Long term measurement of the xylem potential by unpolarizable electrode via liquid junction

Abstract

Long term recordings of bioelectric potential have often been done by a pair of conventional polarizable electrodes, especially in field works. Such electrodes are apt to produce a big potential difference due to incidental polarization. We developed an electrode system which consists of unpolarizable Ag/AgCl type electrodes, liquid salt bridges and paraffin-coated injection needle which was stuck into the xylem apoplast. To avoid the invasion of air which often cuts the liquid junction between the electrodes and the xylem fluid, a tiny hydrostatic pressure (ca. 1 kPa, 10 cm difference in water level) was always applied to the liquid in the salt bridge. This simple device enabled us to continue long ter recording of the xylem potential over two months. We present in this paper the theoretical basis to get informations, without destruction of the plant bodies, about the physiological state of the root system under-ground by the observation of the xylem potential. A model experiment was carried out to warrant the theory.