Water Movement Through Root Systems

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.

[1]  D. Malcolm,et al.  The Plant Root and Its Environment. , 1975 .

[2]  S. A. Barber,et al.  Development and Distribution of the Corn Root System Under Field Conditions1 , 1974 .

[3]  B. Huisinga,et al.  ON THE FUNCTION OF THE CASPARIAN STRIPS IN ROOTS , 1974 .

[4]  W. Thomson,et al.  The Casparian Strip as a Barrier to the Movement of Lanthanum in Corn Roots , 1974, Science.

[5]  H. Ginsburg,et al.  Radial Water and Solute Flows in Roots of Zea mays IV. ELECTRICAL POTENTIAL PROFILES ACROSS THE ROOT , 1974 .

[6]  E. Newman.  PERMEABILITY TO WATER OF THE ROOTS OF FIVE HERBACEOUS SPECIES , 1973 .

[7]  T. Tanton,et al.  Water pathways in higher plants II. water pathways in boots , 1972 .

[8]  M. Cailloux.  Metabolism and the absorption of water by root hairs , 1972 .

[9]  H. Ginsburg,et al.  Radial Water and Solute Flows in Roots of Zea mays: I. WATER FLOW , 1970 .

[10]  D. Aikman,et al.  Excised root exudation—a standing-gradient osmotic flow , 1970, Proceedings of the Royal Society of London. Series B. Biological Sciences.

[11]  E. Newman,et al.  RESISTANCE TO WATER FLOW IN SOIL AND PLANT , 1969 .

[12]  M. Tyree.  The Thermodynamics of Short-distance Translocation in Plants , 1969 .

[13]  W. Anderson,et al.  A Correlation between Structure and Function in the Root of Zea mays , 1967 .

[14]  R. O. Slatyer,et al.  Plant-Water Relationships , 1967 .

[15]  J. Woolley.  Radial Exchange of Labeled Water in Intact Maize Roots. , 1965, Plant physiology.

[16]  W. Lopushinsky,et al.  Effect of Water Movement on Ion Movement into the Xylem of Tomato Roots. , 1964, Plant physiology.

[17]  P. Kuiper.  Water Transport across Root Cell Membranes: Effect of Alkenylsuccinic Acids , 1964, Science.

[18]  M. Tazawa,et al.  WATER PERMEABILITY OF THE CELL WALL IN NITELLA , 1962 .

[19]  W. Emerson.  Water conduction by severed grass roots , 1954, The Journal of Agricultural Science.

[20]  F. M. Scott.  Plasmodesmata in Xylem Vessels , 1949, Botanical Gazette.

[21]  H. F. Rosene.  QUANTITATIVE MEASUREMENT OF THE VELOCITY OF WATER ABSORPTION IN INDIVIDUAL ROOT HAIRS BY A MICROTECHNIQUE. , 1943, Plant physiology.

[22]  H. E. Hayward,et al.  Device for Measuring Entry of Water into Roots , 1942, Botanical Gazette.

[23]  C. Davis.  Absorption of Soil Moisture by Maize Roots , 1940, Botanical Gazette.

[24]  F. Haines.  Observations on the Occurrence of Air in Conducting Tracts , 1935 .

[25]  Paul J. Kramer,et al.  THE INTAKE OF WATER THROUGH DEAD ROOT SYSTEMS AND ITS RELATION TO THE PROBLEM OF ABSORPTION BY TRANSPIRING PLANTS , 1933 .

[26]  D. Clarkson.  Ion transport and cell structure in plants , 1974 .

[27]  P. Kramer,et al.  Diurnal Cycling in Root Resistance to Water Movement , 1974 .

[28]  Charles Randall. House,et al.  Water transport in cells and tissues , 1974 .

[29]  N. Higinbotham,et al.  VII.3 – Some Evidence that Radial Transport in Maize Roots is into Living Vessels , 1973 .

[30]  W. Anderson.  Ion transport in plants , 1973 .

[31]  R. Ehwald,et al.  Die Bedeutung der Diffusion im „Freien Raum‟ für die Konzentrationsabhängigkeit der Aufnahme von Zuckern und Ionen durch pflanzliche Gewebe , 1973 .

[32]  J. Passioura.  The effect of root geometry on the yield of wheat growing on stored water , 1972 .

[33]  H. M. Taylor,et al.  Water Uptake by Cotton Roots During an Irrigation Cycle , 1971 .

[34]  P. Kramer.  Plant and Soil Water Relationships , 1969 .

[35]  G. E. Briggs,et al.  Movement of Water in Plants. , 1967 .

[36]  W. H. Arisz,et al.  Analysis of the Exudation Process in Tomato Plants , 1951 .