Quantifying the effect of two root traits on phosphorus uptake

Organic acid exudation and the ability of plants to respond to heterogeneous nutrient supplies in the soil are two traits that are important for phosphorus acquisition. The ability of plants to react to heterogeneous nutrient supplies is an instance of pheneotypic plasticity, known as root plasticity. This trait is evolutionary beneficial as plants can identify regions of soil that have high phosphorus content and proliferate in those regions, ultimately resulting in lower photosynthate expenditure on acquiring phosphorus. Organic acid exudation is the release of weak acids from the root into the soil. These acids are thought to improve phosphorus acquisition by increasing the solubility of phosphorus in soil. However, quantifying the benefit of these two traits experimentally remains a difficult problem due to challenges in controlling root growth, exudation and root surface area. In this thesis we used mathematical modelling to assess the relative benefits of these two traits. Special attention was paid to the real root geometries and phosphorus movement in the models; both were characterised by complimentary experiments. In particular, we used 3D X-ray computed tomography images of plant roots grown in soil as the geometry in our models. Furthermore, two approaches for measuring soil phosphorus were used to verify the results obtained from the models. One approach was elemental mapping (Scanning Electron Microscopy with Electron Dispersive X-ray Spectroscopy) of thin-sections of soil samples. From these maps, elemental concentrations of phosphorus was determined, which were then compared to model predictions. Another experimental technique was the use of microdialysis probes to sample soil solution and evaluate soil phosphorus concentrations at a range of different times. Microdialysis probes can also exude solutes as they sample soil solution. This functionality is exploited to verify models of organic acids solubilising soil sorbed phosphorus. The verified models were then used to investigate the benefit in terms of phosphorus uptake due to root plasticity and organic acid exudation under realistic soil conditions. Using the root system of a plant that had responded to a fertiliser pellet and a plant that had not as geometries in a phosphorus uptake model determined that the responding plant absorbed more phosphorus from the pellet. Fitting models to the phosphorus uptake/organic acid exudation of microdialysis probes yielded the rate at which organic acids solubilise soil adsorbed phosphorus. This rate was used in a model of a single root exuding organic acids and absorbing phosphorus. We found that for this single root, organic acid exudation did not contribute to phosphorus uptake. This model was then extended to whole root structure geometries extracted from X-ray computed tomography images. We found that roots could collectively exude enough organic acids into the soil to gain a noticeable increase in phosphorus uptake. Moreover, we identified some features of whole root systems which enhance the phosphorus uptake benefits that derive from organic acid exudation.