The mechanism of water absorption due to the forces generated in the root itself is called active absorption. According to this theory, roots are not just passive surfaces, witnessing the passage of water through them, but actually play a dominant role in governing the quantum and the rate of entry of water.
The proponents of active absorption believe that all the events leading to the intake of water into the root cells are controlled by the root surfaces. There seems to be some controversy regarding the actual purport of the term active, whether its use should be restricted to only in instances where metabolic energy is involved, or whether the term can be used in the sense of roots taking an active role in absorption. Hence in order to avoid confusion the term active should be qualified with a prefix indicating the specific source of energy. Accordingly active absorption is of the follow’ I tow categories.
(i) Osmotic active absorption and
(ii) Non osmotic active absorption
Osmotic active absorption:
Atkins (1961) and Priestley (1922) who postulated an osmotic theory for water absorption opined that, the difference in the concentration gradient between cell sap and soil solution is responsible for the entry of water into the roots. According to them, the situation in the soil near the root presents an ideal osmotic system.
There are two solvent systems (soil solution and cell sap) differing in concentration and separated by means of a selectively permeable barrier (cell membrane). As the water potential of cell sap has a higher negative value than the soil solution osmotic migration of the solvent takes place into the cell. Investigations have revealed that the cell sap OP may be about 2 atm and may go up to 8 atm in some circumstances. The OP of soil solution is most cases is less than 1 atm. As a result there is endosmosis.
A number of plant physiologists have supported the osmotic absorption, by demonstrating a close link between the phenomenon of sap exudation (root pressure – see ascent of sap) and water absorption. Some of these scientists are Lundegardh (1950), Broyer (1951), Hoagland (1944) etc.
When an actively water absorbing plant is decapitated, sap exudes out of the cut end of the stem indicating active absorption of water. According to Atkins, the xylem sap always has a high concentration of solutes due to the release of sugars into them by the adjoining parenchyma cells.
Anderson and House (1967) have reported that salts are absorbed by the xylem elements containing living protoplasts. Priestly (1922) on the other hand opined that a cell differentiating into xylem gets higher salt concentration because of its contents.
Objections to osmotic theory:
The main objection comes from the fact, that the sap concentration in xylem is not always high, as a result a gradient may not exist between cortical cells and xylem. (Xylem sap has 2 atm; cortical cell have 3-4). Secondly, root pressure is not universal in plants.
Non osmotic active absorption:
Many physiologists believe that water absorption is driven through the expenditure of metabolic (respiratory) energy. Absorption of water has been observed against the concentration gradient and even when there is a gradient, the rate of absorption far exceeds what is possible due to a concentration gradient.