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  Introduction.                                                                                                                                                           

Plant physiology (Physis = nature of life; logas = study) is the branch of botany which deals with the study of life activities of plants. It include the functional aspects of its processes both at cellular as well as sub-cellular level.

Life process or physiological process may be defined, as any chemical or physiological change occuring within a cell and organism and any exchange of substances between the cell or organism and its environment.

According to the definition of physiological process, imbibition, osmosis, diffusion, plasmolysis, water potential, water conduction, ascent of sap, transpiration, solute absorption and translocation, transport of radiant energy, photomorphogenetic responses, etc. are considered as physiological processes.

 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.

1. Characteristics of imbibition : The phenomenon of imbibition has three important characteristics :

1. 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.

2. 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.
3. 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

1. 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).
2. Surface area of imbibant : If more surface area of the imbibant is exposed and is in contact with liquid, the imbibition will be more.
3. Temperature : Increase in temperature causes an increase in the rate of imbibition.
4. Degree of dryness of imbibant : If the imbibant is dry it will imbibe more water than a relatively wet imbibant.
5. 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.
6. 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

1. The water is first imbibed by walls of root hairs and then absorbed and helps in rupturing of seed coat (made up of cellulose).
2. Water is absorbed by germinating seeds through the process of imbibition.
3. Germinating seeds can break the concrete pavements and roads etc.
4. The water moves into ovules which are ripening into seeds by the process of imbibition.
5. It is very significant property of hydrophilic surfaces.

2. 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.

Or

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.

Or

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.

1. 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.
2. 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 (y) 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

1. Temperature : Increase in temperature leads to increase in the rate of diffusion.
2. 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 µ pressure.
3. Size and mass of diffusing substance : Diffusion of solid is inversely proportional to the size and mass of molecules and ions.

Rate of diffusion µ

1

Size ´ Mass of particles

4. 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 µ 1

(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.

5. 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.
6. 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.

3. Permeability : Permeability is the degree of diffusion of gases, liquids and dissolved substances through a membrane. Different types of membranes may be differentially permeable to different substances. Normally, permeability of a given membrane remains unchanged, but it may change with alteration in the environmental conditions of the cell.
   1. Types of membranes on the basis of permeability

1. Permeable membrane : These membranes allow free passage of solvent (water) and most of the dissolved substances. e.g., cell wall in plant cells. Filter paper is made up of pure cellulose it also functions as permeable membrane.
2. Impermeable membrane : This type of membranes with deposite of waxy substances like cutin and suberin, do not allow the entry of water, dissolved substances and gases. e.g., suberized walls of cork cells, cuticle layer of leaf.
3. Semi-permeable membrane : These membranes permit the movement of solvent molecules only through them, but prevent the movement of solute particles. e.g., egg membrane, animal plasma membrane, parchment membrane, copper ferrocyanide membrane, membranes of collodion.
4. Selectively or Differentially permeable membrane : This type of membranes allow selective passage of solutes along with solvent, through them. e.g., Osmotic diffusion of water through selectively permeable membrane start from higher water potential to lower water potential. Many biological membranes such as cell

membrane (plasmalemma), tonoplast (vacuolar membrane) and the membranes surrounding the sub-cellular organelles are selectively permeable. A non-living selectively permeable membranes is cellophane.

2. Theories of cell permeability : Following theories are given for cell permeability :

1. Sieve theory : Rhouhland and Hoffman described that, small pores are found on membranes.

• The molecules which are small in size than pores of membrane are only passed through these membranes.
• So the molecules of glucose diffused faster as comparatively sucrose (bigger size) molecules.

2. Solubility theory : According to Overtone, formation of membranes take place by fats. Therefore membranes are permeable for those molecules, which are dissolve easily in it.

On the basis of this theory, the permeability of fat insoluble substances like sugar, minerals and amino acids cannot explained.

3. Electro capillary theory : Michaelis, Scorth and Loyad proposed modified theory of sieve theory.

• According to this theory pores found on membranes are surrounded by charged proteins.
• Permeability depends upon the size of charged particles present on pores, size of pore and the charge present on pores.
• So if ionizing substance are smaller than pores, it can pass through membrane.
• In the same way both positive and negative ions pass through the uncharged pores. But positive ions moves through negatively charged pores and negative ions moves through positively charged pores.

4. Carrier concept : According to this concept, movement of substances through membrane required two types of carriers called carrier particle and carrier vesicles.

Carrier particles : These type of particles attached with solutes and forms carrier solute complex. Because it shows chemical affinity to solutes.

• After reaching on inner surface of membrane this carrier, solute complex breakdown.
• Solute enters into the cell and carrier transferred to outer surface.

Carrier vesicles : Wheeler and Hanchey (1971) described that the transportation of substances in higher plants take place by the means of pinosomes.

• Pinosomes originate by infoldings of cells. It shows bulk transportation.

4. 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.

Or

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 :

• 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

5. 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.

Or

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.

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

1. 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.

2. 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

1. 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.
2. 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.

Pin                                                        Pins

1. (B)

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.

3. 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.

1. 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.
2. 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.
3. 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

1. The phenomenon of osmosis is important in the absorption of water by plants.
2. Cell to cell movement of water occurs throughout the plant body due to osmosis.
3. The rigidity of plant organs (i.e., shape and form of organism) is maintained through osmosis.
4. Leaves become turgid and expand due to their OP.
5. Growing points of root remain turgid because of osmosis and are thus, able to penetrate the soil particles.
6. The resistance of plants to drought and frost is brought about by osmotic pressure of their cells.
7. 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.
8. Opening and closing of stomata is affected by osmosis.

5. Osmotic relation of cell : In a plant cell, however, two membranes are present between the cell sap and the surroundings the cell-wall is a permeable membrane that does not interfere with the movement of water and solutes into or out of the cell. The plasma membrane and vacuolar membrane (tonoplast) with the thin layer of

cytoplasm between them behave as differentially permeable membrane. Cell sap of a cell is a mixture of water and soluble substances. Water absorption in root hair from soil is depends on the concentration of cell sap. So a cell behave as a osmotic system in which endosmosis generate following pressures –

1. Turgor pressure (TP) : The plant cell, when placed in pure water, swells but does not burst. Because of

negative osmotic potential of the vacuolar solution (cell sap), water will move into the cell and will cause the plasmalemma be pressed against the cell wall. The actual pressure that develops that is the pressure responsible for pushing the membrane against cell wall is termed turgor pressure.

Osmotic pressure

Cell wall

Plasma membrane

Vacuolar sap

Turgor pressure

In other words, we can say that when water enters the living cell, a pressure is developed within the cell due

Fig : A cell showing turgor pressure, wall pressure and osmotic pressure

to turgidity. The hydrostatic pressure developed inside the cell on the cell wall due to endosmosis is called turgor pressure. It is responsible for growth of young cells.

Significance of turgidity in plants

• It provides stability to a cell.
• Turgidity keeps the cell and their organelles (mitochondria, plastids and microbodies) fully distended. This is essential for plants to live and grow normally.
• Turgor pressure helps in cell enlargement, consequently in stretching of the stems and in keeping leaves erect and fully expanded.
• The turgid cells provide mechanical support necessary for the non woody tissues (maize, sugarcane, banana etc.).
• Loss of turgidity leads to wilting of leaves and drooping of shoots.
• The opening and closing of stomata are regulated by the turgidity of the guard cells.
• Leaf movements (seismonastic movement) of many plants (such as bean, sensitive plant Mimosa pudica) are controlled by loss and gain of cell turgor.
• Due to turgid pressure plumule and radicles force out from seeds at the time of seed germination.
  2. Wall pressure (WP) : Due to turgor pressure, the protoplast of a plant cell will press the cell wall to the outside. The cell wall being elastic, presses back the protoplast with a pressure equal in magnitude but opposite in

direction. This pressure is called wall pressure. Wall pressure (WP) may, therefore, be defined as 'the pressure exerted by the cell wall over the protoplast to counter the turgor pressure. Normally wall pressure is equal and opposite to turgor pressure (WP =TP) except when the cell become flaccid. The value of the two forces continue to rise with the continued entry of water, till the cell becomes fully turgid.

20

Osmotic concentration

10 Diffusion pressure deficit

Turgor pressure

Cell fully turgid (water-saturated)

1.0                1.2               1.4

Relative volume of cell

9            Fig : Relationship between diffusion pressure deficit

and other pressure

6. Interrelationship of DPD, OP and TP (WP) : DPD indicates the sucking power of suction pressure. As water enters into the cell the TP of the cell is increased. Cell wall exerts equal and opposite WP against TP. The actual force responsible for entry of water will be therefore OP–TP

i.e., DPD = OP – WP (As WP = TP) DPD = OP – TP

Consider that a plant cell with OP = 10 atm. is immersed in pure water. In the beginning TP inside the cell is zero i.e.

DPD = OP = 10 atm.

When water enters into the cell, TP increases. Turgidity increases and cell wall develops equal and opposite WP. At the stage of equilibrium TP = 10 atm. and DPD will become zero. It is important to note that OP was same when cell was flaccid and turgid.

DPD = OP – TP

= 10 – 0 = 10 (when flaccid)

= 10 – 10 =0 (when turgid)

The entry of water in cell to cell depends up on the DPD and not on OP and TP. This can be examplified as follows :

1. (B)

Fig : Relation between diffusion pressure deficit and entrance of water in the cell

A cell (A) with OP = 8 and TP = 4 is surrounded by the cells (B) with OP = 10 and TP = 8. Then for cell A, DPD = OP – TP

= 8 – 4

= 4

Similarly for cell B, DPD = OP – TP

= 10 – 8

= 2

Since the DPD of cell A is more, it has less water and, therefore water would diffuse from cell B into the cell A (because that DPD of cell B is less than that of A or it has more water than cell A). The entry of water into the cell A would stop when DPD of both the cells becomes equal. In this way water moves from a cell with less DPD into the cell with more DPD. Thus, DPD is the osmotic parameter, which determines the flow of water from one cell to another.

Under given suitable conditions, the DPD more than OP when TP is negative. DPD of a cell mainly depends upon OP. If two cells have the same OP but differ in TP, the direction of the movement of water from higher TP to lower TP.

7. Plasmolysis (Gr. Plasma = something formed; lysis = loosing) : If a living plant cell is placed in a highly concentrated solution (i.e. hypertonic solution), water comes out of the cell due to exosmosis, through the

plasmamembrane. The loss of water from the cell sap causes shrinkage of the protoplast away from the cell wall in the form of a round mass in the centre. "The shrinkage of the protoplast of a living cell from its cell wall due to exosmosis under the influence of a hypertonic solution is called plasmolysis". The stage of plasmolysis, when the protoplast just begins to contract away from the cell wall is called incipient plasmolysis. The stage when the cell wall has reached its limit of contraction and the protoplast has detached from cell wall attaining spherical shape is called evident plasmolysis. In a plasmolysed cell, the space between the contracted protoplast and the cell wall remains filled with external solution. If a cell with incipient plasmolysis is placed in a hypertonic solution it will show more plasmolysis.

If a plasmolysed cell is placed in pure water or hypotonic solution, endosmosis takes place. The protoplast attains its original shape and the cell regains its original size. "The swelling up of a plasmolysed protoplast due to endosmosis under the influence of a hypotonic solution or water is called deplasmolysis'. Deplasmolysis is possible only immediately after plasmolysis otherwise the cell protoplast becomes permanently damaged. Leaf of Tradescantia is used for demonstration of plasmolysis in laboratory. The value of TP becomes zero at the time of limiting plasmolysis and below zero during incipient and evident plasmolysis.

Chloroplast

Elastic force of cell wall

Turgor pressure

H₂O

H₂O

Nucleus

(A)

Vacuole filled with cell sap

Cell placed in strong salt solution

Result : Plasmolysis (B)

Cell placed in pure water Result : Increased turgor pressure

(C)

Fig : Plasmolysis and deplasmolysis (A) Normal cell (B) Plasmolysed cell

(C) Deplasmolysed cell and increased turgor pressure

Significance of plasmolysis : It proves the permeability of the cell wall and the semipermeable nature of the protoplasm.

• The OP of a cell can be measured by plasmolysis. The OP of a cell is roughly equal to the OP of a solution that causes incipient plasmolysis in the cell.
• Salting of pickles, meat, fishes etc. and addition of sugar to jams, jellies, cut fruits etc., prevent their decay by microbes, as the latter get killed due to plasmolysis or due to high concentration of salt or sugar.
• By salting, the weeds can be killed from tennis courts and the growth of plants can be prevented in the cracks of walls.
• Plasmolysis is helpful in determining whether a particlular cell is living or dead as plasmolysis does not occur in a dead or non living cell.

8. Water potential (y) : The movement of water in plants cannot be accurately explained in terms of difference in concentration or in any other linear expression. The best way to express spontaneous movement of

water from one region to another is in terms of the difference of free energy of water between two regions. Free energy is the thermodynamic parameter, that determine the direction in which physical and chemical changes must occur. The potential energy of water is called water potential. e.g., water is stored behind a dam. When the water runs downhill, its potential energy can be converted to electrical energy. This conversion of energy of water is due to gravity. The other source that provides energy to water is pressure. The increasing pressure increases the free energy there by increasing water potential.

Water running downhill due to gravity can be made to run uphill by overcomming the water potential (energy) by applying pressure. This means that water moves from the point, where water potential is greater to the other, where water potential is less. The difference in water potential between two points is a measure of the amount of work or energy needed to move water from one point to the other. Thus, based on the concept of water potential, the direction of water movement can be predicted. Water potential is measured in terms of pressure.

Measurement unit of water potential is pascal, Pa (1 mega pascal, Mpa = 10 bars). It is represented by Greek letter, Psi (y). Water potential y_w is the difference between chemical potential of water at any point in a system (mw) and that of pure water under standard conditions (mw°). The value of water potential can be calculated by formula :

y_w = (mw) – (mw°) = RT 1 n e/e°

where y_w = water potential, R is gas constant, T is absolute temperature (K), e is the vapour pressure of the solution in the system at temperature T, and e° the vapour pressure of pure water at the same temperature.

The direction in which water will move from one cell to another cell depends on water potential in two regions.

Water potential is measured in bars. A bar is a pressure unit which equals 14.5 lb/in², 750 mm Hg or 0.987 atm.

Water potential of pure water at normal temperature and pressure is zero. This value is considered to be the highest. The presence of solute particles reduces the free energy of water and thus decreases the water potential. Therefore, water potential of a solution is always less than zero or has negative value. External pressure increases the water potential. If a pressure greater than atmospheric pressure is applied to pure water, the water potential can be raised from zero to a positive value. The water potential is equal but opposite in sign to the diffusion pressure deficit (DPD). In terms of DPD, the movement of water takes place from the region of lower DPD to the region of higher DPD, while in terms of water potential (y), the flow of water occurs from the region of higher water potential (less negative) to the region of lower water potential (more negative). The movement of water continue till the water potential in two regions becomes equal.

1. Component of water potential : When a cell is subjected to the movement of water, many factors begin to operate which ultimately determine the water potential of cell sap. For solutions, such as contents of cells, water potential is determined by three major sets of internal factors viz., matric potential (y m), solute potential (y s) and pressure potential (y p). The water potential (y) in a plant cell or tissue can be written as the sum of the matric potential (y m) due to binding of water to cell walls and cytoplasm, the solute potential (y s) due to concentration of dissolved solutes, which by its effect on the entropy components reduces the water potential and the pressure potential (y p) due to hydrostatic pressure, which by its effect on the energy components increases the water potential :

y = y m + y s + y p …… (1) Each component potential is discussed separately below :

1. Matric potential (y m) : Matric is the term used for the surface (such as, soil particles, cell walls, protoplasms, etc.) to which water molecules are adsorbed. The matric potential (y m) is the component of water

potential influenced by the presence of a matrix. It has got a negative value. In case of plant cells and tissues, the matric potential is often disregarded because it is not significant in osmosis. Thus, the above equation (1) may be simplified as follows :                 y = y s + y p …… (2)

In normal cells of mesophytes and hydrophytes it is almost negligible due to presence of large vacuole which leaves little space for matrix in the cell. In herbaceous plants it has been calculate to be only – 0.1 bar by Wiebe (1966). Its value, however, is quite high (–100 to –200 bars) in xeropytes and dryseeds.

2. Solute potential (y s) : Solute potential is also known as Osmotic potential. It is defined as the amount by which the water potential is reduced as a result of the presence of solute. Solute potentials or osmotic potentials (y s) are always in negative values (number). The term solute potential takes the place of osmotic pressure (p; Pi) expressed in bars with a negative sign.

y_s = –p

3. Pressure potential (y p) : Plant cell wall is elastic and it exerts a pressure on the cellular contents. As a result of inward wall pressure, hydrostatic pressure is developed in the vacuole termed as turgor pressure. The pressure potential is usually positive and operates in plant cells as wall pressure and turgor pressure.

Its magnitude varies between +5 bars (during day) and +15 bars (during night).

2. Physical states of cell : Three physical states of cell, according to their water potential, are as follows :

1. In case of fully turgid cell : In case of fully turgid cell, the net movement of water into the cell is stopped. The cell is in equilibrium with the water outside. The water potential in such a case will be zero (0).

Water potential = Osmotic potential + Pressure potential

y = y s + y p

A cell at full turgor has its osmotic potential and pressure potential equal but opposite in sign. Therefore, its water potential will be zero. For example, supposing a cell has its y s of –10 bars and y p of 10 bars the resultant water potential will be zero as follows :

y = y s + y p

y = –10 bars + 10 bars

y = 0 bars

2. In case of flaccid cell : When a plant cell is flaccid, its turgor becomes zero (corresponding to a turgor pressure of a 0 bars). Zero turgor is approached under natural conditions when a tissue is severely wilted. A cell at zero turgor has an osmotic