Some instances water absorption appears to take place through the mediation of forces which originate in the root itself.
Such a process is called active water absorption. It is operative in the form of root pressure, bleeding and guttation.
Roots of plants absorb water from the soil. Water thus absorbed is exuded in the dead xylem ducts of the root and stem under pressure.
The pressure developed inside the roots due to absorption of water is called the root pressure. Such water may exude from the stumps of recently decapitated plants.
If a well-watered tomato plant is detopped and the stump is attached with a rubber sleeve to a glass tube containing some water, manifestation of this pressure can be observed (Fig. 7-6).
In this experiment it will be noticed that water is actually “pushed” up in the glass tube. The maximum pressure developed by the exuding sap can be measured by attaching a closed manometer.
Root pressure is a pressure developed in the tracheary elements of the xylem, as a result of metabolic activities of roots such as the accumulation of salts in the xylem ducts against concentration gradient. Factors which affect respiration such as oxygen tension, auxins and respiration inhibitors, also affect root pressure.
Root pressure is, therefore, referred to as an active process in the sense that living roots are essential for it to occur.
Actually, the movement of water up to the stem is due to osomotic mechanism (passive) which is created as a result of the active absorption of salt by the roots. Two common examples of root pressure are bleeding of veins of sugar maple in spring from its cut branch, and guttation.
It may be stated that root pressure is not universal and is observed in some plants only. Further, it is found during certain periods of growth at a time when the water requirements of the plant are low. Its magnitude rarely exceeds 2 atm and rarely may extend up to 10 atm. Root pressure is also shown to exhibit diurnal rhythm.
Active water absorption is usually explained in two ways:
I. Osmotic active absorption:
No energy is needed in this process. Water from the soil moves in the root hair against a gradient of Ψ. Thus xylem has maximal Ψs but least Ψ. Xylem vessels maintain their high solute concentrations in several ways, diffusion of solutes from the adjacent parenchyma cells or salts absorbed by the roots pass to the xylem vessels.
When the decapitated cotton plants exudate is analysed, its xylem sap has more concentration of solutes than the surrounding solution in the soil. There was also a linear relation between the rate of exudation and the difference of the Ψπ of the xylem sap and the soil water.
II. Non-osmotic active absorption:
According to this view direct energy is expanded in the movement of water against Ψ. The movement does not take place through osmosis. It is usually proposed that energy is made available through respiration. Thus, metabolic state of the root hair plays an important role in the amount of water absorbed. Several evidences are available to support this method of absorption.
For instance, the absorption of water increases when the thin potato slices are transferred from tap water to aerated water. The treatment of roots with metabolic inhibitors reduces the rate of absorption. On the other hand auxins increase the rate of water absorption. Some link between the rate of water absorption and protein synthesis also exists. With respiratory inhibitors diurnal rhythm of root pressure is reduced.