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Title: A study of the effects of powdery mildew (Erysiphe graminis f.sp. avenae) on the growth and development of wild and cultivated oats
Author: Sabri, Nacera
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1993
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This thesis reports a study of the relative tolerances of infection by the powdery mildew fungus (Erysiphe graminis f.sp. avenae) in one line of wild oat (Avena fatua) and two cultivars of cultivated oats (Avena sativa). cvs Lustre and Peniarth . The extent to which mildew developed on the three lines was assessed at various stages of growth by measuring percentage leaf area covered, and conidial production per plant and also amounts of chitin produced per unit fresh weight of leaf tissue. At all stages of plant growth, the line of wild oat supported more fungal growth than did cv. Lustre and even more so than cv. Peniarth. The effects of infection on growth and development of the three lines was assessed by growth analysis. In all lines, infection reduced dry matter production and the development of the different plant structures particularly the leaf. The partitioning of photosynthates to different parts of the plant was also altered. Levels of infection of between about 20 and 25% of leaf area infected reduced dry matter production in the cultivated oats but reductions were not evident in wild oat until about 30% of the leaf area was infected. Infection had little or no effect on the pattern of translocation in wild oat whereas it did in the cultivated oats, especially in cv. Peniarth. Reductions in green leaf area were a consequence of losses due to the enhanced senescence of the lower leaves as well as to reductions in the final sizes of the upper leaves. The reduced size of the upper leaves resulted from reduced cell division and cell expansion. All the reductions occurred to a greater extent in the cultivated oats than in wild oat. The reduction in dry matter production was reflected in reductions in yield components in all lines. Infection reduced numbers of tillers, numbers of fertile tillers and the numbers of grains per panicle. The numbers of grains per plant, hundred grain weight and total grain yield were thus reduced although the proportion of total biomass converted to grain (harvest index) was not affected in any of the three lines. Again all the reductions were greater in the cultivated oats than in wild oat. The reduction in dry matter production was partly due to reductions in green leaf area but reductions in the rate of photosynthesis per unit of green leaf tissue were also apparent. The photosynthetic efficiency was measured in infected and uninfected leaf tissue and also in the adjacent uninfected tissue of the infected leaf in wild oat and the cultivated oats using a Hansatech leaf electrode and by fluorescence analysis. The photosynthetic system in the infected tissue was reduced more by low levels of infection in the cultivated oats than by higher levels of infection in the wild oat. Compensatory photosynthesis in adjacent uninfected tissue was also reduced in all plant lines but not to the same extent as photosynthesis in the infected tissue. This was apparent in cv. Lustre but particularly so in wild oat. Thus infected leaves fixed less carbon than uninfected leaves but the total available to the plant and fungus in infected plants was reduced even more since more was lost through enhanced respiration. Measurements of dark respiration showed that infection increased the respiratory rate of infected and adjacent uninfected tissue in all three lines . The reduced rates of photosynthesis could have been due to parasite induced stomatal closure since this would reduce the amount of CO2 diffusing to the carboxylation sites within the chloroplasts of the mesophyll. The effects of infection on stomatal diffusive resistance were measured using an automatic porometer. An increase in diffusive resistance was found in the infected leaves during the light period in all lines around 5 days after inoculation, when mycelium development became significant. This increase in diffusive resistance could reduce the diffusion of CO2 into the leaf to the mesophyll cells and in fact, a reduction in the amount of CO2 reaching the reaction centres in the chloroplasts was evident particularly in the cultivated oats. In contrast, a decrease in diffusive resistance, almost certainly due to greater than normal stomatal opening since mycelium development was between 1 to 2%, was recorded during the dark period in the infected leaves of cultivated oats and wild oat. This decrease was recorded earlier in the cultivated oats than in wild oat. Some of the reduction in photosynthesis was also probably due to reductions in chlorophylls per unit area of leaf tissue. However, these reductions were not directly related to the reductions in photosynthesis since photosynthesis per milligram of chlorophyll in both infected and adjacent uninfected leaf tissue was not affected in any of the three lines. Thus the mechanisms by which photosynthesis was impaired in infected leaves probably involved effects on photosynthetic electron transport after the PSII Qg binding site and thus on the thylakoid proton motive force. However, the exact site of inhibition of the photosynthetic electron transport is uncertain but inhibition may be due to photoinhibition of the Qg binding-protein or impairment of any subsequent component of the photosynthetic electron transport system including PSI. This study showed that all aspects of growth measured were less reduced in wild oat than in the cultivated oats. Thus wild oat appears to be more tolerant of mildew infection than are the cultivated oats. However, the differences in tolerance between wild and cultivated oats were not great enough to suggest that tolerance is likely to be a useful character for inclusion in a conventional breeding programme for crop improvement.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available