Electrical impedance phase angle as an indicator of plant root stress

This study aimed to demonstrate that single-frequency (1 kHz) measurement of impedance phase angle (Φ) in root-soil systems is suitable for monitoring plant responses to environmental stresses. Potted wheat, soybean and maize plants were exposed to cadmium contamination, alkaline stress, drought or weed competition. Φ was detected at regular intervals between a ground and a plant electrode during plant development, at the end of which root and shoot biomass were measured. Each type of stress significantly reduced both Φ and the root and shoot dry mass, to an extent proportional to the stress level. The decrease in Φ was attributed to various physicochemical changes in root cell membranes, the accelerated maturation of the exo- and endodermis and altered root morphology. These stress responses modified the dielectric properties of the root tissues, influencing the apoplast and symplast pathways of the electrical current inside the roots. The stress-induced increase in the amount of electrically insulating lignin and suberin in root tissues was considered to be an influential factor in decreasing Φ. These results show that in pot experiments the measurement of the impedance phase angle in intact root systems is a potentially useful in situ method for detecting plant responses to stresses affecting roots.