Zones of exudation along plant roots and spatial distribution of micro‐organisms in the rhizosphere

The use of 14C‐labelling of plants in short‐term studies has established that the major zone of release of carbon‐containing materials from roots is the zone of elongation. Much of this material is in an insoluble form, comprising sloughed‐off root cap cells and polysaccharide material, and remains in the immediate vicinity of the root. Electron microscopy of ultra‐thin sections of the root‐soil interface of wheat plants has shown that colonisation by micro‐organisms of the rhizosphere is relatively sparse in the young parts of roots. In the older parts of roots, evidence was obtained that bacteria lyse the walls of the outer root cells and this is followed by dense colonisation of these moribund outer cells by bacteria.