LOCATION OF THE MAJOR BARRIERS TO WATER AND ION MOVEMENT IN YOUNG ROOTS OF ZEA-MAYS L

The main barriers to the movement of water and ions in young roots of Zea mays were located by observing the effects of wounding various cel l layers of the cortex on the roots' hydraulic conductivities and root pressures. These parameters were measured with a root pressure probe. Injury to the epidermis and cortex caused no significant change in hy draulic conductivity and either no change or a slight decline in root pressure. Injury to a small area of the endodermis did not change the hydraulic conductivity but caused an immediate and substantial drop in root pressure. When large areas of epidermis and cortex were removed (15-38% of total root mass), the endodermis was always injured and roo t pressure fell. The hydraulic conductance of the root increased but o nly by a factor of 1.2-2.7. The results indicate that the endodermis i s the main barrier to the radial movement of ions but not water. The m ajor barrier to water is the membranes and apoplast of all the living tissue. These conclusions were drawn from experiments in which hydrost atic-pressure differences were used to induce water flows across young maize roots which had an immature exodermis and an endodermis with Ca sparian bands but no suberin lamellae or secondary walls. The differen t reactions of water and ions to the endodermis can be explained by th e huge difference in the permeability of membranes to these substances . A hydrophobic wall barrier such as the Casparian band should have li ttle effect on the movement of water, which permeates membranes and, p erhaps, also the Casparian bands easily. However, hydrophobic wall dep ositions largely prevent the movement of ions. Several hours after wou nding the endodermis, root pressure recovered to some extent in most o f the experiments, indicating that the wound in the endodermis had bee n partially healed.