Intercellular water transport, bulk movement of water, Soil Plant Atmosphere Continuum (SPAC)
Intercellular water transport:-
Path of Water:- There are two routes of water absorption -
i. Apoplast:- When absorbed water moves through the cell wall and inter-cellular spaces. In this route, water does not enter the cytoplasm inside the cell.
ii. Symplast:- When absorbed water moves through plasma membrane and plasmodesmata. In this route, water enters the cytoplasm inside the cell.
Bulk movement of water (Ascent of Sap):- The upward movement of water from roots to the aerial parts of a plant is known as an ascent of sap.
Theories of Ascent of Sap:-
1. Vital Force Theory
2. Root-pressure Theory
3. Physical Force Theory
1. Vital Force Theory:- Ascent of sap was brought about by the living cells of the stem.
a. Relay-Pump Hypothesis:-
- It was proposed by Godlewski (1884).
- According to this theory, the upward movement of water is due to the pumping activity of the living cells of the xylem parenchyma and medullary rays.
b. Pulsatory Movement Theory:-
- This theory was proposed by Sir J.C. Bose (1923).
- According to this theory, the upward conduction of water is provided by pulsatory motion of the innermost living cortical cells of the root.
2. Root-Pressure Theory:-
- This theory was put forward by Priestley (1916).
- According to this theory, the upward movement of water is due to root pressure. All plants absorb excess water by an active process and tend to build up a positive hydrostatic pressure within the root system called root pressure.
3. Physical Force Theory:- As per this theory, the ascent of sap takes place through the dead xylem vessels.
a. Capillary Force Theory:-
- This theory was proposed by Boehm (1809).
- He believed that the normal atmospheric pressure and the capillarity of the xylem vessels are responsible for the ascent of sap.
- The cohesive force that is present among the water molecules pulls the water upwards through the xylem vessels.
b. Imbibition Theory:-
- This theory was proposed by Sachs(1878).
According to this theory, water is imbibed through the cell wall materials and translocated upwards.
c. Atmospheric Pressure Theory:-
- According to this theory, atmospheric pressure is responsible for the ascent of sap.
- Due to active transpiration, a vacuum is created in the xylem vessels, and to fill this vacuum, and water moves upward from below so as to equalise the pressure.
- But atmospheric pressure can lift the water up to a height of 10 meters, and the operation of atmospheric pressure depends on the free surface at the base, which does not exist in the plants.
d. Transpiration Pull or Cohesion-Tension Theory:-
- This theory was put forward by Dixon and Joly in 1894.
- But it was further improved by Dixon in 1914, so this theory is also known as Dixon’s theory of the ascent of sap.
- This is the most accepted theory for the study of the ascent of sap.
- In the water column, the water molecules remain attracted by the cohesive force and cannot be separated easily from one another.
- There is an attraction between water molecules and the inner wall of xylem ducts, due to which the water column cannot be pulled away from the walls of the xylem ducts.
- Hence, due to strong cohesive and adhesive forces, the continuity of the water column from roots to leaves is maintained.
- Water lost from the mesophyll cells of the leaves creates a strong negative water potential that leads to a negative pressure in the water column. This pressure exerts an upward pull over the water column, which is known as transpiration pull.
- This tension or pull is transmitted up to the roots in search of more water.
- The water column (formed in the xylem elements of roots) now moves upwards under the influence of transpiration pull.
- Thus, the cohesive, adhesive forces and transpiration pull all together help in lifting up water through xylem elements.
- Because of the critical role of cohesion, the transpiration pull is also called the cohesion-tension transpiration pull model of water transport.
- The water inside the xylem vessels forms a continuous column from top to bottom.
- The tension may cause a break in the water column but due to the adhesive and cohesive property of water, the continuous column of water does not break.
Importance of Ascent of Sap:- The ascent of sap serves the following functions:
i. It helps in the upward movement of the water through the xylem vessels in the trees that are several meters high.
ii. It helps in the development of a suction force called transpiration pull that helps the root to absorb more and more water from the soil.
Soil Plant Atmosphere Continuum (SPAC):-
> It is proposed by Huber in 1924.
> All components of field environment i.e. soil, plant and atmosphere, form a physically unified and dynamics networking system in which various water flow processes occur independently. This unified system is called the soil-Plant-Atmosphere-Continuum or SPAC.
> The fundamental principle of SPAC is water moves from higher total water potential to regions of lower total water potential at a rate depending on the hydraulic resistance of the medium.
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