Eco-physiological Effects of Atmospheric Ozone and Polycyclic Aromatic Hydrocarbons (PAHs) on Plants

[Introduction] Tropospheric ozone is one of most concerned air pollutant, by causing damage to trees and crops. Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous contaminants found in various environmental compartments. Photo-induced toxicity of PAHs can be driven from formation of intracellular single oxygen and other reactive oxygen intermediates (ROI) leading to biological damages.(1) In the present study, we measured photosynthesis rate and other variables to investigate the effects of ozone and PAHs on the eco-physiological status of plants such as eggplant, common bean and strawberry. Plants treated with the single or combined air pollutants are expected to exhibit altered physiological, morphological and possibly growth changes. [Materials and Methods] We performed three exposure experiments. Exp.1. Eggplant (Solanum melongena) seedlings, were placed in the open-top chambers (n=6 plants/treatment). Treatment system was ozone (O)(120ppb), phenanthrene (P)(10μM), O+P, fluoranthene (F)(10μM), O+F, mannitol (M)(1mM) and the control (Milli-Q water)(C). P, F and M were sprayed three times weekly on the foliage part of eggplant. Average volume sprayed per seedling was 50mL. The treatment period was 30days and [AOT 40 (Accumulated exposure over a threshold of 40 ppb)]=28.8 ppmh. Exp.2, Common bean (Phaseolus vulgaris L.) seedlings were used (n=5 plants/treatment). The treatment system was the same as Exp.1. The treatment period was 40days and [AOT 40]=38.4ppmh. Exp.3. Strawberry (Fragaria L.) seedlings were used (n=5 plants/treatment). Treatment system was O (120ppb), F(10μM), O+F, F+M, O+M and C. The treatment period was 90days and [AOT 40]=86.4ppmh. Ecophysiological variables examined were photosynthesis rate measured at saturated irradiance (Amax), stomatal conductance to water vapour (gs), internal CO2 concentration (Ci), photochemical efficiency of PS2 in the dark (Fv/Fm), chlorophyll contents, visual symptom assessment and elemental composition in the leaves. [Results and Discussion] The P+O and O treatments caused a large decrease in Amax values of eggplant and common bean leaves and the P treatment made a little decrease in the Amax . Other parameters those were reduced by the O+P, O and P treatments were gs, Fv/Fm, Chlorophyll contents, and elemental composition. The F treatment made more decrease in Amax, gs and chlorophyll contents in eggplant leaves than those in the P treatment. The O treatment significantly decreased Amax, gs, Fv/Fm and chlorophyll contents in the Exp. 3. From the results mentioned above, it was concluded that phenanthrene and fluoranthene deposited onto leaves, combined with ozone fumigations, negatively affected growth and quality of plants. In addition, all plants fumigated with O, P, F, O+P and O+F showed severe visible symptoms on the foliage during the exposure period. We also note that mannitol, a reactive oxygen scavenger, mitigated negative effects of the F+O, O, P and F on plants and therefore mannitol could be useful for protection of plant from air pollutants. (1) Huang X, Lorelei F, Dixon D, Greenberg B (1996) Ecotoxicology and Environmental Safety, Volume 35 (2), 101-197.