Tuesday, November 14, 2006 - 2:00 PM
140-10

Photosynthetic Losses and Peroxidase Induction in Field-Grown Soybean Exposed to Elevated Ground-Level Ozone.

Patrick B. Morgan1, Fitzgerald Booker2, Carl J. Bernacchi3, Kent O. Burkey1, Alan M. Jones4, and Stephen Long5. (1) USDA-ARS-Plant Science Research Unit, 3908 Inwood Rd, Raleigh, NC 27603, (2) USDA-ARS Plant Science Research Unit, 3908 Inwood Road, Raleigh, NC 27603, (3) Illinois State Water Survey, 2204 Griffith Drive, Champaign, IL 61820, (4) University of North Carolina, Departments of Biology and Pharmacology, CB# 3280, Chapel Hill, NC 27599, (5) University of Illinois, 190 Edward R. Madigan Labs, Urbana, IL 61801-3838

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.