Sensing the electrical properties of roots: A review

Abstract

Thorough knowledge of root system functioning is essential to understand the feedback loops between plants, soil, and climate. In situ characterization of root systems is challenging due to the inaccessibility of roots and the complexity of root zone processes. Electrical methods have been proposed to overcome these difficulties. Electrical conduction and polarization occur in and around roots, but the mechanisms are not yet fully understood. We review the potential and limitations of low‐frequency electrical techniques for root zone investigation, discuss the mechanisms behind electrical conduction and polarization in the soil–root continuum, and address knowledge gaps. A range of electrical methods for root investigation is available. Reported methods using current injection in the plant stem to assess the extension of the root system lack robustness. Multi‐electrode measurements are increasingly used to quantify root zone processes through soil moisture changes. They often neglect the influence of root biomass on the electrical signal, probably because it is yet to be well understood. Recent research highlights the potential of frequency‐dependent impedance measurements. These methods target both surface and volumetric properties by activating and quantifying polarization mechanisms occurring at the root segment and cell scale at specific frequencies. The spectroscopic approach opens up a range of applications. Nevertheless, understanding electrical signatures at the field scale requires significant understanding of small‐scale polarization and conduction mechanisms. Improved mechanistic soil–root electrical models, validated with small‐scale electrical measurements on root systems, are necessary to make further progress in ramping up the precision and accuracy of multi‐electrode tomographic techniques for root zone investigation.

Combining Models of Root-Zone Hydrology and Geoelectrical Measurements: Recent Advances and Future Prospects

“…In a synthetic study, Rao et al (2019) evaluated how geometrical anisotropy of root networks impact the effective conductivity of the rhizosphere (the zone near the root). As petrophysical model uncertainty can be influenced by the presence of a root system, a better knowledge of root electrical properties would help define the formulation of a mixed root soil ER in order to refine the petrophysical relationships (Ehosioke et al, 2020).…”

Electrical Capacitance versus Minirhizotron Technique: A Study of Root Dynamics in Wheat–Pea Intercrops

“…In contrast, progressive root cutting and root immersion experiments in hydroponically grown plants supported that C R is highly influenced by the roots located in the media [11]. There is a consensus that the current pathway inside the plant organs is influenced by tissue properties, e.g., suberization [4,6,12,13]. Therefore, one advantage of the capacitance method is that C R is a reflection of root size and physiological status, i.e., water content, membrane integrity and permeability, and endodermal maturation, driven by ontogenic changes and environmental (stress) factors [13].…”

“…Owing to methodological difficulties in assessing the growth and activity of intact root systems in the soil [1,2], the application and development of indirect, non-destructive techniques, including electrical methods, have received increasing attention in recent years [3,4].…”

The potential of electrical imaging for field root zone phenotyping

“…Non-invasive geophysical techniques to study the root system and soil profile have substantially advanced in recent years (Ehosioke et al 2020). Only a few techniques can actually retrieve direct information on the root system architecture (e.g.…”