|Leakey, Andrew D|
Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/24/2006
Publication Date: 11/24/2006
Citation: Bernacchi, C.J., Leakey, A.B., Heady, L.E., Morgan, P., Dohleman, F.G., Mcgrath, J.M., Gillespie, K.M., Wittig, V.E., Rogers, A., Long, S.P., Ort, D.R. 2006. Hourly and seasonal variation in photosynthesis and stomatal conductance of soybean grown at future CO2 and ozone concentrations for three years under fully open air conditions. Plant Cell and Environment. 29:2077-2090. Interpretive Summary: Increasing atmospheric carbon dioxide concentrations, based on the Intergovernmental Panel on Climate Change (IPCC), is predicted to increase leaf photosynthesis by 40% based on previous chambers studies. However increased ground-level ozone decreases leaf photosynthesis based on results from chambers. Often these observations are based on limited numbers of samples over the lifetime of the plant and are taken in ideal light conditions (i.e. saturating). In field grown plants, the effect of the changes in carbon dioxide and/or ground-level ozone may not be as pronounced due to several factors. Using Free-Air gas Concentration Enrichment (FACE) technology, 4700 measurements of plant photosynthesis were made over three entire growing seasons in the field. The increases in photosynthesis due to elevated carbon dioxide was 24% and whereas elevated ozone had no effect on photosynthesis. In combination, elevated carbon dioxide and elevated ground-level ozone resulted in a small increase in photosynthesis only 2/3 of the days sampled. This suggest that the predicted increase carbon storage predicted from increasing carbon dioxide levels will likely be unrealized because of simultaneous increases in ground-level ozone.
Technical Abstract: Numerous controlled environment studies have suggested that elevation of atmospheric [CO2] to 550 ppm, the concentration anticipated for the year 2050, should increase leaf photosynthetic carbon assimilation (A) by ca. 40%. Simultaneously studies of the effects of tropospheric ozone concentration ([O3]), which has been and is expected to continue rising globally has shown significant decreases in A. However, most studies have focused on light-saturated photosynthesis, often measured at one or two developmental stages at one time in the day or under controlled laboratory conditions. Several factors suggest that under fully open air field conditions, covering the full diurnal cycle and complete life cycle, these effects may be diminished. This hypothesis was tested with over 4700 individual measurements of A, photosynthetic electron transport (JPSII) and stomatal conductance (gs) over several complete diurnal cycles and three complete life cycles of Glycine max (L.) Merr. (soybean), the most agronomically important oilseed crop in the U.S. Treatments used Free-Air gas Concentration Enrichment (FACE) technology in a fully replicated factorial complete block design. The mean CO2 uptake rate (A) in the control plots was 15 µmol m-2 s-1. At elevated [CO2], mean A was 24% higher (18 µmol m-2 s-1) and significantly greater on 80% of days. There was a strong positive correlation between daytime maximum temperatures and mean daily integrated A at elevated [CO2] which appeared to account for the variation between days. The elevated [O3] treatment had no significant effect on mean A, gs, or JPSII. Combined elevation of [CO2] and [O3] resulted in a slightly smaller increase in average A than when [CO2] alone was elevated, and was significantly greater than the control on 67% of days. Whole chain electron transport (JPSII) showed similar trends to A, although the proportionate increase in elevated [CO2] was about half that of A.