Let us make in-depth study of the organ, mechanism, factors and methods of water absorption.
Absorption of water and nutrient material is carried on only by the younger portions of a root, near its tip (See Fig. 2.2).
The very tip itself is usually covered with a sheathing root cap of cells which protects the delicate underlying tissues as the root pushes its way through the soil.
Besides root cap, three more zones can be recognized in the young root tip.
These zones are:
Zone of cell division or meristematic zone:
The thin-walled cells of this region are almost alike and they are constantly dividing, thus adding new cells to the root cap and the main part of the root. Through division of cells and the subsequent enlargement of the newly formed cells, root increases in length.
The zone of elongation:
The cells formed in the meristematic region become longer in the region of elongation. The increase in size results largely from an osmotic uptake of water and to a lesser extent by the formation of additional protoplasm.
The zone of maturation & Root Hair Zone:
In this region the cells become specialized in structure and function. Some cells take on structural features which enable them to conduct water. Others become specialized for the conduction of food, and still others for food storage.
Root hairs develop in the younger part of the maturation zone as finger-like extensions of the epiblema cells (Fig. 2.3). This part of maturation zone is often called Root Hair Zone. Root hairs are rarely more than one-hundredth of a millimeter in diameter but may reach a length of several millimeters, forcing their way into the minute crevices of the soil and thus coming in close contact with the soil particles.
Sometimes as many as several hundred root hairs may be borne on a square millimeter of root surface although the number per unit area is usually less. Through the very extensive surface which these root hairs expose to the soil, mainly absorption of water and minerals takes place. They increase the absorbing surface of the root many times.
In conifers the root hairs are either very poorly developed or entirely absent. Such roots are regularly infested with fungal mycelia. Such a root together with its associated fungal hyphae is called mycorrhiza. Mycorrhiza helps roots in water & mineral absorption.
The cell wall of root hair is composed principally of cellulose and pectic compounds. The outer layer of wall is composed of pectic compounds mostly calcium pectate. This coating is responsible for the tenacity with which root hairs and soil particles adhere to each other. The inner wall of the root hair is lined with a thin layer of cytoplasm which is continuous with the cytoplasm of the epidermal cells.
The cytoplasm encloses a large vaculole filled with cell sap. The cell sap is an aqueous solution of mineral salts, sugars and organic acids. Because of the presence of the sugars and acids, the osmotic pressure of the cell sap is much higher than the osmotic pressure of soil solution. The value of osmotic pressure of cell sap of root hairs is 3 to 5 atmosphere, while that of soil solution in a well-watered soil is less than 1 atmosphere.
Mechanism of Water Absorption:
The mechanism of water absorption can be explained by two approaches.
1. Passive absorption:
Root does not play active role. Force responsible for absorption develops due to transpiration. No expenditure of energy for absorption, process.
1. As the leaf cells lose water in transpiration, they develop water deficit (turgor deficit, D.P.D. or Suction force)
2. Their water potential becomes lower and they draw water from the xylem of the veins.
3. This causes the xylem of the veins to develop the S.F. as well as low water potential.
4. Therefore, veins draw water from the petiole, the petiole from the stem, and the stem from the root, and hence water from the soil automatically enters the roots through the root hairs.
5. Thus, the suction force responsible for the absorption of water by the root actually originates
6. The root system merely acts as a physical absorbing system.
2. Active Absorption:
Water is absorbed as a result of activity of root and does not concern with any role of shoot Two theories have been put forward to explain the actual mechanism of active absorption.
(a) Osmotic theory
(b) Non osmotic theory
(a) Osmotic theory:
Proposed by Atkins and Priestley. Water is absorbed due to osmotic difference between soil water and that of tonoplasm. D.P.D. of root hair is increased due to high O.P and low TP of root water is absorbed by endosmosis TP of root hair increase and D.P.D. decreases water moves from root hair to inner cells and finally reaches into the xylem.
(b) Non Osmotic theory:
Proposed by Thimann and Kramer. Water absorption is an active process occurs due to non osmotic reason against the DPD. Process require energy (ATP) comes from respiration.
1. Operate in very slowly transpiring plants.
2. Occurs against the D.P.D. gradient and requires the expenditure of energy released from respiration.
3. There may be some carrier substances in the wall of root cells, which bind with water and carry, it to the inner tissue, (certain bacteria in higher plants).
4. Auxin increases rate of the transpiration as well as water absorption.
Root pressure, guttation and bleeding are the manifestation of active water absorption. The available evidence indicates that passive absorption accounts for most of the water absorbed by plants. Active absorption is important only in slowly transpiring plants growing in soil near field capacity.
Factors Affecting Water Absorption:
1. Available of soil water:
(a) Plant absorbs capillary water, which is present in soil. Absorption of water depends on the amount of capillary water present in the soil. Absorption increases by increasing amount of capillary water.
(b) If, water is present in higher amount in the soil then such type of soil is called “Water logged soil”. This soil is Physiologically dry and lack of oxygen. Because of this anaerobic respiration takes place in roots, and alcohol is formed. Roots can be degenerate due to form action of alcohol.
2. Soil temperature:
(a) Soil temperature affects the following mechanisms:
(i) Low temperature decreases the permeability of cell membrane.
(ii) It is essential for the activity of enzymes for the formation of root hairs.
(iii) At low temperature viscosity of capillary water is increased.
(b) Generally, normal absorption of water take place at temperature of soil between 20 – 35°C. Increasing or decreasing soil temperature of soil between 20 – 35°C inhibit absorption. Cold soil is as physiologically dry.
3. Soil Air:
Absorption of water proceeds more rapidly in well aerated soil. Deficiency of oxygen in soil causes improper respiration in roots & decreases rate of absorption. Poorly aerated soil is physiologically dry.
4. Soil Concentration:
(a) The rate of the absorption is inversely proportional to the concentration of minerals present in soil.
Water Absorption x 1/Concentration of soil minerals
(b) Water absorption is only take place in appropriate soil solution. Soil should be hypotonic & Plant must be hypertonic to carry out the process of endosmosis. If the concentration of 5oil minerals is high, it decreases the rate of absorption & plasmolysis & wilting takes place.
According to Kramer the rate of water absorption is directly proportional to the rate of transpiration. The rate of absorption increases due to increase in the transpiration because passive water absorption increases due to transpiration.
Absorption of Water through Leaves:
Many species of plants can absorb at least limited amount of water through the leaves, particularly the younger ones. Aerial spray of water on leaves as a means of irrigation has become a favorite practice with vegetable and fruit growers.
This way a lot of water is saved and most of the plants give better results than those which are irrigated by traditional methods. In leaves, most of the water enters through the epidermal cells, although in some species hairs and specialized epidermal cells provide regions of high permeability
Other Methods of Water Absorption:
(a) By Mycorrhiza:
1. The root hairs are not developed in some of conifer plants thus water is absorbed with the help of mycorrhizal association (Association between Fungi & higher plant roots)
2. These fungus myecilium absorb water and minerals and transfers to the roots. These fungus mycelium obtain their food from the roots.
(b) By Velamen:
1. Velamens are found in epiphytes such as Orchids.
2. Absorption of water vapour of air takes place in these plants through the hanging roots. These roots have specialized tissue on the outside of their cortex is called velamen.