Radial Transport of Ions and Water Across the Root

There are two parallel pathways or movement of ions (solutes) and water across the cortex towards the stele: one passing through the apoplasm (cell walls and intercellular

Passage Water Through The Root

Rhizodermis

Fig. 2.31 Part of transsection of a maize root showing the symplasmic (A) and apoplasmic (B) pathway of ion transport across the root.

Rhizodermis

Fig. 2.31 Part of transsection of a maize root showing the symplasmic (A) and apoplasmic (B) pathway of ion transport across the root.

spaces, Section 2.2.1) and another passing from cell to cell in the symplasm through the plasmodesmata. In the symplasmic pathway ions, but not water, bypass the vacuoles (Fig. 2.31). As a rule the apoplasmic pathway of ions is constrained by the Casparian band in the walls of the endodermal cells. This band has hydrophobic properties and completely surrounds each cell. Additionally in basal root zones the cell walls of the endodermis become thicker and even lignified (tertiary endodermis). Recently, the key role of the endodermis and the Casparian band as effective barrier also for radial movement of water has been questioned. At least in young roots, despite the hydrophobic properties of the Casparian band, water seems to permeate this band readily (Peterson etal., 1993; Schreiber et al., 1994).

Depending on the plant species and the root zone the apoplasmic pathway may already be constrained or blocked by the exodermis (Fig. 2.31) or suberization of the rhizodermis. Formation of an exodermis is found, for example, in Zea mays, Allium cepa, or Helianthus annuus, but not in Vicia faba or Pisum sativum (Enstone and Peterson, 1992). However, there are somewhat different views on the function of the exodermis as effective barrier for transport of water and solutes in the apoplasm of the root cortex (Section 2.5.1). Termination of the apoplasmic pathway at the exodermis as suggested by Enstone and Peterson (1992) would confine in basal root zones both water influx and ion transport across the plasma membrane into the symplasm to the rhizodermal cells. Although rhizodermal cells play a key role in mineral nutrient uptake in general (Grunwald et al., 1979) and at low external concentrations of potassium and phosphate in particular (Drew and Saker, 1986), it is not possible to generalize on the relative importance of the two pathways in ion and water transport in the root cortex. This depends on: (a) the external concentration compared with the capacity and affinity of the transport system at the plasma membrane for a given ion (e.g., K+ > Na+; NOJ > H3B 03; Section 2.5.5); (b) the root zone considered: depending on the growth rate of the root, the exodermis may develop between 2 cm and 12 cm proximal to the root tip (Perumalla and Peterson, 1986) and may possess 'passage cells' (Storey and Walker,

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Responses

  • gavin
    What is radial transport?
    7 years ago
  • ulrike
    What transports water and ions to the center of the root?
    4 years ago
  • jose
    What is radial movement of ion?
    1 year ago
  • anu
    What is radial movment of water?
    3 months ago

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