Invasion of senescing cereal and grass root tissues by parasitic fungi

The pattern and rate of natural senescence of root cortices (root cortex death, or RCD) was studied for wheat and barley in glasshouse conditions and wheat in root organ culture. Previously reported patterns were confirmed but a further stage of senescence, in which all cortical cell layers died, was associated with severe root impedance. The rate of RCD was significantly delayed by exogenous supply of nitrogen and, to a lesser degree, phosphorus and potassium; form of nitrogen (supplied as NO 3 or NH4 ) also affected RCD. Partial deficiencies of any of these nutrients usually caused as much RCD as did absence of the nutrient. Changes in bulk soil pH within the range 4.0 to 6.0 had no effect on RCD in intact root systems. Exogenous glucose delayed RCD but did not alter its pattern in organ culture, whereas kinetin altered the pattern of RCD, mainly through effects on root branching. Glucose was apparently mobilised, both acroand basipetally to regions of root not in contact with a glucose source. The relationship between RCD and infection by Phialophora graminicola (weak parasite) and vesicular. -arbuscular rnycorrhizal (yAM) fungi (biotrophs) was examined by growing cereals in soils with natural or introduced inocula or in cores of grassland soils. Wheat and barley were compared because of their reported different rates of RCD, and wheat was subjected to shaded or unshaded conditions to alter the rate of RCD. Dark runner hyphae of P. gram:Lnicola were more common in root members and root regions showing advanced RCD, whereas arbuscules of VAM fungi were commonest where cortical cells were nucleate. But not all aspects of VAN distribution on roots could be explained in terms of RCD; in particular, root axes and the proximal regions of primary laterals were poorly infected in conditions in which cortical senescence was unlikely to account for these effects. Population changes of VAM fungi, P. graminicola and root-colonising saprophytes were studied by direct observation of grass roots and by wheat seedling assay in cores of soil from different zones of grassland fairy rings caused by Marasmius oreades, Agaricus arvensis and Entoloma porphyrophaeum. The population of VAM fungi declined markedly in dead zones of rings of M. oreades and did not subsequently recover, whereas populations of P. graminicola declined but then fully recovered as grasses regrew within the rings. Equivalent population changes, though less marked, occurred in non-killing rings caused by A. arvensis and E. porphyrophaeum. It is concluded that these changes were primarily in response to patterns of grass root growth rather than being caused by direct effects of fairy ring fungi on the parasites. P. grarninicola significantly reduced colonisation of wheat roots by the take-all fungus, Gaeumannomyces graminis, in organ culture, consistent with reports that P. graminicola is a natural biocontrol agent of take-all in grasslands. TABLE. OF CONTENTS PAGE ABSTRACT SECTION 1: INTRODUCTION i 1.1 Sources of nutrients for rhizosphere microorganisms 1 1.2 Root cortex death (RCD) in the Gramineae 4 1.2.1 Validation of nuclear staining for assessment ofRCD 8 1.2.2 Factors influencing RCD 9 1.2.3 RCD in detached roots and root pieces II 1.2.4 Summary of the characteristics of RCD in the Gramineae 12 1.3 Implications of RCD for infection by parasitic fungi 13 1.3. 1 Gaeumannomyces graminis and Phialophora graminicola 13 1.3. 2 Vesicular-arbuscular mycorrhizal fungi 19 1.3. 3 Fairy rings in grassland 25 1.4 Aims and scope of work in this thesis 28 SECTION 2: MATERIALS AND METHODS 30 2.1 Fungal cultures 30 2.2 Culture and identification of VA mycorrhizal fungi 32 2.3 Cultivation of plants 33 2.3. 1 Growth in plastic drinking cups 34 2.3. 2 Growth in long perspex tubes 34 2.3. 3 Growth in long polyethylene tubes 35 2.3. 4 Growth of wheat in grassland soil cores 36 2.4 Mineral nutrition of plants 36 2.5 Incubation of detached root pieces 38 2.6 Sampling and assessment methods 40 2.6. 1 Assessment of cortical senescence 40 2.6. 2 Assessment of fungal invasion of roots 41SECTION 1: INTRODUCTION i 1.1 Sources of nutrients for rhizosphere microorganisms 1 1.2 Root cortex death (RCD) in the Gramineae 4 1.2.1 Validation of nuclear staining for assessment ofRCD 8 1.2.2 Factors influencing RCD 9 1.2.3 RCD in detached roots and root pieces II 1.2.4 Summary of the characteristics of RCD in the Gramineae 12 1.3 Implications of RCD for infection by parasitic fungi 13 1.3. 1 Gaeumannomyces graminis and Phialophora graminicola 13 1.3. 2 Vesicular-arbuscular mycorrhizal fungi 19 1.3. 3 Fairy rings in grassland 25 1.4 Aims and scope of work in this thesis 28 SECTION 2: MATERIALS AND METHODS 30 2.1 Fungal cultures 30 2.2 Culture and identification of VA mycorrhizal fungi 32 2.3 Cultivation of plants 33 2.3. 1 Growth in plastic drinking cups 34 2.3. 2 Growth in long perspex tubes 34 2.3. 3 Growth in long polyethylene tubes 35 2.3. 4 Growth of wheat in grassland soil cores 36 2.4 Mineral nutrition of plants 36 2.5 Incubation of detached root pieces 38 2.6 Sampling and assessment methods 40 2.6. 1 Assessment of cortical senescence 40 2.6. 2 Assessment of fungal invasion of roots 41 2.6. 3 Assessment of extent of lignification of roots 42 2.6. 4 Estimation of the number of infective propagules of VAM fungi in soil 42 2.6. 5 Determination of soil pH and available phosphorus content 43 SECTION 3: EFFECTS OF MINERAL NUTRITION AND HYDROGEN ION CONCENTRATION ON SENESCENCE OF THE CORTEX OF WHEAT ROOTS 44 3.