Quantitation of Rates of Transport, Metabolic Fluxes, and Cytoplasmic Levels of Inorganic Carbon in Maize Root Tips during K+ Ion Uptake 1

Our aim was to determine whether fixation of inorganic carbon (C i ), due to phospho enol pyruvate carboxylase activity, is limited by the availability of C i in the cytoplasm of maize ( Zea mays L.) root tips. Rates of C i uptake and metabolism were measured during K 2 SO 4 treatment, which stimulates dark C i fixation. 13 C i uptake was followed by 13 C-nuclear magnetic resonance (NMR); 5 millimolar K 2 SO 4 had no significant effect on 13 C i influx. The contribution of respiratory CO 2 production to cytoplasmic HCO 3 − was measured using in vivo 13 C-NMR and 1 H-NMR of cell extracts; K 2 SO 4 treatment had no effect on respiratory CO 2 production. The concentration of cytoplasmic HCO 3 − was estimated to be approximately 11 millimolar, again with K 2 SO 4 having no significant effect. These experiments allowed us to determine the extent to which extracellularly supplied 14 C i was diluted in the cytoplasm by respiratory CO 2 and thereby measure phospho enol pyruvate (PEP) carboxylase activity in vivo using 14 C i . PEP carboxylase activity in root tips was enhanced approximately 70% over controls within 12 minutes of the addition of 5 millimolar K 2 SO 4 . The activity of carbonic anhydrase, which provides PEP carboxylase with C i , was determined by saturation transfer 13 C-NMR to be more than 200 times that of PEP carboxylase in vivo. The regulation of PEP carboxylase in K 2 SO 4 -treated roots is discussed.