Changes in plant chemical defenses and nutritional quality as a function of ontogeny in Plantago lanceolata (Plantaginaceae)

Age-dependent resistance of a perennial herb, Aristolochia contorta against specialist and generalist leaf-chewing herbivores

Plants need to balance investments in growth and defense throughout their life to increase their fitness. To optimize fitness, levels of defense against herbivores in perennial plants may vary according to plant age and season. However, secondary plant metabolites often have a detrimental effect on generalist herbivores, while many specialists have developed resistance to them. Therefore, varying levels of defensive secondary metabolites depending on plant age and season may have different effects on the performance of specialist and generalist herbivores colonizing the same host plants. In this study, we analyzed concentrations of defensive secondary metabolites (aristolochic acids) and the nutritional value (C/N ratios) of 1^st-, 2^nd- and 3^rd-year Aristolochia contorta in July (the middle of growing season) and September (the end of growing season). We further assessed their effects on the performances of the specialist herbivore Sericinus montela (Lepidoptera: Papilionidae) and the generalist herbivore Spodoptera exigua (Lepidoptera: Noctuidae). Leaves of 1^st-year A. contorta contained significantly higher concentrations of aristolochic acids than those of older plants, with concentrations tending to decrease over the first-year season. Therefore, when first year leaves were fed in July, all larvae of S. exigua died and S. montela showed the lowest growth rate compared to older leaves fed in July. However, the nutritional quality of A. contorta leaves was lower in September than July irrespective of plant age, which was reflected in lower larval performance of both herbivores in September. These results suggest that A. contorta invests in the chemical defenses of leaves especially at a young age, while the low nutritional value of leaves seems to limit the performance of leaf-chewing herbivores at the end of the season, regardless of plant age.

Variation in the Accumulation of Phytochemicals and Their Bioactive Properties among the Aerial Parts of Cauliflower

Vegetables from the Brassicaceae family are excellent sources of bioactive phytochemicals and may reduce the risk of chronic diseases. Variation of phytochemicals in the edible part of cauliflower is known. However, information about the distribution of bioactive and nutritive compounds as well as antioxidant activity among aerial organs of cauliflower is unavailable. Therefore, this study aimed to evaluate the distribution of glucosinolates (GLS), phenolics, flavonoids, chlorophylls, nutritive compounds and antioxidant capacity between the aerial parts of the common variety of cauliflower and to evaluate whether these changes contribute to the differences in the antioxidant capacity between the plant organs. Our study showed that all the aerial organs of cauliflower are a rich source of health-promoting bioactive compounds, including GLS, phenolics and flavonoids, exhibiting antioxidant capacity. The highest contents of phytochemicals and the highest antioxidant capacity were found in leaves. Cauliflower organs were also found to be rich in nutritive compounds, including minerals, proteins and amino acids. Our study showed that the non-edible organs, such as stems and leaves, being neglected parts of cauliflower, if not consumed as the main ingredient, can be used as additives for developing new, functional foodstuff.

Plantago spp. as Models for Studying the Ecology and Evolution of Species Interactions across Environmental Gradients

AbstractA central challenge in ecology and evolutionary biology is to understand how variation in abiotic and biotic factors combine to shape the distribution, abundance, and diversity of focal species. Environmental gradients, whether natural (e.g., latitude, elevation, ocean proximity) or anthropogenic (e.g., land-use intensity, urbanization), provide compelling settings for addressing this challenge. However, not all organisms are amenable to the observational and experimental approaches required for untangling the factors that structure species along gradients. Here we highlight herbaceous plants in the genus Plantago as models for studying the ecology and evolution of species interactions along abiotic gradients. Plantago lanceolata and P. major are native to Europe and Asia but distributed globally, and they are established models for studying population ecology and interactions with herbivores, pathogens, and soil microbes. Studying restricted range congeners in comparison with those cosmopolitan species can provide insight into abiotic and biotic determinants of range size and population structure. We highlight one such species, P. rugelii, which is endemic to eastern North America. We give an overview of the literature on these focal Plantago species and explain why they are logical candidates for studies of species interactions across environmental gradients. Finally, we emphasize collaborative and community science approaches that can facilitate such research and note the amenability of Plantago for authentic research projects in science education.

Interactive Effects of Mycorrhizae, Soil Phosphorus, and Light on Growth and Induction and Priming of Defense in Plantago lanceolata

Increasing demands to reduce fertilizer and pesticide input in agriculture has triggered interest in arbuscular mycorrhizal fungi (AMF) that can enhance plant growth and confer mycorrhiza-induced resistance (MIR). MIR can be based on a variety of mechanisms, including induction of defense compounds, and sensitization of the plant's immune system (priming) for enhanced defense against later arriving pests or pathogens signaled through jasmonic acid (JA). However, growth and resistance benefits of AMF highly depend on environmental conditions. Low soil P and non-limiting light conditions are expected to enhance MIR, as these conditions favor AMF colonization and because of observed positive cross-talk between the plant's phosphate starvation response (PSR) and JA-dependent immunity. We therefore tested growth and resistance benefits of the AMF Funneliformis mosseae in Plantago lanceolata plants grown under different levels of soil P and light intensity. Resistance benefits were assessed in bioassays with the leaf chewing herbivore Mamestra brassicae. Half of the plants were induced by jasmonic acid prior to the bioassays to specifically test whether AMF primed plants for JA-signaled defense under different abiotic conditions. AMF reduced biomass production but contrary to prediction, this reduction was not strongest under conditions considered least optimal for carbon-for-nutrient trade (low light, high soil P). JA application induced resistance to M. brassicae, but its extent was independent of soil P and light conditions. Strikingly, in younger plants, JA-induced resistance was annulled by AMF under high resource conditions (high soil P, ample light), indicating that AMF did not prime but repressed JA-induced defense responses. In older plants, low soil P and light enhanced susceptibility to M. brassicae due to enhanced leaf nitrogen levels and reduced leaf levels of the defense metabolite catalpol. By contrast, in younger plants, low soil P enhanced resistance. Our results highlight that defense priming by AMF is not ubiquitous and calls for studies revealing the causes of the increasingly observed repression of JA-mediated defense by AMF. Our study further shows that in our system abiotic factors are significant modulators of defense responses, but more strongly so by directly modulating leaf quality than by modulating the effects of beneficial microbes on resistance.

The impact of Spodoptera exigua herbivory on Meloidogyne incognita-induced root responses depends on the nematodes' life cycle stages

Induced responses to above-ground and below-ground herbivores may interact via systemic signalling in plants. We investigated whether the impact of above-ground herbivory on root-knot nematode-induced responses depends on the nematode's life cycle stages. Tomato plants were infected with the nematode (Meloidogyne incognita) for 5, 15 or 30 days before receiving Spodoptera exigua caterpillars above-ground. We collected root materials after 24 h of caterpillar feeding. We investigated phytohormones and α-tomatine levels, and the expression of defence and glycoalkaloid metabolism (GAME) marker genes in tomato roots. Nematode infection alone increased the endogenous root levels of jasmonic acid (JA), salicylic acid (SA), abscisic acid (ABA), α-tomatine and the expression of the GLYCOALKALOID METABOLISM 1 (GAME1) gene mostly at 30 days post-nematode inoculation. Caterpillar feeding alone upregulated Lipoxygenase D and downregulated Basic-β-1-glucanase and GAME1 expression in roots. On nematode-infected plants, caterpillar feeding decreased JA levels, but it increased the expression of Leucine aminopeptidase A. The induction patterns of ABA and SA suggest that caterpillars cause cross-talk between the JA-signalling pathway and the SA and ABA pathways. Our results show that caterpillar feeding attenuated the induction of the JA pathway triggered by nematodes, mostly in the nematodes' reproduction stage. These results generate a better understanding of the molecular and chemical mechanisms underlying frequent nematode-plant-caterpillar interactions in natural and agricultural ecosystems.

Combined impacts of prolonged drought and warming on plant size and foliar chemistry

BACKGROUND AND AIMS

Future shifts in precipitation regimes and temperature are expected to affect plant traits dramatically. To date, many studies have explored the effects of acute stresses, but few have investigated the consequences of prolonged shifts in climatic conditions on plant growth and chemistry.

METHODS

Plant size and metabolite profiles were assessed on naturally occurring Plantago lanceolata plants growing under different precipitation (ambient, 50 % less than ambient = drought) and temperature (ambient, +0.8, +2.4 and +4.0 °C above ambient) treatments at the Boston Area Climate Experiment (constructed in 2007).

KEY RESULTS

The analysis of primary and secondary metabolites revealed pronounced effects of drought, and a precipitation × temperature interaction. Strikingly, the effects of precipitation were minimal at the two lower temperatures but marked at the two higher temperatures. Compared with the ambient condition, plants in the drought plots had lower concentrations of foliar nitrogen, amino acids and most sugars, and higher concentrations of sorbitol, citrate and malate, common stress-induced metabolites. This pattern was especially evident at high temperatures. Moreover, drought-exposed plants showed lower concentrations of catalpol, an iridoid glycoside.

CONCLUSIONS

While the effect of warming on the metabolite profiles was less pronounced, differences were marked when combined with drought. Given the interactive effect of environmental variables on leaf chemistry, and the fact that woody and herbaceous plants seem to differ in their responses to temperature and precipitation, future studies should account for the direct and indirect effects of the community response to multifactorial field conditions.

The Ecology of Plant Chemistry and Multi-Species Interactions in Diversified Agroecosystems

Over the past few years, our knowledge of how ecological interactions shape the structure and dynamics of natural communities has rapidly advanced. Plant chemical traits play key roles in these processes because they mediate a diverse range of direct and indirect interactions in a community-wide context. Many chemically mediated interactions have been extensively studied in industrial cropping systems, and thus have focused on simplified, pairwise and linear interactions that rarely incorporate a community perspective. A contrasting approach considers the agroecosystem as a functioning whole, in which food production occurs. It offers an opportunity to better understand how plant chemical traits mediate complex interactions which can enhance or hinder ecosystem functions. In this paper, we argue that studying chemically mediated interactions in agroecosystems is essential to comprehend how agroecosystem services emerge and how they can be guaranteed through ecosystem management. First, we discuss how plant chemical traits affect and are affected by ecological interactions. We then explore research questions and future directions on how studying chemical mediation in complex agroecosystems can help us understand the emergence and management of ecosystem services, specifically biological control and pollination.

Hemiparasites can transmit indirect effects from their host plants to herbivores

Parasitic plants can serve as critical intermediaries between their hosts and other organisms; however these relationships are not well understood. To investigate the relative importance of plant traits in such interactions, we studied the role of the root hemiparasite, Castilleja levisecta (Orobanchaceae), as a mediator of interactions between the host plants it parasitizes and the lepidopteran herbivore Euphydryas editha (Nymphalidae), whose caterpillars feed on Castilleja and sequester iridoid glycosides from it. We tested whether the hemiparasite's size, leaf N concentration, and iridoid glycoside concentrations were influenced by the identity of its host plant, and then whether these traits influenced outcomes for the herbivore. We found that the hemiparasite's size and leaf N depended on the host it parasitized, and these traits in turn affected outcomes for E. editha. Specifically, Euphydryas editha survival increased with hemiparasite size and caterpillar mass increased with leaf N; caterpillars with greater mass were more likely to survive during diapause. We also found preliminary evidence that host identity influenced iridoid glycoside sequestration by the herbivore. Mean iridoid glycoside concentrations in caterpillars ranged from 1-12% depending on the host being parasitized by Castilleja. This study demonstrates that root parasitism can result in strong indirect effects on higher trophic levels, influencing organisms' survival, growth, and chemical interactions.

Genetic stability of physiological responses to defoliation in a eucalypt and altered chemical defence in regrowth foliage

Defoliation may initiate physiological recovery and chemical defence mechanisms that allow a plant to improve fitness after damage. Such responses may result in changes in plant resource allocation that influence growth and foliar chemistry. In this study, we investigated the nature and stability of the defoliation response of juvenile plants from three divergent populations of Eucalyptus globulus Labill. A partial defoliation treatment that removed all upper crown leaves and the apical buds was applied to plants sourced from eight families from each of three populations representing contrasting chemical resistance to mammalian herbivory. Growth, photosynthetic rate and chlorophyll content were assessed pre-defoliation and periodically up to 12 weeks post-defoliation. The content of key plant primary and secondary metabolites was assessed pre-defoliation, at 12 weeks post-defoliation in the old foliage (positioned below the point of defoliation) and in the new foliage of the control plants and regrowth (from axillary buds) on the defoliated plants. There were clear treatment impacts on physiological responses, growth and foliar chemical traits, but despite significant constitutive differences in physiology, growth and chemistry the three E. globulus populations did not vary in their response to foliage loss. Distinct physiological responses to defoliation were observed with treatment plants showing significant up-regulation of photosynthetic rate and increased chlorophyll content in the old foliage remaining in the lower crown. There was a significant increase in the concentrations of a number of foliar chemical compounds in the regrowth arising from previously dormant axillary buds compared with new growth derived from apical meristems. There were changes in biomass allocation; defoliated plants had increased branching and leaf biomass, with changes in regrowth morphology to increase light capture. This study argues for multiple responses of E. globulus juveniles to defoliation involving apical bud loss, including elevated chemical defences matched with increased growth. From a chemical defence perspective, these responses create an enhanced chemical mosaic to the herbivore, with leaves remaining after partial browsing potentially being more palatable than the regrowth. This study demonstrates the multiple independent strategies plants may use to respond to partial defoliation and emphasizes the dynamic interplay between growth and defence in the recovery response.

Bottom-Up Mechanisms Generate the Same Temporal Pattern of Attack by a Specialist and a Generalist Caterpillar on Short-Lived Plants

The local population dynamics of insect herbivores in ephemeral patches of short-lived plants are poorly known. We investigated whether a specialist and a generalist caterpillar exhibit contrasting temporal patterns of attack during plant development and also assessed bottom-up forces related to plant ontogeny that govern such population trends. Immature stages of the polyphagous Trichoplusia ni (Hübner) and the oligophagous Plutella xylostella (L.) were sampled throughout the development of cabbage (Brassica oleracea L. var. capitata L.) crops. We measured protein and glucosinolate contents and insect performance with regard to plant age and leaf strata. The populations of both caterpillar species changed in close parallel throughout plant development, and a nonlinear temporal pattern of egg laying was reproduced in sequential population patterns of the larval stages until pupation. Reduced protein availability and insect performance coincided with a decline in egg laying and subsequent larval abundance in mature plants. By standardizing the plant size, we found that young and nutritious plants support proportionately more insects than large and mature plants. In our models of the population oscillations, the interaction between plant size and quality provided a strong causal explanation for the densities of both oligophagous and polyphagous caterpillars. Patches of fast-growing herbaceous plants are very common worldwide in the form of crop fields, and a generalized temporal pattern of attack may be widespread among caterpillars, regardless of their feeding specialization. Our results highlight the role of bottom-up forces in shaping the population dynamics of caterpillars in such systems.

Effects of the Timing of Herbivory on Plant Defense Induction and Insect Performance in Ribwort Plantain (Plantago lanceolata L.) Depend on Plant Mycorrhizal Status

Plants often are exposed to antagonistic and symbiotic organisms both aboveground and belowground. Interactions between above- and belowground organisms may occur either simultaneously or sequentially, and jointly can determine plant responses to future enemies. However, little is known about time-dependency of such aboveground-belowground interactions. We examined how the timing of a 24 h period of aboveground herbivory by Spodoptera exigua (1-8 d prior to later arriving conspecifics) influenced the response of Plantago lanceolata and the performance of later arriving conspecifics. We also examined whether these induced responses were modulated by the arbuscular mycorrhizal fungus (AMF) Funneliformis mosseae. The amount of leaf area consumed by later arriving herbivores decreased with time after induction by early herbivores. Mycorrhizal infection reduced the relative growth rate (RGR) of later arriving herbivores, associated with a reduction in efficiency of conversion of ingested food rather than a reduction in relative consumption rates. In non-mycorrhizal plants, leaf concentrations of the defense compound catalpol showed a linear two-fold increase during the eight days following early herbivory. By contrast, mycorrhizal plants already had elevated levels of leaf catalpol prior to their exposure to early herbivory and did not show any further increase following herbivory. These results indicate that AMF resulted in a systemic induction, rather than priming of these defenses. AMF infection significantly reduced shoot biomass of Plantago lanceolata. We conclude that plant responses to future herbivores are not only influenced by exposure to prior aboveground and belowground organisms, but also by when these prior organisms arrive and interact.

Does vegetation complexity affect host plant chemistry, and thus multitrophic interactions, in a human-altered landscape?

Anthropogenic land use may shape vegetation composition and affect trophic interactions by altering concentrations of host plant metabolites. Here, we investigated the hypotheses that: (1) plant N and defensive secondary metabolite contents of the herb Plantago lanceolata are affected by land use intensity (LUI) and the surrounding vegetation composition (=plant species richness and P. lanceolata density), and that (2) changes in plant chemistry affect abundances of the herbivorous weevils Mecinus pascuorum and Mecinus labilis, as well as their larval parasitoid Mesopolobus incultus, in the field. We determined plant species richness, P. lanceolata density, and abundances of the herbivores and the parasitoid in 77 grassland plots differing in LUI index in three regions across Germany. We also measured the N and secondary metabolite [the iridoid glycosides (IGs) aucubin and catalpol] contents of P. lanceolata leaves. Mixed-model analysis revealed that: (1) concentrations of leaf IGs were positively correlated with plant species richness; leaf N content was positively correlated with the LUI index. Furthermore: (2) herbivore abundance was not related to IG concentrations, but correlated negatively with leaf N content. Parasitoid abundance correlated positively only with host abundance over the three regions. Structural equation models revealed a positive impact of IG concentrations on parasitoid abundance in one region. We conclude that changes in plant chemistry due to land use and/or vegetation composition may affect higher trophic levels and that the manifestation of these effects may depend on local biotic or abiotic features of the landscape.

The influence of maturity and variety of potato plants on oviposition and probing of Bactericera cockerelli (Hemiptera: Triozidae)

The ecological theory on host plant choice by herbivores suggests that mothers should choose plants that will maximize their offspring's success. In annual host plants, physiology (and therefore host suitability) is sometimes influenced by maturity and growth stage, which may influence female choice. Potato plants were grown under greenhouse conditions and used in choice and no-choice bioassays to determine the effect of plant maturity and variety on oviposition and number of stylet sheaths (which approximate stylet insertions) by tomato/potato psyllids. No-choice bioassays suggested that maturity (time since planting) did not influence oviposition behavior, but oviposition varied significantly among potato plant varieties. There was a significant effect of both maturity and variety on the number of stylet sheaths, which peak toward the middle of the growing season. We also examined tomato/potato psyllid responses to plants grown in a commercial field and again found no effect on oviposition but differences in stylet sheaths. The results suggest that differential susceptibility to zebra chip disease may be associated with unequal feeding rates. Future studies should examine whether the maturity of plants influences larval fitness. Finally, potato variety has an influence on both oviposition and "probing," and has implications for management strategies and the development of resistant potato varieties.

Age versus stage: does ontogeny modify the effect of phosphorus and arbuscular mycorrhizas on above- and below-ground defence in forage sorghum?

Arbuscular mycorrhizas (AM) can increase plant acquisition of P and N. No published studies have investigated the impact of P and AM on the allocation of N to the plant defence, cyanogenic glucosides. We investigated the effects of soil P and AM on cyanogenic glucoside (dhurrin) concentration in roots and shoots of two forage sorghum lines differing in cyanogenic potential (HCNp). Two harvest times allowed plants grown at high and low P to be compared at the same age and the same size, to take account of known ontogenetic changes in shoot HCNp. P responses were dependent on ontogeny and tissue type. At the same age, P-limited plants were smaller and had higher shoot HCNp but lower root HCNp. Ontogenetically controlled comparisons showed a P effect of lesser magnitude, and that there was also an increase in the allocation of N to dhurrin in shoots of P-limited plants. Colonization by AM had little effect on shoot HCNp, but increased root HCNp and the allocation of N to dhurrin in roots. Divergent responses of roots and shoots to P, AM and with ontogeny demonstrate the importance of broadening the predominantly foliar focus of plant defence studies/theory, and of ontogenetically controlled comparisons.

Directed seed dispersal of Piper by Carollia perspicillata and its effect on understory plant diversity and folivory

Directed dispersal occurs when seeds are differentially deposited to sites where offspring survivorship is higher than at randomly chosen sites. Traditionally, characteristics of the dispersal target sites that could increase survivorship of the dispersed plants are thought to be intrinsic to the sites. If directed dispersal is constant over extended periods of time, however, it is likely that nonrandom patterns of dispersal could modify the ecological characteristics of the target site in ways that could increase survivorship and fitness of the dispersed plants. Here we report patterns of Piper diversity (richness, equitability, and similarity) and Piper folivory within plots near natural or artificial roosts of Carollia perspicillata vs. similar plots without bat roosts. Plots with bat roosts, both natural and artificial, had significantly higher Piper species diversity. Additionally, we found that plots with a higher Piper species diversity showed less specialist folivory, higher generalist folivory, and lower total herbivore leaf damage than plots with low Piper diversity. Finally, plots with bat roosts also showed less specialist folivory, lower generalist folivory, and lower total folivory when compared to plots without roosts. We propose that long-lasting nonrandom patterns of seed dispersal can change the local ecological characteristics of target sites via changes in plant diversity, and that these changes are likely to reduce the local rates of folivory and, therefore, increase seed and adult plant survivorship.

Impact of defoliation on the regrowth capacity and the shoot metabolite profile of Plantago lanceolata L

After defoliation challenges, plants should invest their resources in a way that maximizes their fitness, which may lead to trade-offs in investment in physiological versus chemical traits. Thereby, the regrowth capacity of plants may highly depend on the type and the severity of defoliation. In the present study, we investigated the phenotypic plasticity of Plantago lanceolata L. in response to different defoliation events in a comprehensive way, measuring various traits. This herbaceous species grows on ruderal sites and typically suffers from repeated substantial losses of shoot biomass due to mowing and/or herbivory during the growing season. To study treatment-specific effects of defoliation on resource allocation and induction of defence responses, plants were exposed either to (repeated) mechanical clipping or to herbivory by the generalist Grammia incorrupta (Lepidoptera). Next to regrowth and physiological leaf traits such as the water content, the specific leaf area and C/N ratios of these plants, primary and secondary metabolites in leaves were analyzed with a metabolite profiling approach using gas chromatography coupled with mass spectrometry. The different defoliation treatments strongly affected the regrowth capacity of clipped and herbivore-treated plants, leading to additive changes in physiological leaf traits. As a response to the defoliation treatments, clipped and herbivore-treated plants reduced mainly carbon-based primary metabolites such as sorbitol, and glucose, suggesting that the different defoliation challenges similarly limited the ability of carbon fixation. Yet, only in response to herbivory but not to clipping, plants induced defence compounds, which indicates the importance of treatment-specific responses despite severe resource limitations. Overall, the different responses to defoliation by clipping and herbivory may reflect allocation constraints and/or adaptive plasticity.

Role of plant β-glucosidases in the dual defense system of iridoid glycosides and their hydrolyzing enzymes in Plantago lanceolata and Plantago major

The typical defense compounds of Plantaginaceae are the iridoid glycosides, which retard growth and/or enhance mortality of non-adapted herbivores. In plants, glycosidic defense compounds and hydrolytic enzymes often form a dual defense system, in which the glycosides are activated by the enzymes to exert biological effects. Yet, little is known about the activating enzymes in iridoid glycoside-containing plants. To examine the role of plant-derived β-glucosidases in the dual defense system of two common plantain species, Plantago lanceolata and Plantago major, we determined the concentration of iridoid glycosides as well as the β-glucosidase activity in leaves of different age. To investigate the presence of other leaf metabolites potentially involved in plant defense, we used a metabolic fingerprinting approach with ultra-high performance liquid chromatography coupled with time-of-flight-mass spectrometry. According to the optimal defense hypothesis, more valuable parts such as young leaves should be better protected than less valuable parts. Therefore, we expected that both, the concentrations of defense compounds as well as the β-glucosidase activity, should be highest in younger leaves and decrease with increasing leaf age. Both species possessed β-glucosidase activity, which hydrolyzed aucubin, one of the two most abundant iridoid glycosides in both plant species, with high activity. In line with the optimal defense hypothesis, the β-glucosidase activity in both Plantago species as well as the concentration of defense-related metabolites such as iridoid glycosides correlated negatively to leaf age. When leaf extracts were incubated with bovine serum albumin and aucubin, SDS-PAGE revealed a protein-denaturing effect of the leaf extracts of both plantain species, suggesting that iridoid glycosides and plant β-glucosidase interact in a dual defense system.

Effects of insect herbivory on induced chemical defences and compensation during early plant development in Penstemon virgatus

BACKGROUND AND AIMS

The lack of studies assessing the simultaneous expression of tolerance and resistance traits during seedling development and overall seedling defences as compared with adult plants, in general, constitutes a significant research need that can greatly improve our understanding of overall investment in defences during plant ontogeny.

METHODS

Using two seedling and two juvenile stages of the perennial herb Penstemon virgatus (Plantaginaceae) evaluations were made of (a) patterns of investment in constitutive chemical defences [i.e. iridoid glycosides (IGs)], and (b) simultaneous variation in the short-term ability of seedling and juvenile stages to induce resistance traits, measured as induced chemical defences, or tolerance traits, measured as compensatory re-growth following moderate levels of damage by a specialist insect herbivore.

KEY RESULTS

Plants were highly defended during most of their transition from seedling to early juvenile stages, reaching a constant approx. 20 % dry weight total IGs. Furthermore, following 30 % above-ground tissue damage, seedlings and juvenile stages were equally able to induce resistance, by raising their IG concentration by approx. 8 %, whereas compensatory re-growth was only achieved at young juvenile but not seedling stages.

CONCLUSIONS

Two major trends emerged from this study: (1) in contrast to expected and previously observed trends, in this perennial plant species, seedlings seem to be one of the most well-defended stages as compared with adult ones; (2) high levels of constitutive defences did not limit the ability of young developmental stages to induce resistance following damage, although this response may come with a cost (i.e. decreased compensation) in young seedling stages. Hence, as has been previously demonstrated in few other systems, these results points towards an indirect evidence for a trade-off between tolerance and resistance traits at some, but not all, developmental stages; making them often difficult to detect.

Olfactory selection of Plantago lanceolata by snails declines with seedling age

BACKGROUND AND AIMS

Despite recent recognition that (1) plant-herbivore interactions during the establishment phase, (2) ontogenetic shifts in resource allocation and (3) herbivore response to plant volatile release are each pivotal to a comprehensive understanding of plant defence, no study has examined how herbivore olfactory response varies during seedling ontogeny.

METHODS

Using a Y-tube olfactometer we examined snail (Helix aspersa) olfactory response to pellets derived from macerated Plantago lanceolata plants harvested at 1, 2, 3, 4, 5, 6 and 8 weeks of age to test the hypothesis that olfactory selection of plants by a generalist herbivore varies with plant age. Plant volatiles were collected for 10 min using solid-phase microextraction technique on 1- and 8-week-old P. lanceolata pellets and analysed by gas chromatography coupled with a mass spectrometer.

KEY RESULTS

Selection of P. lanceolata was strongly negatively correlated with increasing age; pellets derived from 1-week-old seedlings were three times more likely to be selected as those from 8-week-old plants. Comparison of plant selection experiments with plant volatile profiles from GC/MS suggests that patterns of olfactory selection may be linked to ontogenetic shifts in concentrations of green leaf volatiles and ethanol (and its hydrolysis derivatives).

CONCLUSIONS

Although confirmatory of predictions made by contemporary plant defence theory, this is the first study to elucidate a link between seedling age and olfactory selection by herbivores. As a consequence, this study provides a new perspective on the ontogenetic expression of seedling defence, and the role of seedling herbivores, particularly terrestrial molluscs, as selective agents in temperate plant communities.

Iridoid and secoiridoid glycosides in a hybrid complex of bush honeysuckles (Lonicera spp., Caprifolicaceae): implications for evolutionary ecology and invasion biology

Interspecific hybridization among non-native plant species can generate genotypes that are more reproductively successful in the introduced habitat than either parent. One important mechanism that may serve as a stimulus for the evolution of invasiveness in hybrids is increased variation in secondary metabolite chemistry, but still very little is known about patterns of chemical trait introgression in plant hybrid zones. This study examined the occurrence of iridoid and secoiridoid glycosides (IGs), an important group of plant defense compounds, in three species of honeysuckle, Lonicera morrowii A. Gray, Lonicera tatarica L., and their hybrid Lonicera×bella Zabel. (Caprifoliaceae), all of which are considered invasive in various parts of North America. Hybrid genotypes had a diversity of IGs inherited from both parent species, as well as one component not detected in either parent. All three species were similar in that overall concentrations of IGs were significantly higher in fruits than in leaves, and several compounds that were major components of fruits were never found in leaves. However, specific patterns of quantitative distribution among leaves, unripe fruits, and ripe fruits differed among the three species, with a relatively higher allocation to fruits in the hybrid species than for either parent. These patterns likely have important consequences for plant interactions with antagonistic herbivores and pathogens as well as mutualistic seed dispersers, and thus the potential invasiveness of hybrid and parental species in their introduced range. Methods established here for quantitative analysis of IGs will allow for the exploration of many compelling research questions related to the evolutionary ecology and invasion biology of these and other related species in the genus Lonicera.

Nitrogen enrichment differentially affects above- and belowground plant defense

PREMISE OF THE STUDY

Human nitrogen (N) inputs to terrestrial ecosystems have greatly increased in recent years and may have important consequences for plant growth, reproduction, and defense. Although numerous studies have investigated the effects of nitrogen addition on plants, few have examined both above- and belowground responses within a range of predicted increase and apart from concomitant increases in other nutrients. •

METHODS

We conducted a greenhouse experiment to study the consequences of increased nitrogen inputs, such as those from atmospheric N deposition, on plant performance, chemical defenses, and allocation tradeoffs for an invasive species, Linaria dalmatica. This plant produces iridoid glycosides, which are a group of terpenoid compounds. •

KEY RESULTS

Soil nitrogen enrichment increased growth, reproduction, and whole-plant iridoid glycosides while decreasing some costs of defense. Interestingly, nitrogen addition had varying effects on defense allocation to above- and belowground tissues. Specifically, there was no change in iridoid glycoside concentrations of shoots, whereas concentrations decreased in flowers by ~35% and increased in roots by >400%. •

CONCLUSIONS

Observed increases in plant performance and chemical defenses may have implications for the invasion potential of L. dalmatica. Moreover, our results highlight the importance of evaluating both above- and belowground plant defenses. In particular, findings presented here indicate that research focused on leaf-level defenses may not detect key allelochemical responses, including changes in plant resistance traits that could affect consumers (e.g., herbivores and pathogens) that specialize on different plant tissues as well as plant fitness and invasion success.

Plant induced defenses depend more on plant age than previous history of damage: implications for plant-herbivore interactions

Herbivore-induced plant responses can significantly change as a function of plant developmental stage and previous history of damage. Yet, empirical tests that assess the combined role of multiple damage events and age-dependent constraints on the ability of plants to induce defenses within and among tissues are scarce. This question is of particular interest for annual and/or short-lived perennial plant species, whose responses to single or multiple damage events over a growing season are likely to interact with ontogenetic constraints in affecting a plant's ability to respond to herbivory. Using Plantago lanceolata and one of its specialist herbivores, Junonia coenia, we examined the effect of plant ontogeny (juvenile vs. mature developmental stages) and history of damage (single and multiple damage events early and/or late in the season) on plant responses to leaf damage. Plant responses to herbivory were assessed as induced chemical defenses (iridoid glycosides) and compensatory regrowth, in both above- and below-ground tissues. We found that constitutive concentration of iridoid glycosides markedly increased as plants matured, but plant ability to induce chemical defenses was limited to juvenile, but not mature, plant stages. In addition, induced defenses observed 7 d following herbivory in juvenile plants disappeared 5 wk after the first herbivory event, and mature plants that varied considerably in the frequency and intensity of damage received over 5 wk, did not differ significantly in their levels of chemical defenses. Also, only small changes in compensatory regrowth were detected. Finally, we did not observe changes in below-ground tissues' defenses or biomass a week following 50% removal of leaf tissues at either age class or history of damage. Together, these results suggest that in P. lanceolata and perhaps other systems, ontogenetic trajectories in plant growth and defenses leading to strong age-dependent induced responses may prevail over herbivore-induced indirect interactions.