The essential role of calcium in selective cation transport by plant cells

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Abstract
Excised roots of barley, Hordeum vulgare var. Arivat, absorbed K, Rb, and Na from aerated solutions at pH 4.9 and 30[degree] C, in the presence or absence of Ca, for 1 hour in most experiments. All salts were chlorides. K and Rb inhibit each other''s absorption in the absence as well as the presence of Ca at 0.05 m[image] or 0.1 m[image] concentration, indicating an identical carrier mechanism for which these 2 ions compete. The absorption of either K or Rb from 1 mM solutions is quite indifferent to even a 20-fold excess of Na in the solution, but only when Ca is present. In the absence of Ca, the rate of K-Rb absorption steadily declines in the presence of Na, and falls to nil in about 1 hour. It is concluded that there is a separate Na carrier site and that Ca is essential for the integrity of the separate K-Rb and Na absorption mechanisms. Na absorption is inhibited at low concentrations of K but at higher K concentrations there is little further K effect on Na absorption, in the presence of Ca, again indicating the existence of a Na absorption mechanism distinct from that which carriers K-Rb. Ca is maximally effective in maintaining the integrity of the distinct K-Rb and Na absorbing systems at 0.5 mM concentration. Other divalent cations, notably Mn and Zn, are also effective, though less so than Ca, but Mg is quite ineffective.