Ellagitannins: defences of Betula nana against Epirrita autumnata folivory?

• 1 The induced resistance of the subarctic mountain birch Betula pubescens ssp. czerepanovii is a well‐characterized phenomenon, whereas the induced responses of Betula nana L., one of the parental species of mountain birch, have not yet been characterized. Betula nana is more resistant to several classes of insectivorous herbivores than the mountain birch, although the mechanisms responsible for the better ability to resist herbivores are not known.
• 2 The present study aimed to determine the metabolic changes that are induced by early season herbivory in B. nana leaves and to study the effects of rapidly induced resistance on the growth of Epirrita autumnata larvae.
• 3 Defoliation of B. nana was accomplished by E. autumnata larvae and leaf samples for chemical analyses were collected when the defoliating larvae were at their third and fifth instar. At the same time, laboratory assays for the growth and consumption rates of E. autumnata larvae were conducted.
• 4 The wounding of leaves by E. autumna larvae induced the production of ellagitannins (ETs) in B. nana. Intriguingly, the concentrations of protein‐bound amino acids were also induced by herbivory; however, an increase in proteins was not mirrored in the growth rate of larvae, which was less on the induced foliage. The decreased growth rate of larvae was apparently linked to the increased concentrations of oxidatively‐active ETs and the high concentration of ETs may explain the better resistance of this parental species compared with the hybrid mountain birch with its lower levels of ETs.

     

Morphological, cytogenetic, and molecular evidence for introgressive hybridization in birch.

Extensive morphological variation of tetraploid birch (Betula pubescens) in Iceland is believed to be due to gene flow from diploid dwarf birch (B. nana) by means of introgressive hybridization. A combined morphological and cytogenetic approach was used to investigate this phenomenon in two geographically separated populations of natural birch woodland in Iceland. The results not only confirmed introgressive hybridization in birch, but also revealed bidirectional gene flow between the two species via triploid interspecific hybrids. The populations showed continuous morphological variation connecting the species, but karyotypically they consisted of only three types of plants: diploids, triploids, and tetraploids. No aneuploids were found. Some of the tetraploid plants had B. pubescens morphology as expected, but most of them had intermediate characters. Most of the diploid plants were B. nana, but some were intermediates and a few had B. pubescens morphology. The triploid plants were either intermediates or they resembled one of the two species. Similar introgressive variation was observed among the diploid and triploid progeny of open-pollinated B. nana in a garden. Birch samples including field plants and artificial hybrids were further examined using a molecular method based on genomic Southern hybridization. The experiments verified introgression at the DNA level.     

Extensive sharing of chloroplast haplotypes among European birches indicates hybridization among Betula pendula, B. pubescens and B. nana

Extensive sharing of chloroplast haplotypes among the silver birch, Betula pendula Roth., the downy birch, B. pubescens Ehrh., and the dwarf birch, B. nana L., was discovered using polymerase chain reaction-restriction fragment length polymporphism markers. The geographical component of the genetic variation was stronger than the species component: the species were not significantly different while 11% of the variation could be attributed to differentiation between the two main regions studied, Scandinavia and western Russia. All haplotypes occurring in more than 2% of the individuals were shared among the species and the introgression ratios were quite large: 0.79 between B. pubescens and B. pendula and 0.67 between B. pubescens and B. nana. The data also indicate that B. pendula individuals are more similar to sympatric B. pubescens than to B. pendula individuals from nearby forests. However, this trend is not as pronounced when B. pubescens is considered, suggesting that introgression is not symmetrical. The haplotype sharing among the three Betula species is most likely caused by hybridization and subsequent cytoplasmic introgression.     

Seasonal variation in the content of hydrolysable tannins in leaves of Betula pubescens

The contents of 13 hydrolysable tannins in the leaves of white birch (Betula pubescens L.) trees were analysed at twelve stages throughout the growing season. All individual galloylglucoses, from 1-O-galloyl-beta-D-glucopyranose to 1,2,3,4,6-penta-O-galloyl-beta-D-glucopyranose, accumulated in young leaves, while ellagitannins showed significantly variable seasonal trends. The major ellagitannin during the whole growing season was pedunculagin while 2,3-(S)-HHDP-glucopyranose. the end product of the proposed ellagitannin pathway, accumulated in mature leaves. Relationships between the characteristics of seasonal variation in the contents of individual ellagitannins and their chemical structures were used to unravel the biogenesis of ellagitannins in birch leaves. Evidence of degradation of ellagitannins through hydrolysis during leaf growth and development is presented and implications for herbivory are discussed.     

Changes in leaf trichomes and epicuticular flavonoids during leaf development in three birch taxa

BACKGROUND AND AIMS: Changes in number of trichomes and in composition and concentrations of their exudates throughout leaf development may have important consequences for plant adaptation to abiotic and biotic factors. In the present study, seasonal changes in leaf trichomes and epicuticular flavonoid aglycones in three Finnish birch taxa (Betula pendula, B. pubescens ssp. pubescens, and B. pubescens ssp. czerepanovii) were followed. METHODS: Trichome number and ultrastructure were studied by means of light, scanning and transmission electron microscopy, while flavonoid aglycones in ethanolic leaf surface extracts were analysed by high-pressure liquid chromatography. KEY RESULTS: Density of both glandular and non-glandular trichomes decreased drastically with leaf expansion while the total number of trichomes per leaf remained constant, indicating that the final number of trichomes is established early in leaf development. Cells of glandular trichomes differentiate before those of the epidermis and produce secreted material only during the relatively short period (around 1-2 weeks) of leaf unfolding and expansion. In fully expanded leaves, glandular trichomes appeared to be at the post-secretory phase and function mainly as storage organs; they contained lipid droplets and osmiophilic material (probably phenolics). Concentrations (mg g(-1) d. wt) of surface flavonoids decreased with leaf age in all taxa. However, the changes in total amount ( microg per leaf) of flavonoids during leaf development were taxon-specific: no changes in B. pubescens ssp. czerepanovii, increase in B. pendula and in B. pubescens ssp. pubescens followed by the decline in the latter taxon. Concentrations of most of the individual leaf surface flavonoids correlated positively with the density of glandular trichomes within species, suggesting the participation of glandular trichomes in production of surface flavonoids. CONCLUSIONS: Rapid decline in the density of leaf trichomes and in the concentrations of flavonoid aglycones with leaf age suggests that the functional role of trichomes is likely to be most important at the early stages of birch leaf development.     

Morphological variation among Betula nana (diploid), B. pubescens (tetraploid) and their triploid hybrids in Iceland

BACKGROUND AND AIMS: Introgressive hybridization between two co-existing Betula species in Iceland, diploid dwarf birch B. nana and tetraploid downy birch B. pubescens, has been well documented. The two species are highly variable morphologically, making taxonomic delineation difficult despite stable ploidy levels. Here an analysis is made of morphological variation within each ploidy group with an aim to establishing a reliable means to distinguish the species. METHODS: Plant materials were collected from 14 woodlands in Iceland. The plants were identified based on 2n chromosome numbers. Morphological variation in species-specific characters within each ploidy group was analysed qualitatively and quantitatively. The morphological index was based on eight discrete characters, whereas the multivariate analysis was based on nine leaf variables. KEY RESULTS: Of the 461 plants examined, 9.5 % were found to be triploid hybrids. The three ploidy groups were morphologically distinguishable but their variation overlapped. The diploid, triploid and tetraploid groups had average scores of 1.3, 4.1 and 8.3, respectively, in the morphology index scale from 0 (B. nana) to 13 (B. pubescens). A linear discriminant analysis also revealed significant separation among the three ploidy groups and the model assigned 96 % and 97 % of the B. nana and B. pubescens individuals correctly. The triploid hybrids were difficult to predict since only half of them could be assigned correctly. Leaf length was the most useful variable identifying triploid hybrids. Geographical patterns within the ploidy groups could partly be explained by differences in mean July temperature. CONCLUSIONS: Hybridization between B. nana and B. pubescens is widespread in Iceland. The species can be distinguished from each other morphologically, and from the triploid hybrids. The overlapping morphological variation indicates bidirectional introgression between the two species via triploid hybrids. Iceland could be considered a birch hybrid zone, harbouring genetic variation which may be advantageous in subarctic regions.     

HPLC analysis of leaf surface flavonoids for the preliminary classification of birch species

Flavonoid aglycones found on the surfaces of birch (Betula spp.) leaves may constitute up to 10% of the dry weight of the leaf. A facile extraction and HPLC procedure has been developed that can be used for the preliminary classification of birch species according to the patterns of their leaf surface flavonoids. The procedure involves no complex sample preparation steps, and is able to provide HPLC chromatograms from fresh leaves in less than 30 min. If necessary, leaves do not even need to be removed from the tree. Since the genus Betula is taxonomically complex and separation of different birch species can be problematic, the developed method was applied to 15 Betula species and four sub-species of Betula pendula Seven of the studied species were classified as B. pubescens and eight as B. pendula-type birches. The remaining four species did not belong to either of these two classes on account of their unique pattern of external flavonoids. The difference between the leaf surface flavonoid composition of B. pubescens and B. pendula type birch species was unambiguously clear, and the developed method could reliably distinguish between the two species. Whilst leaf surface flavonoids can be valuable chemotaxonomic markers, they classify birch species differently from morphological markers. Birch species with diploid chromosome sets did not contain any of the flavanones that were present in the leaves of other species. The close relationship between the occurrence of some flavonoid aglycones and the ploidy level of Betula species suggests that these chemotaxonomic markers may be useful both in taxonomic and phylogenetic analyses.     

Reproductive patterns of Betula pendula and B. pubescens coll. along a regional altitudinal gradient in northern Sweden

The Scandinavian birches have different distribution patterns Betula pendula reaches its upper tree limit at the bottom of the valleys in the Scandes, while B pubescens ssp pubescens extends above the B pendula limit, and merges into B pubescens ssp tortuosa at higher elevations In a 6 yr study I quantified the spatial and temporal variation in the reproduction of these species along a regional altitudinal gradient Betula pendula showed a steeper altitudinal decrease of seed germinability and seed germination rate than B pubescens coll Seed weight was strongly and positively correlated with altitude for B pubescens , but not for B pendula The proportion of seeds infected by gall midges Semudobia coll, was much higher in B pendula than in B pubescens coll especially in the mountain area All birch taxa showed large annual variations in quantity and quality of seeds The sapling density was higher in B pubescens ssp pubescens stands than in B pendula and B pubescens ssp tortuosa stands in the mountains The vegetative reproduction was about three times higher for B pubescens ssp tortuosa , than for the other two birches These results indicate that the altitudinal variation in reproduction among the birch taxa mirror their present distribution patterns     

Reproductive variability and pollen limitation in three Betula taxa in northern Sweden

This 3-yr study documents large variation in pollination and seed quality in birches (Betula) in northern Sweden between taxa, high and low altitude populations, and among years Seed quality was positively correlated with pollination degree and improved by pollen supplementation in B pubescens ssp pubescens in the mountain area, indicating pollen limitation Betula pendula showed the same tendency, but less pronounced Reduced sexual reproduction of B pendula in the mountain area might more often be due to limited resources for seed maturation and defense of the seeds against predispersal seed predation In contrast, coastal populations of both species display relatively high pollination rates and supplemental pollination had no effect on seed quality, suggesting that pollen limitation is rare or absent here The mountain birch Betula pubescens ssp tortuosa showed generally lower pollination and seed quality compared with B pubescens ssp pubescens , but not compared with mountain populations of B pendula It is concluded that pollen limitation may occur in Betula , but to what extent varies with taxon, altitude and year The seed quality is also strongly affected by other factors such as infections of gall midges (Semudobia spp, Diptera Cecidomyndae) and incomplete seed maturation     

Allocation of resources within mountain birch canopy after simulated winter browsing

As a response to browsing, birches are known to produce fewer but larger, more nutritious leaves, with enhanced palatability for herbivores. We simulated winter browsing in ramets of mountain birch ( Betula pubescens ssp. czerepanovii ) to find out whether it decreases subsequent foliage biomass and alters the number and type of shoots. After removal of a considerable proportion of buds (up to 35%) in late winter, the birches were able to compensate for the lost leaf biomass in the following summer; there were no differences in total leaf biomass between winter-clipped and control ramets. This indicates that foliage growth was limited by the total amount of stored resources, not by the number of buds. Depending on the position of the buds removed, different mechanisms were responsible for the compensation. After removal of apical buds, the number of leaves decreased significantly but leaves were larger than in control ramets. Removal of the same mass of basal buds – containing similar amount of carbohydrates and proteins as in the treatment removing apical buds – activated dormant buds, especially in apical locations, so that leaf number was similar as in the controls; consequently, size of individual leaves increased only slightly. Thus, while the total leaf biomass in a tree seems to be limited by resources from source organs, the distribution of resources among different canopy sections is controlled by their relative sink strengths. In terms of leaf biomass, apical parts are able to compensate for bud loss by increasing shoot number, basal parts only by increasing leaf size.     

Comparative analysis of leaf trichome structure and composition of epicuticular flavonoids in Finnish birch species

The morphology, ultrastructure, density and distribution of trichomes on leaves of Betula pendula, B. pubescens ssp. pubescens, B. pubescens ssp. czerepanovii and B. nana were examined by means of light, scanning and transmission electron microscopy. The composition of flavonoids in ethanolic leaf surface extracts was analysed by high pressure liquid chromatography. All taxa examined contained both glandular and non-glandular trichomes (short and/or long hairs) but differed from each other in trichome ultrastructure, density and location on the leaf. Leaves of B. pubescens were more hairy than those of B. pendula, but the latter species had a higher density of glandular trichomes. Of the two subspecies of B. pubescens, leaves of ssp. pubescens had more short hairs on the leaf surface and four times the density of glandular trichomes of leaves of ssp. czerepanovii, whereas, in the latter subspecies, short hairs occurred largely on leaf veins, as in B. nana. The glandular trichomes were peltate glands, consisting of medullar and cortical cells, which differed structurally. Cortical cells possessed numerous small, poorly developed plastids and small vacuoles, whereas medullar cells had several large plastids with well-developed thylakoid systems and fewer vacuoles. In B. pubescens subspecies, vacuoles of the glandular cells contained osmiophilic deposits, which were probably phenolic, whereas in B. pendula, vacuoles of glandular trichomes were characterized by the presence of numerous myelin-like membranes. The composition of epicuticular flavonoids also differed among species. The two subspecies of B. pubescens and B. nana shared the same 12 compounds, but five of these occurred only in trace amounts in B. nana. Leaf surface extracts of B. pendula contained just six flavonoids, three of which occurred only in this species. In summary, the structure, density and distribution of leaf trichomes and the composition of epicuticular flavonoids represent good taxonomic markers for Finnish birch species.     

Sources of variability in plant resistance against insects: free caterpillars show strongest effects

Tests with herbivorous animals have shown great variability in foliage quality within individual plants. This may be a product of plant architecture and microclimate as well as a specific response to herbivory. We tested the level of constitutive and herbivore-induced resistance within the canopy of mountain birch ( Betula pubescens ssp. czerepanovii ) stems of different age, using shoots with different architectural position, function and age. We tested foliage both with uncaged larvae of Epirrita autumnata feeding freely on foliage and with growth trials in the laboratory. The only indication of systematic within-tree variability was low consumption by third instar larvae growing on long shoot leaves. This may protect long shoots that would suffer from reduced growth if their first leaves were consumed at an early stage of development. Stem age and architecture did not contribute to within-tree variation and the effects induced by herbivory were similar throughout the tree. A small number of caterpillars was sufficient to elicit responses in larval growth or consumption. This suggests that the presence of caterpillars may act as a cue of increased risk of future herbivory for mountain birch attacked by E. autumnata with poor mobility and regular outbreaks. Previous defoliation and young age of stems increased the disappearance rate of free larvae. The effect of defoliation suggests that induced resistance may act to prevent future buildup of E. autumnata populations. The small size and simple canopy architecture of young stems may in turn increase the disappearance rate by increasing the probability of predators finding prey. This may partly explain the low vulnerability of young mountain birch forests to E. autumnata outbreaks. Overall, changes in the disappearance rate were greater than changes in larval growth and consumption. This suggests that tests in the laboratory or inside cages may underestimate the efficacy of induced resistance against herbivores.     

The role of foliar microfungi in mountain birch - insect herbivore relationships

We studied the effects of epiphytic and endophytic phyllosphere fungi and pathogenic birch rust fungus infection of mountain birch Betula pubescens ssp czerepanovit trees on the larval performance of leaf beetle Phratora potaris We assessed the effects of epiphytic fungi by growing larvae on leaves from trees with manipulated fungal densities We also monitored larval perfonnance and endophytic fungal densities among tree groups classified by herbivory or rust fungus densities The differences in expenmentally manipulated epiphytic fungal densities did not affect larval relative growth rates (RGR) of the species, instead we found significant tree effects Phratora polaris RGR was higher on trees with high level of herbivory than on trees with low herbivory, nevertheless, endophyte densities between these groupings did not differ In the rust fungus expenment, P polarts performance was lowest on trees with low infection compared to no and high infection trees We also did not find correlations among tree-specific endophyte densities and P polaris performance on high and low herbivory trees and trees classified by rust fungus infection Although antagonism among fungi and induction of tree defences cannot be excluded, we suggest that epiphytic and endophytic fungi of mountain birch have negligible effects on P polaris larval performance under natural conditions, probably due to mountain birch variability and a loose ecological connection between mountain birch and its epi- and endophytes Mountain birch and pathogenic birch rust have a more tightly linked relationship, which may also affect insect herbivores Still, leaf properties may play an important role and the effects will depend on the relative timing of the rust infection, herbivore development and changes in leaf quality     

SEASONALLY VARYING DIET QUALITY AND THE QUANTITATIVE GENETICS OF DEVELOPMENT TIME AND BODY SIZE IN BIRCH FEEDING INSECTS

Abstract Genetic variance‐covariance structures (G), describing genetic constraints on microevolutionary changes of populations, have a central role in the current theories of life‐history evolution. However, the evolution of Gs in natural environments has been poorly documented. Resource quality and quantity for many animals and plants vary seasonally, which may shape genetic architectures of their life histories. In the mountain birch‐insect herbivore community, leaf quality of birch for insect herbivores declines profoundly during both leaf growth and senescence, but remains stable during midsummer. Using six sawfly species specialized on the mountain birch foliage, we tested the ways in which the seasonal variation in foliage quality of birch is related to the genetic architectures of larval development time and body size. In the species consuming mature birch leaves of stable quality, that is, without diet‐imposed time constraints for development time, long development led to high body mass. This was revealed by the strongly positive phenotypic and genetic correlations between the traits. In the species consuming growing or senescing leaves, on the other hand, the rapidly deteriorating leaf quality prevented the larvae from gaining high body mass after long development. In these species, the phenotypic and genetic correlations between development time and final mass were negative or zero. In the early‐summer species with strong selection for rapid development, genetic variation in development time was low. These results show that the intuitively obvious positive genetic relationship between development time and final body mass is a probable outcome only when the constraints for long development are relaxed. Our study provides the first example of a modification in guild‐wide patterns in the genetic architectures brought about by seasonal variation in resource quality.     

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.     

Quantitative analysis of polymeric proanthocyanidins in birch leaves with normal-phase HPLC

The proanthocyanidin composition and content in the leaves of nine birch species (Betula albosinensis, B. ermanii B. maximowicziana, B. nana, B. papyrifera, B. pendula, B. platyphylla, B. pubescens, and B. pubescens ssp. czerepanovii) were studied with different methods including colorimetric assay, HPLC coupled with PAD or ESI/MS and NMR. Total proanthocyanidin content was determined using the acid butanol assay. A normal phase-HPLC method was applied for the analysis of polymeric proanthocyanidins. The content of polymeric proanthocyanidins was estimated from a late eluting peak in the chromatogram. With this HPLC method, quantitative analysis of polymeric proanthocyanidins could be performed directly from leaf extracts: no additional purification or preparation steps were required. It was shown that birch leaves contained mainly polymeric proanthocyanidins with a degree of polymerisation greater than 10. Total proanthocyanidin content (expressed as dry weight) was found to vary from 44mg/g (B. papyrifera) to 145mg/g (B. nana), and polymeric proanthocyanidin content from 39mg/g (B. pendula) to 119 mg/g (B. nana).     

Spatial responses of two herbivore groups to a geometrid larva on mountain birch

Direct or plant-mediated interactions between herbivores may modify their spatial distribution among and within plants. In this study, we examined the effect of a leaf-chewing geometrid, the autumnal moth (Epirrita autumnata), on two different herbivore groups, leaf rolling Deporaus betulae weevils and Eriocrania spp. leafminers, both feeding on mountain birch (Betula pubescens ssp. czerepanovii). The exact locations of herbivores within tree canopies were mapped during three successive summers. In the first 2 years, some trees were artificially colonized by eggs of the autumnal moth to induce both rapid and delayed resistance in the foliage. The natural infection levels of the pathogenic rust fungus (Melampsoridium betulinum), potentially involved in species interactions, were also recorded. At the level of the whole tree, the density of D. betulae leaf rolls was lower in trees infested by the autumnal moth in the same year. However, the feeding locations within trees were partly segregated: D. betulae favoured shadier branches, while E. autumnata preferred the sunny parts of the canopy. The autumnal moth did not affect current- or following-year density of leafminers at the tree or branch level. Trees infected by rust had fewer leafminers in the same summer than noninfected trees. There were no interaction effects between defoliation by the autumnal moth and rust infection, and no delayed effects on the abundance of other herbivores the following year. Taken together, these findings suggest that the autumnal moth has a negative, partially plant-mediated impact on D. betulae, and can reduce the extent of current-year defoliation caused by D. betulae. This may be beneficial for the mountain birch, since the greater part of D. betulae damage occurs around or after the end of the larval period of the autumnal moth, which may be a critical time for tree recovery after moth outbreaks.     

Phenolic compounds in seedlings of Betula pubescens and B. pendula are affected by enhanced UVB radiation and different nitrogen regimens during early ontogeny

We studied the ability of tree seedlings to respond to two environmental factors, elevated ultraviolet B (UVB) radiation and availability of nitrogen (N), at the beginning of their development. Seeds of two birch species, Betula pubescens Ehrh. (common white birch) and B. pendula Roth (silver birch), were germinated and the seedlings grown in an experimental field in eastern Finland. The experimental design consisted of a constant 50% increase in UVB radiation (including a slight increase in UVA), a UVA control (a slight increase in UVA) and a control. The seedlings were fertilized with three levels of N. The experiment lasted for 2 months; aboveground biomass was measured and the most mature leaf of each seedling was taken for the analyses of phenolics. Growth of the seedlings was not significantly affected by enhanced UVB, but was increased by increasing N. Elevated UVB induced significant changes in phenolic compounds. Quercetin glycosides were accumulated in the leaves of both species in response to UVB; this is considered to be a protective response. However, the direction of the responses of individual phenolics to different N regimens differed. In addition, concentration of soluble condensed tannins was lower at moderate N than that at lower levels of N in both species; on the contrary, in B. pubescens the concentration of insoluble condensed tannins was highest at moderate N. No significant interaction between UV and N was detected, and the responses of the two species were highly similar to UVB, while the responses to N regimens varied slightly more between species.     

Phenolic and phenolic-related factors as determinants of suitability of mountain birch leaves to an herbivorous insect

We investigated the role of phenolic and phenolic-related traits of the leaves of mountain birch (Betula pubescens ssp. czerepanovii) as determinants of their suitability for the growth of larvae of the geometrid Epirrita autumnata. As parameters of leaf suitability, we determined the contents of total phenolics, gallotannins, soluble and cell-wall-bound proanthocyanidins (PAS and PAB, respectively), lignin, protein precipitation capacity of tannins (PPC), and leaf toughness. In addition, we examined concentrations of soluble carbohydrates and protein-bound amino acids as background variables describing the nutritive value of leaves. The correlation of the leaf traits of our 40 study trees with the tree-specific relative growth rate (RGR) of E. autumnata showed that the only significant correlation with RGR was that of PAS - the largest fraction of total phenolics - and even that explained only 15% of the variation in E. autumnata growth. The nonlinear estimation of the relationship between RGR and PAS by piecewise linear regression divided the 40 study trees into two groups: (i) 19 trees with good leaves for E. autumnata (RGR ranging from 0.301 to 0.390), and (ii) 21 trees with poor leaves (RGR ranging from 0.196 to 0.296). The suitability of leaves within these two groups of trees was determined by different phenolic traits. Within the good group, the suitability of leaves for larvae was determined by the PPC of extracts, which strongly correlated with gallotannins, and by the total content of gallotannins. In contrast, the leaves of poor trees had significantly higher contents of both PAS and PAB, but leaf toughness correlated only negatively with the RGR of E. autumnata larvae. We also discuss the causes of variation in the phenolic and phenolic-related factors that determine the suitability of leaves for E. autumnata larvae in different groups of trees.     

Multiplicity of biochemical factors determining quality of growing birch leaves

Due to rapidly changing physical and biochemical characteristics of growing leaves, correlations between traits of foliage biochemistry and the performance indices of flush feeding herbivores may vary considerably following relatively minor changes in experimental conditions. We examined the effects of the seasonal and inter-tree variation of a comprehensive array of biochemical compounds on the success of an early season geometrid, Epirrita autumnata, feeding on maturing foliage of mountain birch, Betula pubescens ssp. czerepanovii. We monitored the concentrations of individual phenolics, sugars, total nitrogen, nitrogen of proteins, and nitrogen of soluble compounds, water and acetone-insoluble residue. Simultaneously we recorded larval consumption, physiological performance, growth, and pupal mass of E. autumnata. We found significant phenological changes in almost all leaf traits measured. In bioassays with half-grown leaves, leaf gallotannin concentrations showed a nonlinear effect: in trees with high foliar gallotannin concentrations (over 10 mg g⁻¹), physiological performance was strongly reduced by high gallotannin concentrations. In trees with lower gallotannin concentrations, on the other hand, larval growth was reduced by soluble proanthocyanidins, not gallotannins. Differences between high and low gallotannin trees largely depended on phenology, i.e., on the age of leaves. However, not all the differences in leaf traits between late (with high gallotannin concentrations at the time of the bioassay) and early flushing trees disappeared with leaf maturation, indicating that there is also phenology-independent variance in the tree population. In the full-grown leaves of all the study trees, low concentrations of water and of nitrogen of proteins (but not nitrogen of soluble compounds) were the main factors reducing pupal masses of E. autumnata, while neither gallotannin nor proanthocyanidins now played a significant role. The observed change in the factors underlying leaf quality (from gallotannins and proanthocyanidins to nitrogen and water) relate to the activity of the shikimate pathway and the formation of cell walls: gallotannins and proanthocyanidins are both produced in the pathway, and these tannins are assumed to contribute – via binding into cell walls – to tough and durable cell walls. Interestingly, low quality of leaves did not automatically translate into low foliar consumption (i.e., benefits to the tree). On the trees with young, high gallotannin leaves, larvae actually increased consumption on low quality foliage. In the group of trees with slightly more developed, low gallotannin leaves, the quality of leaves did not clearly modify amounts consumed. In full-grown leaves, low leaf quality strongly reduced leaf consumption. These results emphasize the strong influence of tree phenology on the relationships between biochemical compounds and the herbivore. Received: 30 November 1998 / Accepted: 1 March 1999