Electrical Resistances of Corn Root Segments 1 2 Received for publication

Abstract

Longitudinal electrical resistances have been measured on 2-centimeter segments of corn (Zea mays L.) roots, cut at varying distances from the root apex. The segment resistances vary from 400 to 100 kilohms per centimeter along the root length (apex to 18 cm), with the maximum occurring in the 2to 4-centimeter segment, and decreasing thereafter toward the root base. Measurements of isolated root cortical sleeves and steles show that the pathway of least resistance is in the cortex, which has a greater cross-sectional area; the specific resistance of the older stele is less than that of the cortex. The anatomical state of the xylem cannot be inferred from electrical resistance determinations. There have been several recent reports of resistance measurements in plant root segments (3, 4), perhaps triggered by the report of Davis (1) that an electric potential difference was maintained between the xylem exudate fluid of an excised corn root and the bathing solution, even when successive segments were removed from the root tip. The resistance measurements were undertaken to try to understand this effect, since the immediate reaction might be that the observed potential difference in the Davis experiment (1) would be short-circuited through the xylem vessels of a root segment open at each end to ionic solution. In fact, the electrical situation is more complex than this, and was analyzed by Ginsburg (2) who realized that a root segment behaves electrically as an extremely leaky cable. The conclusion reached by him was that the longitudinal resistance of a root segment need not be very large, provided that it is appreciably greater than the radial resistance of the segment, for a stable potential difference to be maintained between the ends of the segment. There is a more general physiological point to these considerations. It rests on the question of whether root pressure exudation from the young, absorbing regions of roots takes place through fully mature metaxylem vessels with perforated cross walls, or whether it is generated in the xylem vessels much as turgor is in other cells. This process requires the presence of membrane-bound cytoplasm in the vessels (e.g. ref. 4). This hypothesis is that in the zone of perforation plate formation, the functioning vessels, i.e., the early maturing metaxylem vessels (5-10 cm from the apex in maize roots), retain intact protoplasm with plasmalemma and tonoplast. They are part of the symplast, but are unique in having the vacuoles extend upward I The research was supported by the National Science Foundation under Grant GB 19201 and in part by funds provided for biological and medical research by the State of Washington Measure No. 171. 2 This paper is dedicated to Leon Bernstein who has served plant physiology well. 3Permanent address: Research School of Biological Sciences, Australian National University, Canberra, A.C.T. 2601, Australia. into the nonliving xylem. In part, resistance measurements have been designed to give evidence on the state of the xylem vessels, but as shown below, it is not possible to infer the anatomical condition of the vessels from resistance measurements. MATERIALS AND METHODS Zea mays (cv. Golden Bantam) seeds were surface-sterilized in 1% Clorox solution, thoroughly rinsed in running water, and set to germinate on moist filter paper in the dark at 20 C. After 2 to 3 days the young seedlings were transferred to a hydroponic culture chamber, and continued to grow in aerated lX solution (4) in thelight at 20 C for a further 2 to 3 days. The solution designated IX has the following composition: 1 mm KCI; 1 mM Ca(NO3)2; 0.25 mm MgSO4; 0.904 mm NaH2PO4; 0.048 mM Na-2HP04; pH 5.8. At the end of this growth period the primary roots were 12 to 18 cm long, essentially without root hairs, and the region of first appearance of laterals was about 10 to 14 cm from the root apex. Root segments of 2-cm length were excised under water at various distances from the tip, and were mounted on paraffin wax blocks, using a paraffin wax-lanolin mixture (m.p. 40 C) to seal the root in place so that a I-mm section at either end of the segment was isolated from the central section (1.6 cm, Fig. 1). The ends and central section of this root preparation were then immersed under drops of IX solution which were contained simply by the high contact angle of the water on the wax. Ag/AgCl electrodes, prepared by electroplating silver wire in 0.1 N HCI at low current density, were then dipped into these drops of lX solution, and were connected to a simple resistance-measuring circuit (Fig. 2). The measuring circuit was calibrated by comparison with known resistance in a high quality resistance box, in the range appropriate for the values obtained on the root segments. By obvious choice of electrodes, the endto-end resistance of the segment, or either end-to-central section resistance could be measured. In an equivalent experimental preparation to determine the radial resistance of the root cortex, a primary root was decorticated by the usual technique of pulling out the stele to give a hollow cylinder of cortical tissue, usually with the root tip in place. Two-centimeter segments of this cylinder were then cut out at the required distances from the root tip, and were mounted in similar fashion to that just described for the whole root segments. A central electrode of Ag/AgCl wire was carefully inserted along the central cavity of the cortical cylinder, with considerable attention being given to avoid damaging the cortex during this operation. Measurements were made between this central electrode (which effectively space-clamps the segment) and an electrode dipping in the external solution bathing the central segment, to determine the radial resistance across the root cortex. Segments of isolated stele, obtained as a by-product in the above preparation, were also mounted in the manner described in the previous paragraph, and the end-toend resistance was determined. These two basic preparations were in some cases modified 137 www.plantphysiol.org on July 19, 2017 Published by Downloaded from Copyright © 1976 American Society of Plant Biologists. All rights reserved. ANDERSON AND HIGINBOTHAM

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@inproceedings{AndersonElectricalRO, title={Electrical Resistances of Corn Root Segments 1 2 Received for publication}, author={Wayne P. Anderson and N. Higinbotham} }