In this article we will discuss about:- 1. Meaning of Passive Water Absorption 2. Evidences for Passive Water Absorption 3. Mechanism 4. Factors.
Meaning of Passive Water Absorption:
Passive water absorption is the most prevalent method of water absorption. In this process the force concerned with this type of absorption emanates from the aerial parts of the plant, especially leaves, and causes a tension in the xylem sap.
The root tip to the apical portion of the plant there is a continuous column of water present in the xylem elements. At many places these are in contact with the living cells. Very little amount of water is needed by these living cells to carry on their metabolism.
On the other hand, leaves and other apical parts of the plant body lose enormous amount of water and, therefore, require huge quantities of water. As a result of active transpiration of the leaves, water is drawn from the cells adjacent to the intercellular spaces below the stomata and these do so from the xylem in turn.
Water in the xylem conducting tissue is put into a great tension. This tension decreases water potential of the xylem sap to several atmospheres. It may be stated that the osmotic potential of xylem is of minor significance. Root hair is present in the soil and is in touch with the water molecules to be absorbed.
As a result the tension of the xylem sap can be remedied in this root hair. Near the area of transpiration the tension of xylem sap is high. The absorption lag is met with when the living plant cells lose water. This normally happens during high period of transpiration. Once the transpiration declines, living cells withdraw water from the xylem sap and regain turgidity.
Evidences for Passive Water Absorption:
Following evidences support the passive water absorption:
(i) Water can be absorbed by a plant even in the absence of roots. In fact, the rate of water absorption in this way is much more than the root system,
(ii) When the transpiration is very high, a negative pressure is observed in the xylem sap of the shoot, and
(iii) The rate of water absorption follows the rate of transpiration. Indeed at the time of intense transpiration the plant displays an absorption lag.
Mechanism of Passive Water Absorption:
The recent thinking is that the path of water absorption from the soil to the xylem is both through free spaces or apoplast of root and the outer parts of the cytoplasm of individual cells.
The apoplast consists of cell walls and intercellular spaces of the root and these are fully permeable. These cells offer least resistance to the water movement.
The cells of endodennis, however, have Casparian bands at several places and are therefore non-permeable.
The movement of water from these cells has to take place through their symplast. In maize roots xylem is lined by some amount of cytoplasm even after the break down of end walls. Thus, water passes from the root cytoplasm in the xylem channel.
Consequently, the tension caused in the xylem, because of transpiration affects the water absorption in the root hair zone.
However, water is moved passively inwards without the help of osmotic forces of the individual cells.
Several factors like temperature, inhibitors, influence the rate of water absorption and its passage through the cytoplasm.
Factors Affecting Passive Water Absorption:
In general, two types of factors affect passive water absorption and these are briefly discussed:
The number and length of root hair as well as the length of root hair zone determine the extent of water absorbed from the soil. In fact, the root hair regulates the surface area which comes in contact with water capillaries.
Root system-extent and depth:
It is also observed that deeper portions of the roots are less efficient for water uptake compared to the less deep portions.
Growth and metabolic pattern of roots:
Since the life span of root hair is short, they dry up and cease to remain functional. Consequently new roots need to be produced. Thus, continuous formation and growth of root hair facilitate water uptake. Also metabolism of the root hair influences the amount of water uptake.
Resistance of conducting system:
The rate of water absorption directly depends upon the resistance to the passage of water. The latter is connected with the cell wall permeability, metabolic state of the protoplasm, nature of endodermis, xylem vessels and their location, distribution and diameter, etc.
Availability of soil water:
The amount of water in the soil influences the rate of water absorption. Soil having poor aeration, low metabolism affects the water uptake.
Concentration of salts:
If the soil water is rich in minerals this increases the osmotic potential of the soil. Such soils are termed physiologically dry. Soils having Ψ up to 2 atm allow the root hair to absorb water. However, halophytes can grow successfully in soils with rich minerals.
The amount of aeration of soil greatly influences the water absorption. A soil which is water logged or has low oxygen is also physiologically dry high percentage of CO2 is detrimental to the absorption of water. In fact poorly aerated soils show decreased root growth and lesser surface is available for permeability.
Water uptake is also closely linked with the rate of transpiration. Transpiration is known to cause tension through the water loss. Hence high rate of transpiration causes increased water absorption.
At temperatures between 15-25°C the absorption of water is maximum. Cold soils are also physiologically dry. Low temperature affects root metabolism, especially its permeability and even its elongation.