Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability

Summary Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.     

The effect of nutrients on pyrrolizidine alkaloids in Senecio plants and their interactions with herbivores and pathogens

The aim of this review is to combine the knowledge of studies on effects of nutrients on pyrrolizidine alkaloids (PAs) in Senecio with those studies of effects of PAs on herbivores and pathogens in order to predict the effects that nutrients may have on herbivores and pathogens via changes in PAs. We discuss whether these predictions match with the outcome of studies where the effect of nutrients on herbivores and insects were measured. PA concentrations in S. jacobaea, S. vulgaris and S. aquaticus were mostly reduced by NPK fertilization, with genotype-specific effects occurring. Plant organs varied in their response to increased fertilization; PA concentrations in flowers remained constant, while shoot and roots were mostly negatively affected. Biomass change is probably largely responsible for the change in concentrations. Nutrients affect both the variety and the levels of PAs in the plant. The reduced PA concentrations after NPK fertilization was expected to benefit herbivores, but no or negative responses from insect herbivores were observed. Apparently other changes in the plant after fertilization are overriding the effect of PAs. Pathogens do seem to benefit from the lower PA concentrations after fertilization; they were more detrimental to fertilized plants than to unfertilized control plants. Future studies should include the effect of each element of nutrients separately and in combinations in order to gain more insight in the effect of specific nutrients on PA content in Senecio plants.     

Chemical diversity and variation of pyrrolizidine alkaloids of the senecionine type: biological need or coincidence?

Laboratory and field tracer experiments with ¹⁴C-labelled senecionine N–oxide (SO) and distant biosynthetic precursors such as [¹⁴C]putrescine revealed that pyrrolizidine alkaloid N-oxides (PAs) in Senecio vernalis Waldstr. & Kit. (Asteraceae) show no significant turnover over periods of up to 29 d. However, PAs are spatially mobile, they are continuously allocated, and labelled PAs are even detectable in leaves and capitula developed weeks after tracer application. Chemical diversification of SO, the common product of PA biosynthesis in roots, was studied in five Senecio species (i.e. S. vernalis Waldstr. & Kit., S. vulgaris L, S. inaequidens DC, two chemotypes of S. jacobaea L. and S. erucifolius L.). Tracer experiments revealed that shoots are capable of transforming [¹⁴ C]SO into the unique species–specific PA patterns. Within a plant, the transformation efficiency of SO can vary quantitatively and qualitatively between shoot organs (i.e. leaves, stems and inflorescences). All transformations proceed position-specifically and stereoselectively. They comprise simple one-step or two-step reactions such as hydroxylations, epoxidations, dehydrogenations, and O-acetylations, as well as the more complex conversion of the retronecine into the otonecine base moiety (e.g. SO into senkirkine). Taking all the evidence together, the qualitative and quantitative composition of the Senecio PA pattern is a dynamic and sensitive equilibrium between a number of interacting processes: (i) constant rate of de-novo synthesis of SO in roots, (ii) continuous long-distance translocation of SO into shoots, (iii) efficiency of SO transformations which may vary between plant organs, (iv) continuous allocation of PAs in the plant, and (v) efficiency and tissue selectivity of vacuolar storage. We suggest that in constitutive plant defence, without significant turnover of its components, such a highly plastic system provides a powerful strategy to successfully defend and possibly escape herbivory. Received: 27 March 1998 / Accepted: 19 May 1998     

Counteractive biomass allocation responses to drought and damage in the perennial herb Convolvulus demissus

Herbivory and water shortage are key ecological factors affecting plant performance. While plant compensatory responses to herbivory include reallocation of biomass from below‐ground to above‐ground structures, plant responses to reduced soil moisture involve increased biomass allocation to roots and a reduction in the number and size of leaves. In a greenhouse study we evaluated the effects of experimental drought and leaf damage on biomass allocation in Convolvulus demissus (Convolvulaceae), a perennial herb distributed in central Chile, where it experiences summer drought typical of Mediterranean ecosystems and defoliation by leaf beetles and livestock. The number of leaves and internode length were unaffected by the experimental treatments. The rest of plant traits showed interaction of effects. We detected that drought counteracted some plant responses to damage. Thus, only in the control watering environment was it observed that damaged plants produced more stems, even after correcting for main stem length (index of architecture). In the cases of shoot : root ratio, relative shoot biomass and relative root biomass we found that the damage treatment counteracted plant responses to drought. Thus, while undamaged plants under water shortage showed a significant increase in root relative biomass and a significant reduction in both shoot : root ratio and relative shoot biomass, none of these responses to drought was observed in damaged plants. Total plant biomass increased in response to simulated herbivory, apparently due to greater shoot size, and in response to drought, presumably due to greater root size. However, damaged plants under experimental drought had the same total biomass as control plants. Overall, our results showed counteractive biomass allocation responses to drought and damage in C. demissus. Further research must address the fitness consequences under field conditions of the patterns found. This would be of particular importance because both current and expected climatic trends for central Chile indicate increased aridity.     

Impact of foliar herbivory on the development of a root-feeding insect and its parasitoid

The majority of studies exploring interactions between above- and below-ground biota have been focused on the effects of root-associated organisms on foliar herbivorous insects. This study examined the effects of foliar herbivory by Pieris brassicae L. (Lepidoptera: Pieridae) on the performance of the root herbivore Delia radicum L. (Diptera: Anthomyiidae) and its parasitoid Trybliographa rapae (Westwood) (Hymenoptera: Figitidae), mediated through a shared host plant Brassica nigra L. (Brassicaceae). In the presence of foliar herbivory, the survival of D. radicum and T. rapae decreased significantly by more than 50%. In addition, newly emerged adults of both root herbivores and parasitoids were significantly smaller on plants that had been exposed to foliar herbivory than on control plants. To determine what factor(s) may have accounted for the observed results, we examined the effects of foliar herbivory on root quantity and quality. No significant differences in root biomass were found between plants with and without shoot herbivore damage. Moreover, concentrations of nitrogen in root tissues were also unaffected by shoot damage by P. brassicae larvae. However, higher levels of indole glucosinolates were measured in roots of plants exposed to foliar herbivory, suggesting that the development of the root herbivore and its parasitoid may be, at least partly, negatively affected by increased levels of these allelochemicals in root tissues. Our results show that foliar herbivores can affect the development not only of root-feeding insects but also their natural enemies. We argue that such indirect interactions between above- and below-ground biota may play an important role in the structuring and functioning of communities.     

The influence of below ground herbivory and plant competition on growth and biomass allocation of purple loosestrife

Experiments investigating plant-herbivore interactions have primarily focused on above-ground herbivory, with occasional studies evaluating the effect of below-ground herbivores on plant performance. This study investigated the growth of the wetland perennial Lythrum salicaria (purple loosestrife) under three levels of root herbivory by the weevil Hylobiustransversovittatus and three levels of plant competition by the grass Phleumpratense in a common garden. Plant growth, flowering phenology, and biomass allocation patterns of purple loosestrife were recorded for two growing seasons. During the first year, root herbivory reduced plant height; plant competition delayed flowering; and the interaction of root herbivory and plant competition resulted in reductions in plant height, shoot weight and total dry biomass. Plant competition or larval feeding did not affect the biomass allocation pattern in the first year. These results indicate the importance of interactions of plant competition and herbivory in reducing plant performance – at least during the establishment period of purple loosestrife. In the second growing season, root herbivory reduced plant height, biomass of all plant parts, delayed and shortened the flowering period, and changed the biomass allocation patterns. Plant competition delayed flowering and reduced the dry weight of fine roots. The interaction of root herbivory and plant competition delayed flowering. Root herbivory was more important than plant competition in reducing the performance of established purple loosestrife plants. This was due, in part, to intense intraspecific competition among the grass individuals effectively preventing shoot elongation of P. pratense and resulting in a carpet like growth. Received: 3 April 1997 / Accepted: 27 July 1997     

Genetic variation in constitutive and inducible pyrrolizidine alkaloid levels inCynoglossum officinale L.

The constitutive pyrrolizidine alkaloid (PA) concentration of both shoots and roots differed significantly between 17 selfed families. The broad-sense heritability accounted for 33–43% of the variation in PA levels. Families also differed significantly in the amount and the direction of PA induction in both shoots and roots, 24 h after punching 15 holes in the leaves. We found a significantly negative relationship between the changes in PA content of the shoots and changes in PA content of the roots. The total PA content of the plants did not increase. We thus concluded that changes in PA distribution over the plant resulted from transport of PAs within the plant. The direction of transport differed between families: some transported PAs to the shoots, others to the roots. This makes it questionable whether PAs act as damage-induced defences. The effect of damage on the PA concentration is far less than the differences found between families in the constitutive PA concentration. This again strongly suggests that damage-induced defences inCynoglossum officinale do not play an important role. We argue that the general lack of attention that is given to genotype in induction experiments, has led to false conclusions.     

Herbivore-induced changes in plant carbon allocation: assessment of below-ground C fluxes using carbon-14

Effects of above-ground herbivory on short-term plant carbon allocation were studied using maize (Zea mays) and a generalist lubber grasshopper (Romalea guttata). We hypothesized that above-ground herbivory stimulates current net carbon assimilate allocation to below-ground components, such as roots, root exudation and root and soil respiration. Maize plants 24 days old were grazed (c. 25–50% leaf area removed) by caging grasshoppers around individual plants and 18 h later pulse-labelled with¹⁴CO₂. During the next 8 h,¹⁴C assimilates were traced to shoots, roots, root plus soil respiration, root exudates, rhizosphere soil, and bulk soil using carbon-14 techniques. Significant positive relationships were observed between herbivory and carbon allocated to roots, root exudates, and root and soil respiration, and a significant negative relationship between herbivory and carbon allocated to shoots. No relationship was observed between herbivory and¹⁴C recovered from soil. While herbivory increased root and soil respiration, the peak time for¹⁴CO₂ evolved as respiration was not altered, thereby suggesting that herbivory only increases the magnitude of respiration, not patterns of translocation through time. Although there was a trend for lower photosynthetic rates of grazed plants than photosynthetic rates of ungrazed plants, no significant differences were observed among grazed and ungrazed plants. We conclude that above-ground herbivory can increase plant carbon fluxes below ground (roots, root exudates, and rhizosphere respiration), thus increasing resources (e.g., root exudates) available to soil organisms, especially microbial populations.     

Rice cultivar evaluation for phosphorus use efficiency

Phosphorus deficiency is one of the most growth-limiting factors in acid soils in various parts of the world. The objective of this study was to screen 25 rice cultivars (Oryza sativa L.) at low, medium, and high levels of soil P. Number of tillers, root length, plant height, root dry weight and shoot dry weight were related to tissue P concentrations, P uptake and P-use efficiency. Shoot weight was found to be the plant parameter most sensitive to P deficiency. Significant cultivar differences in P use efficiency were found. Phosphorus use efficiency was higher in roots than shoots and decreased with increasing levels of soil P. Positive correlations were found among growth parameters such as plant height, tillers, root and shoot weight, and P content of roots and shoots. These results indicate selection of rice cultivars for satisfactory performance under low P availability can be carried out using shoot and root dry weight as criteria.     

Allometric growth, disturbance regime, and dilemmas of controlling invasive plants: a model analysis

Disturbed communities are observed to be more susceptible to invasion by exotic species, suggesting that some attributes of the invaders may interact with disturbance regime to facilitate invasion success. Alternanthera philoxeroides, endemic to South America, is an amphibious clonal weed invading worldwide. It tends to colonize disturbed habitats such as riparian zones, floodplain wetlands and agricultural areas. We developed an analytical model to explore the interactive effects of two types of physical disturbances, shoot mowing and root fragmentation, on biomass production dynamics of A. philoxeroides. The model is based on two major biological assumptions: (1) allometric growth of root (belowground) vs. shoot (aboveground) biomass and (2) exponential regrowth of shoot biomass after mowing. The model analysis revealed that the interaction among allometric growth pattern, shoot mowing frequency and root fragmentation intensity might lead to diverse plant ‘fates’. For A. philoxeroides whose root allocation decreases with growing plant size, control by shoot mowing was faced with two dilemmas. (1) Shoot regrowth can be effectively suppressed by frequent mowing. However, frequent shoot mowing led to higher biomass allocation to thick storage roots, which enhanced the potential for faster future plant growth. (2) In the context of periodic shoot mowing, individual shoot biomass converged to a stable equilibrium value which was independent of the root fragmentation intensity. However, root fragmentation resulted in higher equilibrium population shoot biomass and higher frequency of shoot mowing required for effective control. In conclusion, the interaction between allometric growth and physical disturbances may partially account for the successful invasion of A. philoxeroides; improper mechanical control practices could function as disturbances and result in exacerbated invasion.     

Chemotype of <Emphasis Type="Italic">Senecio jacobaea</Emphasis> affects damage by pathogens and insect herbivores in the field

The evolution of the diversity of plant secondary metabolites is still poorly understood. To determine whether natural enemies could exert selection on plant secondary chemistry, pathogen infestation and invertebrate herbivory were measured on 10 genotypes of Senecio jacobaea (Tansy Ragwort) at two experimental field sites during a 2-year period. The genotypes represented two chemotypes based on the presence of the pyrrolizidine alkaloids (PA) jacobine and erucifoline. At one site, Heteren, mainly generalist herbivores were present. Here, damage was limited and did not differ among genotypes or chemotypes. At the other site, Meijendel, several specialists attacked the plants. Damage increased during the year, with a peak in July when most damage was caused by the specialist moth Tyria jacobaeae. At this peak there was no difference in damage among chemotypes. In the months prior to T. jacobaeae damage, chemotypes with jacobine were more severely attacked by specialists than the chemotypes without jacobine. Total damage during that period was positively correlated with both total PA concentration and jacobine concentration. Probably plant vigor also played a role in host preference since damage per individual plant was positively correlated with plant size. Our results suggest that total PA concentration and specifically jacobine had a positive effect on specialist feeding, indicating ecological costs involved in the production of PAs. Ecological costs related to plant secondary compounds could explain why not all individuals produce high levels of these compounds. In addition, differences in specialist herbivore pressures among sites may contribute to the variation in secondary metabolites among populations.     

Effects of simulated root herbivory and fertilizer application on growth and biomass allocation in the clonal perennialSolidago canadensis

Summary Compensatory growth in response to simulated belowground herbivory was studied in the old-field clonal perennialSolidago canadensis. We grew rootpruned plants and plants with intact root systems in soil with or without fertilizer. For individual current shoots (aerial shoot with rhizome and roots) and for whole clones the following predictions were tested: a) root removal is compensated by increased root growth, b) fertilizer application leads to increased allocation to aboveground plant organs and increased leaf turnover, c) effects of fertilizer application are reduced in rootpruned plants. When most roots (90%) were removed current shoots quickly restored equilibrium between above-and belowground parts by compensatory belowground growth whereas the whole clone responded with reduced aboveground growth. This suggests that parts of a clone which are shared by actively growing shoots act as a buffer that can be used as source of material for compensatory growth in response to herbivory. Current shoots increased aboveground mass and whole clones reduced belowground mass in response to fertilizer application, both leading to increased allocation to aboverground parts. Also with fertilizer application both root-pruned and not root-pruned plants increased leaf and shoot turnover. Unfertilized plants, whether rootpruned or not, showed practically no aboveground growth and very little leaf and shoot turnover. Effects of root removal were as severe or more severe under conditions of high as under conditions of low nutrients, suggesting that negative effects of belowground herbivory are not ameliorated by abundant nutrients. Root removal may negate some effects of fertilizer application on the growth of current shoots and whole clones.     

<Emphasis Type="Italic">In vitro</Emphasis> biosynthesis of antioxidants from <Emphasis Type="Italic">Hemidesmus indicus</Emphasis> R. Br. cultures

Summary Shoot cultures and callus cultures from roots and leaves of Hemidesmus indicus R. Br (Asclepiadaceae) were established on Murashige and Skoog medium with various hormonal combinations. The production of antioxidants (lupeol, vanillin, and rutin) in shoot cultures, callus cultures derived from leaf cells and root cells, was compared with root and aerial portions of the parent plant. Shoot cultures and leaf callus cultures produced more antioxidants than root callus cultures. In vitro culture of this species might ofter an alternative method for production of these important pharmaccuticals, which would reduce the collection pressure on this rare plant.     

Aboveground resource allocation in response to root herbivory as affected by the arbuscular mycorrhizal symbiosis

Aims

Arbuscular mycorrhizal (AM) fungi associate with the majority of terrestrial plants, influencing their growth, nutrient uptake and defence chemistry. Consequently, AM fungi can significantly impact plant-herbivore interactions, yet surprisingly few studies have investigated how AM fungi affect plant responses to root herbivores. This study aimed to investigate how AM fungi affect plant tolerance mechanisms to belowground herbivory.

Methods

We examined how AM fungi affect plant (Saccharum spp. hybrid) growth, nutrient dynamics and secondary chemistry (phenolics) in response to attack from a root-feeding insect (Dermolepida albohirtum).

Results

Root herbivory reduced root mass by almost 27%. In response, plants augmented investment in aboveground biomass by 25%, as well as increasing carbon concentrations. The AM fungi increased aboveground biomass, phosphorus and carbon. Meanwhile, root herbivory increased foliar phenolics by 31% in mycorrhizal plants, and increased arbuscular colonisation of roots by 75% overall. AM fungi also decreased herbivore performance, potentially via increasing root silicon concentrations.

Conclusions

Our results suggest that AM fungi may be able to augment plant tolerance to root herbivory via resource allocation aboveground and, at the same time, enhance plant root resistance by increasing root silicon. The ability of AM fungi to facilitate resource allocation aboveground in this way may be a more widespread strategy for plants to cope with belowground herbivory.

     

Preference for plants damaged by conspecific larvae in ovipositing cabbage root flies: influence of stimuli from leaf surface and roots

In laboratory dual-choice assays females of the cabbage root fly, Delia radicum, prefer for oviposition plants with roots damaged by conspecific larvae to undamaged controls. Cauliflower and kale plants were inoculated with root fly eggs (25 per plant) and the hatching larvae were allowed to feed on the roots for various periods of time (1–17 days). After 4 (cauliflower) or 5 (kale) days of larval feeding the oviposition preference was most pronounced and flies laid between 64% and 68% of their eggs near plants with damaged roots. Later, with increasing damage but fewer surviving, and thus actively feeding, larvae, the magnitude of the preference declined. The preference for plants already damaged by conspecific larvae may contribute to the previously observed aggregated distribution of D. radicum eggs in Brassica crop fields.Further experiments revealed that the sensory cues inducing this oviposition preference originate from the complex consisting of the damaged roots, the surrounding substrate (soil) and associated microbes, rather than from the aerial plant parts. In choice assays using the root-substrate complex of damaged and control plants (aerial parts removed), the observed preference for damaged roots was similar to that found for the entire plant but was more pronounced. The damaged roots alone, compared to control roots, received up to 72% (cauliflower) and 75% (kale) of the eggs. By contrast, surrogate leaves sprayed with methanolic leaf surface extracts from the most preferred plants which had been damaged were not discriminated from surrogate leaved sprayed with extracts of the respective control plants. Analysis of glucosinolate levels in methanolic leaf surface extracts revealed that root damage resulted in enhanced concentrations of indole-glucosinolates on the leaf surface in kale but not in cauliflower. Although indole-glucosinolates are oviposition stimulants for the cabbage root fly, the induced changes were apparently too small to influence oviposition behaviour.     

树木根系碳分配格局及其影响因子

根系作为树木提供养分和水分的“源”和消耗C的“汇”,在陆地生态系统C平衡研究中具有重要的理论意义。尽管20多年来的研究已经认识到根系消耗净初级生产力占总净初级生产力较大的比例,但是,根系（尤其是细根）消耗C的机理以及C分配的去向一直没有研究清楚。主要原因是细根消耗光合产物的生理生态过程相当复杂,准确估计各个组分消耗的C具有很大的不确定性,常常受树种和环境空间和时间异质性、以及研究方法的限制。综述了分配到地下的C主要去向,即细根生产和周转、呼吸及养分吸收与同化、分泌有机物、土壤植食动物,及有关林木地下碳分配机理的几种假说,分析了地下碳分配估计中存在的不确定性。目的是在全球变化C循环研究中对生态系统地下部分根系消耗的C以及分配格局引起重视。     

Effects of plant competition and herbivore density on the development of the turnip root fly (Delia floralis) in an intercropping system

In this study, interactive effects of plant competition and herbivory on plant quality and herbivore development were examined in a greenhouse experiment where cabbage plants [Brassica oleracea L. var. capitata (Brassicaceae)] were intercropped with red clover [Trifolium pratense L. (Fabaceae)]. Cabbages were grown with two red clover densities and attack rates by the root feeding herbivore the turnip root fly, Delia floralis Fall. (Diptera: Anthomyiidae). Above ground and below ground cabbage biomass was reduced through intercropping and larval damage. Intercropping also resulted in lower nitrogen and higher carbon root levels compared with levels in the roots of monocultured cabbage. Furthermore, both root nitrogen and carbon levels increased with herbivory. Root neutral detergent fibre (NDF) and lignin content increased in response to both increased plant competition and higher egg densities. For lignin, an interaction effect was observed in the form of elevated levels in intercropped plants subjected to larval damage, while levels in roots of monocultured cabbage remained unchanged. The quality changes brought about by clover competition affected D. floralis development negatively, which resulted in reduced pupal weight. In addition, increased egg density also decreased larval growth. The effects on the development of D. floralis in relation to host plant quality are discussed. Handling editor: Gimme Walter     

Specific impacts of two root herbivores and soil nutrients on plant performance and insect–insect interactions

Soil‐dwelling insects commonly co‐occur and feed simultaneously on belowground plant parts, yet patterns of damage and consequences for plant and insect performance remain poorly characterized. We tested how two species of root‐feeding insects affect the performance of a perennial plant and the mass and survival of both conspecific and heterospecific insects. Because root damage is expected to impair roots’ ability to take up nutrients, we also evaluated how soil fertility alters belowground plant–insect and insect–insect interactions. Specifically, we grew common milkweed Asclepias syriaca in low or high nutrient soil and added seven densities of milkweed beetles Tetraopes tetraophthalmus, wireworms (mainly Hypnoides abbreviatus), or both species. The location and severity of root damage was species‐specific: Tetraopes caused 59% more damage to main roots than wireworms, and wireworms caused almost seven times more damage to fine roots than Tetraopes. Tetraopes damage decreased shoot, main root and fine root biomass, however substantial damage by wireworms did not decrease any component of plant biomass. With the addition of soil nutrients, main root biomass increased three times more, and fine root biomass increased five times more when wireworms were present than when Tetraopes were present. We detected an interactive effect of insect identity and nutrient availability on insect mass. Under high nutrients, wireworm mass decreased 19% overall and was unaffected by the presence of Tetraopes. In contrast, Tetraopes mass increased 114% overall and was significantly higher when wireworms were also present. Survival of wireworms decreased in the presence of Tetraopes, and both species’ survival was negatively correlated with conspecific density. We conclude that insect identity, density and soil nutrients are important in mediating the patterns and consequences of root damage, and suggest that these factors may account for some of the contradictory plant responses to belowground herbivory reported in the literature.     

Invasive and native populations of common ragweed exhibit strong tolerance to foliar damage

Tolerance and resistance are defence strategies evolved by plants to cope with damage due to herbivores. The introduction of exotic species to a new biogeographical range may alter the plant–herbivore interactions and induce selection pressures for new plant defence strategies with a modified resource allocation. To detect evolution in tolerance to herbivory in common ragweed, we compared 3 native (North America) and 3 introduced (France) populations, grown in a common garden environment. We explored the effect of leaf herbivory on plant vegetative and reproductive traits. Plants were defoliated by hand, simulating different degrees of insect grazing by removing 0%, 50% or 90% of each leaf blade. Total and shoot dry biomasses were not affected by increasing defoliation, whereas root dry biomass and root:shoot ratio decreased significantly for native and introduced populations. Furthermore, defoliation treatments did not affect any of the plant reproductive traits measured. Hence, common ragweed displayed an efficient reallocation of resources in shoot biomass at the expense of roots following defoliation, which allows the species to tolerate herbivory without obvious costs for fitness. We did not detect any difference in herbivory tolerance between introduced and native populations, but significant differences were found in reproduction with invasive populations producing more seeds than native populations. As a result, tolerance to herbivory has been maintained in the introduced plant populations. We discuss some implications of these preliminary results for biological control strategies dedicated to common ragweed.     

Low-level herbivory by root-knot nematodes (Meloidogyne incognita) modifies root hair morphology and rhizodeposition in host plants (Hordeum vulgare)

Low amounts of root infestation by plant parasitic nematodes are suggested to increase nutrient supply and in turn enhance microbial activity and net mineralization rate in the rhizosphere. These effects are generally related to “leakage” of plant-derived metabolites from damaged roots. Besides leakage, the present study examines other nematode–host interactions such as alterations in root exudation and morphology, which were almost not considered yet. This includes undamaged root parts in order to assess systemic plant response. The root-knot nematode Meloidogyne incognita (Kofoid and White 1919; Chitwood 1949) and barley (Hordeum vulgare L. cv. Europa) was used as model system. Host plants were grown in mini-rhizotrons inoculated with 0, 2,000, 4,000 or 8,000 M. incognita for 4 weeks. Root morphology, rhizodeposition (sugars, carboxylates, amino acids), and rhizosphere microbial communities (PLFAs) were assessed. In treatments with 4,000 nematodes, shoot biomass, total N and P content increased by the end of the experiment. Generally, an enhanced release of plant metabolites (sugars, carboxylates, amino acids) from the apical root zone occurred 1 week after inoculation with 4,000 and 8,000 M. incognita, indicating root leakage. Low levels of root herbivory stimulated root hair elongation in both infected and uninfected roots. These systemic changes in root morphology likely contributed to the increased sugar exudation in uninfected roots in all nematode treatments at 3 weeks after inoculation. Root-knots formed a separate microhabitat within the root-system. They were characterised by decreased rhizodeposition and increased fungal to bacterial ratio in the adhering rhizosphere soil. The present study provides the first evidence that, apart from leakage, nematode root herbivory at background levels induces local and systemic effects on root morphology and exudation, which in turn may affect plant performance.