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.

Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) is an invasive worldwide pest of many agricultural, horticultural and ornamental crops. They are difficult to control because of their small size and high resistance to chemical insecticides. The aggregation pheromone of this species is currently used for monitoring, but the full potential for use of this and other pheromones has not yet been explored. Two male-specific headspace volatiles have been previously identified: neryl (S)-2-methylbutanoate which acts as an aggregation pheromone and (R)-lavandulyl acetate, for which the role is unclear. The roles of these compounds were studied to understand how they can be used in pest management. Laboratory bioassays showed that the aggregation pheromone, apart from being an attractant, also increased the activity level of adult F. occidentalis. This could be utilized to activate the thrips out of their concealed spaces within the crop and enhance pickup of chemical insecticides. (R)-lavandulyl acetate reduced the walking and take-off activity of adult females but increased the activity level of adult males. The possible role of this compound as a mating pheromone is discussed. The chemical analysis of male-exposed filter paper discs showed the presence of another compound, 7-methyltricosane, which was shown to act as a contact pheromone for species recognition. Adult females respond by raising their abdomen showing mating rejection towards adult males while abdominal wagging sideways was observed in adult males, a behaviour used in aggressive male-male interactions. This is the first identification of a contact pheromone in the order Thysanoptera.

This research is concerned with the dynamic relationship between agricultural advisers, knowledge and soil in England. On the basis that agricultural advisers have always played a central role in linking research and farming practice and implementing policy on the farm, the thesis explores the role of the adviser in facilitating a shift towards sustainable soil management (which encompasses a range of complex and knowledge intensive practices) and to the realisation of policy objectives in this domain. Specifically it aims to provide detailed empirical evidence of the role that agricultural advisers play in the acquisition, utilisation, generation and transfer of knowledge about soil best management practice and to elicit the factors that enable and constrain these knowledge processes. Conceptually, the research draws on approaches to knowledge and knowledge processes in agriculture from the allied disciplines of rural geography, rural sociology and extension science. An actor-oriented Agricultural Knowledge and Information System (AKIS) approach provides the basis for examining adviser interactions with both the research and farming communities. While the AKIS describes the factors that enable and constrain how advisers engage in knowledge processes in terms of connections across institutional interfaces between research, advice and farming, an actororiented approach, which understands knowledge processes as social processes operating across social interfaces, enables exploration of how individual advisers behaving as autonomous agents resolve these constraining and enabling factors. The- study, combining quantitative and qualitative methods, employs an extensive postal questionnaire survey of a 163 agricultural advisers from across England and three detailed case studies where sustainable soil management is a central theme, namely: the Landcare Project; the UK Soil Management Initiative; and the SUNDIAL Fertiliser Recommendation System. The data describe an advisory community with a range of involvement, concerns and competence in soil management. Patterns of acquisition and utilisation of knowledge about soil best management practice revealed by the questionnaire data suggest that advisers are actively seeking and using knowledge about soil management, although some are more constrained than others in accessing it. These patterns, however, only provide a partial understanding of the complex knowledge processes in which advisers engage as they operate at the boundaries between science and practice. As such, qualitative data from the case studies are used to reveal that, in bridging the different institutional cultures and life worlds of research and practice, advisers encounter different understandings and expectations of soil best management practices. Rather than simply acquiring, utilising and transferring knowledge, the data reveal that advisers negotiate, adapt, transform, generate and integrate knowledge about soil as they struggle to reconcile the principles of research-based soil best management practice with the practical and business constraints of the farm. In doing this advisers, and agronomists in particular, tend to closely align themselves with the interests of the farming community and as such are more likely to reject or question soil best management practice. In addition the apparent lack of advisers' competence and skills in certain knowledge intensive soil best management practices and their reliance on experiential knowledge further explains their reluctance to engage in soil best management practices derived from national research. Integration of knowledge through dialogue and understanding emerges as key to overcoming these tensions and providing the basis for facilitating sustainable soil management. Advisers are shown to have a central role in integrating knowledge from research and from farmers. The processes and relationships that enable this integration are identified. The thesis concludes with some policy relevant suggestions to improve the effectiveness of advisers' participation in the transition to sustainable soil management in England. These include: exploiting a diverse and flexible advisory community; improving advisers' skills and expertise; instilling in them confidence to provide credible and practical soil best management practice; and improving the quality of communication between the advisers, researchers and farmers. Future research directions are reviewed in the context of the proposed implementation of Soil Management Plans on all farms in England as a component of cross compliance within CAP reforms.

This thesis explores how, in the wake of momentous agrarian reforms implemented during the 1980s and 1990s, Vietnam succeeded in generating both a strong increase in agricultural growth, and remarkable trends in poverty reduction. Three specific channels of transmission between agricultural growth and rural poverty reduction are explored and evaluated empirically using the Vietnamese Living Standard Survey. Chapter 2 investigates the impact on agricultural investments of the strengthening in tenure security induced by Vietnam's 1993 Land Law, which set up a new land tenure system based on de jure private property rights. Idiosyncratic characteristics of Vietnam's land reform and the panel nature of the VLSS are taken advantage of to capture the exogenous changes in tenure security brought by the 1993 Land Law. By interpreting the results in the light of Vietnam's agrarian history, I shed light on the role of formal institutions during the process of establishing a new private property right system. Chapter 3 explores the role of education in promoting agricultural growth and documents how, in the 1980s, Vietnam emerged from thirty years of war with literacy levels that are normally achieved by middle-income countries. The differential in educational attainment between North and South Vietnam is used to devise an instrumental variable strategy and to evaluate the contribution of Vietnam's high initial education levels to rice yields. The results confirm the importance of literacy and numeracy skills as pre-conditions for agricultural growth but highlight the importance of non-linearities in the impact of schooling, and of factoring in information on the quality in education in order to better understand the processes through which education affects economic efficiency. Finally, Chapter 4 showed that growth in the agricultural sector had been remarkably pro-poor as it benefited even the poorest of the poor, probably because of favourable initial conditions for pro-poor agricultural growth.

Dairy calves in the U.K. are currently reared on ‘least cost’ principles, with minimal milk feeding and early weaning to solid foods. This has been described as maintaining the calf in ‘a state of chronic hunger’. As well as repercussions on calf health, growth and welfare, human studies suggest that underfeeding the newborn is a major risk factor for metabolic disease in the adult. The aims of this study were to determine current dairy calf rearing practices across the U.K., to investigate the performance of Holstein heifer calves fed increased milk replacer (MR) compared to restricted volumes, and to determine the impact of this on key performance indicators (KPIs) of these animals as calves and growing heifers. A postal questionnaire was offered to one thousand U.K. dairy farmers to determine current calf rearing practices. The response rate was 72% and revealed that housing and feeding practices were variable between farms. The majority of farmers (93%) fed restricted volumes of milk or milk replacer to their pre-weaned calves. The body weight, withers and loin height, heart and belly girth, crown to rump length, hock-fetlock length and body condition score (BCS) were recorded weekly from birth to 12 weeks and monthly from 12 weeks until conception in two groups of Holstein heifer calves on one commercial dairy farm in the north-west of England, U.K. Calves were assigned to a restricted, Group R (n = 50) or ad libitum, Group A (n = 50) MR feeding strategy from birth until weaning. Growth rates were greater for Group A (0.72kg/day) from birth until 3 weeks than Group R (0.17kg/day). Body condition score increased for Group A during this period (0.1 points) while it decreased for Group R (0.3 points). Thereafter, growth rates were similar between dietary groups although no catch-up growth was observed for Group R animals. Changes in morphometric measures were greater for Group A calves than Group R from birth to 12 weeks. From 12 weeks of age onwards, dietary group differences in morphometric measures disappeared but body weight differences remained until conception. The glucose metabolism and insulin sensitivity of a subset of heifer calves (n = 6 Group A, n = 6 Group R) was investigated at 3, 12 and 39 weeks of age and was shown not to be affected by dietary group. The carcass composition of Holstein bull calves assigned to one of the two dietary groups was assessed. Calves were studied at birth (n = 3), 3 weeks, 9 weeks or 12 weeks (n = 3 per dietary group at each age). Carcass composition was assessed using spiral CT technologies. Group A calves had greater internal adipose deposition at all ages but there was no difference in carcass associated adipose tissue. The age at puberty, first service and conception was between 2 and 3 weeks lower for Group A animals than for Group R. Increased MR feeding of Holstein heifers allows for greater growth rates and earlier entry into the milking herd.