Transport Mechanism of movement of water
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.
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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.
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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