Cuticles were isolated enzymatically from the leaves of two maple species (Acer saccharum Marsh and A. platanoides L.) and from orange (Citrus aurantium L.). The cuticles were placed in a plastic cuvette and different concentrations of KCI were perfused over the physiological inner and outer surfaces while the electrical potential (E) that developed across the cuticles and was caused by ion diffusion was measured. E' was always positive, indicating that the permeability of K+ was always greater than that of CW. Measured Eb in cuticles did not fit the Goldman equation, whereas, E' measured during KCI diffusion across selected artificial membranes fit the equation. The magnitude of E' in cuticles and artificial membranes also was dependent on ionic strength, decreasing as ionic strength increased. These observations are explained by combining classical transport equations with equations that describe the equilibrium ion distribution between ionic double layers in the cuticle or membranes and the bathing solution.Relatively little is known about the mechanism of ion permeation through leaf cuticles. More is known about the permeation of nonelectrolytes. On the basis of permeation studies and thermodynamic arguments, Schonherr (6, 7) concluded that small nonelectrolytes permeate through cuticular pores ofup to 0.9 nm in diameter. Since the hydrated diameter of many ions is less than 0.8 nm, it is highly probable that ions also permeate through these pores. Schonherr (6) calculated that pores number about 1010 pores cm-2 of cuticle surface, and they occupy about 6 ppm of the surface area of cuticles. When chloroform-soluble cuticular lipids were extracted, leaving only the cuticular matrix, water permeability increased by two to three orders of magnitude, suggesting 100 to 1000 times more pores were exposed by removal of cuticular lipids. Schonherr and Bukovac (8,9) demonstrated that cuticular pores also have polar regions composed of the following components: (a) polyuronic acids associated with cellulose and pectin in the secondary cuticle, (b) cutin, which is a weakly polar compound due to hydroxyl and unesterified '
Shoot apical meristems of Picea abies seedlings can be cultured on a relatively simple, defined, basal medium. Dome‐like explants initially about 200 μ tall, without externally obvious primordia, and having dry weights of about 3 μg, usually initiate 5–10 new primordia within a week. They typically show 10‐ to 30‐fold dry weight increases in three weeks. None of the 5,000 meristems cultured has produced any basal callus. Growth is strongly influenced by both the type and concentration of agar used to gel the medium. Dry weight yield increases as agar concentration decreases. This is probably partly due to increased diffusion rates of enzymes or other large molecules through more dilute agar gels but possibly also partly ascribable to unknown agarborne inhibitors. About half of the agar concentration effect can be eliminated by substituting glucose and fructose for sucrose in the medium. This suggests that diffusion of invertase through the agar gel in this medium may be a growth limiting factor. Growth of cultures is also promoted by autoclaving sucrose in the presence of the agar. The basis of this effect is not yet understood.
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...
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.