Uptake pathways of polycyclic aromatic hydrocarbons in white clover.

An understanding of the primary pathways of plant uptake of organic pollutants is important to enable the risks from crops grown on contaminated soils to be assessed. A series of experiments were undertaken to quantify the importance of the pathways of contamination and the subsequent transport within the plant using white clover plants grown in solution culture. Root uptake was primarily an absorption process, but a component of the contamination was a result of the transpiration flux to the shoot for higher solubility compounds. The root contamination can be easily predicted using a simple relationship with K(OW), although if a composition model was used based on lipid content, a significant under prediction of the contamination was observed. Shoot uptake was driven by the transpiration stream flux which was related to the solubility of the individual PAH rather than the K(OW). However, the experiment was over a short duration, 6 days, and models based on K(OW) may be better for crops grown in the field where the vegetation will approach equilibrium and transpiration cannot easily be measured. A significant fraction of the shoot contamination resulted from aerial deposition derived from volatilized PAH. This pathway was more significant for compounds approaching log K(OA) > 9 and log K(AW) < -3. The shoot uptake pathways need further investigation to enable them to be modeled separately. There was no evidence of significant systemic transport of the PAH, so transfer outside the transpiration stream is likely to be limited.