Friday, 19 October 2012 at 11:30am
Room 1400 Biomedical and Physical Sciences Bldg.
Refreshments at 11:15
Speaker: Jacob Schaefer, Department of Chemistry, Washington University in St. Louis
Title: Carbon Partitioning in Soybean Leaves by Combined 11CO2 and 13CO2 Labeling
We have labeled soybean leaves with 200- and 600-ppm 13CO2 spiked with 11CO2 and examined the effects of light intensity and water stress on metabolism by using a combination of direct positron imaging and solid-state 13C nuclear magnetic resonance (NMR) of the same leaf. We first made 60-minute movies of the transport of photosynthetically assimilated 11C labels. The positron imaging identified zones or patches within which variations in metabolism could be probed later by NMR. At the end of each movie, the labeled leaf was frozen in liquid nitrogen to stop metabolism, the leaf was lyophilized, and solid-state NMR was used either on the whole leaf, or on various leaf fragments. The NMR analysis determined total 13C incorporation into sugars, starch, proteins, and protein precursors. The combination of 11CO2 and 13CO2 analytical techniques has led to three major conclusions for photosynthetically heterogeneous soybean leaves: (i) transient starch deposition is not the temporary storage of sucrose excluded from a saturated sugar-transport system; (ii) peptide synthesis is reduced under high-light, high CO2 conditions; and (iii) all glycine from the photorespiratory pathway is routed to proteins within photosynthetically active zones when the leaf is water stressed and under high-light and low CO2 conditions. These results are used to gain some insights into why soybean plants respond to high CO2 concentrations with fully increased photosynthetic rates and biomass accumulations (relative to those under ambient CO2 concentrations) in the absence of water stress in open-top chamber experiments, but not in the presence of periodic water stress, either in open-top chamber experiments or free-air carbon-enhancement experiments.