COMPARISON BETWEEN THE UPTAKE OF POTASSIUM BY PLANTS FROM SOLUTIONS OF CONSTANT POTASSIUM CONCENTRATION AND DURING DEPLETION

A comparison was made between two methods of measuring the relationship between the external [K+] and the flux of K+ into whole plants of Lolium perenne and Raphanus sativus. The values of flux obtained from solutions of 1.2 μM K+ held constant around the roots were three and six times greater for Lolium and Raphanus respectively than the values obtained at the same concentration in a depletion experiment in which the solutions, initially 100 μM K+, were depleted to below 1.2 μM K+ by plant uptake. In the depletion experiment with Lolium, the flux was higher into plants grown at low [K+] than into plants grown at 100 μM even though [K+] within the plant was about the same for all groups of plants. It is suggested that Lolium grown at low [K+] has an efficient mechanism for K+ uptake which continues to operate for some time after the plants have been transferred to a higher concentration. With both species, Km was 15-20 μM in the depletion experiment and below 1 μM when concentrations were held constant. © 1979 Oxford University Press.