Long Distance Transport of Water

Absorption of water

Water is absorbed from soil by root system and specially by younger parts (i.e., root tips). In higher plants water is absorbed through root hairs.

Soil water : The chief source of soil water is rain. In soil water is found in different forms. The total amount of water present in the soil is called holard, of this the available to the plant is called chresard 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. These are briefly described below :

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.

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 particles.

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

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. Cohesive and adhesive forces greatly reduce the water protential ($\psi$$\omega$) and thus this type of water in soil is not available to plants.

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. Plants fail to avail this water.

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.

Water vapour : That portion of the pore space in a soil which is not occupied by liquid water contain a soil atmosphere that always includes water vapour.

Water holding capacity : 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.

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.

Soil organisms : The soil fauna includes protozoans, nematodes, mites, insects, earthworms, rats. Protozoans alone are approximately 1 million per gram of soil. Blue green algae and bacteria increase 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, Lycopersicon, Phaseolus, Kochia baosia and Beta.

However, maximum absorption of water is done by the roots.

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.

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.

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 10 mm in diameter. 

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. Munch coined the term apoplast and symplast.

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

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

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.

Mechanism of water absorption : Two distinct mechanisms which are independently operated in the absorption of water in plants. These mechanisms are :

(1) Active absorption     (2) Passive absorption

Renner (1912, 1915) coined the term active and passive water absorption.

(1) Active absorption : Active absorption takes place by the activity of root itself, particularly root hairs. The factor responsible for water absorption is present with in the roots. It utilizes metabolic energy. There are two theories of active absorption :

Osmotic theory : It was proposed by Atkins (1916) and Priestley (1922). It is purely a physical process, which does not directly require expenditure of energy.

A root hair cell functions as an osmotic system. Water is absorbed by the root hair due to osmotic differences between soil water and cells sap. The osmotic pressure of soil water remains below 1 atm, but that of cell sap is usually 2-8 atms. Thus, there exists a great difference in the osmotic pressures of the two sides or in other words there exists, water potential gradient between the soil solution and cell sap. The soil solution having less OP, has higher water potential than the cell sap with more OP (i.e., the cell sap has more negative water potential). Thus, water moves from the region of higher water potential towards the region of lower water potential.

Non-osmotic theory : It was proposed by Thimann (1951) and Kramer (1959). It has been observed that absorption of water still occurs, if the concentration of cell sap in the root hair is lower than that of the soil water, or water is absorbed against concentration gradient (i.e., from higher DPD to lower DPD). Such type of water absorption occurs on the expense of energy obtained from respiration.

Following evidences support the view that energy is utilized during active absorption of water :

(i) Rate of water absorption is directly proportional to the rate of respiration.

(ii) Respiratory inhibitors such as KCN, which inhibit the absorption of water.

(iii) Auxins (growth hormones), which increase respiration also promote water absorption.

(iv) Wilting of plants occur in non-aerated soils such as water logged soils, as roots fail to absorb water in absence of respiration.

(2) Passive absorption : It is the most common and rapid method of water absorption. The factor responsible for water absorption is present some where else than roots. It accounts for about 98% of the total water uptake by plant.

According to this theory, the forces responsible for absorption of water originate not in the cells of roots but in the cells of transpiring shoots. The root cells remain passive.

Due to transpiration, the DPD of mesophyll cells in the leaves increases which causes absorption of water by these cells from the xylem vessels of leaves. As the water column is continuous from leaves to roots, this deficit is transmitted to the xylem elements of roots and finally to root hairs through pericycle, endodermis and cortex. In this way water is continuously absorbed due to transpiration pull created in the leaves. This type of water transport occurs mainly through the apoplast in cortex but through the symplast in endodermis and pericycle.

The path of water from soil upto secondary xylem is :

Soil $\to$ Root hair cell wall $\to$ Cortex $\to$ Endodermis $\to$ Pericycle $\to$ Protoxylem $\to$ Metaxylem.

Factors affecting rate of water absorption : The different factors which influence the rate of water absorption by a plant can be divided into external or environmental and the internal factors.

External or Environmental factors

The amount of soil water : It is optimum at field capacity. Water absorption decreases above it. It begins to decline and stops at PWP.

Concentration of the soil solution : If the concentration of solutes increases in the soil water, its OP also increases which slows down or even inhibits the absorption of water. It happens due to addition of enough fertilizers in the soil increasing its salinity. This is popularly called as physiological dryness.

Soil aeration : Water absorption is done more efficiently in well aerated soil. Any deficiency of oxygen stops the respiration of roots and causes accumulation of CO₂ thus the protoplasm becomes viscous and the permeability of plasma membrane decreases. Due to all these factors the rate of water absorption is reduced. This is the reason for death of plants in flooded areas.

Soil temperature : The optimum temperature for maximum rate of water absorption ranges between 20°C and 30°C. Too high temperature kills the cells. At very low temperatures (4°C) water absorption is reduced or stopped and about O஬ it is almost checked.

Transpiration : The rate of absorption of water is almost directly proportional to the rate of transpiration. A higher rate of transpiration increases the rate of water absorption.

Internal factors

Efficiency of the root system : A plant with deep and elaborate root system can absorb more water. The number of root hairs will be more in a highly branched and elaborate root system, thus its more surface area will be in contact with water.

In gymnosperms, the root hairs are absent, even then they are able to absorb water due to presence of mycorrhizal hyphae.

In epiphytes (orchid), the roots develop a special type of hygroscopic tissue called as velamen which can absorb atmospheric moisture.

Metabolic activity of roots : The poor aeration or use of metabolic inhibitors (e.g., KCN) inhibits the rate of water absorption. The metabolic activities help in proper growth of root system and generation of energy for absorption of certain vital minerals.

Absorption of water through leaves : Many species of plants can absorb at least limited amounts of water through the leaves. Temporary immersion of aerial organs in flood waters takes place in some cases. Also the aerial organs of plants frequently become wet as a result of fog, dew or rain. Most of the water enters through the epidermal cells, although in some species hairs and specialized epidermal cells provide regions of high permeability. In general water absorption is more rapid in young leaves than in old leaves of the same plant.

Ascent of sap

‘The upward transport of water along with dissolved minerals from roots to the aerial parts of the plant is called Ascent of sap’. It is also called translocation of water. The water with dissolved minerals is called sap.

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.

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 :

(1) Vital force theories

(2) Root pressure theory

(3) Physical force theories

(1) 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 :

According to Westermaier (1883), ascent of sap occurs through xylem parenchyma tracheids, and vessels only act as water reservoirs.

Relay pump theory (Clambering 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. Janse (1887) supported the theory and showed that if lower part of the shoot is killed upper leaves were affected.

Criticism

(i) 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.

(ii) 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.

Pulsation theory : Sir J.C. Bose (1923) said that living cells of innermost layer of cortex, just outside the endodermis are in rhythmatic pulsations. Such pulsations are responsible for pumping the water in upward direction. 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).

(2) Root pressure theory : It was proposed by Priestley (1916). According to this theory the water, which is absorbed by the root-hairs from the soil collects in the cells of the cortex. 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”.

Objections

(i) 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.

(ii) The absence of root pressure, ascent of sap continues.

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

In gymnosperms root pressure has rarely been observed.

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

Capillary force 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

(1) For capillarity a free surface is required.

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

(3) Water can rise only up to the height of one meter in xylem vessels having diameter of 0.03mm.

(4) In gymnosperms usually the vessels are absent.

Imbibitional theory : It was proposed by Unger (1868) and supported by Sachs (1879). According to them, water moves upward in the stem through the walls 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.

Atmospheric pressure theory : Due to the loss of water by transpiration, the leaves draw water from the xylem vessels through osmotic pressure. 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 meters. So it is evident that atmospheric pressure alone cannot force water to a height of 100 metres or more.

Jamin’s chain theory : In xylem water and air bubbles are found alternately. Thus upward movement occurs.

Cohesion of water and transpiration pull theory : This is the most widely accepted theory put forth by Dixon and Joly 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 cohesion-tension theory.

This theory depends on the following assumptions :

(1) 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.

(2) 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. Thus a sort of pull is produced in the uppermost xylem cells in the leaves. It is called as the transpiration pull.

(3) 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 develops a negative pressure in the uppermost xylem cells. It 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.

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.