Observation of Cytoplasmic and Vacuolar Malate in Maize Root Tips by 13C-NMR Spectroscopy

The accumulation of malate by maize (Zea mays L.) root tips perfused with KH(13)CO(3) was followed by (13)C nuclear magnetic resonance spectroscopy. In vivo nuclear magnetic resonance spectra contained distinct signals from two pools of malate in maize root tips, one at a pH approximately 5.3 (assigned to the vacuole) and one at a pH > 6.5 (assigned to the cytoplasm). The ratio of cytoplasmic to vacuolar malate was lower in 12 millimeter long root tips than in 2 millimeter root tips. The relatively broad width of the signals from C1- and C4-labeled vacuolar malate indicated heterogeneity in vacuolar pH. During the 3 hour KH(13)CO(3) treatment, (13)C-malate accumulated first primarily in the cytoplasm, increasing to a fairly constant level of approximately 6 millimolar by 1 hour. After a lag, vacuolar malate increased throughout the experiment.