Location of the major barriers to water and ion movement in young roots of Zea mays L.

Abstract

Abstract.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 cell 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 hydraulic 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 root pressure fell. The hydraulic conductance of the root increased but only by a factor of 1.2-2.7. The results indicate that the endodermis is the main barrier to the radial movement of ions but not water. The major barrier to water is the membranes and apoplast of all the living tissue. These conclusions were drawn from experiments in which hydrostatic-pressure differences were used to induce water flows across young maize roots which had an immature exodermis and an endodermis with Casparian bands but no suberin lamellae or secondary walls. The different reactions of water and ions to the endodermis can be explained by the huge difference in the permeability of membranes to these substances. A hydrophobic wall barrier such as the Casparian band should have little effect on the movement of water, which permeates membranes and, perhaps, also the Casparian bands easily. However, hydrophobic wall depositions largely prevent the movement of ions. Several hours after wounding the endodermis, root pressure recovered to some extent in most of the experiments, indicating that the wound in the endodermis had been partially healed.

Apoplasmic barriers and their significance in the exodermis and sheath of Eucalyptus pilularis–Pisolithus tinctorius ectomycorrhizas

“…Most evidence from the literature suggests that the Casparian bands in the exodermis of nonmycorrhizal roots are impermeable to ions, although views differ on their permeability to water (Peterson, 1987 ;Peterson et al, 1993 ;Canny & Huang, 1994 ;Peterson & Cholewa, 1998 ;Steudle & Peterson, 1998 ;Zimmermann & Steudle, 1998). It has been shown here that the exodermal Casparian bands are a barrier to penetration of both tracers into the cortex of all the mycorrhizas, regardless of the degree of permeation of PTS or La into the sheath.…”

Hydraulic conductivity of concentric root tissues of Agave deserti Engelm. under wet and drying conditions

“…I"or separate root tissues, L,ĥ as been calculated from measurements on individual cells and measured for root 'sleeves' (hollow cylinders of epidermal plus hypodermal tissues; Shone & Clarkson, 1988;Schambil & Woermann, 1989). The contribution of the endodermis to radial conductivity has been assessed by puncturing it and other tissues (Peterson, Murrmann & Steudle, 1993), and the radial conductivity of xylem vessels has been measured using steam-killed roots .…”

Hydraulic conductance of root and shoot measured with the transient and dynamic modes of the high-pressure flowmeter

“…We suggest that the conductance of the root system was so low that the water flow needed to compress air bubbles in the xylem or in the root tissue was higher than the water flow crossing root system. This hypothesis is supported by the efficiency of the dynamic mode where a tight linearity was observed between F and P. If we consider that the radial hydraulic resistance of roots is not only due to endodermis but is evenly distributed over the entire root tissue [15,21], air in the root cortex may also have contributed to this elastic perturbation of the measurement. It happened essentially with plants of small size and early in the season, before bubreak was completed.…”