Absorption of Water

Soil Water


The chief source of soil water is rain.


In soil water is found in different forms.


Various terms have been used for soil water according to its availability and non-availability to the plants.


The total amount of water present in the soil is called holard, of this the available to the plant is called chesard and the water which cannot be absorbed by the plants is called echard.


Water occurs freely deep in the soil and above the parent rock, it is called ground water.


Broadly we can recognise five stages of water in the soil which differ in their availability to plants. These are briefly described below :



(a) Gravitational water : When the water enters the soil and passes the spaces between the soil particles and reaches the water table, the type of soil water is called gravitational water. In fact gravitational water is surplus to the water retaining capacity of soil and drains from it to reach in deep saturated zone of earth i.e., ground water, upper surface of which is called water table.



(b) Capillary water : It is the water which is held around soil particles in the capillary space present around them due to force like cohesion and surface tension. This is the water which can be utilised by the plants. It is also called growth water. It occurs in the form of films coating smaller soil particle.

The availability of capillary water to the plant depend upon its diffusion pressure deficit which is termed as the soil moisture stress. The plant cells much have a DPD more than the soil moisture stress for proper absorption of water.



(c) Hygroscopic water : This is the form of water which is held by soil particles of soil surfaces. The water is held tightly around the soil particles due to cohesive and adhesive forces. Hygroscopic water cannot be easily removed by the plants. Cohesive and adhesive forces greatly reduce the water protential (yw) and thus this type of water in soil is not available to plants.

(d) Run away water : After the rain, water does not enter the soil at all, but drained of along the slopes. It is called run away water. The quantity of run away water is controlled by factors like permeability of soil, moisture content of soil, degree of slope and number of ditches present in that area. Plants fail to avail this water.

(e) Chemically combined water : Some of the water molecules are chemically combined with soil minerals (e.g., silicon, iron, aluminium, etc.). This water is not available to the plants.

After a heavy rainfall or irrigation a very little amount of water is retained by the soil, rest of it moves away as surface run away water or gravitational water. The amount of water actually retained by the soil is called field capacity or water holding capacity of the soil. It is about 25–35% in common loam soil. The excess amount of water beyond the field capacity produces water logging.

(iv) Soil atmosphere : In moderately coarse soils as well as in heavy soils (fine textured soil) that are with aggregated particles; there exists large interstitial spaces which facilitate the diffusion of gases. As a result the CO2 produced in a soil by respiration of soil organisms and roots is able to escape rather easily and oxygen used up in this process diffuses into the soil with corresponding case.

(v) Soil organisms : The soil fauna include protozoa, nematodes, mites, insects, earthworms, rats. Protozoons alone are approximately 1 million per gram of soil. Earthworms have the most important effect on the soil structure. Their activities result in a general loosening of the soil which facilitates both aeration and distribution of water. Blue green algae and bacteria increases nitrogen content by nitrogen fixation in soil.

Water Absorbing Organs

Plants absorb water mostly from the soil by their roots, but in some plants even aerial parts like stem and leaves also do the absorption of atmospheric water or moisture. Some important examples of such plants are Vitis, Solanum, Lycopersicum, Phaseolus, Kochia baosia and Beta. The absorption of water by aerial parts is affected by various factors such as structure of epidermis, thickness of cuticle, presence of hair and degree of dryness of epidermal cells.

However, maximum absorption of water is done by the roots. The zone of rapid water absorption usually lies some 20 – 200mm from the root tip behind the meristem, where the xylem is not fully mature and the epiblema as well as the endodermis are still permeable (Kramer, 1956).

This area is usually characterized by the presence of root hairs which serve to increase the area of contact between the root surface and soil. However, presence of root hair is not essential for water absorption. Some roots, such as adventitious roots of bulbs, corms and rhizomes and those of some aquatic plants and gymnosperms do not have root hairs. The zone of rapid water absorption moves along with the growth of root, as the older cells become suberized and lose their ability to absorb water.





The root hairs develop mainly at the tip just above the zone of elongation (cell maturation). A root hair is the unicellular tubular prolongation of the outer wall of the epiblema. The cell wall of root hairs is two layered. The outer layer is made up of pectic substances and is therefore highly hygroscopic. The inner layer is made up of cellulose. Inside the cell wall is a thin layer of cytoplasm which surrounds one or more large vacuoles. The nucleus generally present at the tip.

During water absorption the plasma membrane of root hair, the cytoplasm and the vacuole membrane (tonoplast) behave together as a single differentially permeable membrane. Root-hairs are at the most 1.25 cm in length and never more than 10mm in diameter. As the root progresses through the soil, new root-hairs are formed at the beginning of the zone of maturation, the older hairs further back on the root, dry up and then disappear. Root-hairs elongate very rapidly and may attain full size within few hours.

The number of root-hairs may be simply enormous; it has been estimated that a single rye plant may have 14 billion root-hairs with a total surface area of 4000sq. feet. Thus the root-hairs of plants increase the absorption surface of a root system about 5 to 20 times and because they extend so widely through the soil they make available a supply of water that the plant could not otherwise obtain. Water potential of root hair cells is generally   –1 to –4 atm.

Pathway of Water Movement in Root

Water in the root moves through three pathways such as apoplast pathway, symplast pathway and transmembrane pathways. Munch coined the term apoplast and symplast.

(i) Apoplast pathway : The apoplastic movement of water occurs exclusively through the cell wall without crossing any membrane.



(ii) Symplast pathway : The symplastic movement of water occurs from cell to cell through the plasmodesmata.

(iii) Transmembrane pathway : Water after passing through cortex is blocked by casparian strips present on endodermis. The casparian strips are formed due to deposition of wax like substance, suberin. In this pathway, water crosses at least two membranes from each cell in its path. These two plasma membranes are found on entering and exiting of water. Here, water may also enter through tonoplast surrounding the vacuole i.e., also called as vacuolar pathway.

(4) Mechanism of water absorption : Two distinct mechanism which are independently operate in the absorption of water in plants. These mechanisms are :

(i) Active absorption         

(ii) Passive absorption

Renner coined the term active and passive water absorption.

Ascent of Sap

Land plants absorb water from the soil by their roots. The absorbed water is transported from roots to all other parts of the plants to replace water lost in transpiration and metabolic activities. The stream of water also transports dissolved minerals absorbed by the roots. The water with dissolved minerals is called sap. 'The upward transport of water along with dissolved minerals from roots to the aerial parts of the plant is called ascent of sap'.

(1) Path of ascent of sap : It is now well established that the ascent of sap takes place through xylem. In herbaceous plants almost all the tracheary elements participate in the process, but in large woody trees the tracheary elements of only sap wood are functional. Further, it has been proved experimentally that sap moves up the stem through the lumen of xylem vessels and tracheids and not through their walls.

(2) Theories of ascent of sap : The various theories put forward to explain the mechanism of ascent of sap in plants can be placed in following three categories :

(i) Vital force theories

(ii) Root pressure theory

(iii) Physical force theories





(i) Vital force theories : According to these theories the forces required for ascent of sap are generated in living cells of the plant. These theories are not supported by experimental evidences hence they have been discarded. Some of the important vital force theories are mentioned below :

(a) According to Westermeir (1883), ascent of sap occurred through xylem parenchyma; tracheids, and vessels only acted as water reservoirs.

(b) Relay pump theory : According to Godlewski (1884) ascent of sap takes place due to rhythmatic change in the osmotic pressure of living cells of xylem parenchyma and medullary rays and are responsible for bringing about a pumping action of water in upward direction. Living cells absorb water due to osmosis from bordering vessels (which act as reservoirs of water) and finally water is pumped into xylem vessel due to lowering of pressure in living cells. Thus a staircase type of movement occurs. Janse (1887) supported the theory and showed that if lower part of the shoot is killed upper leaves were affected.

Criticism


Strasburger (1891) and Overton (1911) used poisons (like picric acid) and excessive heat to kill the living cells of the plant. When such twigs were dipped in water, ascent of sap could still occur uninterrupted. This definitely proved that no vital force is involved in ascent of sap.


Xylem structure does not support the Godlewski's theory. For pumping action living cells should be in between two xylem elements and not on lateral sides as found.




(c) Pulsation theory : Sir J.C. Bose (1923) said that living cells of innermost layer of cortex, just outside the endodermis were in rhythmatic pulsations. Such pulsations are responsible for pumping the water in upward direction. He inserted a fine needle into the stem of Desmodium. The needle was connected to a galvanometer and an electric circuit. The fine needle was inserted into the stem slowly. The galvanometer showed slow oscillations which were because of local irritations. But when needle touched the innermost layer of cortex, oscillations turned violent indicating that cells in this layer were pulsating i.e., expanding and contracting alternately. According to Bose, the pulsatory cells pump the water into vessels.

Criticism : Dixon failed to verify the results of Bose. It has been estimated that sap should flow through 230–240 pulsating cells per second to account for normal rate of pulsations. This rate is several times higher as would be possible to the Bose theory (Shull, MacDougal, Benedict).

(ii) Root pressure theory : It is proposed by Priestly. According to this theory the water, which is absorbed by the root-hairs from the soil collects in the cells of the cortex. Because of this collection of water the cortical cells become fully turgid. In such circumstances the elastic walls of the cortical cells, exert pressure on their fluid-contents and force them towards the xylem vessels. Due to this loss of water these cortical cells become flaccid, again absorb water, become turgid and thus again force out their fluid contents. Thus the cortical cells of the root carry on intermittent pumping action, as a result of which considerable pressure is set up in the root. This pressure forces water up the xylem vessels. Thus the pressure, which is set up in the cortical cells of the roots due to osmotic action, is known as the root pressure. This term was used by Stephan Hales. According to Style, root pressure may be defined as "the pressure under which water passes from the living cells of the root in the xylem".

The root pressure is said to be active process which is confirmed by the following facts :

(a) Living cells are essential in root for the root pressure to develop.

(b) Oxygen supply and some metabolic inhibitors affect the root pressure without affecting the semipermeability of membrane systems.

(c) Minerals accumulated against the concentration gradient by active absorption utilising metabolically generated energy lowers the water potential of surrounding cells, leading to entry of water into the cells.



 Objections

Root pressure theory for ascent of sap has following limitations :


Taller plants like Eucalyptus need higher pressure to raise the water up. While the value of root pressure ranges from 2-5 atmospheres, a pressure of about 20 atm. is required to raise the water to tops of tall trees.


Strasburger reported the ascent of sap in plants in which the roots were removed.


Plants growing in cold, drought or less aerated soil, root pressure fails to appear and transport of water is normal.




(iii) Physical force theories : According to these theories the ascent of sap is purely a physical process. Some of the vital force theories are mentioned below :

(a) Capillarity theory : It was proposed by Boehm (1809). According to him, in the fine tubes, the water rises as a result of surface tension to different heights depending on the capillarity of the tube. The finer the tube, the greater will be the rise of water in it. But the xylem vessels are sometimes broader than the capillarity range, and hence the rise due to surface tension will be negligible.

Objections


Capillarity implies free surface but the water in the xylem elements in not in direct contact with the soil water.




Atmospheric pressure can support a column of water only up to the height of 34 feet.


Water can rise only up to the height of one metre in xylem vessels having diameter of 0.03mm.




(b) Imbibitional theory : It was proposed by Unger (1868) and supported by Sachs (1879). According to them, water moves upward in the stem through the wall of the xylem vessels. This theory is not accepted now because it is proved that water moves through the lumen of the xylem vessels and tracheids.

(c) Atmospheric pressure theory : Due to the loss of water by transpiration, the leaves draw water from the xylem vessels through osmotic pressure, which creates a sort of vacuum in the vessels. The atmospheric pressure acting on the water in the soil forces the water to rise up in the xylem vessels to fill the vacuum. But the atmospheric pressure can force the water to a height of only 10 metres. So it is evident that atmospheric pressure alone cannot force water to a height of 100 metres or more.






Root pressure is absent in gymnosperms.


Imbibitional force in pea is 1000 atm.


Cohesive force is called as tensile strength of water.


In soft stem, the ascent of sap can be prevented by applying squeezing pressure which closes the lumen of xylem channels.


Overlapping cuts are given by Preston (1958) in stem in order to break continuity of xylem channels. However, ascent of sap continued.


Manometer (Gk. manos – thin, metron – measure). An instrument for measuring pressure of tension (such as root pressure) in gases and liquids.


Cohesive strength of 47-207 atm. in xylem sap is sufficient to meet the stress of transpiration pull, so that water column does not break.


Adhesion : The attraction between the molecules of dissimilar substances.


Cohesion : The attraction between the molecules of the same substances.


Osmotic pressure is maximum in noon. At this time water contents in the cell are minimum.


In night, root pressure will be maximum because in night transpiration is zero.


Presence of pulsation in the cortical cell was demonstrated by electric probe.


Pressure bomb technique was used by Scholander et al




(d) Cohesion and transpiration pull theory : This is the most widely accepted theory put forth by Dixon and Jolly in 1894, and further supported by Renner (1911, 1915), Curtis and Clark (1951), Bonner and Golston (1952), Kramer and Kozlowski (1960).

It is also known as Dixon's cohesion theory, or transpiration pull theory or cohesion-tension theory.

This theory depends on the following assumptions, which are very near the facts :

(a) The xylem vessels are connected with each other, thus the water in them is in a continuous column from the root hairs to the mesophyll cells.

Walls of tracheids and vessels of xylem are made up of lignin and cellulose and have strong affinity for water (adhesion). The cell wall of adjacent cells, and those between the cells and xylem vessels all through the plant do not affect the continuity of the water column.

(b) Due to the transpiration from leaves, a great water deficit takes place in its cells. As a result of this deficit the water is drawn osmotically from the xylem cells in leaf veins, and by the cells surrounding the veins, and by the cells surrounding the veins. Thus a sort of pull is produced in the uppermost xylem cells in the leaves. It is called as the transpiration pull.

(c) The water molecules have a great mutual attraction with each other or in other words we can say that they have tremendous cohesive power which is sometimes as much as 350 atmospheres. Thus the transpiration pull developed a negative pressure in the uppermost xylem cells is transmitted from there into the xylem of stems, and from there to the roots.

In this way the water rises due to the transpiration pull and the cohesive power of water molecules from the lowest parts of the roots to the highest peaks of the trees. The osmotic pressure in the transpiring leaf cells often reaches to 30 atmospheres whereas only 20 atmospheres are needed to raise the water to the tops of highest known trees.

Objections : This is the most generally accepted theory, yet there are some objections against it which it fails to explain.

The most important objection is that leaving smaller plants, the water column has been found to contain air bubbles, and so their continuity breaks at such places. This phenomenon is known as cavitation and has been demonstrated by Milburn and Johnson (1966). However, Scholander overruled this problem by suggesting that continuity of water column is maintained due to presence of pits in the lateral walls of xylem vessels.

(3) Velocity of ascent of sap : Huber and Schmidt (1936) calculated the velocity of ascent of sap using radioactive 32P, specific dyes and also by heat-pulse transport between two specific points of stem. It varies between 1 and 6 meters per hour but under high transpirational conditions, it may be as high as 45 meters per hour. It is more in ring porous woods having large vessels. It is slowest in gymnosperms.