A PHYSIOLOGICAL BASIS FOR ELECTROPHYTOGRAMS

Abstract

A passive system that may be useful for measuring electrochemical changes occurring in in vivo, extracellular plant tissue has been developed within the past few years. The technique involves the placement of a small (250 μm diameter) noble metal probe into the anatomical region of interest. The electrode potential of this probe is then established relative to a reference electrode. The time variation of this potential (up to 100 mV or more per day) is termed an electrophytogram. The changes are coherent and reproducible. In this dissertation theoretical physiological bases for the voltage fluctuations have been developed. One theory involves modeling the electrophytogram as a chemical thermodynamic system passing through a series of "frozen" equilibria. The electron is considered as the chemical species of interest in this system and changes in the redox potential are interpreted in terms of an electron electrochemical potential (μ̃ₑ).The relationship between changes in the electrochemical nature of the solution contiguous with the probe and the μ̃ₑ intensity variable is then represented for several limiting cases. In the case of a redox reaction accompanied by a proton exchange, the limiting equation shows that pH changes well within the physiological range for extracellular plant fluid can account for the observed voltage fluctuations. An alternative representation of the electrophytogram as an electrostatic field phenomenon is more difficult to analyze due to a lack of information about the three dimensional structure and the probe/tissue interface. However, a cylindrical capacitor model shows that fluxes in the range of a few hundred monovalent ions per um³ of extracellular volume could easily explain the measured voltage changes. The third model involves the polyelectrolyte gel nature of the plant cell wall. The electrophytogram voltage signal is considered to result from surface interactions between the metal probe and the cell wall. This interaction is analyzed by using the theory of interacting electrochemical double layers. Output from a computer simulation shows that if these two surfaces approach within two Debye lengths, a voltage signal will be generated at the electrophytogram probe. Furthermore, slight fluctuations in the surface to surface distance result in voltage fluctuations of a magnitude equal to those observed in vivo. Physiological processes known to occur in plants are discussed with respect to the generation of electrochemical potential gradients and other physical conditions necessary to "drive" the various models. I conclude that the electrophytogram is most likely the result of surface interactions between the probe and the polyelectrolyte gel components of the cell wall. Elucidation of the physiological basis for electrophytograms must also involve an accurate anatomical interpretation of the position of the probe relative to the plant tissue. Therefore the results of a freeze-fracture electron microscopic (FFSEM) examination of the probe/tissue interface after wound healing are included. Electron micrographs show cell wall material appressed directly against the probe, indicating that the electrophytogram provides a method for monitoring the electrochemical status of the cell wall space. Since cell wall material is hygroscopic, it is reasonable to assume that the smallest probe/tissue separation distance observed in the FFSEM's represents a maximum in vivo value. Since this distance is less than 10nm, these data support the hypothesis that the metal surface is within two Debye lengths of the cell wall surface in vivo. Insertion of the probe into mature, fully elongated tissue appears to cause minimal damage to nonxylem tissue beyond the adjacent cell layers and virtually no damage to the xylem region.