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Salt Stimulation of Phosphate Uptake in Maize Root Tips Studied by ³¹P Nuclear Magnetic Resonance ¹

Stanford Magnetic Resonance Laboratory, Stanford University, Stanford, California 94305, United States Salinity Laboratory, United States Department of Agriculture, 4500 Glenwood Drive, Riverside, California 92502

The effects of external salt and inorganic phosphate (Pi) on the concentrations of vacuolar Pi, and cytoplasmic Pi, ATP, glucose-6-phosphate and UDP-glucose in maize root tips were examined using ³¹P nuclear magnetic resonance spectroscopy. We observed a more than two-fold stimulation of Pi uptake from 10 millimolar KH₂PO₄ solutions when root tips were exposed to 100 millimolar NaCl + CaCl₂. This stimulation of Pi uptake was associated with an increase in the concentration of cytoplasmic Pi in root tip cells. Thus, the molar ratio of cytoplasmic Pi to Pi + ATP + glucose-6-phosphate + UDP-glucose increased greatly in root tips exposed to salt and Pi. We speculate that it is this disturbance in relative concentrations of cytoplasmic phosphates (which we show are normally tightly regulated) that is responsible for both the greater rate of uptake of Pi by vacuoles of excised maize root tips, and the previously documented stimulation of Pi translocation from root to shoot in whole maize plants exposed to salt and Pi.