Symplasmic and apoplasmic radial ion transport in plant roots : Cortical plasmalemmas lose absorption capacity during differentiation.

The kinetics and localization of Rb(+) uptake by barley roots were studied as function of the water potential of the medium. At potentials causing plasmolysis, uptake is reduced and becomes restricted to the outer surface of the root. The conclusion is that, at least for Rb(+) in the range of low external concentrations (<1 mM), the mature root cortex has little if any primary absorption(1) capacity. Therefore, the apoplasmic pathway is of little or no significance for radial transport, which occurs by primary absorption at the epidermis and - perhaps - hypodermis followed by symplasmis transport to and through the cortex. Young cortex cells possess a primary absorption capacity comparable to that of the epidermis, but this feature is (largely) lost during differentiation.