Induced resistance in the indeterminate growth of aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis>)

Studies of induction in trees have examined rapid induced resistance (RIR) or delayed induced resistance (DIR), but have not examined induction that occurs in leaves produced by indeterminately growing trees subsequent to, but in the same season as, damage. We refer to induction that occurs during this time period as intermediate-delayed induced resistance (IDIR). We assessed the influences of genetic and environmental factors, and their interactions, on temporal and spatial variation in induction and on tradeoffs between induced and constitutive levels of resistance in indeterminately growing saplings of aspen (Populus tremuloides). We utilized a common garden of 12 aspen genotypes experiencing two levels of defoliation and two levels of soil nutrients. We assessed concentrations of phenolic glycosides and condensed tannins in damaged leaf remnants collected 1 week after defoliation to examine rapid and local induction, and in undamaged leaves produced 8 weeks after defoliation to assess intermediate-delayed and systemic induction. In general, tannins showed RIR, while phenolic glycosides expressed IDIR. For both classes of allelochemicals, we found high estimates of broad-sense heritability and genetic variation in both induced and constitutive levels. Genetic variation may be maintained by both direct costs of allelochemicals and by costs of inducibility (phenotypic plasticity). Such costs may drive the tradeoff exhibited between induced and constitutive levels of phenolic glycosides. IDIR may be important in reducing total-season tissue loss by providing augmented resistance against late summer herbivores in trees that have experienced damage earlier in the season. Herbivore-resistant compensatory growth is especially beneficial to young trees growing in competitive environments.     

Phenolics during early development of <Emphasis Type="Italic">Betula pubescens</Emphasis> seedlings: inhibition of phenylalanine ammonia lyase

We studied phenolic metabolism and plant growth in birch seedlings at the beginning of their development by inhibiting phenylalanine ammonia lyase (PAL), which is the first committed step in phenylpropanoid metabolism. Betula pubescens (Ehrh.) seeds were germinated in inhibitor-free media and the seedlings were transferred to hydroponic culture at the cotyledon stage. They were 6 days old at the start of the experiment, which lasted for 3 weeks. PAL activity was inhibited by three different concentrations of 2-aminoindane-2-phosphonic acid monohydrate (AIP) in the growing media. At the end of 3 weeks, phenolics in all plant parts (roots, stem, cotyledons, first, second and third true leaves) were determined. AIP inhibited strongly the accumulation of phenolic acids, salidroside, rhododendrins, ellagitannins and their precursors, flavan-3-ols, and soluble condensed tannins. The accumulation of lignin and flavonol glycoside derivatives was moderately inhibited. The accumulation of flavonol glycosides, such as quercetin glycosides and kaempferol glycosides, was not generally inhibited, even in leaves that emerged during the experiment, while the accumulation of insoluble condensed tannins was inhibited only slightly and not in all plant parts. This suggests that flavonol glycosides, which may have a UV-B protective role, and insoluble condensed tannins, which may have structural functions, are prioritized in seedling development. Inhibition of PAL with AIP decreased seedling growth and possible reasons for this are discussed. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Genotype and environment determine allocation to and costs of resistance in quaking aspen

Although genetic variability and resource availability both influence plant chemical composition, little is known about how these factors interact to modulate costs of resistance, expressed as negative correlations between growth and defense. We evaluated genotype × environment effects on foliar chemistry and growth of quaking aspen (Populus tremuloides) by growing multiple aspen genotypes under variable conditions of light and soil nutrient availability in a common garden. Foliage was analyzed for levels of nitrogen, phenolic glycosides and condensed tannins. Bioassays of leaf quality were conducted with fourth-stadium gypsy moth (Lymantria dispar) larvae. Results revealed strong effects of plant genotype, light availability and nutrient availability; the importance of each factor depended upon compound type. For example, tannin concentrations differed little among genotypes and across nutrient regimes under low light conditions, but markedly so under high light conditions. Phenolic glycoside concentrations, in contrast, were largely determined by genotype. Variation in phenolic glycoside concentrations among genotypes was the most important factor affecting gypsy moth performance. Gypsy moth biomass and development time were negatively and positively correlated, respectively, with phenolic glycoside levels. Allocation to phenolic glycosides appeared to be costly in terms of growth, but only under resource-limiting conditions. Context-dependent trade-offs help to explain why costs of allocation to resistance are often difficult to demonstrate.     

Induced resistance in the indeterminate growth of aspen (Populus tremuloides)

Studies of induction in trees have examined rapid induced resistance (RIR) or delayed induced resistance (DIR), but have not examined induction that occurs in leaves produced by indeterminately growing trees subsequent to, but in the same season as, damage. We refer to induction that occurs during this time period as intermediate-delayed induced resistance (IDIR). We assessed the influences of genetic and environmental factors, and their interactions, on temporal and spatial variation in induction and on tradeoffs between induced and constitutive levels of resistance in indeterminately growing saplings of aspen (Populus tremuloides). We utilized a common garden of 12 aspen genotypes experiencing two levels of defoliation and two levels of soil nutrients. We assessed concentrations of phenolic glycosides and condensed tannins in damaged leaf remnants collected 1 week after defoliation to examine rapid and local induction, and in undamaged leaves produced 8 weeks after defoliation to assess intermediate-delayed and systemic induction. In general, tannins showed RIR, while phenolic glycosides expressed IDIR. For both classes of allelochemicals, we found high estimates of broad-sense heritability and genetic variation in both induced and constitutive levels. Genetic variation may be maintained by both direct costs of allelochemicals and by costs of inducibility (phenotypic plasticity). Such costs may drive the tradeoff exhibited between induced and constitutive levels of phenolic glycosides. IDIR may be important in reducing total-season tissue loss by providing augmented resistance against late summer herbivores in trees that have experienced damage earlier in the season. Herbivore-resistant compensatory growth is especially beneficial to young trees growing in competitive environments.     

Phenolic glycosides and condensed tannins in Salix sericea, S. eriocephala and their F1 hybrids: not all hybrids are created equal

The performance of hybrids depends upon the inheritance and expression of resistance traits. Secondary chemicals are one such resistance trait. In this study, we measured the concentrations of phenolic glycosides and condensed tannins in parental and F1 hybrid willows to examine the sources of chemical variation among hybrids. S. sericea produces phenolic glycosides, salicortin and 2'-cinnamoylsalicortin, and low concentrations of condensed tannin in its leaves. In contrast, S. eriocephala produces no phenolic glycosides but high concentrations of condensed tannins in its leaves. These traits are inherited quantitatively in hybrids. On average, F1 hybrids are intermediate for condensed tannins, suggesting predominantly additive inheritance or balanced ambidirectional dominance of this defensive chemical from the parental species. In contrast, the concentration of phenolic glycosides is lower than the parental midpoint, indicating directional dominance. However, there is extensive variation among F1 hybrids. The concentration of tannin and phenolic glycosides in F1 hybrid families is either (1) lower than the midpoint, (2) higher than the midpoint, or (3) indistinguishable from the midpoint of the two parental taxa. It appears that the production of the phenolic glycosides, especially 2'-cinnamoylsalicortin, is controlled by one or more recessive alleles. We also observed a two-fold or greater difference in concentration between some hybrid families. We discuss how chemical variation may effect the relative susceptibility of hybrid willows to herbivores.     

Cost of trichome production and resistance to a specialist insect herbivore in <Emphasis Type="Italic">Arabidopsis lyrata</Emphasis>

Theory predicts that trade-offs between resistance to herbivory and other traits positively affecting fitness can maintain genetic variation in resistance within plant populations. In the perennial herb Arabidopsis lyrata, trichome production is a resistance trait that exhibits both qualitative and quantitative variation. Using a paternal half-sib design, we conducted two greenhouse experiments to ask whether trichomes confer resistance to oviposition and leaf herbivory by the specialist moth Plutella xylostella, and to examine potential genetic constraints on evolution of increased resistance and trichome density. In addition, we examined whether trichome production is induced by insect herbivory. We found strong positive genetic and phenotypic correlations between leaf trichome density and resistance to leaf herbivory, demonstrating that the production of leaf trichomes increases resistance to leaf damage by P. xylostella. Also resistance to oviposition tended to increase with increasing leaf trichome density, but genetic and phenotypic correlations were not statistically significant. Trichome density and resistance to leaf herbivory were negatively correlated genetically with plant size in the absence of herbivores, but not in the presence of herbivores. There was no evidence of increased trichome production after leaf damage by P. xylostella. The results suggest that trichome production and resistance to leaf herbivory are associated with a cost and that the direction of selection on resistance and trichome density depends on the intensity of herbivory.     

Genetic variation in resistance of Eucalyptus globulus to marsupial browsers

The evolution of plant defensive traits in response to selection pressures imposed by herbivores is central to co-evolutionary theory. To demonstrate the role of herbivores as selective agents on plant resistance there must be variability in plant resistance to herbivores within a plant population. This variability must be under genetic control, and the variability in plant resistant traits and consequently herbivore damage to plants must reflect variability in plant fitness. We used a common eucalypt species, Eucalyptus globulus, and two major mammalian herbivores, the common brushtail possum (Trichosurus vulpecula) and the red-bellied pademelon (Thylogale billardierii), as a system to investigate intraspecific variation in plant resistance to mammalian herbivores and to investigate if this variation has a genetic basis. We measured mammalian browsing damage on 2,302 individual trees of E. globulus, from 563 families derived from range-wide native stand seed collections of known pedigree and grown in a common environment field trial. Using a selection of trees from the field trial we then conducted a feeding trial with captive herbivores to assess if the genetic variation in plant resistance in the field was reflected in feeding preferences of captive animals, as measured by relative intake. Results from the field trial showed significant genetic variation in plant resistance amongst races, localities and amongst different families. These results were consolidated in the captive trial with similar trends in genetic variation among E. globulus localities. Dry matter intake of foliage by Trichosurus vulpecula was consistently greater than that by Thylogale billardierii; however, the intraspecific preferences of the two herbivores were significantly correlated.     

Long-term effects of defoliation on quaking aspen in relation to genotype and nutrient availability: plant growth,phytochemistry and insect performance

This research tested the long-term effects of defoliation on aspen chemistry and growth in relation to genotype and nutrient availability. We grew saplings of four aspen genotypes in a common garden under two conditions of nutrient availability, and subsequently subjected them to two levels of artificial defoliation. Artificial defoliation suppressed plant growth, and saplings of the four genotypes did not show evidence of genetic variation in tolerance to defoliation. Phenolic glycoside concentrations did not respond to defoliation, but were influenced by genotype and nutrient availability. Condensed tannins responded to defoliation and varied among genotypes. Although defoliation affected condensed tannins, plant quality was not altered in a manner important for gypsy moth performance. Regression analyses suggested that phenolic glycoside concentrations accounted for most of the variation in insect performance. The lack of a strong response important for herbivores was surprising given the severity of the defoliation treatment (nearly 100% of leaf area was removed). In this study, plant genotype was of primary importance, nutrient availability was of secondary importance and long-term induced responses were unimportant as determinants of insect performance.     

Synergies and trade-offs between insect and pathogen resistance in maize leaves and roots

Determining links between plant defence strategies is important to understand plant evolution and to optimize crop breeding strategies. Although several examples of synergies and trade-offs between defence traits are known for plants that are under attack by multiple organisms, few studies have attempted to measure correlations of defensive strategies using specific single attackers. Such links are hard to detect in natural populations because they are inherently confounded by the evolutionary history of different ecotypes. We therefore used a range of 20 maize inbred lines with considerable differences in resistance traits to determine if correlations exist between leaf and root resistance against pathogens and insects. Aboveground resistance against insects was positively correlated with the plant's capacity to produce volatiles in response to insect attack. Resistance to herbivores and resistance to a pathogen, on the other hand, were negatively correlated. Our results also give first insights into the intraspecific variability of root volatiles release in maize and its positive correlation with leaf volatile production. We show that the breeding history of the different genotypes (dent versus flint) has influenced several defensive parameters. Taken together, our study demonstrates the importance of genetically determined synergies and trade-offs for plant resistance against insects and pathogens.     

Feeny revisited: condensed tannins as anti-herbivore defences in leaf-chewing herbivore communities of Quercus

Abstract. 1. Community level oak–tannin–insect patterns have been largely unexplored since Paul Feeny's ground‐breaking research. Two hypotheses were tested for Quercus velutina and Q. alba in the Missouri Ozarks: abundance and richness of leaf‐chewing herbivores are negatively correlated with foliar condensed tannin concentrations and variation in condensed tannin concentrations explains variation in herbivore community structure. 2. In 2001, foliar condensed tannins in the understorey and canopy of these two oak species were quantified simultaneously with censuses of herbivores in May, during leaf expansion, and in June and August, when leaves were fully expanded. Thirty‐eight of the 134 species encountered had densities sufficient to be analysed individually (n = 10). Of those, Acronicta increta (Noctuidae) and Attelabus sp. (Curculionidae), both oak specialists, were negatively correlated with condensed tannins in the canopy of Q. alba. One additional specialist, Chionodes pereyra (Gelechiidae), was marginally negatively correlated with condensed tannins in the understorey of Q. velutina. Understorey species richness of May Q. velutina herbivores was negatively correlated with condensed tannins, as were total canopy insect density and species richness of August herbivores on Q. alba. 3. Principal component analysis (PCA) of insect abundances indicated that understorey and canopy Q. velutina and Q. alba had different communities of leaf‐chewing insects. Furthermore, condensed tannin levels contributed significantly to variation in PCA scores for Q. velutina, explaining 25% of the total variation. 4. Overall, these results indicate that specialists were more likely than generalists both to correlate negatively with condensed tannins and to occur in lower tannin habitats; abundance and richness of both early and late season fauna correlated negatively with tannins; and species were more likely to correlate negatively with condensed tannins when feeding on Q. alba than on Q. velutina and when feeding in the canopy than in the understorey. Future studies of tannin–insect interactions should manipulate leaf quality in combination with manipulations of other factors that likely influence community structure.     

Direct and ecological costs of resistance and tolerance in the stinging nettle

Plant resistance and tolerance to herbivores, parasites, pathogens, and abiotic factors may involve two types of costs. First, resistance and tolerance may be costly in terms of plant fitness. Second, resistance and tolerance to multiple enemies may involve ecological trade-offs. Our study species, the stinging nettle ( Urtica dioica L.) has significant variation among seed families in resistance and tolerance as well as costs of resistance and tolerance to the holoparasitic plant Cuscuta europaea L. Here we report on variation among seed families (i.e. genetic) in tolerance to nutrient limitation and in resistance to both mammalian herbivores (i.e. number of stinging trichomes) and an invertebrate herbivore (i.e. inverse of the performance of a generalist snail, Arianta arbustorum). Our results indicate direct fitness costs of snail resistance in terms of host reproduction whereas we did not detect fitness costs of mammalian resistance or tolerance to nutrient limitation. We further tested for ecological trade-offs among tolerance or resistance to the parasitic plant, herbivore resistance, and tolerance to nutrient limitation in the stinging nettle. Tolerance of nettles to nutrient limitation and resistance to mammalian herbivores tended to correlate negatively. However, there were no significant correlations among resistance and tolerance to the different natural enemies (i.e. parasitic plants, snails, and mammals). The results of this greenhouse study thus suggest that resistance and tolerance of nettles to diverse enemies are free to evolve independently of each other but not completely without direct costs in terms of plant fitness.     

Clonal variation in foliar chemistry of aspen: effects on gypsy moths and forest tent caterpillars

Quaking aspen (Populus tremuloides) exhibits striking intraspecific variation in concentrations of phenolic glycosides, compounds that play important roles in mediating interactions with herbivorous insects. This research was conducted to assess the contribution of genetic variation to overall phenotypic variation in aspen chemistry and interactions with gypsy moths (Lymantria dispar) and forest tent caterpillars (Malacosoma disstria). Thirteen aspen clones were propagated from field-collected root material. Insect performance assays, measuring survival, development, growth, and food utilization indices, were conducted with second and/or fourth instars. Leaf samples were assayed for water, nitrogen, total nonstructural carbohydrates, condensed tannins, and phenolic glycosides. Results showed substantial among-clone variation in the performance of both insect species. Chemical analyses revealed significant among-clone variation in all foliar constituents and that variation in allelochemical contents differed more than variation in primary metabolites. Regression analyses indicated that phenolic glycosides were the dominant factor responsible for among-clone variation in insect performance. We also found significant genetic trade-offs between growth and defense among aspen clones. Our results suggest that genetic factors are likely responsible for much of the tremendous phenotypic variation in secondary chemistry exhibited by aspen, and that the genetic structure of aspen populations may play important roles in the evolution of interactions with phytophagous insects. Received: 14 May 1996 / Accepted: 29 January 1997     

Putting the insect into the birch–insect interaction

Leaf maturation in mountain birch (Betula pubescens ssp. czerepanovii) is characterized by rapid shifts in the types of dominant phenolics: from carbon-economic flavonoids aglycons in flushing leaves, via hydrolysable tannins and flavonoid glycosides, to carbon-rich proanthocyanidins (condensed tannins) in mature foliage. This shift accords with the suggested trade-offs between carbon allocation to plant defense and growth, but may also relate to the simultaneous decline in nutritive leaf traits, such as water, proteins and sugars, which potentially limit insect growth. To elucidate how birch leaf quality translates into insect growth, I introduce a simple model that takes into account defensive compounds but also acknowledges insect demand for nutritive compounds. The effects of defensive compounds on insect growth depend strongly on background variation in nutritive leaf traits: compensatory feeding on low nutritive diets increases the intake of defensive compounds, and the availability of growth-limiting nutritive compounds may modify the effects of defenses. The ratio of consumption to larval growth (both in dry mass) increases very rapidly with leaf maturation: from 2.9 to 9.8 over 2 weeks in June-July, and to 15 by August. High concentrations in mature birch leaves of "quantitative" defenses, such as proanthocyanidins (15-20% of dry mass), presumably prevent further consumption. If the same compounds had also protected half-grown leaves (which supported the same larval growth with only one third of the dry matter consumption of older leaves), the same intake of proanthocyanidins would have demanded improbably high concentrations (close to 50%) in young leaves. The model thus suggests an adaptive explanation for the high levels of "quantitative" defenses, such as proanthocyanidins, in low-nutritive but not in high-nutritive leaves because of the behavioral responses of insect feeding to leaf nutritive levels.     

The effect of elevated CO2 and temperature on the secondary chemistry of Betula pendula seedlings

The effect of elevated atmospheric CO₂ and temperature on resource allocation and secondary chemistry of white birch (Betula pendula Roth) under a non-limiting nutrient and water supply was investigated. Birch seedlings were grown in closed-top chambers exposed to ambient CO₂ and temperature, elevated atmospheric CO₂ (700 ppm), elevated temperature (2°C above ambient) and a combination of elevated CO₂ and temperature for one growing season. Elevated CO₂ significantly increased the total biomass of the seedlings. The combined effect of the elevated CO₂ and temperature treatments further increased the total biomass, but not significantly. The content of nitrogen and water decreased, while some secondary compounds (such as condensed tannins and flavonol glycosides) increased in leaves subjected to CO₂ enrichment. Elevated temperature increased the concentration of total flavone aglycones and decreased that of total HPLC-phenolics in the leaves, due to the decrease in individual flavonol glycosides, cinnamoylquinic acids and (+)-catechin. There were no significant interactive effects between CO₂ and temperature in the phenolic concentrations of the leaves and in the stems, while the number of resin droplets in the top part of the stems showed significant interaction. This clearly implies that carbon allocation into secondary metabolites in the leaves and stems differ under enhanced CO₂ and temperature, and the combined effect of CO₂ and temperature on the herbivore resistance of birches, is lower than that of CO₂ and temperature alone.     

Seedling herbivory by slugs in a willow hybrid system: developmental changes in damage, chemical defense, and plant performance

We evaluated feeding preference and damage by the slug, Arion subfuscus, on seedlings of two willow species, Salix sericea and S. eriocephala, and their F₁ interspecific hybrids. Trays of seedlings were placed in the field and excised leaves were presented to slugs in choice tests. Slugs preferred feeding on and caused the most damage to S. eriocephala seedlings. S. sericea seedlings were least preferred and least damaged. F₁ hybrid seedlings were intermediate in preference and damage. Slug preference of and damage to these seedlings decreased over time, suggesting developmental changes in resistance. Seedlings were sampled for phenolic glycoside and tannin chemistry weekly to coincide with the field and laboratory experiments. Concentrations of phenolic glycosides and tannins increased linearly with seedling age, coincident with changes in slug preference and damage, indicating a developmental change in defense. Slug deterrence was not detected at low concentrations of salicortin when painted on leaves or discs, but both salicortin and condensed tannins deterred slug feeding at concentrations between 50 and 100 mg/g, levels found in adult willows. Seedling performance was related to damage inflicted by slugs. Due to lower levels of damage when exposed to slugs in the field, S. sericea plants had significantly greater biomass than S. eriocephala plants. Biomass of F₁ hybrids was equal to S. sericea when damaged. However, undamaged S. eriocephala and F₁ hybrid plants had the greatest biomass. Because F₁ hybrid seedlings performed as well as the most fit parent in all cases, slugs could be an important selective factor favoring introgression of defensive traits between these willow species.     

Physiological mechanisms underlying the costs of chemical defence in Junonia coenia Hu¨bner (Nymphalidae): A gravimetric and quantitative genetic analysis

Several recent studies document that specialist insect phytophages may be less subject to predation than generalists and suggest that hostplant-derived chemical defences may be an important explanation for the predominance of specialized feeding among insect herbivores. The evolution of such chemical defences depends upon both their advantages versus natural enemies and their physiological costs, but data on these costs, particularly genetic data, are few. Here I report the results of an ecological genetic investigation of food use efficiency and allelochemical sequestration in Junonia coenia Hu¨bner (Nymphalidae). I used standard gravimetric techniques to estimate the efficiency of dry matter incorporation and iridoid glycoside sequestration in the larvae of 37 full-sib families fed artificial diets containing trace, low (2%) and high (10%) concentrations of iridoid glycosides. I found a significant reduction in the efficiency of dry matter incorporation on a high iridoid diet that is entirely attributable to reduced digestibility rather than post-digestive toxic effects. Larvae fed high-iridoid diets sequestered them less efficiently, but this difference was due largely to post-digestive effects. Analyses of genetic variation and architecture of dry matter and iridoid budgets reveal substantial genetic variation in both suites of traits, but only chemical defence showed a significant genotype×environment interaction which would be conducive to the evolution of specialization. Neither group of traits showed across-diet trade-offs in the form of negative correlations of family means among diets. Family means correlations of sequestration indices with dry matter indices within diets reveals that chemical defence comes at a cost to growth, but only in the high diet. I also found evidence of specialized physiological machinery for iridoid glycoside processing. These data indicate that even adapted specialists are negatively affected by plant toxins, but in this species, dietary specialization is more likely to result from selection from natural enemies than from hostplant toxins.     

Evolution of plant resistance and tolerance to frost damage

Plant defence against any type of stress may involve resistance (traits that reduce damage) or tolerance (traits that reduce the negative fitness impacts of damage). These two strategies have been proposed as redundant evolutionary alternatives. A late‐season frost enabled us to estimate natural selection and genetic constraints on the evolution of frost resistance and tolerance in a wild plant species. We employed a genetic selection analysis (which is unbiased by environmental correlations between traits and fitness) on 75 paternal half‐sibling families of annual wild radish [Raphanus raphanistrum (Brassicaceae)]. In an experimental population in southern Ontario, we found strong selection favouring plant resistance to frost, but selection against tolerance to frost. The selection against tolerance may have been caused by a cost of tolerance, as we provide evidence for a negative genetic correlation between tolerance and fitness in the absence of frost damage. Although we found no evidence for the theoretically predicted trade‐off between frost tolerance and resistance among our families, we did detect negative correlational selection acting on the two traits, indicating that natural selection favoured high resistance combined with low tolerance and low resistance coupled with high tolerance, but not high or low levels of both traits together. There were few genetic correlations between the measured traits overall, but frost tolerance was negatively correlated with initial seed mass, and frost resistance was positively correlated with resistance to insect herbivory. Periodic episodes of strong selection such as that caused by the late‐season frost may be disproportionately important in evolution, and are likely becoming more common because of human alterations of the environment.     

Condensed tannins deter feeding by browsing ruminants in a South African savanna

Summary The palatability of 14 species of woody plant was assessed for three species of browsing ruminant, namely kudus, impalas and goats. Results show that palatability was most clearly related to leaf contents of condensed tannins. The effect was a threshold one, with all plants containing more than 5% condensed tannins being rejected as food during the wet season period. In contrast palatability was not influenced by concentrations of protein-precipitating polyphenols, and only weakly related to contents of nitrogen, phosphorus, cations, fibre components and other secondary metabolites. Insect herbivory shows a different pattern. These findings support the hypotheses that (i) condensed tannins function to protect plant cell walls against microbial attack; (ii) hydrolyzable tannins function to inactivate the digestive enzymes of insect herbivores. Large mammalian herbivores are influenced by condensed tannins due to their dependance upon microbial fermentation of plant cell walls for part of their energy needs.     

Induced sink strength as a prerequisite for induced tannin biosynthesis in developing leaves of Populus

Induced defenses occur predominately in young, developing plant tissues that rely upon carbohydrate import to support their growth and development. To test the hypothesis that the induced production of carbon-based defenses is dependent upon photoassimilate import, we examined the response of developing leaves of hybrid poplar (Populus deltoides 2 P. nigra) saplings to wounding by gypsy moth caterpillars (Lymantria dispar L.) and exogenous jasmonic acid (JA). Growth rates, condensed tannin contents and acid invertase activities were measured for individual leaves and the translocation of ¹³C-labeled resources between orthostichous source-sink pairs was quantified. Results showed a substantial increase in the activity of cell wall invertase in sink leaves wounded by gypsy moth caterpillars and treated with JA. JA-induced sink leaves also imported 3-4 times as much ¹³C-labeled carbon from orthostichous source leaves relative to controls and allocated a significant portion of this imported ¹³C to condensed tannin biosynthesis. Reduced carbohydrate flow to these leaves, caused by source leaf removal, resulted in reduced condensed tannin levels and the emergence of a growth-defense tradeoff. These results indicate that (1) induced sink strength is elicited by insect wounding and JA application in hybrid poplar foliage, (2) imported resources are allocated to the production of carbon-based defenses, and (3) the level of induced defense in leaves can be constrained by the ability of leaves to import carbohydrates from source tissues. Together, these results suggest that within-canopy variations in induced resistance may arise in part because of uneven distribution of resources to induced foliage.     

Genotypic variation in response of quaking aspen (Populus tremuloides) to atmospheric CO2 enrichment

Enriched atmospheric CO₂ alters the quantity and quality of plant production, but how such effects vary among plant genotypes is poorly known. We evaluated the independent and interactive effects of CO₂ and nutrient availability on growth, allocation and phytochemistry of six aspen (Populus tremuloides Michx.) genotypes. One-year-old trees, propagated from root cuttings, were grown in CO₂-controlled glasshouses for 64 days, then harvested. Foliage was analyzed for levels of water, nitrogen, starch, phenolic glycosides and condensed tannins. Of seven plant growth/allocation variables measured, four (biomass production, stem growth, relative growth rate and root:shoot ratio) exhibited marginally to highly significant CO₂ 2 genotype interactions. CO₂ enrichment stimulated growth of some genotypes more than others, and this interaction was itself influenced by soil nutrient availability. In addition, enriched CO₂ increased the magnitude of the among-genotype variance for four of the growth/allocation variables. Of six foliar chemical constituents analyzed, CO₂-mediated responses of two (the phenolic glycoside tremulacin and condensed tannins) varied among genotypes. Moreover, enriched CO₂ increased the magnitude of among-genotype variance for four of the chemical variables. Given the importance of these growth and chemical characteristics to the biological fitness of aspen, this research suggests that projected atmospheric CO₂ increases are likely to alter the genetic structures and evolutionary trajectories of aspen populations.