On the evidence of a diffusion barrier in the outer cortex apoplast of cress-roots (Lepidium sativum), demonstrated by analytical electron microscopy

To mark the apoplastic pathway of ions in the root of the dicotyledonous plant Lepidium sativum we used the heavy element lanthanum, which can be identified by analytical electron microscopy (EELS and ESI). In the front root tip, the primary walls of all meristematic cells contained lanthanum. 10-15 mm behind the root apex, lanthanum was found in the cortex cell walls up to the endodermis, but not in the stele. 20-25 mm from the tip, lanthanum was accumulated in the radial cell walls of the hypodermis, which, however, is not a complete diffusion barrier for ions, so that traces of lanthanum also were found in the cortex cell walls up to the endodermis. This study provides evidence for the presence of two apolastic diffusion barriers in the region of highest water uptake in cress roots.