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.

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Possible sources of electrical transmembrane potentials in living cells are examined. The equations of Nernst, Ussing and Goldman cannot predictively explain the origin of membrane potential because they all require knowledge of concentrations of ions both outside and inside the cell, and these internal concentrations are themselves generated by activity of the cell. Therefore a new electrochemical theory for the steady state has been developed, which takes into account both active transport and the Donnan effect. The theory, which should be general to all living cells, successfully predicts membrane potential in examples examined. Certain hitherto unknown effects have been predicted, the most important of which have been named (a) the "nebenion effect", whereby the presence of other ionic species of the same charge sign as the actively transported ionic species depresses membrane potential, and (b) the "Donnan enhancement effect", whereby the membrane potential when both active transport and a Donnan system co-exist is greater than the sum of the potentials produced by each acting separately. Two important consequences follow ; (i) It is impossible for hydrogen or hydroxyl ion transport to generate a significant membrane potential in the face of environmental concentrations of nebenions. Thus the potential found across any membrane must be due to transport of majority ionic species and/or Donnan effects. (ii) The membrane potential in animal cells can only be explained by the "Donnan enhancement effect" in face of the "nebenion effect". Double Donnan systems and both linked and twin independent transports of two ionic species are considered. The effect of fixed charge either in the cell wail or as 3-potential on either side of the membrane is examined. Experimental procedures for the measurement of membrane potential are examined and a new integral microscope- manipulator system design is presented. Investigations are described of causes, and ways of avoiding, artefacts in measurements with micropipette electrodes, by studies both on model systems and directly on maize root cells. Experimental techniques for the measurement of cell membrane resistance and capacity are reviewed, and a new method is introduced to produce AF impedance spectra of cells, from which both membrane resistance and capacity can be calculated. The electronic system used a phase-sensitive detector to simplify analysis of the a.c. bridge network, as well as to remove noise. It is believed this was in 1973 the first use of such a system in electrophysiology. The technique was tested on dummy circuits to represent the living cell. The properties of micropipette electrodes were investigated. Membrane resistance and capacity were successfully measured in maize root cells. This new technique makes these measurements possible on smaller cells than hitherto, since it uses a lone single-barrelled microelectrode. Finally the significance of such measurements in terms of cell and tissue anatomy is considered, and the theory of "vergence" resistance of small connecting bridges between cells is extended to cover the multiperforate septum.

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AbstractSome morphological and physiological changes resulting from prolonged plant growth under plastic screening materials (Lifelite) now being marketed in this country are reported.Lifelite filtered out all light wavelengths between 500 and 580 manometers, lowered the transmission level to 26 percent in the 380 to 500 nanometer range and transmitted up to 62 percent of the wavelengths in the 580 to 700 nanometer range.Lifelite enhanced the germination of spinach seeds, inhibited the germination of lettuce and tomato seeds and had no effect on the germination of cabbage or onion seeds.The only positive morphological change elicited by Lifelite was a considerable increase in stem elongation because of an enlargement of cells. All plants., except cabbage, grown under Lifelite showed a substantial decrease in pigmentation.Ball State UniversityMuncie, IN 47306