Analysis of Root Growth by Impedance Spectroscopy (EIS)

Abstract

Electrical impedance spectroscopy (EIS) is investigated as a non-destructive method for monitoring root growth of tomato. This paper aims to (i) review the basic principles of EIS applied to the characterisation of the different parts of the soil-root-stem-electrode continuum, (ii) experiment the validity of the relationship between root weight and root capacitance taking into account the influence of the soil and plant electrodes position, (iii) describe an EIS analysis of the root growth of tomato plants. All experiments were carried out in 50 dm 3 containers either in hydroponics at 930 lS for the test of root fresh or dry weight and root capacitance relationships, or in a potting mix (oxisol) for electrode placement tests and EIS estimation of root growth. Electrical measurements of the soil-root-stem-electrode continuum were done with a twoelectrode measuring system using unpolarisable Ag-AgCl electrodes. A 'root cutting' and a 'progressively immersed root system' experiments were carried out in order to validate the relationship between root capacitance and root mass at 1 kHz. The effects of soil electrode and plant electrode placement were also tested, pointing out the sensitivity of the method to the insertion height of the ''plant electrode'' into the stem. For the root growth experiment, Impedance Spectra (IS) measurements were made just before harvesting the roots for dry weight and length determination. Measurements were made 14, 22, 26 and 39 days after planting, until flowering. The IS of the soil-root-stem-electrode continuum was modelled by a lumped electric circuit consisting of a series resistor R 0 for the soil and of four parallel resistance (R i )-capacitance (C i ) circuits for the other components of the circuit. The model had nine parameters whose values were estimated by Complex Nonlinear Least Squares curve fitting. A stepwise ascendant regression was used to identify the electrical parameters that better correlated with root dry mass or length increment: C 3 and C 4 were well correlated with root dry mass with a r 2 of 0.975, whereas root length was explained by the combination of 1/R 3 , C 3 , 1/R 2 and 1/R 1 with a r 2 of 0.986. This work may be considered as a new methodological contribution to the understanding of root electrical properties in the non-destructive diagnosis of root systems.

Role of phase angle measurement in electrical impedance spectroscopy

“…The reason is that the amount of energy dissipated is independent of the root system size, thus the measured EI also becomes increasingly independent of the root system size in these cases. Additionally, this phenomenon can be intensified by occurrence of two sources of systematic error such as electrode polarization impedance and parasitic (stray) capacitances apparent at low and high frequencies, respectively (Cao et al, 2010;Ozier-Lafontaine and Bajazet, 2005). As for Experiment 2 (acetochlor treatment), the reduced charge-storage capacity of root membranes and the enhanced rate of energy dissipation caused an increase in the standard deviations of the data groups, leading to the lower significance levels at statistical comparison (Fig.…”

“…The capacitive character of the plant root-soil system proved to be relatively strong within the 700-4 000 Hz frequency range. At low frequencies (mainly below 100 Hz), the current flows through the extracellular (apoplastic) pathway, bypassing the cells: the high EI and low F measured is mostly due to the high resistance and low capacitance of extracellular space, respectively (Ozier-Lafontaine and Bajazet, 2005;Repo et al, 2000). At increasing frequencies, an alternating current starts to penetrate root cells: this intracellular (symplastic) pathway of current generating a capacitance effect is clearly reflected by the growing tendency of F. The further increase in frequency first begins to reduce and then cancels the capacitance of cell membranes, leading to a continuously decreasing F value.…”

“…Conversely, electrical impedance (EI) and electrical capacitance (EC) measurements in a plant-soil system offer good opportunities of rapid in situ investigation of the root system size and root activity without any intrusion into plant life function. By fixing an electrode at the plant stem and embedding the other one in the soil and connecting them by an LCR-instrument, the measured root EI and EC are directly correlated with root mass, root length, or root surface area (Chloupek, 1972;Ozier-Lafontaine and Bajazet, 2005;Rajkai et al, 2002). EI and EC methods have been used for investigation of detached plant tissues and organs subjected to various stress conditions (cold acclimation, freeze-thaw injury, drought, nutrient deficiency, or pathogen infection) as well as for studying intact root systems of plants cultivated in soil or grown in hydroponic solution (Chloupek et al, 2006;Cseresnyés et al, 2012Cseresnyés et al, , 2013Preston et al, 2004).…”

Application of electrical capacitance measurement for in situ monitoring of competitive interactions between maize and weed plants

“…The main limitation of the EC method is the sensitivity of electrical properties to soil water saturation, ionic status, soil texture and to the distance of plant electrode from substrate surface (Ozier-Lafontaine & Bajazet, 2005;Dietrich et al, 2012). Nevertheless, under welldefined conditions (constant soil moisture content, soil salinity and consistent electrode placement) and by the application of an effective current frequency, the method can provide a good estimation of the root system size.…”

Simultaneous monitoring of electrical capacitance and water uptake activity of plant root system

“…Rajkai et al (2005) recommended a two-dielectric (series-connected root and soil dielectric) capacitor model, where the capacitive character of the soil solution was also considered. Edaphic factors having influence on the accuracy of EC measurements (ie water saturation, soil texture and salinity, electrode placement) were investigated as well (Dalton, 1995;Ozier-Lafontaine and Bajazet, 2005).…”