The Effect of Potassium on Cotyledon Expansion Induced by Cytokinins

Potassium has been found to enhance greatly the expansion response of cucumber cotyledons to cytokinins. A reduction of the response to kinetin is obtained with increasing age of the cotyledons. The lesser response is associated with lower levels of potassium remaining in the cotyledon. A high level of KCI in the incubation medium offsets the lower potassium content of the tissue and enables a much larger response to the cytokinins. At 40 mM KCI the response to kinetin is 4.2 times greater than in the absence of KCI. Calcium increases the effect of potassium on the response to kinetin. When incubated in 40 mM KCI and 10 mM CaCI₂ with 10 mg/I 6-benzylamino-purine, the final weight of the cotyledons is 6.8 times the initial weight after just 4 days. This KCI-CaCI₂ combination is also found to promote chlorophyll synthesis in the usual cucumber cotyledon bioassay.