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 the charactization and monitoring of 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 ion uptake and translocation processes in the root systems. We hereby 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 rhizotron laboratory experiment, in which the root system of oilseed plants was monitored in a water–filled rhizotron, that is, in a nutrient-deprived environment. We found a low-frequency polarization response of the root system, which enabled the successful delineation of its spatial extension. The magnitude of the overall polarization response decreased along with the physiological decay of the root system due to the stress situation. 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 the prolonged induced stress situation. 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 the root physiological processes. Given its applicability on both laboratory and field scales, our results suggest an enormous potential of the method for the structural and functional imaging of root systems for various applications. This particularly holds for the field scale, where corresponding methods are highly desired but to date are lacking.

Small-scale characterization of vine plant root water uptake via 3-D electrical resistivity tomography and mise-à-la-masse method

“…Depending on the season, roots carry electrical charges as sap composition is variable and sap flows vary in intensity and direction. Wood composition and physical properties also change with root decay, which implies a variation of electrical properties (Martin, 2012;Martin and Günther, 2013;Weller et al, 2006). Very recently, Rao et al (2018) produced an interesting study aiming at understanding bulk electrical conductivity on individual root segments, incorporating the impact of roots in the pedophysical relations to better infer the real soil water content.…”

Induced polarization applied to biogeophysics: recent advances and future prospects

“…These studies showed that the spatial resolution of IP imaging is not sufficient to resolve single roots, but that the root extent can potentially be reconstructed. Interestingly, the frequency dependence of the electrical properties of root signals was also evident in the imaging results (see Weigand and Kemna 2017), providing potential to improve measurement results by careful selection of measurement frequencies. This will be especially helpful in cases where the electrical properties of both soil and root exhibit strong frequency dependence.…”

“…Time‐lapse IP imaging is required to spatially and temporally differentiate root structure and activity. So far, distinct polarization signatures of non‐woody crop roots have been reconstructed from spectral IP 2D or 3D images in controlled laboratory experiments on root systems embedded in aqueous solutions (Weigand ; Weigand and Kemna , ). These studies showed that the spatial resolution of IP imaging is not sufficient to resolve single roots, but that the root extent can potentially be reconstructed.…”

“…Apart from providing information on the root system extension, it has also been shown that spectral IP imaging can capture information on functional processes within the root C 2019 European Association of Geoscientists & Engineers, Near Surface Geophysics, 17, 595-621 system. For example, Weigand and Kemna (2017) showed a temporal and spatial decrease of polarization strength during prolonged nutrient deprivation of oilseed plants, indicating a sensitivity to nutrient uptake and translocation mechanisms. This is further supported by the observation that decapitated plants, which cannot take up water and nutrients, did not show a decrease in electrical polarization under nutrient deprivation (Weigand and Kemna 2019).…”

Using electrical impedance tomography for rapid determination of starch and soluble sugar contents in Rosa hybrida

“…EIT is mainly used in animal experiments and tentatively in human clinical research related to digestive, respiratory, and craniocerebral issues [13][14][15][16][17]. In recent years, some researchers have used EIT technology to study soil properties, wood decay, root physiological state, and cold resistance of plants [18][19][20][21][22]. There are few reports on the establishment of regression models for the effect of plant physiological state on EIT parameters.…”