Ph.D. (Botany) / Please refer to full text to view abstract

Please read the abstract in the section 00front of this document / Dissertation (M Inst Agrar (Weed Science))--University of Pretoria, 2006. / Microbiology and Plant Pathology / unrestricted

A rhizosphere effect was demonstrated for barley and corn plants at 4 to 6 weeks of age. No qualitative difference between the composition of the rhizosphere and indigenous flora was detected in the response of isolates to nutritional media of varying complexity. Examination of the rhizospheres of barley plants treated with 2,4-D at the rates of 0, 4 and 8 ounces per acre indicated that there was no difference between the total counts of treated and untreated plants. However, evidence was obtained suggesting a qualitative difference between the rhizospheres of the 8 ounce treated plants and those of the 0 and 4 ounce treated plants. On the other hand, corn plants treated with 2,4-D at the rates of 1 and 2 pounds per acre showed a temporary increase in their total rhizosphere count as compared to the total rhizosphere count of untreated plants. The increased total counts appeared to be due to the stimulation of those organisms requiring amino acids and yeast-soil extract for maximum growth. The direct application of 2,4-D to soil at the rate of 100 pounds per acre appears to result in a slight decrease in the total indigenous count. Although this lowering of the total count was due to a decrease in the number of organisms in all nutritional groups, some groups were affected more than others. By comparing the growth response of isolates from the indigenous and rhizosphere flora to 2,4-D at different pH levels, evidence was obtained which indicated that a physiological difference exists between these flora. A study of the rates of the decomposition of 2,4-D in different soils indicates that there is a marked variability in the detoxication rates. An organism was isolated which was capable of decomposing 2,4-D, This isolate showed morphological and cultural characteristics which suited members of the Corynbacterium and Achromobacter genera. / Land and Food Systems, Faculty of / Graduate

Three varieties of spring wheat, Selkirk, Manitou and Pembina, were grown in the field and greenhouse. 2,4-D, bromoxynil and picloram were applied at the 6" stage (4 leaf). The experiments were carried out in a randomized block design. 2,4-D and picloram showed a low concentration stimulation and high concentration inhibition on yield with Selkirk. Manitou and Pembina did not show definite trends with these two herbicides. Bromoxynil decreased the yield in all three varieties and the decrease was enhanced at higher concentrations. Bromoxynil was the only herbicide that increased the percent protein in all three varieties. An inverse relation between the percent protein and yield was seen in Selkirk, Manitou and Pembina treated with bromoxynil. 2,4-D and picloram also produced this relationship in Selkirk but not in Manitou or Pembina. Bromoxynil significantly reduced the yield of protein per plot, whereas 2,4-D and picloram did not. There was no quantitative relation between the protein extracted by 0.1 M Tris-glycine buffer (pH 8.3) or 0.05 N acetic acid (pH 3.5) and the percent protein in wheat. Selkirk was least affected by any of the herbicides, i.e. to the changes in the protein (albumin, globulin and gluten) as seen from the densitometric traces of gels. Manitou and Pembina showed changes as a result of the bromoxynil treatment. / Land and Food Systems, Faculty of / Graduate

This investigation was limited to the preparation of substituted phenoxyacetic acids and related compounds. The types of substances prepared for testing can, in general, be placed in three groups. These are: aryloxyacetic acids, arylmercaptoacetic acids, and those acids containing the thiophene nucleus.

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