Characterization of aqueous pores in plant cuticles and permeation of ionic solutes

Abstract

Plant cuticles are lipid membranes with separate diffusion paths for lipophilic non-electrolytes and hydrated ionic compounds. Ions are lipid insoluble and require an aqueous pathway across cuticles. Based on experimental data, the aqueous pathway in cuticles has been characterized. Aqueous pores arise by hydration of permanent dipoles and ionic functional groups. They can be localized using ionic fluorescent dyes, silver nitrate, and mercuric chloride. Aqueous pores preferentially occur in cuticular ledges, at the base of trichomes, and in cuticles over anticlinal walls. Average pore radii ranged from 0.45 to 1.18 nm. Penetration of ions was a first order process as the fraction of the salt remaining on the cuticle surface decreased exponentially with time. Permeability of cuticles to ions depended on humidity and was highest at 100% humidity. Wetting agents increased rate constants by factors of up to 12, which indicates that the pore openings are surrounded by waxes. The pores in cuticular ledges of Helxine soleirolii allowed passage of berberine sulphate, which has a molecular weight of 769 g mol(-1). Increasing the molecular weight of solutes from 100 to 500 g mol(-1) decreased the rate constants of penetration by factors of 7 (Vicia faba) and 13 (Populus canescens), respectively. Half-times of penetration of inorganic salts and organic ions across Populus cuticles and Vicia leaf surfaces varied between 1 and 12 h. This shows that penetration of ionic compounds can be fairly rapid, and ions with molecular weights of up to 800 g mol(-1) can penetrate cuticles that possess aqueous pores.

Influence of Formulation on the Cuticular Penetration and on Spray Deposit Properties of Manganese and Zinc Foliar Fertilizers

“…As experimental field trials often provide inconsistent results depending on the specific climatic conditions of single seasons, the study of the foliar penetration efficacy of foliar nutrients has long been extended to model systems based on the use of isolated astomatous cuticles [3,[5][6][7]. Any study of the mechanism and efficacy of foliar absorption must consider ion penetration of the cuticle.…”

“…The mechanisms of cuticular penetration of polar and hydrophilic compounds, as usually contained in foliar fertilizers, are currently not fully understood [9]. The cuticle is a lipid semipermeable membrane where the existence of aqueous pores [5,6] has been suggested as a pathway of penetration of ionic or hydrophilic compounds, such as nutrient ions. Although these aqueous pores in the cuticle cannot be evidenced by electron microscope or other optical techniques, sophisticated ion permeability studies provide strong evidence of their existence [3].…”

“…Although these aqueous pores in the cuticle cannot be evidenced by electron microscope or other optical techniques, sophisticated ion permeability studies provide strong evidence of their existence [3]. Any penetration of compounds through these aqueous pores implicates that such compounds or nutrients must be dissolved in water [3,5,6]. Water solubility is a key factor for foliar uptake, since absorption will occur only when the applied compound is dissolved in a liquid phase-usually water-and will subsequently diffuse into the plant organs [3,6].…”

Improving the performance of calcium-containing spray formulations to limit the incidence of bitter pit in apple (Malus x domestica Borkh.)

“…Most research efforts in the last decades focused on investigating the diffusion of substances thorough the plant cuticle (Schönherr, 2006). To explain the mechanisms of cuticular penetration of apolar, lipophilic compounds the "diffusion-dissolution model" was proposed (Riederer and Friedmann, 2006).…”

“…To explain the mechanisms of cuticular penetration of apolar, lipophilic compounds the "diffusion-dissolution model" was proposed (Riederer and Friedmann, 2006). On the other hand, the penetration pathway of hydrophilic solutes through the cuticle is currently not fully understood (Fernández and Eichert, 2009) and the existence of "aqueous pores" as a parallel diffusion mechanism has been hypothesised (Schönherr, 2006). The occurrence of epidermal structures in plant surfaces such as lenticels, stomata or trichomes may significantly influence the rate of absorption of surface-applied agrochemicals.…”