Effects of ouabain and low temperature on the sodium efflux pump in excised corn roots

Ouabain (0.05 millimolar) and low temperature (4 C) both caused the tissue Na⁺ content of excised 5-day-old corn roots to increase, indicating that there is an inhibition of the Na⁺ efflux pump. Na⁺ efflux was measured utilizing three different methods. Each method gave similar results in terms of rate and ouabain sensitivity. With one of these methods, the compartmental efflux method, it was demonstrated that rates for Na⁺ efflux increase as the external Na⁺ concentration is increased; e.g. the efflux rates are 0.529, 1.78, and 3.64 microequivalents per gram fresh weight per hour for external NaCl concentrations of 1, 10, and 30 millimolar, respectively. The data indicate that the Na⁺ efflux pump is located in the plasmalemma of root cells.