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Plant phosphate starvation responses are initiated to reduce Pi demand, conserve Pi reserves and to increase Pi availability. Responses to Pi depravation range from alterations in growth and metabolism to the induction of Pi-inducible genes. Signalling pathways governing starvation responses are unknown, although individual responses have been found to be under the control of local phosphate availability, whilst other responses have been shown to be controlled by shoot-derived systemic signals. A comparison of the kinetics of different phosphate starvation responses, representing growth, physiology and gene expression, suggesting that changes in gene expression and physiology occurred prior to growth responses. The magnitude of all responses was found to be dependent on the duration of starvation and on previous Pi growth conditions, suggesting that internal Pi levels regulate the initiation of responses. Kinetic studies investigating gene expression after Pi re-supply found that down-regulation of expression is rapid and therefore possibly under local control. Enhanced Pi starvation responses were observed under increased sucrose availability, illustrating the importance of a balanced internal C:P ratio for plants. Split-root experiments revealed the systemic down-regulated of Pi-inducible genes and confirmed the existence of long-range signals governed by shoot Pi status in <i>Arabidopsis</i>. Experiments to determine the kinetics of down-regulation in split root plants discovered that the extent of down-regulation was dependent on internal Pi status and on the duration of split-root treatment. Complete down-regulation occurred only after several days. Split-root experiments were conducted with various mutants to further dissect signalling. Wild-type down-regulation was observed in the phosphate response mutant, <i>phr1-1</i>, the cytokinin receptor mutant, <i>cre1</i>and in <i>Arabidopsis</i> plants containing the Pi-inducible At4 gene under the control of the CaMV 35S promoter. Reduced systemic down-regulation was observed in the phosphate translocation mutant, <i>pho1</i>, confirming low shoot Pi status of this mutant.

The aim of this thesis is defined by the title, and two experimental approaches were used to investigate different aspects of the effect of elevated [CO<SUB>2</SUB>] on Sitka spruce; firstly the long term effect on mature tissue using branch bags and secondly, the interaction between [CO<SUB>2</SUB>] and nutrient supply rate on the growth and physiology of seedling trees. Six branches of six 16 year-old Sitka spruce trees were continuously exposed to elevated [CO<SUB>2</SUB>] (˜700 μmol mol<SUP>-1</SUP>) for four years. Branch growth, shoot numbers, needle size, stomatal density, nutrient and carbohydrate concentration, photosynthesis and stomatal conductance were measured throughout the experiment. There was no effect of elevated [CO<SUB>2</SUB>] on the growth of the branches or needles, or on the nutrient or carbohydrate concentrations of needles. Neither was there evidence for an acclimation of photosynthesis or stomatal conductance to growth in elevated [CO<SUB>2</SUB>] in current year needles. However, there was some down-regulation of photosynthesis in one-year old needles coincident with an increase in soluble carbohydrate concentration. In a second experiment one-year old seedlings were re-potted into sand and grown for eight months in open-top chambers in either ambient or ˜700 μmol mol<SUP>-1</SUP> [CO<SUB>2</SUB>]. They were supplied with nutrients at two rates: a high rate designed to permit maximum growth rate, and a low rate 1/10 the high rate. Growth was measured each week and six harvests were made during the experiment. A purpose built whole-tree gas exchange system was used to measure independently above and below ground CO<SUB>2</SUB> fluxes over 24 hours. Shoot photosynthesis responses to [CO<SUB>2</SUB>] and needle nutrient and carbohydrate concentrations were also measured. Elevated [CO<SUB>2</SUB>] enhanced growth and increased allocation to roots at both high and low nutrient supply rates, but growth enhancement was larger at the high nutrient supply rate.

The conditions which prolong viability and the low germination rate of seeds of <I>Afrocarpus gracilior </I>were studied. Different seed lots were stored under various moisture content and temperature combinations. The two seed lots tested had different initial viabilities, probably due to different pre-storage treatments, but generally similar storage requirements to maintain viability at a reasonable level. Tetrazolium tests gave consistently higher viability estimates than germination tests but were well correlated with the latter. Intact seeds were found to have a very low germination rate. However, removal of the seed coat and germination at 30<SUP>o</SUP>C resulted in germination rates of up to 80% after 30 days. The seed coat was found to contain a very powerful inhibitor of germination of <I>Triticum </I>seed. Studies were made on the germination of seeds of <I>Ekebergia capensis</I> and <I>Pygeum africanum. </I>Intact seeds of <I>E. capensis </I>germinated at about 52% under optimum conditions and slight damaging of the seed coat halved the germination rate. Intact seeds of <I>P. africanum </I>germinated at about 20% under optimum conditions and slight damaging of the seed coat doubled the germination rate. Shoots of <I>A. gracilior</I> were found to grow well <I>in vitro</I> on hormone free medium but did not produce axillary buds during normal growth and development. However, it was found that decapitation resulted in the formation of axillary buds at or near the top of the shoots which subsequently developed into shoots. Treatment of intact or decapitated shoots with benzyladenine resulted in the initiation of numerous buds but it proved impossible to extend them into shoots. Extending shoots rooted spontaneously on hormone free medium and were also rooted <I>ex vitro</I> in compost under non-sterile conditions and the rooted plants were weaned to greenhouse conditions.

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There is considerable interest in investigating conserved roles for protease in the Hypersensitive Response (HR), a plant defence response which shares some morphological characteristics with apoptosis. a cysteine protease, with homology to mammalian Cathepsin B proteases, was isolated in a screen for genes up-regulated in the HR. The focus of this current research is to examine the roles of Cathepsin B genes in the model plant <i>Arabidopsis.</i> There are three Cathepsin B homologues in <i>Arabidopsis </i>for which knock-out lines were isolated and genetically crossed using a combination of T-DNA insert lines and RNAi to generate double and triple mutants. These genes were found to act redundantly with triple mutants showing increased susceptibility to both virulent and avirulent strains of <i>Pseudomonas syringae </i>DC3000. Moreover, these genes are also involved in non-host resistance to fungal pathogen <i>Blumeria graminis </i>f.sp. <i>tritici,</i> where they positively regulate the HR but negatively regulate <i>Pathogenesis-Related 1 </i>(<i>PR1</i>) expression downstream of <i>Enhanced Disease Susceptible 1</i>(<i>EDS1</i>). In addition, this work also implicates <i>Cathepsin B </i>genes in senescence, a developmental form of PCD, via regulation of the senescence marker gene <i>Senescence Associated Gene 12 </i>(<i>SAG12</i>). Furthermore, it was shown that NbCathB is localised to the plant apoplast where it is activated upon secretion. Partially purified Cathepsin B protein was inhibited by a variety of peptide inhibitors but evidence of inhibition by several pathogen-derived inhibitors that are secreted during infection was inconclusive.