Structural modifications of the apoplast and their potential impact on ion uptake

Nutrient ions entering the plant root from the soil solution, whether directly or through mycorrhizal fungi, first encounter the apoplast, a chemically complex compartment. The movement of ions in unmodified walls occurs in water‐filled channels (diameter about 10 nm) and is influenced by electrical charges. Some positive charges occur in the wall, but the majority are negative. The permeability of the wall is variable, depending on its age and whether or not it has been further modified. Very young walls are less permeable than older walls, and the permeability of mature walls may be reduced by suberin and/or lignin deposits. It is not clear whether or not the other two main wall‐modifying substances, mucilages and condensed tannins, also significantly reduce wall permeability. Casparian bands, which consist of suberin and lignin within radial and transverse walls, occur in the endodermis and exodermis. These layers form continuous cylinders within the root so that ion movement into the entire areas they surround is prevented. An important corollary is that Casparian bands dictate which cells are accessed by the soil solution and thus are in a position to absorb ions. It is postulated that a Casparian band‐like modification also occurs in the walls of cork cells, based on observations of their impermeability to dyes. Suberin can occur in the form of lamellae which are deposited on all walls of endodermal, exodermal and cork cells. Unlike the Casparian bands, these lamellae limit movement between the apoplast and plasmalemma surface of the individual cells in which they occur. Condensed tannins, an addition to the walls of tree roots which results in them becoming brown, appear to function as antimicrobial agents rather than as permeability barriers. Further research is needed to investigate the structure of both unmodified and modified walls in a variety of species and under a variety of growing conditions.