Give an account of active absorption theories with their demerits.

Explanation

The mechanism of water absorption due to forces generated in the root itself is called active absorption.Active absorption may be osmotic or non-osmotic.
Osmotic active absorption:The theory of osmotic active absorption was postulated by Atkins(1916) and Preistley (1923).According to this theory,the first step in the absorption is soil water imbibed by cell wall of the root hair followed by osmosis.The soil water is hypotonic and cell sap is hypertonic.Therfore,soil water diffuses into root hair along the concentration gradient (endosmosis).When the root hair becomes fully turgid ,it becomes hypotonic and watr moves osmotically to the outer most cortical cell.In the same way,water enters into inner cortex,endodermis,pericycle and finally reaches protoxylem.As the sap reaches the protoxylem a pressure is developed known as root pressure.This theory involves the symplastic movement of water.
Objections to osmotic theory: (i)The cell sap concentration in xylem is not always high (ii)Root pressure is not universal in all plant especially in trees.
Non-Osmotic active absorption:Bennet-Clark (1936) observed absorption of water even if the concentration of cell sap in the root hair is lower than that of the soil water. Such a movement requires an expenditure of enerfy released by respiration (ATP).Thus,there is a link between water absorption and respiration.It is evident from the fact that when respiratory inhibitors like KCN,Chloroform are applied there is a decrease in the rate of respiration and also the rate of absorption of water.