Chapter 17 Transport in Plants by Teaching Care online coaching classes
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.
- 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 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.
- Characteristics of imbibition : The phenomenon of imbibition has three important characteristics :
- 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
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.
- 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). g., during kneading, the flour of wheat gives a warm feeling due to imbibition of water and consequent release of heat.
- 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
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
- Nature of imbibant : Proteins are the strongest imbibants of water, starch less strong, cellulose being the 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).
- Surface area of imbibant : If more surface area of the imbibant is exposed and is in contact with liquid, the imbibition will be more.
- Temperature : Increase in temperature causes an increase in the rate of imbibition.
- Degree of dryness of imbibant : If the imbibant is dry it will imbibe more water than a relatively wet
- 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 If some solute is added into the liquid which is being imbibed, its diffusion pressure decreases and the process of imbibition slows down.
- pH of imbibant : Proteins, being amphoteric in nature, imbibe least in neutral 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
- The water is first imbibed by walls of root hairs and then absorbed and helps in rupturing of seed coat (made up of cellulose).
- Water is absorbed by germinating seeds through the process of
- Germinating seeds can break the concrete pavements and roads
- The water moves into ovules which are ripening into seeds by the process of
- It is very significant property of hydrophilic
- 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 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.
- 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 Alternatively, it may also be defined as the force with which the diffusing molecules move along the concentration gradient.
- 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
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
- Temperature : Increase in temperature leads to increase in the rate of
- Pressure : The rate of diffusion of gases is directly proportional to the So the rate of diffusion increases with increase of pressure. Rate of diffusions µ pressure.
- Size and mass of diffusing substance : Diffusion of solid is inversely proportional to the size and mass of molecules and
Rate of diffusion µ
Size ´ Mass of particles
- 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.
- 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 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.
- 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
(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
- During passive salt uptake, the ions are absorbed by process of
- Diffusion helps in translocation of food
- Gaseous exchange in submerged hydrophytes is takes place by general surface of the cells through diffusion.
- Permeability : Permeability is the degree of diffusion of gases, liquids and dissolved substances through a 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.
- Types of membranes on the basis of permeability
- Permeable membrane : These membranes allow free passage of solvent (water) and most of the dissolved substances. g., cell wall in plant cells. Filter paper is made up of pure cellulose it also functions as permeable membrane.
- 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 e.g., suberized walls of cork cells, cuticle layer of leaf.
- Semi-permeable membrane : These membranes permit the movement of solvent molecules only through them, but prevent the movement of solute particles. g., egg membrane, animal plasma membrane, parchment membrane, copper ferrocyanide membrane, membranes of collodion.
- Selectively or Differentially permeable membrane : This type of membranes allow selective passage of solutes along with solvent, through them. 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.
- Theories of cell permeability : Following theories are given for cell permeability :
- Sieve theory : Rhouhland and Hoffman described that, small pores are found on
- The molecules which are small in size than pores of membrane are only passed through these
- So the molecules of glucose diffused faster as comparatively sucrose (bigger size)
- Solubility theory : According to Overtone, formation of membranes take place by 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.
- Electro capillary theory : Michaelis, Scorth and Loyad proposed modified theory of sieve
- According to this theory pores found on membranes are surrounded by charged
- Permeability depends upon the size of charged particles present on pores, size of pore and the charge present on
- So if ionizing substance are smaller than pores, it can pass through
- 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
- 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
- Solute enters into the cell and carrier transferred to outer
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 It shows bulk transportation.
- 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 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
|(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 membrane.||not||require||a||semi-permeable||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.|
- Osmotic pressure (OP) : Pfeffer coined the term osmotic
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.
- Types of osmosis : Depending upon the movement of water into or outward of the cell, osmosis is of two
- 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 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
- 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. g., shrinkage of grapes, when they are placed in strong sugar solution.
(ii) Demonstration of osmosis
- 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 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.
- Demonstration of osmosis by potato osmoscope : Peel of the skin of large sized potato with the help of 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.
- 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
- 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
- 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
- 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 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
- In xerophytes, the osmotic concentration of cell sap is more than normal. 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
- The phenomenon of osmosis is important in the absorption of water by plants.
- Cell to cell movement of water occurs throughout the plant body due to osmosis.
- The rigidity of plant organs (e., shape and form of organism) is maintained through osmosis.
- Leaves become turgid and expand due to their
- Growing points of root remain turgid because of osmosis and are thus, able to penetrate the soil
- The resistance of plants to drought and frost is brought about by osmotic pressure of their
- Movement of plants and plant parts, for example, movement of leaflets of Indian telegraph plant, bursting of many fruits and sporangia, occur due to osmosis.
- Opening and closing of stomata is affected by osmosis.
- 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 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 –
- 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.
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
- Turgidity keeps the cell and their organelles (mitochondria, plastids and microbodies) fully 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
- The turgid cells provide mechanical support necessary for the non woody tissues (maize, sugarcane, banana ).
- Loss of turgidity leads to wilting of leaves and drooping of
- The opening and closing of stomata are regulated by the turgidity of the guard
- Leaf movements (seismonastic movement) of many plants (such as bean, sensitive plant Mimosa pudica) are controlled by loss and gain of cell
- Due to turgid pressure plumule and radicles force out from seeds at the time of seed
- Wall pressure (WP) : Due to turgor pressure, the protoplast of a plant cell will press the cell wall to the 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.
10 Diffusion pressure deficit
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
- Interrelationship of DPD, OP and TP (WP) : DPD indicates the sucking power of suction 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 :
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
Similarly for cell B, DPD = OP – TP
= 10 – 8
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.
- Plasmolysis (Gr. Plasma = something formed; lysis = loosing) : If a living plant cell is placed in a highly concentrated solution (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.
Elastic force of cell wall
Vacuole filled with cell sap
Cell placed in strong salt solution
Result : Plasmolysis (B)
Cell placed in pure water Result : Increased turgor pressure
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 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 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
- Plasmolysis is helpful in determining whether a particlular cell is living or dead as plasmolysis does not occur in a dead or non living
- Water potential (y) : The movement of water in plants cannot be accurately explained in terms of difference in concentration or in any other linear 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 yw 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 :
yw = (mw) – (mw°) = RT 1 n e/e°
where yw = 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.
- 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 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 :
- Matric potential (y m) : Matric is the term used for the surface (such as, soil particles, cell walls, protoplasms, ) 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.
- 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
ys = –p
- Pressure potential (y p) : Plant cell wall is elastic and it exerts a pressure on the cellular 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).
- Physical states of cell : Three physical states of cell, according to their water potential, are as follows :
- In case of fully turgid cell : In case of fully turgid cell, the net movement of water into the cell is 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
- 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 potential (y s) equal to its water potential (y). For example, supposing a flaccid cell has an osmotic potential of –10 bars and pressure potential (y p) of 0
Water potential = Osmotic potential + Pressure potential
y = y s + y p
y = –10 bars + 0 bars
y = –10 bars
The water potential of the cell will be –10 bars, which is less as compared to the water potential of pure water (0 bars).
- In case of plasmolysed cell : When the vacuolated parenchymatous cells are placed in solutions of sufficient strength the protoplast decreases in volume to such an extent that they shrink away from the cell The cells are plasmolysed. Such cells have negative value of pressure potential (negative turgor pressure). The resultant water potential will be more negative, as for example, a plasmolysed cell has osmotic potential of –10 bars and pressure potential of –2 bars the water potential of the cell will be –12 bars.
Water potential = Osmotic potential + Pressure potential
y = y s + y p
y = –10 + (–2)
y = –12 bars
- Movement of water between two adjacent cells : Suppose A and B are two adjacent plant cells where osmotic movement of water can occur. Cell A has osmotic potential (y s) of –16 bars and pressure potential of 8 bars. The cell B has osmotic potential of –12 bars and pressure potential of 2 bars. The movement of water will be as follows :
|Cell A||Cell B|
|y s = –16
y p = 8
|y s = –12
y p = 2
|y = y s + y p
= –16+8 = –8.
y = y s + y p
= –12+2 = –10.
Cell A has osmotic potential of –16 bars and pressure potential of 8 bars. The water potential will be –8 bars.
y = y s + y p = – 16 + 8 = –8
Cell B has osmotic potential of – 12 bars and pressure potential of 2 bars. The water potential will be –10 bars.
y = y s + y p = – 12 + 2 = –10
Since water moves from higher water potential to lower water potential, the flow of water will be from cell A (–8 bars) to cell B (–10 bars).
Differences between diffusion pressure deficit and water potential
|S.No.||Diffusion Pressure Deficit (DPD)||Water Potential (y)|
|(1)||The DPD was originally described by the term suction force (Saugkraft) by Renner. Other synonyms of the term are suction pressure (SP), enter tendency (E) and osmotic equivalent (E).||Water potential is the chemical potential of water which is equivalent to DPD with negative sign. The term water potential was coined by Slatyer and Taylor (1960).|
|(2)||The diffusion pressure deficit is abbreviated as DPD. The term was coined by Meyer (1938).||The symbol for water potential is a Greek letter psi, which is designated as y.|
- Wilting : A plant usually fails to survive if it is conditioned to water deficiency. The symptoms appear in the plant, plant part or in the cells due to scarcity of water are termed as It is loss of turgidity causing folding and drooping of leaves and other soft aerial parts of the plant. It is of three types :
- Incipient wilting : There is no external symptoms but the mesophyll cells lose a part of their water content during midday due to
- Temporary wilting : It occurs during midday and is visible externally due to drooping of leaves and young At noon the rate of transpirations is quite high as compared to water absorption. Which decreases further due to depletion of water around rootlets. It is corrected in the afternoon when transpiration decreases.
- Permanent wilting : It is the last stage in wilting when the aerial parts do not regain turgidity even if placed in water saturated It is caused by decrease in water content of the soil which increases TSMS (Total soil moisture stress) or resistance to absorption to such an extent that plant roots are unable to absorb water. Permanent Wilting Percentage (PWP) is the percentage of water on the dry weight basis of the soil that is present in the soil when the plants growing in it first touch the condition of permanent wilting. This value varies between 1–15% and depends upon the texture of the soil e.g., clay has higher PWP than sand.
- Stephen Hales is known as father of plant Coined the term root pressure.
- The kinetic energy or free energy possessed by the molecules of a substance is called chemical The chemical potential of water is called water potential.
- The osmotic pressure of a solution can be measured with the help of a apparatus called osmometer.
- Molar solution : 1gm mole of solute plus 1 litre / 1000 cc of solution.
- Molal solution : 1gm mole of solute plus 1 litre / 1000 cc of
- 1M solution containing any solute has water potential of –2.3 bars.
- The value of water potential of solution is lower than that of
- In a pure solvent the value of osmotic potential is zero.
- Plants wilt when turgor pressure inside the cells of such tissue go down below
- Pressure chamber is used for measuring water potential of whole leaves, shoots etc.
- The value of osmotic pressure of the soil solution in a well watered soil is less than 1
- The basic driving force in osmosis is the difference in the free energy of water on the two sides of
- Cryoscopic osmometer measures the osmotic potential of solution by measuring its freezing
- Equimolar concentration of two solution of non-ionising substances will have same osmotic
- In evident plasmolysis, cytoplasm withdraws itself from cell
- P. does not increase by addition of insoluble solute in the solution.
- P.D. can become zero (fully turgid cell). T.P. can also become zero (flaccid cell). However O.P. of a cell can never be zero.
- Plant imbibants. Agar agar imbibes maximum –99 times its weight of water. Other pectic compounds also possess good imbibition capacity. They are followed by proteins (some of them 15 times their volume), starch and Seeds swell more if they are protein rich than starchy seeds. During seed germination the seed coat ruptures because protein/starch rich kernel swells up more than cellulose seed coat.
- Wilting in plant occur when xylem is blocked.
- Osmotic pressure/Osmotic potential : It is measured in atmospheres or bars. Osmotic pressure has a positive value while osmotic potential has a negative 1gm mole of a nonelectrolyte develops an osmotic potential of –22.4 atm or osmotic pressure of 22.4 atm (or 22.7 bars) at 0°C and 24 atm (= 24.3 bars) at 20°C. It increases due to hydration of solutes and ionisation of solute particles.
- Young cells and young fruits absorb a part of their water through
- In hypertonic solution a cell water potential is decrease.
- Under given suitable conditions, the DPD will be more than OP when TP is negative.
- In the process of osmosis in the cell only outer layer of protoplasm act as a membrane.
- Heating kills the cell membrane which lose their selective or differential permeability and become freely permeable to solute as well as
- Wilting of leaves in hot weather is due to excess of transpiration as compared to water absorption.
- Surface tension doesn’t help in molecule transport.
Absorption of water.
- Component of soil : Soil is the superficial layer of the weathered earth crust, which support plant 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
- 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 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.
- 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.
- 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
- 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 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.
- 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 :
- 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 e., ground water, upper surface of which is called water table.
- 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 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.
- Hygroscopic water : This is the form of water which is held by soil particles of soil surfaces. The water is held tightly around the soil particles due to cohesive and adhesive forces. Hygroscopic water cannot be easily removed by the plants. Cohesive and adhesive forces greatly reduce the water protential (yw) and thus this type of water in soil is not available to
- 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 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.
- Chemically combined water : Some of the water molecules are chemically combined with soil minerals (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.
- 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
- Soil organisms : The soil fauna include protozoa, nematodes, mites, insects, earthworms, rats. Protozoons alone are approximately 1 million per gram of Earthworms have the most important effect on the soil structure. Their activities result in a general loosening of the soil which facilitates both aeration and distribution of water. Blue green algae and bacteria increases nitrogen content by nitrogen fixation in soil.
- Water absorbing organs : Plants absorb water mostly from the soil by their roots, but in some plants even aerial parts like stem and leaves also do the absorption of atmospheric water or 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
Zone of cell
Zone of cell elongation
Zone of cell formation
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.
Fig : (A) Tip of a young root of a dicot plant showing different zones and regions of absorption of water and solution (B) Some cells of meristemetic zone showing mitosis
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.
- Apoplast pathway : The apoplastic movement of water occurs exclusively through the cell wall without crossing any
- Symplast pathway : The symplastic movement of water occurs from cell to cell through the
Cytoplasm Apoplast pathway
Symplast pathway Vacuolar pathway
- Transmembrane pathway : Water after passing through cortex is blocked by casparian strips present on 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 mechanism which are independently operate in the absorption of water in These mechanisms are :
(i) Active absorption (ii) Passive absorption
Renner coined the term active and passive water absorption.
- 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 :
- 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.
- 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 (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
- 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 It is faster during day time and slower during night. This fact is also true for respiration.
- Passive absorption : It is the most common and rapid method of water 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
Phloem Passage cell
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 :
Fig : Passive absorption of water through root hair
Soil ® Root hair cell wall ® Cortex ® Endodermis ® Pericycle ® Protoxylem ® Metaxylem.
Differences between active and passive absorption of water
|S.No.||Active absorption||Passive absorption|
|(1)||Force for absorption of water is generated in the cells of root itself.||Force for absorption of water is created in the mesophyll cells.|
|(2)||Osmotic and non-osmotic forces are involved in water absorption.||Water is absorbed due to transpiration pull.|
|(3)||Water is absorbed according to DPD changes.||Water is absorbed due to tension created in xylem sap by transpiration pull.|
|(4)||Water moves through symplast.||Water moves mainly through apoplast.|
|(5)||Rate of absorption is not affected significantly by temperature and humidity.||Its rate is significantly affected by all those factors which influence the rate of transpiration.|
|(6)||Metabolic inhibitors if applied in root cells decrease the rate of water absorption.||No effect of metabolic inhibitors if applied in root cells.|
|(7)||Occurs in slow transpiring plants which are well watered.||Occurs in rapidly transpiring plants.|
|(8)||Rate of absorption is slow.||Very fast rate of water absorption.|
- 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
(i) External or Environmental factors
- 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 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.
- 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 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.
- 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 a temperature kills the At very low temperatures (4°C) water absorption is reduced or stopped due to
- Slower rate of diffusion of
- Decreased permeability of cell
- Increased viscosity of
- Slower rate of metabolism of root
- Slower rate of growth and elongation of
- 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
- 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
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.
- Metabolic activity of roots : The metabolic rate and the rate of water absorption are very closely 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.
- 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. The turgidity of wilted leaves of many species can be restorted by immersing them in water : 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
- Tensiometer is the instrument for measuring soil water potential.
- Apoplast is the non-living continuity of plant body which consists of cell walls, intercellular spaces and water filled xylem
- Symplast pathway consists of the entire network of cell cytoplasm interconnected by
- Auxin treated cells can absorb water even from hypertonic solution by active process.
- Only capillary water is available to
- The amount of water left in the soil after the plant has permanently wilted is the wilted coefficient.
- Humus and clay are two colloidal complexes of
- All organic plant debris which has recently fallen on soil is called litter or A00
- The organic matter is colloidal, due to this water holding capacity is relatively
- Root system in a plant is well developed for increased absorption of
- Cell absorb water by osmosis and imbibition.
- Active uptake of water is affected by sucking power (DPD) of root hairs.
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.
- Path of ascent of sap : It is now well established that the ascent of sap takes place through 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 :
- Vital force theories
- Root pressure theory
- Physical force theories
- 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 Some of the important vital force theories are mentioned below :
- According to Westermeir (1883), ascent of sap occurred through xylem parenchyma; tracheids, and vessels only acted as water
- 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 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.
- Strasburger (1891) and Overton (1911) used poisons (like picric acid) and excessive heat to kill the living cells of the 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
- Pulsation theory : Sir 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.
Fig : Diagrammatic representation of
electricprobe experiment of J.C. Bose.
- Electric probe (B) Plant (C) Galvanometer
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).
- 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 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
Fig : Demonstration of root pressure
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 :
- Living cells are essential in root for the root pressure to
- Oxygen supply and some metabolic inhibitors affect the root pressure without affecting the semipermeability of membrane
- 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
Objections : Root pressure theory for ascent of sap has following limitations :
- Taller plants like Eucalyptus need higher pressure to raise the water 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
- 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 :
- 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 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
- Atmospheric pressure can support a column of water only up to the height of 34
- Water can rise only up to the height of one metre in xylem vessels having diameter of 03mm.
- 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
- 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 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.
- 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 :
- 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
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.
- Due to the transpiration from leaves, a great water deficit takes place in its 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.
- 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 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
Medullary ray and cortex
Root cortex Root hair Osmosis Imbibition
there are some objections against it which it fails to explain.
The most important objection is that leaving smaller plants, the
Fig : Path of ascent of sap showing transpiration pull
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 It is slowest in gymnosperms.
- Root pressure is absent in
- Imbibitional force in pea is 1000
- Cohesive force is called as tensile strength of water.
- In soft stem, the ascent of sap can be prevented by applying squeezing pressure which closes the lumen of xylem
- Overlapping cuts are given by Preston (1958) in stem in order to break continuity of xylem However, ascent of sap continued.
- Manometer (Gk. manos – thin, metron – measure). An instrument for measuring pressure of tension (such as root pressure) in gases and
- Cohesive strength of 47-207 in xylem sap is sufficient to meet the stress of transpiration pull, so that water column does not break.
- Adhesion : The attraction between the molecules of dissimilar
- Cohesion : The attraction between the molecules of the same substances.
- Osmotic pressure is maximum in At this time water contents in the cell are minimum.
- In night, root pressure will be maximum because in night transpiration is
- Presence of pulsation in the cortical cell was demonstrated by electric probe.
- Pressure bomb technique was used by Scholander et al.
Land plants absorb a large quantities of water from the soil, but only a very small fraction of water utilized in various metabolic activities by the plants. The rest amount of it, evaporates from the stem and leaves. About 98 percent of the water absorbed by land plants evaporates from the aerial parts and diffuses into the atmosphere. “The loss of water in the form of vapour from the aerial parts of a plant is called transpiration“. Maximum transpiration occurs in mesophytic plants.
Basically it is an evaporation phenomenon but it differs from the general process of evaporation. Evaporation is referred to the loss of water vapours from a free surface, whereas in case of transpiration of water passes through the epidermis with its cuticle or through the stomata. Transpiration maintained the atmospheric temperature.
Differences between transpiration and evaporation
|(1)||It is a physiological process and occurs in plants.||It is a physical process and occurs on any free surface.|
|(2)||The water moves through the epidermis with its cuticle or through the stomata.||Any liquid can evaporate. The living epidermis and stomata are not involved.|
|(3)||Living cells are involved.||It can occur from both living and non-living surfaces.|
|(4)||Various forces (such as vapour pressure, diffusion pressure, osmotic pressure, etc) are involved.||Not much forces are involved.|
|(5)||It provides the surface of leaf and young stem wet and protects from sun burning.||It causes dryness of the free surface.|
- Magnitude of transpiration : As far as the magnitude of transpiration is concerned, Meyer (1956) had reported that some of the herbaceous plants, under favourable conditions, transpire the entire volume of water which a plant has and it is replaced within a single A tropical palm under well watered conditions may lose as much as 500 litres of water per day. Daily loss of water by an apple tree may be 10-20 litres. A maize plant may lose 3-4 litres of water per day.
Crotolaria juncea evaporates 27 kg of water in its life cycle of 140 days and Helianthus annuus about 56 kg of water. According to estimates at least 1000 gallons of water are lost every month during summer by a 9-10.5 metres
high tree. A birch tree with 200,000 leaves evaporates 300-400 kg of water on a hot day, whereas a 15 years old beech tree evaporates 75 kg water per day during summer. A beech forest of 400-600 trees evaporates some 20,000 barrels of water per day.
- Types of transpiration : Transpiration occurs from all aerial parts of a plant and water is stores in large amount of leaves. However, most of the transpiration takes place through the leaves. It is called foliar transpiration. Stems transpire very little. Transpiration from stem is called cauline transpiration. Depending upon the plant surface involved, transpiration is of three types – cuticular, lenticular and stomatal
- Cuticular transpiration : Cuticle is a layer of wax like covering on the epidermis of leaves and herbaceous stems. It provides a relatively impermeable covering. If it is thin, upto 20 percent of the total transpiration may take place through it, but with the increase in its thickness (g., in xerophytes), the water vapour loss is reduced.
- Lenticular transpiration : Lenticles are the areas in the bark of woody plants which are filled with loosely arranged cells known as complementry
Loss of water vapour through lenticels is called lenticular transpiration. It amounts to about 0.1 percent of the total water loss through transpiration.
- Stomatal transpiration : Stomata are minute pores in the epidermis of leaves, young green The loss of water vapour, which occurs through stomata is called stomatal transpiration. It amounts 80- 90 percent of the total water vapour loss from the plants. It is the most common type of transpiration.
High water vapour content
Stomatal Low water resistance
Arora and Lamba (1982) have reported the presence of stomata on fruit wall of Raphanus sativus var. caudatus and Brassica oleracea var. botrytis.
Fig : Water movement through the leaf to the atmosphere
in the form of vapour
- Structure of stomata : Stomata are the microscopic openings most commonly found in the leaves. These may be present in young stems and sometimes even in fruits (g., citrus, banana, cucumber, etc.). Each stomatal opening is surrounded by two specialised epidermal cells, called as the guard cells.
Because of their small size, the guard cells are rapidly influenced by turgor change and thus regulate the opening and closing of stomata. The guard cells of dicot leaves are kidney-shaped or raniform whereas those of
monocots (family Gramineae) are dumbel-shaped or elliptical. The guard cells are surrounded by epidermal cells called as the accessory cells or subsidiary cells. These are different from the normal cells of epidermis having chloroplasts. The stoma with subsidiary cells is called stomatal apparatus. Each stoma leads into a air space called sub
Fig : Stomatal apparatus (A) Closed (B) Open
stomatal cavity. Both kidney shaped and dumbel-shaped guard cells have been reported in Cyperus. Each guard cell has a thin layer of cytoplasm along the cell wall and a large vacuole. Its cytoplasm contains a distinct nucleus and several chloroplasts. The cell wall of guard cells around the stomatal pores are thickened and inelastic due to presence of a secondary layer of cellulose. Here the cellulose microfibrils are radially arranged and they radiate away from the pore. Rest of the wall is thin, elastic and semipermeable.
The size of stomatal pore varies from species to species – for example of fully opened stomatal pore of Zea mays measures 26mm long and 4mm wide, whereas in Phaseolus it measures 7 × 3mm. The average length of stomata is 20 to 28mm and breadth 3-10mm.
- Number of stomata on leaves : The number of stomata is not equal on both surface of leaves in different
|Name of the plant||Number of stomata/mm2|
|Upper surface||Lower surface|
- Types of stomata : On the basis of orientation of subsidiary cells around the guard cells, Metcalfe and
Chalk classified stomata into following types :
- Anomocytic : The guard cells are surrounded by a limited number of unspecialised subsidiary cells which appear similar to other epidermal e.g., in Ranunculaceae family.
- Anisocytic : The guard cells are surrounded by three subsidiary cells, two of which are large and one is very e.g., in Solanaceae and Cruciferae families.
- Paracytic : The guard cells are surrounded by only two subsidiary cells lying parallel to the guard cells
e.g., Magnoliaceae family.
- Diacytic : The guard cells are surrounded by only two subsidiary cells lying at right angles to the longitudinal axis of the guard e.g., Acanthaceae and Labiatae families.
- Actinocytic : The guard cells are surrounded by four or more subsidiary cells and which are elongated radially to
- Distribution of stomata : The stomata differ in their distribution on the two surfaces of the The leaves are classified into following types on the basis of stomatal distribution on them :
- Epistomatic (Water Lily type) : Stomata are present only on the upper epidermis of leaves. These are found in water Lily, Nymphaea and many other floating
- Hypostomatic (Apple or Mulberry type) : Stomata are present only on the lower surface of leaves.
e.g., Apple, mulberry, peach and walnut.
- Amphistomatic : Stomata are present on both the surfaces of It can further be subdivided into two types :
- Anisostomatic (Potato type) : The number of stomata is more on the lower surface and less on the upper surface. In other words, the lower surface is multistomatic and the upper surface is Such leaves are also called as dorsiventral leaves. e.g., Potato, tomato, bean, pea, and cabbage.
- Isostomatic (Oat type) : The stomata are equally distributed on both the surfaces of leaves. These leaves are also called as isobilateral These are found in monocots e.g., Oat, maize, grasses, etc.
- Astomatic (Potamogeton type) : Stomata are either absent altogether or e.g., Potamogeton
and submerged hydrophytes.
- Daily periodicity of stomatal movement : Loftfield (1921) classified the stomata into four types, depending upon the periods of opening and
- Alfalfa type (Lucerne type) : The stomata remain open throughout the day but close during night, g., Pea, bean, mustard, cucumber, sunflower, radish, turnip, apple, grape.
- Potato type : The stomata close only for a few hours in the evening, otherwise they remain open throughout the day and night g., Cucurbita, Allium, Cabbage, Tulip, Banana etc.
- Barley type : These stomata open only for a few hours in the day time, otherwise they remain closed throughout the day and night, g., Cereals.
- Equisetum type : The stomata remain always open through out the day and night. g., Amphibious plants or emergent hydrophytes.
- Mechanism of opening and closing of stomata : Opening and closing of stomata occurs due to turgor changes in guard cells. Due to endosmosis, an increase in turgor of guard cells takes place which finally results in stretching and bulging out of their outer thin This results in the pulling apart of the opposed inner thicker walls creating an opening or pore in guard cells of stomata. When the turgor pressure of guard cells decreases, inner walls sag, leading to closure of space between them. This is due to the loss of water (exosmosis) from guard cells, resulting in thicker walls to move closer and finally shut the opening. The transpiration is regulated by the movement of guard cells of stomata.
Several theories have been put forth to explain the opening and closure of stomata. Which have been discussed below :
- Photosynthetic theory : According to Von Mohl (1856) the chloroplasts present in guard cells prepare osmotically active substances by photosynthesis. As a result, their osmotic pressure increases and their turgor pressure increases due to This results in opening of stomata.
This theory is not accepted because in many cases, chloroplasts of guard cells are poorly developed and incapable of performing photosynthesis.
- Starch ⇄ sugar interconversion theory : According to Lloyd (1908), turgidity of guard cells depends upon interconversion of starch and This fact was supported by Loftfield (1921) who found that guard cells contain sugar during day time when they are open and starch during night when they are closed. Later Sayre
(1926) observed that stomata open in neutral or alkaline pH which prevails during day time due to constant removal of CO2 by photosynthesis. They remain closed during night when there is no photosynthesis and due to accumulation of CO2, carbonic acid is formed which causes the pH to be acidic, Sayre thus proposed that interconversion of starch and sugar is regulated by the pH. Sayre’s hypothesis was supported by Scarth (1932) and Small et al (1942). This hypothesis was further supported by detection of the enzyme phosphorylase in guard cells by Yin and Tung (1948). This enzyme is responsible for starch-glucose interconversion.
Starch + Pi ¾¾Pho¾sph¾oryl¾ase,¾pH¾=7(D¾a¾y) ®Glucose – 1 – phosphate
¬¾¾¾¾pH¾=¾5 (¾Ni¾gh¾t) ¾¾¾¾¾
In the light of above facts, stomatal opening and closing can be explained in the following way :
pH increases Starch converted
into sugar O.P. of cell sap
CO2 used for photosynthesis
Endosmosis from subsidiary cells
T.P. of Guard cells increases
Guard cells become flaccid
O.P. of guard cells decreases
Sugar converted into starch
Fig : Graphical representation of mechanism of opening and closing of stomata
Criticism : Starch ⇄ Sugar hypothesis has been criticised because of the following objections raised against this theory :
- Starch ⇄ Sugar interconversion is a slow process which can not account for rapid stomatal
- Starch or other polymerised polysaccharide do not occur in
onion plant where stomatal movement occurs.
- Glucose is not detectable in the guard cells when stomatal
- The theory could not explain the extra-effectiveness of blue light at the time of stomatal
+ATP Hexokinase O2
Phosphatase Glucose + phosphate
Fig : Mechanism of opening and closing of stomata according to Steward
Stewards modification : According to Steward’s pH theory CO2 accumulates in guard cells in dark, thus reducing the pH. As a result acidity increases. Sugar to starch conversion is thus favoured. This results in exosmosis causing the closure of stomata. During the day time in sunlight CO2 is consumed (in mesophyll cells). This is responsible for increase in pH and reduction in acidity. Thus hydrolysis of starch to sugar is favoured. Due to increase in osmotic concentration endosmosis occurs in guard cells and stomata open. Steward said that glucose-I- phosphate should be further converted into glucose as glucose-I-phosphate is not capable of changing osmotic pressure. In this process of stomatal opening and closing, enzymes like phosphorylase, phosphoglucomutase, phosphatase and hexokinase are present in guard cells.
(iii) Glycolate theory : Zelitch (1963) proposed that stomata open due to production of glycolic acid by photorespiration in guard cells under low concentration of CO2. The glycolic acid thus produced is converted into soluble carbohydrates which increase the O.P. of guard cells.
This theory is rejected due to following objections :
- It fails to explain the opening of stomata in dark (g., in succulents).
- In some plants stomata have been found to remain closed even during day
- It fails to explain the effect of blue light on stomatal
- Active K+ transport theory : Imamura (1943) and many other scientists found accumulation of K+ in the guard cells when they are exposed to light. Fujino (1967) suggested that stomatal opening and closing occurs due to an active transport of K+ into or out of the guard
- Proton transport theory : It was proposed by Levitt (1974). It incorporates good points of Scarth’s classical pH theory and active K+ -transport According to this theory stomatal opening and closing can be explained in the following manner :
(a) Mechanism of stomatal opening
- During day time due to rapid rate of photosynthesis, the concentration of CO2 decreases in the guard cells. As a result their pH is increased. At higher pH, starch in the guard cells is converted into organic acid by the enzyme phosphoenol pyruvate carboxylase (PEPC). This enzyme was discovered by Willmer etal. (1973). It can convert several others carbohydrate into organic
- The organic acid (g. malic acid) dissociates into H+-ions (protons) and malate ions.
- The protons (H+) are actively transported into subsidiary cells in exchange for K+ with the help of an energy (ATP) driven H+–K+-pump. The uptake of K+-ions is balanced by uptake of Cl and the negative charge on malate-ions.
- Increased concentration of K+ and malate ions in the guard cells increases the P. of guard cells.
- Water enters from adjoining subsidiary cells by
Fig : Role of potassium, chloride and malate
- Turgor pressure of guard cells Turgidity of guard cell is controlled by potassium, chloride and malate.
- Mechanism of stomatal closure : According to Cowan al. (1982) closure of stomata depends upon abscisic acid (ABA) which is in fact an inhibitor of K+-uptake. It becomes functional in presence of CO2 or in acidic conditions (low pH).
- During night photosynthesis stops which results in increased concentration of CO2 which causes lowering of pH.
- At lower pH, ABA inhibits K+-uptake by changing the permeability of guard
- The K+-ions now start moving out of the guard cells which results in lowering of the pH.
- At low pH, organic acids are converted back into starch by
- The P. of guard cells decreases and water moves out of them into subsidiary cells by the process of exosmosis, thus decreasing their turgor pressure.
- The guard cells become flaccid and the stomata
- Stomatal opening in succulent plants : The stomata in succulent plant or CAM plants (like Opuntia, Bryophyllum) open during night (darkness) and remain closed during the day time and found in lower surface. This type of stomatal opening is called ‘Scotoactive type’ and the stomata which open during day are called as photoactive. Stomata closed and open due to the activity of water. This types of stomata is known as hydroactive stomata. The opening and closing mechanism of scotoactive stomata was explained by Nishida (1963). In
succulent plants, during night, there is incomplete oxidation of carbohydrates and accumulation of organic acids (e.g., malic acid) without release of CO2. During day time the accumulated organic acids breakdown rapidly releasing excess amount of CO2 for photosynthesis as well as to keep the stomata closed.
During night :
Glucose Malic acid
During day :
(10) Factors affecting rate of transpiration
- External factors
- Atmospheric humidity : If the atmosphere is humid, it reduces the rate of transpiration. When the air is dry, the rate of transpiration
- Temperature : It affects the rate of transpiration only Increase in the temperature of the air decreases the humidity of the air and therefore more water is vapourised and lost from the transpiring surface. The lowering of the air-temperature, on the other hand, increases the humidity and rate of water-loss as well.
- Light : Light affects the rate of transpiration due to its effect on temperature and During daytime stomata open wide but during night they close. Moreover, during the daytime the light also helps in raising the temperature. Thus increased temperature and presence of wide open stomata increase the rate of transpiration. Light is the most important factor in the regulation of transpiration.
- Atmospheric pressure : The rate of transpiration is inversely proportional to the atmospheric pressure.
- Available soil water : If the available water in the soil is not sufficient the rate of transpiration is Under internal water deficiency the stomata are partially or completely closed.
- Wind velocity : A transpiring surface of leaf continuously adds water vapours to the atmospheric air. Once the immediate area becomes saturated, it reduces the rate of Wind velocity removes the air of that area, which is replaced by fresh air and result in an increases in the rate of transpiration. Wind velocity is measured by anemometer.
(ii) Internal factors/Plant factors
- Leaf area : If leaf area is more, transpiration is However, the rate of transpiration per unit area is more in smaller leaves than in larger leaves due to high number of stomata in a small leaf. Number of stomata per unit area of leaf is called stomatal frequency.
here, I = Stomatal index
S = No. of stomata per unit area
E = No. of epidermal cells in unit area.
- Leaf structure : The anatomical features of leaves like sunken or vestigial stomata; presence of hair, cuticle or waxy layer on the epidermis; presence of hydrophilic substances such as gums, mucilage etc. in the cells; compactly arranged mesophyll cells help in reducing the rate of transpiration.
- Root shoot ratio : According to Parker (1949) the rate of transpiration is directly proportional to the root-shoot ratio.
- Age of plants : Germinating seeds show a slow rate of It becomes maximum at maturity. However, it decreases at senescence stage.
- Orientation of leaves : If the leaves are arranged transversely on the shoot they lose more water because they are exposed to direct If placed perpendicularly they transpire at slower rate.
- Significance of transpiration : The advantages and disadvantages of transpiration are discussed below :
- Transpiration is important for plants because it directly influences the absorption of water from the
- Transpiration exerts a tension or pull on water column in xylem which is responsible for the ascent of sap.
- Transpiration helps in the movement of water and minerals absorbed by the roots to the other parts of the
- The evaporation of water during transpiration contributes to the cooling of leaves (and also the surrounding air) and protects leaves from heat injury particularly under conditions of high temperature and intense
- Transpiration often results in water deficit which causes injury to the plants by
- Rapid transpiration causes mid-day leaf water deficit (temporary wilting). If such condition continues for some time, permanent water deficit (permanent wilting) may develop, which causes injury to
- Many xerophytes have to develop structural modifications to reduce These modifications are extra burden on the plants.
- Excessive rate of transpiration leads to stunted growth of plants.
- Deciduous trees have to shed their leaves during autumn to check
- Since approximately 90 percent of absorbed water is lost through transpiration, the energy used in absorption and conduction of water goes
Besides all the above mentioned disadvantages, the process of transpiration is unavoidable, because of the anatomical structure of the leaves. Since stomata are required for gaseous exchange in photosynthesis and respiration, the loss of water through them cannot be avoided. Therefore, Curtis (1926) truely called ‘transpiration as a necessary evil‘.
- Anti-transpirants : Most of the water absorbed by plants is lost to the atmosphere by transpiration and hence water use by plants is very In recent years efforts have been made to improve the efficiency of water use by the plants. One of the approaches is to reduce transpiration by the application of certain chemical substances. ‘The chemical substances which reduce transpiration (by increasing leaf resistance to water vapour diffusion) without affecting gaseous exchange, are called anti-transpirants‘. Anti-transpirants are of two types metabolic inhibitors and film forming anti-transpirants.
- Metabolic inhibitors : They reduce transpiration by causing partial closure of stomata, without influencing other metabolic processes, the most important of these inhibitors are phenyl mercuric acetate (PMA) and abscissic acid (ABA).
- Film forming anti-transpirants : They check transpiration by forming a thin transparent film on the transpiring surface. They are sufficiently permeable to carbon dioxide and oxygen to allow photosynthesis and respiration, but prevent movement of water vapour through them. The important chemicals of this group are silicon emulsion, colourless plastic resins and low viscosity
- Guttation : The process of exudation of liquid drops from the edges of leaves is called guttation or the process of the escape of liquid from the tip of uninjured leaf is called guttation. Usually it is occur through stomata
like pores called hydathodes. Exudation may some time occur from stem through the scars of leaves and lenticles. Guttation usually occurs when the plant is put in more saturated atmosphere.
Hydathodes are generally present at the tip or margin of leaves. These pores are present over a mass of loosely arranged cells with large intercellular spaces called epithem. This mass of tissue lies above a vein ending. The xylem of a small vein usually terminates among the thin walled parenchymatous cells of
epithem. Guttation is caused due root pressure. It is found in 115 families and
Epithem Vein endings
333 genera of woody and herbaceous plants. e.g., Garden nasturtium
Fig : Vertical section of a leaf showing hydathode
(Tropeolum), Oat (Avena), Calocasia etc. growing in moist, warm soil and under humid conditions. When the absorption of water exceeds that of the transpiration, hydrostatic pressure is built up in xylem ducts. As a result, water is pushed in the xylem ducts and comes out through the hydathodes. The water of guttation contains several dissolved inorganic and organic substance.
Differences between transpiration and guttation
|(1)||It occurs during day time||It usually occurs in the night.|
|(2)||The water is given out in the form of vapour.||The water is given out in the form of liquid.|
|(3)||The transpired water is pure.||Guttated water contains dissolved salts and sugar.|
|(4)||It takes place through stomata lenticel or cuticle.||It occurs through special structure called hydathode found only on leaf tips or margin.|
|(5)||It is a controlled process.||It is uncontrolled process.|
|(6)||It lowers down the temperature of the surface.||It lacks such relationship.|
Differences between stomata and hydathode
|(1)||Stomata occur on epidermis of leaves, young stems, etc.||Hydathodes generally occur at the tip or margins of leaves of those plants that grow in moist shady places.|
|(2)||Stomatal aperture is guarded by two kidney
shaped guard cells.
|The aperture of hydathode is surrounded by a ring
of cuticularised cells.
|(3)||The two guard cells are generally surrounded||The subsidiary cells are absent.|
|by subsidiary cells.|
|(4)||The opening and closing of stomatal
aperture is regulated by guard cells.
|Hydathode pore remains always open.|
|(5)||These are the structure involved in transpiration and exchange of gases.||Hydathodes are involved in guttation.|
- Psychorometer is used for measuring relative humidity as well as
- Hydrometer is used for measuring the density or specific gravity of a
- Barometer is used for measuring atmospheric
- Barograph represents the recording of
- Porometer is used for measuring the size of stomata.
- Atmometer is used for measuring pull caused by evaporation of water from a porous
- Potometer is used for measuring the rate of transpiration.
- Transpiration ratio : It is number of units of water transpired for manufacturing 1 unit of dry Transpiration ratio is 50 in CAM plants, 100–200 in C4 plants, 300–500 for most mesophytes and 900 in Alfalfa.
- Schwendener (1881) was the first to point out that stomatal opening and closing is due to turgor changes in guard cells.
- Photoactive stomata : Stomata open in response to light. The action spectrum consists of red and blue light (blue light is more effective in stomatal opening; Mouravieff, 1958). Since, most of the transpiration is stomatal, the action spectrum of transpiration is red and blue light.
- Stomata remain open at relative humidity above 70% and close below relative humidity of 50%.
- Transpiration of hills : High due to lower atmospheric pressure but low due to lesser hours of light and lower Transpiration is therefore, near normal but the plants show xeromorphy due to lesser water availability.
- The term guttation was coined by Bergerstein (1887).
- In Saxifraga, the rate of guttation is high during
- Lactuca scariota and Syiphium laciniatum are called compass plants as their leaves remain vertically in north-south
- Maximum opening of stomata occurs at about 10:00 AM and 3:00 PM (At 12:00 noon, partial closure of stomata occurs).
- In C3 plant the rate of transpiration is high.
- In angiosperm stomata does not open during at midnight.
- Cobalt chloride paper method was first used by Stahl (1894). It is used to compare rate of transpiration on two surfaces of leaf. Cobalt chloride is blue in anhydrous In contact with water vapour it turns pink.
- Bleeding is the exudation of sap (water along with dissolved organic and inorganic substances) from the injured parts of the plant g., exudation of latex from laticiferous ducts in Euphorbia and members of family moraceae (mulberry family) are the cases of bleeding.
- In many plants like Oleander (Nerium) the stomata are not only sunken but are further protected by the presence of trichomes or epidermal
- In many plants like India-rubber, cabbage, sugar-cane etc., the leaves are covered with “bloom”, which is a waxy substance and as such prevents a certain amount of cuticular
- The rate of transpiration will greatly depends upon position of stomata.
- The presence of gums, mucilage, latex , in the tissues of the leaf also checks transpiration of water.
- When transpiration is very low and absorption is high, the root pressure is maximum.
- In aquatic and submerged plants stomata are absent g., Vallisnaria.
Translocation of organic solutes.
The synthesis of carbohydrate food materials, mainly through the process of photosynthesis, occurs in green cells of plant. The non-green cells are therefore, dependent on photosynthetic cells for their carbohydrate supply. The organic food mainly from the leaves, is transported to the non-green parts where it is needed for respiration and biosynthesis. “This movement of organic food or solute in soluble form, from one organ to another organ is called translocation of organic solutes.”
It has been now well established that carbohydrates are translocated from leaves to roots and storage organs (tubers, bulbs, fruits, etc.) along the phloem in the form of sucrose. They are transported through living sieve elements of phloem (chiefly sieve tube members in seed plants). The process of translocation requires expenditure of metabolic energy and the solute moves at the rate of 100cm/hr.
(1) Directions of translocation
- Downward translocation : It is of most important type, e., from leaves to stem and roots.
- Upward translocation : From leaves to developing flowers, buds, fruits and also during germination of seeds and tubers,
- Radial translocation : From pith to cortex and
Thus we find that the translocation of food takes place from organs where food is in high concentrations (e.g., leaf, tuber, rhizome) to organs where it is in low concentration (e.g., roots). The first are called supply ends and the later as consumption ends.
(2) Path of translocation
- Downward translocation of organic solutes : Phloem is the path for downward translocation of organic Following evidences are in support of it :
- Elimination of other tissues : Tissues other than phloem cannot account for downward translocation of Because xylem is responsible for upward movement of water and minerals, so it cannot account for downward translocation of solute at the same time. Cortex and pith are not structurally suitable for this purpose.
Thus only phloem is left (where there is end to end arrangement of sieve tubes united by sieve pores). Which is responsible for translocation of solutes in downward direction.
- Chemical analysis of phloem sap and xylem sap : Chemical analysis of sieve tube sap proves that concentrated solution of sucrose is translocated from the place of synthesis to other parts of the plant Glucose and fructose are sometimes found in traces only. The amount of sucrose is more in phloem sap during the day and less in night. In xylem the amount of sucrose is in traces and also there is no diurnal fluctuation.
- Isotopic studies : If leaf of potted plant is illuminated in the presence of
it forms radioactive products of photosynthesis which are then
transported to stem. It was detected by autoradiographic studies that these substances are translocated through phloem particularly sieve tubes. Radioactivity is found below
and used for growth synthesis and respiration
Fig : Diagram representing the translocation of sugars through phloem
and above the nodes of the leaf to which radioactive carbon was provided. Burr and others (1945) allowed bean leaf to photosynthesize in an atmosphere of carbon isotope (13C or 14C) and observed that labelled sugar moved in the phloem.
- Blocking of phloem : Blocking of sieve pores by ‘callose’ during winter blocks translocation of
- Ringing or Girdling experiment : It was first performed by Hartig (1837). On removing the ring of bark (phloem + cambium) above the root at the
base of stem, accumulation of food occurs in the form of swelling just above the ring, which suggests that in absence of phloem, downward translocation of food is stopped.
- Structure of phloem : The structure of phloem tissue is well modified for conduction of solutes. Phloem tissue of an angiosperm consists of sieve tubes, companion cells several kinds of parenchyma cells, fibres and Of
these sieve tubes are involved in sugar
Sieve tube member
Mitochondria Sieve area (occasionally present on the lateral wall)
Plastid Starch grain
Companion cell Vacuoles
Golgi apparatus Free ribosomes
Mitochondria Phloem protein
Continuity of cytoplasm through pores of the sieve plate
translocation. Sieve tubes are elongated cells arranged longitudinally end to end.
Middle lamella Plasma membrane
Parenchyma cells are closely associated with them and remain connected through fine cytoplasmic thread called plasmodesmata.
Fig : Phloem structure : (A) Sieve tube with companion cells
- S. of sieve tube through sieve plate showing cytoplasmic connections through the pore
During maturation of a sieve tube, cell wall undergoes certain distinctive changes. It develops pores in its transverse wall. Each pore has a single strand of cytoplasm extending through it and connecting the protoplast of adjoining sieve tube. These pores may also be present in lateral walls in certain cases. In general, these sieve areas (pores) are localised on the end walls and are called sieve plates. The border of each pore becomes impregnated at maturity with callose (a polysaccharide) and thus cytoplasmic strand within pore remains encased in a cylinder of callose.
In addition to the changes in the cell wall, the protoplast of sieve element also undergoes remarkable changes during maturation. Nucleus, tonoplast and vacuole undergoes disintegration and disappear Esau (1966) believes that in case of mature sieve tubes, the cytoplasm and vacuole become one system called mycotoplasm. A proteinaceous component called P-protein (phloem protein) makes its appearance in the cytoplasm of young sieve tube as discrete slime bodies. The slime bodies consist of thread-like filaments.
- Upward translocation of organic solutes : According to Dixon the upward conduction of foods takes place through the xylem. However scientists are not in agreement with him. According to Curtis upward conduction of foods also takes place through phloem. This view is based upon ringing experiments. He took three woody In plant A ringing was done as described in ringing experiments. In plant B xylem was injured in a ring but phloem was left intact. In plant C xylem and phloem were in normal position. In plant A and B all leaves above the ring were removed. In A there was no growth above the ring and also the dry weight of this part was less. This
proves that upward conduction of food takes place through phloem. So organic food moves upwardly and downwardly through the phloem.
(3) Mechanism of translocation
- Diffusion hypothesis : Mason and Maskell (1928) working on cotton plant demonstrated that the translocation of foods occurs from the place of high concentration (place of manufacture or storage) to the place of lower concentration (place of consumption). But this concept can not be supported considering the translocation The actual rate of translocation is many time faster than the rate of diffusion. Hence, Mason and Phillis (1936) modified this concept and proposed activated diffusion hypothesis. According to this concept the food particles are first energy activated then translocated. This hypothesis is not accepted due to lack of experimental evidence.
- Protoplasmic streaming hypothesis : This concept was proposed by de Vries (1885). According to him the food is transported across by streaming current of protoplasm. The cell protoplasm shows a special locomotion movement called cyclosis. It is of two types, rotation and circulation. While rotation is circular movement of protoplasm, circulation is radial movement forming eddies around the vacuoles. The hypothesis involves two phenomenon, such as streaming of sieve protoplasm and diffusion of metabolites through sieve
This hypothesis not only explains faster rate of translocation but also the bidirectional movement of metabolites across a single sieve element. This hypothesis was supported by Curtis (1950).
- Transcellular streaming : Thaine (1964) suggested modification to cytoplasmic streaming He observed the presence of transcellular strands in sieve tubes which contains particles. These strands move up and down. He
defined transcellular streaming as “the movement of the particulate and fluid constituents of cytoplasm through linear files of longitudinally oriented plant cells. “He further proposed that transcellular strands are proteinaceous and characteristic microtubules to afford rhythmic contraction. Thus, transcellular streaming is an attractive mechanism as it would explain the phenomenon of bidirectional translocation.
- Interfacial hypothesis : According to Van den Honert, (1932) food is translocated by interfacial flow along transcellular strands which provide greater surface The solute molecules are absorbed on the interface and as a result
Substance moving down
Substance moving up
Fig : Representation of protoplasmic streaming hypothesis
the surface tension decreases. This concept is comparable to spreading of oil drop on water surface. By this concept we can explain faster rate of translocation.
- Contractile proteins : Fensom and Williams (1974) observed a
Up stream Companion cells
network of interlinked microfibrils in the sieve tube which oscillated in a manner resembling moving flagella in other organisms. They suggested that particles attached to microfibrils move with a bouncing motion. These movements suggested that the microfibrils were composed of contractile threads of P-protein.
- Electro-osmotic hypothesis : A mechanism involving electro- osmosis was proposed independently by Fensom (1957) and Spanner
Water and solutes
Sieve tube member K+ ions are
pumped back to the upstream side
K+ through adjoining companion cells
Fig : Electro-osmotic flow of solutes through sieve plate
(1958). According to this hypothesis the solute moves in the positive direction of the electrical gradient along with
K+ ions. Important features of the mechanism are as follows :
- The sieve plates are negatively charged, hence they repel negatively charged ions (anions).
- There is high concentration of potassium ions (K+) in the sieve tube solution. These and other cations can pass through the sieve
- An electrical gradient builds up across the sieve plate in the direction of the flow; when the solute is moving in downward direction anions begin to accumulate below the sieve plate and cations above it (when solutes move in upward direction, K+ ions may accumulate below the sieve plate).
- A current of K+ passes through the sieve pores by electro-osmosis, and sugar and water molecules adhered tightly to K+ are carried along with them. Thus each sieve plate is an ‘electro-osmotic pumping station‘ which induces mass flow of solution along with the movement of K+ The energy for this movement is supplied by ATP from the companion cells and sieve tubes.
- The K+ ions from the downstream side of the sieve plate are pumped back to the upstream side of the sieve plate through adjoining companion
The evidence in support of electro-osmosis are as follows :
- High concentration of K+ ions is found in the sieve
- Role of companion cells in supplying ATP has been
- The charged porous surface of the sieve plates is suitable for the flow of solutes by The hypothesis has, however, been rejected on following grounds.
- The hypothesis fails to explain the bidirectional transport of metabolites in phloem.
- Considerable energy would be required to maintain a continuous circulation of potassium
(vii) Munch mass flow or pressure flow
hypothesis : The mass flow or pressure flow mechanism was first proposed by Hartig (1860). It was later modified by Munch (1930). Crafts elaborate it further in (1938). Munch assumed that the protoplasm of sieve tube is connected through plasmodesmata and forms a continuous system, called as the symplast. The
Producer cells (green mature cells of the leaves) photosynthesis maintains high osmotic concentration of the solutes.
translocation of solutes occur in a mass alongwith cell sap through the sieve tubes form a region of higher turgor pressure to low turgor pressure (i.e., along a turgor pressure gradient).
The principle of mass flow can be explained with the help of following laboratory model. Two osmometers A and B are bounded by semipermeable
Phloem cells (comparable to tube T)
membrane and are interconnected by tube P. They are now dipped in two separate troughs X and Y
Consumer cells in root cells
photosynthetic products are utilized in growth, respiration or converted into storage compounds resulting into low osmotic potential
Fig : Diagrammatic representation of the mechanism of solute translocation
respectively. The troughs are also interconnected by a tube T. It is assumed that initially entire system is filled with water and is at equilibrium. If sugar is added in osmometer A
its osmotic pressure and suction pressure increase, as a result water enters into it by the process of endosmosis. This causes an increase in the turgor pressure in osmometer A due to which water starts flowing in mass towards osmometer B. The water molecules also carry solutes alongwith them. This movement of water and sugar molecules from A to B will continue until the concentration of entire system becomes uniform and will stop once a state of equilibrium is achieved.
Sugar produced in photosynthesis
But if at B sugar molecules are either removed or converted into starch, the movement will continue endlessly.
The above said system is comparable to the system of transport operating in a living plant. The osmometer A
Sugar removed X T Y Concentrated
Fig : A model demonstrating the Munch mass flow hypothesis
represent the leaves where mesophyll cells manufacture sugar by photosynthesis. It increases their osmotic pressure and suction pressure due to which water is drawn in from the adjoining cells thus the turgor pressure of mesophyll cells is increased. Tube P represents the phloem which transmits the solutes along the turgor pressure gradient to osmometer B representing the storage or consumption end (e.g., roots, fruits and other living cells). Tube T represents xylem which transfers water from roots to leaves. This hypothesis satisfactorily explains the flow of solutes through sieve tubes under high pressure.
Munch’s hypothesis has been supported further by the following :
- When a woody or herbaceous plant is girdled, the sap containing high sugar content exudates from the cut
- Positive concentration gradient disappears when the plants are
- Movement of viruses and growth hormones is fast in illuminated leaves as compared to shaded
Objections to the Munch’s hypothesis
- The hypothesis fails to explain bidirectional movement of metabolites which is common in
- Osmotic pressure of mesophyll cells and that of root hair do not confirm the
- Munch’s hypothesis gives a passive role to the seive tube elements and the
(4) Factors affecting translocation
- Temperature : Swanson and Whitney (1953) reported that translocation out of the leaf was highly sensitive to temperature. Optimum temperature for translocation ranges between 20-30°C. The rate of translocation increases with the increase of temperature upto an upper limit and then starts At low temperature, the rate of translocation decreases but such an effect is only transient because the resultant steeper concentration gradient quickly brings about readjustment of translocation rate.
- Light : Hartt and his coworkers (1964) proposed that the movement of assimilates of a leaf can depend upon radient energy. The increase in light intensity more food starts being translocated to roots than to shoots. At lower intensity the growth of root and shoot is inhibited thereby the rate of translocation also
- Hormones : Cytokinins have a pronounced effect on the translocation of water soluble nitrogen
- Oxygen : Oxygen is necessary during transfer of food from mesophyll cells into phloem which is called as
- Minerals : Boron is highly essential for translocation of sugar. Translocation of sucrose occurs in the form of sucrose-borate Phosphorus also helps in translocation of solutes.
- Water : Translocation of photosynthates out of the leaves is highly sensitive to the amount of water in the plant However, it does not have much effect on movement of solutes through phloem.
- Metabolic inhibitors : The metobolic inhibitors which inhibit the process of respiration (g., iodoacetate, HCN, carbon monoxide etc.) adversely affect the process of translocation because phloem loading and unloading require ATP.
- Mittler (1958) develop a technique for the collection of phloem sap using an aphid stylet.
- In a girdled plant, root die first and ultimately shoot dies.
- Medullary rays is the path of radial translocation of organic
- The principle pathway by which water is translocated in angiosperm is xylem and vessels.
- The food stored in the ripening fruit is derived from nearest leaves.
- Marshall and Wardlaw (1973) proposed the solution flow hypothesis.
- Active mass flow in which oxygen is required was proposed by Mason and Phillis (1936).
- Bimodal theory of translocation was putforth by Fenson (1971).
- Mature sieve tubes do not show streaming movemen