Electrical resistance and ion movement through excised discs of sugar beet root tissue

Abstract

AbstractElectrical and tracer techniques were used to investigate the movement of Na+, K+ and Cl− through discs of a range of thicknesses cut from the root tissue of sugar beet, Beta vulgaris L. cv. Amono. At low external concentration the electrical resistance across a discs is less than that of an equivalent volume of solution. This does not appear to be due to a low resistance symplastic pathway but rather, to an enhanced concentration of cation in the apoplast. The resistance is proportional to the thickness of tissue. Although measurement of diffusion potential gives about 25 mV difference across the disc for a ten‐fold change in cation concentration, there is little discrimination between K+ and Na+.The observed tracer kinetics of 86Rb through the disc are consistent with those of diffusion, with a coefficient of diffusion, D, of 0.19 × 10−9 m2 s−1 and a tissue partition coefficient, k, of 0.27 (or of 2.7 if referred to the cell wall phase only). 36Cl gives a similar value for D, but has a k reduced by a factor of 3.3, a result that is consistent with the diffusion potential observation. However, a much larger discrimination would have been expected from the chemically measured cation exchange capacity.