Effect of K+, its Counter Anion, and pH on Sodium Efflux from Barley Root Tips

Sodium efflux from ²²Na⁺-loaded root tips root tips of Hordeumvulgare L. was markedly increased by replacing 10mM Na₂SO₄ inthe washing solution by K₂SO₄ with the same electrical conductivity.This increase was inhibited by both an uncoupler and an inhibitorof oxidative phosphorylation but not by ouabain. Potassium ionsdid not enhance Na⁺ efflux in the presence of a rapidly absorbedcounter anion, such as Cl^–, instead of . Efflux of ²²Na⁺ could also be enhanced by a low pH in theabsence of K⁺; this was prevented by uncouplers, but not byan inhibitor of the mitochondrial ATPase. It seems that K⁺ indirectly enhances Na⁺ efflux. It is suggestedthat metabolic K⁺ uptake in excess of the counter anion resultsin a proton gradient across the plasmalemma (acid outside) inducingH⁺/Na⁺ antiport.     

Non-specificity of Salt Effects on Mg2+dependent ATPase from Grass Roots

The effect of tris, choline, and ethanolamine chlorides on theactivity of Mg^2–dependent ATPase in membrane fractions(cell walls, mitochondria, and microsomes) of Zea mays L. (cv.Neve Yaar 22), Avena saliva L. (cv. Mulga), and Hordeum vulgareL. (cv. Omer) was compared with the effect of KC1 and NaCl.Considerable salt effects on apparent Mg²⁺ATPase activity werefound only at relatively high pH values (8.2) at which Mg²⁺.ATPaseactivity was low in the absence of monovalent cation salts.The Mg²⁺-dependent ATP hydrolysis by ATPases from all the membranefractions increased in the presence of at least one of the organiccations to the same extent as in the presence of KCI or NaCl.The monovalent organic cations are only very slowly absorbedby corn roots in comparison with K⁺ and Na⁺. It is concluded that monovalent salt effects on ATPase fromthese plant roots are not cation specific and not related tothe capability of root cells to absorb cations. Present evidencefor the existence of a cation-transport ATPase in plant tissueis critically reviewed.