| Increasing atmospheric carbon dioxide concentration ([CO2]) is widely accepted, but the concomitant rise in tropospheric ozone concentration ([O3]) is less well publicized. Global [CO2] is expected to reach 550 ppm by the middle of this century and [O3] predicted to reach a global mean of 63 p.p.b. by 2050. Meta-analytic techniques were used to provide an objective, quantitative summary of the responses of soybean to chronic ozone exposure in 53 peer-reviewed studies. On average, elevated [O3] decreased shoot biomass at maturity and seed yield by 34% and 24%, respectively. SoyFACE (SOYbean Free Atmosphere gas Concentration Enrichment, www.soyface.uiuc.edu), a facility that elevates [O 3] (1.2x current) or [CO2] (550 p.p.m.), provided a unique opportunity to analyze the effects of these two gases under open-air conditions within an agricultural field. Growth in elevated [O3] decreased above-ground net primary productivity (ANNP, ∼11%) and reproductive production (14%) without significantly altering mass partitioning. Conversely, growth in elevated [CO2] over 3 years increased ANPP (17--18%) and yield (15%). Although ANPP and yield responses are remarkably consistent across three growing seasons and two cultivars (cv. Pana and Pioneer 93B15), they are less than expected based on projections from previous chamber experiments. Simultaneously with analysis of production, measurements of fluorescence and photosynthetic gas-exchange were made on excised leaves. In contrast to expectations from previous chamber studies, elevated [O3] did not alter light-saturated photosynthesis (Asat), carboxylation capacity (Vc,max) or maximum electron transport ( Jmax) of the most-recently fully-expanded leaf, but losses were apparent as the leaves aged. Quantitative measurements (real-time PCR) of a sucrose-regulated sucrose/H+ symporter (GmSUT2) expression showed transcriptional down-regulation for soybean grown in both elevated [CO2] and elevated [O3] suggesting potential limitations to carbon allocation. This first analysis of the effects of elevated [O3] and [CO2] on soybean under open-air conditions suggests that chamber experiments may have over-estimated the stimulatory effect of [CO2], but not the damaging effect of [O3]. This corresponds to decreases in both photosynthetic capacity and possibly capacity for phloem loading. These results suggest that projections of future world food supply based on chamber studies are likely over-optimistic. |
