Monovalent Cations and Growth Regulation. I. Growth Responses in Cucumber Hypocotyl Segments

The elongation of etiolated cucumber (Cucumis sativus L.) hypocotyl segments was stimulated by KCl and a number of other potassium salts at a concentration of 0.02 N K+. The effect of KCl was not evident until 14 hours after the beginning of treatment. NaCl, LiCl, and RbCl enhanced elongation of the segments, and their dosage-response curves were similar to that for KCl. At supraoptimal concentrations (≥ 0.05 M), LiCl inhibited segment growth. CsCl was inhibitory at all concentrations tested and was thus the only alkali metal cation to lack growth-promoting activity in this system. NH4Cl also promoted elongation, but not as effectively as did the alkali cations. CaCl2, CoCl2, and MgCl2 did not enhance growth when tested at the same concentrations as the monovalent cations. CoCl2 promoted strongly at 10-3 M, and NiCl2 was slightly active at 10-4 M. It is suggested that the nutritional requirement for K in higher plants results from a specific involvement in certain enzyme systems and from a relatively nonspecific role related to the elongation response described here.