The research undertaken was designed to investigate the potential use of coppice woodland for the clean up and remediation of heavy metal contaminated soil. Particular attention was given to high yielding coppice woodland species, especially willow and poplar. This was because breeding and field trials have reported that some hybrid willow clones can produce biomass yields of up to 60 t ha-1 y-1. The experimental works undertaken comprised three experiments which have been referred to as the 'Field Studies', 'Pot Studies' and 'Hydroponic Studies' and describes the medium in which the trees were grown and studied. The experiments were devised to study the variation in the uptake of metals between different tree species growing in different environments. The findings of the studies generally indicate that metal uptake rates determined in the field were lower that the uptake levels recorded in the pot and hydroponic studies. These differences were attributed to the increased availability of the study metals in the pot and hydroponic studies. Zinc concentrations in the above ground tissue of willow determined from the three experiments ranged from 159 to 223 mg kg-1 in the field study, 281 to 2995mg kg-1 in the pot study and 40 to 5530 mg kg-1 in the hydroponic study. Zinc was the only metal accumulated to significant concentration within the biomass of fields samples. Zinc, copper, cadmium, nickel and chromium were accumulated in the biomass of seedlings grown in the pot studies and zinc, copper, cadmium, nickel, chromium and to a lesser extent lead were accumulated by some of the hydroponic study trees. Some of the uptake levels recorded were not dissimilar from accumulation levels reported in hyperaccumulater species and highlights the potential of some tree species to accumulate metals in above ground tissues (leaves, twigs and stem). The pot and hydroponic studies suffered high seedling fatality rates which were attributed to metal toxicity and/or salinity. These findings could indicate possible establishment problems when trying to plant trees on contaminated sites.

Aspects of the biology, epidemiology and control of Rhizoctonia solani from potato were investigated using a range of laboratory and field-based experiments. In vitro experiments revealed nutritional factors including a range of carbon sources, and inorganic and organic nitrogen did not affect significantly mycelial growth or sclerotial germination. Carbon and nitrogen sources including cellobiose, glucose, glycerol and potassium nitrate significantly increased sclerotial biomass production in vitro. Mycelial growth, sclerotial production and germination occurred over a temperature range of 10-30oC, with an optimum of 25oC for both AG 2-1 and AG 3 isolates. Mycelial growth and sclerotial germination occurred at pH 4-9 with an optimum of pH 5.6, whereas sclerotial production occurred between pH 4-6 for AG 2-1 isolates and pH 4-8 for AG 3 isolates. Mycelial growth, sclerotial biomass production and germination declined with decreasing osmotic, matric and soil water potential, with mycelial growth prevented between -3.5 MPa and -4.0MPa on osmotically adjusted media, at -2.0 MPa on metrically adjusted media and -6.3 MPa in soil. Sclerotial production ceased prior to the limits for mycelial growth and germination for all isolates, between -1.5 MPa and -3.5 MPa on osmotically adjusted media and -1.5 MPa on metrically adjusted media. AG 3 isolates produced significantly more well-formed sclerotia during all in vitro experiments compared to the loosely constructed sclerotia produced by AG 2-1 isolates. A pathogenicity bioassay, coupled with staining and microscopic examination of stem tissues, showed all AGs formed infection cushions as a prerequisite to infection, with clear differences in the extent of infection cushion formation and subsequent stem lesion severity. AG 2-1 produced small, infrequent infection cushions, causing stem lesions only 1-2 mm in length which did not increase in size or severity after initial formation.

Most of the fieldwork for this study was conducted on the island of Islay, in the Inner Hebrides, the stronghold of the chough, Pyrrhocorax pyrrhocorax L., in Scotland. The aims of this study were to provide baseline data on the phenology of potential invertebrate foods of the chough, and to provide a greater understanding of the factors affecting these invertebrate populations. The literature concerning (a) the chough in Britain and its feeding ecology, (b) the invertebrate fauna of pasture, (c) the invertebrates associated with cow dung, (d) ivermectin and its effect on the invertebrate fauna associated with cattle dung, and (e) the multivariate analysis methods used in this study, is reviewed. An area of heather moorland and four pastures were selected on Islay. Invertebrates were collected from these sites between January 1988 and November 1989 using pitfall traps, and by sampling soil and cow pats. The data obtained was analysed using two multivariate analysis methods -Two-Way-Indicator-Species- Analysis (TWINSPAN) and Detrended Correspondence Analysis (DECORANA). Information on 62 surface-active taxa was obtained from pitfall trapping. Although seasonal taxa assemblages were recognized, the distribution of the invertebrate communities was primarily related to soil moisture content. Grazing intensity and seasonality were also important factors determining the composition of the invertebrate fauna at each site. The taxa active during the summer and winter at the two sand grassland sites, did not appear to differ as markedly as at the other sites sampled. Figures showing the seasonal activity of some of the frequently occurring taxa at each site considered potential chough prey items are provided. Surface-active potential chough food items were present, at all the sites investigated, throughout the year. Soil-sampling provided information on 34 taxa. As with the surface-active fauna, the primary factor influencing the soil fauna was soil moisture content. The time of year was also an important factor governing the soil fauna composition, with the majority of taxa occurring in low numbers during the summer months at all the sites sampled. Figures indicating the seasonal occurrences of some of the taxa considered potential chough prey items at each site are provided. Soil did not appear to be a good source of potential prey items for the chough during the summer months, although, as a result of seasonal increases in size, certain taxa, e.g. Tipulidae larvae, may have been more 'worthwhile' prey items at this time of year than at any other. Information on 54 taxa was obtained from sampling cow dung. Seasonality and age of the dung were very important in determining the composition of the dung fauna. The seasonal variations in the fauna associated with the cow pats are described. Potential chough prey items were associated with cow dung, in any stage of decay, throughout most of the year. Only during the period from October/November to January did there appear to be a lack of suitably sized potential prey items in the dung. The 'summer' months, when fresh dung contained large numbers of beetle adults and developing fly larvae, and late autumn, when pats deposited during the summer months are old enough for the large numbers of Aphodius spp. larvae present to have attained a reasonable size, were considered to be the times at which cow dung presented the best feeding opportunities for the chough. Fifty taxa were identified in samples of chough faeces. Multivariate analysis of these data indicated that the seasonal availability of prey items was the most important factor influencing chough diet throughout the year. Soil-dwelling Tipulidae (January to July) and Bibionidae (January to April) larvae, dung-associated insects (during the spring, and late summer and autumn), and surface-active insects (during the summer) were important invertebrate components of the diet. Cereal grains were extremely important supplementary food items during the early winter months, when invertebrate availability was low. An experiment was conducted at the College to investigate the effects on the dung fauna of spiking cow dung with 2.0, 1.0, or 0.5 mg/kg dung of ivermectin. Pats were placed on pasture between May and September 1988. The pats were lifted, and the soil beneath sampled, after 15 to 90 days exposure. A total of 65 taxa were identified. These data were analysed using TWINSPAN, DECORANA and Canonical Correspondence Analysis (CANOCO). The major factors determining the invertebrate fauna of the pats were length of exposure, exposure date, and ivermectin presence/absence. Ivermectin markedly affected the fauna associated with the pats. Pats exposed in June and August degraded faster than those exposed in May or September. In June, the ivermectin-treated pats degraded significantly slower than the control pats. An attempt to extract ivermectin from cow dung for analysis by high-performance liquid chromatography is described. This proved unsuccessful and the reasons for this failure, and possible improvements, are discussed. The main conclusions of this study are: (1) that Tipulidae larvae are extremely important components of the chough's diet on Islay, and that the climatic conditions of the island favour these insects; (2) livestock farming on Islay, especially the out-wintering of cattle, provides essential feeding opportunities for the chough, as, (a) gazing animals produce the short sward preferred by the chough as a feeding habitat, (b) large numbers of insects are associated with the dung of these animals, and (c) supplementary feed provided for the cattle in winter also provides an essential alternative food source for the chough at a critical time; (3) the chough's preference on Islay for feeding in sandy, coastal pasture is due to the fact that these sites, (a) contain a variety of suitable invertebrate prey items throughout most of the year, (b) are normally intensively grazed and so contain large amounts of dung with its associated fauna, and (c) are used for out-wintering cattle and therefore cereal grains can be found there; (4) treating cattle with ivermectin could have an adverse effect on the chough as it reduces the number and variety of invertebrates associated with the dung, an important food source for the birds, especially in spring and autumn.

One of the greatest proximate threats facing biodiversity is habitat change as a result of the combined effects of agricultural development and livestock grazing. Extensive livestock systems are increasingly competing with wildlife for access to land and natural resources in African rangelands. Ethiopia has the highest numbers of livestock in Africa with most livestock production taking place in highland systems. The Bale Mountains contain the largest extent of afroalpine habitat in Africa and are the most important conservation area in Ethiopia as one of thirty-four Conservation International Biodiversity Hotspots. The Bale Mountains National Park was established forty years ago to protect the endemic, and rodent-specialist Ethiopian wolf (Canis simensis) and the afroalpine habitats upon which they depend. I use a 21 year time-series of livestock counts in the park to detect changes in the Oromo traditional livestock production system or godantu. I show that a seasonal transhumant livestock production regime, akin to godantu, only appears to persist in the Web valley today and I also find some evidence that the type of livestock is changing with smaller stock, such as sheep and goats, being more frequently kept. Despite reportedly growing numbers of households in the Web valley, I do not detect an overall increase in livestock numbers, suggesting that the area may have reached its carrying capacity in terms of livestock. In contrast, the number of livestock on the Sanetti Plateau increased over the monitoring period, including the remote and inhospitable western section of the plateau. Furthermore, I detect an increase in the risk of contact and disease transmission between Ethiopian wolves and free-roaming dogs in wolf optimal habitats. I subsequently estimate, through the use of transects, the densities of livestock in the Web valley, Morebawa and Sanetti and calculate the ratio of observed to maximum sustainable livestock density (based on rainfall and vegetation productivity). The conservative results suggest that the Web valley and Morebawa are overstocked given the rangeland predicted productivity for those areas, and that the problem of overstocking is at its worst during the dry season. I then establish critical relationships between vegetation conditions, livestock grazing pressure and rodent populations (Tachyoryctes macrocephalus, Lophuromys melanonyx and Arvicanthis blicki) in the afroalpine, under both natural and controlled experimental conditions. Rodent biomass declines as the livestock index increases along a natural grazing gradient, but rodents‟ body condition, use of the habitat or breeding ecology and most vegetation variables measured do not significantly vary across the livestock grazing gradient. Rodent populations do respond to the experimental removal of livestock inside exclosures, although this response is site and species-specific, and I find some evidence that these responses are concurrent with changes in the vegetation. Grazed plots have higher plant species richness and diversity compared to the exclosure sites which harbour reduced bareground cover and taller vegetation. Differences among rodent species in their responses to grazing may be mediated by interaction between the direct effects of grazing on habitat quality and species-specific habitat requirements, although the specific mechanism of this interaction could not be tested. Livestock may negatively affect rodents by increasing their predation risk (through removing vegetation cover), by reducing the soil suitability for maintaining burrow systems (through trampling) or, less likely, by competing for food resources. A series of simple dynamic food chain models are developed to explore the interactions between Ethiopian wolves, rodents and vegetation and how they may be affected by increasing levels of livestock grazing. I also explore how predictions made about these trophic dynamics are affected by the type of functional response linking the different levels. The models suggest that the pastures of the Web valley and Morebawa are likely to be incapable of maintaining wolves at current livestock densities, while the Sanetti plateau may be able to sustain only slight increases in livestock density before tipping into a trophic configuration unable to sustain wolves. This model is a first step in assessing the seriousness of conflict between pastoralists and wildlife in BMNP. Resource selection functions are developed and validated for cattle and sheep/goats grazing in the Bale afroalpine in an attempt to understand some of the drivers behind the heterogeneous use of the landscape by livestock. Habitat use by livestock is focused on lower-lying pastures in the vicinity of water sources (rivers or mineral springs). Only cattle strongly select for/against particular vegetation types. The probability of habitat use is also linked to the distance from the nearest villages. The models developed highlight a strong association between livestock use and rodent biomass in Morebawa and Eastern Sanetti, suggesting that livestock grazing poses a threat to Ethiopian wolves‟ persistence in marginal habitats in which rodent availability is already limited. Furthermore, the concentration of livestock around water sources has serious implications for the degradation of the park‟s hydrological system and the livelihoods and food security of the millions of people living in the dependent lowlands. Integrating research and practice is a fundamental challenge for conservation. I discuss how the methodological tools developed and the insights gained into the dynamics of the afroalpine system can contribute to the management of livestock grazing pressure in Bale Mountains National Park and highlight gaps in the knowledge of the afroalpine ecosystem where more research is needed.

Eukaryotic cells maintain a compartmental cellular organization of membrane-enclosed organelles that communicate with each other through the exchange of trafficking vesicles. Members of a superfamily of membrane proteins, the so-called SNAREs, are essential for the necessary fusion of vesicle membranes to the membrane of target organelles. SNAREs are needed to overcome the energy barrier that prevents spontaneous membrane fusion events. A number of studies from the past decade indicated that SNARE proteins might fulfill a function beyond merging membranes. The mammalian plasma membrane SNARE Syntaxin1A was shown to directly interact with and through this interaction modify the activity of, for example, a calcium ion channel and a potassium ion channel. In its classical function as SNARE protein, Syntaxin1A mediates specialized vesicle fusion events such as synaptic transmission in neurons or secretion of insulin from pancreatic cells. These specialized vesicle fusion events require precise timing that is controlled by intracellular signaling events. These intracellular signaling events involve the coordinated action of members from different families of ion channels. Current models suggest that the dual functions of a SNARE protein in ion channel regulation and membrane fusion serve to fine-tune highly regulated vesicle fusion events. This thesis provides evidence for the first direct interaction between a SNARE protein and an ion channel from plants and suggests a function for this interaction in Arabidopsis potassium nutrition. Three different protein-protein interaction assays for full-length membrane proteins that comprised a yeast mating based split-ubiquitin assay, co-immunoprecipitation after expression in insect cells and bi-molecular fluorescence complementation after transient Arabidopsis root transformation, confirm that the Arabidopsis plasma membrane SNARE SYP121 interacts in vitro and in vivo with the Shaker ion channel subunit KC1. Furthermore, the interaction between KC1 and SYP121 is specific over the closest homologue of Syp121, namely SYP122. Shaker channels are plasma membrane proteins with four subunits that transport the essential macronutrient potassium in response to changes in membrane voltage. The KC1 subunit is unique among the Shaker channels. It can only act as a regulatory subunit that modifies channel properties when forming heterotetramers with other Shaker subunits such as AKT1, not as functional homotetramer. AKT1 is expressed predominantly in the root epidermis, i.e. root hairs, where it overlaps with the more broadly expressed KC1 and SYP121. Previous publications showed that a low external potassium concentration combined with high levels of ammonium that is used to block all root potassium uptake systems apart from AKT1, causes akt1 null mutants to display strongly reduced main root length as well as whole plant potassium content compared to wild type plants. It is shown here that the phenotype of both syp121 and kc1 null mutants is identical to the akt1 mutant under these growth conditions. The design of new antibodies against native AKT1 and KC1 and an optimized protocol for root plasma membrane protein enrichment and solubilisation allowed for the first time visualization of native Arabidopsis AKT1 protein. This technical advance made it possible to confirm that both Shaker channel subunits are present in equal amounts in the plasma membrane of roots cells from syp121 mutant and wild type plants. It is concluded that the potassium uptake phenotype of the syp121 mutant is not caused by the absence of channel proteins from the plasma membrane due to a disruption of the vesicle trafficking function of the SNARE SYP121. An alternative function for SYP121 in potassium nutrition that involves direct interaction with AKT1-KC1 heterotetrameric channels is supported by electrophysiological measurements after heterologous expression in Xenopus leavis oocytes. SYP121 modifies the voltage-dependent potassium uptake characteristics of AKT1-KC1 heterotetramers in a way most easily understood in context of a conformational change within the voltage sensing protein parts of the Shaker channel that are caused by the direct interaction with the SNARE protein. It is concluded that the identical potassium uptake phenotype of the akt1, kc1 and syp121 mutants is caused by the inability to form a functional tripartite complexes. As KC1 is able to form heterotetrameric channels with several different Shaker channel subunits, for example KAT1 that is highly expressed in guard cells, it is likely that this novel interaction between KC1 and SYP121 might modulate channel activities in different tripartite complexes to affect various cellular functions.

Law and State, especially forms of landed property and contract, have played an important mediatory role in the internationalisation of agricultural capital in Ghana. The establishment of cocoa production in Ghana in the late nineteenth and the early part of the twentieth century established the predominance of small holder peasant production in Ghanaian agriculture. The production and export of cocoa also established a specific form of internationalisation of agricultural capital in Ghana. This involved the subsumption of peasant commodity producers within the circuit of international capital. Because capital did not directly control production its relations with the peasantry centred around struggles over both the conditions of labour. in the sphere of production and over the realisation of the value of the peasants' product, in the sphere of circulation. These struggles were moulded by legal forms of landed property controlled by the direct producer and the character of the contractual relationship between peasant and the representatives of capital. The transformation induced by cocoa production included changes in forms of landed property, a process in which the colonial state played an important role. These changes have been a significant influence on the subsequent forms of internationalisation of agricultural capital in the post colonial period. The thesis shows through an analysis of the post colonial sugar and oil palm industries the nature of this influence. It also shows ho«- the shift in the proclaimed objectives of the state from the colonial concern with export agriculture to the "nationalist" post colonial goal of seif reliance came to be co-opted by new forms of international capital and the mediatory role of legal forms, especially contract, in this process of co-optation. This work is based mainly on written primary and secondary sources, complemented by intcrviews with some officials of the some of the institutions covered in the thesis. My secondary sources include unpublished essays and thesis, books, articles, reports, studies by companies, government bodies and similar such published material. Most of the primary material used in the parts of the work that deal with the colonial period conic from the British Public Records Office and the Ghana National Archives in Accra. For the post colonial period a substantial part of the primary information was gathered using personal contacts in various state institutions, particularly the Ministry of finance and Economic Planning, the Attorney General Department and the Ghana Investment Centre.

The work reported here aimed to provide a comprehensive database of core information to support the development and validation of process-based models of resource capture and growth in semi-arid overstorey agroforestry systems. Intensive field studies were carried out in Kenya over a 30 month period and the results obtained were combined with data from a previous project to produce a dataset spanning a 4.5 year period. This dataset was then used to verify output from the HyPAR model. Allometric procedures developed from the pipe model theory (Lott et al., 1998) were used to estimate tree growth non-destructively throughout the observation period. Significant differences in tree size between the sole (Td) and dispersed agroforestry (CTd) treatments were established during the first 130 days after planting, probably because of competition with the associated crops. The above-ground biomass and trunk length and taper characteristics of the CTd trees remained inferior to those of Td trees throughout the observation period, seriously undermining the economic potential of this agroforestry system. The biomass and grain yield of CTd understorey crops were similar to the corresponding sole crops during the first three seasons, but were negligible in three of the final four seasons, with maize yields reaching 50 % of the equivalent sole crop values only when seasonal rainfall was well above average. This observation suggests that water availability was the primary limitation for CTd maize during the final seasons of the trial, a conclusion supported by the superior performance of maize grown under net enclosures which simulated tree shade in the absence of below-ground competition. Cowpea and maize were grown concurrently in two seasons to examine the impact of grevillea on C4 and C3 crops with contrasting responses to shade; biomass and grain yield were less affected in cowpea than in maize. The tree canopy in the dispersed agroforestry (CTd) treatment reduced the daily mean quantity of radiation incident upon the understorey crops by c.30 % during the final four growing seasons, although the discontinuous nature of the tree canopy caused substantial local variation in shading intensity. Seasonal mean fractional interception was greater for the combined canopies of the CTd treatment when soil moisture status was relatively high than for either of the sole canopies, suggesting the occurrence of spatial complementarity. Tree shade had a substantial moderating influence on meristem temperature since the mean diurnal temperature range was reduced from a maximum of 20°C in sole maize to 13 °C under the trees, and maximum meristem temperature was decreased by up to 6 °C relative to sole maize. However, the non-uniform shading provided by the trees caused substantial spatial variation in thermal time accumulation and hence crop development. Grevillea continued to grow during dry seasons and was therefore able to capture off-season rainfall which might otherwise have been lost from productive use. In addition, adaptation of heat balance gauges for use on grevillea roots (Lott et al., 1996) showed that substantial quantities of water could be extracted from deep-seated reserves below the crop rooting zone during dry periods, indicating the potential for spatial and temporal complementarity. However, transpiration by grevillea greatly exceeded rainfall during the dry season, rapidly depleting residual water supplies which might otherwise have been available for crop growth. In addition, approximately two thirds of the water used by the trees during cropping seasons was extracted from the soil surface horizons by lateral roots at distances of up to 2 m from the trunk. Thus, the potential for above and below-ground complementarity may be seriously undermined by the extensive capture of water by tree roots from the crop rooting zone. Comparison of output from the HyPAR model against the observed results provided information pertinent to future model development. The model proved to be insufficiently flexible for end-users wishing to simulate the growth of different crops during the same simulation cycle, or to use model output to aid management decisions such as the timing of pruning. The allometric procedures used by the model to estimate canopy size from trunk diameter at breast height also proved incapable of accounting for reductions in canopy size resulting from pruning. Estimates of tree height are rounded to the nearest metre within the model, representing a potentially serious loss of resolution when annual increments often do not exceed 2 m. In addition, the numerous parameters required by the model would force most end-users to rely heavily on published information, potentially undermining the reliability of simulations.

Gaseous nitrous oxide (N₂0) undergoes physical and chemical reactions in the atmosphere, contributing to both global warming and the catalytic destruction of stratospheric ozone. This chemically reactive greenhouse gas is produced both naturally and anthropogenically. The greatest source of N₂0 is from the microbial transformation of N compounds during the processes of nitrification and denitrification in natural and cultivated soils. However, there is some uncertainty in the strength of these emission sources. Therefore one of the directives of the Terrestrial Initiative in Global Environmental Research programme, of which the following work was a part, was to elucidate the factors which influence the emission rates of N₂0 from these systems. It is essential that these factors are quantified, in order to correctly assess the effect of N₂0 as an environmental determinant. A reliable automated soil core headspace gas analyser system for the continuous measurement of N₂0 at the laboratory scale was developed. The system determined N₂0 evolution rates from reconstructed soil cores consisting of re-packed aggregates of known diameters, incubated under different environmental conditions. There was an increase in N₂0 emission rate (range = 0.5-61 x 10-7 mol N m-2 h-l) with aggregate size, soil N0₃-concentration and soil water content under unsaturated conditions. However, the extent of these trends was masked by the variability in emission rates. One source of variability in N₂0 emissions from unsaturated soil, was related to localized organic (e.g. faunal) residues. Subsequent investigations involving the incorporation of discrete faunal residues, DFRs (dead Earthworms), was found to greatly stimulate N₂0 emission from unsaturated re-packed soil cores. These N₂0 emission rates approached those attained when the soil was under saturated conditions, which were up to 3 orders of magnitude greater than emission rates from unamended, unsaturated soil. There was no apparent influence of DFR on N₂0 emissions from soil under saturated conditions suggesting that the effect of DFRs under aerobic conditions was the creation of localized anoxic zones. N₂0 emission rates increased with increasing soil water content reaching a maximum under fully saturated conditions for three different soils (range = 0.25-1.8 x 10-4 mol N m-2 h-1). The emissions of N₂0 from the three soils were different under both unsaturated and saturated conditions and appeared to be related to soil parameters, specifically organic matter content, clay content and soil pH. The contrast in rates of N₂0 emission from unsaturated and saturated soil prompted a test of the hypothesis that wetting/draining cycles increase the total emission rate. During the saturated phase, N₂0 is produced, but its egress is restricted by saturated transmission pores. Rapid drainage causes a flush of N₂0 from saturated aggregates by providing open emission channels. The rapid increase in N₂0 flux that was observed during the draining of saturated soil occurred in all three soil types (range = 1-5 x 10-3 mol N m-2 h-1). This almost instantaneous N₂0 pulse, which in some cases lasted less than 2 hours, occurred repeatedly, emitting similar rates of N₂0 during 10 cycles of flooding and draining. An attempt was made to simulate N₂0 emission using the results gained from these investigations to parameterize a reaction-diffusion model. The model successfully predicted N₂0 emission from soil undergoing a transformation from unsaturated to saturated conditions. However, model deficiencies were found during simulations involving the sequential rise and fall in water table height. The inability of the model to accurately predict the rapid increase in flux that occurred following core drainage, exposed gaps in knowledge and areas of future research regarding the short-term fluxes of N₂0 from soil.

Soil compaction has been described as the most serious environmental problem caused by conventional agriculture, as it results in several stresses which may interact simultaneously, including increased soil strength, decreased aeration and reduced hydraulic conductivity. Root system architecture (RSA) is the arrangement of roots within the soil matrix and is important because the specific deployment of roots within the soil can determine soil exploration and resource uptake. As roots deliver water and nutrients to growing plants, whilst also providing anchorage, their importance cannot be overstated. Yet, our understanding of how roots interact with the surrounding soil, especially at the micro-scale level, remains limited because soil is an opaque medium, so preventing roots from being visualised without disturbing them. Destructive techniques are commonly employed for the analysis of RSA, however this can result in the loss of key information concerning root architecture, such as elongation rates and root angles and important soil characteristics such as soil structure and pore connectivity. However, X-ray Computed Tomography (CT) has been shown to be a promising technique for visualising RSA in an undisturbed manner. The species considered in this thesis were wheat (Triticum aestivum L.) and tomato (Solanum lycopersicum L.). Further information regarding the response of roots to soil compaction has been achieved through the use of X-ray CT, automatic root tracing software and novel image analysis procedures. Soil compaction significantly affected root length, volume, surface area, angle, diameter, elongation rates and root path tortuosity, however the influence of soil texture on root responses to soil compaction was significant. Moderate compaction benefits root growth in clay soil, possibly due to the greater nutrient and water holding capacity, but adversely affected root growth in loamy sand. The results suggest that there is an optimum level of soil compaction for the different soil types. Roots elongated rapidly between 2-3 days after germination (DAG), it is hypothesised that is related to the mobilization of seed storage substances to the growing roots. The use of transgenic mutants of tomato with altered levels of abscisic acid (ABA) has provided a greater insight into the role of ABA in mediating root responses to soil compaction. This work will enable better phenotyping of plant varieties with enhanced root system traits for resource foraging and uptake. Knowledge of the responses of root systems in heterogeneous soil is vital to validate root phenotypes and overcome future food security challenges.

Economic, political and environmental factors have prioritized the need for research on phosphate (Pi) acquisition efficiency (PAE), Pi use efficiency (PUE) and Pi fertilizer uptake efficiency in crops. However, the coordination of molecular responses to Pi starvation and the mechanisms of Pi starvation tolerance have been investigated predominantly in model plants but remain elusive in grain crops, especially in wheat. This project investigates transcriptional profiles in wheat, particularly in the roots, as a response to nutrient availability focusing on phosphate (Pi). Furthermore, appropriate screening approaches and the difficulties in crop improvement, particularly for wheat, are discussed. Pi acquisition by plants is mediated by members of Pi transporter families. The roles of these Pi transporters in Pi partitioning and re-translocation is complex and the knowledge about their functioning in wheat still limited. Here, members of the Pht1 family in wheat were identified, their expression profiles determined when exposed to different nutrient regimes in roots and ear tissues at various developmental stages and their potential role as targets for genetic improvement discussed. In addition to Pi transporters, regulatory genes including transcription factors, signalling pathways and apparently other Pi-responsive genes with unknown function are also of critical importance. Therefore, the genome-wide responses to limited nutrient availability were investigated for the first time in roots of field-grown wheat exposed to limited nutrient availability resulting in the identification of several candidate genes for PAE/PUE improvement on the molecular level. These data were validated against other studies and across a wider wheat germplasm. Furthermore, the correlation of candidate gene expression to the nutritional status, Pi availability and PAE/PUE properties revealed four potential target genes which may be major contributors to genotypic diversity of this trait. However, there are still some agronomic bottlenecks which impede implementing Pi efficient crops and the application of molecular tools and marker genes.