Mechanism of Water Absorption: Kramer (1959) explained how water enters in the root hair and the precise mechanism of water absorption. Accordingly absorption of water in plants is by two independent processes.
1. Active absorption
2. Passive absorption
1. Mechanism of Active Absorption:
- Absorption of water caused by forces present in the root is called active absorption of water.
- Only 2 to 4% of total water absorbed by plants is absorbed by this method.
- This type of absorption takes place mainly during humid nights when the rate of transpiration is very low.
- Inactive absorption, the cells of root play an important role and due to the activity of cells a positive force develops in the xylem of roots which is called as root pressure.
- This theory assumes that water from soil enters into root against a gradient of DPD and the process requires the expenditure of energy which is made available by respiration.
2. Mechanism of Passive Absorption:
- This is the main method of absorption of water in plants.
- 96 to 98% of water is absorbed by this method.
- During passive absorption, the factors of absorption operate in the transpiring surface of the plant.
- Loss of water from the cells of leaves results in a shortage of water in these parts and this results in a tension in the water column present in the xylem of veins of the leaf.
- This pull is transmitted to the water column of the stem and finally to the roots.
- This tension is negative and is called suction force or transpiration pull.
- On account of this, soil water enters root hair and is pulled through root, stem and leaves.
- It should be noted that in passive absorption of water there is no active participation of vital activity of cells of the plant.
- The strongest evidence in the flavour of this principle is that during the high rate of transpiration, absorption of water also takes place rapidly.