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Pharmaceuticals have been classified as an environmental concern due to their increasing consumption globally and potential environmental impact. We examined the toxicity of sediment‐associated diclofenac and citalopram administered as both single compounds and in a mixture to the sediment‐living amphipod Corophium volutator. This laboratory‐based study addressed the following research questions: (1) What is the toxicity of sediment‐associated diclofenac and citalopram to C. volutator? (2) Can the mixture effect be described with either of the two mixture models: concentration addition (CA) or independent action (IA)? (3) What is the importance of the choice of (i) exposure measure (start concentration, time‐weighted average [TWA], full exposure profile) and (ii) effect model (concentration–response vs. the toxicokinetic–toxicodynamic model general unified threshold model for survival in its reduced form [GUTS‐RED]) for the derived effect concentration values? Diclofenac was more toxic than citalopram to C. volutator as a single compound (10‐day exposure). Diclofenac exposure to C. volutator provided median lethal concentrations (LC50s) within the same range (11 µg g−1 dry wt sediment) using concentration–response based on TWA and both GUTS‐RED models. However, concentration–response based on measured start concentrations provided an approximately 90% higher LC50 (21.6 ± 2.0 µg g−1 dry wt sediment). For citalopram, concentration–response parameters were similar regardless of model or concentration used (LC50 85–97 µg g−1 dry wt sediment), however, GUTS‐RED with the assumption of individual tolerance resulted in a lower LC50 (64.9 [55.3–74.8] µg g−1 dry wt sediment). The mixture of diclofenac and citalopram followed the CA quite closely, whereas the result was synergistic when using the IA prediction. In summary, concentration–response based on TWA and GUTS‐RED provided similar and reasonably good fits compared to the data set. The implications are that GUTS‐RED will provide a more flexible model, which, in principle, can extend beyond the experimental period and make predictions based on variable exposure profiles (toxicity at different time frames and at different variable exposure scenarios) compared to concentration–response, which provides contaminant toxicity at one point in time. Environ Toxicol Chem 2024;43:1767–1777. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. [ABSTRACT FROM AUTHOR]

We propose benchmark dose estimation for event-time data, using a two-step approach. This approach avoids estimation of complex models and has been previously shown to give robust results for summarizing relevant parameters for risk assessment. In the first step, the probability of the event of interest to occur (in a certain time interval) is described as a function of time, resulting in an event-time model; such a model is fitted allowing an individual curve for each dose, and relevant estimates are extracted. In the second step, a dose--response model is fitted to the estimates of t50 obtained from the event-time model in the first step. Given a predefined benchmark response, the benchmark dose is then estimated from the resulting model. This novel approach is demonstrated in two examples. Our application of the time-to-event model showed a gain in power compared to the traditional analysis of end-of-study summary data. [ABSTRACT FROM AUTHOR]

Mathematical models within the General Unified Threshold models of Survival (GUTS) framework translate time‐variable chemical exposure information into expected survival of animals. The GUTS models are species and compound specific and explicitly describe the internal exposure dynamics in an organism (toxicokinetics) and the related damage and effect dynamics (toxicodynamics), thereby connecting the external exposure concentration dynamics with the simulated mortality or immobility over time. In a recent scientific opinion on toxicokinetic–toxicodynamic (TKTD) models published by the European Food Safety Authority (EFSA), the GUTS modeling framework was considered ready for use in the aquatic risk assessment for pesticides and aquatic fauna. The GUTS models are suggested for use in risk assessment, if they are sufficiently validated for a specific substance–species combination. This paper aims to illustrate how they can be used in the regulatory environmental risk assessment for pesticides for a specific type of refinement, that is, when risks are triggered by lower tiers in acute as well as in chronic risk assessment and mortality or immobility is the critical endpoint. This approach involves the evaluation of time‐variable exposure regimes in a so‐called "Tier‐2C" assessment. The insecticide chlorpyrifos was selected as an example compound because a large data set was available. The GUTS models for 13 different freshwater arthropods and 8 different theoretical aquatic exposure profiles were used to calculate a series of GUTS‐based risk estimates, including exposure profile‐specific multiplication factors leading to 50% mortality or immobility at the end of the tested profile (LP50/EP50) as "margins of safety." To put the use of GUTS models within the tiered aquatic risk assessment into perspective, GUTS models for the 13 aquatic arthropods were also used to predict the environmental risks of a measured chlorpyrifos exposure profile from an experimental ditch study (Tier‐3 approach), and the results are discussed in the context of calibration of the tiered approach. Integr Environ Assess Manag 2021;17:243–258. © 2020 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC) KEY POINTS: This paper illustrates how General Unified Threshold models of Survival (GUTS) models can be used in the regulatory environmental risk assessment for pesticides for the evaluation of time‐variable exposure regimes in a so‐called Tier‐2C assessment.For chlorpyrifos as an example compound, GUTS models for 13 different freshwater arthropods and 8 different theoretical aquatic exposure profiles were used to calculate a series of GUTS‐based risk estimates, including exposure profile‐specific "margins of safety" (LP50/EP50) values.GUTS models were put in perspective within the tiered aquatic risk assessment by comparing the predicted environmental risks of a measured chlorpyrifos exposure profile for 13 aquatic arthropods to results from an experimental ditch study.For the example data set (chlorpyrifos) the results of the Tier‐2C assessment using GUTS models were not in conflict with the principle of the tiered approach in that lower tiers should be more conservative than higher tiers. [ABSTRACT FROM AUTHOR]

The negative effects of pesticides on beneficial terrestrial invertebrates have received much attention, and in this context quantification of the activity of metabolic enzymes provides an important tool. Unfortunately, current methodologies are limited in their use.In this study, the grain beetle, Tenebrio molitor was used to illustrate why in vitro methods are ineffective to quantify metabolic enzymes in some insects. It was further used to optimize an ex vivo assay that can replace in vitro methods when these are ineffective. The ex vivo method can quantify metabolic enzymatic activities of cytochrome P450 mixed function oxidases (CYPs), general esterases (EST) and glutathione S‐transferases (GST).In detail we show, that T. molitor produces inhibitory and/or degrading agents, which are released during tissue homogenization. As these agents inhibit the in vitro CYP activity of rat liver microsomes we suggest, that they are also responsible for the lack of in vitro CYP activity of T. molitor. We tested the effectiveness of broad‐spectrum protease inhibitors as protective agents and found, that they were only effective in concentrations that also interfered directly with the activity of CYP enzymes. As an alternative to in vitro measurements, we optimized an ex vivo method for quantification of CYP, EST and GST on individual gut systems of T. molitor. Our ex vivo method advances previous ex vivo methods, by allowing quantification of three major enzyme families, all in a single individual, and by exploiting real‐time kinetic measurements of the enzymatically produced product.Subsequently, our method was successfully tested for its use on three different life stages: larvae, pupae, and adult insects from T. molitor and Apis mellifera. The method shows promise for expansion to other species and potentially other enzyme families and/or substrates. [ABSTRACT FROM AUTHOR]

BACKGROUND: Combining low doses of chemical insecticides with entomopathogens constitutes a sustainable pest control method, but the significance of the timing and sequence of exposures needs clarification. We studied lethal effects of combinations of the entomopathogenic fungus Beauveria bassiana (KVL03‐122) and the pyrethroid alpha‐cypermethrin on the beetle Tenebrio molitor under varying timing and sequence of exposure. Synergy over time was evaluated in relation to the model of independent action (IA). We expected that increased progression of disease caused by B. bassiana would make beetles more susceptible to the insecticide, leading to enhanced synergy. RESULTS: Synergistic effects between B. bassiana and alpha‐cypermethrin were observed when B. bassiana was applied first, but only when the interval between applications was >48 h. With 72 h between exposures, mortality had increased to 100% after 8 days, in contrast to the 60% mortality expected. No synergy was observed when the insecticide was applied prior to fungal exposure within 24 h. CONCLUSION: The sequence and timing of exposure do matter to achieve synergistic mortality by combining B. bassiana and alpha‐cypermethrin, and the IA model proved to be a strong tool with which to evaluate the interactions of the two stressors over time. Pest control strategies could include B. bassiana followed by low‐dose exposures to alpha‐cypermethrin after 2–3 days. © 2018 Society of Chemical Industry This study shows that synergy between an insecticide and an entomopathogenic fungus depends on both application sequence and the time intervals between these in Tenebrio molitor as a model insect. [ABSTRACT FROM AUTHOR]

Two of the main classes of pesticides commonly used in agriculture are azole fungicides and pyrethroid insecticides. Because azoles have been shown to synergize the effect of pyrethroids, the effect of their mixture is of concern. The aim of the present study was to investigate the effect of sublethal concentrations of epoxiconazole and α-cypermethrin and their mixture on growth, reproduction, and in vivo cytochrome P450 activity of the aquatic crustacean Daphnia magna over 42 d. Continuous exposure to nonlethal concentrations of α-cypermethrin at 20 ng/L negatively affected adult growth and number and size of neonates within the first 14 d of exposure. Exposure to epoxiconazole at 25 μg/L increased protein content of adults within 1 to 3 d after initiating exposure and increased cumulative number of offspring at exposure times >31 d. Epoxiconazole enhanced the negative effect of α-cypermethrin up to 3-fold leading to decreased growth, cytochrome P450 activity, and reproduction of D. magna within the first 14 d of exposure. After 14 d, the synergistic interactions disappeared. The reported effects, although lacking direct negative consequence in the long term, might have cumulative toxicity with other stressors such as food scarcity, predation, and pathogens, posing an additional hazard for the organisms at the beginning of their life cycle. Environ Toxicol Chem 2017;36:2155-2166. © 2017 SETAC [ABSTRACT FROM AUTHOR]

The article focuses on a research related to role of nitrate as a endocrine disruptor. Topics discussed include association between aquatic nitrate concentrations and hormonal changes such as decreased reproduction, intracellular conversion of nitrate to nitric oxide in Daphnia magna, relevance of thyroid hormones for metamorphosis and nitrogen available in the form of nitrate, nitrite and ammonia for biological uptake.

Temperature is an important factor affecting toxicity, determining chemical toxicokinetics and toxicodynamics in poikilothermic organisms. Because metabolic rates are also affected by temperature, interactions between the emergence of toxic effects and time are very likely. The aim of the present study was to investigate how temperature affects the toxicity of copper toward the nematode Caenorhabditis elegans when measured during short, fixed time frames compared with full life cycles. Copper toxicity was tested in 2 experiments at 4 or 6 temperatures in the range of 11 °C to 24 °C, with Cu concentrations spanning from 1 mg Cu/L agar to 40 mg Cu/L agar, respectively. Reproduction and mortality were determined across the entire life cycle, and the time to production of first egg and the population growth rate were calculated. The results showed that the 50% effect concentrations (EC50s) of Cu increased 1.5-fold to 2.5-fold with increasing temperature within the tested range, depending on endpoint. When calculating EC50 on reproduction after 24 h or 96 h, the typical setup for temperature-chemical interaction studies, results ranged from no temperature effect to effects much larger than those for the full life cycle. Studies of temperature effects on toxicity must therefore be carefully designed in relation to the research question being investigated. Environ Toxicol Chem 2014;33:1363-1371. © 2014 SETAC [ABSTRACT FROM AUTHOR]

Knowledge about how toxicity changes with temperature is important for determining the extent of safety factors required when extrapolating from standard laboratory conditions to variable field scenarios. In the present study, the authors evaluated the toxicity of Cu and Cd to the potworm Enchytraeus crypticus at 6 temperatures in the range of 11 °C to 25 °C. For both metals, reproductive toxicity decreased approximately 2.5-fold with increasing temperature. This is contrary to what most other studies have found. Measurements of the bioavailable fraction of the metals in the soils and the internal metal concentrations in the worms over time showed that the major cause of change in toxicity with temperature for Cu was the worms' ability to regulate internal concentration at high temperatures. Uptake of Cd increased with time at all temperatures and with higher rates at high temperatures. Hence, the lower toxicity of Cd at high temperatures is proposed to be due to the E. crypticus being more efficient at immobilizing Cd and/or repairing damages at high compared to low temperatures. The present study concludes that no consistent relationship between metal toxicity and temperature across species can be made. The metabolic dependence of the species in terms of regulating metal uptake, excretion, immobilization, damage, and repair processes, will be crucial factors in determining species susceptibility to metals at varying temperatures. Environ Toxicol Chem 2013;32:2274-2283. © 2013 SETAC [ABSTRACT FROM AUTHOR]

Environmental factors, in particular temperature, have been shown to affect the toxicity of chemicals. In the present study the authors exposed the nematode Caenorhabditis elegans to five concentrations of Cd (2, 4, 6, 8, and 10 mg Cd/L agar) at four constant temperatures (11, 15, 18, and 21°C) and monitored survival and reproduction on a daily basis. Data were incorporated in a population matrix model to determine the population growth rate (PGR). An additional experiment at 15 and 20°C and 0, 1, 5, and 10 mg Cd/L was performed to include growth measurements in order to relate changes in reproduction to resource allocations between investments in growth and reproduction. The impacts of Cd on PGR increased with increasing temperature, shifting the median effective concentration (EC50) for PGR from 11.6 ± 5.4 and 9.2 ± 1.3 at 11°C and 15°C, to 2.1 ± 0.1 and 1.7 ± 0.4 at 18°C and 21°C. Cadmium and temperature decreased growth rates, but Cd also increased maturation times and decreased final body size. It is hypothesized that Cd toxicity leads to a decrease in nutrient assimilation and that this 'chemical anorexia' is more severe at high temperatures, where energy demands for growth and reproduction are the highest. Environ. Toxicol. Chem. 2012;31:787-793. © 2012 SETAC [ABSTRACT FROM AUTHOR]

Butyltins (BTs), such as tributyltin (TBT) and dibutyltin (DBT), are toxic to aquatic organisms, but the presence of the strong adsorbent, black carbon (BC), can markedly influence BT toxicity and uptake in organisms. In the present study, the acute toxicity and uptake of TBT and DBT in the crustacean, Daphnia magna, were investigated with and without addition of nano-charcoal at different pHs and water hardnesses. The results showed that the toxicity of TBT and DBT increased by lowering the pH from 8 to 6. This reflects a relatively higher toxicity of cationic BT species than of the neutral species. At pH 6, by enhancing the water hardness of the media from 0.6 to 2.5 mM, the toxicity of TBT and DBT consistently decreased due to competitive binding of bivalent cations (Mg2+, Ca2+) to biotic ligands of D. magna. Furthermore, the toxicity of TBT to D. magna significantly decreased in the presence of nano-charcoal compared with experiments without nano-charcoal at pH 6 and 8, while no significant decrease in toxicity of DBT was observed in the presence of nano-charcoal. This can be attributed to the insignificant decrease of free DBT concentration in the presence of nano-charcoal compared with that for TBT. Conversely, it was observed that more TBT and DBT were taken up in D. magna in the presence of nano-charcoal due to the uptake of TBT or DBT associated with nano-charcoal by Daphnia in gut systems, as seen by light microscopy. This indicated that only free nonadsorbed BTs were toxic to D. magna, at least during short periods of exposure. Environ. Toxicol. Chem. 2011;30:2553-2561. © 2011 SETAC [ABSTRACT FROM AUTHOR]

Bioactive compounds produced by plants are easily transferred to soil and water and may cause adverse ecosystem effects. Cyclotides are gene-encoded, circular, cystine-rich mini-proteins produced in Violaceae and Rubiaceae in high amounts. Based on their biological activity and stability, cyclotides have promising pharmaceutical and agricultural applications. We report the toxicity of the cyclotides: kalata B1, kalata B2, and cycloviolacin O2 extracted from plants to green algae ( Pseudokirchneriella subcapitata), duckweed ( Lemna minor L.), lettuce ( Lactuca sativa L.), and bacteria extracted from soil measured as [H]leucine incorporation. Quantification by liquid chromatography-mass spectrometry demonstrated up to 98% loss of cyclotides from aqueous solutions because of sorption to test vials. Sorption was prevented by adding bovine serum albumin (BSA) to the aqueous media. Cyclotides were toxic to all test organisms with EC50 values of 12 through 140 µM (algae), 9 through 40 µM (duckweed), 4 through 54 µM (lettuce), and 7 through 26 µM (bacteria). Cycloviolacin O2 was the most potent cyclotide in all assays examined. This report is the first to document toxic effects of cyclotides in plants and soil bacteria and to demonstrate that cyclotides are as toxic as commonly used herbicides and biocides. Hence, cyclotides may adversely affect soil and aquatic environments, which needs to be taken into account in future risk assessment of cropping systems for production of these highly bioactive compounds. Environ. Toxicol. Chem. 2011; 30:1190-1196. © 2011 SETAC [ABSTRACT FROM AUTHOR]

The article presents a study that investigates the time course of changes in CO2 assimilation, conductance and chlorophyll fluorescence of glyphosate to affect photosynthetic electron transport chain. The study uses Kautsy curve parameters to measure gas exchange and chlorophyll fluorescence on barley plants exposed to increasing glyphosate doses. Results show that conductance and CO2 assimilation are affected at lower doses than chlorophyll fluorescence.

The plant Artemisia annua L. is cropped in many countries for production of the antimalarial drug artemisinin.Artemisinin is phytotoxic and has insecticidal activity. Large-scale cultivation of A. annua may cause transfer of artemisinin to soil and, hence, may affect both soil organisms and the aquatic environment if the compound is leachable. In the present study, a new method for extraction of artemisinin from soil was developed, and field concentrations and degradation kinetics of artemisinin in sandy and loamy soils were measured. The soil concentrations in a Danish A. annua field were up to 11.7 mg/kg. The degradation kinetics could be modeled as the sum of two first-order reactions, a fast initial degradation followed by a reaction that was 11- to 25-fold slower. It took at least 35 d before artemisinin could not be detected (<0.36 mg/kg) at 20°C, classifying artemisinin as being relatively persistent in the environment. Combined with its water solubility of 49.7 ± 3.7 mg/L, this makes it potentially leachable. In soil, artemisinin repelled the earthworm (Eisenia fetida; the 10 and 50% effect concentrations [EC10s and EC50s, respectively] were 5.24 ± 2.64 and 21.57 ± 4.73 mg/kg, respectively) and inhibited growth of lettuce (Lactuca sativa L.; EC50, 2.48 mg/kg). Springtails (Folsomia candida) were not affected in the tested concentration range of 1 to 100 mg/kg. Artemisinin had toxicity to the freshwater algae (Pseudokirchneriella subcapitata; EC50, 0.24 ± 0.01 mg/L) and duckweed (Lemna minor; EC50, 0.19 ± 0.03 mg/L) similar to that of the commercial herbicide atrazine. Based on the presented data, the risks of adverse environmental effects because of cultivation of A. annua are high and comparable to those when using commercial pesticides. [ABSTRACT FROM AUTHOR]

From a theoretical point of view, it has often been argued that the model of independent action (IA) is the most correct reference model to use for predicting the joint effect of mixtures of chemicals with different molecular target sites. The theory of IA, however, relies on a number of assumptions that are rarely fulfilled in practice. It has even been argued that, theoretically, the concentration addition (CA) model could be just as correct. In the present study, we tested the accuracy of both IA and CA in describing binary dose-response surfaces of chemicals with different molecular targets using statistical software. We compared the two models to determine which best describes data for 158 data sets. The data sets represented 98 different mixtures of, primarily, pesticides and pharmaceuticals tested on one or several of seven test systems containing one of the following: Vibrio fischeri, activated sludge microorganisms, Daphnia magna, Pseudokirchneriella subcapitata, Lemna minor, Tripleurospermum inodorum, or Stellaria media. The analyses showed that approximately 20% of the mixtures were adequately predicted only by IA, 10% were adequately predicted only by CA, and both models could predict the outcome of another 20% of the experiment. Half of the experiments could not be correctly described with either of the two models. When quantifying the maximal difference between modeled synergy or antagonism and the reference model predictions at a 50% effect concentration, neither of the models proved significantly better than the other. Thus, neither model can be selected over the other on the basis of accuracy alone. [ABSTRACT FROM AUTHOR]

The article discusses the combination outcome of herbicides on plants and algae. It aims to provide a comparative analysis of these combination effects throughout aquatic and terrestrial test systems and to determine whether terrestrial plants have higher chances of synergistic interactions of herbicide mixtures compared to aquatic plants and algae. Some of the mixtures considered that were examined on the terrestrial species include Tripleurospermum inodorum, Stellaria media, Lemna minor, and Pseudokirchneriella subcapitata. The authors stress that the concentration addition model of the mixtures is useful in approximating the impact of the mixtures of herbicides on terrestrial and aquatic species.

Binary-mixture studies often are conducted with the aim of elucidating the effect of one specific chemical on the biological action of another. The results can be interpreted in relation to reference models by the use of response-surface analyses and isobolograms. The amount of data needed for these analyses is, however, extensive, and the experiments therefore rarely are repeated. In the present study, we investigate the reproducibility of isobole shapes of binary-mixture toxicity experiments in terms of deviation from the reference model of concentration addition (CA), dose-level dependence, and isobole asymmetry. We use data from four herbicide mixtures tested in three to five independent experiments on the aquatic test plant Lemna minor and the terrestrial plant Tripleurospermum inodorum. The results showed that the variation both within and among experiments was approximately half the size for the aquatic test system compared to the terrestrial system. As a consequence, a consistent deviation from CA could be obtained in three of four herbicide mixtures for L. minor, whereas this was only the case for one or two of the herbicide mixtures tested on T. inodorum. For one mixture on T. inodorum, both CA synergism and antagonism were detected. Dose-dependent effects could not be repeated consistently, just as the asymmetry found in some isoboles could not. The study emphasizes the importance of repeating mixture toxicity experiments, especially for test systems with large variability, and using caution when drawing biological conclusions from the test results. [ABSTRACT FROM AUTHOR]

The joint action of soil-applied herbicide mixtures with similar or different modes of action has been assessed by using the additive dose model (ADM). The herbicides chlorsulfuron, metsulfuron-methyl, pendimethalin and pretilachlor, applied either singly or in binary mixtures, were used on rice (Oryza sativa L.). The growth (shoot) response curves were described by a logistic dose-response model. The ED50 values and their corresponding standard errors obtained from the response curves were used to test statistically if the shape of the isoboles differed from the reference model (ADM). Results showed that mixtures of herbicides with similar molecular targets, i.e. chlorsulfuron and metsulfuron (acetolactate synthase (ALS) inhibitors), and with different molecular targets, i.e. pendimethalin (microtubule assembly inhibitor) and pretilachlor (very long chain fatty acids (VLCFAs) inhibitor), followed the ADM. Mixing herbicides with different molecular targets gave different results depending on whether pretilachlor or pendimethalin was involved. In general, mixtures of pretilachlor and sulfonylureas showed synergistic interactions, whereas mixtures of pendimethalin and sulfonylureas exhibited either antagonistic or additive activities. Hence, there is a large potential for both increasing the specificity of herbicides by using mixtures and lowering the total dose for weed control, while at the same time delaying the development of herbicide resistance by using mixtures with different molecular targets. [ABSTRACT FROM AUTHOR]

Widely used herbicides sometimes inadvertently contaminate surface waters. In this study we evaluate the toxicity of herbicides to aquatic plants and algae and relate it to environmental herbicide concentrations and exposure scenarios, herbicide formulation and mode of action. This was done experimentally for ten herbicides, using the aquatic macrophyte Lemna minor L. and the green alga Pseudokirchneriella subcapitata (Korshikov) Hindak, supplemented with a database study comprising algae toxicity data for 146 herbicides. The laboratory study showed that herbicide formulations in general did not enhance herbicide efficacy in the aquatic environment. The Roundup formulation of glyphosate proved to be the only exception, decreasing the EC50 of the technical product for both L. minor and P. subcapitata approximately fourfold. Comparison of the sensitivity of L. minor and P. subcapitata revealed up to 1000-fold higher sensitivity of L. minor for the herbicides categorized as weak acids (pKa <5), emphasizing the importance of higher plants in hazard assessment. Database analyses showed that no herbicide group, categorized by site of action, was significantly more toxic than another. Synthetic auxins were the exception as they are virtually non-toxic to unicellular algae. There was no strong correlation between toxicity to algae and Kr,,, of the herbicides, not even within groups having the same site of action. Evaluating all data, few herbicides were toxic at concentrations below 1 μ litre-¹, which is the 99.9th percentile of the herbicide concentrations measured in the Danish surveillance programme. Joint action of several herbicides cannot however be excluded. © 2005 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

The regulation of NO3- uptake kinetics and reduction in relation to long- and short-term changes in irradiance was explored in roots and photosynthetic tissues of Lemna minor. The NO3- uptake kinetics, nitrate reductase activity, plant morphology, chlorophyll and tissue NO3-, organic-N, starch and sugars were measured on roots and fronds of L. minor grown at four combinations of irradiance- and NO3- availability. Long-term acclimatizations paralleled those of terrestrial plants. Short-term changes in irradiance, however, changed frond NO3- uptake proportionally with frond chlorophyll and N content, indicating a relationship between responsiveness and the metabolic potential of the plants. Root uptake changed to balance the change in frond uptake keeping whole plant uptake varying by < 40%. Nitrate reductase activity was primarily located in the roots and was correlated with frond uptake, indicating a transport of NO3- from shoot to root before reduction. This study shows that irradiance can affect the contribution of root and leaf uptake by aquatic plants and that roots play a major role in NO3- reduction despite large foliar uptake. [ABSTRACT FROM AUTHOR]

Summary • Both roots and leaves of free-floating plants can potentially take up nutrients. In this study, the ability and relative contribution of roots and fronds for N uptake by the floating macrophyte Lemna minor was investigated. • The NH 4 + and NO 3 - uptake kinetics of roots and fronds were measured on plants acclimated to three different NH 4 NO3 concentrations. • Lemna had the capacity to take up NH 4 + and NO 3 - through both roots and fronds; uptake kinetics for the two tissue types were comparable on an area basis. The overall contribution of root and frond to whole-plant uptake, estimated from measured kinetic characteristics, varied depending on plant N status (the root contribution increased from 32 to 73% for N-satiated and N-depleted plants, respectively). • The shift in the balance between root and frond contribution to whole-plant uptake resulted from a 1.5–38 times greater increase in the area-specific uptake capacity and affinity of roots relative to fronds, combined with a larger decrease in the minimum concentration for uptake ( C min ) for roots than fronds. At the morphological level, root–frond surface area increased with declining N supply, which might be beneficial to the plants since the area return per unit biomass invested was nine times greater for roots than for fronds. [ABSTRACT FROM AUTHOR]

1. The relative contribution of roots and leaves to nutrient uptake by submerged stream macrophytes was tested in experiments where plants were grown in an outdoor flow-channel system. Water was supplied from a nutrient-rich stream with inorganic nitrogen and phosphorus concentrations typical of Danish streams. 2. Four submerged macrophyte species were tested, Elodea canadensis, Callitriche cophocarpa, Ranunculus aquatilis and Potamogeton crispus, and all species were able to satisfy their demand for mineral nutrients by leaf nutrient uptake alone. This was evident from manipulative experiments showing that removal of the roots had no negative impact on the relative growth rate of the plants. Further, the organic N and P concentrations of the plant tissue was constant with time for the de-rooted plants. 3. Enrichment of water and/or sediment had no effect on the relative growth rate of two species, E. canadensis and C. cophocarpa, indicating that in situ nutrient availability was sufficient to cover the needs for growth. Despite the lack of a response in growth rate, a reduced root/shoot biomass ratio was observed with nutrient enrichment of water and/or sediment, and an increased tissue-P concentration in response to open-water enrichment. 4. The open-water nutrient concentrations of the stream in which the experiments were performed are in the upper part of the range found for Danish farmland streams (the majority of Danish streams). Still, however, the negligible effect of nutrient enrichment on the growth of submerged macrophytes observed suggests that mineral nutrient availability might play a minor role in controlling macrophyte growth in most Danish streams. [ABSTRACT FROM AUTHOR]

In this article the author discusses the prediction of the size and dose range of hormesis in mixtures. The author relates the results of the study on the response of root length in lettuce and growth rate in duckweed exposed to mixtures pesticides giving biphasic concentration-response curves of varying degrees. She adds that there were no significant differences between the estimated values depending on the type of model used as long as risk assessment is based on an adverse effect endpoint.

During the past two decades, the phenomenon of hormesis has gained increased recognition. To promote research in hormesis, a sound statistical quantification of important parameters, such as the level and significance of the increase in response and the range of concentration where it occurs, is strongly needed. Here, we present an improved statistical model to describe hormetic dose-response curves and test for the presence of hormesis. Using the delta method and freely available software, any percentage effect dose or concentration can be derived with its associated standard errors. Likewise, the maximal response can be extracted and the growth stimulation calculated. The new model was tested on macrophyte data from multiple-species experiments and on laboratory data of Lemna minor. For the 51 curves tested, significant hormesis was detected in 18 curves, and for another 17 curves, the hormesis model described that data better than the logistic model did. The increase in response ranged from 5 to 109%. The growth stimulation occurred at an average dose somewhere between zero and concentrations corresponding to approximately 20 to 25% of the median effective concentration (EC50). Testing the same data with the hormesis model proposed by Brain and Cousens in 1989, we found no significant hormesis. Consequently, the new model is shown to be far more robust than previous models, both in terms of variation in data and in terms of describing hormetic effects ranging from small effects of a 10% increase in response up to effects of an almost 100% increase in response. [ABSTRACT FROM AUTHOR]

Theoretically, the effect of two independently acting compounds in a mixture will depend on the slope of the dose- response curves of the individual compounds if evaluated in relation to the model of concentration addition (CA). In the present study, we explored development of the shape of the dose-response relationship for four different recommended endpoints (surface area, frond number, fresh weight-specific, and dry weight-specific relative growth rates [RGRA, RGRF, RGRFW, and RGRDW, respectively]) and for two differently acting herbicides (metsulfuron-methyl and terbuthylazine) over time (3-15 d) on the standard test plant Lemna minor to identify endpoints and experiment times for which predictions of independent action (IA) would depart the most from those of CA. After a test time of 6 d, predictions of IA based on RGRA and RGRFW showed antagonism in relation to CA. Based on RGRDW, synergy was predicted, whereas IA based on RGRF was indistinguishable from CA. To test the prediction of choice of endpoint giving different results in mixture-toxicity experiments, three endpoints and six combinations of independently acting herbicides were evaluated using isobolograms. The experiments showed that in four of six herbicide combinations, different conclusions were reached depending on endpoint. The contradictory isoboles did not follow the theory of IA and, therefore, are more likely to be related to differences in susceptibility of the physiological processes affecting each endpoint than to the shape of the dose-response curve. [ABSTRACT FROM AUTHOR]