Studies on the behaviour of bracken control chemicals in plant-soil systems
This thesis is principally an investigation on behavioural aspects of bracken control herbicides in some plant and soil systems with a view to improving their effectiveness. The main chemicals investigated were asulam and aminotriazole. As the work developed broader aspects were studied, including the behaviour of iodide and thiocyanate in plants. The work can be sub-divided as follows:- 1. A brief discussion of bracken and its control was made with some emphasis being placed on its control by systemic herbicides and possible approaches to improving their efficiency. 2. An investigation into the mechanism of action of asulam in plants. The findings can be summarized as follows:- Preliminary work involving lAA-oxidase assays and plant growth studies revealed an interesting interaction between e-aminobenzoic acid and asulam which indicated sulphonamide-type activity for asulam. A series of experiments were carried out whereby a range of species sown in beakers containing vermiculite were treated with asulam solutions. This resulted principally in stunting of the root system which could be partially or totally overcome by the simultaneous addition of either e-aminobenzoic acid or folic acid. Compounds related to e-aminobenzoic acid had no such activity, indicating the specificity of the antagonism. The results led to the conclusion that a possible mechanism of action of asulam is the inhibition of folic acid synthesis resulting in impairment of biological methylations and hence inhibition of protein and nucleic acid synthesis. The selectivity of asulam may be due to differential e-aminobenzoate or folate concentrations. 3. An investigation into the possibility of achieving pre-emergence activity for asulam in bracken control. as follows:- An appraisal of asulam soil behaviour was made, whereupon it The results can be summarized was considered that the use of additives would probably be required to regulate movement and prolong persistence for sufficient quantities to achieve contact with the rhizome buds. Asulam adsorption experiments, carried out using surfactant solutions and three acid-organic soils found under bracken, revealed that, on the whole, the anionic surfactants employed (sodium dodecyl sulphate and sodium dodecylbenzene sulphonate) significantly reduced asulam adsorption, whilst cetyl trimethylammonium bromide (cationic surfactant) significantly increased adsorption. These trends were reflected in leaching experiments using a thick layer soil plate method. Most surfactant treatments decreased persistence of asulam in nonleaching degradation experiments, sodium dodecylbenzene sulphonate being an exception. persistence. In a field trial, pre-emergence application of asulam No carbamate-derived additive had any effects on asulam (G.7 kg/ha) had no effect on bracken. The use of 1% cetyl trimethylammonium bromide or 1% sodium dodecylbenzene sulphonate in the spray formulations had no beneficial effect. 4. An investigation into the use of ammonium thiocyanate and other additives in bracken control formulations to reduce herb Established work had shown that scorching of bracken fronds by aminotriazole was a result of detoxification via a free radical mechanism and led to subsequent poor performance due to decreased translocation. A bioassay, involving floating bracken leaflets on aminotriazole solutions, was developed for assessing the effects of the free radical scavenger ammonium thiocyanate on aminotriazole scorching and assessing the activities of other potential additives. The results indicated that the concentration of ammonium thiocyanate, rather than the 1:1 ratio with aminotriazole at present used commercially, is the critical factor in determining the reduction in scorching. Further results revealed that the concentration of ammonium thiocyanate required could vary depending on the environment under which ~he bracken was growing. In a field experiment using 8.4 kg/ha aminotriazole and ammonium thiocyanate at a range of ratios from 1:0 to 1:1.25, only the 1:0.5 aminotriazole to ammonium thiocyanate gave significantly greater control than the aminotriazole alone after 3 years (74% control). Of other potential additives tested in the bioassay, thioacetamide was selected as promising for aminotriazole formulations, however, in a field trial it proved ineffective at the level used. Successful incorporation of asulam into the scorching bioassay proved difficult. In a field trial, an ammonium thiocyanate-asulam mixture (10:1) had no beneficial effect over asulam alone (2.2 kg/ha) at 2 spraying dates. Preliminary experiments involving the use of dock and potato leaflets, as material for the scorch bioassay, produced promising results for the tailoring of aminotriazole-ammonium thiocyanate formulations for improved effective control. 5. An investigation into the selective herbicidal activity of iodide. The results can be summarized as follows:- In initial studies, the rate of iodide oxidation in an in-vitro hydrogen peroxide/horseradish peroxidase system was found to be reduced by the addition of ammonium thiocyanate. The selective herbicidal activity of sodium iodide towards dwarf bean, pea, kale and cabbage was tested while the potential thiocyanate content of these plants was measured. A good relationship between increasing thiocyanate content and increasing resistance to iodide was observed. Further studies achieved limited success in conferring resistance to iodide in bean with thiocyanate additions, while the mechanism of inhibition of thiocyanate in the enzyme system was found to be ma~nly through chemical reduction of the iodine product - the toxic moiety. Evidence was gathered to suggest that other compounds present in plants, including glucosinolates, thiols and ascorbate, and some pesticidal additions, may contribute to the overall selectivity through their effect on iodine formation and/or persistence. 6. An investigation into the mode of action of iodide and thiocyanate in plants. The results can be summarized as follows:- Whole plant studies using dwarf bean reaffirmed that of the halides and pseudohalide thiocyanate, only iodide and thiocyanate possess defoliant properties. Established work had indicated their possible involvement with IAA (indole-3-acetic acid). In-vitro enzyme studies and photochemical and chemical oxidations of IAA, suggested that IAA destruction is not through a direct interaction between IAA and the halide and pseudohalide but depends on their conversion to the halogen or pseudohalogen which can then destroy lAA. This may be accomplished by the peroxidase enzyme system for iodide only and does not provide a mechanism for thiocyanate activity. However, both iodide and thiocyanate have chemical oxidation potentials suitable for their conversion to the halogen or pseudohalogen respectively via the photosynthetic apparatus. Hence, this provided an explanation for their activity and the apparent inactivity of bromide, chloride and fluoride which would be oxidised very slowly or not at all. 7. Some conclusions as to the outcome of the studies were drawn and some suggestions made for further work.