Light, especially UV radiation, greatly elevates the toxicity of PAHs to plants. We have previously shown that chlorosis is an excellent indicator of PAH toxicity. Usually when chlorophyll (Chl) content is diminished, photosynthesis is also inhibited. Therefore, we tested if photosynthesis was indeed inhibited by PAHs in the presence of Simulated Solar Radiation. A Commonly used measure of in vivo photosynthetic activity in plants is chlorophyll a (Chl a) fluorescence, an assay of electron transport in photosystem II (PSII). This assay can be performed in situ on intact plants within 24 h of application of the chemical. In this study, two methods of collecting Chl a fluorescence data are described. Fluorescence induction was used to measure maximal PS II efficiency and photosynthesis downstream from PS II, with the finding that both were predictive of growth inhibition. Pulse amplitude modulated (PAM) fluorometry which measures steady state PS II efficiency was also used. This technique also detected negative impacts of photomodified anthracene on the plants.