Resistance of Plant Surfaces to Water Loss: Transport Properties of Cutin, Suberin and Associated Lipids

Cutin, a biopolyester composed of hydroxy and epoxy fatty acids, is the barrier between the aerial parts of higher plants and their environment. Suberin a polymer containing aromatics and polyesters, functions as a barrier in underground parts, wound surfaces, and a variety of internal organs. The composition and probable structure of these polymers are discussed. The biosynthesis of the hydroxy, epoxy, and dicarboxylic acids of the polyesters from the common cellular fatty acids is elucidated. An extracellular enzyme transfers the hydroxy and epoxyacyl moieties from their coenzyme A derivatives to the growing polyester. The enzymes acting in the biodegradation of the polyesters have been isolated from fungi, pollen, and mammals and characterized. The function and possible practical implications of these polyester barriers are briefly discussed.

The water permeability of astomatous cuticular membranes isolated from Citrus aurantium L. leaves, pear (Pyrus communis L.) leaves and onion (Allium cepa L.) bulb scales was determined before and after extraction of cuticular waxes with lipid solvents. In pear, the permeability coefficients for diffusion of tritiated water across cuticular membranes (CM) prior to extraction [P d(CM)] decreased by a factor of four during leaf expansion. In all three species investigated P d(CM) values of cuticular membranes from fully expanded leaves varied between 1 to 2×10(-7) cm(-3) s(-1)·P d(CM) values were not affected by pH. Extraction of cuticular waxes from the membranes increased their water permeability by a factor of 300 to 500. Permeability coefficients for diffusion of THO across the cutin matrix (MX) after extraction [P d(MX)] increased with increasing pH. P dvalues were not inversely proportional to the thickness of cuticular membranes. By treating the cutin matrix and cuticular waxes as two resistances acting in series it was shown that the water permeability of cuticles is completely determined by the waxes. The lack of the P d(CM) values to respond to pH appeared to be due to structural effects of waxes in the cutin matrix. Cuticular membranes from the submerse leaves of the aquatic plant Potamogeton lucens L. were three orders of magnitude more permeable to water than the cuticular membranes of the terrestrial species investigated.