Sodium Does Not Compete with Calcium in Saturating Plasma Membrane Sites Regulating ²²Na Influx in Salinized Maize Roots

Abstract

Half maximal inhibition of sodium ((22)Na(+)) influx into maize (Zea mays L.) root segments incubated in solutions containing from 0.25 to 100 millimolar NaCl was consistently attained with external calcium activity at 0.26 +/- 0.10 millimolar. Sodium ions do not appear to compete with calcium during initial binding to sites on the plasma membrane that participate in the regulation of sodium influx under saline conditions.