Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants (POPs) that demonstrate carcinogenic and mutagenic toxicities 
. These contaminants are frequently detected at relatively high concentrations (mg/kg) in soils worldwide 
. Plants can absorb these contaminants from soil, causing deleterious effects on human and animal health via the consumption of contaminated vegetables 
. Therefore, an improved understanding of plant uptake of PAHs is essential for assessment of both the exposure of humans and other animal species and the risk represented by PAH-contaminated sites.
Recently, plant uptake of organic contaminants such as PAHs has attracted considerable attention 
. PAHs enter plants via foliage uptake from the atmosphere 
and root uptake from contaminated soil 
. Gao and Collins (2009) quantified the contributions of these two PAH uptake pathways in white clover. A significant fraction of shoot contamination resulted from the aerial deposition of volatilized PAHs, particularly of compounds with log KOA
>9 and log KAW
: octanol–air partition coefficient, KAW
: dimensionless air–water partition coefficient) 
. PAH uptake by plants from the soil to roots is a major pathway; the subsequent transport to shoots via the transpiration stream flux favors compounds with greater aqueous solubility 
. The magnitude of root uptake depends primarily on the lipid contents of plant roots, which is itself dependent on the protein, fat, nucleic acid, and cellulose contents; these contain lipophilic components and serve as the major domains accommodating PAHs after penetration of plant root surfaces. Recently, Kang et
. (2010) reported that the lipid contents of intracellular components determined the accumulation of lipophilic compounds; e
. PAHs, and that the corresponding diffusion rate was determined by the concentration gradient between the cell wall and intracellular organelles 
Many organic chemicals, including PAHs, are metabolized by plants, and exhibit a reduced concentration in plant tissues 
. However, these metabolic processes have been studied for only limited types of organic contaminants; for example, trichloroethylene, benzene, explosives and herbicides 
. The metabolic processes vary according to the type of contaminant and plant species. However, little information is available on the metabolism of PAHs by plants 
. A recent study showed that phenanthrene (PHE) was metabolized into other polar products in Zea mays
. In another study, anthracene (ANT) was metabolized primarily in cell walls, and the formed products were bound to cell wall components such as pectin, lignin, hemicellulose, and cellulose 
Metabolic processes are controlled by a variety of enzymes. For instance, cytochrome P450 monooxygenase could detoxify herbicides such as fenoxaprop-ethyl, diclofop-methyl, and bentazon in plants 
. Polyphenol oxidase (PPO), commonly found in fungi and plants, refers to a group of enzymes that catalyze the oxidation of phenolic compounds 
. Peroxidase (POD), another type of oxidative enzyme commonly present in plant and animal tissues, can oxidize phenols and aromatic amines in the presence of hydrogen peroxide. In contrast, the oxidation of phenolic compounds by PPO requires the presence of oxygen gas 
. Both PPO and POD play important roles in the metabolism of aromatic compounds in soil and water 
. However, little information is available regarding their function in the metabolism of PAHs by plants.
Inhibitors are commonly utilized in agricultural production to control enzyme activities and the metabolism of organic components, such as herbicides, by plants. Sterling and Balke (1990) reported reduced effects of monooxygenase inhibitors (1-aminobenzotriazole, tetcyclasis, piperonyl butoxide and cinnamic acid) on the oxidative metabolism of bentazon in rice and in soybean cell cultures 
. This inhibition has also been reported for the herbicides fenoxaprop-ethyl and diclofop-methyl in wheat or barley by tetcyclacis and tridiphane 
. Gronwald and Connelly (1991) reported that the cytochrome P450 inhibitor phenylhydrazine significantly diminished bentazon metabolism; moreover, other inhibitors–such as 3(2,4-dichlorophenoxy)-1-propyne and aminobenzotriazole–also reduced bentazon metabolism, albeit to a lesser extent 
. A significant decrease in the activities of arginine decarboxylase and ornithine decarboxylase occurred when DL-α-difluoromethylornithine (DFMO) and DL-α-difluoromethylarginine (DFMA) were added to a maize (Zea mays
L.) callus culture medium, and resulted in irreversible inhibition of putrescine synthesis 
. However, to our knowledge, most previous studies of the effects of enzyme inhibitors on plant metabolism focused primarily on herbicide applications in agricultural production, and little is known about the effects of inhibitors on plant metabolism of absorbed POPs such as PAHs.
Ascorbic acid (AA) is a naturally occurring, water-soluble compound with desirable characteristics as an enzyme inhibitor. It is the most abundant antioxidant in plants, and is used in agriculture to enhance plant stress-resistance 
. A recent in vitro
study reported that AA inhibits the activity of PPO in Mangifera indica
. However, few studies have investigated the effects of AA on enzyme activities and the metabolism of PAHs by plants.
To this end, the objective of this study was to evaluate the influence of the commonly used inhibitor, AA, on plant enzyme activities and PAH uptake. Naphthalene (NAP), PHE, and ANT, as representative 2- and 3-ringed PAHs, were the PAHs used. Tall fescue (Festuca arundinacea
Schreb.) is a common pasture plant for livestock production, and is also used in phytoremediation due to its fibrous root system and large root-specific surface area 
. In addition, the uptake of PAHs by this plant has been reported 
. Hence, tall fescue was chosen as a test plant in this investigation. These findings suggest that the common use of enzyme inhibitors in agricultural production may promote the accumulation of organic contaminants in plants, hence increasing risk in terms of food safety and quality.