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Replacement of calcium ion with magnesium ion in the cell wall of the pea epicotyl makes the wall more extensible. A possible role of this differential effect of Ca2+ and Mg2− in regulating cell elongation in pea epicotyl is discussed. The ratio of the content of calcium ion to that of magnesium ion (Ca2+/Mg2+) in the walls decreased markedly in the order of the first > the second > the third internodes of the pea epicotyl. The capacity of the walls for cation exchange increased in the same order, whereas the calcium-magnesium ion exchange selectivity of the walls was virtually constant. Our results indicate that the changes in the Ca2+/Mg2+ ratio among the internodes is not attributable to the ion exchange properties of the walk per se, but is due to other physiological conditions which regulate the activities of free calcium and magnesium ions in the environment with which the walls are in equilibrium.
Nakajima et al. (Sun,) studied this question.