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ENVIRONMENTAL STRESS AND ADAPTATION

An In Vivo Analysis of the Effect of Season-Long Open-Air Elevation of Ozone to Anticipated 2050 Levels on Photosynthesis in Soybean¹

Departments of Plant Biology and of Crop Sciences, University of Illinois, Urbana, Illinois 61801 (P.B.M., D.R.O., S.P.L.); and United States Department of Agriculture, Agricultural Research Service, Urbana, Illinois 61801 (C.J.B., D.R.O.)

Rising atmospheric carbon dioxide concentration ([CO₂]) is widely recognized, but less appreciated is a concomitant rise in tropospheric ozone concentration ([O₃]). In industrialized countries, [O₃] has risen by 0.5% to 2.5% per year. Tropospheric [O₃] is predicted to reach a global mean of >60 nL L^–1 by 2050 with greater averages locally. Previous studies in enclosures suggest that this level of [O₃] will decrease leaf photosynthesis, thereby limiting growth and yield of Glycine max L. Merr. SoyFACE (Soybean Free Air gas Concentration Enrichment) is the first facility to elevate atmospheric [O₃] (approximately 1.2x current) in replicated plots under completely open-air conditions within an agricultural field. Measurements of gas exchange (assimilation versus light and assimilation versus intercellular [CO₂]) were made on excised leaves from control and treatment plots (n = 4). In contrast to expectations from previous chamber studies, elevated [O₃] did not alter light-saturated photosynthesis (A_sat, P = 0.09), carboxylation capacity (V_c,max, P = 0.82), or maximum electron transport (J_max, P = 0.66) for the topmost most recently fully expanded leaf at any stage of crop development. Leaves formed during the vegetative growth stage did not show a significant ozone-induced loss of photosynthetic capacity as they aged. Leaves formed during flowering did show a more rapid loss of photosynthetic capacity as they aged in elevated [O₃]. A_sat, V_c,max, and J_max (P = 0.04, 0.004, and 0.002, respectively) were decreased 20% to 30% by treatment with ozone. This is noteworthy since these leaves provide photosynthate to the developing grain. In conclusion, a small (approximately 20%) increase in tropospheric [O₃] did not significantly alter photosynthetic capacity of newly expanded leaves, but as these leaves aged, losses in photosynthetic carbon assimilation occurred.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.043968.

^* Corresponding author; e-mail stevel{at}life.uiuc.edu; fax 217–244–7563.

Received April 1, 2004; returned for revision June 10, 2004; accepted June 11, 2004.

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