Since the industrial revolution, tropospheric ozone concentrations ([O₃]) have risen by 0.5–2.5% per year (faster than CO₂).  Nearly 25% of the earth's surface is currently at risk from ground-level ozone in excess of 60 ppb despite improvements in US air quality over the past 30 yrs.  The US soybean-corn agricultural system covers approximately 62 million hectares making it arguably one of the largest ecosystems.  Utilizing SoyFACE (SOYbean Free Atmosphere gas Concentration Enrichment) to elevate [O₃] (1.2x ambient) provided a unique opportunity to analyze the ozone-impact in the field.  In 2002 & 2003, simultaneous measurements of chlorophyll fluorescence and photosynthetic gas-exchange were made on two groups of excised leaves: the newest topmost fully-expanded leaves and two leaf cohorts over their lifetimes.  Total peroxidase activity (measurement of H₂O₂ detoxification capacity) was made for mature leaves in 2006.  Elevated [O₃] accelerated leaf senescence hastening losses in carboxylation efficiency (an in vivo measure of Rubisco activity) and photosynthetic electron transport for regeneration of ribulose 1,5-bisphosphate (RuBP).  Lost leaf photosynthetic capacity due to elevated [O₃] decreased harvest yield by 20%.  Following only two, 8-hour fumigations with low-level [O₃] in controlled environment chambers, total peroxidase activity increased in wild-type Arabidopsis leaves.  Field-grown soybean in open-top chambers also increased total peroxidase activity upon exposure to ozone compared to low-ozone environments.  Peroxidase activity increased over the growing season in SoyFACE plots; however, elevated ozone did not further increase leaf peroxidase activity compared to ambient concentrations.  These findings emphasize the need to discover how plants respond mechanistically to elevated [O₃] at the leaf level as a means of protecting against damage both today and for the future.  With the increasing dependence placed upon model projections of crop production, assessing the impacts of ground-level [O₃] on crops in the field is crucial to understanding future food production.