Relay pump theory
Relay pump theory was put forward by Godlewski (1884). According to this, upward conduction of water is due to the
pumping activity of xylem parenchyma cells and the cells of medullary rays.
Root pressure theory
Root pressure theory was put forward by Priestley (1916). Root pressure refers to positive hydrostatic pressure
which sometimes develops in the xylem sap of roots as a result of metabolic activities of roots. It is a manifestation of
active water absorption. Root pressure is observed in certain seasons which favour optimum metabolic activity and reduce
transpiration.
Root pressure is maximum during rainy season (in tropical regions) and spring season (in temperate habitats). It is retarded or
absent under conditions of starvation, low temperature, drought and reduced availability of oxygen. Root pressures are usually
not more than + 1 to + 2 bars. Therefore, it could account for the ascent of sap only to a height of about 20 m.
Hence, this theory can account for the ascent of sap only in the herbaceous plants. The magnitude of pressure developed
is too small to push the water to the apical regions, in the tall trees. Besides, root pressure is not found in all plants. No
or little root pressure is found in gymnosperms which have some of the tallest trees in the world. In rapidly transpiring
plants, no root pressure is found, instead a negative pressure is found under such conditions.
Capillary force theory
Capillary force theory was given by Boehm (1809). According to this theory, water rises up in narrow tubes of xylem
vessels by surface tension or adhesion and cohesion. The upward movement of water will continue till the forces of
adhesion and cohesion are balanced by the downward pull of gravity.
The value of capillarity is very small which can raise water
to a height of about 1 metre in vessels of normal diameter
(0.03 mm). Therefore, if operational, it will be useful to
only small sized plants and cannot operate in plants having
tracheids due to the presence of end walls.
Transpiration pull or cohesion-tension
theory
The most widely accepted theory for ascent of sap is
transpiration pull theory or cohesion-tension theory. This
theory was proposed by Dixon and Jolly in 1894.
This theory is based on the following points:
- Transpirational pull exerted on the water column.
- Cohesive and adhesive properties of water molecules
so as to form an unbroken continuous column of
water in the xylem.
Transpiration pull
Water is lost from mesophyll cells to the intercellular spaces
of leaves as a result of transpiration. The water vapours
move out of the plant through stomata. As a result of
loss of water from mesophyll cells, the diffusion pressure
deficit (DPD) increases.