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Title: Investigation of the phytoalexin response of Arachis hypogaea L. and its possible involvement in resistance to foliar pathogens
Author: Edwards, Christine
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 1993
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Natural infection of groundnut foliage with Cercospora arachidicola and Phoma arachidicola resulted in the accumulation of at least six antifungal compounds. The dominant antifungal compound was the pterocarpan medicarpin. Abiotic agents including ultra-violet irradiation, salts of heavy metals and detergents were used to elicit medicarpin and other antifungal compounds. Abiotic elicltors induced low concentrations of compounds compared with those in infected material; for example leaves treated with silver nitrate accumulated medicarpin in the range 0-25 μg/g fresh weight whereas concentrations in infected leaves ranged from 150-800 μg/g fresh weight. The more abundant antifungal compounds in extracts from infected leaves were isolated and purified by a combination of flash chromatography and semi-preparative high performance liquid chromatography (HPLC). Purified compounds were identified by their UV absorption, mass spectrometry and proton nuclear magnetic resonance spectroscopy with confirmation by on-line spectroscopic techniques including; liquid chromatography with diode array detection, gas chromatography mass spectrometry (GC-MS) and liquid chromatography mass spectrometry (LC-MS). Besides medicarpin other major phytoalexins were the isoflavone formononetin, the pterocarpan demethylmedicarpin and an isoflavanone. In addition minor phytoalexins were tentatively identified as 7-6'-dihydroxy-4'methoxy isoflavanone, an isoflavanone with unknown substitution and the isoflavone daldzein. An antifungal a beta-unsaturated aldehyde, occasionally found in leaves was identified as trans-trans 2-4 decadienal. Greenhouse experiments with two cultivars originating in Zimbabwe, P84/5/244 and Egret, infected with C. arachidicola and Puccinia arachidis, causal agents of early leafspot and rust respectively, resulted in differential accumulation of phytoalexins. In all experiments cv. P84/5/244 exhibited greater susceptibility and produced only small amounts of phytoalexins. On the other hand cv. Egret appeared to be more resistant and displayed a corresponding increase in phytoalexin accumulation. Field data demonstrated that accumulation of phytoalexins reflected plant genotype and was not associated with resistance of some cultivars to P. arachidicola. Analysis of the composition of phytoalexins indicated that demethylmedicarpin could be a metabolite of medicarpin. This was supported by demonstration of the ability of Cercospora arachidicola to degrade medicarpin in vitro. Phoma arachidicola also modified and degraded medicarpin but the major metabolites were not identified.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available