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Background information, essential to a full appreciation of the research presented in the thesis, is submitted in Chapters Two, Three and Four in the form of literature reviews. Chapter Two includes a review of the geology, climate, soils, past management, vegetation history and nature conservation value of Bodmin Moor. The chapter culminates by discussing the proposed future management of grazing on Bodmin Moor. Chapter Three comprises a review of the mechanisms by which grazing affect vegetation. Particular reference is made to the effects of 7 General Introduction variation in grazing practices and interactions between grazing and other environmental variables. Chapter Four provides a review of successional processes with particular reference to the effects of grazing on upland plant community succession and the limits of current knowledge of grazing management in upland habitats. Chapter Five presents the results of phytosociological classification and explores the relationships between environmental variables and vegetation. Chapter Six examines spatial variation in the seed bank of Bodmin Moor in a range of communities at different depths. Chapter Seven presents the results of a Countryside Stewardship monitoring scheme established on Bodmin Moor North SSSI. Chapter Eight investigates the effects of variation in timing, frequency and severity of defoliation, on Molütia caerulea, along a soil moisture gradient. Chapter Nine reviews the preceeding work focusing on the overall implications of the thesis. The structure of the thesis is presented diagrammatically in Figure 1.1.

This thesis presents structural and biochemical studies of a phytochrome, Agp1, a bilin-binding red-light receptor protein. Crystallographic studies were undertaken in order to obtain structural insight into the mechanism of photoconversion and signal transduction from the sensor domain to the signalling domain, the latter hypothesised to be universally conserved among two-component histidine kinases. Using Agp1 from a non-photosynthetic plant pathogen, Agrobacterium tumefaciens as a model phytochrome, structural determination of both Pr and Pfr-forms was attempted, in order to permit unbiased structural comparison of the two. Application of the surface entropy reduction strategy led to determination of the structure of Agp1 in its Pr-form and crystals of Agp1 in the Pfr-like form were obtained for the first time. Biochemical studies were undertaken to probe the conformational differences between Agp1 as apoprotein, Pr-form, and Pfr-like form. Limited differences in secondary structure exist between the forms of Agp1. Conformational differences between the Pr and the Pfr-like form seem to underlie the fact that the space group of Agp1 crystals in the Pfr-like form is different from that of Agp1 in the Pr-form. The ability of the Agp1 apoprotein to form a dimer via a disulphide bond at the N-terminal chromophore-binding cysteine residue implies flexibility of the N-terminal region which allows for the initial bilin incorporation during holoprotein formation.

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The form and seasonal development of in vivo mycorrhizas of P. sylvestris seedlings growing in forest nursery plots 458FY70 (Kennington) and 83FY7O (Wareham) have been characterised; this enabled various characters of P. sylvestris seedlings (for example non mycorrhizal short roots, mycorrhizal roots, 'pioneer' and 'mother' and subordinate 'mother' roots) to be compared for different soil conditions. Examination of the heteroihyzic root systems of P. sylvestris and P. nigra showed that long roots, for example 'pioneer' roots, are possibly more capable of adapting to a variety of soil conditions. The effects of Simazine on measured aspects of the root systems of nursery grown P. sylvestris and P. nigra are unclear; equivalent Pines growing in the same soils contained in plant pots showed a demonstrable increase in both numbers of short roots and in mycorrhizas on the application of Simazine equivalent to 4 lbs Simazine per acre. Growth measurements of known mycorrhizal and root pathogenic fungal isolates,on agar plates with added herbicides, showed that increase in growth of some isolates ocourred in the presence of higher concentrations of Simazine or Atrazine. In contrast, equivalent experiments using Diehlobenil, Dichlobenzoate, Chlorthiamid and 245T indicated that identical isolates, growing in the presence of these herbicides, were inhibited. These observations of herbicidal activity on mycorrhizas and their mycobionts were confirmed by measurement of respiration rates for Boletus elegans and Amanita rubescens, both mycobionts (the latter in very exceptional circumstances), and mycorrhizal roots of P. sylvestris. The extent of phosphate uptake by P. sylvestris mycorrhizas, estimated from buffered phosphate solutions containing herbicide, was also in agreement with the results. Using a number of established techniques, attempts to produce reliable numbers of in vitro mycorrhizas were of limited success.

Following a brief introduction to this field of research (Part 1), physiological and ultramicroscopical studies are presented (Part 2), in which the influence of thermal and chemical treatments upon solute translocation in the phloem is analysed. It is felt that the results of these studies find their plainest interpretation in terms of those theories which invoke a mass flow of sieve tube sap and sometimes, more specifically, in terms of the Pressure-Flow Hypothesis of Munch. Thus it would appear (Section 2.2) that the 'path' region was remarkably insensitive to metabolic inhibitors (with the exception of extremely concentrated KCN where an indirect influence would seem probable). Thermal treatments to this region (Sections 2.3 and 2.4) influence translocation rate in a manner that appears to be mediated by changes in the physical characteristics of the system, such as the viscosity of the sap and the hydraulic conductivity of the conduit. Becoming despondent with the lily which neither toils nor spins but notwithstanding accomplishes so much in so small a body, attention was directed to a consideration of models of the sieve tube (Part 3), in which hitherto impossible measurements became straightforward. The first of these were not satisfactory for one reason or another (Section 3.2), but a later 'Working Model' drew attention to certain interesting possibilities, although problems of scale ruled out quantitative predictions. A rather different approach, involving hydrodynamic equations and computer methods, (Sections 3.4 and 3.5) permitted the consideration of the flow of solution along conduits which were of the scale of the sieve tubes and in which the probable and possible characteristics of the sieve tube sap and the sieve tubes themselves could be investigated. This work led to rather more valuable suggestions concerning the water relations of the phloem tissues, and perhaps to a more rigorous feasibility test of Munch's Hypothesis than had been made before; from this the Hypothesis emerged unscathed. Before drawing the thesis to a conclusion (Part 4), it was felt that mention should be made (Section 4.2) of an attempt to correlate the work of Parts 2 and 3; while largely unsuccessful, this avenue offers a most interesting prospect for subsequent study. Some new material was introduced (Section 4.3) in which the possibility of a further role for the sieve plate is advanced. This is followed by a discussion (Section 4.4) of certain less straightforward possibilities that should be considered in an analysis of sap flow in the phloem.

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The enhanced warming effect possibly due to anthropogenic green house gas emissions has led to the derangement of global climatic mechanisms (especially at the interannual scale). This has led to the disturbances to the equilibrium of the Earth System and the interconnected self-regulatory processes. Terrestrial vegetation takes an active role in the regulation of the equilibrium of the Earth System by the processes of resistance and resilience. Whilst comprehensive and extensive modelling studies that investigate the effects of climatic change in terrestrial systems have been undertaken, few investigations have focused on the change and evolution in these systems from a holistic geophysiological perspective. In the first part of this thesis, econometric time-series modelling techniques were applied to National Oceanographic and Atmospheric Administration Advanced Very High Resolution Radiometer derived Normalized Difference Vegetation Index data sets in order to evaluate the responses of terrestrial South American vegetation to the interannual El Nifio Southern Oscillation climatic perturbations. Lags between vegetation response and the El Nifio Southern Oscillation perturbations are identified and quantified. The results indicate that the terrestrial vegetation loses its sensitivity to El Nifio Southern Oscillation perturbations in the post 1993 period, leading to the hypothesis that the terrestrial system maybe showing a Gaian behaviour that would enable homeostasis in the system of concern. The null hypothesis of this was tested using a stochastic Auto Regressive Integrative Moving Average model, which further strengthens the argument put forward by the hypothesis. Further comprehensive analysis was performed by using the Hybrid version 4.1, a mechanistical model of vegetation dynamics to test the effects of varying changes in the phase and amplitudes of the El Nino Southern Oscillation on terrestrial vegetation. Simulations of different interannual El Nino Southern Oscillation climatic scenarios under varying trends for increases in atmospheric C02 concentrations confirm the possibility of such a homeostatic property in the terrestrial vegetation system within its geophysiological limits.