Soil Water Relation:
Plants absorb water mainly from the soil. Five stages of water in the soil —
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1. Gravitational water.
2. Capillary water.
3. Imbibed water.
4. Runaway water.
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5. Chemically combined water.
Mechanism of water Absorption:
1. Active absorption of water
2. Passive absorption of water
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1. Active Absorption of Water:
When the water is absorbed due to the activity of root itself, particularly root hairs, it is termed active absorbtion.
(a) Osmotic Absorption:
The first step in the absorption of water is its imbibition on the cell wall surface. The water enters through the cell wall and comes in contact with differentially permeable plasma membrane and cytoplasm. The cell sap, filled inside the vacuole is now separated from external soil water by a differentially permeable membrane.
(b) Non-osmotic absorption:
Sometimes, it has been observed that absorption of water takes place even when the osmotic pressure 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.
2. Passive Absorption of Water:
Passive absorption of water takes place when rate of transpiration is usually high. Rapid evaporation of water from the leaves during transpiration creates a tension in water in the xylem of leaves. This tension is transmitted to water in xylem of roots through the xylem of stem and the water rises upward to reach the transpiring surfaces.
Pathway of water across root cells —
There are three possible pathways for the movement of water across the cortex:
(i) Through a system of inter connecting cell walls and intercellular spaces.
(ii) Through the living cytoplasm via plasmodesmata.
(iii) Through the living cytoplasmic membranes, cytoplasm and vacuoles of the critical cells.
Table. 2. Difference between active and passive water absorption:
| Active water absorption | Passive water absorption |
| i. It occurs mainly due to the activity of root hairs. Needs energy. ii. The water is absorbed by the osmotic and non osmotic mechanisms. iii. The osmotic process involves diffusion pressure dificit of the cells. iv. The water first enters into the cell sap and then passes from one cell to another (symplast movement). | i. It occurs mainly due to the activity of upper part of plant such as shoot and leaves. ii. The water is absorbed due to the process of active transpiration in the upper part. iii. The passive absorption occurs due to the tension created in xylem sap by transpiration pull. iv. The water in passive absorption moves probably through the free spaces (apoplast movement) |
Factors affecting the rate of water absorption:
1. Available soil water: If water in the soil is present below the permanent wilting percentage the absorption is affected.
2. Temperature of the soil: The low temperature of the soil reduces absorption of water.
3. Concentration of soil solution.
4. Aeration of Soil: Poorly aerated soil retards absorption of water.
Transpiration and Guttation:
The loss of water in the form of vapours from the living tissues of aerial parts of the plant is termed as transpiration.
Types of Transpiration
1. Stomatal transpiration
2. Cuticular transpiration
3. Lentricular transpiration
Factors affecting the rate of transpiration:
External Factors —
1. Atmospheric Humidity:
The rate of transpiration decreases with the increase in relative humidity of atmosphere.
2. Temperature:
The rate of transpiration increases with the increase in atmospheric temperature.
3. Light:
The rate of transpiration increases in light and decreases in dark.
4. Atmospheric Pressure:
Lowering of atmospheric pressure permits more rapid diffusion of water vapours.
5. Available soil water:
If the available water in the soil is not sufficient the rate of transpiration is decreased.
Internal Factors —
1. Leaf area:
By reducing the size of leaves, plants decrease their rate of transpiration.
2. Leaf structure:
Thick cuticle on the surface of plant parts, compact mesophyll cells in the leaves and reduction in number of stomata reduce the rate of transpiration.
3. Age of plants:
Rate of transpiration is slow at the seedling stage and maximum at maturity state.
Advantages of transpiration —
i. It controls the rate of absorption of water.
ii. It helps in the mineral absorption and ascent of sap.
iii. It regulates the plant temperature.
Disadvantages of Transpiration —
i. Nearly 97% of the total water absorbed by the plants is lost in the form of vapours.
ii. Transpiration often results in water deficits which causes injury to the plants by desiccation.
iii. Rapid transpiration causes temporary wilting.
iv. Deciduous trees have to shed their leaves to check transpiration.
Guttation:
The process of exudation of liquid drops from the edges of leaves is called Guttation.
Table. 3. Difference between Transpiration & Guttation
| Transpiration | Guttation |
| i. Occurs in day ii. Water is given out in the form of vapour iii. Occurs through stomata, lenticle or cuticle iv. It is a controlled phenomenon | i. Night ii. In the form of liquid iii. Occurs through hydathodes iv. It is a uncontrolled phenomenon |
Ascent of Sap:
The water is absorbed mainly by the roots and is moved upward to all the parts of plants via stem. Thus the upward movement of water through stem to tree top is called ascent of sap.
Path of Ascent of Sap:
After absorption by root hair cells, the water moves through several root tissues such as cortex, endodermis, pericycle and finally enters the xylem tissue. Once the water it reaches the mesophyll tissues of the leaves. The bulk of water enters the mesophyll cells and finally evaporates and transpires through the stomata, only small amount of water is used in metabolism and growth.
Many theories have been put forward to explain the upward movement of water.
1. Root pressure theory
2. Vital theories
3. Physical force theories
1. Root pressure theory – stocking
2. Vital theories of ascent of sap – Godlewski, Bose and Westermeir
3. Physical force theories
(a) Imbibition theory – Uinger and Sachs
(b) Capillary theory – Boehm
(c) Transpiration pull and cohesion of water theory. This theory was first proposed by Dixon and Joly in 1894.
The theory is based on the following features:
(i) Cohesive and Adhesive properties of water molecules to form a continuous water column in the xylem.
(ii) Transpiration pull exerted on this water column.
Water molecules remain joined to each other due to the presence of H-bonds between them.
Although H-bond is very weak but when they are present in enormous numbers as in case of water, a very strong mutual force of attraction or cohesive force develops between water molecules and hence they remain in the form of a continuous water column in the xylem.
The adhesive properties of water further ensure the continuity of water column in Xylem.
When transpiration takes place in leaves at the upper parts of the plant, water evaporates from the intercellular spaces of the leaves to the outer atmosphere through stomata. More water is into the intercellular spaces from the mesophyll cells. Due to all ^his tension is created in water in the xylem elements of the leaves. This tension is transmitted downward to water in xylem elements of the roots through the xylem of petiole and stem and the water is pulled upward in the form of continuous unbroken water column to reach the transpiring surfaces upto to the top of the plants.
Factors affecting Ascent of Sap:
High temperature, low atmospheric humidity, high atmospheric pressure and wind velocity permit more rapid transpiration and thus these factors also promote upward transport of water. Deficiency of water in the soil also cause decrease in the ascent of sap indirectly by influencing the absorption of water. Stomatal opening and closing also regulates movement of water in plants.