An in vivo analysis of the effect of season-long open-air elevation of ozone to anticipated 2050 levels on photosynthesis in soybean

Rising atmospheric carbon dioxide concentration ([CO₂]) is widely recognized, but less appreciated is a concomitant rise in tropospheric ozone concentration ([O₃]). In industrialized countries, [O ₃] 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 [CO ₂]) were made on excised leaves from control and treatment plots (n = 4). In contrast to expectations from previous chamber studies, elevated [O ₃] 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 [O ₃]. 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.