Cation Penetration through Isolated Leaf Cuticles

Abstract

The rates of penetration of various cations through isolated apricot Prunus armeniaca L. leaf cuticles were determined.Steady state rates were measured by using a specially constructed flow-through diffusion cell. The penetration rates of the monovalent cations in group IA followed a normal lyotropic series, i.e., CS+ > Rb+ > K+ > Na+ > Li+. The Leaf cuticle has a major influence on the absorption and ultimate impact of foliarly applied materials. Not only do pesticides, nutrients, and growth regulators enter plants through their leaves, but many pollutants also enter plants via this route. Foliar absorption may therefore either increase or decrease plant yield depending on conditions. The monographs of Hull (7) and Martin and Juniper (10) are especially valuable in bringing together and analyzing the many reports about cuticles and absorption. Both reviews reveal apparent gaps in our knowledge of the mechanisms and kinetics of cuticular penetration. Schumacher and Lambertz (17) described structures in the outer cell wall of leaf epidermal cells, and they and others assigned to them an important role in transcuticular penetration. These structures, which were observed adjacent to the cuticle, were later called ectodesmata and have been the focus of much attention. Recently, Schbnherr and Bukovac (16) showed that ectodesmata appear only at sites of penetration in the cuticle and, in fact, demonstrated similar structures in agar resulting from the penetration of material through an isolated cuticle. Thus, the appearance of ectodesmata was It was the purpose of this research to determine the rates and mechanism of cation penetration through isolated cuticles. In order to ascertain whether or not simple unhindered diffusion satisfactorily explains cuticular penetration, penetration rates were compared to those of simple unhindered diffusion in aqueous solutions. The effects of concentration and pH were also investigated in order to obtain data pertinent to the mechanism involved. Kinetic data from this research reveal some interesting characteristics about cuticular pores. MATERIALS AND METHODSCuticles were separated from the upper (nonstomatous) surface of apricot leaves (Prunus armeniaca L.) using the method described by Holloway and Baker (5). In brief, it consisted of selecting large leaves and punching two discs, 3 cm in diameter from each side of the midrib. The leaf discs were floated on a solution of ZnCl, in concentrated HCl (1 g in 1.7 ml) with the top surface up. The acid solution was infused into the leaf by vacuum infiltration. After 3 hr the cuticle was stripped, washed, and floated onto distilled water for storage in the refrigerator at 4 C until used.A diffusion cell was constructed by making a master in brass and casting a die with silicone rubber (General Electric . From the die, half-cells were cast using Ferris No. See-Thru flexible mold compound. The resulting half-cells were clear semihard rubber. The surfaces adhered to each other so that no grease was needed to seal the system. S...

Movement of Cations through Cuticles of Citrus aurantium and Acer saccharum

“…The order of permeabilities observed was K+ .> Cs > Na+ > Li'. By comparison, McFarlane and Berry (2) determined that the relative permeability coefficients of monovalent cation penetration of apricot (Prunus americana) leaf cuticles was in the order of: Cs' > Rb+ > K+ > Na+ > Li+. The ranking order was close to Prunus (however, we did not include Rb+), which followed the order of decreasing ionic radius and increasing hydrated ionic radius for the Group IA alkali metals.…”

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

“…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. Evidence from studies of the diffusion of ions (Yamada et al, 1964;MacFarlane and Berry, 1974) and more recently from studies of diffusion potentials across the cuticle (Tyree et al, 1990a) confirm that cuticles are asymmetrical in nature, with the pores lined with negative charges on the inner (cutin) portion of the cuticle. Such an arrangement invites interaction with cations, which are normally part of the interna1 environment of leaves.…”

“…Both the wax layer and the cutin contain hydrophilic pores (Schonherr and Schmidt, 1979;Seymour, 1980). Yamada et al (1964) and MacFarlane and Berry (1974), measuring the penneability of cuticles to ions, demonstrated considerable discrimination between ions on the basis of ion size. 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.…”

“…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.…”

Low Proton Conductance of Plant Cuticles and Its Relevance to the Acid-Growth Theory

“…The ability of the cuticle to limit movement of cations such as K+ (7,21), organic substances (4), and water (17) has been established, but the proton conductance2 of a cuticle has been examined only after long periods (3). Knowledge about the proton conductance of the cuticle is of importance in regard to the acid-growth theory of auxin-induced growth (15).…”