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    Phosphate absorption by Arabidopsis thaliana : the effects of phosphorus nutritional status : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Plant Biology and Biotechnology at Massey University, Palmerston North, New Zealand
    (Massey University, 1997) Clark, Gregory Thomas
    The effect of phosphorus nutritional status on phosphate uptake within the concentration range of the high affinity uptake mechanism, and subsequent translocation to the shoot was investigated in the plant species Arabidopsis thaliana. Plants of different nutritional status were generated by exposure to different set phosphate concentrations throughout an aseptic hydroponic growing period. Alternatively phosphorus deficiency was induced by growth at high concentrations of phosphate followed by a period of 5 days in phosphate-free hydroponic solution. In effect these growth conditions resulted in plants of distinguishable phenotypic character with respect to phosphate absorption, phosphate translocation, arsenate sensitivity and root-shoot ratio. To determine absorption kinetics nutrient depletion trials were carried out in which phosphate uptake was measured by monitoring the loss of phosphate from depletion solutions of set initial phosphate concentration to which the root systems of intact plants were exposed. Km and Vmax kinetic parameters were calculated from the depletion trial data using the software package "Igor Pro". Influx and net phosphate uptake was determined by setting the initial phosphate concentration of the depletion trials using either 32 P labelled KH 2 P0 4 or non-labelled KH 2 P0 4 respectively. Radioactivity was measured by counting the Cerenkov radiation in a scintillation counter. Non-labelled phosphate depletion was measured by either spectrophotometric assay or ion chromatography. To asses the effect of the phosphate analogue arsenate on phosphate influx, 32 P labelled phosphate uptake was measured with arsenate (KH 2 AsO 4 ) present in the depletion solution at a concentration of 20 µM. Phosphate translocation was determined by counting the Cerenkov radiation in the roots and shoots separately of plants that had been exposed to the 32 P labelled depletion solutions. Under the conditions of this project, phosphorus deficient plants exhibited alterations in the kinetic parameters Km and Vmax for phosphate uptake that were dependent on how the deficiency was induced. For plants that were grown continuously at low phosphate concentrations Km was decreased without a concomitant change in Vmax. For plants that were grown at high concentrations of phosphate followed by a 5 day period of phosphate starvation, a significant increase in Vmax was recorded without an associated change to Km. Phosphate uptake was found to be severely inhibited by the presence of arsenate in the depletion solution. Greatest inhibition however was found not to occur at the level of absorption into the plant root system but rather appeared to be at a site involved in phosphate loading into the xylem. Inhibition at this site was also found to be greatest in low phosphorus status plants. From these results it is suggested that plants of low phosphorus status possess high affinity phosphate xylem loading mechanisms, induced under conditions of phosphorus deficiency, which have a greater susceptibility to arsenate competitive inhibition and toxicity than equivalent xylem loading mechanisms in high phosphorus status plants.
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    A study of growth, development and N-fixation of several white clover (Trifolium repens L.) cultivars under different water deficit and phosphorus levels : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University, Palmerston North, New Zealand
    (Massey University, 1991) Wang, Xinjun
    White clover (Trifolium repens L.) is one of the most important pasture plants in New Zealand. It contributes nitrogen, high quality forage and seasonal spread of production. However, it has high requirements for phosphate and does not persist well under moisture stress condition.s In this study the effects of water deficit level and phosphorus (P) level on the growth, development and N-fixation of different white clover cultivars have been studied. Several New Zealand and overseas white clover cultivars of contrasting morphological characteristics were selected for this study. These cultivars were Kopu, Pitau, Dusi, Haifa, Huia, Tahora and Whatawhata. The main objectives of this study were to determine the effects of water stress and phosphorus levels on growth, development, and N-fixation of these cultivars and to identify the plant characteristics most effective in distinguishing the cultivars used in the study. The research approach was to measure the responses of these cultivars to water deficit and P fertility treatments; and the responses were then related to selected plant characteristics, particularly morphological characteristics. Attempt was also made to distingish the cultivars using multivariate analysis techniques. The results of this study showed that the cultivars had responded differently to water deficit treatment and P treatment. The retardation they suffered in their growth and development, expressed by both dry weight (DW) and growth rate of stonon components (leaf, petiole and branch), was significantly different and, in many cases, could be related back to their morphology and development characteristics. Cultivars of small stature, such as Whatawhata and Tahora, were generally less affected by the stress treatments. Cultivars grown well under control conditions, such as Kopu and Haifa, suffered more severely under stress conditions. For all cultivars, parameters association with leaf area were more sensitive to water deficit stress than those associated with growing points. The study of plant water status of these cultivars under progressively decreasing water availability failed to identify any significant difference between the cultivars in their ability to avoid dehydration at medium to high water deficity stress levels. So it could be suggested that the diference between the cultivars represented their ability to tolerate water stress and was unlikely to be associated with their dehydration avoidance ability but more likely to be associated with morphological characteristics such as deep root and reducing leaf area when water deficit stress occurred. The cultivats were different in their N-fixation ability, as measured by the acetylene reduction analysis (ARA), and this difference was related strongly to leaf size. But when expressed as ARA per unit DW, the difference between the cultivars was non-significant. Overall, under control conditions, mainly the morphological characteristics, such as leaf size, leaf weight and petiole length, caused the difference between cultivars, other characteristics, such as the plant DW components, DW partitioning, and P and N partitioning in plant components, were less important. Among the morphological characteristics, leaf size and leaf weight per stolon were the most important characteristics differentiating between cultivars. These two characteristcs were also the most important for determining plant yield. Under water deficit stress and P deficiency treatment, root DW and branch number were the two most important characteristics differentiating between the cultivars. ABSTRACT White clover (Trifolium repens L.) is one of the most important pasture plants in New Zealand. It contributes nitrogen, high quality forage and seasonal spread of production. However, it has high requirements for phosphate and does not persist well under moisture stress conditions. In this study the effects of water deficit level and phosphorus (P) level on the growth, development and N-fixation of different white clover cultivars have been studied. Several New Zealand and overseas white clover cultivars of contrasting morphological characteristics were selected for this study. These cultivars were Kopu, Pitau, Dusi, Haifa, Huia, Tahora and Whatawhata. The main objectives of this study were to determine the effects of water stress and phosphorus levels on growth, development, and N-fixation of these cultivars and to identify the plant characteristics most effective in distinguishing the cultivars used in the study. The research approach was to measure the responses of these cultivars to water deficit and P fertility treatments; and the responses were then related to selected plant characteristics, particularly morphological characteristics. Attempt was also made to distinguish the cultivars using multivariate analysis techniques. The results of this study showed that the cultivars had responded differently to water deficit treatment and P treatments. The retardation they suffered in their growth and development, expressed by both dry weight (DW) and growth rate of stolon components (leaf, petiole and branch), was significantly different and, in many cases, could be related back to their morphology and development characteristics. Cultivars of small stature, such as Whatawhata and Tahora, were generally less affected by the stress treatments. Cultivars grown well under control conditions, such as Kopu and Haifa, suffered more severely under stress conditions. For all cultivars, parameters associated with leaf area were more sensitive to water deficit stress than those associated with growing points. The study of plant water status of these cultivars under progressively decreasing water availability failed to identify any significant difference between the cultivars in their ability to avoid dehydration at medium to high water deficit stress levels. So it could be suggested that the difference between the cultivars represented their ability to tolerate water stress and was unlikely to be associated with their dehydration avoidance ability but
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    Dissolution and plant-availability of phosphate rocks in selected New Zealand and Indonesian soils : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 1992) Tambunan, Donald
    Use of phosphate rocks (PRs) as direct-application fertilizers has received considerable attention in countries that have large areas of acidic soils. Properties of acidic soils generally favour dissolution of PRs and increase their effectiveness as direct-application fertilizers. In this study, the dissolution and effectiveness of several PRs, North Carolina (NCPR), Moroccan (MPR) and Pati (PPR) phosphate rocks, was investigated in a range of New Zealand and Indonesian soils. The main objective of the thesis was to provide information that could assist in improving recommendations on their use in field situations. Laboratory studies showed that the extent of PR dissolution could be estimated using sequential P fractionation techniques to measure amounts of residual (undissolved) PR in soils. In New Zealand soils, residual PR was accurately estimated from the increase in HCl-extractable P (∆HCl-P) between NCPR-fertilized and unfertilized soils following sequential extraction of soil and soil/NCPR mixtures with 0.5 M NaCl/TEA (30 min), 1 M NaOH (16 h) and 1 M HCl (16 h). The ∆HCl-P method, however, was not suitable for use on strongly weathered Indonesian soils because of low recovery P in the HCl extractant following NaOH extraction. Tri-acid (HNO3:HCl:HClO4) digestion or H2SO4 (0.5-1 M) extraction overcame this problem. A ∆H2SO4-P method involving 0.5 M NaCl/TEA, 1 M NaOH and 0.5 M H2SO4 extractions was subsequently tested and shown to be suitable for measuring residual PR in acidic New Zealand and Indonesian soils. Measurement of 32P-labelled synthetic francolite dissolution in these soils confirmed the accuracy of the new ∆H2SO4-P method. Considerable evidence exists from this study to indicate that the capacity of soil to supply acid and remove Ca from the site of PR dissolution are most important in determining the extent of PR dissolution. The extent of NCPR dissolution in New Zealand soils was found to decrease with increasing additions of CaCO3 or NaHCO3 due to increases in soil pH (for NaHCO3, and CaCO3-amended soils) and exchangeable Ca (for CaCO3-amended soils). The maximum extent of PR dissolution occurring in the range of acidic New Zealand and Indonesian soils incubated with NCPR and MPR was found to be negatively correlated with initial amounts of exchangeable soil Ca (r=0.83-0.92) and the percentage Ca saturation of the cation exchange capacity (r=0.78-0.92). Also, increases in soil pH, and possibly solution concentration of Ca, were the main reasons for decreases in synthetic francolite dissolution in soils amended with increasing rates of plant residue. And finally, field trials conducted in Indonesia showed that the extent of PR (NCPR, MPR and PPR) dissolution was greater in the more acidic Ultisol (pH H2O=4.8) than in the Entisol (pHH2O=5.3). Laboratory incubation studies showed that the key factors determining the chemical-availability (i.e. extractable with Olsen, Bray 1 and resin tests) of P derived from soluble P fertilizer or PRs in New Zealand and Indonesian soils were rate of addition, soil pH and P sorption characteristics and the nature of soil test. A short-term (30 days) glasshouse study using a range of New Zealand soils showed that the plant-P uptake from soil fertilized with NCPR was low, relative to monocalcium phosphate (MCP), indicating the low extent of NCPR dissolution. The plant-availability of soluble P and dissolved P from PR, however, was more dependent on soil P adsorption characteristics than on other soil properties. Field trials in Indonesia showed that PRs were more effective agronomically than triple superphosphate (TSP) for maize in a P deficient Ultisol only when the PRs were applied to Calopogonium caeruleum cover crop 6 to 18 months prior to sowing maize. In an Entisol, PRs were less effective than TSP irrespective of application time. In the Ultisol, PR effectiveness was not affected by liming, provided that the PRs were applied 6 to 18 months prior to the addition of lime. Results of the Indonesian field trials showed that Bray 1 test was a better predictor of plant growth responses than either Olsen or resin tests in PR-fertilized Ultisol, where high effectiveness of PRs was observed. Three PR dissolution models of increasing complexity (Mitscherlich, Cubic, Kirk and Nye) were tested using NCPR and MPR dissolution data generated from a laboratory incubation study. Only Mitscherlich and Kirk and Nye models adequately described PR dissolution in the soils studied. A sensitivity analysis showed that any differences between observed and simulated PR dissolution by the Kirk and Nye model could be attributed to problems in obtaining a representative measure of soil solution pH. The Kirk and Nye model was modified to simulate PR dissolution in the field and tested using data from the Ultisol field site. The model adequately predicted NCPR and MPR dissolution over 545 days. In this case the accuracy of predictions was found to be dependent on the value of the initial soil pH and the accuracy of simulating daily soil water contents. The model showed potential for use in a wider range of soil-plantclimat conditions in order to assist with the selection of soils suitable for the use of direct-application PR fertilizers.