Diffusion and Electric Mobility of KCI within Isolated Cuticles of Citrus aurantium

Abstract

Fick's second law has been used to predict the time course of electrical conductance change in isolated cuticles following the rapid change in bathing solution (KCI) from concentration C to 0.1 C. The theoretical time course is dependent on the coefficient of diffusion of KCI in the cuticle and the cuticle thickness. Experimental results, obtained from cuticles isolated from sour orange (Citrus aurantium), fit with a diffusion model of an isolated cuticle in which about 90% of the conductance change following a solution change is due to salts diffusing from polar pores in the wax, and 10% of the change is due to salt diffusion from the wax. Short and long time constants for the washout of KCI were found to be 0.11 and 3.8 hours, respectively. These time constants correspond to KCI diffusion coefficients of 1 x 1015 and 3 x 10 1 square meters per second, respectively. The larger coefficient is close to the diffusion coefficient for water in polar pores of Citrus reported elsewhere (M Becker, G Kerstiens, I Schonherr [1986] Trees 1: 54-60). This supports our interpretation of the washout kinetics of KCI following a change in concentration of bathing solution.drift velocity (v), but at a slow flux density (mol s-1 m-2), because the internal ionic concentration is low. This follows because flux density = vC. Fick's law predicts that the average distance, x, that a molecule can diffuse in a time t is given by x = (2Dt)°5. Even if D in cuticles were one-hundredth that in water (2 x 10-9), the time for the average molecule to traverse 1 ,m of cuticle would be t = x2/2D = (1 x 10-6)2/2 x 2 x 10`1 = 0.025 s. Because cuticles took several hours to equilibrate, it suggested to us that the drift velocity must be low and that the D of salts must be many orders of magnitude lower than that in water.In this paper, we analyze the time course of X change in cuticles. This is the first estimate we know of in the literature of the D of a salt in a plant cuticle. Our approach is to use Fick's law to relate the D to the time course to approach equilibrium following a change in C°. A similar approach has been used (1) to calculate the D of water in isolated cuticles, but in that study, Fick's law was used to analyze the approach to a steady-state flux of water after imposing a constant concentration difference across a cuticle.In a previous study of the electrical X2 of isolated cuticles equilibrated in various salt solutions (8), we noted that it took several hours for the electrical X of a cuticle to reach an equilibrium X following a change in C°. Ionic Ps are low in cuticles (8). In theory, P equals aD/6. Because cuticles are very hydrophobic, we would anticipate a very small a that equals the ratio of the concentrations of the ionic species in the cuticle to that in the aqueous medium at equilibrium. We had presumed that the low P was due primarily to a low a. If this is true, then ions should pass through cuticles at a high 1 This research was supported by funds from the U.S. Department of Agriculture, Forest Service, ...

Transport of Water and Solutes across Maize Roots Modified by Puncturing the Endodermis (Further Evidence for the Composite Transport Model of the Root)

“…It is clear that, for a proper function of the Casparian band as an effective barrier for solutes, the D, would have to be considerably smaller. Thus, it could become comparable to D, values of univalent salts in cuticles (1 X 10-15 to 3 X l O -I 7 m2 s-' for KCl; Tyree et al, 1992). The calculations show that, if the Casparian band were almost impermeable, its D, would have to be even less than that of a waxy layer like the cuticle.…”

Effect of Cations on Effective Permeability of Leaf Cuticles to Sulfuric Acid

“…This situation is also likely in the case of flux of ions in which only monovalent cations are involved (Tyree et al,. 1991(Tyree et al,. , 1992.…”

“…The maximum possible EP is determined by the combination of resistances due to the wax layer and the fully protonated cutin. In contrast to the diffusion of water through the cuticle, where the wax layer accounts for almost a11 of the resistance to movement (Schonherr 1976a), and to situations involving the flux of ions in environments consisting solely of monovalent ions (Tyree et al, 1991(Tyree et al, , 1992, the diffusion of acid (and presumably other ionic substances) is substantially influenced by the cutin layer. Lower EP values are likely caused by incompletely protonated cutin or blocked pores in the cutin.…”

“…The contrast between species in both EP values and the effect of cations on these values may be due to differences in the wax layer, the cutin layer, or both. Because it has been shown under similar circumstances for another Citrus species that it is the wax layer that largely limits flux of ions (Tyree et al, 1991(Tyree et al, , 1992, small differences in the pore density or length (or other properties) could easily account for species differences in EP.…”

Determination of Epidermal Transpiration in Four Cultivars ofNicotiana TabacumL. Using Epidermal Strips in a Quasi-Steady State System

“…Probably for the same reasons, most of the information available regarding the sorption of water by plant cuticles (Chamel et al 199 1) , permeability of cuticles to acids (Hauser et al 1993), rheological properties (Petracek and Bukovac 1995), water permeability , and diffusion and mobility of ions in cuticles (Tyree et al 1992). have been obtained using isolated cuticles.…”