Transport in Plants
Page No.
Topic
1.
2.
3.
4.
5.
Diffusion
Osmosis
Plasmolysis
Transpiration
How do Plants absorb
water?
6. Water Movement up a
Plant (Ascent of sap)
7. Guttation
01
04
08
10
14
18
20
Syllabus
Transport in Plants
Diffusion, Osmosis,Plasmolysis,Transpiration, Hoe do plants absorb
water?,Water movement up a plant, Guttation.
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PLANT PHYSIOLOGY
The study of various vital activities and metabolism of plant is known as Plant physiology.
Stephan Hales is known as father of plant physiology.
J.C. Bose is known as father of Indian plant physiology.
TRANSPORT IN PLANTS
In a flowering plant substances that would need to be transported are water, mineral nutrients, organic nutrients and plant growth regulators/ hormones.
The small distance transport means transport with in the cell or across the membrane or from cell to
cell in a tissue occurs by diffusion, facilitated diffusion (passive transport) and by Active transport.
Transport over longer distances proceeds through the vascular system (the xylem and the phoem)
and is called translocation.
In rooted plants transport in xylem (of water and minerals) is unidirectional, from roots to stem.
Transport in phloem means transport of organic compounds synthesised in the photosynthetic leaves
is Bidirectional (from leaves to storage organs and later from storage organs to other growing
parts).
Means or types of transport
Transport in plants is of two types :
(A) Short distance transport
(B) Long distance transport
(A)
(1)
Short distance transport
If transport occurs within the cell or from one cell to another cell then it is considered as short
distance transport. Further it is of two types
(1) Passive transport
(2) Active transport
Passive transport
If transport occurs according to concentration gradient (High concentration to low concentration)
without expenditure of ATP. It occurs by following methods
(i) Diffusion
(ii) Facilitated diffusion
DIFFUSION
“The movement of molecules or atoms or ions of a material from an area of higher concentration to
an area of thier lower concentration is called diffusion”.
The diffusion is continue till the dynamic equilibrium is not established. At this stage the movement of
molecules is equal in both direction so net movement become zero.
The kinetic energy, which is present in the molecules of material is distributed equally in thier available space by their nature.
Diffusion is obvious in gases & liquids but diffusion in solid is more likely rather than of solid.
Diffusion rate  Gas > Liqiud > Solid
Diffusion of a substance is independent from the diffusion of other substance.
Diffusion of each substance is free from diffusion other substance like diffusion of CO2 & O2 occurs
at the same time in different directions through stomata.
Diffusion can occur in both manners  without any membrane or through the membrane.
Diffusion is a slow process and is not dependent on a ‘living system’.
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Diffusion is very important to plant since it is the only means for gaseous movement within the plant
body.
Diffusion rates are affected by the gradient of concentration, the permeability of the membrane
separating them, temperature, pressure.
The diffusion molecules or ion exert a pressure, on the substance or medium in which diffusion takes
place, known as diffusion pressure.
Water molecules move from their higher concentration (high DP) to their lower concentration (low
DP) in plants.
SIGNIFICANCE OF DIFFUSION
 Exchange of gases like CO2, O2 takes place through diffusion. It is the only means for gaseous
movement within the plant body.
 The process of transpiration is also a diffusion. The evaporation of water from the intercellular
space is linked with diffusion during the transpiration.
FACILITATED DIFFUSION
Diffusion of any substance is depends upon solubility in main constitution of membrane, lipid. Lipid
soluble substance rapidly diffuse through membrane.
Moiety of which substance is hydrophilic, diffusion diffcultly through membrane. So there is the need
to simplify its movement. Membrane proteins provide space for transfer of these molecules. This
process of diffusion with the help of membrane proteins is called facilitated diffusion.
In facilitate diffusion, specific proteins helps in transfer of substances across the membrane and no
ATP consume. These specific proteins do not setup a concentration gradeint, a concentration gradeint
must already be present for molecules to diffuse even if facilitated by the proteins.
Extracellular molecules bind with transport proteins and later this transport proteins release the
molecules inside the cell by rotation movement.
Transport rate reaches a maximum when all of the protein transporters are being used (saturation).
Facilitated diffusion is very specific, it allows cell to select substances for uptake. It is sensitive to
inhibitors which react with protein side chains.
The proteins form channels in the membrane for molecules to pass through. Some channel are
always open others can be controlled. Some are large, allowing a veriety of molecules to cross.
The porins are proteins that form huge pores in the outer membranes of the plastids, mitochondria
and some bacteria allowing molecules up to the size of small proteins to pass through.
Eg. Water channels made up of 8 different types of aquaporins.
In symport, two molecules move across the membrane in similar direction, while in antiport they
move in opposite directions. When a molecule moves freely across the membrane then this method
is called uniport.
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(2)
Active transport
Active transport uses to pump molecules against a concentration gradeint. Active transport is carried
out by membrane proteins. Hence different proteins in the membrane play a major role in both active
as well as passive transport.
Pumps are proteins that uses energy to carry substances across thecell membrane. These pumps
can transport substance from a low concentration to a high concentration (‘uphill’ transport).
Transport rate reaches a maximum when all the protein transporters are being used or are saturated.
Like enzymes the carrier protein is very specific in what it carries across the membrane. These
protein are sensitive to inhibitors that reac. with protein side chains.
Comparison of Different Transport Mechanisms
Property
simple Diffusion Facilitated Transport Active Transport
Requires special membrane proteins
NO
Yes
Yes
Highly selective
NO
Yes
Yes
Transport saturates
NO
Yes
Yes
Uphill transport
NO
NO
Yes
PERMEABILITY

The exchange of materials in and out through the membrane is called pereability :
The membranes are divided in the following types on the basis of permeability
(i)
Permeable membrane :Such membrane are permeable for both - soltes and solvent e.g. cell wall, filter paper
(ii)
Semipermeble membranes :Such membrane allow diffusion of solvent molecules, but do not allow the passage of solutes e.g.
artficial membranes like Cellophane and Copperferrocyanide membranes, parchment paper, goat
bladder.
(iii)
Selective permeable membrane or differentially permeable membrane :Such membranes allow some selective solutes to pass through them along with the solvent molecules. e.g. Cell membrane , tonoplast, Organeller membrane.
These membrane are permeable for CO2, N2, O2 gases alcohol, ether and water, but impermeable
for polysaccharides and proteins.
(iv)
Impermeable membrane :membrane which do not allow solute & solvent to pass through it.
e.g. Rubber membrane, Al-foil, Suberised cell wall, lignified cell wall.
TYPES OF SOLUTIONS
(i)
Isotonic solution :
if solution in which a cell is placed, has equal osmotic concentration to that of cell sap, then this
solution is called isotonic solution.
(ii)
Hypotonic solution :
If the osmotic concentration of a solution is lesser than that of the cell sap, then this solution is called
hypotonic solution. If a cell is placed in such solution endosmosis takes place, and as result cell swells
up and become turgid. e.g. Swelling of dried grape (resins) in water.
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(iii)
Hypertonic solution :
If the osmotic concentration of a sloution is higher than that of the cell sap then this solution is known
as hypertonic solution. If a cell placed in this type of solution, exomosis takes place. It means water
of the cell sap diffused out into the outer solution and as result cell v\become plasmoysed.
OSMOSIS

“Osmosis is defined asd the special diffusiion of solvent *(water in this context) from the
solution of lower concentration (Hypotonic) to the solution of higher concentration (Hypertonic) when both the solutions are separated by a semipermeable membrane”.
The water moves into the cell during the osmosis is called endosmosis.
Ex. Resin placed in water.
When the water starts moving out of the cell then it is called exomosis.
Ex. grapes kept in salt solution
The net direction & rate of osmosis depends on both the pressure gradient & concentration gradient.
“O.P. of solution is equal to that external pressure, which required to be applied on a hypertonicsolution
in order to prevent osmosis or entry of water in it when the hypotonic and hypertonic solution are
separated by a semipermeable membrane”.
The osmotic pressure of pure water is zero. O.P. is due to presence of solute into the solution.
The osmotic pressure of a solution is directly proportional to the concentration of solute in it.
Seuence of OP = Hydrophytes < Mesophytes < Xerophytes < Halophytes.
The highest osmotic pressure is found in the halophytes group. Atriplex confertifolia (202 atm.)
Osmotic pressure of a solution is measured by osmometer.
According to Harris, the osmotic pressure is highest in leaves & lowest in roots.

The formula of Vont Hoff for measuring O.P. :
OP = mRT
Here - m = moler concentration
R = Gas constant [0.082 mole/molecules]
T = Absolute temperature
the osmotic pressure of 1 mole glucose solution at 0oC
OP  1 × 0.082 × 273
 22.4 atm., for non electrolytes
The O.P. of electrolytes is find out by the following formula
OP = MRTI
Where I is the constant of ionisation of electrolytes.
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The osmotic pressure of electrolytes is higher than that of non electrolytes.
For example-solution of 1 M NaCl and 1 M glucose. The molar concentration ofboth solution are
equal but
O.P. of 1 M NaCl is higher O.P. towards the higher O.P.
TURGOR PRESSURE OR T.P. AND WALL PRESSURE OR W.P

“When a cell is immersed in water, then enter into the cell because osmotic pressure of
the cell sap is higher. The cell content press upon the wall or develop a pressure against
the cell wall, which is called turgor pressure.”
The turgor pressure is balanced by an equal but opposite pressure of the thick cell wall, it is known
as wall pressure. Wall pressure & turgor pressure are equal but the direction is opposite.
TP =WP
Turgor pressure is not applicable for free solution. This is only applicable for osmotic system. Turgor
pressure is also known as hyfrostatic pressure.
Plant cell does not burst, when placed in a pure water due to wall pressure, but an animal cell burst
when placed in pure water because wall pressure is absent due to absence of cell wall. It can be
demonstrated by placeing RBCs of human blood in distilled water. When examined after some time,
the RBCs are found to have burst.
The value of turgor pressure is normally from zero to in between the osmotic pressure in plants cell.
A flaccid cell has zero turgor pressure.
The highest value of turgor pressure is found in fully turgid cell and it is equal to the osmotic pressure
Fully turgid cell has OP = TP
The value of turgor pressure is assumed as negative (–ve) during the plasmolysis of the cell.

Significance of T.P.
1.
2.
Maintains the normal shape of the cell
Turgorpressure helps in cell elongation or growth of cell.
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DIFFUSION PRESSURE DEFICT (DPD) OR SUCTION PRESSURE (SP)

DPD : The decrease inthe diffusion pressure of any system due to addition of solute is
called DPD
DPD determines the direction of osmosis and it is the power of absorption of water for
the cell. (Suction pressure)
This is also known as demand of water in cell
DPD  concentration of solute
The diffusion of water takes place from the region of lower DPD to the region of higher DPD in the
process of osmosis.
SPM
Lower DPD H O Higher DPD
2
SPM = Semi Permeable Membrane
Normally, osmotic pressure is greater than the tugor pressure in a cell. The difference between
osmotic pressure and turgor pressure is called suction pressure or DPD.
DPD = OP – TP or WP
The DPD of any free solution is equal to the osmotic pressure of that solution because for free
solution TP = O
DPD = OP
(i) DPD in partially turgid or normal cell :
DPD = OP – TP
(ii) DPD for fully turgid cell :
When a cell is palced in pure water or hypotonic solution then water enter into the cell, result turgor
pressure develop in the cell. THe cell starts swelling due to the turgor pressure. Simultaneously ,
concentration of cell sap this, when value of TP will be equal to the OP then DPD will be zero.
At this stage cell becomes fully turgid. Therefore in a fully turgid cell
DPD = OP – TP
When, OP = TP or OP – TP = O
So that
DPD = 0
(iii) DPD in flaccid cell :
If, the cell is in placed state then its T.P or W.P would be zero and value of DPD would be equal to
OP.
TP or Wp = O
Therefore,
DPD or S.P = OP
If a flaccid cell placed in water then waters enter into cell because DPD of the cell sap is higher.
(iv) DPD for plasmolysed cell :
The value of turgor pressure is negative in plasmolysed cell. In this state
DPD = OP – TP
 [TP = –Ve]
DPD = OP – [–TP] OP + TP
DPD = OP + TP
Demand of water = Plasmolysed cell > Flaccid cell > Partially turgid cell >? Fully turgid cell
A - Cell
B - Cell
OP = 25 atm
TP = 10 atm
OP = 30
OP = 25
DPD = 15 atm
Greater DPD
DPD = 5
H2O
Lesser DPD
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WATER POTENTIAL OR

W
“The difference between the free energy of molecules of pure water and free energy of
the solution is called water potential of the system”.
Now a days according to concept of free energy and thermodynamics DPD of a solution is also
representd by water potential.
The water potential of pure water is maximum because the pure water has a great free energy. The
free energy, lower down by additional of solute. THe value of water potential of pure water is taken
to be zero.
Water potential is represented by Greek word  (Psi)  W and it is measured in bars or Pascal (Pa)
water potential is equal to DPD but opposite in sign. Its value is negative.
 W = – DPD
Water potential has following components :
1.
Osmotic potential/ Solute potential ( S ) :
Osmotic potential or solute potential represents the concentration of the solute. Water potential
(  W ) is negative negative in the presence of solutes. So that osmotic potential is also negative.
More the solute less the solute potential.
2.
Pressure potential (  P ) :
Turgor pressure is also known as pressure potential according to concept of free energy.
S = Solute potential = – O.P..
 P = Pressure potential = T.P..
 W = S   P   g   m
 g ,  m is negligible
so
 W  S   p
 W = –ve
S = –ve
 p = +ve or –ve
Water always move from higher water potential towards towards the lower water potential.
For example if the water potential of ‘A’ cell is 10 bars and water potential of ‘B’ cell is - 12 in two
cells, then water will be flow from ‘A’ cell to the ‘B’ cell.
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PLASMOLYSIS

If a plant cell placed in a hypertonic solution, water molecules diffused out from the cell.
As a result of exosmosis, the protoplasm of the cell detached from the cell wall and starts
shrinking. This is called plasmolysis.
various steps of plasmolysis are as follows
(i)
Limiting Plasmolysis : This is the first step of plasmolysis. At this stage the cell wall has contracted to maximum.
(ii)
Incipient Plasmolysis : Continued exosmosis beyond the limiting plasmolysis decrease the size of
protoplast beacuse no further concentration in cell wall is there, so gap is produced between cell wall
& cell membrane. The concentration of cell membrane is intially from the corners.
(iii)
Evident Plasmolysis : The shrinking of protoplast is continued due to continuous exosmosis so it
become detached from the cell wall & assume a spherical shape.
 Hypertonic solution get filled between the cell wall & shrunken protoplast in plasmolysed cell.
SIGNIFICANCE OF PLASMOLYSIS :
 If plasmolysis continues for long duration in a cell then it dies. To destory weeds, salt is used
around their root.
 Fishes & meat are prevented from spoilage by salting ,which inhibit the growth of bacteria &
fungus.
 Higher concentration of sugar in jams & jellies stop the growth of bacteria & fungi.
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 Higher amount of chemical fertilizer near the root cause death or browing of plant due to plasmolysis.
IMBIBITION
(i)
(ii)
(iii)
(iv)
Adsorption of liquid by any solid material is called imbibition or adsorption of water by
hydrophilic colloids is known as imbibition.
This is a physical process by which a dry solid colloid material swells up by adsorption of water. The
cell wall is made up of colloidal substances as cellulose, pectin, hemicellulose etc. and they imbibe
water. Proteins, Agar- agar, starch etc, these are all imbibant materials. Agar -agar can adsorbs 99
times more water than that of its weight. Some of the proteins adsorb 15 times more water.
Imbibition power = Agar – agar > Pectin > Protein > Starch > Cellulose
Water or any other liquid which is adsorbed is known as imbibate & the solid adsorbent which cause
imbibition or adsorption is called imbibant.
Affinity is must between imbibant & liquid material & movement of water occurs according to water
potential gradeint. (These are 2 pre-requistie for imbibition)
Imbibition is also a type of diffusion since water movement is along a concentration gradient ;
the seeds and other such materials have almost no water hence they adsorb water easily.
Water potential gradient between the adsorbent and the liquid imbibed is essential for
imbibition. In addition, for any substance to imbibe any liquid, affinity between the adsorbent and
the liquid is also a prequisite.
The heat released during the imbibition is called heat of wetting.
A huge pressure is developed in material due to imbibition. This pressure is called Imbibition pressure (IP).
IP is also called as matric potential with respect to water potential. DPD = IP or  w   m
The imbibition is less in compact arranged material like wood, and more in lighter or soft material like
gelatin.

Significance of Imbibition :
(1)
Example of imbibition are adsorption of water by seeds and dry wood. The pressure is produced by the swelling of wood had been used by prehistoric man to split rocks and boulders.
If it were not the pressure due to imbibition, seedings would not have been able to emerge
out of the soil into the open; they probably would not have been able to establish.
Breaking of seed coat during the seed germination is due to imbibition process. Proteins, fats and
starch is present in the kernel. This kernel swells up more as compared to the seed coat which
breaks the seeds coat.
Initial process of water absorption in roots by the root hairs is imbibition.
Resurrection in many plants like Selaginella, Lichen, takes place due to the process of imbibition.
The water enter into the aerial roots due to imbibition.
(2)
(3)
(4)
(5)
Movement of water molecules :
Higher D.P.
Lower D.P.

Lower O.P.
Higher O.P.

Lower DPD 
Higher DPD
Higher (less –ve)  w
Lower (more –ve)  w

Higher T.P.
Hypertonic

Lower conc. of solution
Higher conc. of solution.

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TRANSPIRATION

Loss of water in form of vapour , from the aerial parts (organs) of living plants is known
as Transpiration.
Only 1-2 % of absorbed water is used by the plants , while remaining [98-99 %] of water lost
in atmosphere.
“Transpiration is an essential evil” - by Curtis
“Transpiration is an unavoidable evil” - by Stewards.
The minimum transpiration is found in succulent xerophytes.
Maximum transpiration is found in mesophytes.
→
Transpiration is absent in algae, fungi & submerged aquatic plants.
TYPES OF TRANSPIRATION

(i)
Transpiration is of the following three types :
Stomatal transpiration :
Transpiration takes place through the stomata which are present on the leaves of the plants and
delicate organs, is called stomatal transpiration. The maximum amount of water is lost by this
transpiration. About 80 % to 90 % transpiration is occurs through the stomata.
(ii)
Cuticular Transpiration :
Loss of water through the cuticle which present on the herbaceous stem and leaves. Cuticle is a
wax like thin layer present on epidermis. About 9 % to 9.9 % transpiration is cuticular.
(iii)
Lenticular Transpiration :
Minute pore like structure found on the stem of woody plants and epidermis of some fruits called
lenticles. Some amount of water is lost by lenticles is known as lenticular transpiration. However it
is approximately 0.1 % to 1 % of the total water lost.
Folier transpiration : Total transpiration takes place through the leaves is called as foliar transpiration.
Foliar transpiration (from the leaves) = Stomatal + Cuticular
Stomata are found on the aerial delicate organs and outer surface of the leaves in the form of
minute pores. Stomatal pore is surrounded by two specialised epidermal cells called as guard
cell. They are kidney shaped or crescent shape.
The shape of guard cells in monocots (Gramineae) is dumbel shaped
Guard cells are epidermal cells. But due to presence of chloroplast they are differ from
other epidermal cells.
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The outer wall of the guard cells is thin and elastic, while inner wall is thick and elastic.
Guard cells are surrounded by some specialised epidermal cells called subsidiary cells or accessory cells.
Stomata are found on both upper and lower surface. Stomata attached with air chambers which
forms a cavity that is called sub-stomatal-cavity.
MECHANISM OF OPENING AND CLOSING OF STOMATA
 The cause of the opening or closing of stomata is a change in turgidity of the guard cell. When
guard cell become turgid, stomata opens and when guard cell become flaccid, stomata closes.
 When osmotic concentration (OP) of guards cell is high, then guard cell become turgid by absorption of water through endosmosis from near by cells (subsidary cells)
 When OP of guard cell decreases then guard cell become flaccid. Due to flaccidity of guard cell,
inner thick wall of both guard cell become fused & stomata closes.
 The opening of stomata is also aided due to orientation of the microfibrils in the cell wall of the
guard cell. Cellulose microfibrils are oriented radially rather than longitudnally making it easier for
the stomata to open.

Active K+  H+ exchange theory or active proton transport mechanism
This is mordern & most accepted theory for stomatal movement
 First of all Fujino observed influx of K+ ions in guard cell during stomatal opening. Detail study of
this phenomenon was done by Levitt. According to him stomata open by following mechanism :
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Closing of stomata : Plant hormone ABA-acts on guard cells, which interfere the exchange of
K+ 
H+ ions in guard cells, results in reserve of reaction of opening of stomata, hence stomata closed.
 High concentration of K+ ion in guard cell is electrically balanced by uptake of (–ve & malate
ions which are (+ve) in guard cells.
FACTORS AFFECTING STOMATAL OPENING AND CLOSING
(1)
Light :
In most of the plants stomata open during the day and close during the dark (except succulent
xerophytic plants). Opening of stomata completes in the presence of blue and red light.
Blue light is most effective and causing stomatal opening.
(2)
Temperature :
Loft field show temperature quotient of opening of stomata is [Q10] = 2
(3)
CO2 concentration :
Stomata opens at low concentration of CO2 while closed at high concentration of CO2
CO2 is antitranspitrant gas.
(4)
Growth Hormones :
Cytokinin hormone induce opeing of stomata. It increase the influx of K+ ions and stimulate the
stomata for opening.
While ABA stimulate the stomata for closing. This hormone oppose the induction effect of
cytokinin.
ABA affects the permeability of the guard cells. It prevent the out flux of H+ ions and increase the
out flux of K+ ions. Because of this pH of the guard cells decreased.
FACTORS AFFECTING THE RATE OF TRANSPIRATION
(A)
EXTERNAL FACTORS :
(1)
Atmospheric humidity : Tr  Relative humidity
1
This is the most important factor. The rate of transpiration is higher in low atmospheric humidity
while at higher atmospheric humidity, the atmosphere is moistened, resluts in decrease of rate of
transpiration.
Therefore, the rate of transpiration is high during the summer and low in rainy season.
(2)
Temperature : Tr  Temperaturee
The value of Q10 for transpiration is 2. It means by increasing 10oC temperature, the rate of transpiration is approximately double. (By Loftfield)
Water vapour holding capacity of air increased at high temperature, resulting therate of transpiration
increased. On contrary vapour holding capacity of air decreased at low temperature so that the rate
of transpiration is decreased.
(3)
Light :
Light stimulates, transpiration by heating effect on leaf.
Action spectrum of transpiration is blue and red.
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Rate of transpiration is faster in blue light than of red light. Because stomata are completely
opened as their full capacity in the blue light.
(4)
Wind velocity : Tr  Wind velocity
Transpiration is less in constant air but if wind velocity is high the rate of transpiration is also high,
because wind removes humid air (saturated air) around the stomata.
Transpiration increases in the begining at high wind velocity [30 - 35 km/hour] But latter on it cause
closure of stomata sue to water stress and transpiration decrease.
(5)
Atmospheric Pressure :
At low atmospheric pressure, rate of the diffusion increase which increase the rate of transpiration.
By carrying a plant from Kota, to hill station, rate of transpiration increased.
Transpiration ratio (TR) : Moles of H2O transpired/moles of CO2 assimilated
Ratio of the loss of water to the photosynthetic CO2 fixation is called TR.
TR is low for C4 plants (200-350) while high for C3 plants (500-1000). It means C4 conserve
water with efficient photosynthesis
CAM plants posses minimum TR (50-100)
(B)
INTERNAL OR PLANT FACTORS :
(1)
(2)
(3)
(4)
(5)
Number & distribution of stomata
Number of opened stomata
Water status of the plant
Canopy structure
CO2 concentration : stomata are sensitive towards the internal CO2 conc. in the leaves.
 Stomata open at low concen. of CO2 while close at high conc. of CO2
Anti transpirants :
Chemical substance which reduce the rate of transpiration are known as antitranspirants. Anti
transpirants are as follows :
Phenyl Mercuric Acetate [PMA], Aspirin, (Salicylic acid), Abscisic Acid [ABA], Oxi-ethylene, Silicon oil,CO2 and low viscous wax
Antitranspirantss are used in dry farming.
TRANSPIRATION AND PHOTOSYNTHESIS A COMPROMISE
(1)
(2)
(3)
(4)
(5)
(6)
Inregulation of temperature :
Cooling effect on the surface of leaves is produced by the process of tranpiration, due to which
temperature remains constant in plants.
Translocation of minerals in plants body
In ascent of sap
Supplies water for photosynthesis
In water absorption
Maintain the shape and structure of the plants by keeping cells turgid.
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HOW DO PLANT ABSORB WATER ?
ORGAN FOR WATER ABSORPTION

(i)
(ii)
(iii)
(iv)
Roots have four different regions :
Root cap region
Meristematic region
Elongation region
Root hair region
The maximum absorption of water takes place from root hair region. These root hairs increases
the absorption area of root.
PATH OF WATER ABSORPTION
Soil solution  Root hairs  Epiblema/Epidermis  cortex  Cortex  Endodermis
(passage cells)  Pericycle cells  Protoxylem  Metaxylem.
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 The responsibility of absorption of water & minerals is more specifically the function of the root
hairs that are present in millions at the tips of the roots.
 Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the
surface area for absorption.
 Once water is absorbed by the root hairs, it can move deeper into root layers by 2 distinct
pathway:
* apoplast pathway
* symplast pathway
 The apoplast is the system of adjacent cell walls that is continuous throughout the plant, except at
the casparian strips of the endodermis in the roots.
 Apoplastic movement of water occurs exclusively through the intercellular space & the walls of
the cells.
* Movement through apoplast does not involve crossing the cell membrane.
* This movement is dependent on the gradient.
 The apoplast does not provide any barrier to water movement & water movement is through
mass flow.
 Mass flow of water occurs due to adhesive & cohesive property of water.
 The symplastic system is the system of interconnected protoplast.
Neighbouring cells are connected through cytoplasmic strands that extend through Plasmodesmata.
 During symplastic movement, the water travels through thecells their cytoplasm ; intercellular
movement is through the plasmodesmata.
 Water has to enter the cells through the all membrane, hence the movement is relatively slower.
Movement is again down a potential gradient.
* symplastic movement may be aided by cytoplasmic streaming.
 Most of the water flow in the roots occur via apoplast since the cortical cells are loosely packed,
& hence offer no resistance to water movement.
* However, the inner boundary of the cortex, the endodermis is impervious to water because of a
band of suberised matrix c/d the casparian strip water molecules are unable to penetrate the layer, so
they are directed to wall regions that are not suberised, into the cells proper through the membranes.
 The water then moves through the symplast & again crosses a membrane to reach the cells of
the xylem.
 The movement of water through the root layers is ultimately symplastic in the endodermis.This is
the only way water & other solutes can enter the vascular cylinder.
 Once inside the xylem, water is again free to move between cells as well as through them. In
young roots, water enters directly into the xylem vessels and/or tracheids. These are non-living
conduits & so are part of apoplast.
 Some plants have additional structures associated C them that help in water (& mineral) absorption.
 A mycorrhiza is a symbiotic association of a fungus C a root system.
The fungus provides minerals & water to the roots, in turn the roots provide sugar & N-containing
compounds to the mycorrhizae.
Some plants have an obligate association & the mycorrhizae. For eg : Pinus seed cannot germinate
& estabilish C out the pressure of mycorrhizae.
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Factors affecting water absorption
(1)
Available soil water :
Plant absorbs capillary water, which is present in soil. Absorption of water depends on the amount
of capillary water present in the soil. Absorption increases by increasing amount of capillary water.
If, water is present in higher amount in the soil then such type of soil is called “ Water logged soil”
. This soil is physiologically dry and lack oxygen. Because of this anaerobic respiration takes place
in roots, and alcohol is formed. Roots can be degenerate due to form alcohol. (Dry soil is physically
dry.)
(2)
Soil temperature (Suitable temperature - 20 to 35oC) :
Soil temperature affects the following mechanisms :
Low temperature decreases the permeability of cell membrane.
It is essential for the activity of enzymes for the formation of root hairs.
At low temperature viscosity of capillary water is increased.
Cold soil is as physiologically dry.
(i)
(ii)
(iii)
(3)
Soil Air :
Absorption of water proceeds more rapidly in well aerated soil.
Deficiency of oxygen in soil causes improper respiration in roots. Poorly aerated soil is physiologically dry.
(4)
Soil Concentration :
The rate of the absorption is inversely proportional to the concentration of minerals present in soil.
1
Water Absorption  concentration of soil minerals
Water absorption is only take place in appropriate soil solution. If the concentration of soil minerals is
high. It decreases the rate of absorption. Therefore saline soil is physiologically dry. Only Halophytes can grow in this soil because they maintain high OP.
(5)
Transpiration :
According to Kramer the rate of water absorption is directly proportional to the rate of
transpiration.
The rate of absorption increases due to increase in the transpiration. Because passive water absorption increases due to transpiration.
LONG DISTANCE TRANSPORT OF WATER :
 Long distance transport of substance within a plant cannot be by diffusion alone. Diffusion is a
slow process. It can account for only short distance movement of molecules.
 In large & complex organisms, often substance have to be moved across very large
distancesometimes the site of production or absorption & site of storage are too far from each other,
diffusion or active transport would not suffice.
 Special long distance transport systems become necessary so as to move substances across long
distance & at a much faster rate.
Mass flow is the movement of substance in bulk from one point to another as a result of pressure
differences between 2 points.
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 It is a characteristic of mass flow that substance, whether in solution or in suspension are swept
along at the same pace, as in flowing river.
 This is unlike diffusion where different substance move independently depending on their concentration gradients.
 Bulk flow can be achieved either through a positive hydrostatic pressure gradient (pushing pressure) or a negative hydrostatic pressure gradient (e.g. suction through a straw).
 The bulk movement of substance through the conducting or vascular tissue of plants is called
translocation.
 The higher plants have highly specialized vascular tissue - xylem & phloem
WATER MOVEMENT UP A PLANT (ASCENT OF SAP)
Upward movement of absorbed water against the gravitational force upto top parts of plants
is called ascent of sap.
Experiment on Balsam plant by using eosin dye proved that xylem is path of ascent of sap.
MECHANISM OF ASCENT OF SAP :
Cohesion - tension - transpiration pull model : By Dixon & Jolly
 Most accepted or universally accepted theory for explaining mechanism of ascent of sap.
* According to it following components are involved in ascent of sap :
(a)
Cohesion : Mutual attraction between the wate molecules is K/a cohesion which helps to form a
continuous water column in xylem elements.
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(b)
Adhesion : Attraction between xylem walls & water molecules is C/d adhesion force, which help in
upward maintenance of water column in the xylem.
(c)
Surface tension
(d)
Transpiration Pull : A negative pressure (pulling pressure) develop in xylem due to rapid transpiration
in leaves, this is c/d transpiration pull which is responsible for pulling of water column in xylem.
 Ascent of sap is constitutive effect of cohesion, surface tension, adhesion & transpiration pull.
Measurements reveal that the force generated by transpiration can create pressure sufficient to lift
a xylem sized column of water over 130 metres high.
 Cohesion, adhesion & surfacetension properties give water high tensile strength means ability to
resist pulling force & also give water high capillarity means ability to rise in thin tubes.
* In plants capillarity is aided by small diameter of tracheids & vessels.
Root pressure :
As various ions from the soil are actively transported into the vascular tissue of the roots,
water follows (its potential gradient) and increases the pressure inside the xylem. The
positive pressure is called root pressure, and can be responsible for pushing up water to small
height in the stem.
Root pressure can, at best , only provide a modest push in the overall process of water
transport.
They obviously do not play a major role in water movement up tall trees.
Thre greatest contribution of root pressure may be to re-establish the continuous chains
of water molecules in the xylem which often break under the enormous tensions created
by transpiration.
Root pressure does not account for the majority of water transport; most plants meet their
need by transpiratory pull.
PHLOEM TRANSPORT (TRANSLOCATION IN PLANTS)
Food/organic material condition in plants mainly occurs by phloem. (Proved by Girdiling experiment).
Food conduction occurs in between source and sink.
Generally green photosynthetic plant parts like leaves acts as source while non photosynthetic parts
like root, shoot, fruits acts as sink.
Food conduction may may be in any required direction unlike the water coduction which is a unidirectional process.
Translocation of food mainly occurs in the form of sucrose because it is non-reducing sugar and
chemically inert in pathway of coduction.
Phloem sap is mainly water & sucrose but other sugar hormones & amino acid are transported or
translocated through phloem.

Pressure flow/mass flow hypothesis of food/sucrose translocation - Given by E. Munch.
This is the most accepeted theory of food conduction in plants.
According to it food translocation occurs in between source and sink in order of turgor
pressure gradient i.e., high T.P to low T.P
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Leaf Mesophyll Cells
Photosynthesis
Glucose
Sucrose
Rise in osmotic pressure
Endosmosis
Endosmosis
Rise in Turgor pressure of mesophyll cell
'en mass' comes in Sieve tube cell from Xylem and increase
Turgor pressure Gradient
LOADING
Mass flow of H2O & Sucrose due to Turgor Gradient
ATP
ADP
Water comes in Sieve tube cell from Xylem and increase
Turgor pressure Gradient
Water comes in Sieve tube cell from Xylem and increase
Turgor pressure Gradient
UNLOADING
Sucrose comes to Root cell and convert into starch or
energy or get consumed in Respiration
Decrease in O.P of Root Cells
H2 O
Water moves to Root Xylem
Sucrose translocation in plants
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GUTTATION

Loss of water from the uninjured part or margin of leaves of the plant in the form of water
droplets is called guttation.
Guttation occurs when transpiration is low and water absorption is high.
Liquid of guttation along with water contains some organic and inorganic (dissolved) substances.
It means it is not pure water.
Normally, guttation process is found in herbacious plants like grasses, tomats, balsum, Naustertium,
Colocasia, Sexifraga and in some of the plants of Cucurbitacease family.
Guttation occurs from the margins of the leaves through the special pore (always open) like structures called hydathodes or water stomata. Generally guttation occurs during mid night or early
morning.
Parenchymatous and loose tissue are lie beneath the hydathode, which are known as epithem or
transfer tissues.
The process of guttation take place due to root pressure.
EXUDATION/BLEEDING

If liquid ooze out from the injured or cut parts of the plants is called bleeding or exudation.
This process takes place due to high root pressure.
Sugar is obtained from the sugar mapple by this process. Opium. Latex of rubber is obtained by
this process.
The highest bleeding is found in Caryota urens (Toddy palm) (about 50 liter per day).

Wilting :
SOME EXTRA POINTS
Cobalt-chloride test : This method is used for the comparision of transpiration at both the surface
of the leaves.
Porometer is used to find out the area of stomata on the leaf.
Transpiration measuring instrument is called potometer. The rate of absorption of water is measured through this instrument because rate of water absorption is proportional to the rate of
transpiration.
Manometer is used to measure root pressure.
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Cell & Cell Division # 20