Patterns of Effective Permeability of Leaf Cuticles to Acids

Abstract

Plants in the field are frequently exposed to anthropogenic acid precipitation with pH values of 4 and below. For the acid to directly affect leaf tissues, it must pass through the leaf cuticle, but little is known about the permeability of cuticles to protons, or about the effect of different anions on this permeability. We investigated the movement of protons through isolated astomatous leaf cuticles of grapefruit (Cifrus x paradisi Macfady.), rough lemon (Cifrus limon [l.] Burm. fils cv Ponderosa), and pear (fyrus communis 1.) using hydrochloric, sulfuric, and nitric acids. Cuticles were enzymically isolated from leaves and placed in a diffusion apparatus with pH 4 acid on the morphological outer surface of the cuticle and degassed distilled water on the inner surface. Changes in pH of the solution on the inner surface were used to determine rates of effective permeability of the cuticles to the protons of these acids. Most cuticles exhibited an initial low permeability, lasting hours to days, then after a short transition displayed a significantly higher permeability, which persisted until equilibrium was approached. The change in effective permeability appears to be reversible. Effective permeabilities were higher for sulfuric acid than for the others. A model of the movement of protons through the cuticle is presented, proposing that dissociated acid groups in channels within the cutin are first protonated by the acid, accounting for the low initial effective permeability; then protons pass freely through the channels, resulting in a higher effective permeability.

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

“…This equation is in the form of a simple linear regression, with the proton concentration difference across the membrane presented as a function of time, plus a constant term. The slope at any point gives EP of the cuticle to protons of the acid (Nobel, 1991;Hauser et al, 1993). EP does not represent the true permeability of the cuticle to the protons, because the calculation here, based solely on the proton conceritration difference, does not take into account that portion of the driving force due to diffusion potential controlled by the relative permeabilities of the anion and cation.…”

“…isolated cuticles have shown that, although the permeability to acids is low (Dreyer et al, 1981), protons from acids eventually can pass through the cuticle, establishing equilibnum across it (Hauser et al, 1993). Most investigations of the movement of ionic substances through isolated cuticles have used enzymic isolation procedures that expose the cuticles to pH values of 4 or less (MacFarlane and Berry, 1974;Schonherr, 1976a;Hauser et al, 1993).…”

“…Most investigations of the movement of ionic substances through isolated cuticles have used enzymic isolation procedures that expose the cuticles to pH values of 4 or less (MacFarlane and Berry, 1974;Schonherr, 1976a;Hauser et al, 1993). Based on comparison with results from intact (Seymour, 1980) or mechanically isolated (Dreyer et al, 1981) cuticles, and on the chemical composition of the tissue, it seems unlikely that the properties of the cuticle are substantially altered by the treatment.…”

“…While investigating the movement of water through isolated cuticles, Schonherr (1976b) established a pore size near 0.9 nm, which can typically accommodate hydrated mineral ions. Although the outer layer of the cuticle is largely composed of a mixture of hydrophobic waxes (Tulloch, 1970), the inner layer, inherently more porous (Schonherr, 1976a), has a more complex chemistry, including dissociable acidic groups (Schonherr and Huber, 1977;Kolattukudy, 1980) that appear to be associated with the pores through which mineral acids pass (Hauser et al, 1993). The acid groups in the pores act as fixed buffers, "absorbing" protons for considerable periods before the pores become protonated and allow the free and rapid passage of protons through the cuticle to the inside.…”

“…Measurements of pH were continued at 20-min intervals until the pH of the inner solution approached that of the outer solution; a computer-controlled data acquisition system (ADC-1 data acquisition board; Remote Measurements Systems, Seattle, WA) was used for sampling pH. In a previous study, Hauser et al (1993) detennined that rates of diffusion through the cuticle are much lower than those through the unstirred layers within this apparatus; thus, the lack of stirring does not limit measurement of proton flux through the cuticle.…”