Many authors have reported linear correlations between electrical capacitance, measured between an electrode inserted at the base of a plant and an electrode in the rooting substrate, and root mass. The measured capacitance is conventionally interpreted using the electrical model of F.N. Dalton in which roots are regarded as cylindrical capacitors wired in parallel. This model was tested for barley (Hordeum vulgare) grown hydroponically using treatments that included: raising roots out of solution, and cutting roots at positions below the solution surface. Although good linear correlations were found between capacitance and mass for whole root systems, when roots were raised out of solution, capacitances were non-linearly related to submerged root mass. Excision of roots in the solution had negligible effect on measured capacitance.The latter observations conflict with Dalton’s model. Capacitance correlated linearly with the cross-sectional area of root tissue at the solution surface, and inversely with distance between plant electrode and solution surface. A new model for capacitance was proposed and tested with cereal plants growing in solid substrates. Capacitances of plants in various substrates were measured under contrasting water regimes. Substrate capacitances increased with increasing water content. At water contents approaching field capacity, substrate capacitances were at least an order of magnitude greater than those of plant tissues. Wetting the substrate locally around a plant stem base was both necessary and sufficient to record maximum capacitance, which was correlated with stem cross-sectional area. Capacitance measured between two electrodes could be modelled as an electrical circuit in which component capacitors (plant tissue/ substrate) are wired in series, with capacitances of components connected to the same electrode acting in parallel. All results were consistent with the new model. Whilst the measured capacitance can, in some circumstances, be correlated with root mass, it is not a direct assay of root mass.
|Date of Award||2013|
|Supervisor||Hamlyn Jones (Supervisor)|