Water movement through root systems

Abstract

The root system provides a considerable resistance to water flow through it. This resistance is important because it influences the water deficits which can build up in the above-ground parts of the plant even when the roots are in moist soil. The permeability of roots per unit surface area varies between species, and between different parts of the same root (depending on age). It is sensitive to environmental conditions such as temperature, oxygen and salinity; this suggests that a major part of the resistance lies in living material in the ‘radial-flow pathway’, i.e. between the root surface and entry to the xylem. It is uncertain which is the main pathway of radial flow. Possible alternatives are: (1) the vacuolar pathway, crossing the cortex and other tissues by passing from vacuole to vacuole; (2) the free-space/endodermis pathway, water moving in the cell walls except at the endodermis where it enters the protoplasm; or (3) the symplasm pathway, water moving in the protoplasm and passing from cell to cell by plasmodesmata. Calculations are presented, based on the primary root of maize whose cell dimensions and root permeability are known, to show what permeabilities of membranes, wall material and plasmodesmata would be necessary to allow each of the three pathways to predominate. These calculations predict the symplasm as the most likely pathway. The free-space/endodermis pathway is unlikely because the permeability of cell wall material is too low. Several pieces of experimental evidence also favour the symplasm pathway. There are large variations between species in the permeability of roots for longitudinal flow in the xylem. Usually this xylem flow will present much less resistance than the radial flow pathway. However, there is no adequate evidence on whether the resistance may be greatly increased by bubble formation in vessels under water stress.