To investigate the seasonal changes in the physiological activity of the roots in field trees, Diospyros kaki, we carried out simultaneous and continuous measurements of the trans-root electric potential (TRP), of the rate of sap-flow with transpiration and of the xylem pressure (Px) through several years. The amplitude of the diurnal change and its temperature dependency of both TRP and Px began to increase in late January. These parameters attained their maxima in the late defoliate phase just before unfolding of new leaves at the end of March, thereafter rapidly went down to their minimal levels with increasing transpiration. The low levels continued during the foliate stage until December, and the pattern of the seasonal change in Px was inverse of that in the sap-flow rate. The role of the electrogenic ion pump activity across the interface between root-symplast and xylem vessel as a driving force for acropetal water transport is discussed in relation to those seasonal changes.
A convenient system via a perfect liquid junction was developed for measuring the trans-root electric potential (TRP) of a tree in the field. This system enabled continuous measurement of the TRP of an old kaki tree (Diospym kaki, Japanese persimmon), which showy clear diurnal oscillation throughout two successive years. The level of the TRP on the average for every ten days had two minima (ca. -70mv) in mid winter and mid summer, and two maxima (ca. -30 mv) in October and May. A regular seasonal change in the amplitude of the oscillation was observed, it was at its maximum (ca. 70 mv) in early spring just before the sprouting of new leaves, and at its minimum (ca. 10 mv) during summer and early autumn when leaves were fully expanded. The relationship between the TRP, water transport and rainfall is discussed. change -Trans-root electric potential -Tree root Continuous recording of trans-root electric potential (TRP) of trees in the field is needed by researchers of arboridulture to diagnose the physiological state of root systems in situ. Also, in the field of earth science, availability of trees as sensors for the earth electric potential (or current) has been investigated (Ito 1997(Ito , lwamoto 1991. In previous measurements of bioelectric potential of trees in the field, direct contact with metal wires has been used in most cases to induce the electric potential difference between the plants and soil (e.g. Toriyama and Matteucig 1992). However, such conventional methods often cause serious errors because of the irregular polarization that occurs across the interface between the metal and the liquid phase of a plant body. Sometimes the observed potential exceeds the ordinary level of membrane potential in plant cells, i.e. over -300 mV (e.g. It0 1997). Although these results could partly reflect a change in the bioelectric potential of multicellular plants, Abbreviations: ERG, electro-radiccigram; V, , membrane potential across organ surface; Vpx, membrane potential across xylem/ symplast interface; V,, electric potential of the bulk phase surrounding the surface of an organ; V, or V, , xylem electric potential with respect to V, (when applied to plant roots, it is often called TRP, trans-root electric potential), N.B. Vx=Vw -Vpx, the difference between the two membrane potentials.they are obviously overlapping with other electric phenomena. The most reliable results can not be obtained without a pair of unpolarizable electrodes and a perfect liquid junction which connects these electrodes with the plant and the earth. For these purposes, a simple and reliable system for continuous measurement was devised and tested on one kaki tree in a garden setting. Materials and Methods Plant MaterialA kaki tree (Diospyros kaki Thunb. ex Murray cultivar, jiro), whose trunk diameter is about 28cm (planted just on the northside of the Mori Laboratory of Plant Physiology) was used for this experiment. DeviceAs shown in Fig. 1, an experimental system for continuous recording of TRP over years in the field was developed. The...
Re-examination of the electro-radicogram (ERG) obtained during past 10 years research (Masaki and Okamoto in Trees (Berl) 21: [433][434][435][436][437][438][439][440][441][442] 2007) enabled us to discriminate the excess activity of the electrogenic ion pump in the root surface cell membrane over that of the xylem pump during most of the foliate phase. The transroot electric potential (TRP) is defined as the difference between V ps (electric potential difference between symplast and bulk water phase surrounding the root) and V px (electric potential difference between symplast and xylem apoplast). The diurnal oscillation of TRP followed that of the air temperature and/or light intensity with a delay of several hours during defoliate phase. This means the superiority of the electrogenic activity of the xylem pump over that of the root surface pump. However, after leaf expansion, TRP began to oscillate inversely with the temperature change with a short delay, indicating the superiority of the electrogenic activity of the surface ion pump over that of the xylem pump. An experimental lumbering of the surroundings of the kaki tree in foliate phase prominently increased the ERG amplitude, keeping the inverted phase relation, with the increase in transpiration caused by the increased illumination. An incidental sudden fall of the temperature and illumination caused an inverse reaction.Keywords Diurnal oscillation of trans-root electric potential Á Phase relation Á Polarity in electro-radicogram Á Root surface ion pump Á Transpiration Abbreviations TRP Trans-root electric potential ERG Electro-radicogram V ps Electric potential difference between symplast and bulk water phase surrounding the root V px Electric potential difference between symplast and xylem apoplast
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