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  • Author or Editor: T. Nacsa x
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The electrical capacitance method was applied for the examination of living root systems in a pot experiment. The measured root capacitances gave an unambiguous indication of the development of root mass and length. The root capacitances measured using needle and clamp plant electrodes were closely similar when the roots of whole plants were placed in water, while increasing differences were observed with a decrease in soil water saturation. The difference in capacitance between the plant electrodes is outlined by interpreting the action mechanism of the clamp electrode. The capacitance and electrical impedance spectra (30 Hz-1 MHz) were determined for roots in soil, for pieces of roots washed free of soil, and for the soil itself. The root capacitance was smaller than that of the soil and higher than that of root pieces at 1 kHz, while the capacitance of the soil became equal to that of roots in soil at about 2 kHz. This calls attention to the importance of the measuring frequency when determining root capacitance. A capacitor model with two dielectric media is proposed besides Dalton's model in order to interpret the behaviour of root and soil capacitances. However, its validity requires further verification.

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A new method is introduced to agricultural practice for measuring the living active root of the plants. The measured root capacitance is interpreted in electro-chemical principles. In addition to the electrochemical interpretation of the measurements we aimed to find a non-wounding electrode instead of the needle plant electrode. Another reason for dealing with the tweezer plant electrode was to decrease the relatively high standard deviation of the root capacitance readings due to the relatively high uncertainty of hitting the xylem with the needle plant electrode. To improve and standardize the contact between the tweezer plant electrode and the stem a high electrical conductivity gel (UNIGEL) was applied on the stem before clipping the tweezers.   Experiments for the root capacitance measurements were made in temperature and light controlled climate chambers (Conviron, Canada) in 2 litre plastic pots filled with 4:1 soil:sand mixture and water culture. Comparison of the root capacitances of five-week old sunflower plants measured with the needle and the tweezer plant electrodes proved identical in water culture and capillary water saturated soil. However, the applicability of the tweezer plant electrode needs further study for other plants and environmental conditions. The effect of measurement frequency on root capacitance and resistance with the HP4284A impedance bridge was also studied to see the effectiveness of polarization (Figure 1).  From Figure 1 it can be seen that root capacitance decreased at frequencies above 1 kHz, while it increased up to the dielectric constant of water at lower frequencies. An interpretation of measurable root capacitance in the soil-plant system is given using separate measured plant tissue and soil capacitances. We established that root capacitance in the soil-plant system approximates the capacitance of the root tissue.  Good correlation was found between root capacitance and the calculated root surface area (RA) for sunflower plants (Figure 2).   The GW LCR-814 was found suitable for making root capacitance measurements.  Finally, further experimental work is needed to collect information for the more general and extended applicability of the method before it becomes a routine tool in ecological and agricultural practice. 

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