Fate of polycyclic aromatic hydrocarbons in plant-soil systems : plant responses to a chemical stress in the root zone

Plant uptake and translocation of polycyclic aromatic hydrocarbons (PAHs) from soil was investigated to explore plant-microbial interactions in response to a chemical stress in the root zone. Plant uptake of individual PAHs was examined under laboratory conditions selected to maximize root exposure. Specially constructed flasks that isolated roots and soil from aboveground tissues, provided a single, unambiguous pathway for PAH uptake. White sweetclover, Melilotus alba, was grown in soils dosed with ¹⁴C-naphthalene, -phenanthrene, -pyrene, and -fluoranthene. The highest ¹⁴C concentrations were associated with roots, with decreasing concentrations observed in stems and leaves; however, the greatest percentage of recoverable ¹⁴C remained in the soil (≥ 86%) for all four PAHs. No evidence of bioaccumulation of the individual PAHs was found in M. alba over a 5-day exposure period.

Root uptake and translocation of PAHs from soil to aboveground plant tissues, as indicated by ¹⁴C-residues in M. alba tissues, proved to be a limited mechanism for PAH transport into terrestrial food chains. However, root surface sorption of PAHs may be important for plants in soils containing elevated concentrations of PAHs. Consequently, the root-soil interface may be important for plant-microbial interactions in response to a chemical stress. ¹⁴CO₂ pulse labeling studies provide evidence of a shift in ¹⁴C-allocation from aboveground tissue to the root zone when plants were exposed simultaneously to phenanthrene in soil. In addition, soil respiration and heterotrophic plate counts of rhizosphere microorganisms increased in plants exposed to phenanthrene as compared to controls.

This study demonstrates the Importance of the root-soil interface for plants growing in PAH contaminated soil and provides supportive evidence for a plant-microbial defense response to chemical toxicants in the root zone. Both plants and rhizosphere microorganisms responded quickly to the presence of a PAH in soil: plant exudation and rhizosphere microbial numbers both increased in response to phenanthrene in soil. Thus, lipophilic toxicants in soils may reach high concentrations in the root zone, but rhizosphere microbial communities under the influence of the plant may reduce the amount of the compound that is actually taken up by the root.