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

Packer, Kirsten F

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

Master Thesis

2018

Publisher

University of Cape Town

Department

Department of Biological Sciences

Faculty

Faculty of Science

Abstract

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.

Keywords

Biological Sciences

Reference:

Collections

Masters

Full item page