Submitted to: Plant Physiology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/15/2004
Publication Date: 10/1/2004
Citation: Morgan, P.B., Bernacchi, C.J., Ort, D.R., Long, S.P. 2004. An In Vivo Analysis of The Effect of Season-Long, Open-Air Elevation of Ozone to Anticipated 2050 Levels On Photosynthesis in Soybean. Plant Physiology. 135:2348-2357. Interpretive Summary: In the northern mid-latitudes, surface ozone () has risen from an estimated pre-industrial 10ppb to an average regional concentration of almost 60ppb today. Although once considered a problem of conurbations, long-distance and even intercontinental ozone transport has resulted in a steady increase in rural areas hundreds and thousands of kilometers from the original sources of pollution. Mean July ozone levels are projected to increase over this century by 30ppb in the Midwest US and by 50ppb in eastern China, two of the largest soybean production areas of the globe. We have used an open-air treatment facility to determine how photosynthetic capacity in soybean responded to elevated [O3] exposure in the field. This first investigation of elevation of [O3] to anticipated 2050 levels under fully open-air field conditions suggests that the effects on photosynthesis in a sensitive plant, such as soybean, are substantially less than those predicted by prior chamber studies; particularly during vegetative growth. This raises doubts about the efficacy of screens for resistance germplasm based on growth stages prior to grain filling. However, the results do show that elevated [O3] accelerates loss of photosynthetic capacity in the late-season cohort of leaves, which are critical in providing assimilate to the developing seeds, and that the most significant loss is in apparent in vivo Rubisco activity. This information will help predict the amount of crop yield loss that can be expedted as ozone pollution increased over the next several decades.
Technical Abstract: 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 - 2.5% per year. Tropospheric [O3] is predicted to reach a global mean of > 60ppb 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 Concentration Enrichment) is the first facility to elevate atmospheric [O3] (approx. 1.2x current) in replicated plots under completely open-air conditions within an agricultural field. Measurements of gas-exchange (assimilation vs. light and assimilation vs. 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 (Asat, p = 0.09), carboxylation capacity (Vc,max, p = 0.82) or maximum electron transport (Jmax, 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]. Asat, Vc,max and Jmax (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 (~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.