Evaluating the importance of root abscission versus efflux to plant N-loss: consequences for plant N-isotope composition

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dc.contributor.advisorCramer, Michael Den_ZA
dc.contributor.authorPacker, Kirsten Fen_ZA
dc.date.accessioned2018-05-25T07:49:17Z
dc.date.available2018-05-25T07:49:17Z
dc.date.issued2018en_ZA
dc.description.abstractThe common observation that plant δ¹⁵N values are lower than those of associated soil is generally attributed to transporter-facilitated efflux of ¹⁵N-enriched N. N efflux tends to occur under specific conditions, for instance, when the external N concentration is high, when the external medium is acidic and when roots experience mechanical stress. While efflux is presumed to act as a regulator of cytoplasmic N concentrations, it is energetically costly for plants to take up N only to release it back into the rhizosphere. A link between root tissue loss (e.g. root turnover or rhizodeposition) and plant δ¹⁵N has not been suggested, although root abscission is likely to be more ubiquitous than N efflux. This thesis questions the extent to which N efflux and root abscission contribute to plant N-loss and plant δ¹⁵N values. I hypothesized that: (1) plants supplied with more N would have more negative δ¹⁵N relative to the source, and greater root abscission from a relatively larger root biomass (2) the aeration necessary for hydroponic culture can act as a mechanical stressor on roots, accentuating plant N-loss through root abscission and N efflux. Wheat was grown in sand with NO₃- supplied at five relative addition rates (RAR) and in hydroponics with three physical disturbance regimes (direct aeration, aeration constrained within a pipe and circulation of nutrient solution through sand). The δ¹⁵N of roots and shoots, as well as the plant-derived N accumulation in both growth mediums, were determined. When the N supply matched the plant N demand, as determined by the relative growth rate, there was no discrimination between plant and source δ¹⁵N. N-loss here, although negligible, was in the organic form, which implies root abscission. By contrast, when N supply exceeded plant N demand, plant δ¹⁵N values decreased (e.g. after 47 d, plant δ¹⁵N of RAR 0.075 d⁻¹ was 0.4‰ but was −4.1‰ at RAR 0.175 d⁻¹) because they lost ¹⁵N-enriched N. This N was largely inorganic and presumably lost through efflux. In disturbed hydroponic conditions (i.e. direct and pipe treatments), root 'fragments' were a major biomass- (six-fold greater than root dry weight) and N-loss (two-fold greater than plant net N uptake) pathway. Plants from all treatments lost more N within root fragments than through efflux, although the cumulative N-loss was significantly smaller from plants grown in relatively undisturbed hydroponic conditions (i.e. sand). This suggests that root abscission is likely to be an important N-loss pathway for plants and thus contributes to the global offset between plant and soil δ¹⁵N values. Moreover, efforts to improve nitrogen use efficiency of crop plants, though reduced efflux, need to take cognizance of root abscission because it is an unavoidable artefact of root growth.en_ZA
dc.identifier.apacitationPacker, K. F. (2018). <i>Evaluating the importance of root abscission versus efflux to plant N-loss: consequences for plant N-isotope composition</i>. (Thesis). University of Cape Town ,Faculty of Science ,Department of Biological Sciences. Retrieved from http://hdl.handle.net/11427/28152en_ZA
dc.identifier.chicagocitationPacker, Kirsten F. <i>"Evaluating the importance of root abscission versus efflux to plant N-loss: consequences for plant N-isotope composition."</i> Thesis., University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2018. http://hdl.handle.net/11427/28152en_ZA
dc.identifier.citationPacker, K. 2018. Evaluating the importance of root abscission versus efflux to plant N-loss: consequences for plant N-isotope composition. University of Cape Town.en_ZA
dc.identifier.ris TY - Thesis / Dissertation AU - Packer, Kirsten F AB - The common observation that plant δ¹⁵N values are lower than those of associated soil is generally attributed to transporter-facilitated efflux of ¹⁵N-enriched N. N efflux tends to occur under specific conditions, for instance, when the external N concentration is high, when the external medium is acidic and when roots experience mechanical stress. While efflux is presumed to act as a regulator of cytoplasmic N concentrations, it is energetically costly for plants to take up N only to release it back into the rhizosphere. A link between root tissue loss (e.g. root turnover or rhizodeposition) and plant δ¹⁵N has not been suggested, although root abscission is likely to be more ubiquitous than N efflux. This thesis questions the extent to which N efflux and root abscission contribute to plant N-loss and plant δ¹⁵N values. I hypothesized that: (1) plants supplied with more N would have more negative δ¹⁵N relative to the source, and greater root abscission from a relatively larger root biomass (2) the aeration necessary for hydroponic culture can act as a mechanical stressor on roots, accentuating plant N-loss through root abscission and N efflux. Wheat was grown in sand with NO₃- supplied at five relative addition rates (RAR) and in hydroponics with three physical disturbance regimes (direct aeration, aeration constrained within a pipe and circulation of nutrient solution through sand). The δ¹⁵N of roots and shoots, as well as the plant-derived N accumulation in both growth mediums, were determined. When the N supply matched the plant N demand, as determined by the relative growth rate, there was no discrimination between plant and source δ¹⁵N. N-loss here, although negligible, was in the organic form, which implies root abscission. By contrast, when N supply exceeded plant N demand, plant δ¹⁵N values decreased (e.g. after 47 d, plant δ¹⁵N of RAR 0.075 d⁻¹ was 0.4‰ but was −4.1‰ at RAR 0.175 d⁻¹) because they lost ¹⁵N-enriched N. This N was largely inorganic and presumably lost through efflux. In disturbed hydroponic conditions (i.e. direct and pipe treatments), root 'fragments' were a major biomass- (six-fold greater than root dry weight) and N-loss (two-fold greater than plant net N uptake) pathway. Plants from all treatments lost more N within root fragments than through efflux, although the cumulative N-loss was significantly smaller from plants grown in relatively undisturbed hydroponic conditions (i.e. sand). This suggests that root abscission is likely to be an important N-loss pathway for plants and thus contributes to the global offset between plant and soil δ¹⁵N values. Moreover, efforts to improve nitrogen use efficiency of crop plants, though reduced efflux, need to take cognizance of root abscission because it is an unavoidable artefact of root growth. DA - 2018 DB - OpenUCT DP - University of Cape Town LK - https://open.uct.ac.za PB - University of Cape Town PY - 2018 T1 - Evaluating the importance of root abscission versus efflux to plant N-loss: consequences for plant N-isotope composition TI - Evaluating the importance of root abscission versus efflux to plant N-loss: consequences for plant N-isotope composition UR - http://hdl.handle.net/11427/28152 ER - en_ZA
dc.identifier.urihttp://hdl.handle.net/11427/28152
dc.identifier.vancouvercitationPacker KF. Evaluating the importance of root abscission versus efflux to plant N-loss: consequences for plant N-isotope composition. [Thesis]. University of Cape Town ,Faculty of Science ,Department of Biological Sciences, 2018 [cited yyyy month dd]. Available from: http://hdl.handle.net/11427/28152en_ZA
dc.language.isoengen_ZA
dc.publisher.departmentDepartment of Biological Sciencesen_ZA
dc.publisher.facultyFaculty of Scienceen_ZA
dc.publisher.institutionUniversity of Cape Town
dc.subject.otherBiological Sciencesen_ZA
dc.titleEvaluating the importance of root abscission versus efflux to plant N-loss: consequences for plant N-isotope compositionen_ZA
dc.typeMaster Thesis
dc.type.qualificationlevelMasters
dc.type.qualificationnameMScen_ZA
uct.type.filetypeText
uct.type.filetypeImage
uct.type.publicationResearchen_ZA
uct.type.resourceThesisen_ZA
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