06 Plant Water Relation - part 09 - Translocation of water

06 Plant Water Relation - part 09 - Translocation of water


Translocation of water: 
  • The transport of water with dissolved minerals from root to other aerial parts like stem and leaves, against the gravity, is called translocation or ascent of sap. 
  • Translocation of water occurs through the lumen of conducting elements of xylemtracheids and vessels, in all vascular plants. 
  • Ringing experiment has proved that xylem is the path of ascent of sap. 
  • Several mechanisms/ theories have been put forth to explain the mechanism of translocation of water. 
  • The theories include - 
  1. Vital force theory
  2. Relay pump theory
  3. Physical force theory
  4. Root pressure theory, etc. 
  • We shall consider following three theories : 
1. Root Pressure Theory (Vital Theory) : 

  • According to this theory, the activity of living cells of root is responsible for translocation of water. 
  • J. Pristleyproposed this theory. 
  • When a stem of potted plant is cut few inches above the soil by a sharp knife, xylem sap is seen flowing out/ oozing out through the cut end. 
  • This exudation at the cut end of stem is a good proof for the existence of root pressure. 
  • As water absorption by roots is constant and continous process, a hydrostatic pressure is developed in the living cells of cortex of root.This is termed as root pressure by S. Hales. 
  • It is due to root pressure water along with dissolved minerals is not only forced into xylem but it is also conducted upwards against the gravity. 
  • Root pressure seems to be largely an osmotic phenomenon and its development is an active process. 
  • The value of root pressure is +1 to +2 bars which is enough to pump water to a height of 10 to 20 meters. 
  • The factors like oxygen, moisture, temperature of soil, salt contents, etc. influence the root pressure.
Objections/ limitations of root pressure theory: 
  1. It is not applicable to plants taller than 20 meters. 
  2. Ascent of sap can also occur even in the absence of root system. 
  3. Root pressure value is almost nearly zero in taller gymnosperm trees.
  4. In actively transpiring plants, no root pressure is developed.
  5. Xylem sap under normal condition is under tension i.e. it shows negative hydrostatic pressure or high osmotic pressure. 
  6. To sum up therefore, root pressure is not the sole mechanism explaining the ascent of sap in all plants of varying heights..
2. Capillarity theory (physical force theory): 
  1. According to this theory, physical forces and dead cells are responsible for ascent of sap. This theory was put forth by Bohem in (1863). 
  2. Wick dipped in an oil lamp, shows capillarity due to which oil is raised upwards. 
  3. The conduction of water in a straw dipped in water, is raised to a certain height because of capillarity. 
  4. The height to which water is raised depends on the diameter of the straw. 
  5. Capilarity is because of surface tension, and forces of cohesion (attraction between like molecules) and adhesion (attraction between unlike molecules). 
  6. Xylem vessel/ tracheid with its lumen is comparable with straw. 
  7. Water column exist because of combined cohesive and adhesive forces of water and xylem wall, due to capillarity. 
  8. It is because of capillarity water is raised or conducted upwards against the gravity, to few centimeters only. 
Objections/ Limitations of capillarity theory: 
  1. Capillary tube (xylem) must be continously and completely hollow from one end to the other end but tracheids in the xylem show closed end-walls. 
  2. The lower end of capillary tube i.e. xylem must bein direct contact with soil water. 
  3. However, there exists a barrier of root cortexbetween xylem and soil water. 
  4. Narrower the capillary tube, greater is the height to which water column is raised. 
  5. Thus, taller trees should show xylem vessels with very narrow bore (diameter). 
  6. However, in nature the tall trees show xylem vessels having wider bore. 
  7. Hence, to sum up capillarity can not be the sole mechanism to explain ascent of sap in all the plants of varying heights. 
3. Cohesion- tension theory (Transpiration pull theory) : 
  • This is presently widely accepted theory explaining ascent of sap in plants. 
  • It was putforth by Dixon and Joly (1894). 
  • This theory is based on two principles. 
1. Cohesion and adhesion, and transpiration pull : 
  • A strong force of attraction between water molecules, is called cohesive force. 
  • While a strong force of attraction between water molecules and lignified wall of lumen of xylem vessel, is called adhesive force. 
  • Due to combined cohesive and adhesive forces a continous water column is developed (formed) in the xylem right from root upto the tip of the topmost leaf in the plant. 
2. Transpiration pull : 
  • The transpiration pull developed in the leaf vessel is transmitted down to root and thus accounts for the ascent of sap. 
  • Excess water is lost in the form of vapour, mainly through the stomata found on leaf. 
  • This water loss increases D. P. D. of mesophyll cells. 
  • These cells withdraw water ultimately from xylem in the leaf. 
  • In otherwords, due to continous transpiration, a gradient of suction pressure (i.e. D. P. D.) is developed right from guard cells up to the xylem in the leaf. 
  • This will create a tension (called negative pull or transpiration pull) in the xylem. 
  • Consequently, water column is pulled out of xylem. 
  • Thus, water is pulled upwards passively against the gravity leading to the ascent of sap. 
Objections/ Limitations of transpiration pull theory: 
  1. For transpiration pull to operate, water columnshould be unbroken and continous. 
  2. However, due to temperature fluctuations during day and night, gas bubbles may enter in water column breaking the continuity.
  3. This mechanism assumes that tracheids are more efficient than the vessels, as their end walls support water column. 
  4. However, vessels are more evolved than tracheids and are more efficient.
  5. If plant is smeared with vaseline in order to stop transpiration, even then ascent of sap occurs. 
  6. Ascent of sap also occurs in deciduous plants that have shed all of their leaves. 
  7. These observations point to the fact that besides physical forces, activity of living cells seems to be necessary for lifting the water column up.

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