Polymorphism in Plant Defense Against Herbivory: Constitutive and Induced Resistance in Cucumis sativus

Theory predicts that plant resistance to herbivores is determined by both genetic and environmentally induced components. In this study, we demonstrate that the phenotypic expression of plant resistance to spider mite herbivory in Cucumis sativus is determined by genetic and environmental factors and that there is an interaction between these factors. Previous feeding by spider mites induced systemic resistance to subsequent attack over several spatial scales within plants, reducing the population growth of mites compared to that on control plants. Effects of induction were effective locally over the short term, but resulted in local increased susceptibility to spider mite attack after several days. However, this local induced susceptibility on the damaged leaf was associated with induced resistance on newer leaves. Induced resistance was correlated with increases in cucurbitacin content of leaves, but was not associated with changes in the density of leaf trichomes. Induced resistance to herbivory was not detected in plants of a genotype lacking constitutive expression of cucurbitacins, which were in general highly susceptibile to mite attack. Allocation trade-offs between growth and defense are often invoked to explain the maintenance of variation in the levels of plant resistance. Contrary to current thinking, neither constitutive nor herbivore-induced plant resistance were associated with reductions in plant allocation to root and shoot growth. However, plants that had high levels of induced resistance to spider mites were the most susceptible to attack by a specialist beetle. Such ecological trade-offs between resistance to generalist herbivores and susceptibility to specialist herbivores may be important in the maintenance of variation of plant resistance traits. In summary, C. sativus exhibits strong genetic variation for constitutive and induced resistance to spider mites, and this variation in resistance is associated with ecological trade-offs.     

Induction of Direct and Indirect Plant Responses by Jasmonic Acid, Low Spider Mite Densities, or a Combination of Jasmonic Acid Treatment and Spider Mite Infestation

Jasmonic acid (JA) and the octadecanoid pathway are involved in both induced direct and induced indirect plant responses. In this study, the herbivorous mite, Tetranychus urticae, and its predator, Phytoseiulus persimilis, were given a choice between Lima bean plants induced by JA or spider mites and uninduced control plants. Infestation densities resulting in the induction of predator attractants were much lower than thus far assumed, i.e., predatory mites were significantly attracted to plants that were infested for 2 days with only one or four spider mites per plant. Phytoseiulus persimilis showed a density-dependent response to volatiles from plants that were infested with different numbers of spider mites. Similarly, treating plants with increasing concentrations of JA also led to increased attraction of P. persimilis. Moreover, the duration of spider mite infestation was positively correlated with the proportion of predators that were attracted to mite-infested plants. A pretreatment of the plants with JA followed by a spider mite infestation enhanced the attraction of P. persimilis to plant volatiles compared to attraction to volatiles from plants that were only infested with spider mites and did not receive a pretreatment with JA. The herbivore, T. urticae preferred leaf tissue that previously had been infested with conspecifics to uninfested leaf tissue. In the case of choice tests with JA-induced and control leaf tissue, spider mites slightly preferred control leaf tissue. When spider mites were given a choice between leaf discs induced by JA and leaf discs damaged by spider mite feeding, they preferred the latter. The presence of herbivore induced chemicals and/or spider mite products enhanced settlement of the mites, whereas treatment with JA seemed to impede settlement.     

The Dynamics of Carbon–Nutrient Balance: Effects of Cottonwood Acclimation to Short- and Long-Term Shade on Beetle Feeding Preferences

The carbon–nutrient balance hypothesis (CNBH) predicts that shading should increase leaf palatability to herbivores by decreasing concentrations of carbon (C) -based chemical defenses and increasing nitrogen (N). We measured cottonwood (Populus deltoids) growth, leaf chemistry, and beetle (Plagiodera versicolora) feeding preferences on saplings grown in either continuous high (HH) or low (LL) light, and saplings switched from high to low (HL) or low to high (LH) light for nine days. As expected, based on the CNBH, shading increased total N and decreased total phenol glycoside (C-based secondary metabolites) concentrations in plants from all shade treatments (LL, HL, and LH), relative to HH plants, with plant growth and gross leaf chemistry being affected by initial and final light regime. In contrast, while specific phenol glycoside concentrations were affected by the initial and final light regime, they also showed an initial × final light interaction. Beetles tended to prefer LL to HH plants. Beetles unexpectedly preferred HH to either HL or LH switched plants, most likely because high concentrations of a specific phenol glycoside – salicin – occurred in both switched treatments and inhibited beetle feeding. Plant chemical allocation during light acclimation led to unpredictable changes in specific C-based compounds, even though plant growth and gross chemistry conformed to expectations for shading effects and the CNBH. The response of this herbivore to altered concentrations of a specific compound confounded predictions based on average dynamics of suites of chemicals. Our findings may help explain why relationships between light availability and herbivory in field studies, where light varies on many time scales, can differ from those predicted by the CNBH. Understanding both dynamic plant chemical responses to altered resource availability and controls over allocation to specific compounds would likely enhance future predictability of specific environment-plant-herbivore interactions.     

Effects of mite resistance mechanism of geraniums on mortality and behavior of foxglove aphid (Acyrthosiphon solani Kaltenbach)

Geraniums (Pelargonium xhortorum Bailey) possess a pest-resistance mechanism, based on glandular trichomes and the exudate they produce, that has been shown to be effective against the two-spotted spider mite (Tetranychus urticae Koch). Using an intact plant bioassay, the effectiveness of the resistance mechanism was determined for another potential pest, the foxglove aphid (Acyrthosiphon solani Kaltenbach). Comparisons were made between plant lines previously analyzed for their degree of resistance to mites, as well as their glandular trichome density and trichome exudate production. Over 100 aphid adults were bioassayed on each of the five plant lines used in the experiment. In addition to adult aphid survival, the production and survival of nymphs was determined in this bioassay. The results indicate that plant lines that are resistant to the two-spotted spider mite are also resistant to the foxglove aphid, while lines susceptible to mites are susceptible to the aphids. To evaluate the physical impediment features of the trichome exudate, the behavior of foxglove aphid nymphs was compared on two geranium lines, one a resistant line with high trichome densities and large quantities of exudate and the second a susceptible line with few trichomes and reduced exudate. A third leaf surface type was produced by washing the exudate from resistant leaves using a mildly basic buffer solution prior to the bioassay. Aphid behavior was divided into five categories: feeding or probing, resting, wandering, struggling, and immobilized. On both susceptible leaves and resistant leaves from which the exudate had been removed by washing, the aphids settled quickly and were observed with inserted stylets during most of the observation intervals. In contrast, aphids on the unwashed resistant leaf surfaces often became ensnared in the sticky trichome exudate and had difficulty in settling to probe the leaf. Physical entrapment by glandular trichome exudate appears to be an important aspect of aphid resistance in geraniums.     

Role of cucurbitacin C in resistance to spider mite (Tetranychus urticae) in cucumber (Cucumis sativus L.)

Cucurbitacins are bitter triterpenoid compounds that are toxic to most organisms and occur widely in wild and cultivated Cucurbitaceae. The only cucurbitacin identified in Cucumis sativus is cucurbitacin C. The bitter taste of cucumber has been correlated with resistance to the spider mite Tetranychus urticae, but a quantitative relationship has not been established. We determined the spider mite resistance and cucurbitacin C content in the dihaploid progeny derived from the F₁ generation of a cross between a bitter, spider-mite-resistant cucumber line and a bitter-free, spider-mite-susceptible line. The ratio of the number of bitter to bitter-free dihaploids conformed to the expected 1:1 ratio, based on a monogenic segregation pattern. Genetic analysis ascribed 69% of the variance of the difference in spider mite survival rate to the bitterness locus. Within the group of bitter dihaploids, cucurbitacin C content was significantly correlated with spider mite resistance. Thus, a quantitative relationship between cucurbitacin C content and spider mite resistance could be established.     

Effects of nitrogen fertilization on secondary chemistry and ectomycorrhizal state of Scots pine seedlings and on growth of grey pine aphid

Effects of nitrogen availability on secondary compounds, mycorrhizal infection, and aphid growth of 1-year-old Scots pine (Pinus sylvestris L.) seedlings were studied during one growing season. Seedlings were fertilized with nutrient solutions containing low, optimum, and two elevated (2 × and 4 × optimum) levels of NH₄NO₃. At the end of growing season foliar nitrogen concentration, needle biomass, needle length, water contents of needles, root collar diameter, and number of buds increased with enhanced nitrogen availability. Addition of nitrogen did not have effect on concentrations of monoterpenes in growing needles, but in mature needles significantly decreased concentrations of some individual and total monoterpenes were detected. In growing needles the concentrations of some individual resin acids decreased, and in mature needles concentrations of some individual and total resin acids increased with increased nitrogen fertilization. Higher numbers of resin ducts were found in mature needles with nitrogen fertilization. Nitrogen fertilization decreased total phenolic concentrations in growing and mature needles of the current year, but in needles of the previous year no significant differences occurred. Mycorrhizal infection was highest at medium (optimum and 2 × optimum) nitrogen fertilization levels. The relative growth rate (RGR) of grey pine aphid [Schizolachnus pineti (F.)] responded positively to the increase in foliar nitrogen content. However, the increase in aphid performance between optimum and the highest fertilization level was slight. This may indicate a deterring effect of resin acids on aphids. The results indicate that carbon/nutrient balance hypothesis fails to predict directly the effects of nitrogen availability on concentrations of carbon-based defensive compounds in mature foliage. Altered nitrogen supply affects allocation to secondary metabolites differently, depending on the developmental state of the plant and the biosynthesis pathway, cost of synthesis, and storage of compounds.     

The Response of Phytoseiulus persimilis to Spider Mite-Induced Volatiles from Gerbera: Influence of Starvation and Experience

When leaves of the ornamental crop Gerbera jamesonii are damaged by the spider mite Tetranychus urticae, they produce many volatile compounds in large quantities. Undamaged gerbera leaves produce only a few volatiles in very small quantities. In the headspace of spider mite-damaged gerbera leaves many terpenoids are present, comprising 65% of the volatile blend. In addition, a number of nitrogen containing compounds, such as oximes and nitriles, are produced.We studied the attraction of P. persimilis to the volatiles from spider mite-damaged gerbera leaves and how attraction is affected by starvation and previous experience. Phytoseiulus persimilis that were reared on spider mites (T. urticae) on Lima bean were not attracted to spider mite-induced volatiles from gerbera. Starvation did not influence the predator's response to these volatiles. In contrast, predators that were reared on spider mites on gerbera leaves were strongly attracted to volatiles from spider mite-infested gerbera. This was found also for predators that originated from a culture on spider mite-infested bean and were offered six days of experience with spider mites on gerbera leaves.     

Genetic and Environmental Variation in Acyl Glucose Ester Production and Glandular and Nonglandular Trichome Densities in Datura wrightii

Natural populations of Datura wrightii in southern California are dimorphic for trichome type. Some plants within populations produce greater than 85% glandular trichomes, whereas other produce mainly nonglandular trichomes. Glandular trichome exudates in D. wrightii consist of glucose esterified with straight chain C₆–C₉ acids. These exudates, and similar exudates in other species, confer resistance to several insect herbivore species. We tested the hypothesis that water was limiting sugar ester production and examined the extent to which trichome density was determined by environmental factors by measuring the concentrations of sugar esters and the densities of trichomes on leaves of plants grown under different irrigation treatments. Water did not limit sugar ester production, as unwatered plants produced 36% more millimoles of glucose esters per square centimeter of leaf surface than did watered plants. Although the addition of water increased leaf size, densities of both nonglandular and glandular trichomes did not change with leaf length or area, suggesting that plants having larger leaves initiated more trichomes in order to maintain nearly constant densities. Millimoles of sugar esters produced did not correlate with densities of glandular trichomes, suggesting that other factors in addition to glandular trichome number govern the production of sugar esters for plant defense.     

Herbivory induces systemic production of plant volatiles that attract predators of the herbivore: Extraction of endogenous elicitor

It was previously shown that in response to infestation by spider mites (Tetranychus urticae), lima bean plants produce a volatile herbivoreinduced synomone that attracts phytoseiid mites (Phytoseiulus persimilis) that are predators of the spider mites. The production of predator-attracting infochemicals was established to occur systemically throughout the spider mitein-fested plant. Here we describe the extraction of a water-soluble endogenous elicitor from spider mite-infested lima bean leaves. This elicitor was shown to be transported out of infested leaves and was collected in water in which the petiole of the infested leaf was placed. When the petioles of uninfested lima bean leaves were placed in water in which infested leaves had been present for the previous seven days, these uninfested lima bean leaves became highly attractive to predatory mites in an olfactometer when an appropriate control of uninfested lima bean leaves was offered as alternative. The strength of this effect was dependent on the number of spider mites infesting the elicitor-producing leaves. Higher numbers of spider mites resulted in an elicitor solution with a stronger effect. In addition, spider mite density was important. The elicitor obtained from one leaf with 50 spider mites had a stronger effect on the attractiveness of uninfested leaves than the elicitor obtained from three leaves with 17 spider mites each. This suggests that the stress intensity imposed on a plant is an important determinant of the elicitor quantity. While the elicitor has a strong effect on the attractiveness of uninfested leaves, spider mite-infested leaves are still much more attractive to predatory mites than elicitor-exposed leaves. The data are discussed in the context of systemic effects in plant defense and the biosynthesis of herbivore-induced terpenoids in plants.     

Fertility,Root Reserves and the Cost of Inducible Defenses in the Perennial Plant <Emphasis Type="Italic">Solanum carolinense</Emphasis>

We examined the relationship between internal resources (root reserves), external resources (soil fertility), and allocation to defense vs. growth in the clonal, perennial herb Solanum carolinense. In a short-term (9 d) greenhouse experiment, plants were treated once with jasmonic acid (JA) to determine if polyphenols and glycoalkaloids were inducible by simulated herbivory. In a longer-term (4 wk) greenhouse experiment, we measured the cost, in terms of growth, of treatment with JA every 3 d, to determine if the induced response was due more to carbon limitation or nitrogen limitation. We manipulated the resources available to the plants by varying soil fertility and the size of root cuttings from which plants were grown, and assessed how different resource levels affected the growth and production of polyphenols and alkaloids under JA treatment or control conditions. In the short term, JA increased the concentration of polyphenols in both above- and belowground plant parts, as well as alkaloid concentrations in the roots. In the long term, the only significant secondary chemistry response to JA was an increased polyphenol concentration in above ground tissues. The total amount of polyphenols produced was the same for JA and control plants, indicating that the higher concentration was a result of the lower biomass of treated plants. In contrast, alkaloid concentrations in plants treated with JA for 4 wk did not differ from controls, but JA-treated plants contained lower total amounts of alkaloids in above ground tissues, as a result of decreased growth. Fertilizer level and root cutting size had effects on growth and the production of secondary compounds and influenced the cost of induction. Plants grown under high fertility had a greater reduction in growth in response to JA than plants grown under low fertility, indicating a greater trade-off between growth and defense for high fertility plants. Plants from larger root cuttings grew bigger without any reduction in the concentration of polyphenols and alkaloids. We demonstrated that the phenotype of S. carolinense was plastic in response to simulated herbivory, fertility level, and root cutting size, and that there was a significant growth cost to induction that varied with the environment and appears to be due in large part to the allocation of limited carbon reserves.     

Effect of nitrogen and water treatment on leaf chemistry in horsenettle (Solanum carolinense), and relationship to herbivory by flea beetles (Epitrix spp.) and tobacco hornworm (Manduca sexta)

We studied the interaction between plants (horsenettle; Solanum carolinense) and herbivorous insects (flea beetles; Epitrix spp., and tobacco hornworm; Manduca sexta) by focusing on three questions: (1) Does variation in nitrogen availability affect leaf chemistry as predicted by the carbon-nutrient balance (CNB) hypothesis? (2) Does variation in plant treatment and leaf chemistry affect insect feeding? (3) Is there an interaction between the insect herbivores that is mediated by variation in leaf chemistry? For three successive years (1998-2001), we grew a set of clones of 10 maternal plants under two nitrogen treatments and two water treatments. For each plant in the summer of 2000, we assayed herbivory by hornworms in both indoor (detached leaf) and outdoor (attached leaf) assays, as well as ambient flea beetle damage. Estimates of leaf material consumed were made via analysis of digitized leaf images. We also assayed leaves for total protein, phenolic, and glycoalkaloid content, and for trypsin inhibitor, polyphenol oxidase, and peroxidase activity. Despite strong effects of nitrogen treatment on growth and reproduction, only total protein responded as predicted by CNB. Leaf phenolic levels were increased by nitrogen treatment, polyphenol oxidase activity was decreased, and other leaf parameters were unaffected. Neither hornworm nor flea beetle herbivory could be related to plant treatment or genotype or to variation in any of the six leaf chemical parameters. A negative relationship between flea beetle and hornworm herbivory was found, but was not apparently mediated by any of the measured leaf chemicals. Because leaf resistance was maintained in low nitrogen plants at the apparent expense of growth and reproduction, our results support the concept of a fitness cost of defense, as predicted by the optimal defense hypothesis.     

Inhibition of feeding by a generalist insect due to increased volatile leaf terpenes under nitrate-limiting conditions

Nitrogen-limited plants ofHeterotheca subaxillaris accumulate greater quantities of leaf volatile terpenes than do nitrogen-rich plants. A series of feeding trials were performed to determine if such nitrate-limited plants are better defended against generalist-feeding insect herbivores. Soybean looper (Pseudoplusia includens) larvae were fed leaves fromH. subaxillaris rosettes grown under high and low nitrate supply regimes. Larval consumption, growth, and survival declined as the leaf volatile terpene content increased. Larval consumption and growth were enhanced by higher plant nitrate supply and with increasing leafage. The results suggest that the higher quantity of volatile terpenes in the leaves of nitrate-limited plants may better defend these leaves against generalist-feeding insects.     

Change in Behavioral Response to Herbivore-induced Plant Volatiles in a Predatory Mite Population

Damage by herbivorous spider mites induces plants to produce volatiles that attract predatory mites that consume the spider mites. A clear attraction to volatiles from Lima bean plants infested with the spider mite Tetranychus urticae has been consistently reported during more than 15 years for the predatory mite Phytoseiulus persimilis. We have monitored the response to volatiles from spider-mite infested Lima bean plants for a laboratory population of the predatory mite from 1991 to 1995 on a regular basis. A reduction in the level of attraction in the laboratory population of P. persimilis was recorded in mid-1992. The attraction of the laboratory population was weaker than that of a commercial population in the latter part of 1992, but the responses of these two populations were similarly weak in 1994 and 1995. Therefore, a behavioral change has also occurred in this commercial population. Experiments were carried out to address the potential causes of this change in attraction. The attraction of predators from a commercial population with a strong response decreased after being reared in our laboratory. Within a predator population with a low degree of attraction, strongly responding predators were present and they could be isolated on the basis of their behavior: predators that stayed on spider-mite infested plants in the rearing set-up had a strong attraction, while predators that had dispersed from the rearing set-up were not attracted to prey-infested bean plants. From our laboratory population with a low degree of attraction, isofemale lines were initiated and maintained for more than 20 generations. All isofemale lines exhibited a consistently strong attraction to spider mite-induced plant volatiles, similar to the attraction recorded for several populations in the past 15 years. Neither in a population with a strong attraction nor in two with a weak attraction was the response of the predators affected by a starvation period of 1–3 hr. Based on these results, possible causes for the observed reduction in predator attraction to spider mite-induced bean volatiles are discussed. The predatory mite P. persimilis is a cornerstone of biological control in many crops worldwide. Therefore, the change in foraging behavior recorded in this predator may have serious consequences for biological control of spider mites.     

Effects of Herbivore Damage and Nutrient Level on Induction of Iridoid Glycosides in Plantago lanceolata

Damage by larvae of the buckeye butterfly (Junonia coenia) resulted in removal of 15–25% of Plantago lanceolata leaf area. Plants grown under high nutrients were larger than those grown under low nutrients. Twenty-eight days after herbivory, plants grown under high nutrients were still larger than those grown under low nutrients, and plants exposed to herbivores were significantly smaller than those not exposed to herbivores, regardless of the nutrient treatment. Damage by larvae also increased the iridoid glycoside content in the leaves and reproductive tissues of these Plantago lanceolata relative to undamaged controls. Whether damaged or undamaged, the iridoid glycoside content of P. lanceolata was highest in the reproductive tissues and lowest in the roots. Although initial concentrations of iridoid glycosides were significantly higher in plants grown under low nutrient conditions than in plants grown under high nutrient conditions, nutrient availability did not alter the phytochemical response of plants to herbivore damage. These results provide additional support for the defensive role of the iridoid glycosides in Plantago lanceolata by demonstrating that phytochemical variation is not always an incidental effect of nutrient stress but can be a direct response to damage by herbivores.     

Comparison of furanocoumarin concentrations of greenhouse-grownRuta chalepensis with outdoor plants later transferred to a greenhouse

Ruta chalepensis contained concentrations of furanocoumarins 25–50% of those found inR. graveolens both in the whole leaf and on its surface. On the leaf surface of plants grown all year indoors in a greenhouse, they increased steadily between November 1 and December 14 on mature upper and lower leaves. New growth upper leaves on December 14 contained less than mature upper leaves. Plants transferred from outdoors to the greenhouse showed decreased concentrations after the first two weeks, followed by recovery both in the whole leaf and on the leaf surface. Proportions of xanthotoxin and bergapten to psoralen changed during the experiment. On the leaf surfaces and in the whole upper leaves of the indoor plants, the proportions were often similar, but in the transferred plants, in most cases, psoralen was less than bergapten or xanthotoxin in the upper leaves and markedly less in the lower leaves. Implications of these findings for possible effects of environmental changes on secondary plant metabolism are discussed.     

Effects of Light and Nutrient Availability on Aspen: Growth, Phytochemistry, and Insect Performance

This study explored the effect of resource availability on plant phytochemical composition within the framework of carbon–nutrient balance (CNB) theory. We grew quaking aspen (Populus tremuloides) under two levels of light and three levels of nutrient availability and measured photosynthesis, productivity, and foliar chemistry [water, total nonstructural carbohydrates (TNC), condensed tannins, and phenolic glycosides]. Gypsy moths (Lymantria dispar) and forest tent caterpillars (Malacosoma disstria) were reared on foliage from each of the treatments to determine effects on insect performance. Photosynthetic rates increased under high light, but were not influenced by nutrient availability. Tree growth increased in response to both the direct and interactive effects of light and nutrient availability. Increasing light reduced foliar nitrogen, while increasing nutrient availability increased foliar nitrogen. TNC levels were elevated under high light conditions, but were not influenced by nutrient availability. Starch and condensed tannins responded to changes in resource availability in a manner consistent with CNB theory; levels were highest under conditions where tree growth was limited more than photosynthesis (i.e., high light–low nutrient availability). Concentrations of phenolic glycosides, however, were only moderately influenced by resource availability. In general, insect performance varied relatively little among treatments. Both species performed most poorly on the high light–low nutrient availability treatment. Because phenolic glycosides are the primary factor determining aspen quality for these insects, and because levels of these compounds were minimally affected by the treatments, the limited response of the insects was not surprising. Thus, the ability of CNB theory to accurately predict allocation to defense compounds depends on the response of specific allelochemicals to changes in resource availability. Moreover, whether allelochemicals serve to defend the plant depends on the response of insects to specific allelochemicals. Finally, in contrast to predictions of CNB theory, we found substantial allocation to storage and defense compounds under conditions in which growth was carbon-limited (e.g., low light), suggesting a cost to defense in terms of reduced growth.     

Plasticity in allocation of nicotine to reproductive parts inNicotiana attenuata

Although little is known about the patterns of chemical defense allocation in reproductive tissues, optimal defense theory predicts a high constitutive allocation due to the tissues' high fitness value. To examine this prediction, we quantified the short- and long-term changes in the nicotine pools of reproductive tissues in response to both floral and leaf damage. Recently opened flowers (stage 5 capsules) do not alter their nicotine pools within a day in response to herbivory byManduca sexta larvae or mechanical damage to the corolla. Similarly, leaf damage during both vegetative and reproductive growth does not influence the nicotine pools of the first three stage-5 capsules produced. However, the nicotine pools of capsules produced later in reproductive growth were significantly larger (1.2- to 1.9-fold) on plants with leaf damage. These differences in floral nicotine pools were a result of both increases in nicotine pools of capsules on damaged plants and decreases in the nicotine pools of capsules on undamaged plants during reproductive growth. Leaf damage did not affect the rate of capsule maturation or the mass of stage-5 capsules at any time during reproductive growth. An allometric analysis of nicotine pools and biomass of reproductive parts in all stages of development from damaged and undamaged plants demonstrates that damaged plants allocated a significantly larger quantity of nicotine to reproductive parts in all stages of development than did undamaged plants. Given that nicotine is thought to be synthesized in the roots and transported to leaves and reproductive parts, nicotine could be allocated to reproductive parts in proportion to the number of developing capsules on a plant. We excised the first 27 stage-5 capsules on plants with and without leaf damage, with the expectation that plants with fewer capsules would allocate a larger amount of nicotine to the remaining capsules. In contrast to the prediction of this passive allocation model, floral excision did not affect nicotine pools on plants with or without leaf damage. These results demonstrate that the allocation of nicotine to reproductive parts is more strongly influenced by damage to vegetative rather than reproductive tissues. Reproductive parts are constitutively defended over the short term, but the set points for defense allocation are apparently increased by damage to vegetative tissues during reproductive growth. The decrease in allocation of nicotine to reproductive parts in undamaged plants during reproductive growth suggests an optimization of resource allocation as plants realize their potential fitness.     

Vicia faba–Lygus rugulipennis Interactions: Induced Plant Volatiles and Sex Pheromone Enhancement

The profiles of volatile chemicals emitted by Vicia faba plants damaged by Lygus rugulipennis feeding, and by feeding plus oviposition, were shown to be quantitatively different from those released by undamaged plants. Samples of volatile chemicals collected from healthy plants, plants damaged by males as a consequence of feeding, plants damaged by females as a consequence of feeding and oviposition, plants damaged by feeding with mated males still present, and plants damaged by feeding and oviposition with gravid females still present, showed significant differences in the emission of hexyl acetate, (Z)-β-ocimene, (E)-β-ocimene, (E)-β-caryophyllene, and methyl salicylate. In particular, treatments with mated females present on plants had a significant increase in emission levels of the above compounds, possibly due to eggs laid within plant tissues or active feeding, compared with undamaged plants and plants damaged by males feeding, with or without insects still present. Furthermore, the pheromonal blend released by mated L. rugulipennis females, mainly comprising hexyl butyrate, (E)-2-hexenyl butyrate, and (E)-4-oxo-2-hexenal, was enhanced when females were active on broad bean plants, whereas such an increase was not observed in males. Both sexes gave electroantennogram responses to green leaf volatiles from undamaged plants and to methyl salicylate and (E)-β-caryophyllene emitted by Lygus-damaged plants, suggesting that these compounds may be involved in colonization of host plants by L. rugulipennis. In addition, mated males and females were responsive to hexyl butyrate, (E)-2-hexenyl butyrate, and (E)-4-oxo-2-hexenal released by mated females on V. faba, indicating that these substances could have a dual function as a possible aggregation pheromone in female–female communication, and as a sex pheromone in female–male communication. An erratum to this article can be found at     

Effect of Nitrogen Availability on Expression of Constitutive and Inducible Chemical Defenses in Tomato, Lycopersicon esculentum

Young tomato plants (Lycopersicon esculentum) grown in sand in a greenhouse and subjected to different fertilization regimes were used to test the effects of nitrogen availability on constitutive levels of phenolics and on constitutive and inducible activities of polyphenol oxidase and proteinase inhibitors. Theories that emphasize physiological constraints on the expression of phytochemicals predict an increase in levels of carbon-based allelochemicals under moderate nitrogen stress but predict, under the same conditions, an attenuation of chemical responses involving nitrogen-containing compounds such as proteinase inhibitors and polyphenol oxidase. We found that nitrogen availability had a strong effect on constitutive levels of phenolics; weaker effects on constitutive polyphenol oxidase activity, constitutive proteinase inhibitor activity, and inducible polyphenol oxidase activity; and no effect on inducible proteinase inhibitor activity. These results point to a need for the integration of theories that emphasize physiological influences on secondary metabolism with those that emphasize ecological influences on secondary metabolism and suggest that current theories of plant defense do not adequately account for enzymatic and proteinaceous defenses against arthropods.     

The Significance of Ectomycorrhizas in Chemical Quality of Silver Birch Foliage and Above-Ground Insect Herbivore Performance

We tested whether the ectomycorrhizal (ECM) infection level of roots of silver birch (Betula pendula) affects performance of above-ground insect herbivores by increasing available plant biomass, by enhancing availability of nutrients, or by modifying concentration of defense compounds, i.e., phenolics, in birch foliage. Insect performance was determined for a phloem-feeding generalist (Lygus rugulipennis, the European tarnished plant bug), a phloem-feeding specialist (Calaphis flava, the birch aphid), and a chewing generalist (Epirrita autumnata, the autumnal moth larva). Silver birch plantlets had either natural ECM infection level (on average 24% of short roots with ECM), reduced ECM infection level with fungicide (F−, 9% ECM), or enhanced ECM infection level after inoculation with the fungus Paxillus involutus (PI+, 45% ECM) or Leccinum versipelle (LV+, 42% ECM). In general, the most pronounced effect of ECM was observed on growth of plantlets, i.e., stem growth decreased. In PI+-treated plants, leaf biomass also decreased. The effect of mycorrhizal colonization on the host plant’s nitrogen (N) and phosphorous (P) concentration was dependent on the mycorrhizal species and experiment. Fungicide treatment did not cause a consistent decrease in nutrients. Finally, defense of birch against herbivory, expressed as foliar phenolic concentration in plantlets, was not modified by ECM. However, E. autumnata had a significantly higher relative growth rate on PI+ plantlets with high leaf N concentration than on LV+ plantlets with low leaf N concentration. The birch aphid C. flava produced significantly less nymphs on birches with enhanced ECM infection levels (PI+ and LV+ plantlets) than on controls. In summary, our data show that the ECM infection level mainly affects the growth parameters of plantlets, whereas effects on leaf chemical quality are minor. Our data show that effects of ECM infection of birch roots on aboveground herbivores are multifaceted and depend on the fungal species forming ectomycorrhiza and also on the degree of specialization and feeding guild of insects.