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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52999 |
| Date | 03 1900 |
| Creators | Viktor, Aleysia |
| Contributors | Cramer, M. D., Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 113 p. : ill. |
| Rights | Stellenbosch University |
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