Halophytes, Succulence and Sodium in Plants—a Unified Theory

S ummary Reference to the relevant literature shows that sodium ions can bring about increased succulence and, with regard to their response to sodium, halophytes differ from mesophytes only in degree, not in kind. Attention is drawn to the fact that the effect of sodium parallels the effect of increased light and aridity. It is proposed that these three factors exert the same effect on plant cells, namely to bring about an increased ATP synthesis. The mechanism whereby sodium ions bring this about is thought to be due to the reversal of a membrane‐bound ATPase which can otherwise act as a sodium pump. Aridity brings about increased concentration of sodium ions in the xylem sap and light brings about increased ATP synthesis via photophosphorylation. It is pointed out that the cell membranes must possess certain other physiological properties, if sodium is to act in the manner described. Attention is drawn to the fact that such an action of sodium ions would provide a homeostatic mechanism whereby the plant cell could avoid the detrimental effect of high concentrations of toxic ions.