Abstract: | A comparison was made between two methods of measuring the relationshipbetween the external K+] and the flux of K+ into whole plantsof Lolium perenne and Raphanus sativus. The values of flux obtainedfrom solutions of 1.2 µM K+ held constant around the rootswere three and six times greater for Lolium and Raphanus respectivelythan the values obtained at the same concentration in a depletionexperiment in which the solutions, initially 100 µM K+,were depleted to below 1.2 µM K+ by plant uptake. In thedepletion experiment with Lolium, the flux was higher into plantsgrown at low K+] than into plants grown at 100 µM eventhough K+] within the plant was about the same for all groupsof plants. It is suggested that Lolium grown at low K+] hasan efficient mechanism for K+ uptake which continues to operatefor some time after the plants have been transferred to a higherconcentration. With both species, Km was 1520 µMin the depletion experiment and below 1 µM when concentrationswere held constant. |