Introductory Crop Physiology (1+1)

Active absorption of water

Active osmotic absorption of water

Atkins (1916) and Priestley (1922) postulated the active osmotic absorption of water. The first step in osmotic absorption of water is the imbibition of soil water by the hydrophilic cell walls of root hairs.
Osmotic pressure (OP) of the cell sap of root hairs is usually higher than the OP of the soil water. Therefore, the diffusion pressure deficit (DPD) and suction pressure in the root hairs become higher and water from the cell walls enters into them through plasma membrane by osmotic diffusion. As a result, OP, suction pressure and DPD of root hairs now become lower, while their turgor pressure is increased.

Now, the cortical cells adjacent to root hairs have high OP, SP and DPD in comparison to the root hairs. Therefore, water is drawn into the adjacent cortical cells from root hairs by osmotic diffusion. In the same way, by cell to cell osmotic diffusion water gradually reaches the inner most cortical cells and the endodermis.

Osmotic diffusion of water into endodermis takes place through special thin walled cells called passage cells because the other endodermal cells have casparian strips (is a band of cell wall material deposited on the radial and transverse walls of the endodermis, which is chemically different from the rest of the cell wall. It is used to block the passive flow of materials, such as water and solutes into the stele of a plant) on thin walls which are impervious to water. Water from endodermal cells is drawn into the cells of pericycle by osmotic diffusion which now become turgid and their suction pressure in decreased.
In the last step, water is drawn into xylem from turgid pericycle cells. It is because in the absence of turgor pressure of the xylem vessels, the SP of xylem vessels becomes higher than SP of the cells of the pericycle when water enters into xylem from pericycle and a positive pressure is developed in the xylem of roots which can raise the water to a certain height in the xylem. This pressure is called as root pressure.

Active non-osmotic absorption of water

Sometimes, it has been observed that absorption of water takes place even when OP of soil water is high than OP of cell sap. This type of absorption which is non-osmotic and against the osmotic gradient requires the expenditure of metabolic energy probably through respiration.

Bennet Clark et al (1936), Thimann (1951) and Bogen and Prell (1953) observed that absorption of water against the gradient requires extra energy supplied by cellular respiration. However, Kramer (1969) reported that active absorption of water has negligible importance in the water economy of almost all plants.

Aquaporins (are proteins embedded in the cell membrane that regulate the flow of water) are also associated with non osmotic absorption of water.

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