of larvae fed on non-induced leaves. Adult oviposition behavior was not influenced by systemic induction.?We conclude that systemically induced responses in cabbage might reduce P. brassicae fitness both directly, by affecting their development and feeding behavior and indirectly by making caterpillars and pupae more vulnerable to attack by carnivores. The occurrence of a possible relationship between direct and indirect defence is discussed. Received 24 January 2001; accepted 3 May 2001.     

Glycohistochemistry of a marine sponge, Chondrilla nucula (Porifera, Desmospongiae), with remarks on a possibly related antimicrobial defense strategy and a note on exopinacoderm function

Based on carbohydrate histochemistry, including the use of lectins, and TEM, the study describes the distribution of terminal sugars in different structures of the demosponge Chondrilla nucula. The results of the general and specific carbohydrate histochemical approaches confirmed the presence of acidic and neutral glycoconjugates in the cells, and, with declining amounts from the ectosome to the mesohyl, in the extracellular matrix (ECM). AB-PAS staining indicated acidic complex carbohydrates particularly in the exopinacoderm, and more neutral ones in the cells and the ECM of the mesohyl. The PO-lectins applied demonstrated a general spectrum of free saccharide residues (α-d-mannose, α-/β-d-N-acetylglucosamine, α-d-N-acetylgalactosamine, α-d-galactose, β-d-galactose) in both sponge parts; α-l-fucose was only distinct in the ectosome. Sialic acids [siaα(2,3)-galactose, siaα(2,6)-N-acetylgalactosamine] were dominant in the very thin exopinacoderm, indicating O-linked high molecular weight glycoproteins. In this way a glycophysiologically ‘rigid’ outer mucus cover is developed as protection against mechanical hazards. Some of the free sugars (α-d-mannose, N-acetylglucosamine, N-acetylgalactosamine β-d-galactose, α-l-fucose) are known to prevent the adherence of different bacteria and fungi to cellular surfaces. Thus a high concentration of such sugars, may impede massive attacks of micro-inhabitants on mobile sponge cells, pinacocytes, and the exopinacoderm layer.     

Increased nitrogen availability influences predator–prey interactions by altering host-plant quality

Little is known about how plant nutritional and defensive qualities interact to influence predator–prey interactions. To address this need, we provided the neo-tropical milkweed, Asclepias curassavica, with two levels of nitrogen availability and examined how altered host-plant quality influenced the responses of a specialist aphid, Aphis nerii, and a coccinellid predator, Harmonia axyridis. Aphis nerii uses A. curassavica for multiple resources, including nutrition and sequestration of cardenolides for defense against natural enemies. Increased nitrogen availability improved A. curassavica quality by decreasing carbon-to-nitrogen (C:N) ratios and cardenolide concentrations, resulting in A. nerii that also had lower C:N ratios and cardenolide concentrations. Aphis nerii population growth was higher on plants with high nitrogen availability, compared with aphids on plants with low nitrogen availability. In no-choice feeding trials, Harmonia axyridis consumed more high C:N ratio aphids, suggesting a potential compensatory response to reduced aphid nutritional quality. Additionally, H. axyridis were able to consume more low-quality aphids at the expense of increasing exposure to increased cardenolide concentrations, suggesting that interactions between H. axyridis and A. nerii may be strongly influenced by prey nutritional quality. This work highlights the need to consider how variation in plant quality influences herbivore nutritional and defensive quality when examining mechanisms that influence predator–prey interactions.     

Glucosinolates and other metabolites in the leaves of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> from natural populations and their effects on a generalist and a specialist herbivore

Because many secondary metabolites in plants act as defense against herbivores it has been postulated that these compounds have evolved under selective pressure by insect herbivores. One explanation for the within-species variation in metabolite patterns in a particular species is that different populations are under selection by different herbivores. We tested this hypothesis, using Arabidopsis thaliana plants that originated from dune and inland areas. We analyzed Arabidopsis thaliana leaves using NMR spectroscopy and multivariate data analysis. Major differences in chemical composition were found in water-methanol fractions and were due to higher concentrations of sinigrin and fumaric acid in dune plants. Inland plants showed lower levels of glucose. Quantitative analysis of glucosinolates was performed with HPLC. Individual plants and populations demonstrated differences in glucosinolate composition and concentration. In growth chamber experiments, the generalist herbivore, Spodoptera exigua grew significantly better on the inland plants, while the specialist herbivore Plutella xylostella performed equally well on plants of both origins. Aliphatic glucosinolate as well as total glucosinolate concentrations negatively correlated with larval mass of Spodoptera exigua. No significant correlations, however, were found between larval mass of Plutella xylostella and glucosinolates in the leaves. A specialist and a generalist herbivore were responding differently to plant secondary chemistry, as was also found in several other studies. This is an important indication that differences in glucosinolate concentrations among populations may result from differential selection by different guilds of herbivores.     

Pyrrolizidine alkaloids on three trophic levels – evidence for toxic and deterrent effects on phytophages and predators

Summary. Pyrrolizidine alkaloids (PAs) present a model system in the investigation of tritrophic interactions mediated by plant secondary compounds. However, their toxicity for insect herbivores has never been experimentally proven. Here, we demonstrate the toxic effects of a PA on growth and survival of the eri silk moth Philosamia ricini. In a feeding experiment, larvae of this generalist herbivore fed with an artificial PA diet gained weight significantly slower than control animals, and died as pupae. We suggest that derivatives of the ingested PA N-oxide damage developmental functions during metamorphosis. A tracer test with [¹⁴C]senecionine N-oxide revealed that the caterpillars lack adaptations that would prevent conversion of the chemical into the pro-toxic free base. In contrast, the PA adapted leaf beetle Longitarsus anchusae accumulates PAs as N-oxides. We tested the purpose of sequestration in this species as defence against predators. Through a series of prey choice experiments with three carabid predator species, chemically non-protected bark beetle pupae were chosen almost uniformly over L. anchusae pupae. In a following choice test with one of these predators, artificially PA-treated mealworm segments deterred the predator from feeding. Overall the study corroborates the immediate toxic effect of PAs on non-adapted herbivores and the protective effect that adapted insects may gain by sequestering them. It thereby underlines the potential for PAs to play a central role in multitrophic interactions between plants, phytophages and their predators.     

Diet induced differences in carbon isotope fractionation between sirenians and terrestrial ungulates

Carbon isotope differences (Δ¹³C) between bioapatite and diet, collagen and diet, and bioapatite and collagen were calculated for four species of sirenians, Dugong dugon (Müller), Trichechus manatus (Linnaeus), Trichechus inunguis (Natterer), and the extinct Hydrodamalis gigas (Zimmerman). Bone and tooth samples were taken from archived materials collected from populations during the mid eighteenth century (H. gigas), between 1978 and 1984 (T. manatus, T. inunguis), and between 1997 and 1999 (D. dugon). Mean Δ¹³C values were compared with those for terrestrial ungulates, carnivores, and six species of carnivorous marine mammals (cetaceans = 1; pinnipeds = 4; mustelids = 1). Significant differences in mean δ¹³C values among species for all tissue types were detected that separated species or populations foraging on freshwater plants or attached marine macroalgae (δ¹³C values < −6‰; Δ¹³C_{bioapatite–diet} ∼14‰) from those feeding on marine seagrasses (δ¹³C values > −4‰; Δ¹³C_{bioapatite–diet} ∼11‰). Likewise, Δ¹³C_{bioapatite–collagen} values for freshwater and algal-foraging species (∼7‰) were greater than those for seagrass-foraging species (∼5‰). Variation in Δ¹³C values calculated between tissues and between tissues and diet among species may relate to the nutritional composition of a species’ diet and the extent and type of microbial fermentation that occurs during digestion of different types of plants. These results highlight the complications that can arise when making dietary interpretations without having first determined species-specific Δ¹³C_{tissue–diet} values. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

d-Pinitol as a key oviposition stimulant for sulfur butterfly, Colias erate: chemical basis for female acceptance of host- and non-host plants

The sulfur butterfly, Colias erate, utilizes various legumes as host plants. We examined the chemical constituents of its primary host plant, Trifolium repens (white clover), to identify phytochemicals inducing oviposition by C. erate females. Since one of the four aqueous subfractions prepared from a methanolic extract of the plant has previously been shown to be the most responsible for the oviposition-stimulatory activity exerted by the plant, chemical analyses were conducted of the fraction concerned. Activity-directed fractionation of the subfraction by ion-exchange chromatography revealed that the key substance(s) resided in the neutral fraction. Preparative TLC of the neutral fraction and subsequent spectral analyses identified d-(+)-pinitol, glycerin, methyl β-d-glucoside, and myo-inositol as characteristic components together with ubiquitous sugars (e.g., sucrose and glucose). Of these, only pinitol singly evoked significant oviposition responses at concentrations over 0.05%. In dual-choice bioassays, however, females laid significantly more eggs on pinitol solutions admixed with glycerin or methyl β-d-glucoside than on pinitol alone. Two cyanoglucosides, linamarin, and lotaustralin, occurring in the other aqueous subfractions, also synergistically increased the oviposition response in combination with pinitol. The results clearly indicated that pinitol is a crucial oviposition stimulant involved in host recognition, while glycerin, methyl β-d-glucoside, linamarin, and lotaustralin function as synergists. We further examined the oviposition responses of C. erate females to aqueous fractions, along with their chemical compositions, that had been prepared from five other host plants and a non-host plant, Aristolochia debilis (Aristolochiaceae), on which oviposition occasionally took place in an outdoor cage during the experiments. The plant species accepted by ovipositing females were all found to contain pinitol in amounts enough to induce egg laying by the butterfly, thus leading to the conclusion that pinitol serves as the essential mediator in recognizing and accepting potential host plants.     

Jasmonic acid treatment and mammalian herbivory differentially affect chemical defenses and growth of wild mustard (Brassica kaber)

Summary. Jasmonic acid (JA) is a wound-related hormone found in most plants that, when applied exogenously, can induce increases in levels of chemical defenses in patterns similar to those induced by mechanical damage or insect feeding. Relative to responses to insect and pathogen attack, chemical responses of herbaceous plants to mammalian herbivore attack have been little studied. In a field experiment, we compared the effects of JA treatment and naturally occurring mammalian herbivory on the expression of trypsin inhibitors, glucosinolates, peroxidase activity and growth of wild mustard (Brassica kaber). Exogenous JA significantly increased trypsin inhibitor activity and glucosinolate concentration, and moderately increased peroxidase activity in the eighth true leaves of five-week-old plants, relative to untreated controls. In contrast, levels of these chemical defenses in the eighth true leaves or in regrowth foliage of plants that had ∼80% of their leaf area removed by groundhogs (Marmota monax) did not differ from that in undamaged and untreated controls. Although exogenous JA significantly elevated levels of chemical defenses, it did not affect height of plants through the season and only slightly reduced time to first flower. Groundhog herbivory significantly reduced height and delayed or abolished flowering, but these effects were not substantial unless coupled with apical meristem removal. We hypothesize that the lack of effect of groundhog herbivory on chemical defenses may be due in part to the speed and pattern of leaf area removal by groundhogs, or physiological constraints caused by leaf area loss. Despite having no effect on chemical defense production, leaf area loss by groundhogs was more costly to growth and fitness than the effects of JA application in this study, but only substantially so if coupled with apical meristem removal. We suggest that in general, costs of defense production in plants are likely to be minimal when compared to the risk of losing large amounts of leaf area or primary meristematic tissue. Thus, if they are effective at deterring herbivory, the benefits of inducible defense production likely outweigh the costs in most cases. Received 20 December 2000; accepted 3 May 2001     

The roles of nicotine and natural enemies in determining larval feeding site distributions of Manduca sexta L. and Manduca quinquemaculata (Haworth) on tobacco

Summary. Field observations indicated that hornworms select feeding sites non-randomly on tobacco. We tested the hypotheses that differences in feeding site locations of larvae of Manduca sexta L. and Manduca quinquemaculata (Haworth) (Lepidoptera: Sphingidae) on tobacco could be explained by differential nicotine concentrations within plants and leaves, species-specific responses to nicotine, or pressure exerted by natural enemies. Results showed that third-instar larvae of M. sexta fed more proximally and centrally on the leaf, whereas M. quinquemaculata fed more distally. Within-plant selection of leaves did not differ; both species selected leaves in the middle region of the plant. Nicotine concentrations in a high nicotine genotype, NC95, varied within each leaf, increasing 2—3 fold from the basal to apical portion of the leaf, and within each plant, increasing 7—10 fold from the first fully expanded leaf to the twelfth (lowest) leaf. In laboratory bioassays, both Manduca species responded to nicotine as a feeding deterrent. Electrophysiological studies demonstrated that gustatory organs of both species responded to nicotine at concentrations found in tobacco leaves and that M. quinquemaculata generally showed a less vigorous response to nicotine than M. sexta. Field mortality of M. sexta due to parasitism by Cotesia congregata (Say) and to parasitism and predation combined differed among feeding sites; predation alone did not. Results suggest that although nicotine concentration and species specific responses to nicotine play a role in determining feeding site locations, pressure exerted by natural enemies, especially parasitism by C. congregata, is more important. Received 22 February 2000; accepted 20 July 2001.     

Chitobiase activity in the epidermis and hepatopancreas of the fiddler crab Uca pugilator during the molting cycle

The activity of chitobiase, also known as N-acetyl-β-glucosaminidase, in the epidermis and hepatopancreas of the fiddler crab Uca pugilator (Bosc, 1802), during the molting cycle, was investigated. A pH optimum of 5 to 6 was found for the enzymatic activity in both the epidermis and hepatopancreas. The temperature optimum for epidermal and hepatopancreatic chitobiase activities was 50 to 60 °C. The K _m values for epidermal and hepatopancreatic chitobiase activities at 19 °C were 0.190 ± 0.027 and 0.203 ± 0.016 mM 4-methylumbelliferyl-N-acetyl-β-glucosaminide, respectively. Hepatopancreatic chitobiase activity was significantly higher than epidermal enzymatic activity in all the molt cycle stages tested except Postmolt Stage A-B. Chitobiase activity varied significantly during the molting cycle, with the epidermal enzymatic activity in Premolt Stage D_3–4 significantly higher than in Stage C (intermolt) and Premolt Stage D₀, whereas hepatopancreatic chitobiase activity in Premolt Stage D_3–4 was significantly higher than in all other molt stages tested. The patterns of chitobiase activity in the epidermis and hepatopancreas correlate well with the hemolymph titer of ecdysteroids in U. pugilator during the molting cycle; this suggests that chitobiase activity in both tissues is regulated at least in part by the steroid molting hormones. Received: 6 May 1998 / Accepted: 12 September 1998     

Differential responses in American (<Emphasis Type="Italic">Castanea dentata</Emphasis> Marshall) and Chinese (<Emphasis Type="Italic">C. mollissima</Emphasis> Blume) chestnut (Falales: Fagaceae) to foliar application of jasmonic acid

Summary.  Tannins are plant defense compounds that exhibit antibiotic (e.g. toxic) and antixenotic (e.g. repellent) effects against an array of plant pests. They are broadly divided into two major groups, hydrolysable tannins (gallotannins and ellagitannins) and proanthocyanidins, each with an undetermined number of compounds. We investigated constitutive levels of hydrolysable tannins and proanthocyanidins in the leaves and stems of American (Castanea dentata Marshall) (Fagales: Fagaceae) and Chinese (C. mollissima Blume) chestnut. American chestnut contained more proanthocyanidins in leaves and stems than Chinese chestnut, but Chinese chestnut contained more foliar hydrolysable tannins. Regardless of these differences, gypsy moth (Lymantria dispar L, Lepidoptera: Lymantriidae) performance did not differ when fed American and Chinese chestnuts. We also investigated the effects of jasmonic acid (JA) treatment on differential tannin induction in American and Chinese chestnut leaves and stems. JA treatment increased proanthocyanidins in American chestnut stems and hydrolysable tannins in both tissue types of American chestnut, but did not influence tannin concentrations in Chinese chestnut leaves or stems. HPLC-ESI-MS analysis of pooled samples suggested that hydrolysable tannins in each tissue were qualitatively comprised primarily of ellagitannins, and JA generally increased the number of hydrolysable tannins that could be detected by ESI-MS. Third, we investigated the performance of gypsy moths on JA treated and untreated American and Chinese chestnut. Caterpillar relative growth was not influenced by JA treatment on Chinese chestnut, but decreased in response to JA application on American chestnut. Our results indicate that JA-dependent defenses differ between these chestnut species. This study improves our understanding of ecologically important differences in tannin induction and herbivore susceptibility in Castanea, and has important implications in efforts for American chestnut restoration and commercial chestnut production.     

Soil nitrogen availability and herbivore attack influence the chemical defenses of an invasive plant (Linaria dalmatica; Plantaginaceae)

Chemical defenses are thought to contribute to the invasion success and impacts of many introduced plants; however, for most of these species, little is known about these compounds and how they vary in natural environments. Plant allelochemical concentrations may be affected by a variety of abiotic and biotic factors, including soil nutrients and herbivores. Moreover, such quantitative variation is likely to play an important role in species interactions involving these invasive plants. The purpose of this study was to examine patterns of variation in iridoid glycoside concentrations of the invasive plant Linaria dalmatica (Plantaginaceae). We conducted a greenhouse experiment to investigate the effect of soil nitrogen availability on iridoid glycoside concentrations. Results from this experiment showed that plant iridoid glycoside concentrations decreased with increased nitrogen availability. Additionally, plants were collected from multiple field sites in order to characterize the influence of population, soil nitrogen availability, and herbivore attack on iridoid glycoside variation. Results from field studies indicated that plants demonstrated considerable seasonal variation, as well as variation within and among populations, with iridoid glycoside concentrations ranging from approximately 1 to 15% dry weight. The relationship between soil nitrogen and plant iridoid glycosides varied among populations, with a strong negative correlation in one population, a marginally significant negative relationship in a second population, and no relationship in the remaining two populations. Additionally, we found a negative relationship between iridoid glycoside concentrations and plant injury by an introduced biocontrol agent, the stem-mining weevil Mecinus janthinus (Cucurlionidae). These results show that plant allelochemical concentrations can vary widely in natural environments and suggest that levels of plant defense may be reduced by increased soil nitrogen availability and herbivore attack in this invasive plant species.     

Is flower scent influencing host plant selection of leaf-galling sawflies (Hymenoptera, Tenthredinidae) on willows?

Though it is known that flower scent not only attracts pollinators but also herbivores, little is known about the importance of flower scent on the distribution of leaf herbivores among individuals within a plant species. In this study we determined the distribution of galls induced by the sawfly Pontania proxima (Serville 1823) (Hymenoptera, Tenthredinidae, Nematinae) on flowering and non-flowering representatives of several clones belonging to Salix fragilis and S. × rubens (Salicaceae). Further, amounts and composition of scent of flowering and non-flowering twigs were compared (dynamic headspace-gas chromatography–mass spectrometry, DHS-GC–MS), and a scent sample collected from flowering twigs of S. fragilis was tested by coupled gas chromatography and electroantennographic detection (GC-EAD) on the antennae of P. proxima females. The results show that the presence of flower catkins on plants led to a higher degree of allocation with galls, but the number of galls differed not between flowering and non-flowering plants. The DHS-GC–MS analyses revealed that the total amount of flower scent emitted per flowering twig is about 90 times higher than the scent emitted by a non-flowering twig. Further, several compounds were emitted only by flowering but not by non-flowering twigs. In the GC-EAD analyses, antennae consistently responded not only to green leaf volatiles, but also to compounds emitted only by the flowers (e.g. 1,4-dimethoxybenzene). These flower scent compounds are suggested to affect the host plant choice by attracting more sawflies from the distance to flowering plants compared to non-flowering plants. The EAD-active compounds emitted from vegetative plant parts are assumed to act as long-distance signals especially when flowers are absent on host plants, e.g. during the oviposition period of the second generation of P. proxima.