Studies on the behaviour of bracken control chemicals in plant-soil systems

Stephen, Norman H. (1983) Studies on the behaviour of bracken control chemicals in plant-soil systems. PhD thesis, University of Glasgow.

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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.
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.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: Q Science > QD Chemistry
Colleges/Schools: College of Science and Engineering > School of Chemistry
Supervisor's Name: Duncan, Dr. H.J.
Date of Award: 1983
Depositing User: Ms Mary Anne Meyering
Unique ID: glathesis:1983-4972
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Feb 2014 11:45
Last Modified: 19 Feb 2014 11:49

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