Carbon partitioning in soybean (Glycine max) leaves by combined 11C and 13C labeling

We labeled soybean (Glycine max) leaves with 200 and 600 ppm (CO2)-C-13 spiked with (CO2)-C-11 and examined the effects of light intensity and water stress on metabolism by using a combination of direct positron imaging and solid-state C-13 nuclear magnetic resonance (NMR) of the same leaf. We first made 60-min movies of the transport of photosynthetically assimilated C-11 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 C-13 incorporation into sugars, starch, proteins, and protein precursors. The combination of C-11 and C-13 analytical techniques has led to three major conclusions regarding photosynthetically heterogeneous soybean leaves: transient starch deposition is not the temporary storage of sucrose excluded from a saturated sugar-transport system; peptide synthesis is reduced under high-light, high CO2 conditions; and 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.