Plant Cuticles Are Polyelectrolytes with Isoelectric Points around Three

Abstract

The isoelectric points of isolated cutides from Citrus aurantium L. Transport properties of membranes depend strongly on the nature and density of charges fixed to the membrane matrix (6,11). Plant cuticles are biological polymer membranes of heterogeneous composition (12). Their transport properties are of great biological and practical importance and interest, as is evident from an extensive literature dealing with cuticular transpiration (21), leaching, uptake of nutrients, growth regulators, herbicides, fungicides, and insecticides applied to the foliage (9).Early indirect evidence suggested that cuticles may function as weak acid ion exchangers. Sorption of organic ions such as dyes, 2,4-D, and naphthaleneacetic acid was found to be pH-dependent; sorption of anions decreased while sorption of cations increased with increasing pH (2,5,14,16). When sorption of anionic and cationic dyes was plotted against pH, the curves had a point of inflection between pH 2.8 and 3.2 (2), which was believed to represent the pK of the cuticle. Further, surfaces of intact leaves selectively bound divalent over monovalent ions (10). From studies concerning the effect of pH on cuticular transpiration, Hartel (4) properties of isolated tomato fruit cuticular membranes directly and quantitatively. Three dissociable groups in the pH ranges 3 to 6 (0.2 meq g-'), 6 to 9 (0.3 meq g-'), and 9 to 12 (0.55 meq g-') were observed. The first two groups were tentatively identified as -COOH groups and the third as phenolic -OH group. The cuticles contained 0.65% (w/w) nitrogen, and above pH 9 more OH-ions were consumed in titration than Ca2+ bound, indicating titration of some basic groups, possibly -NH3+. Cuticles of apple fruits had previously been shown to contain small amounts of protein (8).Thus, there is some evidence that cuticles are bipolar polyelectrolytes, but quantitative data are lacking. In this paper, we present quantitative data on the isoelectric point, on permselectivity of cuticles to ions, and on the amounts of positive and negative charges fixed to the matrix of various isolated cuticles. (18); (c) immersion at room temperature in a solution of ZnC12 in concentrated HCI (1 g in 1.7 ml) (7). Cellular debris still adhering to the cuticles after isolation was removed with a jet of water, exchangeable cations were removed by agitating the cuticles at room temperature in 1 N HCI for 10 to 20 min (three changes) followed by washing with deionized H20 to remove sorbed HCI. Cuticular waxes were removed from a portion of the pear and Citrus leaf cuticular membranes by successive extraction at room temperature with methanol (three changes), 1 part methanol and 1 part chloroform (three changes), and chloroform (five changes) for 10 min each. All cuticular membranes were air-dried and stored until used. In the following, the method of isolation (enz., oxalate, ZnCl2/HCI) and the removal of waxes (-wax) will be stated after the name of the species from which the cuticle was isolated. MATERIALS AND METHODSMembrane Potential...

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

“…This explanation is consistent with the presence of organic acid groups within the pores of the cutin (Schonherr and Huber, 1977) and the tendency of divalent and trivalent cations to bond with such acid groups (Martin, 1984). (The chemical composition of the wax does not suggest modification with change in pH.)…”

“…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.…”

Structural changes and permeability of ivy (Hedera helix L.) leaf cuticles in relation to leaf development and after selective chemical treatments

“…HCl hydrolysis removes non-lipid components such as polysaccharides and polypeptides (Schonherr & Huber, 1977) and the insoluble residue represents the cutin polymer (CUT). The percentage of acidhydrolysed components (HYD) was given by the following relationship: (HYD"o) = 100-(CUT%) -(SCL%) (Chaumat & Chamel, 1990).…”

“…Intracuticular waxes are embedded in the cutin polymer; httle information is available on their composition (Baker, 1982;Baker, Bukovac & Hunt, 1982). Plant cuticles also contain non-lipid constituents such as polysaccharides (WattendorfF & Holloway, 1980), phenolics (Riley & Kolattukudy, 1975;Hunt & Baker, 1987) and polypeptides (Schonherr & Huber, 1977).…”

Influence of marine salts on the localization and accumulation of surfactant in the needles of Pinus halepensis Mill

“…The relative amounts of solute contained in cuticles and waxes would also depend on the time of exposure. Uptake of chemicals into conifer needles proceeds in two distinct phases (Screiber and (Schönherr and Huber, 1977;Chamel et al, 1992 (Bussotti et al, 1995) and Quercus ilex (Moricca et al, 1993). It has been shown that the highly ordered and crystalline waxes limit the sorption of solutes across isolated plant cuticles (Bukovac and Petracek, 1993).…”