Measurement of partition coefficients for selected polycyclic aromatic hydrocarbons between isolated plant cuticles and water

Partition coefficients between plant cuticles and water (K_cutw) were measured for 10 selected polycyclic aromatic hydrocarbons (PAHs) to evaluate the sorption capacity of plant cuticular layers for hydrophobic organic chemicals. The partitioning properties of PAHs between cuticles and water were evaluated by using (1) isolated cuticular layers and (2) leaf homogenate. The abaxial and adaxial cuticular layers of Euonymus japonicus were isolated by enzymatic digestion. A third-phase partitioning method using poly(dimethylsiloxane) (PDMS) was used to obtain K_cutw. The K_cutw values for the selected PAHs showed no significant differences between the abaxial and adaxial cuticular layers and ranged between 10^4.1 and 10^7.6. These values are close to or slightly higher than their 1-octanol/water partition coefficient (log K_ow), indicating high sorption capacity of plant cuticles. On the contrary, partition coefficients between the lipid tissues of homogenized leaves and water were lower than those obtained using isolated cuticular layers by factors of 3.7-190, which is likely due to the breakdown of lipid layers. This indicates that the sorption of hydrophobic organic chemicals by plant leaves is better evaluated using isolated cuticles and that the sorption potential of plant leaves may be underestimated when leaf homogenates are used.