Table II. Chase-phase quantification of substrate compartmentalization in GR horseweed leaf tissue following a 10-h pulse phase and 2-h wash phase

Standard in-magnet perfusion conditions were employed: 20°C, 100% oxygen equilibration, and dark. Data are reported as means ± se (n = 3) and reflect the ratio of individual resonance amplitudes to that of the external reference (MDP). Glyphosate data were reported previously in bar graph format (Ge et al., 2010). 31P-NMR chemical shifts were measured relative to an external reference (MDP) contained in a capillary tube placed within the 10-mm NMR tube along with the leaf tissue. MDP is assigned a 31P chemical shift of 18.2 ppm relative to the 31P chemical shift of 85% phosphoric acid, which is taken as 0.0 ppm. The differing chemical shifts for the two compartments, cytosol versus vacuole, reflect the differing ionization states for the phosphonate group in the presence of high-pH (cytosol) versus low-pH (vacuole) environments. For additional insight into the 31P chemical-shift dependence on pH, see Ackerman et al. (1996).

SubstratesCompartmentalization31P-NMR Chemical Shift
Early Chase Phase (Hours 12–14)Late Chase Phase (Hours 18–20)CytosolVacuole
Glyphosate0.36 (0.03)0.41 (0.03)0.26 (0.05)0.53 (0.04)8.08.6
AMPA0.86 (0.07)3.15 (0.38)0.60 (0.03)3.12 (0.39)9.810.5
NMG0.36 (0.05)0.25 (0.02)0.21 (0.06)0.36 (0.11)6.56.9