SUBCELLULAR-DISTRIBUTION OF INORGANIC-PHOSPHATE, AND LEVELS OF NUCLEOSIDE TRIPHOSPHATE, IN MATURE MAIZE ROOTS AT LOW EXTERNAL PHOSPHATE CONCENTRATIONS - MEASUREMENTS WITH P-31-NMR

Maize plants were grown in nutrient solution without phosphate, or in which inorganic phosphate (Pi) was maintained at nearly constant concentrations of 1 muM, 10 muM or 0.5 mM. In vivo P-31-NMR measurements showed that there was no discernible difference in the cytoplasmic Pi content (mumol cm-3 root volume) of the mature roots of plants exposed to 1 muM, 10 muM or 0.5 mM external phosphate for up to 12 d. However, the vacuolar Pi content of the mature roots varied about 10-fold between these three groups. The cytoplasmic Pi content of roots receiving no external phosphate decreased significantly after about 7 d total growth, and at about this time the vacuolar pool of Pi became too small for accurate measurement. The presence of 1 muM Pi in the nutrient solution completely prevented this decline in cytoplasmic Pi, and there was some evidence that it also raised the Pi content of the root vacuoles above the almost undetectable level found in the totally P-starved roots. During the first 7-9 d of growth, the nucleoside triphosphate content of the mature roots was unaffected by the concentration of phosphate in the nutrient solution. The results highlight the close control of cytoplasmic concentrations of certain important phosphorus metabolites in roots growing in soil of normal agricultural fertility.