Sampling and analysis
The soils were collected by soil auger from the surface (0–20 cm) in the vicinity of the root 60 days after ger- mination. Soils were manually crushed and homogenized, then passed through a sieve (2 mm). The subsamples were stored at 4°C for assessing soil enzymes and enumeration of soil microorganisms. Shoots and roots were harvested, respectively. Root fragments were collected by sieving the soil and adding them to the root samples. Roots were carefully washed with tap water to remove any adhering soil particles. Then shoots and roots were freeze- dried and weighed. Soil dehydrogenase activity (DH) was determined by the reduction of triphenyltetrazolium chloride (TTC) to triphenylformazan (TPF) as described by Tabatabai (1994). PAHs degraders were enumerated using the most-probable-number (MPN) method with five replicates per dilution (Wrenn and Venosa, 1996). A mix- ture of phenanthrene (10 g/L), anthracene (1 g/L), fluorene (1 g/L) and fluoranthene (1 g/L) was supplied as the sole