Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1994. / Vita. Abstract. Includes bibliographical references. Also available via the Internet.

No description available.

Thesis (M.S.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (February 9, 2007) Includes bibliographical references.

The CERES-Wheat Crop Growth and Development model treats temperature, nitrogen and water stresses as limiting factors. For each day the model calculates a stress index for temperature, N and water, compares the magnitude of the indices, and then adjusts the calculated daily potential growth using the index of the most severe stress, while ignoring the other stresses. Under the conditions in Oregon, however, mild N and water stress will often be present together in about equal degree of stress. Some published results suggest that both stresses affect growth and yield under those conditions. Therefore, this work was undertaken to evaluate the combined effects of N and water stress on growth and development of wheat (Triticum aestivum L.). To make such an evaluation one must be able to control both N and water supplies to the plant and the response of the plant to these two variables must be measured at different growth stages. A system of imposing controlled plant water stress developed by Snow and Tingey (1985) was adapted and evaluated for its potential to impose controlled levels of both N and water stress to single wheat plants. Using a 12 mmol N and 4 cm pathlength as optimum N and water supply treatment, 2 mmol N and 12 cm pathlength and a 1 bar standard ceramic disc in the floral foam column as N and water stress treatments during tillering resulted in 15 % reduction in tillers/plant for limits to the N supply alone, a 39 % reduction for limits to the water supply alone, and a 52 % reduction when both stresses were imposed simultaneously. There was no effect of N or water supply treatments on the leaf appearance rate on the main stem, a measure of the rate of progress toward flowering. Both N and water supplies had a strong effect on tillers/plant which, in turn, affected plant biomass and its constituent parts. The effects were independent, suggesting that, to accurately model the wheat canopy development when both mild N and water stresses are present, both stresses must be considered. A 'law of minimum' concept as currently used in the CERES-Wheat model would not be an accurate model for the process of tillering. In an experiment where stress was imposed during flowering and grain-filling, the grain yield/plant varied significantly with both N and water supplies. The interaction between N and water treatments on grain yield was also significant and was due primarily to their significant interaction on mean kernel weight. The major determinant of grain yield was tillers/plant at harvest. Both N and water supplies affected kernels/ear and N stress caused a reduction of 12% in both fertile spikelets/ear and kernels/fertile spikelet. At an optimum water supply, the difference between the effect of optimum and medium N supply on grain yield was not significant but yield in low N supply was reduced by 54%. In the medium water supply, the grain yield at both medium and low N treatments were significantly lower than at optimum N supply. There was no significant difference in yield between N supply treatments in the low water supply treatments. Thus, under severe water stress, a 'law of minimum' concept appeared to be valid, but under less severe stress both N and water supply affected grain yield. The data on leaf water potential and leaf temperature showed that plants in low water supply treatments maintained consistently lower leaf water potential and higher leaf temperature than in optimum water supply treatment. / Graduation date: 1993

Thesis (Ph.D.)--University of Nebraska-Lincoln, 2008. / Title from title screen (site viewed Mar. 10, 2009). PDF text: xix, 194 p. : ill. (some col.) ; 14 Mb. UMI publication number: AAT 3336804. Includes bibliographical references. Also available in microfilm and microfiche formats.

Thesis (Ph.D.) - University of Queensland, 2003. / Includes bibliography.

Thesis (MSc)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: The aim of this study was to determine (l) the influence of elevated dissolved inorganic carbon (DIC) on the nitrogen use efficiencies (NUE) of tomato seedlings grown with different nitrogen sources, (2) how changes in the regulation and activities of nitrate reductase (NR), phosphoenolpyruvate carboxylase (PEPc), carbonic anhydrase (CA) and subsequent changes in metabolites would account for observed changes in NUE, and (3) to what extent elevated DIC contributed to the carbon budget of plants grown with different nitrogen sources. Lycopersicon esculentum cv. Fl44 seedlings were grown in hydroponic culture (pH 5.8) with 2 mM of either N03- or NH4 + and the solutions were aerated with either 0 ppm or 5000 ppm CO2 concentrations. The similar NUEs of NH/-fed plants grown with either root-zone CO2 concentration were largely due to their similar RGRs and N uptake rates. Elevated root-zone DIC had an initial stimulatory effect on N~ + uptake rates, but it seems as if this effect of DIC physiological processes was cancelled out by the toxic effect of unassimilated NH/. The NUE for N03--fed plants supplied with 5000 ppm root-zone CO2 was higher relative to 0 ppm root-zone CO2 and it was possibly due to the higher relative growth rates for similar N uptake rates of 5000 ppm compared to 0 ppm root-zone CO2. Nitrate-fed plants grown with 5000 ppm compared to 0 ppm root-zone CO2 had higher in vivo NR and in vitro NR and PEPc activities. These increases in enzymes activities possibly lead to increases in organic acid synthesis, which could have been used for biomass accumulation. This would account for the increased relative growth rates of N03--fed plants grown with 5000 ppm compared to 0 ppm root-zone CO2. The increasing rootzone CO2 concentrations resulted in the Ó15N values of NH/-plants becoming more positive indicating an absence of enzymatic discrimination. This may have been due to the inhibitory effect of DIC on Nll,+ uptake, causing plants to utilise both internal isotopes equally. The Ól3C studies showed that PEPc contributed equally to both N03-- and NH/-fed plants over the long term. From this it can be concluded that the lower NUE of NH/-compared to N03--fed plants grown with 5000 ppm root-zone C02 was due to increased N uptake and exudation of organic compounds into the nutrient solution. Experiments with 813C also showed that at increasing rootzone CO2 concentrations, PEPc made a bigger contribution to the carbon budget via the anaplerotic reaction. / AFRIKAANSE OPSOMMING: Die doel van hierdie studie was om (1) die invloed van verhoogde opgeloste anorganiese koolstof dioksied (DIC) op die stikstofverbruiksdoeltreffenheid (NUE) van plante wat op verkillende stikstofbronne gekweek is, te bepaal. (2) Veranderinge in die regulering van nitraat reduktase (NR), fosfo-enolpirovaatkarboksilase (PEPc) en karboonsuuranhidrase (CA) is bestudeer en gekorreleer met waargeneemde verskille in NUE. (3) 'n Beraming van die mate waartoe verhoogde DIC bydra tot die koolstofbegroting van plante, gekweek op verskillende stikstofbronne, word bespreek. Lycopersicon esculentum cv. F144 saailinge is in waterkultuur (pH 5.8) met 2 mM N03- of NH/ gekweek en die oplossings is alternatiewelik met 0 ppm of 5000 ppm CO2 belug. Die NUEs van plante gekweek met NH/ en belug met albei C02 konsentrasies was vergelykbaar grootliks as gevolg van hulooreenkomstige relatiewe groeitempo's en Nopname. DIC het aanvanklik NH/ opname gestimuleer, maar enige latere stimulerende effek van DIC op fisiologiese prosesse was klaarblyklik uitgekanselleer deur N~ + toksiteit veroorsaak deur vertraagde assimilasie. Die NUE van plante gekweek met N03- en 5000 ppm CO2 was hoër as dié van plante gekweek met N03- en 0 ppm CO2. Dit is moontlik gekoppel aan hoër relatiewe groeitempo's teenoor onveranderde N opname tempo's. Plante gekweek met N03- en 5000 ppm CO2 het hoër in vivo NR en in vitro NR en PEPc aktiwiteite getoon as plante gekweek met N03- en 0 ppm CO2. Bogenoemde toenames in ensiem aktiwiteite word verbind met biomassa toename deur verhoogde organiese suur sintese. Dit bied 'n moontlike verklaring vir die hoër relatiewe groeitempo's van plante gekweek met N03- en 5000 ppm CO2 teenoor plante gegroei met N03- en 0 ppm CO2. Die 015N waardes van plante gekweek met NH/ en 5000 ppm CO2 was meer positief as dié van plante gekweek met Nl-l,+ en 0 ppm CO2 wat gedui het op die afwesigheid van ensiematiese diskriminasie. Dit kon as gevolg gewees het van die vertragende effek van DIC op Nl-la + opname wat daartoe sou lei dat die plante beide isotope eweveel inkorporeer. Eksperimente met ol3C het getoon dat PEPc oor 'n lang tydperk eweveel begedra het tot die koolstofbegroting van plante gekweek met beide N03- and N~+. Hiervan kan afgelei word dat die laer NUE van plante gekweek met NH4 + en 5000 ppm C02 in vergelyking met dié van plante gekweek met N03- en 5000 ppm CO2 die gevolg was van verhoogde NH/ opname en uitskeiding van aminosure in die voedingsoplossing. Eksperimente met 0"c het ook getoon dat verhoogde DIe konsentrasies die bydrae van PEPc tot die plant se koolstofbegroting laat toeneem.

Thesis (MSc)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The generally low concentrations of P and N in the soil, causes most plants to experience nutrient deficiency during their life cycle. Lupins can rely on both cluster roots and nodules for P acquisition and biological nitrogen fixation (BNF) respectively. The legume Lupinus albus is able to survive under low nutrient conditions, because it has two specialized belowground organs for the acquisition of N and P. In this regard, cluster roots increase P uptake and root nodules acquire atmospheric N2 via biological nitrogen fixation (BNF). Although these organs normally tolerates low P conditions, very little is known about their physiological and metabolic flexibility during variations in P supply. Furthermore, the resource allocation (C, N and P) between cluster roots and nodules has also been largely understudied. The aim of this investigation was therefore to determine the resource allocation, physiological and metabolic flexibility of these organs during variations in P supply. Although variation on P supply had no effect on the total biomass, there were significant differences in specialised below-ground organ allocation to cluster roots and nodule formation. Cluster root formation and the associated C-costs increased during low P supply. In contrast to the cluster root decline at high P supply, there was an increase in nodule growth allocation and corresponding C-costs. Since cluster roots were able to increase P acquisition under low P conditions, this below-ground investment may also have benefited the P nutrition of nodules. These findings provide evidence that when lupins acquire N via BNF in their nodules, there may be a trade-off in resource allocation between cluster roots and nodules. The short-term elevated P supply, caused an increased allocation of C and respiratory costs to nodules, at the expense of cluster roots. This alteration was also reflected in the increase in nodule enzyme activities related to organic acid synthesis, such as Phosphoenol-pyruvate Carboxylase (PEPC), Pyruvate Kinase (PK), Malate Dehydrogenase (NADH-MDH) and Malic Enzyme (ME). In cluster roots, the elevated P conditions, caused a decline in these organic acid synthesizing enzymes. This suggests that during short-term elevated P supply, there is a great degree of physiological and metabolic flexibility in the lupin nutrient acquiring structures. / AFRIKAANSE OPSOMMING: Die algemeen lae konsentrasies van fosfaat en stikstof in die grond , veroorsaak dat die meeste plante voedingstekorte ervaar tydens hul lewensiklus. Lupiene kan staatmaak op beide groep-wortels en wortel-knoppies vir P verkryging en biologiese stikstofbinding onderskeidelik. Die peulplant Lupinus albus is in staat om te oorleef onder lae voedings toestande , as gevolg van die twee gespesialiseerde ondergrondse organe vir die verkryging van stikstof en fosfaat. In hierdie verband verhoog groep-wortels fosfaat opname en wortel-knoppies verkry atmosferiese stikstof via biologiese stikstofbinding. Alhoewel hierdie organe normaalweg lae fosfaat toestande verdra , is baie min bekend oor hul fisiologiese en metaboliese buigsaamheid tydens variasies in fosfaat aanwending. Daar is verder ‘n tekort aan die studie van hulpbron toekenning tussen groep-wortels en wortel-knoppies. Die doel van hierdie ondersoek was dus om die toekenning van hulpbronne , fisiologiese en metaboliese buigsaamheid van hierdie organe tydens variasies in fosfaat aanwending te bepaal. Variesie in fosfaat verskaffing het geen invloed op die totale plant biomassa gehad nie, maar daar was wel ‘n beduidende verskil in gespesialiseerde ondergrondse toekenning tussen groep- wortels en wortel-knoppies. Groep-wortel vorming en die gepaardgaande koolstof koste het toegeneem met lae fosfaat verskaffing. In teenstelling met die groep-wortel daling met hoë fosfaat verskaffing, was daar 'n toename in groei van wortel-knoppies en die ooreenstemmende koolstof koste daarvan. Aangesien groep-wortels in staat was om fosfaat verkryging te verhoog onder lae fosfaat toestande, mag hierdie ondergrondse belegging bygedra het tot die voeding van wortel-knoppies . Hierdie bevindings bewys dat lupiene afhanklik van wortel-knoppies ‘n wisselwerking in toekenning van hulpbronne, tussen groep-wrotels en wortel-knoppies handaaf. Kort termyn verhoogde fosfaat aanwending veroorsaak 'n verhoogde toekenning van koolstof en respiratoriese energie na wortel-knoppiess, ten koste van groep-wortels . Hierdie verandering is ook weerspieël in die toename in wortel-knoppie ensiem aktiwiteit in verband met organiese suur sintese (PEPC PK,MDH,ME) . In groep-wortels, het die verhoogde P toestande verder 'n afname in die organiese suur produserende ensieme veroorsaak. Dit dui aan dat tydens kort termyn verhoogde P aanwending, daar 'n groot mate van fisiologiese en metaboliese buigsaamheid in die lupiene voedingstowwe verkryging strukture plaasvind.

[Typewritten] Includes bibliographical references.

Thesis (M. Sc.)(Microbiology)--University of Pretoria, 2005. / Includes bibliographical references. Available on the Internet via the World Wide Web.