A Comparison of Acid-induced Cell Wall Loosening in Valonia ventricosa and in Oat Coleoptiles

The acid-induced loosening of cell walls of Valonia ventricosa has been compared to that of frozen-thawed oat coleoptiles. The two acid extension responses are similar in regard to the shape of the pH response curve and the increase in plastic compliance induced by acid treatment. In both systems the acid response can be inhibited by Ca²⁺ and in both the removal of the protons leads to a rapid termination of wall loosening. The two responses differ in several significant ways, however. The acid-induced extension of Valonia walls is more rapid than that of coleoptile walls, but of smaller total magnitude. Acid-induced loosening can occur in Valonia without the wall being under tension, but not in coleoptiles. The acid-induced extension of Valonia walls is not inhibited by 8 molar urea, whereas the response in oat coleoptiles is completely inhibited by this treatment. Ethylenediaminetetraacetate (EDTA) can cause wall loosening in Valonia comparable to that produced by low pH, whereas in coleoptiles EDTA causes a much smaller response. These results with Valonia are consistent with a mechanism of acid-induced wall loosening in which a central role is played by the displacement of Ca²⁺ from the wall, while the larger part of acid-induced wall loosening in oat coleoptiles appears to be via a different mechanism.