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Fluorescence imaging system (FIS) developed in our laboratories was used to study steady state fluorescence characteristics of plants subjected to chronic ozone stress. The imaging system consisted of four ultraviolet (UV) fluorescent lamps as an excitation source, an automated filter wheel with band pass filters, and a cooled charge coupled device (CCD) camera. Fluorescence images were captured at blue (F450), green (F550), red (F680), and far-red (F740) region of the spectrum centered at 450 nm, 550 nm, 680 nm, and 740 nm, respectively. Four different concentration schemes of tropospheric O3 and CO2 interactive environments were considered for this investigation. Soybean plants were grown full-season in 3 m diameter open-top chambers (OTCs) purged with the following gaseous treatments: charcoal filtered (CF) air; CF plus CO2; nonfiltered (NF) air plus O3; and NF plus O3 plus CO2. Cultivars 'Forrest' (O3 sensitive) and 'Essex' (O3 tolerant) were planted in each chamber treatment. The mean seasonal (7 h/day) CO2 and O3 concentrations monitored for these treatments were: 331 (mu) l/l CO2 and 22 nl/l O3; 472 (mu) l/l CO2 and 21 nl/l O3; 327 (mu) l/l CO2 and 63 nl/l O3; and 479 (mu) l/l CO2 and 64 nl/l O3, respectively. The most pronounced differences among the treatments were noted in the F450 and F550 images of leaves in both cultivars exposed to elevated O3 with more pronounced irregular appearances (white spots) in the O3 sensitive cultivar. Changes in cellular membrane integrity caused by chronic exposure to elevated O3 may have attributed to the increase in F450 and F550 intensities observed on the leaves grown in the elevated O3 environment. Significantly higher F680 and F740 fluorescence image intensities were observed in the elevated O3 exposed leaves in the presence and in the absence of elevated CO2 in both soybean cultivars. These observations suggested that elevated O3 affected the photosynthetic efficiency of the plants even in the environment with elevated CO2. The O3 sensitive 'Forrest' was found to have higher fluorescence intensities in all four bands compared to those of the more O3 tolerant 'Essex.' Although visible stress symptoms such as chlorosis, discoloration, or necrosis were not evident in the leaves used in this study, FIS demonstrated the capability of detecting the effects of chronic exposures to different air quality treatments. The OTC system air treatment environments in conjunction with FIS enhanced our understanding of the interactions between plant stresses and fluorescence responses. These findings with FIS represent a new approach in the studies of plant-pollutant interactions by providing a rapid nondestructive assessment.
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