Environmental pollution is getting a serious problem throughout the world and their deleterious effect on plants attracts more attention recently. Polycyclic aromatic hydrocarbons (PAHs) is one of the hazardous group of environmental pollutants which are of highly concern with respect to their carcinogenicity and mutagenicity. Fluoranthene (FLU) and phenanthrene (PHE) are four and three ring PAHs respectively, which negatively affect animal, plant and human cells, ranging from acute to chronic toxicity. This study was designed to investigate stress responses of Marigold (Calendula officinalis) and Strawberry (Fragaria x ananassa) seedlings to these PAHs fumigation, and to evaluate the mitigative efficacy of Mannitol (Mann) and UV-A (black light) while combined with the PAHs. Marigold and strawberry seedlings were pot-grown in a greenhouse in Hiroshima University. To assess PAHs stress on the seedlings, physiological measurements such as photosynthesis at near saturating irradiance, stomatal conductance and Ci were measured during experiments using Li-6400 (LI-COR, USA) and photochemical efficiency of PS II in dark adapted state (Fv/Fm) was measured by MINI-PAM (Heinz-Walz Inc., Germany). After harvest, chlorophyll (chl a, and chl b) contents, carotenoids, proline contents and enzymes (Catalase (CAT) and Ascorbate peroxidase (APX)) activities were measured using spectrophotometer (UV-2400, Shimadzu Co., Japan).
Overall, FLU fumigation negatively affected gas exchange parameters and chlorophyll contents in marigold seedlings while PHE application did so with lesser extent compared to control. However, no significant changes were noticed in gas exchange parameters and chl b in strawberry seedlings, whereas Fv/Fm, chl a and carotenoid were significantly decreased by FLU fumigation. Moreover, exposure of strawberry to UV-A in combination with the FLU had no significant effect on any of the measured parameters except for Fv/Fm which was further decreased and was significantly different from both FLU alone as well as control. In marigold, lowest proline contents and highest enzymes activities in the treatment fumigated with only FLU indicated oxidative stress through generation of reactive oxygen species (ROS). The negative effect of FLU fumigation on marigold was mitigated by the addition of Mann to the solutions which was not significantly different from the control anymore. It was evident from the findings that Mann was an efficient quencher of ROS generated by FLU and PHE in marigold whereas UV-A showed no mitigation effects against FLU in strawberry.