The effect of elevated carbon dioxide and ozone on soybean development, leaf anatomy and productivity

Atmospheric [CO2] continues to increase at an accelerated rate and is expected to reach 550 mumol mol-1 by 2040 and greater than 750 mumol mol-1 by the end of the 21st century. At such levels, C3 plants have been shown to respond directly to the increased availability of CO2 through an increase in net photosynthesis followed by a variety of downstream responses. Soybean (Glycine max [L.] Merr.) is one of the most well-studied species for elevated [CO2] response and is the most important oil seed crop produced globally. Detailed information on how this vital agricultural commodity responds to elevated [CO2] in an open-field agricultural setting is still limited. The Soybean Free-Air Concentration Enrichment (SoyFACE) research facility provides an unparalleled opportunity to evaluate soybean response to elevated [CO2] in an open-air setting. The research focus of this dissertation was to evaluate some of the neglected aspects of soybean responses to elevated [CO2] and ozone in an open-air setting. Specifically soybean development, leaf anatomy and gross primary productivity (GPP) were examined.;In order to evaluate developmental responses to elevated atmospheric [CO2], soybeans were tracked throughout their life-cycle using well established developmental scoring protocols. Since elevated [CO 2] causes an increase in canopy temperature through decreased crop transpiration, it was predicted that this would accelerate development. However, soybean grown in elevated [CO2] showed a highly significant delay in reproductive development. Soybean grown in elevated [CO2] required ∼57 more growing degree days (GDD, °Cd) to complete full bloom stage and ∼53°Cd more to complete the beginning seed stage In contrast, soybean grown in elevated [CO2] needed ∼51 °Cd fewer GDD to complete seed filling. Soybeans grown in elevated atmospheric [CO2] produced significantly more nodes on the main stem than those grown under current atmospheric conditions. This difference was consistent across all reproductive stages, but most prominent after the beginning of pod development. The practical effect of elevated [CO 2] on soybean was to delay crop maturation by over two days, and to also extend the growing season.;These results reflect changes in morphology at the organ level due to elevated [CO2]. Soybean response to elevated [CO2] was also evaluated at the anatomical level, along with its response to elevated [O3]. Ozone is another trace gas increasing globally, whose detrimental effects on plants are also of considerable interest. It was predicted that both elevated [CO2] and elevated [O3], because of the decreases in transpiration they induce, might decrease both stomatal index and xylem cross-sectional area. Soybeans grown in elevated [CO2] and/or [O3] exhibited a significant decreased stomatal index, but no change in xylem cross-sectional area, when compared to those grown in current ambient conditions. It was also predicted that changes in photosynthetic rate would result in an increase in phloem cross-sectional area under elevated [CO2] and the opposite in elevated ozone. However, a decrease was observed under both treatments. Further statistical analysis showed that change in phloem cross-sectional area combined with changes in stomatal density provided the strongest separation between the two gaseous treatments and the combined treatment.;Leaf area is significantly reduced in elevated [O3]; however, it is significantly increased in elevated [CO2]. This increase in leaf area, along with the increased number of main stem nodes, result in the previously reported increase in leaf area index observed in soybeans grown in elevated [CO2]. This increased photosynthetic tissue, along with increases in light-saturated photosynthesis observed under elevated atmospheric [CO21 is expected to result in a proportional increase in canopy photosynthesis or gross primary production (GPP). A draw-back of FACE is that GPP cannot be measured directly. Measurements of micrometeorology, leaf photosynthetic properties and canopy structure were combined in an adapted version of the Integrated Scientific Assessment Model (ISAM) to estimate GPP. The results of this model showed an increase in diurnal GPP, which resulted in a season-long 30% stimulation of both GPP and net primary production (NPP). Although the NPP modeled in this way agreed closely in absolute terms with that estimated from biomass measurements, the stimulation in NPP due to elevated [CO 2] was very much higher than the biomass measurements showed. The potential causes of this discrepancy are discussed and indicate new questions to be addressed.