The interaction of divalent cations with pectic substances and their influence on acid-induced cell wall loosening

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Abstract
Certain divalent cations are potent inhibitors of acid-induced cell wall loosening in both higher plants and green algae. It has been proposed that this inhibition involves the formation of pectic gels in the cell wall. To test this hypothesis, we have looked for a relationship between the ability of various ions to inhibit growth and (i) their ability to cause gelation of isolated pectins, (ii) their binding affinity for isolated cell walls, and (iii) their binding affinity for purified pectin. There is good correspondence between the effectiveness of an ion as a growth inhibitor and its binding affinity for the cell wall. There is very poor correspondence between growth inhibition and the ability to induce the gelation of pectin. Our results indicate that pectic gel formation is not involved in cation-induced growth inhibition, but that the major cation binding site in the cell wall may be involved in a nongel-forming capacity.