Permeability studies on epidermal–hypodermal sleeves isolated from roots of Allium cepa (onion)

Abstract

Suberized sleeves of epidermal–hypodermal cells have been isolated by enzymic digestion from roots of onion. The average permeability coefficients of the sleeves to calcium and phosphate ions were 1.8 × 10⁻³ mm s⁻¹ and 1 × 10⁻³ mm s⁻¹. respectively, while the diffusive permeability coefficient for tritiated water was 2.6 × 10⁻³ mm s⁻¹. If the cell walls in vivo have properties similar to those in the isolated sleeves, then it is concluded that diffusion of small molecules in the apoplast is subject to less resistance than diffusion across plasma membranes. Using a range of compounds differing in lipid solubility and molecular weight (MW), it was found that permeability was inversely related to MW. It is concluded that the suberized walls of the sleeves are microporous, having hydrophilic channels traversing them. The results accord with some published information on onion roots in vivo but contrast with the very low permeability of the hypodermis found in some other situations.