Permeation of lipophilic chemicals across plant cuticles: prediction from partition coefficients and molar volumes

Abstract

Permeability of astomatous cuticular membranes isolated fromCitrus aurantium leaves was determined with the following test compounds: 4-nitrophenol, atrazine [6-chioro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine],2,4-D [(2,4-dichlorophenoxy)acetic acid], 2,4,5-T [(2,4,5-trichlorophenoxy)acetic acid], PCP [pentachlorophenol], HCB [hexachlorobenzene], perylene and DEHP [1,2-benzenedicarboxylic acid bis(2-ethyl-hexyl)ester]. Permeance coefficients (flow per unit area, time and driving force) and extrapolated hold-up times were measured. There was a fair linear correlation (r=0.91) between the logarithms of the permeance coefficients (log P) and the logarithms of the octanol/water partition coefficients (log Ko/w). With this simple equation permeance coefficients for compounds having log Ko/wvalues ranging from 2 to 7.9 can be estimated to within an order of magnitude of values determined experimentally. Precision of prediction was improved (r =0.95), when log P was related to the logarithm of the cuticle/water partition coefficient (log kcm/w) but the best agreement between measured and calculated permeances was obtained (r=0.98) when the molar volumes (MV) of the test compounds were also taken into consideration by relating log P to log [KCM/W]/MV.