Contribution à l'évaluation écotoxicologique du Tébuthiuron - un herbicide de la classe des urées substituées

Une étude écotoxicologique a été menée à l'égard du Tébuthiuron (TB), un phytocide homologué inhibant la croissance de végétation nuisible, afin de mieux cerner son impact sur le milieu aquatique susceptible d'être affecté par des épandages terrestres. La toxicité du TB a été évaluée en réalisant des bioessais à trois paliers écologiques, soit avec la truite arc-en-ciel (Salmo gairdneri), avec l'algue Selenaslrum capricornutum et avec la bactérie (Photobacterium phosphoreum) du système Microtox®. Parmi ces trois indicateurs, les algues se sont montrées les plus sensibles (C150 = 0,08 mg • L-1), suivies des truites (CL50 = 115 mg • L-1) et enfin, des bactéries du système Microtoxe (C150 - 328 mg • L-1). Des résidus maximaux variant de 0,091 à 0,18 mg • L-1 rapportés pour le TB en milieu aquatique, suite à des applications expérimentales, laissent donc croire que seules les algues pourraient étre victimes d'une agression marquée. D'autre part, les essais réalisés avec le SOS Chromotest ont démontré que le TB était faiblement génotoxique sans activation métabolique. En revanche, des algues exposées à 1 mg • L-1 de TB durant 4 heures n'ont pu ni (dés)actlver ni bioaccumuier l'herbicide. En général, notre enquéte corrobore certaines données générées pour fins d'homologation de ce produit, lesquelles concluaient en faveur de son innocuité relative à l'égard de l'environnement aquatique. Les effets chroniques que pourrait avoir une longue exposition de faibles concentrations de TB sur certains paliers écologiques devraient cependant faire l'objet d'lnvestigations futures.A wide array of chemical products are commonly used to inhibit the growth of a diversity of undesirable vegetation types for numerous purposes. In forestry applications, herbicides can improve ranges, contribute to sylviculture and facilitate rights-of-way management, for example. Among congeners of the substituted urea class herbicides, Tebuthiuron (TB) has proven efficient for such activities. Since its commercial appearance in 1974, this broad-spectrum weed killer was employed to control a variety of herbaceous and woody plants. When applied on soil before or during the onset of plant emergence, TB irreversibly affects photosynthesis after being absorbed by roots and translocated to its target sites. Prior to its registration as a herbicide, TB had undergone extensive (eco)toxicological testing which had generally indicated low potential for environmental concern, with regards to terrestrial and avian fauna. Although TB is generally purported to be unproblematic towards fish, the overall impact of substituted urea class herbicides is still not fully documented, as far as various members of the aquatic community are concerned. The experimental results presented herein - specifically on TB - contribute both confirmatory as well as some new information in this respect.In our study, TB toxicity was investigated at three ecological levels by undertaking acute bioassays with rainbow trout (Salmo gairdeneri), algae (Selenastrum capricornutum) and bacteria (Photobacterium phosphoreum). Among these bio-indicators, algae proved to be the most sensitive (EC50 = 0.08 mg • L-1), followed by rainbow trout (LC50 = 115 mg • L-1) and bacteria (EC50 = 328 mg • L-1). Since maximum TB residues lying between 0.091 and 0.18 mg • L-1 have been reported for aquatic systems following experimental terrestrial applications, our toxicity results suggest that only algae could be adversely affected following acute exposure to the herbicide.Additional tests performed with the SOS Chromotest, a bacterial colorimetric assay for detecting DNA-damaging agents, first showed that TB is weakly genotoxic without metabolic activation. Since recent genotoxicity studios have revealed that vegetal systems can either detoxify, activate or uptake specific chemicals, we then explored this possibility by exposing S. capricornutum (106 cells • mL-1) to 1 mg • L-1 of TB for 4 h. Results of this acute exposure indicated an absence of positive (detoxication) or negative (activation, accumulation) phytoplanktonic interactions. Indeed, the genotoxic characteristics of TB, before and after algal exposure, were unaltered, as demonstrated by SOS Chromotest assays. In this same experiment, a similar assay on TB-exposed algal cells (i.e. SOS Chromotest on an algal cell solvent extract) detected no genotoxic activity.In conclusion, our study corroborates existing data generated for TB registration purposes and essentially supports the notion that this chemical is relatively harmless towards the aquatic environment under normal use conditions. Nevertheless, an important caveat remains concerning chronic affects on specific organisms, which could result from long term exposure to low concentrations of TB. Since such potential effects have not yet been adequately addressed, further studios are warranted in this area

Case studies of job access and reverse commute program: 2009-2010

This report presents perceptual, mobility and employment outcomes self-reported by 573 users of 26 transportation services funded by the Job Access and Reverse Commute (JARC) program. The respondents were predominantly low income with 42 percent reporting 2008 personal incomes less than $10,000 and two-thirds of the respondents earning$20,000 or less for the same year. Nearly half the respondents have no household vehicles. Nearly three in five respondents reported that their travel has become reliable and convenient after using the services. Workers using the services have benefitted from overall reductions in the cost of commuting to work. Close to 94 percent rated the service as being important or very important in keeping their jobs. Respondents also self-reported that the services allowed them to access a job with better pay or better working conditions, and to improve their skills. Both median hourly wages and median weekly earnings are reported to have increased since using the service for those workers who use the service to commute to work and were employed in the one-month period prior to starting use of the service. Alternative reasons may exist for these wage changes, including overall changes in the economic conditions of the locations where the services operate, as well as changes in the personal conditions of the workers that are unrelated to the JARC program in the period between starting use of the service and the time of the survey, such as graduation from job-training or school, residential relocation and so on. Because of the lack of a probability sample of services, the results cannot be generalized to the entire JARC program. Detailed case studies of the 26 services yield insights into the types of benefits that are being provided overall in these cases and the planning and programmatic environment within which they operate

Highly charged ions with E1, M1, and E2 transitions within laser range

Level crossings in the ground state of ions occur when the nuclear charge Z and ion charge Z_ion are varied along an isoelectronic sequence until the two outermost shells are nearly degenerate. We examine all available level crossings in the periodic table for both near neutral ions and highly charged ions (HCIs). Normal E1 transitions in HCIs are in X-ray range, however level crossings allow for optical electromagnetic transitions that could form the reference transition for high accuracy atomic clocks. Optical E1 (due to configuration mixing), M1 and E2 transitions are available in HCIs near level crossings. We present scaling laws for energies and amplitudes that allow us to make simple estimates of systematic effects of relevance to atomic clocks. HCI clocks could have some advantages over existing optical clocks because certain systematic effects are reduced, for example they can have much smaller thermal shifts. Other effects such as fine-structure and hyperfine splitting are much larger in HCIs, which can allow for richer spectra. HCIs are excellent candidates for probing variations in the fine-structure constant, alpha, in atomic systems as there are transitions with the highest sensitivity to alpha-variation

History and sensitivity comparison of two standard whole-sediment toxicity tests with crustaceans : the amphipod Hyalella azteca and the ostracod Heterocypris incongruens microbiotest

The review first details the development of the test procedures with Hyalella azteca which historically emerged as one of the recommended test species for whole-sediment assays and its gradual standardization and endorsement by national and international organizations. The sensitivity and precision of the H. azteca test for application on chemicals and on real world sediments is discussed. The review subsequently addresses the development of the whole sediment microbiotest with the ostracod crustacean Heterocypris incongruens with larvae of this test species hatched from dormant eggs (cysts), rendering this assay stock culture/maintenance free. The application of the 6-day ostracod microbiotest on sediments in Canada and in Belgium is discussed, as well as its endorsement by the ISO subsequent to an extensive international inter-laboratory ring test. The sensitivity of the amphipod and ostracod tests is compared by data from studies in which both assays were applied in parallel. A comparison of more than 1000 ostracod/amphipod data pairs of a 12-year river sediment monitoring study in Flanders/Belgium confirmed that both whole-sediment assays have a similar sensitivity and that the 6-day ostracod microbiotest is a valuable and cost-effective alternative to the 10-14 day amphipod test for evaluation of the toxic hazard of polluted sediments

Quantification of the influence of the downy mildew (Plasmopara viticola) epidemics on the compensatory capacities of Vitis vinifera 'Merlot' to limit the qualitative yield damage

A preview study on the analysis of the impact of downy mildew (Plasmopara viticola) epidemics on the plant growth and yield quality has shown no correlation between disease severity progress on the canopy and sugar accumulation in the berries from veraison until harvest, indicating the capacity of the vine to compensate a stress situation induced by the downy mildew damage on leaf canopy. In this study the compensation capacity of the plant was analysed during three years under field conditions comparing three different downy mildew control strategies: A = “Untreated canopy” (to prevent quantity losses, the clusters were treated once with a contact fungicide at the discovery of the first downy mildew sporulation); B = “Reduced fungicide schedule” (based on a first treatment at the appearance of the first symptoms, to avoid yield quantity losses followed by one or two additional fungicide applications during the early epidemic phase with the aim of delaying the epidemic). C = “Standard schedule” (schedule normally applied in the vineyard). The experimental plot was moved each year to avoid stress influence due to a repetition of the trials on the same place. The grapevine compensated for the carbohydrate requirements of the cluster by mobilizing the starch reserves stored in the woody parts. Roots were the most important site of carbohydrate accumulation used from the grapevine and the mobilisation was higher in treatment A followed by treatment B. This compensation did not completely exhaust the reserves of each woody part and lets suppose a hierarchical pattern of the mobilisation; first a mobilisation of the reserves stored in the roots, and secondly those from the trunk, cane and shoot. The downy mildew could be considered as a stress factor for the grapevine.

Quantitative effect of leaf damage caused by downy mildew (Plasmopara viticola) on growth and yield quality of grapevine 'Merlot' (Vitis vinifera)

The impact of downy mildew (Plasmopara viticola) epidemics on the plant growth and yield quality was analysed during three years under field conditions in order to show a relationship between disease severity on leaves and yield quality losses. Three different treatments were compared: A = “Untreated canopy” (to prevent quantity losses, the clusters were treated once with a contact fungicide at the discovery of the first downy mildew sporulation); B = “Reduced fungicide schedule” (based on a first treatment at the appearance of the first symptoms, to avoid yield quantity losses followed by one or two additional fungicide applications during the early epidemic phase with the aim of delaying the epidemic). C = “Standard schedule” (schedule normally applied in the vineyard). The experimental plot was moved each year to avoid stress influence due to a repetition of the trials on the same place. The epidemic progress in the treatment A has shown the same tendency during the three years with an increase starting from the beginning of the ripening phase. The disease severity was more important on the lateral than on the main leaves. During the three years of the study, the disease did not influence the amount of total healthy leaf area per plant until veraison. From this phenological stage until harvest, the healthy leaf area per plant decreased rapidly at the same time as the epidemic increased. The yield quantity was not affected indicating that a single fungicide application at the finding of the first sporulation was enough to preserve the crop production. Among the yield quality parameters, the sugar content has been negatively influenced by the downy mildew leaf damage. The difference was particularly evident between the treatments A and C with differences from 1.4 to 2.04 °Brix. Generally, treatment B didn’t differ from C. Sugar uptake in the berries begun to show a different dynamic between 7 and 14 days after the onset of ripening. No correlation between disease severity progress on the canopy and sugar accumulation in the berries from veraison until harvest was found, indicating the capacity of the vine to compensate a stress situation induced by the downy mildew damage on leaf canopy.

Influence of Plasmopara viticola on gas exchange parameters on field-grown Vitis vinifera 'Merlot'

The impact of downy mildew (Plasmopara viticola) epidemic on main and lateral leaf assimilation capacity of Vitis vinifera 'Merlot' has been quantified, under field conditions during the ripening phase, by means of leaf gas exchange measurements. The aim was to describe the impact of different disease severity levels on the gas exchange rate of symptomless portions of main and lateral leaves and of the sporulating parts in comparison with healthy leaves. The measurements were carried out on plants normally treated and on plants where only the clusters were treated with a contact fungicide to prevent quantity yield losses. A drastic reduction in the photosynthetic rate was observed on the sporulating area of main and lateral leaf tissues. Stomatal and mesophyll conductance decreased and stomatal resistance increased, indicating the difficulty of CO2 diffusing through the stomata into the mesophyll to the site of carboxylation. Downy mildew affected more negatively the gas exchange parameters on the symptomless parts of a diseased lateral leaf than of a main leaf, indicating a greater susceptibility of lateral leaves. A decrease of stomatal conductance and, consequently, of the photosynthetic rate, transpiration and water use efficiency was observed already at low severity level with increments of the disease severity on the leaf. At the same time an increase of stomatal resistance on the symptomless area of a lateral leaf was measured. Visual assessment of the diseased leaf area didn’t reflect the actual part colonized by the pathogen and at least a portion of the leaf area determined as healthy has in fact a latent lesion. Therefore, the visual estimation of downy mildew infection may not give a good indication of the effect of the pathogen on host physiology. The results also emphasized the important role of downy mildew as a stress element for the plant during ripening phase, a source element for carbohydrate production.

Response of 'Merlot' (Vitis vinifera) grapevine to defoliation caused by downy mildew (Plasmopara viticola) during the following growing season

The downy mildew (Plasmopara viticola) epidemics on leaf canopy during the ripening phase causes a stress situation for the grapevine. The plant compensates the carbohydrate request of the berries mobilising the carbohydrate reserves stored in the woody parts. In this fourth study the impact of the reserves reduction on the growth and fertility and the recovering capacity of the plant were analysed during two consecutive periods of two years (first year = stress; second year = recovering). Two treatments were compared: “Untreated canopy” (to prevent quantity losses, the clusters were treated once with a contact fungicide at the discovery of the first downy mildew sporulation) and “Standard schedule” (schedule normally applied in the vineyard). The impact of decreased reserve contents in the following growth season has negatively influenced only the shoot elongation and the potential crop yield quantity of the “Untreated canopy” treatment. Nevertheless, a single recovery year was enough to rebuild the reserve pool particularly in the roots, confirming the acclimation potential of the grapevine.