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Thesis (MScAgric (Plant Pathology))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Maize is the most important cereal crop produced in southern Africa. Maize producers, processors, and consumers in the region, however, are affected by Fusarium ear rot, a disease caused primarily by the fungal pathogen Fusarium verticillioides that reduces grain quality and potentially contaminates the grain with mycotoxins (fumonisin). Due to the threat of fumonisin to human and animal health, and the economic losses associated with reductions in grain quality, strategies aimed at the prevention of Fusarium ear rot and fumonisin contamination are required. These preventative strategies should be focused on protecting the crop prior to harvest, as damage is known to occur in the field before storage. Chapter 1 provides the reader with a broad overview of maize production in southern Africa, the disease Fusarium ear rot caused by F. verticillioides, and the contamination of grain with fumonisins. Potential disease management practices are summarised, and the role of host resistance and its underlying mechanisms emphasised. Finally, the use of plant breeding and resistance elicitors as methods to enhance host resistance in maize towards Fusarium ear rot and fumonisin contamination are discussed in detail. The planting of maize genotypes with enhanced host resistance potentially offers the most efficient method to reduce Fusarium ear rot and mycotoxin contamination. If plant breeding is to be used to enhance resistance, sources of genetic resistance are required. These sources would ideally be in the form of locally adapted maize genotypes, such as inbred lines. In Chapter 2, maize inbred lines used in local breeding programmes, which are adapted to the production conditions in southern Africa, were evaluated as potential sources of resistance to Fusarium ear rot and fumonisin contamination. If inbred lines with good genetic resistance were to be identified they could be used by breeding programmes to develop commercial maize cultivars with resistance to Fusarium ear rot and fumonisin. Activation of resistance responses in normally susceptible maize genotypes using resistance elicitors could provide a novel management strategy for Fusarium ear rot control, as no commercial cultivars with complete resistance to this disease have been identified in southern Africa. Elicitors have previously been found to induce resistance to plant pathogens, mostly in dicotyledonous crops, but the ability of a range of elicitors to reduce Fusarium ear rot and fumonisin contamination in maize has not been investigated. In Chapter 3, a variety of chemical elicitors that induced resistance in other plant-pathogen systems were selected based on the different defence pathways that they stimulate, and evaluated in field and greenhouse trials. Three commercial maize hybrids were included in the trial, conducted at two different field sites, and the elicitors were tested for their ability to reduce Fusarium ear rot and fumonisin contamination of grain, as well as for their effect on yield. These elicitors could be applied in the field as part of an integrated disease management programme, are environmentally friendly, and would be affordable to commercial producers that produce the majority of maize in South Africa.

Investigations are described which were carried out to analyse the way in which certain mineral element deficiencies restrict the growth and development of plants. The plant system used in this work was excised pea roots grown in sterile culture media, and the deficiencies studied were those of iron, magnesium and molybdenum. Growth was measured at the cell level and related to other characteristics of the system; two different experimental designs being employed to assess the effects of deficiencies. In the first, roots were grown in full nutrient and in deficient media and growth was measured on samples taken after growing periods of 0, 3, 5, 7, 9 and 11 days; while in the second, roots were grown for 7 days in full nutrient and in deficient media and serial one centimetre sections taken from these roots were compared. The first approach assessed the effects of the deficiency on overall growth, and the second gave an indication of the effects of the deficiency on the longitudinal differentiation of pea roots. Both experimental approaches were employed when examining iron and magnesium deficient roots, but only the second when examining molybdenum deficient roots. Roots deficient in iron and magnesium were obtained by culturing tips cut from germinated seeds in deficient media, but two successive tip passages were necessary to obtain roots deficient in molybdenum. Growth was assessed basically in terms of length, cell volume, protein nitrogen, and rate of oxygen uptake. However with iron deficient roots measurements of invertase activity, sensitivity of the oxygen uptake to cyanide, and the frequencies of cells in the different stages of division were also made. The techniques involved in the culture of deficient and full nutrient roots, and the analytical techniques are described. It has been shown that iron deficiency markedly affects the growth and development of excised pea roots. Growth in terms of length and cell number per root is stopped after 7 days and no further increases occur between days 7 and 11. Although iron deficiency stops cell division, measurements made at day 7 indicate that this deficiency does not restrict the process of cell expansion. In fact 7 day old iron deficient roots carry larger cells in the terminal centimetre than full nutrient roots. By 11 days the iron deficient roots have a pronounced swelling at the terminal end, and it is suggested that this is brought about by an abnormal expansion of the cells in the lateral direction. Some cells containing mitotic figures are present in the tips of 7 and 11 day old iron deficient roots. However there are fewer cells in the division stages of prophase and metaphase and practically no cells in the stages of telophase and anaphase in the deficient roots when comparisons were made with full nutrient roots. The protein nitrogen content of iron deficient roots is lower than that of full nutrient roots at day 7, but there is a considerable increase in both deficient and full nutrient roots between days 7 and 11. The trend of the derived quantity, average protein nitrogen content per cell, is the same in both groups of roots up to day 7, but from day 7 to day 11 it increases sharply in the deficient roots but does not change in the full nutrient roots. This result indicates that cell division was not stopped in the deficient roots by a shortage of protein nitrogen as such. At the day 7 stage the distribution of protein nitrogen along the length of deficient roots is different to that in full nutrient roots. The front sections of the deficient roots contain an increased content and the back sections a decreased content when compared with full nutrient roots. On a per cell basis the situation is the same, as the cells in the front sections of deficient roots have a higher average protein content and those in the back sections a lower content when compared with the cells of full nutrient roots. The accumulation of protein nitrogen in the front sections of iron deficient roots is most probably associated with the cessation of active cell division in the meristem. Evidence is available which suggests that under normal conditions the formation and development of cells in the apex of the root is dependent on substrates synthesised in the mature regions of the root and translocated forward. It is considered that in iron deficient roots precursors of protein are no longer removed by the demands of the meristem and they condense to form protein in the regions adjacent to the apex. Invertase activity per unit protein nitrogen is the same in both full nutrient and iron deficient roots at all stages. Further, there is no difference in invertase activity when the corresponding sections of full nutrient and deficient roots are compared at day 7. It is clear that in this one respect the protein of iron deficient roots is similar to that of full nutrient roots. The rate of oxygen intake per root of iron deficient roots is lower than that of full nutrient roots at the early day 3 stage, but there are large increases in the rates in both deficient and full nutrient roots between days 3 and 11. It is of some significance that iron deficiency clearly reduces the rate of oxygen uptake at a stage before the process of cell division is stopped. On a per unit protein nitrogen basis the rate of oxygen uptake of deficient roots is lower than that of full nutrient roots after day 3. It is suggested that the effects of days 3 and 5 are a direct effect of iron deficiency but the effects at days 9 and 11 are influenced by the fact that cell division stops at day 7. The results from 7 day roots show that the effect of iron deficiency in reducing the rate of oxygen intake per unit protein nitrogen is confined to the front three sections of the root as iron deficiency does not alter the rates in the back three sections. Iron recovery experiments show that iron deficient roots 7, 9 and 11 days old can resume cell division and grow when they are transferred to a full nutrient medium. It is of interest that in these experiments the recovery in terms of an increased rate of oxygen uptake is greater than the recovery in terms of length and protein nitrogen. Experiments in which the rate of oxygen uptake of deficient and full nutrient roots were measured in the presence and absence of cyanide show that in both groups of roots there is a large fraction of the respiration insensitive to cyanide. The activity of this cyanide insensitive system increases considerably from day 3 to day 11 in both the full nutrient and iron deficient roots. Increases, after day 3 in the activity of this cyanide insensitive system, which would not contain iron, account for the large increase in the total rate of oxygen uptake of iron deficient roots between days 3 and 11. The activity of the cyanide sensitive system involved in respiration decreases in both groups of roots between days 0 and 5. It increases from day 5 to 11 in full nutrient roots, but does not increase in deficient roots over this period. That synthesis of a cyanide sensitive system involved in respiration stops at or about the same stage as cell division in iron deficient roots is considered to be highly important. This cyanide sensitive system most probably corresponds to the iron containing cytochrome/cytochrome oxidase system, and there is other circumstantial evidence that this system is important in the process of cell division. It is important to note that the activity of the cyanide sensitive system was the same in the tips of deficient and full nutrient roots at the day 7 stage. It may be that a certain minimum level of activity per cell is necessary to maintain division; a slight reduction stopping cell division completely, but not being capable of detection by the method of measurement.

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2v. : / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (D.Sc.)--University of Adelaide, Dept. of Botany, 1975

Typescript (Photocopy) Bibliography: leaves i-xii. (3rd paging sequence)

Thesis (M. Phil.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 104-107).

Thesis (Ph. D.)--University of Queensland, 2002. / Includes bibliographical references.