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Title: Rust uredospore germination studies on leaf surfaces
Author: Parker, Ann
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 1982
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The work contained in this thesis investigates aspects of the physical, chemical and microbial components of the phylloplane of broad beans and snapdragons in relation to rust diseases of these plants caused by Uromyces fabae and Puccinia antirrhirii respectively. Although experiments were performed with both rusts initially, most work was done with uredospores of U. fabae. The microflora of uredospores and leaves of field-grown plants was sampled at frequent intervals during two summers using a variety of techniques. These studies showed that the dominant microorganisms in the phylloplane of both plants were bacteria and yeasts of the genus Cryptococcus. However composition of the phylloplane microflora varied with the host species, the age and severity of rust infection on sampled leaves, and the time of year. Isolations from uredospores collected from field-grown plants usually included a higher proportion of bacteria than occurred from isolations made from the phylloplane. Experiments using 14C-labelled exogenous nutrients indicated that utilisation by Cryptococcus cells was much greater than by germinating uredospores. As solutions of various exogenous nutrients of similar type and concentration to those found in the phylloplane failed to increase the germination and growth of germ tubes of mature uredospores, and as these uredospores can germinate in deionized water, it was deduced that competition for nutrients by microorganisms in the phylloplane would not inhibit germination of nutrient-independent uredospores. However it was found that solutions of yeast extract or various vitamins and growth substances could stimulate germination and growth of germ tubes of older uredospores. The effect of addition of various microbial epiphytes isolated from leaves and uredospores to germinating uredospores was investigated. Various reactions were seen including stimulation, inhibition or no effect on uredospore germination and growth of germ tubes. No isolate was shown to consistently inhibit uredospore germination and hence a microbial antagonist capable of controlling rust infection was not found. Addition of isolates of phylloplane microorganisms to leaves prior to inoculation of uredospores or addition of nutrients did not increase antagonism. However, interaction studies indicated that uredospore germination, growth of germ tubes and infection could be increased by the presence of cells of Cryptococcus, Sporobolomyces and Aureobasidium pullulans. The greatest stimulation of uredospore germination was achieved with cells of Cryptococcus and hence this interaction was studied in more detail. Addition of Cryptococcus cells stimulated uredospore germination and pustule formation on both attached and detached broad bean leaves when inoculated simultaneously with or prior to uredospores. Leachate from Cryptococcus cells also stimulated uredospore germination, growth of germ tubes and infection on detached leaves but to a lesser extent than the presence of the yeast cells. Uredospores utilised only a small proportion of substances leached from 14C-labelled Cryptococcus cells, whereas they could utilise a greater proportion of the membrane-sterilised leachate from similar yeast cells. However, the presence of uredospores appeared to affect the partitioning of the endogenous reserves of 14C-labelled yeast cells, increasing amounts of 14C in compounds leaked into the external solution and as CO2 but reducing the proportion remaining in the yeast cells. In interactions in which the 14C-label originated in uredospores, addition of Cryptococcus cells decreased the amount of 14C in the external solution without significantly affecting the proportions of label in the cells or as CO2. However, addition of membrane-sterilised leachate from Cryptococcus cells did not alter the partitioning of the 14C-label from uredospores to the external solution or as CO2.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available