Electrical capacitance of bean (Vicia faba) root systems was related to tissue density—a test for the Dalton Model

Abstract

The capacitance method offers a rapid and non-destructive method for root measurement. We tested a four terminal (4 T) capacitance (C, Farads) measurement circuit, which removes parasitic errors. We also tested the plausibility of Dalton Model’s assumptions that roots act as cylindrical capacitors and that their entire length contributes to measured C. Faba bean (Vicia faba) was grown in sand and harvested at different ages to determine fresh root mass. Length and radii were determined using scanning software. Tissue density ρ was determined from observing buoyancy in water and from scanned dimensions. Relative permittivity e r was calculated using an empirical model fitted to the data. Parasitic errors were small. C was a poor predictor of root dimensions. An empirical model L ∝ (C/ρ k )m, was a reasonable predictor (R2 = 0.56; P < 0.05) of root length L and was not related to root allometry. This relation also allowed calculation of a plausible average value of e r . The well-watered conditions ensured that contact resistances were low. It appeared that the entire root system was contributing to measured C and average calculated e r was plausible for cortex tissue, if roots were assumed to be cylindrical capacitors (i.e. the Dalton Model)