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Importance of water for crops

Water plays a crucial role in life of plants for every gram of organic matter produced byplant; 500 gm of water is absorbed by roots of plants, transported through the plant body

and lost to atmosphere.

Even slight imbalance in this flow of water can cause water deficits and severemalfunctioning of many cellular processes. Therefore plant must balance the uptake and

loss of water, but this balance is serious challenge because for photosynthesis plants need

to draw CO2 from atmosphere but open stomata exposes them to water loss and threat of

dehydration.

Water makes up most of the mass of the plant cell, each cell contains large water filledvacuoles in such cells cytoplasm makes only 5-10% of the volume. Water typically

consists of 80-95% of the mass of growing plant tissues. Wood has 35-75 of water in

tissues. Seeds are driest plant tissue and have 5-15% water but they absorb large amount

of water before germination. Water is most abundant and best solvent known. As solvent it makes up medium for

movement of molecules within and between cells and greatly influence the structure of

proteins, nucleic acids, polysaccharides and many other cell constituents.

Water forms the environment in which most of the biochemical reactions of cell occursand it is directly participates in essential chemical reactions.

Water is the most abundant and at the same time the most limiting resource foragricultural productivity.

The amount of yield reduction in the absence of water is affected by genotype, severity ofwater deficit and the stage of development, therefore an understanding of the uptake and

losses of water by plants is very important.

The structure and properties of water

1. Polarity: Water has special property that enables it to act as solvent and to be readilytransported through body of plant.

These properties are largely due to polar structure of water. The water molecule consists of

oxygen atom which is covalently bonded to two hydrogen atoms. The two O-H bonds form

an angle of 105.

Because the oxygen atom is more electronegative than hydrogen, it tends to attract the

electrons of covalent bond.

This attraction result in a partial negative charge at oxygen end of the molecule and a partial

positive charge at each hydrogen.

These partial charges are equal, so the water molecule carries no net charge.

This separation of partial charges, together with the shape of water molecule makes a polar

molecule and opposite partial charges between neighboring water molecules tends to attract

each other.

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The weak electrostatic attraction between water molecules is known as hydrogen bond and is

responsible for many unusual properties of water.

2. Excellent solvent: Water is an excellent solvent; this is due to small size of water moleculeand due to its polar nature. Polar nature makes it good solvent for ionic substances and for

molecules such as sugars and proteins, that contain polar OH or NH2 groups.

3. Thermal properties: Thermal properties include specific heat and latent heat ofvaporization. The extensive hydrogen bonding between water molecules result in unusual

thermal properties such as high specific heat and high latent heat of vaporization.

Specific heat: It is the heat energy required to raise the temperature of a substance by

specific amount. Water requires a relatively large energy to break hydrogen bonds and to

raise its temperature. This large energy input is requirement is important for plants, because it

helps them to buffer temperature fluctuations.

Latent heat of vaporization: It is the energy needed to separate molecules from the liquidphase and move them into the gas phase at constant temperature- a process that occurs during

transpiration. For water at 25 C, the heat of vaporization is 44 kJmol-1, the highest known

value for any liquid. Most of this energy is used to break hydrogen bond between water

molecule. The high latent heat of vaporization of water enables plant to cool themselves by

evaporating water from leaf surface. Transpiration is an important component of temperature

regulation in plants.

4. Cohesive & adhesive properties of water:Water molecule at an air water interface are more strongly attracted to neighboring water

molecule than to gas phase in contact to water surface.As a consequence of this unequal attraction, am air water interface minimizes the surface

area. To increase the area of an air-water interface, hydrogen bond must be broken which

require input of energy. The energy required to increase surface area is known as surface

tension.

The extensive hydrogen bonding in water also gives rise to cohesion, which is the mutual

attraction between molecules and adhesion which is the attraction of water molecules to solid

phase such as cell wall or glass surface.

Cohesion, adhesion and surface tension gives rise to capillarity, which the movement of

water in capillary tube.

5. Tensile strength:Cohesion gives water a high tensile strength, so tensile strength is the maximum force per

unit area that a column of water can withstand before breaking.

It has been demonstrated that water in small capillary can resist tension more than -30 Mpa.

The presence of gas bubbles reduce the tensile strength of water column. If a tiny gas bubble

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forms in a column of water under tension, the gas bubble may expand until the tension in the

liquid phase collapse, this phenomenon is known as cavitation. Cavitation can disrupt water

transport through xylem.

Water transport process

Under field conditions roots are present in relatively moist soil while the stem and leaves grow

under relatively dry atmosphere. When water moves from soil through plant to atmosphere, it

travels through membrane a widely variable medium such as cell wall, cytoplasm, membrane

and air spaces and the mechanism of water transport varies with the type of medium it is passing

through.

Major processes of water transport

1. Diffusion: Water molecule in a solution are not static they continuously colloid and intermingle with

each other and exchange kinetic energy with each other. This random motion is called

diffusion.

As long as other forces are not acting on the molecule, diffusion causes the net movementof molecules from region of high conc. to region of low conc. that is down a conc.

gradient.

The rate of diffusion is directly proportional to the conc. gradient.

However diffusion is an extremely slow process, it has been calculated that average time

required for a glucose molecule to diffuse across a cell with a diameter of 50 micrometeris 2.5 seconds.

Therefore diffusion in a solution can be effective within cellular dimensions but it is tooslow for the mass transport over large distances.

2. Mass flow / Bulk flow: Second process by which water moves is known as mass flow or bulk flow. Bulk flow is movement of group of molecules en masse, most often in response to

pressure gradient.

Pressure derived by bulk force of water is the predominant mechanism responsible forlong distance transport of water in xylem. It also accounts for much of the water flow through soil and through cell walls of plant

tissue. In contrast to diffusion pressure driven bulk flow is independent of solute conc.

gradient.

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3. Osmosis: Membranes of plant cells are selectively permeable i.e. allowing movement of water and

other small uncharged substances across them more easily than the movement of large

solutes and charged substances.

Osmosis occurs spontaneously in response to a driving force. The direction and rate of water flow across membrane are determined not only by the

conc. gradient or pressure gradient but sum of these two driving forces.

The system that describes the behavior of water and water movement in soil and plant isbased on the potential energy relationship.

Water has capability to do work it will move from area of high potential energy to an areaof low potential energy.

The potential energy in an aqueous system is expressed by comparing it with potentialenergy of pure water.

Since water in plants and soil is usually not chemically pure due to the solutes and isphysically controlled by forces such as gravity, polar attraction and pressure.

The potential energy is less than that of pure water. It is called water potential w and itexpressed as force per unit area and its units are pascal (Pa).

There are major factors that influences the water potential in plants are concentration,pressure and gravity.

The water potential of a solution may be dissected into individual components and maybe expressed as:

w = S + p + g + m

S = Solute potential

p = Pressure potential

g = Gravitational potential

m = Matric potential

1. Solute potential (S) : Also known as osmotic potential. It represents the effect of dissolved solutes on water potential. Solutes reduce the free energy of the water by diluting the water. Osmotic potential is independent of the specific nature of solutes. It has a ve value. The value can vary considerably with in cells of well watered garden

plants such as lettuce, cucumber S may be as high as 0.5 MPa, the normal range is -0.8

to -1.2 Mpa.

The upper limit for cell S is set by the minimum conc. of dissolved ions, metabolitesand proteins in cytoplasm of living cells.

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At other extreme, plants under drought conditions attain much lower S for e.g. waterstress typically leads to an accumulation of solutes in cytoplasm, vacuole, this allows

plant to maintain turgor pressure despite of low water potential.

Plants that grow in saline environment are halophytes, typical have very low value of S . As low S lowers water potential to extract water from salt water without allowing toenter excessive salts to enter the plant tissue at the same time. Most crop plants can not survive in sea water because of dissolved salts sea water has low

water potential than the plant tissue can attain.

2. Pressure potential (p): The term p is the hydrostatic pressure of the solution. It can be +ve or ve. p is also called as pressure potential. The positive hydrostatic pressure within cells is called turgor pressure. The value of p can also be negative in case of xylem and in wall between cells where

ve hydrostatic pressure can develop. A +ve turgor pressure is important for 2 main reason, first growth of plants requires

turgor pressure to stretch cell wall and second reason is that the +ve turgor pressure

increases the rigidity of cells and tissues.

This is important for young lignified tissues which can not support mechanically withouthigh internal pressure.

A plant wilts when turgor pressure inside the cell tissues fall towards zero.3. Gravitational Potential (g):

Gravity causes water to move downwards the unless the force of gravity is opposed by an

equal and opposite force. When dealing with water transport at cell level, the gravitational component (g) is

generally omitted because it is negligible compared to the osmotic potential and the

hydro static potential.

4. Matric potential (m): Matric potential is the force with which water is held to plant and soil, constituted by

forces of adsorption of water and capillarity.

It is important in dry soil, seed, cell wall. It can be removed by forces. This is always negative value.

Water potential is important because cell growth, photosynthesis and crop productivity are allstrongly influenced by water potential and its components.

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Water flow across a membrane is a passive process i.e. water moves in response to physicalforces, towards the region of low water potential or low free energy and there are no

metabolic pumps that push water from one place to another.

This rule is important only when water is being transported, when solutes are transported atshort distance across the membrane or for long distance along phloem then water transport

may be coupled to solute transport and water may move against water potential gradient, but

water will move across membrane only in response to water potential gradient

And the rate of water movement is proportional to magnitude of driving gradient. Theconcept of water potential has two principle uses:

y First water potential governs transport across cell membrane.y Secondly water potential is used as a measure of water status of plant. Because of

transpirational losses to atmosphere plants are rarely fully hydrated..

the processes which are most effected by water deficit are cell growth, accumulation of

solutes

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SPAC ( Soil plant water continum)

Water in soil :

Water content and rate of water movement in soil depends upon the soil type and soil texture.

In case of sandy soils, soil particles are 1 mm or more in diameter. Sandy soil have relatively

low surface area per gram of soil and have large spaces or channels between particles.

While Clay particle are smaller than 2 m in diameter. Clay soil have much greater surface area

and smaller channels between particles.

When soil is heavy watered by rain or by irrigation the water percolates downwards by gravity

through spaces between soil particles.

Water in the soil may exist as a film adhering to the surface of soil particles or it may fill the

entire channel between particles.

The water holding capacity of soil is called field capacity.

Filed capacity is the water content of soil after it has been saturated with water and excess waterhas been allowed to drain away.

Clay soils or soils with high humus content have a large field capacity.

So such after being saturated may retain 40% water by volume. In contrast sandy soils typically

retain 3% water by volume after saturation.

Like water potential of plant cells, water potential of soils may be dissected into components:

The osmotic potential and hydrostatic pressure

The osmotic potential of the soil water is normally negligible because solute concentrations are

low. Osmotic potential may be 0.2 MPa. For the soils that have good conc. of salts OP should

be significantly less than 0.2 MPa.

Hydrostatic pressure for wet soil is very close to zero. As the soil dries out it becomes negative.

Water movement in the soil is largely by bulk flow driven by a pressure gradient. As plant

absorb water from soil they deplete soil of water near the surface of the roots. This depletion

reduces the pressure potential in the water near the surface and establishes a pressure gradient.

Because the water filled pore spaces in the soil are interconnected, water moves to the root

surface by bulk flow through these channels down the pressure gradient. In very dry soil the

water potential may fall below the permanent wilting point. At PWP water potential of this soil is

so low that plants can not regain turgor pressure even if all water loss through transpiration

ceases.

Water absorption by roots

Close contact between the surface of the root and the soil is essential for effective water

absorption by root.

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This contact provide surface area needed for water uptake and is maximized by the growth of

the root and root hairs into the soils.

Root hairs are microscopic extensions of root epidermal cells that greatly increase the surface

area of root thus providing greater capacity for absorption of ions and water from soil.

Close contact between the soil and the root surface is easily ruptured when soil is disturbed so it

is for this reason that newly transplanted seedling and plants need to be protected from water loss

for first few days after transplantation.

There after new root growth into the soil reestablishes soil-root contact and the plant can better

withstand water stress.

When water comes in contact with root surface the nature of transport becomes more complex.

From epidermis to endodermis of the root there are 3 pathways through which water can flow the

apoplast, transmembrane and symplast pathways

In apoplast pathway water moves exclusively through the cell wall without crossing anymembranes. The apoplast is the continuous system of cell walls and intercellular air

spaces in plant tissues.

The transmembrane pathway is the route followed by water where it enters the cell onone side and exits the cell on other side, enters the next in series and so on. In this

pathway water crosses least two membranes for each cell in its path. Transport across the

tonoplast may also be involved.

In the symplast pathway water travels from one to next via the plasmodesmata. Thesymplast consists of the entire network of cell cytoplasm interconnected by

plasmodesmata.

It has been shown that apoplast pathway is more important for uptake. At the endodermis water movement through apoplast pathway is obstructed by casparian

strip. Casparian strip is a band of radial walls in the endodermis and made up of hydrophobic

substance called suberin.

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Casparian strip breaks the continuity of the apoplast pathway and forces the water and solutesto cross the endodermis by passing through the plasma membrane.

Even though apoplast patway is important to the roots the water movement across theendodermis occurs through the symplast.

Water uptake by roots decreases when roots are subjected to low temperature or anaerobicconditions or treated with respiratory inhibitors such as cyanide. These treatments inhibits root respiration and the roots transport less water. The exact explanation for this effect is not clear. On the other hand the decrease in water transport in the root provide an explanation for

wilting of plants in waterlogged soils.

Submerged roots soon run out of oxygen which is normally provided by diffusion through airspaces in the soil.

The anaerobic roots transport less water to the shoots which consequently suffer net waterloss and begin to wilt.

Root pressure

Roots generate positive hydrostatic pressure by absorbing ions from the soil solution andtransporting them to xylem.

Root pressure occurs when soil water potentials are high and transpiration rates are low. When transpiration rates are high water is transferred rapidly into leaves and lost to

atmosphere, so positive pressure nerve develop in xylem.

Plants that develop root pressure frequently produce liquid droplets on the edge of theirleaves and this phenomenon is called guttation.

It occurs through specialized pores called hydathodes.Water transport through the xylem

In most plants, the xylem constitutes the longest pathway of water transport.

Compared with the complex pathway across the root tissue, xylem is simple pathway of low

resistance.

The conducting cells in the xylem have a specialized anatomy that enables them to transport

large quantities of water with great efficiency.

There are two important types of tracheary elements in xylem :Tracheids & Vessel elements.

Vessel elements are found only in angiosperms, while tracheids are present both in angiosperm

and gymnosperms. Both these water conducting cells are dead cells they have no membrane and

no organelles, have thick lignified walls which forms hollow tube through which water can flow

easily with little resistance.

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Tracheids are elongated spindle shaped cells that are arranged in overlapping vertical files.

Water flows between tracheids by means of the numerous pits in their lateral walls.

Pits of one tracheids are typically located opposite to the pits of an adjoining tracheids forming

pit pairs.

Pit pairs constitutes a low resistance pathway for water movement between tracheids.

Vessel elements are shorter and wider than the tracheids and have perforated end walls that form

perforation plates at end of each cell.

Vessels elements also have pits on their lateral walls.

The perforated end walls allow vessels members to be stacked from end to end to form a large

tube called vessel. Vessels varies in length both within and between species, length varies from

10cm to many meters.

Because of their open end walls the vessels provide very efficient low resistance pathway for

water movement.

The vessel member found at extreme ends of a vessel lack perforation at the end walls andcommunicate with neighboring vessels via pit pairs.

Theoretically pressure gradient needed to move water through the xylem could result fromthe generation of positive pressure at the base of the plant or negative pressure at the top of

plant. However root pressure which is usually less than 0.1 M Pa is inadequate to move water

till top of a tree.

However water at top of tree develops a large tension and this tension pulls the water throughthe xylem.

This mechanism is called Cohesion Tension Theory of Sap Ascent, because it requires

cohesive properties of water to sustain large tension in xylem water column. Latest instrumentation such as pressure probe & pressure chamber has concluded that the

cohesion and tension theory is acceptable.

However the long tension that developed in xylem of trees & other plants can create someproblems.

The water under tension transmits an inward force to the wall of xylem if the walls are weakthey collapse under the influence of tension.

The secondary wall thickening & lignifications of tracheids and vessels are adaptation thatare prevents this tendency to collapse.

Secondly as the tension in water increases, there is an increased tendency for air to be pulledthrough the microscopic pores in the xylem cell walls this phenomenon is called air seeding.Another mode by which bubbles can form in xylem vessels is due to reduced solubility of

gases in ice.

The freezing of xylem pipes can therefore lead to bubble formation. This phenomenon ofbubble formation is called Cavitation or embolism.

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Cavitation breaks the continuity of water column & prevents water transport in xylemleading to dehydration and death of the tissue.

The impact of cavitation on the plant is minimized by several means. Because of the tracheary elements in the xylem are interconnected; one gas bubble does not

completely stop water flow.

The water can detour around the blocked point by travelling through the neighboringelement.

Gas bubbles can also be eliminated from xylem. At night when transpiration is low xylem pressure potential increases and water vapour &

gases may simply dissolve back into xylem solution.

Many plants have secondary growth in which new xylem form each year. New xylem become functional before the old xylem ceases to function because of gas

bubbles or by substances secreted by the plants.

The tension needed to pull water through xylem are result of evaporation of water throughleaves.

On its way from leaf to atmosphere water is pulled from xylem into cell walls of mesophyllwhere it evaporates into air spaces of leaves.

The water vapour then exists the leaf through stomatal pores. So water moves along this pathway mainly by diffusion, so this water movement is

controlled by concentration gradient of water vapour.

All land plants face competing demand of taking up CO2 from atmosphere while limitingwater losses i.e. there is a conflict between the need for water conservation and need for CO2

assimilation.

Transpiration is regulated by guard cells which regulates the stomatal pores size to meetphotosynthetic demand for CO2 uptake while minimizing water loss to atmosphere.

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Transpiration

It is the process of loss of water in the form of water vapours from aerial portion of living plants

is known as transpiration

Type of transpiration

Depending upon the plant part and structure involved transpiration is of various types:

1. Stomatal transpiration: It is the transpiration which occurs through the stomata, epidermallayer of leaf. Green cell has numerous stomata. Stomata re responsible for 80-90% of total

transpiration. It also known as foliar transpiration.

2. Cuticular transpiration: Water vapours are lost from outer wall of epidermal cells through

cuticle. Cuticle is wax like layer of cutin that cover the epidermal cells of leaves. It reduces

water loss but / because it is not completely permeable to water vapours. The thicker cuticle,

less water transpiration in plants. The plant have thick cuticle, the cuticular transpiration will

be insignificant. The thickness of cuticle is affected by environmental factors. It accounts

10% of total transpiration.

3. Lenticular transpiration: Lenticels are small opening in cork (bark) of woody stems, thug,fruit, water lost through these openings. The amount of water vapour lost through lenticels

transpiration i.e. 0.1 % of total transpiration. But in deciduous plants which shed their leaves

in autumn , lenticular transpiration can be significant

Stomatal Transpiration

Stomatal Transpiration constitute more than 90% of the total transpiration. Water absorbed by

roots is translocated to mesophyll cells of roots through xylem elements. These cells have large

intercellular spaces, water evaporates from surface of mesophyll cells occur in the intercellular

spaces and then escape to the outer atmosphere through stomata.

Structure of Stomata

Stomata are openings found on epidermis of leaves, each stomata is bound by twospecialized epidermal cells called guard cells. Guard cells differ from epidermal cells in their shape and presence of chlorophyll. Guard

cells can be kidney shaped/ bean shaped or dumbbell shaped.

Kidney shaped guard cells are mostly present in dicots while dumbbell shaped in monocots. The inner wall of guard cells is thick and inelastic where as outer wall is thin and elastic. This structural feature of guard cell controls the opening and closing of stomata.

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Guard cells are bounded by one or more modified epidermal cell called subsidiary cells oraccessory cells.

When osmotic concentration of guard cells increases they absorb water from the surroundingepidermal cells and become turgid.

Increased turgidity causes thin elastic wall of guard cells to stretch the wall towards outerside. The thick & inelastic wall also gets pulled along with & assumes a concave shape. The stoma or pore between two guard cells is open in this condition, the loss in turgidity of

guard cells result in stomatal closure.

This turgidity of guard cells which is regulated by osmotic conc. is the main cause foropening & closing of stomata.

The leaf surface depending upon species may contain 1000 to 6000 stomata per square cm. The size of each stomata pore when fully open may measure 3-12 in diameter and 10-40

in length. On an average fully open stomata occupy 2% of the total leaf area.

Distribution of Stomata

Stomata mostly found on lower surface of dorsiventral leaf (dicot) but may occur on both

surfaces in isobilateral leaf (monocot). With the exception of few submerged hydrophytes

stomata are widely distributed among Angiosperms and Gymnosperms.

Based on the their distribution stomata can are grouped into 5 types:1. Apple or Mulberry type: Stomata are present only on lower surface e.g. apple. Such leaves

are called hypostomatic.

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2. Potato type: Stomata occur on both the leaf surfaces but more stomata are present on lowersurface for e.g. potato, tomato, brinjal. Such leaves are called amphistomatic.

3. Oat type: Stomata occur on both surfaces of the leaves and are almost equally distributed.E.g wheat, rice, grasses. Also called amphistomatic.

4. Water Lily type: Stomata are found only on upper surface of leaf for e.g. in water lily.Leaves of such plants are found floating on water. Also called as epistomatic.

5. Potamageton type: in submerged aquatic plants the stomata are generally absent, if presentthey are non functional.

Types of stomata based on Stomatal Movement

1. Alfa Alfa type: Stomata remain open throughout day and closed throughout night. Found inall thin leaved mesophytes e.g. peas, beans, grapes, mustard.

2. Potato type: stomata remain open throughout day and night except few hours after sunset.e.g. onion, potato, cabbage, banana.

3. Barley type: stomata open for only few hours a day and remain closed for rest of the period.e.g barley, maize.

4.

Equisetum type: stomata remain open throughout day and night

Mechanism of stomatal movement

i. Opening of stomata in nightStomata open in night due to following reactions:

In light the starch in guard cells is metabolized in to PEP which is later on converted toorganic acids particularly Malic acid. This reaction is catalyzed by PEP carboxylase.

Malic acid dissociate into malate and H ions in guard cells Hydrogen ions from guard cells are transported to epidermal cells and potassium ions

from epidermal cells are absorbed in to guard cells through hydrogen potassium ion

exchange pump.

In the guard cells potassium ions are balanced by Malate anion additionally small amountof chloride ions are absorbed which neutralize a small percentage of potassium.

Process of potassium exchange requires ATP and it is active process. Increased potassium and anion conc. in guard cells increase the osmotic conc. hence

water enters guard cells by end osmosis.

Turgor pressure in guard cells increased due to end osmosis and stomata opens.

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ii. Closing of stomata in dark In there is no photosynthesis so CO2 is not utilized and its conc. in substomatal cavityincreases. An inhibitor hormone like ABA and Auxins function in presence of the CO2. It inhibits

the potassium uptake by changing the diffusion and permeability of guard cells

The potassium ions are transported back to epidermal cells or subsidiary cells from guardcells.

The osmotic conc. of guard cells decreases this result in movement of water out of guardcells by exosmosis

The guard cells become flaccid and stomata are closed.

Starch Organic Acid K+ absorption Osmotic conc. decreases

(product of photosynthesis)

Decrease in CO2 conc. LIGHT Endosmosis

Photosynthesis Turgor pressure increases

Stomata Opens

Stomata closes

Turgor pressure decreases Photosynthesis stops

Exosmosis DARK CO2 conc. increases

Osmotic conc. Decreases K+ efflux Organic acids

Starch

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FACTORS AFFECTING THE STOMATA MOVEMENT

1. Carbon di oxide: Stomata will close if concentration of CO2 in external atmosphereincreases, both in light and dark. It is because higher conc. of CO 2 reduces the pH of guard

cells, which promotes conversion of sugar into starch. Stomata open in light because in light

CO2 is utilized in photosynthesis and hence its conc. decreases.

2. Light: The opening and closing of stomata in the light is most common phenomenon.Stomata begin to opens shortly after exposure to light because the conc. of CO2 in guard cells

decreases and the starch is converted to sugar. The blue and red wavelengths of light are

most effective because these wavelengths are most efficient for photosynthesis. On return todarkness these begin to close, the stomata also close in infrared and UV lights and there is

opening in green wavelength of light

3. Temperature: Stomatal opening is possible even in the dark if temperature is high; this isdue to the increased activity of the hydrolyzing enzymes. Very high temperature leads to

closure of stomata which is observed as mid day closure. A reduction in temperature below

30 C often result in stomatal opening. However in most plants stomata are closed at or

below 0 C even in continuous light.

4. Water content of leaf: Water content of leaf must be high when stomata open if availabilityof water is poor i.e. under water deficient conditions the stomata are partially or completely

closed.

5. pH of guard cells: The stomata open with a rise in pH of guard cell and closes when pHdecreases.

FACTOR AFFECTING THE RATE OFTRANSPIRATION

EXTERNALFACTORS

1. Light: It affects transpiration because it is directly involved in opening and closing ofstomata. Stomata usually open in the light and closes in the dark. Therefore bulk of

transpiration takes place during the day time and at night only small amount of water is lost

by cuticular and lenticular transpiration. Besides this light also affect the rate of transpiration

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indirectly by effecting temperature and membrane permeability of cell. High intensity and

long duration of light increase temperature and hence there is an increase in rate of

transpiration. High intensity of light also increases permeability of cell membrane, resulting

in easier diffusion of water vapour into atmosphere.

2. Atmospheric humidity: The rate of transpiration depends upon the difference in vapourpressure between internal atmosphere of leaf and air out side the leaf. Since the relative

humidity is the actual amount of vapour present in air at any given time, the rate of

transpiration is mainly dependent upon it. Low humidity i.e. low vapour pressure outside leaf

favours transpiration because in such situation vapour pressure gradient between internal leaf

atmosphere and external atmosphere is very sharp. As the conc. of vapour pressure in the

external atmosphere rises the vapour pressure gradient becomes less steep and the rate of

transpiration also decreases. The vapour pressure of atmosphere decreases with altitude

therefore high altitude plants as show zeromorphic adaptations to reduce transpiration.

3. Temperature: The rate of transpiration increases with rise in temperature this is because athigher temperature there is more evaporation of water from mesophyll cells & there is greater

saturation of leaf atmosphere with water vapour. At the same time a rise in temperature

lowers the relative humidity of air outside the leaf consequently there is rapid diffusion of

water from humid atmosphere inside the leaf to outer dry atmosphere.

4. Wind: In still air water vapour from highly saturated air around the leaf reduces thesteepness of vapour pressure gradient and the rate of transpiration is also reduced. Breeze

removes humid air present around leaves and rate of transpiration increases. Stronger winds

causes bending and fluttering of leaves forces water vapour present in intercellular spaces

come out. Therefore rate of transpiration increases. Winds of very high velocity increases

transpiration initially, due to excessive loss of water the guard cells become flaccid as a result

stomata closes and transpiration stops.

5. Availability of soil water: In a dry soil the soil solution becomes more conc., therefore thereis less tendency of water to enter by osmosis. It means that there is less uptake of water by

root and hence rate of transpiration is also lowered.

Internal / Plant Factors

1. Structure of leaf surface: The rate of transpiration depends on structure of leaf surface.Zerophytic characters such as presence of thick cuticle wax layer on trichones of leaf surface

reduce the rate of transpiration.

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2. Area of transpiring surface: Leaf is main organs of transpiration therefore any decrease inleaf surface area will reduce transpiration and conserve water absorption by roots. Many

grasses roll up their leaves there by exposing less surface to air, thus reducing transpiration.

Reduction in leaf surface is achieved when leaves are reduced to needles for in pines, cacti.

The shedding of leaves in dry or cold season i.e when soil water is get frozen by deciduous

plants is also a xerophytic adaptations to reduce transpiration.

3. Stomata: Number, distribution and structure of stomata greatly affect transpiration. Ingeneral the greater the no: of stomata per unit area greater is stomatal transpiration. How ever

distribution is also important for example Dicotyledonous plants have more stomata on lower

surface of leaf than on the upper surface, where as monocot are held vertically and have

equal distribution on both surfaces. Generally loss of water is more from dicot leaves.

Xerophytic plants have sunken stomata, in this condition water vapour diffusing through

stomata accumulate in pit where stomata is located this reduces the diffusion rate and hence

rate of transpiration.

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DROUGHT

It is defined as deficiency of water severe enough to check plant growth. Drought leads to an

internal deficit of water in plants. Some of the physiological effects of drought are as follows:

Reduced availability of water result in decreased activity of protoplasm, transportation ofmaterials including mobilization of enzymes, diffusion of gases.

Water stress stimulates the synthesis of Abscisic acid and ethylene. Stomata get closed and hill reaction activity is reduced. All the metabolic activities work at reduced rate i.e. rate of photosynthesis and respiration is

reduced, sometimes growth is completely stopped.

Natural immunity to diseases is lost, survival becomes difficult.

Though drought is mainly due to deficiency of water in soil it accentuated by atmosphericconditions such as increased transpiration.

Drought may be classified into two types:1. Atmospheric drought: It is caused by atmospheric humidity, high wind velocity and

temperature. It causes plants to lose most of its water.

2. Soil drought: Soil does not provide water in quantity sufficient to replace amount lost intranspiration. The internal deficit of water increases and plants undergo permanent

wilting.

Classification of plants according to response of available water1. Hydrophytes: Grow where water is always available. E.g. in ponds or marshes.2. Mesophytes: Grows where water availability is intermediate.3. Xerophytes: Water is scarce most of the times.4. Glycophytes: Plants those are sensitive to high salt concentration.5. Haleophytes: Plants which can grow in high salt concentrations. Xerophytes: These have four categories

1. Drought escaping2. Drought evading3. Drought enduring4. Drought resistant

1. Drought escaping: Ephemerals, the plants which complete their life cycle in few days,weeks etc when water availability is high or during the rainy season. They pass dry season in

the form of seeds which repeat cycle in next rainy season.

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2. Drought evading: Annuals of dry area are called drought evading plants. They grow infavourable rainy season and have mechanism to reduce transpiration and prolong their

lifecycle for some drought period.

3. Drought resistant: Succulents have morphological adaptations for reducing transpiration fore.g. thick cuticle, sunken stomata, reduced surface area. Succulents also accumulate water

which is used slowly during dry periods. Such plants are called drought resistant. So drought

resistance is ability of plant to grow normally in dry habitat and yield the maximum. The

property of drought resistance can be used to increase agricultural output of developing

countries where vast area of agricultural land .. this is called dry farming.

Drought resistance can be induced artificially, drought resistant plants have high

photosynthesis and other metabolic activities, good retention of water for proper hydration of

protoplasm. Good synthetic rate even during wilting conditions. Ability to endure

dehydration and high temperature.

4. Drought enduring plants: these are non succulent perennials plants and they endure droughtwithout any mechanism to ensure continued water supply. For e.g. true xerophytes,

euxerophytes. They exhibit dehydration tolerance of hardiness rather than avoidance. They

lose large quantities of water. Their protoplasm is subjected to extremely negative water

potential yet they are not killed. These plants are drought endurers these have many

characters of drought avoiders such as small leaves, sunken stomata. For most of the plants

water level below 50-70% are lethal but in euxerophytes water level drop to as little as 30%

before leaf dies. Euxerophytes can take up water directly from dew, rain or even moist

atmosphere. Higher plants have all these features and outer adaptations such as Heteroblasty.

It is the ability of single plant to produce morphological and physiological different seeds

which can germinate over varieties of environment conditions and sometimes over years.

Salt secreted on leaves of some plants help to absorb moisture from atmosphere. And this

moisture is then absorbed by leaf. Water use efficiency (It is ratio of dry matter produced to

water consumed) increases as soil water availability decreases. Allelochemicals are produced

which restrict the germination or growth of plants completely.

Second type of classification1. Water spenders2. Water collectors3. Water savers

1. Water spenders: Desert plants such as palms that grow in oasis where their roots reachwater table or plants like alfa-alfa that have their roots that extend 7-10 m down to water

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table. Never experience extremely negative water potential. They are water spender as they

avoid drought such plants are able to use soil water while extending roots to water table.

2. Water collectors: Succulent species such as cacti, agave and various other CAM plants arewater collectors. They resist drought by storing water in their succulent tissues. Sufficient

water is stored and its rate of escape is low due to thick cuticle and stomatal closure during

day time. Because their protoplasm is not subjected to extremely negative water potential,

succulents are drought avoiders and they are not truly drought tolerant.

3. Water savers: Many non succulents desert plants have adaptations that reduce water loss.They called are water savers. For e.g. desert shrubs have small leafs, sunken stomata, leaf

hairs etc, these modification reduce loss of water but never prevents it. As water evaporates

from plants salt in protoplasm reach level that can damage crucial enzymes. Important

adaptations found in many organisms subjected to water stress and other stresses is

accumulation of organic compounds such as sucrose, amino acids (proline) that lowers the

osmotic potential and water potential in cell without limiting enzyme function. As water

stress increases such compounds appear in cell of many xerophytes to drop osmotic potential

this is called osmotic adjustment or osmoregulation.

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YIELD AND PLANT NUTRITION

High agricultural yield depends strongly on available nutrients. Yield of most crop plants increase linearly with amount of fertilizer they absorbed to meet

the increased demand for food.

World consumption of primary fertilizers mineral elements N, P & K is rising constantly. Most chemical fertilizers contain inorganic salts of macro nutrients N, P, K. Fertilizers that contain one of these three nutrient are called straight fertilizers. Some fertilizers like Superphosphate, Ammonium Nitrate, Muriate of Potash fertilizers that

contain two or more nutrients called compound or mixed fertilizer and contain N, P2O5 &

K2O in various proportions.

With long term agricultural production consumption of micro nutrients can reach a point atwhich they too must be added to soil as fertilizers.

Micro nutrients may be necessary to correct preexisting deficiency. Chemical fertilizers may also be applied to soil to modify pH. Soil pH affects the availability of all mineral nutrients. Addition of lime can raise the pH of acidic soil, addition of sulphur lowers the ph of alkaline

soils.

Organic fertilizers:

These originated from residue of plants and animals. These must be broken down usually by activity of micro-organisms through the process of

mineralization.

Mineralization depends upon temperature, water, oxygen availability and type & no : of

microorganism in soil. Consequently rate of mineralization is highly variable and nutrients become available to

plants over a period ranging from days to month to years.

But residue from organic fertilizers improves physical structure of soil, enhancing waterretention during drought and drainage in wet conditions.

Foliar application:

Mineral nutrients can be applied to leaves as foliar application. It can reduce the uptake time of

nutrient by plant which is important during a phase of rapid growth uptake of Fe, Mn & Cu is

more efficient through foliar application than soil application, where they adsorb to soil particles

and are less available.

Requirement of Mineral Nutrients:

Quantitative requirement of essential nutrients depends upon crop, yield level and nutrient. The

status of nutrient in plant tissue and plant growth can be described as deficient, transitional and

adequate. These levels vary with genetical and environmental factors.

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In deficient zone adding increment of nutrients result in increased dry matter production.

In adequate zone adding increment of nutrient result in increased element content in plant tissue

but little or no yield increase. This part of response curve is called luxury consumption.

In transition zone adding increment of nutrient increased both yield and nutrient conc. the yield

response to application of most nutrient follows a law of diminishing returns, that is each addedfertilizer produces a small yield increase finally reaching plateau. The economic benefits of

fertilizers are function of yield response in relation to fertilizer cost.

Efficiency is relationship output to input

Input Resource cycling Output

Waste

In case of crops supply or availability on amount of mineral nutrient input and plant growth,

physiological activity or yield are typical outputs. So in crop nutrient efficiency is defined asability of system to convert nutrient input into desired output & minimum nutrient waste. By

definition each plant has its own nutrient efficiency i.e. they have nutrient efficient and non

efficient genotypes.

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1. Line 1: A nutrient efficient genotype has greater outcome as nutrient input increases. Soresponse of genotype to high level of input (nutrient) was improved. This was basis for green

revolution in which short statured high yielding varieties of rice and wheat were developed

that could response to high N without lodging. Green revolution was single breakthrough in

last century in agriculture. It is basically improvement of nutrient use efficiency, but it was

criticized in one aspect as new varieties were less efficient at low level of nutrients. It

disproportionally benefited rich farmer who could afford fertilizers.

2. Line 2: An alternative approach where yield response to nutrient over low range of nutrientavailability increase without affecting the response to high rate of inputs. This is more useful

in low nutrient availability. Common bean is classic example of crop. This is grown in

marginal land with few inputs.

Efficiency and responsiveness: Efficiency is ability to grow or yield at low nutrient

availability & responsiveness is capacity to response to increase level of nutrient. The

common bean becomes very responsive.

3. The third approach is superior yield response in all levels of nutrient availability. Genotypeare highly evaluated in selection and breeding programme. Broad adaptation across diverse

environment may be due to factors that are not directly related to nutrient utilization. Such as

photoperiods, temperatures, adaptation, better shoot articulation, more appropriate price,

phenology etc. it could also result from traits. Nutrient improving utilization of plant nutrient

.

Notes by: GURJEET SINGH BHULLAR(9872351607)L-2009-A61-M

M.Sc( Horti)