ABSORPTION AND TRANSLOCATION OF WATER AND SOLUTES

Many workers opined that active absorption is the main mechanism of water absorption and gave very little importance to passive absorption. However, according to Kramer (1969), the active absorption of water is of negligible importance in the water economy of plants. He stated that the root pressure and the related phenomena involved in the active absorption of water are mere consequences of salt accumulation in the xylem of different kinds of roots. Following are the reasons for treating the active absorption as unimportant : 1. Root pressure is not observed in tall plants like conifers and other gymnosperms. 2. Root pressure is not observed in fast transpiring plants. 3. Amount of water exuded by the cut end of stumps due to root pressure is not equal to that lost by transpiration. 4. The plant as a whole absorbs more water than an excised system. 5. In submerged plants, water is absorbed from the general surface, through roots are present. It appears, therefore, that the passive abs...

It is taking place in two ways : 1. Active Absorption of water In this process, the root cells play an active role in the absorption of water and metabolic energy released through respiration is consumed. Active absorption may be of two kinds. a. Osmotic absorption: When water is absorbed from the soil into the xylem of the roots along the osmotic gradient. b. Non-osmotic absorption: When water is absorbed against the osmotic gradient. 1a. Active Osmotic Absorption of Water The first step in the 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 DPD and the suction pressure in the root hairs become higher and water from the cell walls enters into them through the plasma membrane (semi-permeable) by osmotic diffusion. As a result, the OP, suction pressure, and DPD of root hairs now lower, while their turgor pressure...

  Plants absorb water from the soil by their roots. The water is found in different forms in the soil. The chief source of water to the soil is rain. After the rains, a part of the water drains away. This is called runaway water and it is not available to the plants. A part of the water percolates downwards through large pores between the soil particles under the influence of gravity and is known as gravitational water. It reaches the low water table and goes beyond the root zone. Alternatively, a good amount of water is retained by the soil particles and is called Field Capacity moisture, i.e., the water retained due to the water-holding capacity of soil particles. This water may be present in three forms: Capillary water, Hygroscopic water, and Chemically combined water. Of all these, the water available to the plants is capillary water which fills the space between the non-colloidal smaller particles of soil. Hydroscopic water is held by the soil particles of colloidal smaller p...