We examined some biophysical mechanisms of ion migration across leaf cuticles enzymatically isolated from Acer saccharum L. and Citrus aurantium L. leaves. Diffusion potential measurements were used to calculate the permeabilities of Cl-, Li+, Na+, and Cs+ ions all as a ratio with respect to the permeability of K+ in cuticles. In 2 millimolar ionic strength solutions the permeability sequence from high to low was K = Cs > Na > Li >> Cl. When the outer and inner surfaces of cuticles were bathed in artificial precipitation and artificial apoplast, respectively, diffusion potentials ranging from -52 to -91 millivolts were measured (inside negative). The Goldman equation predicted that the measured potentials were enough to increase the driving force on the accumulation of heavy metals by a factor of 4 to 7. Other ions migrate with forces 3 to 10 times less than predicted by the Goldman equation for concentration differences alone. Our analysis showed that Ca2+, and perhaps Mg2+, might even be accumulated against concentration gradients under some circumstances. Their uptake was apparently driven by the diffusion potentials created by the outward migration of monovalent salts. We feel that future models predicting leaching of nutrients from trees during acid rain events must be modified to account for the probable influence of diffusion potentials on ion migration.The objective of our research program is to determine the effects of environmental stress on tree growth and development, and includes the effects of air pollutants on the uptake and leaching of nutrients from leaves of trees. Within that context, this study was designed to develop a foundation for conducting research into the mechanisms of movement of multiple ion species through cuticles. As noted earlier (8), little is known about the mechanism of ion permeation through leaf cuticles; therefore, our previous research was focused on and hypothesized a charged-pore model that explains the asymmetric properties of cuticles reflected by diffusion potentials across cuticles of Acer and Citrus. That work was based exclusively upon diffusion potentials generated by using KCI solutions, and all experiments were conducted with salt-bridge electrodes of a type traditionally used in electrophysiological studies, i.e. by embedding Ag-AgCl wires in a KCl-gel (agarose) matrix. Such a system has limitations because of slow response times, intermittently unstable readings, and the inability to make reliable measurements of divalent cations or mixed ion species; therefore, we developed a new technique for making rapid, reliable determinations of diffusion potentials by using bare Ag-AgCl electrodes.We used Ag-AgCl electrodes to determine the effects of cations other than K+ on diffusion potentials across cuticles. We present results of experiments conducted with the monovalent cations Li', Na+, and Cs' in mixed salt solutions, using K+ as the reference species. We also measured diffusion potentials using artificial solutions designed to approximate precipitati...
We report a new method for measuring cation and anion permeability across cuticles of sour orange, Citrus aurantium, leaves. The method requires the measurement of two electrical parameters: the diffusion potential arising when the two sides of the cuticle are bathed in unequal concentrations of a Cl salt; and the electrical conductance of the cuticle measured at a salt concentration equal to the average of that used in the diffusionpotential measurement. The permeabilities of H+, Li+, Na+, K+, and Cs+ ranged from 2 x 10-8 to 0.6 x 10-8 meters per second when cuticles were bathed in 2 moles per cubic meter Cl-salts. The permeability of Cl-was 3 x 10-9 meters per second. The permeability of Li+, Na+, and K+ was about five times less when measured in 500 moles per cubic meter Cl-salts. We also report an asymmetry in cuticle-conductance values depending on the magnitude and the direction of current flow. The asymmetry disappears at low current-pulse magnitude and increases linearly with the magnitude of the current pulse. This phenomenon is explained in terms of transport-number effects in a bilayer model of the cuticle. Conductance is not augmented by current carried by exchangeable cations in cuticles; conductance is rate limited by the outer waxy layer of the cuticle. ion migration. In the most recent of those studies (12), we used diffusion-potential measurements exclusively to obtain data from which we calculated ionic-permeability ratios relative to K+, which we used as a reference ion. Here, we report on experiments that combine two different electrical measurements, diffusion potentials and electrical conductance, and use these data to calculate ionic permeabilities. MATERIALS AND METHODSCuticles of sour orange, Citrus aurantium L., were prepared in a manner similar to that described earlier (11). Briefly, adaxial cuticles were isolated by enzymatic techniques (incubated in pectinase and cellulase in acetic acid at pH 4.5 and at 37°C) and mounted between the cylindrical wells of a flow cell similar to that used previously (1 1). The cylindrical well had a diameter of 6.4 mm, and two Pt disk electrodes were added (Fig. 1, left side, and Fig. 2). The disks were platinum blacked and mounted onto the back surface of each well. Holes were drilled to allow solution flow through the inlet and outlet ports. A small hole was drilled parallel to the inner face of each half-cell to allow the insertion of a tight-fitting Ag wire within 200 ,m of the cuticle. Exposed segments of the Ag wires within the wells were chlorinated.As noted in an earlier paper (12), the objective of our research program is to determine the effects of environmental stress on tree growth and development. One component of our current research is to develop a basic foundation for conducting research on the movement ofions through cuticles so that we may develop a better understanding of the uptake and leaching of nutrients from leaves of trees. Previously (1 1, 12), we reported results of experiments in which diffusion potentials were meas...
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, ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.