The research described in this týesis was focused on achieving a better understanding of the genetics of resistance of wheat to septoria tritici blotch (Mycosphaerella graminicola). Firstly, a detached leaf technique that could be a useful complement to field trials and an alternative to whole seedling assays in assessing cultivar resistance and investigating the genetics of the host-pathogen interaction was developed. Sets of inter-varietal substitution lines, developed at the John Innes Centre, involving known and possible sources of resistance to septoria tritici blotch, were tested with several single-pycnidium isolates in both the seedling and the adult plant stage. Two specific resistance genes were identified on chromosomes of `Synthetic 6x' and `Bezostaya 1'. A resistance gene, named Stb5, was identified using the M. graminicola isolate IP094269 and mapped on the short arm of chromosome 7D of `Synthetic 6x'. `Bezostaya I's specific resistance gene to IP0323 seems to be located in the same region as Stb6, and is indeed likely to be the same gene. `Bezostaya 1' and `Cappelle Desprez' also seemed to carry components for partial resistance. Triticum macha resistance to septoria tritici blotch on the other hand was both of a specific and isolatenon- specific nature. The specific components carried by T. macha seemed to be the Stb6 gene and an additional resistance gene, but it was not possible to identify their chromosomal location. To evaluate the relationship of heading date and plant height components to severity of septoria tritici blotch, an F6 single seed population of `Apollo' x `Thesee' was studied in natural conditions. Septoria tritici blotch levels were substantially lower in later-heading than in earlier-heading lines. Total plant height had comparatively little effect on disease severity, but increased distance between the two upper leaves increased disease levels. `Apollo' seems to carry partial resistance involving more than one QTL.

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Ericaceous plants grow preferentially on soils with a low pH, and generally perform poorly in areas with high pH calcareous soils. The reasons for the calcifuge behaviour of species of one genus, Erica, was studied in a calcareous rendzina collected from the South Downs, with a view to identifying physiological characteristics which could be used to select for improved lime-tolerance in the Ericaceae. Varieties of E. carnea and E. x darleyensis were shown to be relatively resistant to chalk-soil compared with varieties of E. vagans. In soil-based screening experiments, E. vagans 'Lyonesse' and 'Mrs D. F. Maxwell' rapidly developed severe foliar chlorosis, or lime-induced chlorosis, and their growth in soil was depressed by up to 7-fold compared with plants in an acid (pH 4) peat compost. E. carnea and E. x darleyensis varieties were resistant to chlorosis and remained green in chalk soil. The addition of nutrients (NPK and trace elements) increased the growth of the lime-resistant varieties in both peat and chalk soil. In nutrient solution, the source of nitrogen (nitrate or ammonium ions) had little effect on the growth of either lime-resistant or lime-sensitive varieties, although some varieties performed better when a component of the N was in the form of N03'. In an Fe-free nutrient solution, 1 mM NaHCO3 markedly reduced the root and shoot growth of both lime-tolerant and sensitive varieties. The clear differences between lime-resistant and lime-sensitive varieties found in soil experiments were not apparent with NaHCO3 in solution culture indicating that low concentrations of bicarbonate in nutrient solution may not be an appropriate method of screening for improved lime-tolerance in Erica. Bicarbonate and high pH in nutrient solution depressed the uptake of the radioisotopes 59Fe and 86Rb in both lime-resistant and lime-sensitive cultivars. Iron deficiency, induced by withholding Fe in nutrient solution, resulted in an elevated rate of root Fe(III)EDTA (ethylenediaminetetraacetic acid) reduction compared with Fesufficient plants. The maximum induced Fe(III)EDTA reduction rate (Vn,. ) of limeresistant cultivars (0.96-0.107 pmol. g"' FWT. h'') was lower than that of lime-sensitive varieties (0.125-0.404 tmol. g' FWT. h-1), suggesting that roots of lime-sensitive varieties have a greater capacity for Fe absorption than lime-tolerant varieties. Spraying or watering with Fe chelate (FeEDDHA - ethylenediamine di(ohydroxyphenylacetic acid)) remedied chlorosis in the lime-sensitive varieties and increased leaf chlorophyll by up to six fold and whole plant biomass by up to two fold. Chlorosis was associated with a reduced concentration of o-phenanthroline-extractable ('active') Fe, and a high concentration of total Fe, compared with green tissue. It is concluded that in chalk soil, lime-induced chlorosis, caused by the immobilisation of Fe in the shoot and a reduction in 'physiologically active' Fe in the leaf tissue, characterises lime-sensitive heather varieties. Resistance to chalk soil in Erica appears not to be related to root physiology. Rather the ability to transport and distribute Fe within the shoot under calcareous conditions is a major factor contributing to limeresistance in heathers.

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