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No description available.

Laboratory studies were conducted to investigate the mode of action of a new herbicide, isouron (N-(5-(1,1-dimethylethyl)-3-isoxazolyl)-N,N-dimethylurea). These studies included two plant metabolites of isouron, a monomethylurea (N-(5-(1,1-dimethylethyl)-3-isoxazolyl)-N-methylurea) and a urea derivative (N-(5-(1,1-dimethylethyl)-3-isoxazolyl)-urea). The compounds were examined for their effects on four metabolic processes of enzymatically isolated soybean (Glycine max L.) leaf cells. The processes examined were photosynthesis, protein, ribonucleic acid (RNA) and lipid synthesis. Relative incorporation of radiolabeled precursors was the parameter measured to assess herbicidal effects. Results indicate that isouron and its monomethylurea derivative possess similar herbicidal properties. The urea derivative revealed substantially reduced herbicidal effectiveness. The primary mode of action of these compounds appears to be inhibition of photosynthesis. Experiments to determine an I₅₀ value for isouron and experiments to evaluate the influence of an antioxidant EDU (N-(2-(2-oxo-1-imidazolidinyl)ethyl)-N’-phenylurea) and a singlet oxygen quencher DABCO (1,4-diazobicyclo-(2,2,2)-octane) on the photosynthetic inhibition and chlorophyll breakdown induced by isouron were also conducted. An I₅₀ value of 0.51 uM was calculated for isouron. There was no significant influence of EDU or DABCO on isouron toxicity. Greenhouse experiments were conducted to assess the interactions of isouron and four herbicide antidotes on two varieties of corn (Zea mays L.), Dekalb 'XL72AA' and 'XL67', and wheat (Triticum aestivum L.), 'Tyler' and 'McNair 1003'. Antidotes used were naphthalic anhydride (1,8 naphthalic anhydride), R-25788 (N,N-diallyl-2,2-dichloroacetamide), CGA-43089 (α-((cyanomethoxy)imino)benzene acetonitrile) and CGA-92194 (α-((1,3-dioxolan-2-yl-methoxy)imino)benzene acetonitrile. Statistically significant antagonistic interactions between herbicide and antidote were observed in both wheat and corn. Synergistic interactions were also recorded. The order of antidotal effectiveness on corn was determined to be: NA > CGA-43089 > CGA-92194 > R-25788 and in wheat: NA > CGA-43089 > R-25788 (CGA-92194 severely limited germination). Visual observation of both corn and wheat plants indicated that nonantidoted isouron was very toxic to these crops. / Master of Science

The biological actions of the herbicides SC-0224 (trimethylsulfonium carboxymethylaminomethylphosphonate) and glyphosate [N-(phosphonomethyl)glycine] (PMG) were compared. In each study trimethylsulfonium iodide (TMS-I) was included as a treatment because the trimethylsulfonium ion is a constituent of the SC-0224 molecular structure. In inflated duckweed (Lemna gibba L.), both formulated and technical grade forms of SC-0224 were found to be much more phytotoxic to duckweed than either formulated or technical grade forms of glyphosate. The growth inhibition caused by glyphosate was partially prevented by different combinations of the aromatic amino acids phenylalanine, tyrosine, and tryptophan; whereas, the duckweed growth inhibition caused by SC-0224 could not be reduced by the same amino acid combinations. TMS-I and SC-0224 were found to be equally phytotoxic to duckweed. SC-0224 caused larger increases than glyphosate in the pool levels of amino acids; the increases caused by SC-0224 were similar, however, to those caused by trimethylsulfonium iodide. Expressed on a per gram fresh weight basis none of the chemical treatments caused significant changes in soluble protein or the incorporation of ¹⁴C-leucine into soluble protein. On a per flask basis (allowing for decreased growth in treated flasks), both herbicides and TMS-I caused significant decreases in soluble protein and ¹⁴C-leucine incorporation. SC-0224 and TMS-I caused larger decreases than glyphosate in both cases but the SC-0224 and TMS-I treatments were not significantly different. These data indicate that differences in the phytotoxicity of SC-0224 .and glyphosate may be due to the action of the trimethylsulfonium ion of the SC-0224 structure. The effects of these herbicides on the conversion of shikimate to anthranilate in a cell-free extract of Klebsiella pneumoniae ATCC 25306 were also compared. SC-0224 and glyphosate equally inhibited the production of anthranilate indicating that SC-0224 has action similar to glyphosate on the shikimate pathway. The effects of these herbicides on photosynthetic electron transport (the Hill reaction) was determined using isolated thylakoids from Alaska pea (Pisum sativum L.). The action of SC-0224 was compared with the action of glyphosate, TMS-I and diuron [3-(3,4-dichorophenyl)-1,1-dimethylurea]. SC-0224, glyphosate and TMS-I did not inhibit the Hill reaction at concentrations up to 10 mM; whereas, diuron caused an almost total inhibition at 0.10 mM. The results of this study indicate that SC-0224 is not an inhibitor of photosynthetic electron transport. These studies indicate that both constituents of the SC-0224 structure, TMS and PMG, are phytotoxic and may act independently. / Ph. D.

The physiological responses of Myriophyllum spicatum to 2,4-D, diquat and copper were quantified using a plant tissue viability assay, and daily measures of dissolved oxygen and pH. Correlations of herbicide tissue residues to physiological response measures were determined and the relationship was used to develop exposure-response models. Diquat and copper had a greater effect on plant tissue viability than was observed for 2,4-D. Diquat produced greater reductions in dissolved oxygen concentrations and pH values than 2,4-D or copper. Copper exposure had the least effect on these parameters. Exposure-response models developed for 2,4-D predicted effective control at plant tissue residues ranging from 4000 to 4700 mg/kg. Aqueous exposure concentrations necessary to produce effective control plant tissue residues ranged from 0.20 to 0.40 mg/L. Exposure-response models developed for diquat predicted effective control at plant tissue residues ranging from 225 to 280 mg/kg. Aqueous exposure concentrations necessary to produce effective control plant tissue residues ranged from 0.113 to 0.169 mg/L. Exposure-response models developed for copper predicted effective control at plant tissue residues ranging from 680 to 790 mg/kg. Aqueous exposure concentrations necessary to produce effective control plant tissue residues ranged from 0.32 to 0.64 mg/L. Model predictions for 2,4-D, diquat and copper were within 0.5 mg/L of the manufacturers' label recommendations for these herbicides. The use of laboratory microcosms in development of exposure-response models for diquat and copper produced results comparable to those using the larger-scale greenhouse systems. Diquat effectively controlled M. spicatum at lower tissue residues than 2,4-D or copper. In addition, initial aqueous exposure concentrations were also lower for diquat. Use of these models in field situations should be coupled with considerations of quantity of biomass present and environmental conditions, such as turbidity, in order to accurately calculate exposure concentrations necessary for effective tissue residues. Thus, the use of these models can be used to optimize the impact on the target species while minimizing exposure for nontarget species.

Master of Science

Glyphosate (N-phosphonomethylglycine) is an organophosphate compound which was developed by the Monsanto Company in 1971 and is the active ingredient found in several herbicide formulations. The use of glyphosate-based herbicides in South Africa for the control of alien invasive plants and weeds is well established, extensive and currently unregulated, which vastly increases the likelihood of glyphosate contamination in environmental water systems. Although the use of glyphosate-based herbicides is required for economic enhancement in industries such as agriculture, the presence of this compound in natural water systems presents a potential risk to human health. Glyphosate and glyphosate formulations were previously considered safe, however their toxicity has become a major focal point of research over recent years. The lack of monitoring protocols for pesticides in South Africa is primarily due to limited financial capacity and the lack of analytical techniques.