Photoinduced Effects of Polycyclic Aromatic Hydrocarbons onBrassica napus(Canola) during Germination and Early Seedling Development

It has recently been demonstrated that light dramatically enhances the toxicity of polycyclic aromatic hydrocarbons (PAHs) to the duckweedLemna gibbaL. G-3 (L. Ren, X.-D. Huang, B. J. McConkey, D. G. Dixon, and B. M. Greenberg, 1994,Ecotoxicol. Environ. Saf.28,160–171). To extend this research to terrestrial plants,Brassica napusL. (oil seed rape) seeds were germinated in the presence of three PAHs; anthracene (ANT), benzo[a]pyrene (BAP), and fluoranthene. The chemicals were applied both in intact form and following photomodification in UV-B radiation; toxicity was assessed in simulated solar radiation (SSR), a light source with a visible light:UV-A:UV-B ratio similar to that of sunlight. Germination efficiency, root and shoot growth, and chlorophyll content, measured after 6 days of exposure, were used as toxicity endpoints. Intact and photomodified PAHs had little impact on shoot fresh weight or chlorophyll content, but markedly inhibited root fresh weight, with the photomodified PAHs having greater impacts than the intact PAHs. The decline in root fresh weight was not attributable to a decline in germination frequency or delayed germination. However, the seedlings produced shorter roots in the presence of either intact or photomodified PAHs. To explore the role of actinic radiation on PAH toxicity, seedlings were incubated in SSR, visible light and darkness with either intact or photomodified PAHs. Inhibition of root growth was only achieved by the intact chemicals if actinic radiation was present. However, with photomodified ANT or photomodified BAP, root fresh weight accumulation was inhibited in SSR, visible light and darkness. Thus, intact PAHs are hazardous to terrestrial plants in the presence of light, but once the compounds are photomodified, actinic radiation is no longer an absolute requirement for phytotoxic activity.