Interspecies variability of the plant/air partitioning of polychlorinated biphenyls

The plant/air partition coefficients (K-PA) of a range of PCB congeners were determined in five different grass and herb species common to Central Europe [ryegrass (Lolium multiflorum), clover (Trifolium repens), plantain (Plantago lanceolata), hawk's beard (Crepis biennis), and yarrow (Achillea millefolium)]. The measurements were conducted between 10 and 35 degrees C using a solid-phase fugacity meter. Large differences in the partitioning behavior between the plants were observed, with the partition coefficient varying by up to a factor of 20 between the five species. There was also considerable interspecies variability in the enthalpy of phase change (plant to air), but these differences were not related to the differences in the partition coefficients. A good linear relationship between log K-PA and experimentally determined log K-OA values (octanol/air partition coefficients) was obtained for each plant species (r(2) between 0.86 and 0.98). However, the slope of the regression lines ranged from 0.57 to 1.15. Using a solvent analogy, this indicates that the lipophilicity of the contaminant storage compartment is often different from octanol and varies widely among plants. The variability in the partition coefficients was primarily due to these differences in the quality of the lipophilic contaminant storage plant compartment, not to differences in its quantity or size. Most current models of organic contaminant accumulation in plants assume that the lipophilic storage compartment behaves like octanol, but this was not the case for any of the plants studied here, Before we can create models capable of accurately predicting concentrations of lipophilic trace organics in plants, more research into the nature of the plants' contaminant storage properties is necessary.