Plant Water Retention

A. Concept of water relation.

Water is the most important constituent of plants and is essential for the maintenance of life, growth and development. Plants lose huge amount of water through transpiration. They have to replenish this lost water to prevent wilting. Water is mainly absorbed by the roots of the plants from the soil, than it moves upward to different parts and is lost from the aerial parts, especially through the leaves. Before taking up the absorption and movement of water in plants, it is worthwhile to understand the phenomenon of imbibition, diffusion and osmosis involved in the water uptake and its movement in the plants.

(1) Imbibition : The process of adsorption of water by solid particles of a substance without forming a solution is called 'imbibition'. It is a type of diffusion by which movement of water take place along a diffusion gradient. The solid particles which adsorb water or any other liquid are called imbibants. The liquid which is imbibed is known as imbibate. Cellulose, pectic substances, protoplasmic protein and other organic compound in plant cells show great power of imbibition.

(i) Characteristics of imbibition : The phenomenon of imbibition has three important characteristics :

(a) Volume change : During the process of imbibition, imbibants increase in volume. It has been observed that there is an actual compression of water. This is due to arrangement of water molecules on surface of imbibant and occupy less volume than the same molecules do when are in free stage in the normal liquid. During the process of imbibition affinity develops between the adsorbant and liquid imbibed. A sort of water potential gradient is established between the surface of adsorbant and the liquid imbibed.

e.g. If a dry piece of wood is placed in water, it swells and increases in its volume. Similarly, if dry gum or pieces of agar agar are placed in water, they swell and their volume increases. Wooden doors and windows adsorb water in humid rainy season and increase in their volume so that they are hard to open or close, in gram and wheat the volume increase by adsorption of water, in plant systems are adsorption of water by cell wall.

(b) Production of heat : As the water molecules are adsorbed on the surface of the imbibant, their kinetic energy is released in the form of heat which increase the temperature of the medium. It is called heat of wetting (or heat of hydration). e. g., during kneading, the flour of wheat gives a warm feeling due to imbibition of water and consequent release of heat.

(c) Development of imbibitional pressure : If the imbibing substance (the imbibant) is confined in a limited space, during imbibition it exerts considerable pressure. The bursting of seed coats of germinating seeds is the result of imbibition pressure developed within the seeds as they soak the water. Imbibition pressure can be defined as the maximum pressure that an imbibant will develop when it is completely soaked in pure water. Imbibition pressure is also called as the matrix potential because it exists due to the presence of hydrophilic substances in the cell which include organic colloids and cell wall.

Resurrection plants of Selaginella, lichens, velamen roots and dry seeds remain air dry for considerable periods because they can absorb water from the slight downpour by the process of imbibition.

(ii) Factors influencing the rate of imbibition

(a) Nature of imbibant : Proteins are the strongest imbibants of water, starch less strong, cellulose being the weakest. That is why proteinaceous pea seeds swell more than the starchy wheat seeds. During seed germination seed coat rupture first because it is made up of cellulose (weak imbibant) and kernel is made up of protein, fat and starch (strong imbibant).

(b) Surface area of imbibant : If more surface area of the imbibant is exposed and is in contact with liquid, the imbibition will be more.

(c) Temperature : Increase in temperature causes an increase in the rate of imbibition.

(d) Degree of dryness of imbibant : If the imbibant is dry it will imbibe more water than a relatively wet imbibant.

(e) Concentration of solutes : Increase in the concentration of solutes in the medium decreases imbibition due to a decrease in the diffusion pressure gradient between the imbibant and the liquid being imbibed. It is due to the fact that imbibition is only a special type of diffusion accompanied by capillary action. If some solute is added into the liquid which is being imbibed, its diffusion pressure decreases and the process of imbibition slows down.

(f) pH of imbibant : Proteins, being amphoteric in nature, imbibe least in neutral medium. Towards highly acidic or highly alkaline pH, the imbibition increases till a maximum is reached, there after it starts slowing down.

(iii) Significance of imbibition

(a) The water is first imbibed by walls of root hairs and then absorbed and helps in rupturing of seed coat (made up of cellulose).

(b) Water is absorbed by germinating seeds through the process of imbibition.

(d) The water moves into ovules which are ripening into seeds by the process of imbibition.

(e) It is very significant property of hydrophilic surfaces.

B. Diffusion :

The movement of the molecules of gases, liquids and solutes from the region of higher concentration to the region of lower concentration is known as diffusion.

Diffusion is the net movement of molecules or ions of a given substance from a region of higher concentration to lower one by virtue of their kinetic energy.

It is the movement of molecules from high diffusion pressure to low diffusion pressure.

Phenomenon of diffusion can be observed everyday.

It may occur between gas and gas (e.g., diffusion of ammonia into air), liquid and liquid (e.g., diffusion of alcohol into water), or solid and liquid (e.g., diffusion of sugar into water). The diffusion of one matter is dependent of other. That is why many gases and solutes diffuse simultaneously and independently at different rates in different direction at the same place and time, without interfering each other. From soil, water and ions of simple inorganic salts pass into plants through the root cells by a process which is basically diffusion, though greatly modified by other factors. The water and solutes pass through the dead and living vessels and also from cell to cell by diffusion. When a crystal of copper sulphate is placed in a beaker containing water, a dense blue colour is seen around the crystal.

(i) Diffusion pressure : It is a hypothetical term coined by Meyer (1938) to denote the potential ability of the molecules or ions of any substance to diffuse from an area of their higher concentration to that of their lower concentration. Alternatively, it may also be defined as the force with which the diffusing molecules move along the concentration gradient.

(ii) Diffusion pressure deficit (DPD) or Suction pressure (SP) : The term diffusion pressure (DP) and diffusion pressure deficit (DPD) were putforth by B.S. Meyer in 1938. Originally, the DPD was described by the term suction force (Saugkraft) or suction pressure (SP) by Renner (1915). Now a days, the term water potential (ψ) is used which is equal to DPD, but negative in value.

Each liquid has a specific diffusion pressure. Pure water or a pure solvent has the maximum diffusion pressure. If some solute dissolved in it, the water or solvent in the resulting solution comes to attain less diffusion pressure than that of the pure water or pure solvent. In other words, diffusion pressure of a solvent, in a solution is always lower than that in the pure solvent. 'The amount by which the diffusion pressure of water or solvent in a solution is lower than that of pure water or solvent is known as diffusion pressure deficit (DPD)'. Because of the presence of diffusion pressure deficit, a solution will always tend to make up the deficit by absorbing water. Hence, diffusion pressure deficit is the water absorbing capacity of a solution. Therefore, DPD can also be called suction pressure (SP).

(iii) Factors influencing rate of diffusion

(a) Temperature : Increase in temperature leads to increase in the rate of diffusion.

(b) Pressure : The rate of diffusion of gases is directly proportional to the pressure. So the rate of diffusion increases with increase of pressure. Rate of diffusions ∝ pressure.

(c) Size and mass of diffusing substance : Diffusion of solid is inversely proportional to the size and mass of molecules and ions.

Rate of diffusion

(d) Density of diffusing substance : The rate of diffusion is inversely proportional to the square root of density of the diffusion substance. Larger the molecules, slower will be the rate of diffusion. This is also called Graham's law of diffusion.

(D = Diffusion and d = Density of diffusing substance).

According to the density the diffusion of substances takes place in following manner –

Gas > Liquid > Solid

The vapours of volatile liquids (sent or petrol) and solids (camphor) also diffuse like gases.

(e) Density of the medium : The rate of diffusion is slower, if the medium is concentrated. Thus, a gas would diffuse more rapidly in vacuum than in air. Substances in solution also diffuse but at a much slower rate than gases. Substances in solution diffuse more rapidly from regions in which their concentration is higher into regions of low concentration. If two solutions of sugar (or of any other substance) of different concentrations are in contact, sugar molecules diffuse from the higher to the lower concentrations of sugar and water molecules diffuse from the higher to the lower concentrations of water, until equilibrium is attained when the two solutions become of equal concentration.

(f) Diffusion pressure gradient (DPG) : The rate of diffusion is directly proportional to the difference of diffusion pressure at the two ends of a system and inversely proportional to the distance between the two.

(iv) Significance of diffusion

❒ Gaseous exchange during the processes of photosynthesis and respiration takes place with the help of diffusion.

❒ The process of diffusion is involved in the transpiration of water vapours.

❒ Aroma of flowers is due to diffusion of volatile aromatic compounds to attract pollinating animals.

❒ During passive salt uptake, the ions are absorbed by process of diffusion.

❒ Diffusion helps in translocation of food materials.

❒ Gaseous exchange in submerged hydrophytes is takes place by general surface of the cells through diffusion.

C. Osmosis :

Osmosis (Gr. Osmos = a pushing or impulse) was discovered by Abbe Nollet in 1748 and also coined the term 'osmosis'. First of all Traube (1867) use copper ferrocyanide and develop semipermeable membrane to show its utility in the osmosis of plant physiology. First time Pfeffer in (1887) develop osmoscope by using semipermeable membrane.

Osmosis is special type of diffusion of a liquid, when solvent moves through a semipermeable membrane from a place of higher diffusion pressure to a place of lower diffusion pressure.

It is the migration of solvent from a hypotonic solution (of lower concentration) to hypertonic solution (of higher concentration) through a semi-permeable membrane to keep the concentration equal.

In osmosis, the water (or solvent) molecules moves as follows :

From the region of	To the region of
Pure solvent (water)	Solution
Dilute solution	Concentrated solution
High free energy of water molecules	Low free energy of water molecules
Higher chemical potential (or water potential)	Lower chemical potential (or water potential)
Higher diffusion pressure of water	Lower diffusion pressure of water

❒ In formalin preserved Spirogyra filament, selective permeability of plasmamembrane is lost and hence no effect on placing in hypertonic solution.

❒ If salt presents in higher concentration in a cell than outer side, water will enter in the cell by osmosis.

(iv) Differences between diffusion and osmosis

S.No.	Diffusion	Osmosis
(1)	It is the movement of particles, molecules or ions from the region of their higher free energy to the region of their lower free energy.	It is the movement of solvent of water from the area of its higher free energy or chemical potential to the area of its lower free energy or chemical potential through a semi-permeable membrane.
(2)	It can occur in any type of medium.	It occurs only in liquid medium.
(3)	The diffusing molecules may be solids, liquids or gases.	It involves the movement of solvent molecule only.
(4)	It does not require a semi-permeable membrane.	A semi-permeable membrane is required for the operation of osmosis.
(5)	It is purely dependent upon the free energy of the diffusing substance only.	It depends upon the free energy chemical potential of the solvent present on the two sides of the semi-permeable membrane.
(6)	An equilibrium in the free energy of diffusing molecules is achieved in the end.	An equilibrium in the free energy of solvent molecule is never achieved.

(v) Osmotic pressure (OP) : Pfeffer coined the term osmotic pressure.

Osmotic pressure of a solution is the pressure which must be applied to it in order to prevent the passage of solvent due to osmosis.

Osmotic pressure is that equivalent of maximum hydrostatic pressure which is produced in the solution, when this solution is separated from its pure solvent by a semipermeable membrane.

It can also be defined as "the excessive hydrostatic pressure which must be applied to it in order to make its water potential equal to that of pure water". Osmotic pressure is equal to the pressure which is needed to prevent the passage of pure water into an aqueous solution through a semi-permeable membrane. In other words, it is that pressure which is needed to check the process of osmosis.

(i) Types of osmosis : Depending upon the movement of water into or outward of the cell, osmosis is of two types.

(a) Endosmosis : The osmotic inflow of water into a cell, when it is placed in a solution, whose solute concentration is less than the cell sap, is called endosmosis e.g., swelling of raisins, when they are placed in water.

❒ When a fish of marine water kept in fresh water than it will be die due to endosmosis.

❒ An animal cell placed pure water will swell up and brust.

❒ Pollen grains of some of plants germinate on stigma soon but they burst in water or dilute sugar solution.

(b) Exosmosis : The osmotic outflow of water from a cell, when it is placed in a solution, whose solute concentration is more than the cell sap, is called exosmosis. e.g., shrinkage of grapes, when they are placed in strong sugar solution.

(ii) Demonstration of osmosis

(a) Thistle funnel experiment to show osmosis : Tie the mouth of a thistle funnel with an egg membrane or animal bladder which are semi-permeable in nature. Put sugar solution (hypertonic solution) inside the thistle funnel. Thistle funnel is dipped in water with the help of a stand. A rise in level is noticed after some time. This is due to the diffusion of water into thistle funnel through semi-permeable membrane by the process of osmosis.

(b) Demonstration of osmosis by potato osmoscope : Peel of the skin of large sized potato with the help of scalpel. Cut its one end to make the base flat. Make a hallow cavity in the potato almost up to the bottom. Put sugar solution into the cavity and mark the level by inserting a pin in the wall of the cavity of tuber. Place the potato in beaker containing water. After some time, it will be noticed that level in cavity rise. It is due to phenomenon of osmosis. The experiment demonstrates that living cells of potato act as differentially permeable membrane.

Osmosis cannot be demonstrated by a potato osmoscope using a solution of NaCl instead of sugar because the potato tissue is permeable to salt solution.

(iii) Osmotic concentrations (Types of solutions) : A solution can be termed as hypotonic, hypertonic and isotonic depending upon its osmotic concentration, with respect to another solution or cell sap.

(a) Hypotonic solution (hypo = less than). A solution, whose osmotic concentration (solute potential) is less than that of another solution or cell sap is called hypotonic solution. If a cell is placed in such a solution, water start moving into the cell by the process of endosmosis, and cell become turgid.

(b) Hypertonic solution (hper = more than). A solution, whose osmotic concentration (solute potential) is more than that of another solution or cell sap is called hypertonic solution. If a cell is placed in such a solution, water comes out of the cell by the process of exosmosis and cell become flaccid. If potato tuber is placed in concentrated salt solution it would become shrink due to loss of water from its cell.

(c) Isotonic solution (iso = the same). A solution, whose osmotic concentration (solute potential) is equal to that of another solution or cell sap, is called isotonic solution. If a cell is placed in isotonic solution, there is no net changes of water between the cell and the solution and the shape of cell remain unchanged. The normal saline (0.85% solution of NaCl) and 0.4 m to 0.5 m solution of sucrose are isotonic to the cell sap.

❒ Osmotic concentration of a solution may governed by concentration of solute, temperature of solution, ionization of solutes and hydration of the solute molecules.

❒ In xerophytes, the osmotic concentration of cell sap is more than normal. e.g., A molar solution of sucrose separated from pure water by such a membrane has an OP of approximately 22.4 atmospheres at 0°C. The osmotic pressure of given solution can be calculated by using the following relationship.

Osmotic pressure = CST

Where, C = Molar concentration of solution, S = Solution constant, which is 0.082 and T = Absolute temperature i.e., 273°C.

Sucrose is non-ionizing substance while NaCl is ionizing substance. Osmotic pressure of a solution of ionizing substance is greater than that of equimolar concentration of non-ionizing substance. e.g., 0.1M sucrose solution has an OP of 2.3 bars while 0.1M sodium chloride solution has value of 4.5 bars.

(vi) Significance of osmosis in plants

(a) The phenomenon of osmosis is important in the absorption of water by plants.

(b) Cell to cell movement of water occurs throughout the plant body due to osmosis.

(d) Leaves become turgid and expand due to their OP.

(e) Growing points of root remain turgid because of osmosis and are thus, able to penetrate the soil particles.

(f) The resistance of plants to drought and frost is brought about by osmotic pressure of their cells.

(g) Movement of plants and plant parts, for example, movement of leaflets of Indian telegraph plant, bursting of many fruits and sporangia, etc. occur due to osmosis.

(h) Opening and closing of stomata is affected by osmosis.

D. Absorption of water

(1) Component of soil : Soil is the superficial layer of the weathered earth crust, which support plant life. Generally soil is the combination of various component such as mineral matter (inorganic component), organic matter, soil water, soil atmosphere and soil organisms. On an average the ratio and proportion of the above mentioned components is as follows :

Mineral matter : 40% of volume

Organic matter : 10% of volume

Soil water : 25% of volume

Soil atmosphere : 25% of volume

Soil organisms : Some

(i) Mineral matter : The soil is produced by the breakdown of parent rocks by a process called weathering. Weathering is a result of three kinds of processes physical, chemical and biological. Physical weathering involves fragmentation of rocks due to freezing and thawing, movement of earth (as earth-quakes) and other mechanical processes. Chemical weathering involves reactions between e.g., carbonic acid(H₂O + CO₂ → H₂CO₃) with minerals of rocks. Biological weathering is due to action of living organisms specially microbes.

The characteristic of soil depend on its texture and structure.

(a) Soil texture : Texture depend upon the size of particles in a soil. On the basis of texture, soils are usually classified as gravel, sand, silt and clay. Clay particles are tiny and sticky in nature, hence holding capacity is highest in clay soil.

(b) Soil structure : The arrangement of particles in a soil is called soil structure. The smaller particles become crowded into spaces between the larger and colloids form coatings over all the larger particles, binding them together into various types of structural units.

(ii) Organic matter : Both plants and animals contribute to the organic matter of the soil. Some of the material is derived from dead roots and soil organisms and is therefore well distributed through the soil from the beginning. On the other hand much organic matter is deposited upon the soil surface as leaves, twigs, etc., and becomes incorporated into the mineral matter only through the activities of micro organisms.

After the normal biological processes of decay, decomposition of litter through the above stages, the resultant production becomes incorporated into the mineral soil imparting a dark colour to it. Such finely divided amorphous organic matter as has become mixed with the mineral materials is called humus and the process leading to its formation humification. Humus usually is homogenous, dark coloured and odourless. Humus and clay the two colloidal components of the soil are called 'colloidal complex' of soil. This complex increases water holding capacity of sandy soil.

(iii) Soil water : The chief source of soil water is rain. In soil water 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 (ψω) 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.

E. 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.

F. 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.

(i) Active absorption : Active absorption takes place by the activity of root itself, particularly root hairs. It utilizes metabolic energy. There are two theories of active absorption :

(a) Osmotic theory : It was proposed by Atkins (1916) and Priestly (1922). It is purely a physical process, which does not directly required 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. Water continues to enter the root hair cell as long as the water potential of the root cell sap is more negative than that of the soil solution, until the elasticity of stretched cell wall is sufficient to balance the osmotic potential or OP of the cell. Water moves from cell to cell along the water potential gradient and reach up to endodermis and pericycle. Finally water enters into the xylem. This type of absorption involves symplast i.e., movement of water occurs through the living cytoplasm of the cells. The cells between the xylem and the soil solution may be considered as a single complex semipermeable membrane.

(b) 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. The exact mechanism of utilization of energy is not well understood. It may be used directly or indirectly.

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

❒ Rate of water absorption is directly proportional to the rate of respiration.

❒ Factors like low temperature, deficiency of oxygen, respiratory inhibitors such as KCN, which inhibit respiration also inhibit the absorption of water.

❒ Auxins, which increase respiration also promote water absorption.

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

❒ The occurrence of distinctive diurnal variation in water uptake and root pressure. It is faster during day time and slower during night. This fact is also true for respiration.

(ii) Passive absorption : It is the most common and rapid method of water absorption. It account 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 cell of roots but in the cells of transpiring shoots. In other words in this type of absorption of water, the roots 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 → Root hair cell wall → Cortex → Endodermis → Pericycle → Protoxylem → 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.

(i) External or Environmental factors

(a) The amount of soil water : If the amount of water in the soil is between its field capacity and permanent wilting percentage, the rate of water absorption remains more or less uniform. But a decrease in the soil water below the permanent wilting percentage causes decrease in the absorption of water. If the soil water is increased much beyond the field capacity, as happens during floods, the air pores between soil particles are filled with water, and water absorption stops.

(b) Concentration of solutes in the soil water : 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 and is different from physical dryness which is caused due to virtual lack of water as in xerophytes.

Water absorption is done more efficiently in well aerated soil. Any deficiency of oxygen stops the respiration of roots and causes accumulation of CO₂thus 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.

(d) Soil temperature : The optimum temperature for maximum rate of water absorption ranges between 20°C and 30°C. Too high a temperature kills the cells. At very low temperatures (4°C) water absorption is reduced or stopped due to

(i) Slower rate of diffusion of water.

(ii) Decreased permeability of cell membrane.

(iii) Increased viscosity of protoplasm.

(iv) Slower rate of metabolism of root cells.

(v) Slower rate of growth and elongation of roots.

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

(ii) Internal factors

(a) Efficiency of the root system : A plant with deep and elaborate root system can absorb more water than one having a shallow and superficial root system because deep roots are always in contact with ground water at different levels. Moreover, 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. These fungal hyphae retain water and make it continuously available to roots.

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

(b) Metabolic activity of roots : The metabolic rate and the rate of water absorption are very closely related. The direct evidence in favour of this comes from the fact that 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.

F. 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'. At mid day hours the xylem sap is in a state of tension because the rate of transpiration is very high.

(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.

❒ 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.

(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.