Damage-induced changes in woody plants and their effects on insect herbivore performance: a meta-analysis

A genetic basis for the manipulation of sink-source relationships by the galling aphid Pemphigus batae

We examined how the galling aphid Pemphigus batae manipulates resource translocation patterns of resistant and susceptible narrowleaf cottonwood Populus angustifolia. Using carbon-14 ((14)C)-labeling experiments in common garden trials, five patterns emerged. First, although aphid galls on resistant and susceptible genotypes did not differ in their capacity to intercept assimilates exported from the leaf they occupied, aphids sequestered 5.8-fold more assimilates from surrounding leaves on susceptible tree genotypes compared to resistant genotypes. Second, gall sinks on the same side of a shoot as a labeled leaf were 3.4-fold stronger than gall sinks on the opposite side of a shoot, which agrees with patterns of vascular connections among leaves of the same shoot (orthostichy). Third, plant genetic-based traits accounted for 26% of the variation in sink strength of gall sinks and 41% of the variation in sink strength of a plant's own bud sinks. Fourth, tree susceptibility to aphid gall formation accounted for 63% of the variation in (14)C import, suggesting strong genetic control of sink-source relationships. Fifth, competition between two galls was observed on a susceptible but not a resistant tree. On the susceptible tree distal aphids intercepted 1.5-fold more (14)C from the occupied leaf than did basal aphids, but basal aphids compensated for the presence of a distal competitor by almost doubling import to the gall from surrounding leaves. These findings and others, aimed at identifying candidate genes for resistance, argue the importance of including plant genetics in future studies of the manipulation of translocation patterns by phytophageous insects.

Effect of clone selection, nitrogen supply, leaf damage and mycorrhizal fungi on stilbene and emodin production in knotweed

BACKGROUND

Fallopia japonica and its hybrid, F. xbohemica, due to their fast spread, are famous as nature threats rather than blessings. Their fast growth rate, height, coverage, efficient nutrient translocation between tillers and organs and high phenolic production, may be perceived either as dangerous or beneficial features that bring about the elimination of native species or a life-supporting source. To the best of our knowledge, there have not been any studies aimed at increasing the targeted production of medically desired compounds by these remarkable plants. We designed a two-year pot experiment to determine the extent to which stilbene (resveratrol, piceatannol, resveratrolosid, piceid and astringins) and emodin contents of F. japonica, F. sachalinensis and two selected F. xbohemica clones are affected by soil nitrogen (N) supply, leaf damage and mycorrhizal inoculation.

RESULTS

1) Knotweeds are able to grow on substrates with extremely low nitrogen content and have a high efficiency of N translocation. The fast-spreading hybrid clones store less N in their rhizomes than the parental species. 2) The highest concentrations of stilbenes were found in the belowground biomass of F. japonica. However, because of the high belowground biomass of one clone of F. xbohemica, this hybrid produced more stilbenes per plant than F. japonica. 3) Leaf damage increased the resveratrol and emodin contents in the belowground biomass of the non-inoculated knotweed plants. 4) Although knotweed is supposed to be a non-mycorrhizal species, its roots are able to host the fungi. Inoculation with mycorrhizal fungi resulted in up to 2% root colonisation. 5) Both leaf damage and inoculation with mycorrhizal fungi elicited an increase of the piceid (resveratrol-glucoside) content in the belowground biomass of F. japonica. However, the mycorrhizal fungi only elicited this response in the absence of leaf damage. Because the leaf damage suppressed the effect of the root fungi, the effect of leaf damage prevailed over the effect of the mycorrhizal fungi on the piceid content in the belowground biomass.

CONCLUSIONS

Two widely spread knotweed species, F. japonica and F. xbohemica, are promising sources of compounds that may have a positive impact on human health. The content of some of the target compounds in the plant tissues can be significantly altered by the cultivation conditions including stress imposed on the plants, inoculation with mycorrhizal fungi and selection of the appropriate plant clone.

Use of quality control charts for detection of outliers and temporal trends in cumulative meta-analysis

Cumulative meta-analysis (CMA) aims to aggregate accumulating evidence. Essentially a visual tool, CMA should be supplemented by formal statistical methods for assessment of the significance of the accumulating evidence, and for detection of temporal trends in effect sizes. These methods should also take into account multiple testing inherent in CMA. We review the existing methods for detection of temporal trends in effect sizes and suggest a new approach, namely the use of standard quality control (QC) charts, in particular X charts and CUSUM charts, to detect possible outliers and trends over time. We discuss the application of the QC charts to four popular measures of effect size: the odds ratios, the relative risks, the correlation coefficients and the standardized mean differences. Applications of QC charts are illustrated by three meta-analysis examples from medicine, ecology and evolutionary biology. Copyright © 2011 John Wiley & Sons, Ltd.

The effect of disturbance on an ant-plant mutualism

Protective ant–plant mutualisms—where plants provide food or shelter to ants and ants protect the plants from herbivores—are a common feature in many ecological communities, but few studies have examined the effect of disturbance on these interactions. Disturbance may affect the relationship between plants and their associated ant mutualists by increasing the plants’ susceptibility to herbivores, changing the amount of reward provided for the ants, and altering the abundance of ants and other predators. Pruning was used to simulate the damage to buttonwood mangrove (Conocarpus erectus) caused by hurricanes. Pruned plants grew faster than unpruned plants, produced lower levels of physical anti-herbivore defenses (trichomes, toughness), and higher levels of chemical defenses (tannins) and extrafloral nectaries. Thus, simulated hurricane damage increased plant growth and the amount of reward provided to ant mutualists, but did not have consistent effects on other anti-herbivore defenses. Both herbivores and ants increased in abundance on pruned plants, indicating that the effects of simulated hurricane damage on plant traits were propagated to higher trophic levels. Ant-exclusion led to higher leaf damage on both pruned and upruned plants. The effect of ant-exclusion did not differ between pruned and unpruned plants, despite the fact that pruned plants had higher ant and herbivore densities, produced more extrafloral nectaries, and had fewer physical defenses. Another common predator, clubionid spiders, increased in abundance on pruned plants from which ants had been excluded. I suggest that compensatory predation by these spiders diminished the effect of ant-exclusion on pruned plants.

Induced chemical defenses in a freshwater macrophyte suppress herbivore fitness and the growth of associated microbes

The freshwater macrophyte Cabomba caroliniana induces a chemical defense when attacked by either the crayfish Procambrus clarkii or the snail Pomacea canaliculata. Induction by either consumer lowers the palatability of the plant to both consumers. When offered food ad libitum, snails feeding on non-induced C. caroliniana grew 2.6-2.7 times more than those feeding on induced C. caroliniana. Because snails fed less on induced plants, this could be a behavioral effect (reduced feeding), a physiological effect of the induced metabolites on the consumer, or both. To assess these possibilities, we made artificial diets with lipid extracts of induced versus non-induced C. caroliniana and restricted control snails to consuming only as much as treatment snails consumed. Growth measured as shell diameter was significantly lower on the diet containing extract from induced, as opposed to non-induced, plants; change in snail mass was more variable and showed a similar, but non-significant, trend. Thus, snails may reduce feeding on induced plants to avoid suppression of fitness. The induced defenses also suppressed growth of co-occurring microbes that might attack the plant through herbivore-generated feeding scars. When two bacteria and three fungi isolated from C. caroliniana surfaces were cultured with the lipid extract from induced and non-induced C. caroliniana, both extracts inhibited the microbes, but the induced extract was more potent against three of the five potential pathogens. Thus, induced plant defenses can act against both direct consumers and microbes that might invade the plant indirectly through herbivore-generated wounds.

Effects of sap-feeding insect herbivores on growth and reproduction of woody plants: a meta-analysis of experimental studies

The majority of generalisations concerning plant responses to herbivory are based on studies of natural or simulated defoliation. However, effects caused by insects feeding on plant sap are likely to differ from the effects of folivory. We assessed the general patterns and sources of variation in the effects of sap feeding on growth, photosynthesis, and reproduction of woody plants through a meta-analysis of 272 effect sizes calculated from 52 papers. Sap-feeders significantly reduced growth (-29%), reproduction (-17%), and photosynthesis (-27%); seedlings suffered more than saplings and mature trees. Deciduous and evergreen woody plants did not differ in their abilities to tolerate damage imposed by sap-feeders. Different plant parts, in particular below- and above-ground organs, responded similarly to damage, indicating that sap-feeders did not change the resource allocation in plants. The strongest effects were caused by mesophyll and phloem feeders, and the weakest by xylem feeders. Generalist sap-feeders reduced plant performance to a greater extent than did specialists. Methodology substantially influenced the outcomes of the primary studies; experiments conducted in greenhouses yielded stronger negative effects than field experiments; shorter (<12 months) experiments showed bigger growth reduction in response to sap feeding than longer experiments; natural levels of herbivory caused weaker effects than infestation of experimental plants by sap-feeders. Studies conducted at higher temperatures yielded stronger detrimental effects of sap-feeders on their hosts. We conclude that sap-feeders impose a more severe overall negative impact on plant performance than do defoliators, mostly due to the lower abilities of woody plants to compensate for sap-feeders' damage in terms of both growth and photosynthesis.

Induced resistance in a brown alga: phlorotannins, genotypic variation and fitness costs for the crustacean herbivore

In the marine littoral, strong grazing pressure selects for macroalgal defenses such as the constitutive and inductive production of defense metabolites. Induced defenses are expected under spatiotemporally varying grazing pressure and should be triggered by a reliable cue from herbivory, thereby reducing grazing pressure via decreased herbivore preference and/or performance. Although induced resistance has frequently been demonstrated in brown macroalgae, it is yet to be investigated whether induced macroalgal resistance shows genetic variation, a prerequisite for evolutionary responses to selection. In addition, consequences of induced resistance on herbivore performance have rarely been tested while the role of brown algal phlorotannins as inducible defense metabolites remains ambiguous. Using preference bioassays, we tested various cues, e.g., natural grazing, waterborne cues or simulated grazing to induce resistance in the brown alga Fucus vesiculosus. Further, we investigated whether there are induced responses in phlorotannin content, genetic variation in induced resistance or incurred performance costs to the mesoherbivore isopod, Idotea baltica. We found that both direct grazing and waterborne grazing cues decreased the palatability of F. vesiculosus, while increasing the total phlorotannin content. Since the sole presence of the herbivore also increased the total soluble phlorotannins, yet failed to stimulate deterrence, we concluded that phlorotannins alone do not explain increased resistance. Induced resistance varied between algal genotypes and thus showed potential for evolutionary responses to variation in grazing pressure. Induced resistance also incurred performance costs for female I. baltica via reduced egg production. Our results show that the induced resistance of F. vesiculosus decreases grazing pressure by deterring herbivores as well as impairing their performance. Resistance may be induced in advance by waterborne cues and spread effectively throughout the F. vesiculosus belt. Through lowering herbivore performance, induced resistance may also reduce future grazing pressure by decreasing the population growth rate of I. baltica.

Induced accumulation of phenolics and sawfly performance in Scots pine in response to previous defoliation

Phenolic compounds often accumulate in foliar tissues of deciduous woody plants in response to previous insect defoliation, but similar responses have been observed infrequently in evergreen conifers. We studied the effects of defoliation on the foliar chemistry of Scots pine (Pinus sylvestris L.) and cocoon mass, and survival of the pine sawfly (Diprion pini L.). In two successive years, needles were excised early in the season leaving only the current-year shoot intact (defoliated trees); untreated entire shoots served as controls (control trees). A year after the second defoliation, pine sawfly larvae were transferred to the trees. Delayed induced resistance in Scots pine in response to defoliation was indicated by (1) reduced cocoon mass in defoliated trees and (2) increased concentrations of phenolics and soluble condensed tannins in the foliage of defoliated trees compared with controls. Myricetin-3-galactoside, which showed the strongest induced response (104% and 71% increase in current-year (C) and previous-year (C+1) needles) of the compounds analyzed, also entered the regression model explaining variation in sawfly performance. Other compounds that entered the model, e.g., (+)-catechin, showed weaker responses to defoliation than myricetin-3-galactoside. Hyperin, condensed tannins and quercitrin showed strong induced responses in C or C+1 needles, or both, but these compounds did not explain the variation in sawfly performance. Accumulation of phenolics is sometimes associated with the reduced foliage nitrogen (N) concentrations in deciduous trees, and our results suggest that this may also be the case in evergreen conifers. Based on the earlier findings that defoliation reduces needle N concentration and N deficiency results in the accumulation of the same phenolic compounds, i.e., myricetin and quercetin glycosides, and soluble condensed tannins, we suggest that the accumulation of phenolics in defoliated trees occurred in response to the reduced foliar N concentration.

Methyl jasmonate does not induce changes in Eucalyptus grandis leaves that alter the effect of constitutive defences on larvae of a specialist herbivore

The up-regulation of secondary metabolic pathways following herbivore attack and the subsequent reduction in herbivore performance have been identified in numerous woody plant species. Eucalypts constitutively express many secondary metabolites in the leaves, including terpenes and formylated phloroglucinol compounds (FPCs). We used clonal ramets from six clones of Eucalyptus grandis and two clones of E. grandis x camaldulensis to determine if methyl jasmonate (MeJA) treatment could induce changes in the foliar concentrations of either of these groups of compounds. We also used bioassays to determine if any changes in the performance of larvae of Paropsis atomaria, a chrysomelid leaf beetle, could be detected in treated ramets versus the untreated controls, thus indicating whether MeJA induced the up-regulation of defences other than terpenes or FPCs. We found no significant effects of MeJA treatment on either the foliar concentrations of terpenes and FPCs or on herbivore performance. We did, however, detect dramatic differences in larval performance between Eucalyptus clones, thereby demonstrating large variations in the levels of constitutive defence. Larval feeding on clones resistant to P. atomaria resulted in high first instar mortality and disruption of normal gregarious feeding behaviour in surviving larvae. Histological examination of larvae feeding on a resistant clone revealed damage to the midgut consistent with the action of a toxin. These findings concur with mounting evidence that most evergreen perennial plants lack foliar-induced defences and suggest that constitutively expressed secondary metabolites other than those commonly examined in studies of interactions between insect herbivores and Eucalyptus may be important in plant defence.

Herbivore-induced shifts in carbon and nitrogen allocation in red oak seedlings

* A dual-isotope, microcosm experiment was conducted with Quercus rubra (red oak) seedlings to test the hypothesis that foliar herbivory would increase belowground carbon allocation (BCA), carbon (C) rhizodeposition and nitrogen (N) uptake. Plant BCA links soil ecosystems to aboveground processes and can be affected by insect herbivores, though the extent of herbivore influences on BCA is not well understood in woody plants. * Microcosms containing 2-yr-old Q. rubra seedlings and soil collected from the Coweeta Hydrologic Laboratory (NC, USA) were subjected to herbivory or left as undamaged controls. All microcosms were then injected with 15N-glycine and pulsed with 13CO2. * Contrary to our hypothesis, herbivore damage reduced BCA to fine roots by 63% and correspondingly increased allocation of new C to foliage. However, 13C recoveries in soil pools were similar between treatments, suggesting that exudation of C from roots is an actively regulated component of BCA. Herbivore damage also reduced N allocation to fine roots by 39%, apparently in favor of storage in taproot and stem tissues. * Oak seedlings respond to moderate insect herbivore damage with a complex suite of allocation shifts that may simultaneously increase foliar C, maintain C rhizodeposition and N assimilation, and shift N resources to storage.

Why plant volatile analysis needs bioinformatics--detecting signal from noise in increasingly complex profiles

Plant volatile analysis may be the oldest form of what now is called plant "metabolomic" analysis. A wide array of volatile organic compounds (VOCs), such as alkanes, alcohols, isoprenoids, and esters, can be collected simultaneously from the plant headspace, either within the laboratory or in the field. Increasingly faster and more sensitive analysis techniques allow detection of an ever-growing number of compounds in decreasing concentrations. However, the myriads of data becoming available from such experiments do not automatically increase our ecological and evolutionary understanding of the roles these VOCs play in plant-insect interactions. Herbivores and parasitoids responding to changes in VOC emissions are able to perceive minute changes within a complex VOC background. Plants modified in genes involved in VOC synthesis may be valuable for the evaluation of changes in plant-animal interactions compared to tests with synthetic compounds, as they allow changes to be made within the context of a more complex profile. We argue that bioinformatics is an essential tool to integrate statistical analysis of plant VOC profiles with insect behavioural data. The implementation of statistical techniques such as multivariate analysis (MVA) and meta-analysis is of the utmost importance to interpreting changes in plant VOC mixtures. MVA focuses on differences in volatile patterns rather than in single compounds. Therefore, it more closely resembles the information processing in insects that base their behavioural decisions on differences in VOC profiles between plants. Meta-analysis of different datasets will reveal general patterns pertaining to the ecological role of VOC in plant-insect interactions. Successful implementation of bioinformatics in VOC research also includes the development of MVA that integrate time-resolved chemical and behavioural analyses, as well as databases that link plant VOCs to their effects on insects.

Plant defense, growth, and habitat: a comparative assessment of constitutive and induced resistance

The growth rate (GR) hypothesis relates the evolution of plant defense to resource availability and predicts that plants that have evolved in abiotically stressful environments grow inherently more slowly and are more constitutively resistant to herbivory than plants from more productive habitats. Stress-adapted plants are also predicted to have reduced inducibility, but this prediction has not been previously tested. To evaluate this hypothesis, I compared the growth of nine species of herbaceous plants from Missouri glade habitats to congeners from more productive non-glade habitats. I also conducted bioassays using larvae of the generalist herbivore Spodoptera exigua to estimate constitutive and inducible resistance in these congeners. Glade congeners tended to grow more slowly and have higher constitutive resistance and lower inducibility than non-glade species. However, none of these comparisons was statistically significant due to the conflicting response of one congeneric pair (Salvia azurea and S. lyrata). Analyses without this genus were consistent with the GR hypothesis, as were analyses that categorized congeners by relative growth rate. These results highlight the complexity in searching for factors that determine plant growth rates and resistance traits across multiple genera and support the hypothesis that both constitutive and induced resistance may be influenced by selection on traits that alter plant growth rates. Future studies should attempt to determine whether variation in inducibility is better explained by habitat or relative plant growth rates.

Proteomic profiling of aphid Macrosiphum euphorbiae responses to host-plant-mediated stress induced by defoliation and water deficit

Abiotic and biotic host-plant stress, such as desiccation and herbivory, may strongly affect sap-sucking insects such as aphids via changes in plant chemicals of insect nutritional or plant defensive value. Here, we examined (i) water deprivation and (ii) defoliation by the beetle Leptinotarsa decemlineata as stresses indirectly affecting the aphid Macrosiphum euphorbiae via its host plant Solanum tuberosum. For plant-induced stress, aphids were reared on healthy vs. continuously stressed potato for 14 days (no watering; defoliation maintained at approximately 40%). Aphid performance under stress was correlated with metabolic responses monitored by profiling of the aphid proteome. M. euphorbiae was strongly affected by water stress, as adult survival, total aphid number and biomass were reduced by 67%, 64%, and 79%, respectively. Aphids performed normally on defoliated potato, indicating that they were unaffected or able to compensate any stress induced by plant defoliation. Stressed aphid proteomes revealed 419-453 protein spots, including 27 that were modulated specifically or jointly under each kind of host-plant stress. Reduced aphid fitness on water-stressed plants mostly correlated with modulation of proteins involved in energy metabolism, apparently to conserve energy in order to prioritize survival. Despite normal performance, several aphid proteins that are known to be implicated in cell communication were modulated on defoliated plants, possibly suggesting modified aphid behaviour. The GroEL protein (or symbionin) of the endosymbiont Buchnera aphidicola was predominant under all conditions in M. euphorbiae. Its expression level was not significantly affected by aphid host-plant stresses, which is consistent with the high priority of symbiosis in stressed aphids.

Browse Quality in Quaking Aspen (Populus tremuloides): Effects of Genotype, Nutrients, Defoliation, and Coppicing

The consequences of interactions among genetic, ontogenetic, and environmental factors for the quality of winter-dormant tissues as food for browsing herbivores is poorly understood. We conducted two sequential common garden studies to assess the impacts of intraspecific genetic variation, nutrient availability, prior defoliation, and ontogenetic stage on the chemical quality of winter-dormant tissue in quaking aspen (Populus tremuloides Michx.). In the first study, saplings of 12 aspen genotypes were grown under low and high soil nutrient conditions, with or without two successive seasons of defoliation. Quantity and quality of current year's twig growth were assessed. Twig production varied among genotypes and declined under low nutrient availability, but showed little response to prior defoliation. Chemical quality of sapling twigs varied substantially among genotypes, and in response to nutrient availability and prior defoliation. Overall, browse quality improved (nitrogen levels increased while phenolic glycoside and condensed tannin levels decreased) after defoliation. Growth and chemical variables exhibited low to moderate clonal repeatability (broad sense heritability) values. Our second study employed the same 12 genotypes, grown under high-nutrient conditions and with or without two seasons of defoliation. The trees were coppiced to produce root sprouts, which were chemically assessed 1 yr later. Rejuvenation via coppicing led to increased levels of nitrogen, phenolic glycosides (salicortin), and tannins in root sprouts, and the magnitude of change varied among aspen genotypes. Signatures of defoliation nearly 2 yr earlier persisted in terms of elevated levels of phenolic glycosides in root sprouts of previously defoliated trees. Aspen forests likely present browsing herbivores with chemically heterogeneous environments because of the interactions of genetic, ontogenetic, and environmental factors that vary over space and time.

Leaf litter decomposition differs among genotypes in a local Betula pendula population

Ecosystem processes, such as plant litter decomposition, are known to be partly genetically determined, but the magnitude of genetic variation within local populations is still poorly known. We used micropropagated field-grown saplings of 19 Betula pendula genotypes, representing genetic variation in a natural birch population, to examine (1) whether genotype can explain variation in leaf litter decomposition within a local plant population, and (2) whether genotypic variation in litter decomposition is associated with genotypic variation in other plant attributes. We found that a local B. pendula population can have substantial genotypic variation in leaf litter mass loss at the early stages of the decomposition process and that this variation can be associated with genotypic variation in herbivore resistance and leaf concentrations of soluble proteins and total nitrogen (N). Our results are among the first to show that fundamental ecosystem processes can be significantly affected by truly intraspecific genetic variation of a plant species.

Interactions between gray-sided voles (Clethrionomys rufucanus) and bilberry (Vaccinium myrtillus), their main winter food plant

We compared the abundance, population structure and palatability of bilberry ramets on vole-free islands, islands with voles but no predators (predator-free islands) and mainland sites with both voles and predators. As expected, bilberry biomass was strongly correlated with the herbivory pressure exerted by the voles, since it was significantly lower on the mainland, and much (>80%) lower on the predator-free islands, than on the vole-free islands. However, another finding, which conflicts with hypotheses postulating that herbivory generally induces plant defenses, was that voles preferred ramets from predator-free islands. Bilberry plants were fairly tolerant to grazing since they compensated for some of the lost tissue by producing more new ramets. This response should promote stability in the plant-herbivore interaction by reducing the impact of past grazing on current food production and thus minimizing time delays in the interactions that could potentially generate population cycles.

Simulated winter browsing may lead to induced susceptibility of willows to beavers in spring

Browsing may lead to an induced resistance or susceptibility of the plant to the herbivore. We tested the effect of winter browsing by Eurasian beavers ( Castor fiber L., 1758) on food quality of holme willows ( Salix dasyclados Wimm.) in and after the following growth season. Shrubs were pruned in February, and new shoots from these (cut) shrubs were compared with those of untreated (uncut) ones in May and November. The shoots were analysed for dry matter, nitrogen, acid detergent fibre, and total phenolics. In May, the leaves from the cut treatment had a better food quality (more water, more nitrogen, and less phenolics) than those from the uncut one. There was in part also a systemic response, with lower total phenolics in both the cut and untreated parts of pruned shrubs (uncut–cut) than in the uncut shrubs. In November, we did not find significant differences in biochemistry of bark among cut, uncut, or uncut–cut treatments. These results are in accordance with a cafeteria experiment in the field: in May the beavers preferred shoots from the cut treatment, but in November they showed no preference. The results suggest that willows invest in compensatory growth rather than a defence response early in the regrowing phase.

Recycling of nitrogen in herbivore feces: plant recovery, herbivore assimilation, soil retention, and leaching losses

Herbivores directly and indirectly affect ecosystem functioning in forests. Feces deposition is a direct effect that supplies ephemeral N pulses to soils. Herbivore-mediated changes in plant N allocation and uptake are indirect effects that can also influence soil N availability. These effects may interact if defoliation influences the ability of plants to recover fecal N, and this may affect subsequent generations of herbivores. We added (15)N-enriched insect feces (frass) to a series of replicated red oak, Quercus rubra, mesocosms that had been damaged experimentally and then followed the frass N over the course of 2 years. In the first season, some frass N was mineralized in the soil and leached in organic form from the mesocosms within 1 week of deposition. Within 1 month, frass N had been acquired by the oaks and enriched the foliage; late-season herbivores assimilated the frass N within the same growing season. In the second season, herbivore damage from the previous year lowered total leaf N contents and (15)N recovered in the foliage. A subsequent cohort of early-season herbivores fed on this foliage consequently derived less of their N from the previous year's frass, and feral leaf rollers colonized fewer of these saplings. The 0- to 5-cm soil fraction was the largest N sink measured, and 42% of the frass N was recovered in the soil. The results demonstrate that: (1) some frass N can be recycled rapidly into foliage and assimilated by successive cohorts of herbivore within the same season; (2) damage can affect N allocation in the following year's foliage, influencing N availability to and host selection by herbivores; and (3) leaching losses occur soon after deposition but are buffered by soil pools, which are the largest sinks for frass N.

Shoot development and extension of Quercus serrata saplings in response to insect damage and nutrient conditions

BACKGROUND AND AIMS

Plants have the ability to compensate for damage caused by herbivores. This is important to plant growth, because a plant cannot always avoid damage, even if it has developed defence mechanisms against herbivores. In previous work, we elucidated the herbivory-induced compensatory response of Quercus (at both the individual shoot and whole sapling levels) in both low- and high-nutrient conditions throughout one growing season. In this study, we determine how the compensatory growth of Quercus serrata saplings is achieved at different nutrient levels.

METHODS

Quercus serrata saplings were grown under controlled conditions. Length, number of leaves and percentage of leaf area lost on all extension units (EUs) were measured.

KEY RESULTS

Both the probability of flushing and the length of subsequent EUs significantly increased with an increase in the length of the parent EU. The probability of flushing increased with an increase in leaf damage of the parent EU, but the length of subsequent EUs decreased. This indicates that EU growth is fundamentally regulated at the individual EU level. The probabilities of a second and third flush were significantly higher in plants in high-nutrient soil than those in low-nutrient soil. The subsequent EUs of damaged saplings were also significantly longer at high-nutrient conditions.

CONCLUSIONS

An increase in the probability of flushes in response to herbivore damage is important for damaged saplings to produce new EUs; further, shortening the length of EUs helps to effectively reproduce foliage lost by herbivory. The probability of flushing also varied according to soil nutrient levels, suggesting that the compensatory growth of individual EUs in response to local damage levels is affected by the nutrients available to the whole sapling.

Direct and indirect chemical defence of pine against folivorous insects

The chemical defence of pine against herbivorous insects has been intensively studied with respect to its effects on the performance and behaviour of the herbivores as well as on the natural enemies of pine herbivores. The huge variety of terpenoid pine components play a major role in mediating numerous specific food web interactions. The constitutive terpenoid pattern can be adjusted to herbivore attack by changes induced by insect feeding or oviposition activity. Recent studies on folivorous pine sawflies have highlighted the role of induced pine responses in herbivore attack and have demonstrated the importance of analysing the variability of pine defence and its finely tuned specificity with respect to the herbivore attacker in a multitrophic context.

The evolutionary consequences of ecological interactions mediated through phenotypic plasticity

Phenotypic plasticity describes the capacity of a genotype to exhibit a range of phenotypes in response to variation in the environment. Environmental variation encompasses both abiotic and biotic components of the environment,including interactions among organisms. The strength and outcome of many ecological interactions, ranging from antagonism to mutualism, are mediated through the phenotypically plastic responses of one or more players in the interaction. Herein, three broadly defined, non-mutually exclusive,evolutionary consequences of ecological interactions mediated through phenotypic plasticity are discussed. (1) The predictable plastic response of one partner can favor behaviors, physiological responses, and life history traits of an interacting partner that manipulate, circumvent, or ameliorate the response of that partner. (2) Phenotypic plasticity can generate substantial spatial and temporal variation within and among populations. Such phenotypic variation can depend on the density and identity of interacting players in an ecological community, and can ultimately affect the evolutionary outcome of ecological interactions. (3) Phenotypic plasticity affects the strength and direction of natural selection. Ecological interactions mediated through phenotypic plasticity are ubiquitous in nature, and the potential evolutionary consequences of these interactions illustrate the complexity inherent in understanding evolution in a community context.

Plant-mediated effects in insect-pathogen interactions

Interactions between insect herbivores and their pathogens can be modulated by host plants. Inter- and intraspecific differences in plant chemistry and structure can alter the susceptibility of insects to infection and the production and environmental persistence of pathogens. Whether plants can manipulate insect pathogens to act as "bodyguards" and increase their own fitness remains to be shown. Reduced insect performance owing to poor plant quality can enhance the susceptibility of an insect to disease while these same phytochemicals can also reduce the effectiveness of entomopathogens in killing the host. As we discuss here, plants have an important role in the evolution of insect-pathogen relationships and a tritrophic perspective should thus be incorporated into the study of insects and their pathogens.

The differential effects of herbivory by first and fourth instars of Trichoplusia ni (Lepidoptera: Noctuidae) on photosynthesis in Arabidopsis thaliana

The effect of different feeding behaviours of 1st and 4th instar Trichoplusia ni on photosynthesis of Arabidopsis thaliana var. Columbia was characterized using spatially resolved measurements of fluorescence and leaf temperature, as well as leaf gas exchange,. First instars made small holes with a large perimeter-to-area ratio and avoided veins, while 4th instars made large holes with a low perimeter-to-area ratio and consumed veins. Herbivory by 1st instars reduced photosynthesis more strongly in the remaining leaf tissue than that by 4th instars. Photosystem II operating efficiency (PhiPSII) was correlated with the rate of CO2 exchange, and reductions in PhiPSII in areas around the missing tissues contributed to a 15.6% reduction in CO2 assimilation on the first day following removal of 1st instars. The corresponding increases in non-photochemical quenching and greater rates of non-stomatal water loss from these regions, as well as the partial reversal of low PhiPSII by increasing the ambient CO2 concentration, suggests that localized water stress and reduced stomatal conductance contributed to the inhibition of photosynthesis. Damage by 1st but not 4th instars reduced the maximum quantum efficiency of photosystem II photochemistry (Fv/Fm) by 4-8%. While herbivory by both 1st and 4th instars increased dark respiration rates, the rates were too low to have contributed to the observed reductions in CO2 exchange. The small holes produced by 1st instars may have isolated patches of tissue from the vascular system thereby contributing to localized water stress. Since neither 1st nor 4th instar herbivory had a detectable effect on the expression of the Rubisco small subunit gene, the observed differences cannot be attributed to changes in expression of this gene. The mode of feeding by different instars of T. ni determined the photosynthetic response to herbivory, which appeared to be mediated by the level of water stress associated with herbivore damage.

Significance of flavonoids in plant resistance and enhancement of their biosynthesis

The roles of flavonoids in plant defence against pathogens, herbivores, and environmental stress are reviewed and their significant contribution to plant resistance is discussed. The induction of flavonoids is of particular interest for gathering evidence of their roles. Tools are mentioned which may enhance flavonoid biosynthesis and accumulation. These include metabolic engineering and UV light. The induction of defence-related flavonoids is modified by other determining factors and competition between growth and secondary metabolism may exist. In an evolutionary context, stress-related oxidative pressure may have been a major trigger for the distribution and abundance of flavonoids. UV protection is one of their most significant, or even the most significant, functional role for flavonoids. The multi-functionality of these compounds, however, often complicates the interpretation of experimental results but, overall, it supports the importance of flavonoids.