Two modes of rubidium uptake in sunflower plants.

The Rb(+)-uptake kinetics in K(+)-starved sunflower (Helianthus annuus) plants can be explained by the addition of two Michaelis-Menten equations. In contrast, Rb(+) uptake can be described by a single Michaelis-Menten equation in normal-K(+) plants. Differences in the K(m)s and in the Arrhenius plots of Rb(+) uptake in the two types of plants suggest two uptake modes.