Nikolaou et al., 1984) . The properties of PAHs can be altered It has recently been demonstrated that light dramatically eneither abiotically (e.g., photochemically) or biotically (e.g., hances the toxicity of polycyclic aromatic hydrocarbons (PAHs) cytochrome P450-mediated monooxygenation) (Yang et al., to the duckweed Lemna gibba L. G-3 (L. Ren, X.-D. Huang, B. Ren et al., 1994). plants, Brassica napus L. (oil seed rape) seeds were germinated in Photochemical activation occurs when the

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... ve p-orthe presence of three PAHs; anthracene (ANT), benzo[a]pyrene bital systems of PAHs absorb light in the ultraviolet-B (UV-(BAP), and fluoranthene. The chemicals were applied both in in-B; 290-320 nm) and/or the ultraviolet-A (UV-A; 320-400 tact form and following photomodification in UV-B radiation; toxnm) regions of the solar spectrum (Suess, 1976; Cook et al., icity was assessed in simulated solar radiation (SSR), a light source 1983; Nikolaou et al., 1984; Huang et al., 1993; Ren et with a visible light:UV-A:UV-B ratio similar to that of sunlight. Germination efficiency, root and shoot growth, and chlorophyll al., 1994). The compounds are reactive following photon content, measured after 6 days of exposure, were used as toxicity absorbance, which can result in photomodification of the endpoints. Intact and photomodified PAHs had little impact on PAH, defined here as photooxidation and/or photolysis; the shoot fresh weight or chlorophyll content, but markedly inhibited process is rapid under environmentally relevant levels of root fresh weight, with the photomodified PAHs having greater actinic radiation (Katz et al., 1979 ; Zepp and Schlotzhauer, impacts than the intact PAHs. The decline in root fresh weight was Nikolaou et al., 1984; Huang et al., 1993 Ren not attributable to a decline in germination frequency or delayed et al., 1994). Moreover, PAHs have high quantum yields for germination. However, the seedlings produced shorter roots in the