What is the Mechanism of Water Absorption Process in Plants?

The mechanism of absorption of water is of two types: active and passive often operating independently (Mramer, 1949). In active mechanism, water is absorbed by the force developing in the root system.

Active absorption generally takes place in slowly transpiring and well-watered plants. In passive absorption mechanism, the forces responsible for the absorption of water develop in the shoot system, which are ultimately transmitted to the root system.

The roots play a passive role in this mechanism. Passive absorption is, generally found in rapidly transpiring and ill-watered plants.

Active absorption of water:

Two types of theories such as osmotic theory and Xon Osmotic theory have been proposed to explain active absorption of water by plants.

(i) Osmotic Theory:

The ostomatic theory explains that the root hairs act as ohmmeters and there exists a typical condition so that water can move across osmotic ally from the soil solution to the root xylem.

The theory assumes that translocation of water takes place by means of diffusion along a gradient of decreasing water potential (\j/) or increasing solute potential (v|/s). Absorption of water means translocation of water from soil solution to the xylem of root.

The xylem sap, according to this theory, possesses a higher osmotic potentiality thin that of soil solution. So water moves from the soil to the root xylem by cells to cell osmosis (Priestley, 1922). Because osmosis is a function of living cell, this process of osmotic water absorption is included under active process.

(ii) Non osmotic theory:

The non-osmotic theory assumes that when the driving forces are zero, water absorption takes place at the expenditure of metabolic energy released in respiration. Thus this theory explains the absorption of water by roots when the osmotic potential of soil water is greater than that of the root hair cell sap.

The non-osmotic active water uptake can also be supported by the following facts:

(i) There is a correlation between the rate of water absorption and respiration,

(ii) Auxin-induced water uptake

(iii) The factors such as low temperature, oxygen tension and respiratory inhibitors which affect respiratory rate and in turn the absorption rate.

The serious draw bck of the above theory is that it fails to explain the exact manner of utilization of metabolic energy in the process of water uptake.