Accumulation and transport of (2,4-dichlorophenoxy)acetic acid in plant cuticles. II. Permeability of the cuticular membrane.
Diffusion of (2,4-dichlorophenoxy)acetic acid (2,4-D) across plant cuticles from 10 species was investigated at 25 degrees C using enzymatically isolated cuticles. Permeance coefficients (P) and extrapolated holdup times (te) were measured and permeability (P), diffusion (D), and partition coefficients (K) were calculated. Permeance coefficients ranged from 2.72 X 10(-8) (Capsicum fruit) to 1.00 X 10(-10) m/sec (Ficus leaf). Extraction of soluble cuticular lipids from cuticular membranes increased permeances by up to four orders of magnitude. This demonstrates that permeance is determined by the soluble cuticular lipids associated with the cutin, rather than by cutin alone. Mean diffusion coefficients calculated from holdup times were 4.0 X 10(-15) (fruit cuticular membranes) and 1.71 X 10(-16) m2/sec (leaf cuticular membranes), respectively. Since a common diffusion coefficient exists for both leaf and fruit cuticles, differences in permeability coefficients between species can be attributed to differences in the partition coefficients. However, partition coefficients calculated from transmembrane diffusion are lower than those determined directly in a sorption experiment by a factor of from 6 to 200. Thus, the high resistance of plant cuticles to transport of 2,4-D can be attributed to both low diffusion and partition coefficients in the transport-limiting layer made up of cutin and soluble lipids which are densely packed and highly ordered. A linear equation is derived and permits the prediction of permeability coefficients of plant cuticles from partition coefficients determined in a simple sorption experiment.