Carbon-nutrient Balance Hypothesis Research Articles

Leaf defense traits of birch, beech, and oak saplings grown under two types of soil in a free-air ozone exposure system

ABSTRACT Since most leaf defense traits are derived from photosynthates, the increase of feeding damage in woody plants can be partially attributed to the recent increase in ground-surface O3 (elevated O3; eO3), which has suppressed the photosynthetic capacity of the plants. In different soil fertility conditions (fertile brown forest soil or infertile volcanic ash soil), we made research on leaf defense traits of the three species; birch (an early successional species), beech (a late successional species), and oak (an intermediate species). The results showed that except for lignin, the defensive capacity of birch leaves was reduced by eO3 as well as by a fertile soil condition. In beech as an O3-sensitive species, C/N was slightly higher in fertile soil; especially total phenolics and lignin concentrations, but increased with eO3. Therefore, carbon-nutrient balance hypothesis was not applicable in beech, which may be due to a different nutrient translocation system from birch and other trees. Oak, an O3-tolerant species, was less affected by eO3 and soil condition; C/N slightly tended to increase with eO3 due to a decrease in N. In addition, total phenolics of oak leaves tended to decrease by eO3 in infertile soil. From these results, O3 tolerance, which differs among tree species, can lead to new and altered plant-insect relationships through impact on defense functions, which can also lead to new relationships in biodiversity.

Analysis and correlations of the protein content and selected 'antinutrients' of faba beans (Vicia faba) in a German sample set of the cultivation years 2016, 2017, and 2018.

Faba beans (Vicia faba) experienced a significant revival in cultivation in Western Europe in the last decade. In this study, potential correlations between protein content (PC), trypsin inhibitory activity (TIA), and tannin content were investigated in a large German sample set with bean samples obtained from 50 different farms present in 11 German federal states. Three consecutive cultivation years (2016, 2017, and 2018) were included. The faba bean samples were grown under real cultivation conditions without any specific experimental design and finally marketed by the farmers. This enabled researchers to identify the relationship and extent of the three quality parameters towards the varying cultivation conditions and practices. Moreover, the correlations observed between the parameters were brought into the context of well-known theoretical plant hypotheses such as the carbon-nutrient balance hypothesis (CNBH), the growth-differentiation balance hypothesis (GDBH), as well as the protein competition model (PCM) for evaluating the potential for use in predictions. The study showed a significant negative correlation between PC and tannin content in faba beans over each cultivation year, whereas a positive correlation between TIA and tannin content was found. No clear correlation was observed between PC and TIA. The three plant hypotheses (CNBH, GDBH, and PCM) seem to be not fully valid. Nonetheless, these findings might be a useful guideline for predicting the composition of selected compounds, and sustainable recommendations about cultivation and exploitation for the feed and food sector can be derived. © 2022 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Herbivore resistance in congeneric and sympatric Nothofagus species is not related to leaf habit.

Two fundamental hypotheses on herbivore resistance and leaf habit are the resource availability hypothesis (RAH) and the carbon-nutrient balance hypothesis (CNBH). The RAH predicts higher constitutive resistance by evergreens, and the CNBH predicts higher induced resistance by deciduous species. Although support for these hypotheses is mixed, they have rarely been examined in congeneric species. We compared leaf constitutive and induced resistance (as leaf polyphenol and tannin concentrations, and as damage level in non-choice experiments) and leaf traits associated with herbivory of coexisting Nothofagus species using (1) a defoliation experiment and (2) natural defoliation caused by an outbreak of a common defoliator of Nothofagus species. In the defoliation experiment, polyphenol and tannin concentrations were similar between deciduous and evergreen species; regardless of leaf habit, polyphenols increased in response to defoliation. In the natural defoliation survey, N. pumilio (deciduous) had significantly higher herbivory, lower carbon/nitrogen ratio and leaf mass per area, and higher nitrogen and phosphorus concentrations than N. betuloides (evergreen); N. antarctica (deciduous) had intermediate values. Polyphenol concentrations and herbivore resistance indicated by the non-choice experiment were lower in N. pumilio than in N. antarctica and N. betuloides, which had similar values. Higher herbivory in N. pumilio was associated with a higher nutritional value and a lower level of leaf carbon-based defenses compared to both the evergreen and the other deciduous species, indicating that herbivore resistance in Nothofagus species cannot be attributed to only leaf habit as predicted by the RAH or CNBH.

Arbuscular mycorrhiza facilitates the accumulation of glycyrrhizin and liquiritin in Glycyrrhiza uralensis under drought stress.

Liquorice (Glycyrrhiza uralensis) is an important medicinal plant for which there is a huge market demand. It has been reported that arbuscular mycorrhizal (AM) symbiosis and drought stress can stimulate the accumulation of the active ingredients, glycyrrhizin and liquiritin, in liquorice plants, but the potential interactions of AM symbiosis and drought stress remain largely unknown. In the present work, we investigated mycorrhizal effects on plant growth and accumulation of glycyrrhizin and liquiritin in liquorice plants under different water regimes. The results indicated that AM plants generally exhibited better growth and physiological status including stomatal conductance, photosynthesis rate, and water use efficiency compared with non-AM plants. AM inoculation up-regulated the expression of an aquaporin gene PIP and decreased root abscisic acid (ABA) concentrations under drought stress. In general, AM plants displayed lower root carbon (C) and nitrogen (N) concentrations, higher phosphorus (P) concentrations, and therefore, lower C:P and N:P ratios but higher C:N ratio than non-AM plants. On the other hand, AM inoculation increased root glycyrrhizin and liquiritin concentrations, and the mycorrhizal effects were more pronounced under moderate drought stress than under well-watered condition or severe drought stress for glycyrrhizin accumulation. The accumulation of glycyrrhizin and liquiritin in AM plants was consistent with the C:N ratio changes in support of the carbon-nutrient balance hypothesis. Moreover, the glycyrrhizin accumulation was positively correlated with the expression of glycyrrhizin biosynthesis genes SQS1, β-AS, CYP88D6, and CYP72A154. By contrast, no significant interaction of AM inoculation with water treatment was observed for liquiritin accumulation, while we similarly observed a positive correlation between liquiritin accumulation and the expression of a liquiritin biosynthesis gene CHS. These results suggested that AM inoculation in combination with proper water management potentially could improve glycyrrhizin and liquiritin accumulation in liquorice roots and may be practiced to promote liquorice cultivation.

Response of extrafloral nectar production to elevated atmospheric carbon dioxide

Extrafloral nectar attracts ants, whose presence provides protection for the plant against herbivores. Extrafloral nectar is thus a critical component of many plant–insect mutualisms worldwide, so environmental perturbations that alter extrafloral nectar production or composition could be disruptive. The carbon–nutrient balance hypothesis predicts that under elevated CO2 the total volume of extrafloral nectar will increase but the proportion of the foliar carbohydrate pool secreted as extrafloral nectar will decrease, without any change in the sugar composition of the extrafloral nectar. We investigated the impact of elevated atmospheric CO2 on extrafloral nectar in an Australian wild cotton species, Gossypium sturtianum J.H.Willis. Under elevated CO2 there was an increase in the proportion of leaves actively producing nectar and a decrease in the nectar volume per active leaf. Elevated CO2 did not affect the total volume or composition of extrafloral nectar, but there was a change in how the nectar was distributed within the leaf canopy, as well as evidence of increased turnover of leaves and earlier onset of flowering. By the end of the study, there was no difference in the total resources allocated to extrafloral nectar under elevated CO2, which contrasts with the predictions of the carbon-nutrient balance hypothesis. Developmental changes, however, could affect the timing of extrafloral nectar production which could, in turn, alter the foraging patterns of ants and their defence of plants.

Does the Growth Differentiation Balance Hypothesis Explain Allocation to Secondary Metabolites in Combretum apiculatum , an African Savanna Woody Species?

The growth differentiation balance hypothesis (GDBH) provides a framework that predicts a trade-off between costs of secondary metabolites (SMs) relative to the demand for photosynthate by growth. However, this hypothesis was developed using empirical evidence from plant species in northern boreal and temperate systems, leaving its applicability to species under different abiotic and biotic conditions questionable and generalizations problematic. The objective of this study was to investigate whether the GDBH explains allocation to SMs in the deciduous African savanna woody species C. apiculatum along a 6-point N gradient. The cornerstone prediction of the GDBH, i.e., the parabolic response in SMs along the N gradient, was not observed, with secondary metabolism showing compound-specific responses. Quercetin, myricetin, and kaempferol glycoside concentrations, all produced via the same pathway, responded differently across the N gradient. Flavonol glycoside, cinnamic acid, and quercetin glycoside concentrations decreased as N increased, which provides partial support for the carbon nutrient balance hypothesis. Simulated herbivory had no effect on photosynthesis, decreased foliar N and consequently increased C:N ratio, but did not induce an increase in SMs, with condensed tannins and flavonol glycosides being unaffected. Defoliated plants at low N concentration compensated for lost biomass, which suggests a tolerance response, but as predicted by the limiting resource model, plants at higher N concentration were evidently C limited and thus unable to compensate. Our results show that the GDBH does not explain allocation to SMs in C. apiculatum, and suggest that mechanistic explanations of plant allocation should consider the integrative defensive effect of changed SMs.

Testing for Bottom—up Effects in an Overbrowsed Boreal Landscape

On Anticosti Island (Quebec, Canada), overbrowsing by white‐tailed deer Odocoileus virginianus has substantially modified plant communities and reduced the recruitment of balsam fir Abies balsamea seedlings over most of the territory. An exception to this phenomenon has been observed in localised patches occurring on a single geological deposit named Chicotte, where the natural recruitment of balsam fir is occurring even in the presence of a large white‐tailed deer population. We hypothesized that edaphic properties within the Chicotte deposit could result in lower forage quality, which in turn could reduce browsing pressure and allow fir regeneration to occur (i.e. bottom—up effects). To test this hypothesis, we measured soil properties and foliage chemistry of four forage species (balsam fir, white spruce Picea glauca, Canada mayflower Maianthemum canadense and Canada bunchberry Cornus canadensis) collected on each of three geological deposits on Anticosti Island: Chicotte, Becscie and Jupiter (the latter two considered as controls). Contrary to expectation, results from principal component analysis suggested that Chicotte was the most fertile, whereas Becsie was the least fertile, of the three deposits. Furthermore, balsam fir foliage chemistry did not respond to geological deposit. Conversely, Mantel et Procrustes tests revealed a significant correlation between soil properties and forage quality for white spruce, consistent with the carbon—nutrient balance hypothesis. Univariate tests confirmed that neutral detergent fiber concentrations in white spruce were higher on the Becscie than on the Chicotte deposit. Likewise, in vitro true digestibility of both white spruce and Canada bunchberry foliage were lower on the Becscie than on the Chicotte deposit. Although we failed to demonstrate why balsam fir recruitment occurs on the Chicotte deposit, our data demonstrate that edaphic properties may affect the quality of some forage types, which potentially affect foraging patterns in overbrowsed boreal landscapes.

The effect of successive years of defoliation by the larch sawfly (Pristiphora erichsonii (Hartig)) on the foliage properties of the Japanese larch (Larix kaempferi (Lamb.) Carr.), with particular reference to the CN balance hypothesis

The carbon/nutrient balance hypothesis (CNBH) attempts to explain the mechanism of induced changes in plant properties. The responses of the Japanese larch (Larix kaempferi) to defoliation by the larch sawfly (Pristiphora erichsonii) were examined from the perspective of the CNBH. This study was conducted in seven Japanese larch plantations in central Hokkaido, Japan. The defoliation intensity was determined from canopy photos taken from 2009 to 2012. The chemical and physical properties of the foliage were determined from 2010 to 2012. Severe insect defoliation was found at two sites in 2009 and at all seven sites in 2010 and 2011. A decrease in foliar nitrogen and increases in phenolics, tannins, and the CN ratio were found in the years following severe defoliation and were significantly influenced by the 2009 defoliation intensity. The influence of defoliation in 2010 and 2011 was weaker. These results indicated that the past defoliation history additively affected the foliage properties in the 2 years following insect defoliation. In addition to the 2009 defoliation effects, site effects were found on phenolics, sugars, and the CN ratio. Relative to the other sites, the CN ratio was high at both sites where severe defoliation was found in 2009. Phenolics and sugars did not increase linearly with the CN ratio, indicating that limitations affected their synthesis. These results suggest that the induced changes in L. kaempferi properties are partially up-regulated under nitrogen limitation, but that secondary compound synthesis was, most likely, influenced by external site-dependent factors other than nitrogen limitation.

Increased cellular brevetoxins in the red tide dinoflagellate Karenia brevis under CO2 limitation of growth rate: Evolutionary implications and potential effects on bloom toxicity

Karenia brevis blooms impair human health, marine ecosystems, and coastal economies in the Gulf of Mexico via their production of carbon‐based neurotoxins (brevetoxins), which contain no nitrogen (N) or phosphorus (P). N and P limitation of growth rate substantially increases brevetoxins in this dinoflagellate, consistent with predictions of the carbon nutrient balance (CNB) hypothesis. This hypothesis further predicts that an increase in carbon‐based brevetoxins should not occur if growth rate is limited by carbon dioxide (CO2). We tested this prediction by examining the effect of CO2 limitation of K. brevis growth rate on cellular brevetoxins. In contradiction to the prediction of the CNB hypothesis, brevetoxins normalized to cell carbon were on average 81% higher in CO2‐limited cells growing at a rate of 0.1 d−1 than in cells growing at their maximum rates (0.4–0.5 d−1). This increase in brevetoxin : C values in the CO2‐limited cells, however, was 23–42% lower than that previously observed for comparable growth rate limitation by phosphate. The CO2‐limited cells also exhibited 40% higher cellular N : C and 60–100% higher chlorophyll a : C than observed in P‐limited cells at equivalent growth rates. These effects were likely due to the up‐regulation of the cell's CO2‐concentrating mechanism under CO2 limitation, which increased the demand for photosynthetically produced adenosine‐5′‐triphosphate. The results indicate that anthropogenic increases in CO2 concentrations in surface ocean waters are likely to increase the toxicity of K. brevis blooms due to potential increases in bloom biomass yield and to a greater likelihood that dense blooms will become N or P limited rather than CO2 limited.

Effects of nutrient enrichment and herbivory on morphology, reproduction and chemical content of Turbinaria conoides (Phaeophyceae)

SummaryThe effects of nutrient enrichment and herbivory on resource allocation patterns among morphology, reproduction, and chemical content of the brown alga, Turbinaria conoides (J. Agardh) Kützing were tested in the shallow subtidal zone of the Gulf of Thailand. The field experimental design comprised 36 plots (50 × 50 cm2) with and without herbivores, and two nutrient levels. Cages (uncaged and fully caged plots) were used to exclude herbivorous fishes and two nutrients levels were achieved by experimental enrichment above ambient nutrient concentrations. For morphology of Turbinaria, the maximum length (holdfast to frond apex), the width of the base of the stipe, number of blades, holdfast and stipe diameter were measured. Biomass, reproductive output, and tissue nutrient (carbon: nitrogen: phosphorus) content of T. conoides were examined. Phlorotannin concentrations were examined using a modified Folin‐Ciocalteu method. The results showed that herbivory had no effect on morphology, reproduction, or phlorotannin concentrations. This could be due to the structural and morphological deterrents of alga, which might minimize grazing effects from herbivorous fishes. Nutrient enrichment had no effect on morphology and reproduction of T. conoides, possibly due to low nutrient demand in Turbinaria. However, nutrient enrichment did affect phlorotannin concentrations, as phlorotannins in the enriched plots were lower than the ambient plots. These results support, in part, the carbon–nutrient balance hypothesis that states algae will allocate fewer resources to production of anti‐herbivore chemical compounds when enriched with potential growth‐limiting nutrients.

Costs of defense and a test of the carbon-nutrient balance and growth-differentiation balance hypotheses for two co-occurring classes of plant defense.

One of the goals of chemical ecology is to assess costs of plant defenses. Intraspecific trade-offs between growth and defense are traditionally viewed in the context of the carbon-nutrient balance hypothesis (CNBH) and the growth-differentiation balance hypothesis (GDBH). Broadly, these hypotheses suggest that growth is limited by deficiencies in carbon or nitrogen while rates of photosynthesis remain unchanged, and the subsequent reduced growth results in the more abundant resource being invested in increased defense (mass-balance based allocation). The GDBH further predicts trade-offs in growth and defense should only be observed when resources are abundant. Most support for these hypotheses comes from work with phenolics. We examined trade-offs related to production of two classes of defenses, saponins (triterpenoids) and flavans (phenolics), in Pentaclethra macroloba (Fabaceae), an abundant tree in Costa Rican wet forests. We quantified physiological costs of plant defenses by measuring photosynthetic parameters (which are often assumed to be stable) in addition to biomass. Pentaclethra macroloba were grown in full sunlight or shade under three levels of nitrogen alone or with conspecific neighbors that could potentially alter nutrient availability via competition or facilitation. Biomass and photosynthesis were not affected by nitrogen or competition for seedlings in full sunlight, but they responded positively to nitrogen in shade-grown plants. The trade-off predicted by the GDBH between growth and metabolite production was only present between flavans and biomass in sun-grown plants (abundant resource conditions). Support was also only partial for the CNBH as flavans declined with nitrogen but saponins increased. This suggests saponin production should be considered in terms of detailed biosynthetic pathway models while phenolic production fits mass-balance based allocation models (such as the CNBH). Contrary to expectations based on the two defense hypotheses, trade-offs were found between defenses and photosynthesis, indicating that studies of plant defenses should include direct measures of physiological responses.

Effects of fertilization and drought stress on tannin biosynthesis of Casuarina equisetifolia seedlings branchlets

Nutrient, water, and their interactions influence the allocation of investment by plants to resistance and tolerance traits. We used a completely crossed randomized-block design experiment to examine the independent and interactive effects of nutrients and water availability on tannin production of C. equisetifolia seedlings. The results showed that nitrogen and phosphorus fertilizer have significant effects on total phenolics (TP) and extractable condensed tannins (ECT) concentrations of branchlets. TP and ECT concentrations decreased with fertilizer addition and increased in arid condition. This pattern lends to support source-sink hypothesis such as the carbon-nutrient balance (CNB) hypothesis and the growth-differentiation balance (GDB) hypothesis. Soluble sugars or starch concentrations were both inversely related to TP concentrations. However, there was no significant correlation between them and ECT concentrations. In addition, chlorophyll concentration had a positive linear correlation with TP and no significant correlation with ECT. On the contrary, chlorophyll a/b ratios were negatively correlated with TP and positively correlated with ECT. The discrepancy of relationship between carbohydrates and TP or ECT showed that the biosynthetic routes of different tannins were different. In this study, no significant correlation between TP and N, or ECT and N, did not support protein competition model (PCM). TP:N and ECT:N ratios were higher in nutrient deficiency and arid conditions, which were one of the important nutrient conservation strategies for C. equisetifolia.

Tree competition and defense against herbivores: currency matters when counting the cost

Browsing by herbivores profoundly affects the nature and extent of woody plant communities through a variety of mechanisms, including alterations to disturbance regimes (such as fire frequency), indirect effects on nutrient cycling and soil fertility, and changes to net primary production (Hobbs 1996). Plants have evolved myriad ways to avoid being eaten, ranging from the development of spiny, unpalatable foliage to the production of secondary metabolites as defense compounds (Bennett and Wallsgrove 1994). In analyzing the responses of plants to herbivory and the consequences of these responses for plant–plant competition, there has often been an implicit assumption that such defense strategies come at a cost to the individual (more defense means less growth or reproduction), thereby providing the basis for fundamental ecological and evolutionary hypotheses about allocation of limited resources (Strauss et al. 2002). Several hypotheses have been used as a framework for investigating the patterns and trade-offs of plant defense against herbivores (Stamp 2003), especially in the ecological literature. These hypotheses have been used singularly, or even in combination, to try and produce a unifying theory of plant– herbivore ecology and evolution. All of these hypotheses have been criticized in the literature. Although there are many instances when hypotheses have predicted empirical observations correctly, there are also many instances when either they have not, or they have been misinterpreted (Stamp 2003). One of these hypotheses, the carbon/nutrient balance (CNB) hypothesis, was originally developed to explain the influence of soil nutrients and shade on plant defense chemistry (Bryant et al. 1983). The CNB hypothesis suggests that species adapted to fertile sites (such as early-successional species) will respond to herbivory by utilizing stored resources for compensatory growth, while slower-growing species adapted to relatively infertile soils will instead protect their leaves by investing more carbon in anti-herbivory defense compounds. Such a hypothesis is appealing, as it makes specific predictions about patterns of allocation to plant secondary metabolite production, which have often been confirmed by empirical observation (reviewed by Stamp 2003). However, the CNB hypothesis has also been criticized, as there are many instances when predictions do not match observations (e.g., Hamilton et al. 2001, Koricheva 2002). In this issue, Calder et al. (2011) report that conifer expansion reduces the competitive ability of Populus tremuloides (trembling aspen). The transition from aspen to conifer forests in western North America often coincides with a transition to soils with lower pH and lower nitrogen concentrations (Jerabkova et al. 2006), at least partly mediated by the chemical composition of conifer needles being added to the litter. The loss of early-successional species, such as aspen, in these systems over time has been attributed to their physiological preference for nitrogen uptake as NO3, which may become less available in soils as conifer dominance increases (Kronzucker et al. 1997, Min et al. 1998, Kronzucker et al. 2003). While changes in nutrient availability may partly explain this successional trajectory, the canopies of evergreen conifers and deciduous broad-leaf trees also provide very different light environments for seedlings. Calder et al. (2011) show that by reducing light availability and altering soil characteristics, a conifer species (Abies lasiocarpa, subalpine fir) inhibited the growth of competing aspen trees. However, these abiotic changes induced by conifer dominance had another effect: they led to lower concentrations of phenolic glycosides (PGs) and condensed tannins (CTs) in aspen leaves, two groups of Commentary

Plant defense: warding off attack by pathogens, herbivores, and parasitic plants

Plant defense: warding off attack by pathogens, herbivores, and parasitic plants

Impacts of elevated CO2 on expression of plant defensive compounds in Bt-transgenic cotton in response to infestation by cotton bollworm

1 The allocation of defensive compounds of transgenic Bt (cv. GK-12) and nontransgenic cotton (cv. Simian-3) grown in elevated CO2 in response to infestation by cotton bollworm Helicoverpa armigera (Hübner) was studied in closed-dynamics CO2 chambers. 2 A significant reduction in foliar nitrogen content and Bt toxin protein occurred when transgenic Bt cotton grew under elevated CO2. A significantly higher carbon/nitrogen ratio as well as condensed tannin and gossypol contents was observed for transgenic Bt (cv. GK-12) and nontransgenic cotton in elevated CO2, in partial support of the carbon nutrient balance hypothesis as a result of limiting nitrogen and excess carbon in cotton plants in response to elevated CO2. 3 The CO2 level and infestation time significantly affected the foliar nitrogen, condensed tannin, gossypol and Bt toxin protein contents of cotton plants after feeding by H. armigera. The interaction between CO2 levels × cotton variety had a significant effect on foliar nitrogen content after injury by H. armigera.

Plant allocation to defensive compounds of transgenic Bt cotton in response to infestation by cotton bollworm under conditions of elevated CO2

Allocation of defensive compounds of transgenic Bt cotton grown under conditions of elevated CO2 in response to infestation by cotton bollworm, Helicoverpa armigera (Hübner) was studied in two successive years (2004 and 2005). Both CO2 levels and bollworm infestation affected allocation of defensive compounds of non-transgenic cotton and transgenic Bt cotton. Both significantly higher foliar condensed tannin and a higher carbon/nitrogen ratio for non-transgenic cotton and transgenic Bt cotton were recorded under elevated CO2 compared to ambient CO2. This result accords with the Carbon Nutrient Balance hypothesis linking limiting nitrogen and excess carbon in cotton plants in response to elevated CO2. Furthermore, significantly higher levels of catalase (CAT) and superoxide dismutase (SOD) and a significantly lower level of true choline esterase (TChE) were also observed in H. armigera fed on transgenic Bt cotton compared with those fed on non-transgenic cotton. Also, there were significant interactions between cotton variety and bollworm infestation time on CAT and SOD activities. Our results indicate that plant allocation to defensive compounds will be affected sufficiently to affect plant–herbivore populations interactions.

The ecological stoichiometry of toxins produced by harmful cyanobacteria: an experimental test of the carbon‐nutrient balance hypothesis

The elemental composition of primary producers reflects the availability of light, carbon and nutrients in their environment. According to the carbon-nutrient balance hypothesis, this has implications for the production of secondary metabolites. To test this hypothesis, we investigated a family of toxins, known as microcystins, produced by harmful cyanobacteria. The strain Microcystis aeruginosa HUB 5-2-4, which produces several microcystin variants of different N:C stoichiometry, was cultured in chemostats supplied with various combinations of nitrate and CO(2). Excess supply of both nitrogen and carbon yielded high cellular N:C ratios accompanied by high cellular contents of total microcystin and the nitrogen-rich variant microcystin-RR. Comparable patterns were found in Microcystis-dominated lakes, where the relative microcystin-RR content increased with the seston N:C ratio. In total, our results are largely consistent with the carbon-nutrient balance hypothesis, and warn that a combination of rising CO(2) and nitrogen enrichment will affect the microcystin composition of harmful cyanobacteria.

Seasonal and interannual dynamics of polyphenols in Myriophyllum verticillatum and their allelopathic activity on Anabaena variabilis

In 4 successive years, we investigated the seasonal and interannual variability of the total polyphenolic pool and of the individual polyphenolic compounds in Myriophyllum verticillatum, as well as their allelopathic activity in a small eutrophic lake. We tested whether nutrient availability explained interannual and seasonal changes in the production of polyphenols. There were no strong interannual variations in plant tissue carbon, nitrogen and phosphorus concentrations, while total phenolic compounds (TPC) significantly differed between years, especially in apical meristems (range: 38–122 mg g −1 dry weight (DW)). Seasonal patterns, with maxima between May and July, changed between years. Partially confirming the carbon-nutrient balance hypothesis sensu Bryant et al. [Bryant, J.P., Chapin III, F.S., Klein, D.R., 1983. Carbon/nutrient balance of boreal plants in relation to vertebrate herbivory. Oikos 40, 357–368], we found correlations between TPC and the C/N (carbon/nitrogen) ratio in some but not all years, especially in apical meristems. Plant tissue phosphorus content accounted also for the variability in TPC in some years. Crude extracts of apical meristems always inhibited the growth of Anabaena variabilis, used as a target cyanobacterium. Plant TPC concentration and allelopathic activity were significantly correlated in all years except in 2005. Bioassay-directed fractionation of M. verticillatum extracts coupled with LC–MS analyses of the respective fractions revealed several isomers of HHDP-di- and -tri-galloylglucose apparently responsible for the allelopathic effects. The individual active compounds revealed a more distinct seasonal pattern compared to the pool of phenolic compounds in M. verticillatum, with a clear maximum in May, the ecologically most relevant period for inhibitory effects of submerged macrophytes on phytoplankton.

Transgenic Bacillus thuringiensis (Bt) cotton (Gossypium hirsutum) allomone response to cotton aphid, Aphis gossypii, in a closed-dynamics CO2 chamber (CDCC)

Allocation of allomones of transgenic Bacillus thuringiensis Gossypium hirsutum (Bt cotton) (cv. GK-12) and non-Bt-transgenic cotton (cv. Simian-3) grown in elevated CO(2) in response to infestation by cotton aphid, Aphis gossypii Glover, was studied in a closed-dynamics CO(2) chamber. Significant increases in foliar condensed tannin and carbon/nitrogen ratio for GK-12 and Simian-3 were observed in elevated CO(2) relative to ambient CO(2,) as partially supported by the carbon nutrient balance hypothesis, owing to limiting nitrogen and excess carbon in cotton plants in response to elevated CO(2). The CO(2) level significantly influenced the foliar nutrients and allomones in the cotton plants. Aphid infestation significantly affected foliar nitrogen and allomone compounds in the cotton plants. Allomone allocation patterns in transgenic Bt cotton infested by A. gossypii may have broader implications across a range of plant and herbivorous insects as CO(2) continues to rise.

Nitrogen availability, growth, carbon partition and resin content in Grindelia chiloensis

Grindelia chiloensis (Asteraceae) is a shrub native to the Patagonia and Monte regions of Argentina and is presently under study as a resin crop. The production rate of diterpene resin acids is, partly, determined by environmental conditions and can be controlled by changing water and radiation availability or temperature. Although theory predicts that resin content should increase with low nitrogen availability, the responses reported in the literature are few, mainly in conifers and are not always consistent with predictions by the carbon nutrient balance hypothesis (CNBH). The objective of this work was to test whether resin production is linked to nitrogen availability as predicted by the CNBH. Plants of G. chiloensis were grown in a greenhouse under four nitrogen availability conditions (0, 1.5, 3.0, and 6.0 mM of N) generated using a modified Hoagland solution. Structural and functional characters were measured. Leaf resin and root total non-structural carbohydrate content (TNC) were higher in plants grown with low nitrogen availability, whereas assimilation rate was slightly affected, in accordance with predictions. Under low nitrogen availability, shoot biomass, and leaf–stem ratio were lower. Resin composition was only slightly affected by nitrogen availability, suggesting that this character is under strong genetic control.