Kinetics of Rb Absorption by Excised Barley Roots under Changing Rb Concentrations. I. Effects of Other Cations on Rb Uptake

The absorption of Rb86applied as RbCl (10-5 M) by excised barley roots (Hordeum vulgare L., var. Atlas 6) was studied by following the concentration changes and the amounts taken up by the roots during its absorption. Rb absorbed by roots was determined after they were washed several times with 2 x 10-5 M KCl to remove all Rb adsorbed on superficial sorption sites. In the absence of interfering ions, the quantity of Rb disappearing from the solution agreed with that absorbed by the roots. K ions (2 x 10-5 M) reduced equally both the amount of Rb disappearing from the solution and that taken up by the roots, resulting in both showing good agreement. In the presence of 2 x 10-5 M Na, Ca, or Mg ions, however, Rb disappearance and absorption into roots were not only slowed down slightly, but the two quantities did not agree, less Rb was taken into the roots than that disappearing from the solution. A postuiate involving a chain of negative sites as possible pathways for cation entry into root cells is given to explain the results.