Showing papers in "Journal of Chemical Ecology in 2000"

Investigating the mode of action of natural phytotoxins

Abstract: The potential use of natural phytotoxins (including allelochemicals) to develop novel tools for weed management is enhanced by the elucidation of their modes of action. This approach has not been emphasized by the agrochemical industry, although the possibility of discovering new target sites may be promising, since natural products tend to have modes of action different from synthetic herbicides. The approach of testing a compound on all known herbicide molecular target sites for commercial herbicides and other potent phytotoxins is feasible. However, this would preclude the discovery of new mechanisms of action. Discovering new target sites requires more challenging holistic approaches, initiated with physiological and biochemical studies that use whole plant assays. Studying basic plant responses to a compound may yield important clues to the specific physiological processes affected by the compounds and uncover novel mechanisms of action.

Allelopathic Effects of Volatile Cineoles on Two Weedy Plant Species

Abstract: The volatile monoterpene analogs, 1,4-cineole and 1,8-cineole, have been identified as components of many plant essential oils, but relatively little is known about their biological activities. We compared the effects of 1,4- and 1,8-cineole on two weedy plant species by monitoring germination, mitosis, root and shoot growth, chlorophyll content, and photosynthetic efficiency. 1,4-Cineole severely inhibited growth of roots and shoots, causing cork-screw shaped morphological distortion, whereas 1,8-cineole caused a decrease in root growth and germination rates. Chlorophyll fluorescence data (yield and F v / F m) indicated that 1,4-cineole caused significantly higher stress (P ≤ 0.001) to photosynthesis when compared to controls. Mitotic index data showed that 1,8-cineole severely decreased (P ≤ 0.001) all stages of mitosis when compared with controls, while 1,4-cineole only caused a decrease in the prophase stage (P ≤ 0.05). Although superficially similar in structure, these two cineoles appear to have different modes of action.

Feeding and growth of Plutella xylostella and Spodoptera eridania on Brassica juncea with varying glucosinolate concentrations and myrosinase activities.

Abstract: We measured feeding behavior, feeding damage, and larval growth of the crucifer specialist, Plutella xylostella and the generalist, Spodoptera eridania, on the cotyledons of 14 homozygous lines of Brassica juncea differing in myrosinase activity and glucosinolate profiles. The proportion of time feeding and area damaged by P. xylostella were lower on lines with high myrosinase activities [0.49–0.73 nmol glucose released/mg tissue(fresh weight, FW)/min] than on lines with low myrosinase activities [0.20–0.31 nmol glucose released/mg tissue(FW)/min]. In contrast, the proportion of time feeding and area damaged by S. eridania were not related to myrosinase activity, but were lower on cotyledons of lines with high glucosinolate concentrations [6.8–21.3 μg/g(FW)] than on lines with low glucosinolate concentrations [0.09–0.61 μg/g(FW)]. Relative growth rates (RGR) of both insect species were lower on lines with high glucosinolate concentrations, but were not related to myrosinase activity in the lines. In toxicity experiments that used artificial diets, allyl isothiocyanate, but not allyl glucosinolate, was lethally toxic to neonate P. xylostella (LC50s of 1.54 μmol/g, and ≫100 μmol/g, respectively), whereas isothiocyanate and the glucosinolate were lethally toxic to neonate S. eridania (LC50s of 3.42 and 6.73 μmol/g, respectively). We interpret these results to indicate that myrosinase activity might be more important for plant defense against specialist insects that have adaptations to intact glucosinolates, but less important for defense against generalists, which are susceptible to the intact glucosinolates.

Effects of four monoterpenes on germination, primary root growth, and mitochondrial respiration of maize.

Abstract: The effects of four monoterpenes—camphor, eucaliptol, limonene, and α-pinene—on the oxidative metabolism of mitochondria isolated from maize root (Zea mays), on maize seed germination, and on primary root growth were investigated. The effects of individual monoterpenes on respiration were variable. α-Pinene concentrations of 0.05–1.0 mM stimulated respiration with a mixture of substrates composed of NADH, L-malate, succinate, and L-glutamate, and in the absence of exogenously added ADP (basal respiration). However, at concentrations higher than 1.0 mM, α-pinene inhibited respiration both in the absence (basal respiration) and presence of ADP (coupled respiration). Limonene at 0.1 mM or above stimulated basal respiration and inhibited in parallel the coupled respiration. Similar effects were promoted by eucaliptol, but at a higher concentration range (1.0 mM or above). Camphor was less active. At 10 mM concentration, it caused stimulation of basal respiration but did not affect coupled respiration. In the concentration range 0.1–10.0 mM, limonene was inactive on seed germination and primary root growth. Camphor and eucaliptol did not inhibit germination but reduced fresh and/or dry weight of roots at 5.0 mM and above. α-Pinene inhibited both seed germination and fresh weight of primary roots at 10.0 mM concentration. The results indicate that intact seeds and primary roots are less sensitive than isolated mitochondria. The relatively more lipophilic monoterpenes α-pinene and limonene had less activity than the more water-soluble oxygenated monoterpenes camphor and eucaliptol in inhibiting seed germination and/or primary root growth, despite the fact that they had a higher activity on the oxidative metabolism of isolated mitochondria. The findings suggest that the solubility of monoterpenes may be the major factor implicated in these differences.

Induction of Plant Synomones by Oviposition of a Phytophagous Insect

Abstract: Earlier investigations of host habitat location in the egg parasitoid Oomyzus gallerucae have shown that oviposition of the elm leaf beetle (Xanthogaleruca luteola) induces the field elm (Ulmus minor) to emit volatiles that attract the egg parasitoid. In this study we investigated the mechanism of this induction by testing the effects of differently treated elm leaves on O. gallerucae in a four-arm olfactometer. First we investigated which sequence of the herbivore oviposition behavior is necessary for the synomone induction. The following major sequences were observed: (1) Prior oviposition, the gravid female gnawed shallow grooves into the leaf surface. (2) After gnawing upon the leaf surface, the female attached about 20–30 eggs with oviduct secretion in the grooves. We experimentally mimicked the shallow grooves on the leaf surface by scratching the leaf surface with a scalpel (= scratched leaves). Volatiles from such scratched leaves did not attract the egg parasitoid. However, as soon as eggs with oviduct secretion, or only oviduct secretion, was applied to these scratched leaves, they emitted attractive volatiles. Application of oviduct secretion and eggs on undamaged leaves did not elicit release of attractive synomones. Thus, an elicitor is located in the oviduct secretion, but becomes active only when the leaf surface is damaged. Jasmonic acid is known as a mediator of plant responses induced by feeding of herbivorous arthropods, and we demonstrate that it mediates production of elm synomones that attract O. gallerucae. The plant's reaction to oviposition was systemic, and leaves without eggs near leaves with eggs emitted attractants.

Assessing the Attractiveness of Volatile Plant Compounds to Western Flower Thrips Frankliniella occidentalis

Abstract: The responses of walking adult female western flower thrips, Frankliniella occidentalis, to plant volatiles at several concentrations were investigated in a Y-shaped glass tube olfactometer. The simple experimental design allowed comparing the effect of numerous volatiles and differentiated clearly between attractive and repellent compounds. Western flower thrips were attracted by the benzenoids benzaldehyde and pand o-anisaldehyde; the monoterpenes geraniol, nerol, linalool, and (+)-citronellol; the sesquiterpene (E)-β-farnesene, eugenol, and 3-phenylpropionaldehyde; two phenylpropanoids; and the nonfloral odor ethyl nicotinate. p-Anisaldehyde, nerol, ethyl nicotinate, and (E)-β-farnesene elicited positive responses at several concentrations; all other volatiles were attractive at a specific concentration. Salicylaldehyde, a benzenoid, elicited negative responses at two concentrations. Experimental series with several other volatiles belonging to the above mentioned chemical groups and the essential oils from rose and geranium did not result in either clearly positive or negative responses by tested thrips.

Ostariophysan Alarm Pheromones: Laboratory and Field Tests of the Functional Significance of Nitrogen Oxides

Abstract: In laboratory and field-trapping studies, we exposed fathead minnows (Pimephales promelas) and finescale dace (Chrosomus neogaeus) to hypoxanthine-3-N-oxide and a suite of structurally and functionally similar compounds in order to determine if: (1) hypoxanthine-3-N-oxide functions as a chemical alarm signal in ostariophysan fishes and (2) the purine skeleton, a structural component, or the nitrogen oxide, a functional component, or both act as the molecular trigger in this chemical alarm signaling system. Minnows and dace exhibited significant antipredator responses when exposed to conspecific skin extract or hypoxanthine-3-N-oxide (increased shoaling and decreased area use) and the functionally similar pyridine-N-oxide (increased shoaling) but not to structurally similar molecules lacking a nitrogen oxide functional group or to a swordtail skin extract control. Field-trapping studies revealed similar results. Traps labeled with fathead minnows skin extract, hypoxanthine-3-N-oxide, or pyridine-N-oxide caught significantly fewer fish than did those labeled with distilled water. These data strongly suggest that the nitrogen oxide functional group acts as the chief molecular trigger in the Ostariophysan alarm pheromone system and that, contrary to previous research, hypoxanthine-3-N-oxide may be one of several possible molecules that function as a chemical alarm signal. Here we report the first example of a single functional group capable of eliciting a suite of behavioral responses.

Identification of olfactory stimulants for Anopheles gambiae from human sweat samples.

Abstract: The behavioral and electroantennogram (EAG) responses of female Anopheles gambiae mosquitoes to pooled samples of freshly collected human sweat and human sweat incubated for 42–52 hr were tested. No behavioral or EAG response was obtained to pooled fresh sweat samples, whereas incubated pooled sweat samples produced a behavioral as well as an EAG response. GC-MS analysis of the headspace composition of the fresh sweat revealed ethanol (15.1% of the total amount of volatiles trapped), acetic acid (10.9%), and 3-hydroxy-2-butanone (9.5%) as the most abundant compounds; a wide range of ethyl esters was present as well. None of the ethyl esters was detected in the headspace collections from incubated sweat, while the relative amounts of ethanol, acetic acid, and 3-hydroxy-2-butanone were strongly reduced. In the latter collections, indole (27.9%), 1-dodecanol (22.4%), and 3-methyl-1-butanol (10%) were present in high amounts, while they were absent or present in only minor amounts in the headspace collections from fresh sweat. Geranyl acetone (6%) and 6-methyl-5-hepten-2-one (1.9%) were relatively abundant in both the fresh and incubated headspace samples. EAG responses were observed in response to indole, 6-methyl-5-hepten-2-one, and geranyl acetone.

Harvester Ants Utilize Cuticular Hydrocarbons in Nestmate Recognition

Abstract: Cuticular hydrocarbons appear to play a role in ant nestmate recognition, but few studies have tested this hypothesis experimentally with purified hydrocarbon extracts. We exposed captive colonies of the harvester ant Pogonomyrmex barbatus to small glass blocks coated with whole cuticular lipid extracts and the purified hydrocarbon portion of extracts from nestmate and nonnestmate workers. As an estimate of agonistic behavior, we measured the proportion of ants in contact with blocks that flared their mandibles. Blocks coated with cuticular extracts from nonnestmates were contacted by more workers in one of two experiments and elicited higher levels of aggression in both experiments than blocks bearing extracts from nestmates. The cuticular hydrocarbon fraction of extracts alone was sufficient to elicit agonistic behavior toward nonnestmates. The results demonstrate that harvester ants can perceive differences in cuticular hydrocarbon composition, and can use those differences in nestmate recognition.

Volicitin, an elicitor of maize volatiles in oral secretion of Spodoptera exigua: isolation and bioactivity

Abstract: Plants respond to insect-inflicted injury by systemically releasing relatively large amounts of several volatile compounds, mostly terpenoids and indole. As a result, the plants become highly attractive to natural enemies of the herbivorous insects. In maize, this systemic response can be induced by the uptake via the stem of an elicitor present in the oral secretions of caterpillars. Such an elicitor was isolated from the regurgitant of Spodoptera exigua larvae, identified as N-(17-hydroxylinolenoyl)-L-glutamine, and named volicitin. Here we present details on the procedure that was used to isolate volicitin and the biosasays that demonstrate its potency as an elicitor of maize volatiles that attract parasitoids. With a series of liquid chromatography purification steps, volicitin was separated from all other inactive substances in the regurgitant of larvae of the noctuid moth S. exigua. Maize seedlings that were incubated in very low concentrations of pure natural volicitin released relatively large amounts of terpenoids and became highly attractive to the parasitoid Microplitis croceipes. The identification of this and other insect-derived elicitors should allow us to determine their precise source and function, and better understand the evolutionary history of the phenomenon of herbivore-induced volatile emissions in plants.

Are laboratory bioassays for allelopathy suitable for prediction of field responses

Abstract: One concern often voiced by researchers of allelopathic interactions is that many laboratory bioassays do not adequately predict the responses observed in field situations. The questions that arise are: (1) What criteria should be implemented to design ecologically relevant bioassays? (2) What species (crops or weeds) are involved in the interaction? (3) Are we investigating allelopathy of debris/residues or interactions involving living plants? (4) Which plant indicator species are actually cohabiting with the species under investigation? and (5) What are appropriate experimental controls? It is difficult to design a bioassay that can be used to examine responses in all species. In fact, each bioassay must be designed specifically to assess species interactions after careful consideration of growth habit, biotic characteristics, and ecophysiological factors. The objective of this paper is to discuss the significance of bioassays designed to study a particular aspect of allelopathy. We conclude that through a laboratory bioassay we can not demonstrate that allelopathy is operational in natural settings. An investigator should consider allelopathy as one component in a multifaceted approach to ecology and address key questions to determine the relevance of a particular assay. Due to the complexity of field interactions and responses, one can only hope to predict and describe some of the cause-and-effect relationships observed in a field setting. An accurate assessment of these main effects will prove invaluable in directing the focus of future research emphasis.

Different levels of protective alkaloids in grasses with stroma-forming and seed-transmitted Epichloë/Neotyphodium endophytes

Abstract: The three alkaloid groups—lolines, ergopeptides, and peramine— are typically associated with endophyte infection of grasses, with the main function to protect hosts against herbivores. We determined levels of N-formylloline, N-acetylloline, ergovaline, and peramine in 18 European grasses naturally infected with seed-transmitted Neotyphodium endophytes or sexual Epichloe species. Peramine was the most common alkaloid, whereas lolines and ergovaline were only detected in Festuca hosts infected with E. festucae, N. coenophialum, or N. uncinatum. Only ten of the grass species analyzed contained detectable amounts of one or more of these alkaloids. There was a clear tendency for plants associated with stroma-forming Epichloe species to be free of alkaloids, and those that did produce alkaloids contained only small levels of peramine. In contrast, plants infected with seed-transmitted Neotyphodium endophytes often contained extremely high levels of lolines. Lolines enhance host survival through increased protection from herbivores and, thus, may be particularly favored in asexual endophytes that depend on host seed-production for their dispersal.

Response of the seven-spot ladybird to an aphid alarm pheromone and an alarm pheromone inhibitor is mediated by paired olfactory cells.

Abstract: Electrophysiological responses of adult seven-spot ladybirds, Coccinella septempunctata, to (E)-β-farnesene, an aphid alarm pheromone, and (−)-β-caryophyllene, a plant-derived alarm pheromone inhibitor, were investigated by recording from single olfactory cells (neurons) on the antenna. Cells having high specificity for each of the two compounds were identified. Furthermore, these two cell types were frequently found in close proximity, with a larger amplitude consistently recorded for the cell responding specifically to (E)-β-farnesene. Preliminary behavioral studies in a two-way olfactometer showed that walking adults were significantly attracted to (E)-β-farnesene; this activity was inhibited with increasing proportions of (−)-β-caryophyllene. The possible ecological significance of colocation or pairing of olfactory cells for semiochemicals with different behavioral roles is discussed.

Effects of Temperature and Photoperiod on Phytotoxic Root Exudates of Cucumber (Cucumis sativus) in Hydroponic Culture

Abstract: In order to elucidate the effects of temperature and photoperiod on the quality and quantity of plant root exudates, a Japanese cucumber (Cucumis sativus, cv. Shougoin-Aonaga-Fushinari) was grown hydroponically in growth chambers under controlled temperature and photoperiod conditions with or without the addition of activated charcoal (AC) to the nutrient solutions. Fresh AC was used to trap the organic compounds exuded from cucumber roots every two weeks. Cucumber plants without AC were severely retarded in root growth and in the accumulation of dry matter, especially at high temperature and long photoperiod, compared to those with AC. The growth inhibitors, adsorbed on the AC or accumulated in the nutrient solution without AC, were extracted by organic solvents and analyzed by GC-MS. Benzoic acid and its derivatives, cinnamic acid derivatives, and fatty acids were identified. The rate of root exudation in vegetative and reproductive stages for some of these organic acids increased with the elevation of temperature and the elongation of photoperiod, and the mean rate was two or more times higher than the minimum exudation at low temperature with short photoperiod. Some of the identified compounds significantly inhibited the germination and/or root growth of lettuce and cucumber.

Vascular Architecture Generates Fine Scale Variation in Systemic Induction of Proteinase Inhibitors in Tomato

Abstract: Systemic induction following damage has been found in many plant species. Despite this widespread appreciation for the importance of induction, few studies have characterized the spatial variability of induction. We used tomato, Lycopersicon esculentum, to examine how damage to a single leaf affected the spatial distribution of systemic induction of proteinase inhibition in leaves above the damaged leaf. We crushed each leaflet of the second true leaf with forceps and measured the spatial distribution of proteinase inhibition in leaves 3, 4, and 5 at 8, 16, 24, 48, 72, and 120 hr. Constitutive levels of proteinase inhibitor activity were quantified in undamaged plants. We hypothesized that, due to vascular control of signal movement, systemic induction would show both among and within leaf variability. Following damage to leaf 2, induction was most pronounced in leaf 5 and minimal in leaf 3. In general, proteinase inhibitor activity was greatest at 24 hr and then declined. As predicted by vascular architecture, the near side of leaves in adjacent orthostichies showed higher induction than the far side of leaves. There was no increase in proteinase inhibitor activity in the undamaged neighboring plants. Overall our results demonstrate that systemic induction of proteinase inhibitors is partially controlled by vascular architecture and that future studies on systemic induction should examine the vascular architecture of the plants being studied. We argue that this spatial variation may influence the performance of herbivores sensitive to induced chemical changes.

Evaluation of methods for measuring polyphenols in conifer foliage

Abstract: There is a resurgence of interest in the quantification of polyphenols in plant tissues because of their presumed ecological importance in plant–litter–soil and plant–animal interactions The influence of sample preparation, extracting solvent, foliage quality, and assay method was investigated for the quantification of total phenols and condensed tannins in conifer foliage Our results suggest that it is not possible to recommend a single optimal protocol for quantification of total phenol and condensed tannin fractions from plant materials In general, the use of aqueous acetone (50–70% v/v) with freeze-dried materials gave the highest recovery The Folin-Ciocalteau method for total phenols and the butanol–HCl hydrolysis method for condensed tannins appear superior to other common assays tested There were large differences (14–22 times) in the reactivity of purified condensed tannins among species, indicating the importance of an appropriate standard for polyphenol quantification A solid-state 13C NMR method with an improved "interrupted decoupling" pulse sequence yielded the highest concentrations for condensed tannins Assuming that 13C NMR provides an accurate measure of total condensed tannin, the other extraction/assay methods used in this study recovered 50–86% of the condensed tannin fraction The recovery rate is correlated with the nitrogen content of the foliage, which suggests that the formation of protein–tannin complexes may limit the extractability of condensed tannins While 13C NMR condensed tannin values may give the best value for total condensed tannin concentrations, the water-soluble fraction may have the greatest physiological and/or ecological significance

INDUCTION OF PHENOLIC COMPOUNDS IN TWO CULTIVARS OF CUCUMBER BY TREATMENT OF HEALTHY AND POWDERY MILDEW-INFECTED PLANTS WITH EXTRACTS OF Reynoutria sachalinensis

Abstract: Accumulation of phenolic compounds (p-coumaric, caffeic, and ferulic acids and p-coumaric acid methyl ester) was followed in susceptible (Mustang) and tolerant (Flamingo) cucumber (Cucumis sativus) cultivars. The objective was to determine whether these compounds played a role in resistance against powdery mildew following a prophylactic treatment with Milsana (leaf extracts from the giant knot weed Reynoutria sachalinensis, polygonaceae). This treatment significantly reduced the incidence of powdery mildew in both cultivars. Phenolic compounds were extracted from leaves. In the hydrolyzed fraction containing phenolic aglycones, levels of p-coumaric, caffeic, and ferulic acids and of p-coumaric acid methyl ester increased in all treatments (with leaf extracts of R. Sachalinensis, powdery mildew, or both) except the control, one or two days after treatment. In the fraction containing free phenolics, from the tested compounds, only ferulic acid showed an increase in cv. Flamingo (tolerant), and was particularly evident following treatments. On the other hand, the amounts of hydroxycinnamic acids increased rapidly in the two cultivars following Milsana treatment, suggesting their role in disease reduction. All compounds showed antifungal activity when tetsed against common pathogens of cucumber (Botrytis cinerea, Pythium ultimum, and P. aphanidermatum), but in general methyl esters were more fungitoxic than their corresponding free acids. This study suggests that cucumber is able to release antifungal compounds that are instrumental in repressing powdery mildew infection. This response is seemingly independent from the level of genetic resistance associated with each cultivar.

Inhibition of fungal spore germination by Nasutitermes: evidence for a possible antiseptic role of soldier defensive secretions.

Abstract: The antifungal property of two of the principal components of the terpenoid frontal gland secretions of nasute termite soldiers was studied by incubating spore suspensions of the fungus Metarhizium anisopliae with α-pinene and limonene singly or in combination at different concentrations. In vitro assays showed that these substances reduced spore germination through direct and indirect (vapor) contact. To determine if the frontal gland secretions protected termites from fungal infection in vivo, the effect of M. anisopliae on the time course of survival of Nasutitermes costalis and N. nigriceps was studied by exposing termites to either a 4.3 × 107 spores/ml or a control sporeless suspension. The caste composition of experimental groups was manipulated to create mixed-caste subcolonies and monocaste groups. Relative to Coptotermes formosanus, a species that relies on the mechanical defenses of soldiers, N. costalis and N. nigriceps were less susceptible to fungal infection. Spore-exposed N. costalis and N. nigriceps termites had 1.2 times the hazard ratio of death of controls, while the hazard ratio of death of spore-exposed C. formosanus was 11.4 times that of controls. Although the lower susceptibility to infection in Nasutitermes may be explained in part by the antifungal properties of α-pinene and limonene, group composition also played a major role in the survival of spore-exposed termites. Mixed-caste and soldier monocaste groups had 3.4 and 4.7 times the hazard ratio of death, respectively, relative to the worker monocaste treatment. These results suggest that although Nasutitermes terpenoid secretions may have antifungal properties, the caste composition of groups and the social interactions of termites also play a role in determining susceptibility to fungal infection.

Distribution and Exudation of Allelochemicals in Wheat Triticum aestivum

Abstract: Wheat allelopathy has potential for weed suppression. Allelochemicals were identified in wheat seedlings, and they were exuded from seedlings into agar growth medium. p-Hydroxybenzoic, trans-p-coumaric, cis-p-coumaric, syringic, vanillic, trans-ferulic, and cis-ferulic acids and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) were identified in both the shoots and roots of 17-day-old wheat seedlings and their associated agar growth medium. Wheat accessions with previously identified allelopathic activity tended to contain higher levels of allelochemicals than poorly allelopathic ones. The allelopathic compounds present in the shoots generally also were identified in the roots and in the agar medium. Allelochemicals were distributed differentially in wheat, with roots normally containing higher levels of allelochemicals than the shoots. When the eight allelochemicals were grouped into benzoic acid and cinnamic acid derivatives, DIMBOA, total coumaric, and total ferulic acids, the amount of each group of allelochemicals was correlated between the roots and the shoots. Most of the allelochemicals identified in the shoots and roots could be exuded by the living roots of wheat seedling into the agar growth medium. However, the amounts of allelochemicals in the agar growth medium were not proportional to those in the roots. Results suggest that wheat plants may retain allelochemicals once synthesized. The presence of allelochemicals in the agar growth medium demonstrated that wheat seedlings were able to synthesize and to exude phytotoxic compounds through their root system that could inhibit the root growth of annual ryegrass.

Mutual reproductive benefits between a wild orchid, Bulbophyllum patens, and Bactrocera fruit flies via a Floral synomone

Abstract: The solitary flower of Bulbophyllum patens selectively attracts male fruit flies of several Bactrocera species with a specific fragrance in the rain forest of Malaysia. It temporarily traps flies between its hinged see-saw lip and column for pollination. The attractant component is zingerone [4-(4-hydroxy-3-methoxyphenyl)-2-butanone], a pungent essence of ginger. Zingerone has a structure resembling two major fruit fly attractants (methyl eugenol and raspberry ketone) and shows potency to attract a wide range of fruit fly species (B. carambolae, B. caudata, B. cucurbitae, B. tau, and B. umbrosa). A fruit fly visitor is rewarded by feeding on zingerone, and in return it removes the pollinarium and then transfers it to another flower. Males of the melon fly acquire the floral essence and selectively store it in the pheromone gland to attract conspecific females. Males of B. papayae, however, convert zingerone to zingerol in the crop. The latter compound is stored in the rectal gland and subsequently released to attract females. This provides a rare example of a mutualistic interaction between insects and plants via a floral synomone, in which both organisms obtain advantages directly benefiting their reproductive systems.

Identification and synthesis of volicitin and related components from beet armyworm oral secretions

Abstract: Oral secretion of beet armyworm caterpillars (BAW), when applied to damaged tissues of corn seedlings, induces the seedlings to emit volatile compounds that attract the natural enemies of the caterpillars. The key elicitor present in BAW oral secretions is N-[17-hydroxylinolenoyl]-L-glutamine (volicitin). Analysis of the oral secretion showed that it also contained N-[17-hydroxyolinoleoyl]-L-glutamine, free 17-hydroxylinolenic, and 17-hydroxylinoleic acid, the glutamine conjugates of linolenic and linoleic acid as well as free linolenic and linoleic acid. Here we present the identification and synthesis of the hydroxy acids and of glutamine conjugates.

Induction and/or selection of phenolic acid-utilizing bulk-soil and rhizosphere bacteria and their influence on phenolic acid phytotoxicity.

Abstract: Bulk-soil and rhizosphere bacteria are thought to exert considerable influence over the types and concentrations of phytotoxins, including phenolic acids, that reach a root surface. Induction and/or selection of phenolic acid-utilizing (PAU) bacteria within the bulk-soil and rhizosphere have been observed when soils are enriched with individual phenolic acids at concentrations ≥0.25 μmol/g soil. However, since field soils frequently contain individual phenolic acids at concentrations well below 0.1 μmol/g soil, the actual importance of such induction and/or selection remains uncertain. Common bacteriological techniques (e.g., isolation on selective media, and plate dilution frequency technique) were used to demonstrate in Cecil Ap soil systems: (1) that PAU bacterial communities in the bulk soil and the rhizosphere of cucumber seedlings were induced and/or selected by mixtures composed of individual phenolic acids at concentrations well below 0.25 μmol/g soil; (2) that readily available carbon sources other than phenolic acids, such as glucose, did not modify induction and/or selection of PAU bacteria; (3) that the resulting bacterial communities readily utilize mixtures of phenolic acids as a carbon source; and (4) that depending on conditions (e.g., initial PAU bacterial populations, and phenolic acid concentration) there were significant inverse relationships between PAU bacteria in the rhizosphere of cucumber seedlings and absolute rates of leaf expansion and/or shoot biomass. The decline in seedling growth could not be attributed to resource competition (e.g., nitrogen) between the seedlings and the PAU bacteria in these studies. The induced and/or selected rhizosphere PAU bacteria, however, reduced the magnitude of growth inhibition by phenolic acid mixtures. For a 0.6 μmol/g soil equimolar phenolic acid mixture composed of p-coumaric acid, ferulic acid, p-hydroxybenzoic acid, and vanillic acid, modeling indicated that an increase of 500% in rhizosphere PAU bacteria would lead to an approximate 5% decrease (e.g., 20–25%) in inhibition of absolute rates of leaf expansion. As far as we know, this is the first time that such a relationship has been quantified.

Role of Toxins in Intake of Varied Diets by Sheep

Abstract: Herbivores foraging on toxic plants may consume a variety of foods that contain different toxins to increase food intake and to avoid toxicosis We studied whether lambs offered two foods, each containing a different toxin, could ingest more food than lambs offered one food with a single toxin Thirty-two lambs were allotted to four groups that received: (1) a ration with toxin A, (2) a ration with toxin B, (3) two rations, one with toxin A and the other with toxin B, and (4) a ration with no toxins Toxin pairs used in the study were (1) amygdalin and lithium chloride (LiCl), (2) LiCl and LiCl, (3) sparteine and saponin, (4) oxalate and nitrate, and (5) tannin and saponin For an hour each morning, lambs were offered their ration(s) and intakes were measured Lambs were maintained on an alfalfa pellet or grass hay diet Each trial lasted either five or six days Whether or not lambs ate more when offered foods with different toxins depended on the kind and amount of toxin in the food Lambs offered rations with amygdalin and LiCl or oxalate and nitrate consumed more food than lambs offered a ration with only one of these toxins Lambs offered rations with sparteine and saponin or tannin and saponin did not eat more food than lambs offered a ration with either saponin or sparteine or tannin alone Nor did lambs eat more when offered two rations both containing LiCl In all trials, lambs offered toxins showed no signs of toxicosis, and they ate less food than lambs offered rations without toxins Our results indicate that in some cases ruminants can increase intake of toxic foods by consuming foods containing different toxins However, currently the only way to determine how specific toxins may interact in the body to influence intake would be to conduct feeding trials using plants or ground diets that contain toxins

Variation in Phenolic Compounds within a Birch (Betula pendula) Population

Abstract: In previous studies, the qualitative and quantitative variation found in defense chemistry among birch populations and even among individual clones has been considerable. However, information about variation among adult, naturally regenerated birch trees from natural populations is still lacking. In this study, the phenolic composition of leaves of 30 naturally regenerated 20-year-old birch (Betula pendula) trees was analyzed for two successive years in order to characterize the chemical composition of individual trees, analyze the annual variation, and determine chemical similarities among individual trees within a population. The main phenolic compounds were flavonoid glycosides, myricetin, and quercetin derivatives. Annual variation in concentration among leaves was large. In most trees, concentrations were markedly higher in 1998 than in 1997; for certain compounds, the detected increase was as much as a 50%. However, for some individual trees, there were no differences between years in chemical quantity. Thus, when selection or grouping of trees is based on secondary chemistry, quantitative variation should be considered carefully. With the qualitative UPGMA method of classification, four chemotypes were found. The grouping was similar for both years, and qualitatively the results of an individual tree seem to be independent of sampling year. The stability in chemical profile of individual trees suggests that quality is tightly controlled by genotype, which provides a recognition tool for chemotaxonomy. The high within-population variation found in leaf defense chemistry may provide protection against different types of insects (generalists or specialists) and, thus, have positive effects on population survival.

Glucosinolates and differential herbivory in wild populations of Brassica oleracea

Abstract: Glucosinolates are known to elicit responses from Brassica herbivores in laboratory studies. To study their importance in interactions with herbivores in the field, glucosinolate profiles and levels of herbivory were ascertained for wild cabbage plants growing in four neighboring populations in the UK. Glucosinolate profiles differed between plant populations, but not between different habitats within populations. Within habitats, there was no link between individual plant glucosinolate profiles and herbivory by Pieris spp., slugs and snails, flea beetles or aphids. Plants attacked by the micromoth, Selania leplastriana, contained higher levels of 2–hydroxy-3–butenylglucosinolate and 3–indolylmethylglucosinolate than plants within the same population that were not attacked. It is concluded that the differences in glucosinolate profiles between the plant populations are unlikely to be due to differential selection pressures from herbivores feeding on the mature plants over the two years studies.

Effects of Alfalfa Saponins on the Moth Spodoptera littoralis

Abstract: Alfalfa saponins administered to Spodoptera littoralis in the larval diet caused prolongation of the larval and pupal stages, retarded growth, increased mortality, and reduced fecundity and fertility. At least some of these effects were probably due to digestion problems manifested by longer food retention in the gut. Preliminary data indicated that the efficiency of food utilization was not altered. Saponin aglycones exerted similar developmental derangements; medicagenic acid proved most active; hederagenin, soysaponogenol A, and soysaponogenol B exhibited moderate activities; and soysaponogenol E was inactive. It is proposed that saponins become active only when the sugar component is cleaved off by the gut glycosylases and that substrate specificity of these enzymes is decisive for the activity of ingested saponins. For example, all tested α-L-arabinopyranosyl glycosides were inactive, while the corresponding aglycones or glucosides were active. The liberated aglycones are apparently deposited in the tissues and exert post-feeding disturbances such as delay of imaginal ecdysis and reduced egg hatchability.

Plant Secondary Compounds and Grasshoppers: Beyond Plant Defenses

Abstract: Modern grasshoppers probably evolved from polyphagous ancestors endowed with the ability to tolerate many plant secondary compounds. This tolerance involves various behavioral and anatomical adaptations. Polyphagous grasshoppers have a relatively low level of sensitivity to the taste of many secondary compounds, and, if they do respond to the taste, have the capacity to habituate. This gives time for the induction of detoxifying enzymes so that unpalatable but potentially nutritious plants may be eaten safely. Associative learning involving secondary compounds may be important in food aversion learning, enabling the insects to avoid foods that have inappropriate nutrients, for example. Learning is also involved when grasshoppers develop associations between the taste of chemicals in the surface waxes of plants and internal leaf chemistry, enabling them to make faster decisions about the acceptability of a plant. Anatomically, the midgut ceca of polyphagous grasshoppers have well-developed posterior arms, and it is possible that these are especially important in detoxification, while some species, in addition, have a specialized pocket region in which macromolecules accumulate to be eliminated from the body when the lining of peritrophic envelope is drawn out. Polyphagous species also have thick peritrophic envelopes to which various phenolics become adsorbed. Finally, the midgut environment contains surfactants that reduce tannin–protein complexing except at very high tannin concentrations. Some polyphagous species can utilize secondary compounds as defensive substances or, in one case, in cuticular sclerotization. Grass feeding has evolved on numerous occasions from these polyphagous ancestors, and it has been associated with a loss of many of the characters providing protection from secondary compounds.

Taste Sensitivity of Insect Herbivores to Deterrents is Greater in Specialists Than in Generalists: A Behavioral Test of the Hypothesis with Two Closely Related Caterpillars

Abstract: Sensitivity of caterpillars of Heliothis virescens, a generalist, and H. subflexa, a specialist, to eight different plant secondary compounds was examined behaviorally. The compounds were nicotine hydrogen tartrate, hordenine, caffeine, sinigrin, linamarin, arbutin, chlorogenic acid, and salicin. All compounds deterred feeding, at least at the higher concentrations, but the generalist was less affected than the specialist. Thus the hypothesis that specialists have greater sensitivity to deterrents than generalists was supported. In most cases deterrence occurred on first encounter, indicating that the response was sensory; in some cases short-term postingestive effects also appeared to play a role. The larger quantities of deterrent-containing food ingested by H. virescens sometimes resulted in measurable postingestive effects during the second control test. This did not occur in H. subflexa, which more commonly rejected the deterrent-containing food on first contact. The contrast between the species is discussed in relation to tradeoffs involved in different diet breadths.

Identity and function of scent marks deposited by foraging bumblebees

Abstract: Foraging bumblebees can detect scents left on flowers by previous bumblebee visitors and hence avoid flowers that have been depleted of nectar. Tarsal secretions are probably responsible for this repellent effect. The chemical components of the tarsal glands were analyzed by combined gas chromatography-mass spectrometry for three species of bumblebee. Bombus terrestris, B. lapidarius, and B. pascuorum. The hydrocarbons identified were similar for each species. although there were interspecific differences in the relative amounts of each compound present. The tarsal extracts of all three species comprised complex mixtures of long-chain alkanes and alkenes with between 21 and 29 carbon atoms. When B. terrestris tarsal extracts were applied to flowers and offered to foraging bumblebees of the three species. each exhibited a similar response; concentrated solutions produced a repellent effect, which decreased as the concentration declined. We bioassayed synthetic tricosane (one of the compounds found in the tarsal extracts) at a range of doses to determine whether it gave a similar response. Doses greater than or equal to 10(-12) ng/flower resulted in rejection by foraging B. lapidarius. Only when less than or equal to 10(-14) ng was applied did the repellent effect fade. We bioassayed four other synthetic compounds found in tarsal extracts and a mixture of all five compounds to determine which were important in inducing a repellent effect in B. lapidarius workers. All induced repellency but the strength of the response varied: heneicosane was most repellent while tricosene was least repellent. These findings are discussed in relation to previous studies that found that tarsal scent marks were attractive rather than repellent.

Jasmonate-induced responses of Nicotiana sylvestris results in fitness costs due to impaired competitive ability for nitrogen.

Abstract: We present the first evidence for a fitness cost of an inducible response that is detectable in a nitrogen (N) currency. Nicotine is an induced defense in Nicotiana sylvestris that can utilize 5–8% of the plant's total N, an investment that apparently cannot be recouped by metabolism. Induced nicotine production is endogenously regulated by jasmonic acid (JA), and we treated leaves with the methyl ester of this wound hormone (MeJA) in quantities (0, 25, 250 μg) known to elicit changes in endogenous JA and subsequent nicotine responses comparable to those elicited by mechanical wounding and herbivory in this species. We grew plants in competition chambers (CCs) in which three same-sized plants could compete for a communal but fixed pool of 15NO3 to quantify the outcome of competition for this fitness-limiting resource that is used both in defense and seed production. Competition profoundly increased all measures of growth and reproductive performance measured per milligram of N acquired. While plants acquired all the N supplied to them in the hydroponic solution, plants grown in CCs (as compared those grown in individual chambers—ICs) retained more of this N and produced more biomass, had larger nicotine contents, allocated less of their N to nicotine, produced larger floral stalks with more flowers, aborted fewer flowers, matured more capsules, and produced a greater mass of seed. Plants grown in ICs produced heavier seed, but this difference did not translate into a difference in seed viability.