Multi-frequency electrical impedance tomography as a non-invasive tool to characterize and monitor crop root systems

Abstract

Abstract. A better understanding of root-soil interactions and associated processes is essential in achieving progress in crop breeding and management, prompting the need for high-resolution and non-destructive characterization methods. To date such methods are still lacking, or restricted by technical constraints, in particular for characterizing and monitoring root growth and function in the field. A promising technique in this respect is electrical impedance tomography (EIT), which utilizes low-frequency (< 1 kHz) electrical conduction and polarization properties in an imaging framework. It is well established that cells and cell clusters exhibit an electrical polarization response in alternating electric current fields due to electrical double layers which form at cell membranes. This double layer is directly related to the electrical surface properties of the membrane, which in turn are influenced by nutrient dynamics (fluxes and concentrations on both sides of the membranes). Therefore it can be assumed that the electrical polarization properties of roots are inherently related to nutrient uptake and translocation processes in the roots. We here propose broadband (mHz to hundreds of Hz) multi-frequency EIT as a non-invasive methodological approach for the monitoring and physiological, i.e. functional, characterization of crop root systems. The approach combines the spatial resolution capability of an imaging method with the diagnostic potential of electrical impedance spectroscopy. The capability of multi-frequency EIT to characterize and monitor crop root systems was investigated in a laboratory rhizotron experiment, in which the root system of oilseed plants was monitored in a water-filled rhizotron under ongoing nutrient deprivation. We found a low-frequency polarization response of the root system, which enabled the successful delineation of the spatial extension of the root system. The magnitude of the overall polarization response decreased along with the physiological decay of the root system due to the nutrient deprivation. Spectral polarization parameters, as derived from a pixel-based Debye decomposition analysis of the multi-frequency imaging results, reveal systematic changes in the spatial and spectral electrical response of the root system. In particular, quantified mean relaxation times (of the order of 10 ms) indicate changes in the length scales on which the polarization processes took place in the root system, as a response to prolonged nutrient deficiency. Our results demonstrate that broadband EIT is a capable non-invasive method to image root system extension as well as to monitor changes associated with root physiological processes. Given its applicability at both laboratory and field scales, our results suggest an enormous potential of the method for the structural and functional imaging of root system for various applications. This particularly holds for the field scale, where corresponding methods are highly desired but to date lacking.