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Title: The effect of herbicides on soil micro-organisms
Author: Shennan, Jean L.
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1972
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The w-substituted phenoxyacetic and gamma-phenoxybutyric acid herbicides (4-chloro-2methylphenoxyacetic acid, 2,4-dichloro-phenoxyacetic acid, etc., and their gamma-butyric homologues) are widely used as selective weedkillers. During crop treatment these chemicals can reach the soil in some quantity and therefore it is of interest to consider the effect, they may have in the microbiological activity of the soil environment. Results of investigations on in vitro culture with a selection of micro-organisms (fungi, yeasts and green algae) agreed with published reports that such herbicides were harmless at concentrations (100 ppm), these chemicals showed a consistent and striking difference in effect between the acetic and gamma-butyric acid forms at equivalent concentrations, the latter being toxic whilst the former had little or no effect on growth of fungi. This was a reversal of the effect in higher plants where plant growth regulating activity of the gamma-butyric acid form is dependent on its beta-oxidative breakdown by the plant to the active acetic acid homologue. The influence of chemical structure on the fungitoxicity of the gamma-butyric acid form of the w-substituted phenoxycarboxylic acid herbicides exemplified by gamma-MCPB (gamma-4-chloro-2-methylphenoxybutyric acid) was investigated in homologous series of these chemicals from the acetic to the valeric acid forms, and in other phenyxycarboxylic acids variously substituted in the side chain and the ring structure of the molecule. It appeared that activity toward micro-organisms was largely a function of the length of the side chain rather than its total molecular weight and was enhanced by increase in the number of substituted groups on the ring. Furthermore the activity of the gamma-butyric acids and higher members of the homologous series was shown not to be dependent on their breakdown to simpler forms by beta-oxidation. The toxicity and growth inhibition in Penicillum notatum by ?-MCPB was paralleled by inhibition of endogenous and mitochondrial respiration. It is concluded that gamma-MCPB at relatively high concentrations has an intrinsic toxicity towards micro-organisms in addition to its hormonal effect on higher plants when converted to MCPB by beta-oxidation.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available