Mechanism of Water Absorption in Plants
Mechanism of Water Absorption in Plants
Water is absorbed by the plants according to the potential gradient, that is water moves from soil to the roots when water potential of soil solution is more than that of plant cell s sap. However, the gradient is generated differentially in slowly and rapidly transpiring plants. In show transpiring plants, the roots play an essential role in generating the gradient and the mechanism is called active absorption. In rapidly transpiring plants, the roots play a passive role; the water is just pulled in through them and the mechanism is called passive absorption.
Active absorption of water by the roots occurs by the process of osmosis according to the gradient in water or osmotic potential. The osmotic potential of the xylem sap is generally-2 bars or less where as that of soil solution is in the order of-0.1 bar. In other words, the concentration of solutes in the cell sap is generally higher than that in the soil solution. As long as capillary water is available in the soil, the water will move from soil solution to the cell sap i.e. from the dilute solution to the concentrated solution. Obviously, this inward flow of water will continue until the concentration gradient is maintained. This is achieved by the development of an internal pressure in the roots called root pressure.
The phenomenon of root pressure has been observed for more than 100 years, by the botanists. It can be observed as exudation of sap from cut stem of a small herbaceous plant. The amount of pressure generated can be measured if a mercury manometer is attached to the cut stem. In demonstration, normally we take actively growing well watered potted plant of a herb such as that of balsam. Pressures of 5 to 6 bars have been recorded, although in most cases, values do not exceed 1 bar. Root pressure in most plants develops only when ample soil moisture is available and transpiration is low because of humidity.
The natural visible phenomenon associated with root pressure and exudation of sap is guttation. In this process, liquid water is exuded by the leaves where it forms droplets. Active absorption, root pressure and guttation are usually considered to be different aspects of same phenomenon.
It is not clearly understood, how the root pressure develops. However, three theories have been proposed. Experimental evidences have been presented far and against each of the theories.
Some plant physiologists have suggested that root pressure is produced by the secretion of water into xylem. This secretion results from a higher permeability of water on the inner side than on the outer side of the root cells. Alternatively, the secretion could be achieved by spending some respiratory energy of the root cells.
Many physiologists have attributed root pressure to electro-osmotic movement of water into root xylem. It has been experimentally demonstrated that water can move across a membrane, if an electric current is applied. However, attempts to cause water flow into roots by applying electric current have been unsuccessful.
According to this theory, root pressure develops as a result of osmotic process. The root functions as an osmometer because of the accumulation of solutes. Water moves from dilute solution (soil solution) to concentrated solution (xylem sap) through various tissues of root and it causes the development of root pressure.
Water absorption is rapidly transpiring plants can occur by passive mechanism. In this mechanism, root functions as a passive absorption surface and water moves through the roots rather than by them. Indeed passive absorption can occur even through dead or anaesthetized roots or even when no roots are present.
The passive absorption results from the development of tension in xylem water as a consequence of water loss from leaves and other evaporating surfaces. The tension is then transmitted to the root surfaces through root cells. This causes the mass flow of water from soil to the xylem of roots. In living roots, this mass flow occurs primarily through the free spaces of root cells. Thus, no root cells are involved in this process of water flow. As expected, the rate of passive absorption is well correlated to the rate of water loss or transpiration. However, during some periods (specially during noon) of the day, the rate of absorption does not cope with the rate of transpiration, primarily because of the resistance of water flow across the root cells. This causes the temporary wilting of the plants. The plants recover themselves from temporary wilting, later in the day, when rate of transpiration is lowered.
Passive mechanism is a more common means of water absorption than active absorption.
Sometimes, roots are able to absorb water from the soil in vapour form as well specially when the soil is dry and when all liquid water is in the vicinity of the root hairs has dried up. If the water potential in root is more negative than that in the soil (although at some distance from root hairs) then water diffuses into root in vapour form. Obviously the solutes do not move inside with this kind of water absorption as the water molecules can move through vapour phase but no the solutes. Under these conditions, the uptake of water is not coupled with the uptake of solutes.
Factors Affecting Rate of Water Absorption
Some important plant and environmental factors affecting the rate of water absorption are as follows:
Rate of transpiration-
It is the most important factor affecting rate of water absorption. Absorption increases with the increase in the rate of transpiration. When the rate of transpiration is low, then only active absorption of water (which has a lower rate) occurs.
Absorption of water also depends upon the extension of root system. Deep rooted plants can absorb more water than those with shallow roots obviously because of their better access to the available water. Further, the permeability of water to root cells depends upon their age and maturation also. Generally the permeability and consequently the rate of water absorption decreases with age, as older tissues are suberized.
Soil composition and soil water content-
Another factor affecting the rate of water uptake directly, in the soil composition especially in relation to its water content. Water becomes unavailable to the plants when soil reaches its permanent wilting percentage. The salt concentration in the soil solution and its aeration also affect water absorption. Plants either growing in salty marshes or those supplied with excessive fertilizer are unable to absorb water because of excessive salt concentration in the soil. Such plants can show sign of wilting or ‘burning’ even though the soil is at field capacity. Such a situation in the soil is sometimes referred to as physiological dryness.
Low temperature inhibits rate of water absorption which is brought about by complex factors such as increase in the viscosity of water, decrease in the permeability of root cells, decrease in root growth and metabolism and decreased availability of soil water at low temperature.
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