Related Experiment Video
Updated: Jul 2, 2026

The Calibration and Use of Capacitance Sensors to Monitor Stem Water Content in Trees
Published on: December 27, 2017
Source of sustained voltage difference between the xylem of a potted Ficus benjamina tree and its soil
Christopher J Love1, Shuguang Zhang, Andreas Mershin
1Center for Biomedical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States of America.
Abstract:
It has long been known that there is a sustained electrical potential (voltage) difference between the xylem of many plants and their surrounding soil, but the mechanism behind this voltage has remained controversial. After eliminating any extraneous capacitive or inductive couplings and ground-mediated electric current flows, we have measured sustained differences of 50-200 mV between the xylem region of a Faraday-caged, intact, potted Ficus benjamina tree and its soil, as well as between its cut branches and soils and ionic solutions standardized to various pH values. Using identical platinum electrodes, no correlation between the voltage and time of day, illumination, sap flow, electrode elevation, or ionic composition of soil was found, suggesting no direct connection to simple dissimilar-metal redox reactions or transpirational activity. Instead, a clear relationship between the voltage polarity and magnitude and the pH difference between xylem and soil was observed. We attribute these sustained voltages to a biological concentration cell likely set up by the homeostatic mechanisms of the tree. Potential applications of this finding are briefly explored.
Related Concept Videos
DC Battery
Xylem and Transpiration-driven Transport of Resources
Potentiometer
Suppose the emf of a battery needs to be measured. If the battery is directly connected to a standard voltmeter, the measured quantity is...
Tonicity in Plants
Tonicity
Tonicity describes the capacity of a cell to lose or gain water depending on the solute...
Tonicity in Plants
Water and Mineral Acquisition

