III yr BOTANY HONS

1. Water and water absorption by
plants
Dr. Sujit GhoshDr. Sujit Ghosh
J K College
Purulia

3. Plant
Paths
•Symplastic ­through
plasmodesmata
•Apoplastic ­cell walls and
intercellular spaces
•Transmembrane ­by crossing the
plasma membranes
Mechanism
•Active absorption­root cells –
respiratory energy invested
•Osmotic ­to the osmotic gradient
•Nonosmotic ­against the osmotic
gradient.
•Passive
•Transpiration
Routes
•Next page
Plant
water
relation
•Transpiration
Cause
•Transpiration
•Root pressure
•Cohesion and adhesion
•Capillarity
Factors
•Plant factors
•Environmental factors

4. Water in soil Root hair Root cortex
Endodermis
(biological check
post)
Pericycle Root Xylem
Upward
movement
through stem
xylem
Leaves Transpiration
Diffusion through
stomata

5. Factor affecting
water absorption
Plant factors
Root system
Branching of root
Length and
number root hair
and root hair
zone
walls of root
hairs are
permeable and
consist of pectic
substances and
cellulose which
are strongly
hydrophilic
High rate of root
hair formation
Lacking water
resistance
cuticle
Resistance of
conducting
system
Availability of
soil water
Environmental
factors
Concentration of
salt
Soil air
Transpiration
Soil temperature

6. Functions
Is a major component of cells.
Is a solvent for the uptake and
transport of materials
Is a good medium for
biochemical reactions.
Is a reactant in many
biochemical reactions (i.E.,
Functions
of water:
biochemical reactions (i.E.,
Photosynthesis).
Provides structural support via
turgor pressure (i.E., Leaves).
Is the medium for the transfer of
plant gametes (sperms swim to
eggs).
In water, some aquatic plants shed
pollen underwater.

7. Site of water
absorption
The successively
smaller branches
of the root
system of any
plant terminate
ultimately in the
root tips, of
which there may
Most absorption
of water occurs
in the growing
root tip regions,
and especially in
Older portions of
most roots
become covered
with cutinized or
suberized layers
through which
Whenever the
water potential
in the peripheral
root cells is less
than that of the
soil water,
movement ofwhich there may
be thousands
and often
millions on a
single plant.
and especially in
the root hair
zone.
through which
only very limited
quantities of
water can pass.
movement of
water from the
soil into the root
cells occurs.

8. Mechanism of water absorption is of two types:
Active Absorption of Water:
In this process the root cells play active
role in the absorption of water and
metabolic energy released through
Passive Absorption of Water:
It is mainly due to transpiration, the root
cells do not play active role and remain
metabolic energy released through
respi­ration is consumed.
Osmotic absorption i.e., when water is ab­sorbed from
the soil into the xylem of the roots according to the
osmotic gradient.
Non­osmotic absorption i.e., when water is absorbed
against the osmotic gradient.
cells do not play active role and remain
passive.

9. 1. Water potential term was authored by Slatyer and Taylor (1960). . It
is present day term which is utilized as in place of DPD.
2. The movement of water in plants cannot be accurately explained in
terms of difference in concentration or in other linear expression.
3. 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 (from higher free energy level to lower
free energy level).
4. As indicated by standards of thermodynamics, every components of
system is having definite amount of free energy which is measure ofsystem is having definite amount of free energy which is measure of
potential work which the system can do.
5. Water Potential is the difference in the free energy or chemical
potential per unit molar volume of water in system and that of pure
water at the same temperature and pressure.
6. Water always moves from the area of high water potential to the
area of low water potential. Water potential of pure water at normal
temperature and pressure is zero. This value is considered to be the
highest. The presence of solid particles reduces the free energy of
water and decreases the water potential. Therefore, water potential
of a solution is always less than zero or it has negative value.

10. MATRIX POTENTIAL
(due to binding of
water to cell and
cytoplasm)
SOLUTE POTENTIAL
(due to concentration
of dissolve solutes
which by its effect on
the entropy
components reduces
the water potential)
PRESSURE
POTENTIAL
(due to hydrostatic
pressure, which by its
effect on energy
components increases
the water potential)
COMPONENTS OF WATER POTENTIAL
(A typical plant cell consists of a cell wall, a vacuole
filled with an aqueous solution and a layer of
cytoplasm between vacuole and cell wall. When such a
cell is subjected to the movement of water then many
factors begin to operate which ultimately determine
the water potential of cell sap.)

11. Ψw = Ψs + Ψp + Ψm
Ψs= As the amount by which the water potential is reduced as
the result of the presence of the solute, are always in negative
values and it is expressed in bars with a negative sign.
Ψp = Plant cell wall is elastic and it exerts a pressure on the
cellular contents. As a result the inward wall pressure,
hydrostatic pressure is developed in the vacuole it is 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 barsmagnitude varies between +5 bars (during day) and +15 bars
(during night).
Ψm =Due to hydrostatic pressure, which by its effect on energy
components increases the water potential
In case of plant cell, m is usually disregarded and it is not
significant in osmosis. Hence, the above given equation is
written as follows.
Ψw = Ψs + Ψp

12. Pure water has the
maximum water
potential which by
definition is zero.
Water always moves
from a region of higher
Ψw to one of lower Ψw.
All solutions have
lower water potential
than pure water
Osmosis in terms of
water potential occurs
as a movement of
water molecules from
the region of higher
water potential to a
region of lower water
potential through a
semi permeable
membrane

13. Osmotic Relations of
Cells According to
Water Potential:
In case of fully
turgid cell:
In case of flaccid
cell:
In case of
plasmolysed cell:
The net movement of water
into the cell is stopped. The
cell is in equilibrium with the
water outside. Consequently
the water potential in this
case becomes zero. Water
potential is equal to osmotic
potential + pressure
potential.
The turgor becomes zero. A
cell at zero turgor has an
osmotic potential equal to its
water potential.
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 wall and the cells are
plasmolysed. Such cells are
negative value of pressure
potential (negative turgor
pressure).

14. Active Osmotic Absorption of Water:
First step in the osmotic absorption of water is the imbibition of
soil water by the hydro­philic cell walls of root hairs.
Osmotic Pressure (O.P.) of the cell-sap of root hairs is usually
higher than the O.P. of the soil water.
Therefore, the Diffusion Pressure Deficit (D.P.D.) and the suction
pressure in the root hairs become higher and water from the cell
walls enters into them through plasma-membrane (semi-
permeable) by osmotic diffusion.permeable) by osmotic diffusion.
As a result, the O.P., suction pressure and D.P.D. of root hairs now
become lower, while their turgor pressure is increased.
Now, the cortical cells adjacent to root hairs have higher O.P.,
suction pressure and D.P.D. in comparison to the root hairs.
Therefore, water is drawn into the adjacent cortical cells from the
root-hairs by osmotic diffusion.
In the same way, the water by cell to cell osmotic diffusion
gradually reaches the inner­most cortical cells and the
endodermis. Osmotic diffusion of water into endodermis takes
place through special thin walled pas­sage cells because the other

15. Water absorption
Active
absorption
Passive
absorption
Mass flow Diffusion
Osmotic theory
(Atkins and
Prisely)
Non osmotic
theory(Thimann
and Krammer)

16. Process of Water movement : There are two major ways to move molecules:
Bulk (or Mass) Flow Diffusion
This is the mass movement of
water molecules in response to a
pressure gradient. The molecules
move from high to à low pressure,
following a pressure gradient.
The net, random movement of
individual molecules from one area to
another. The molecules move from
[high] à [low], following a
concentration gradient. Another way of
stating this is that the molecules move
from an area of high free energy (higher
concentration) to one of low free energy
(lower concentration). The net
movement stops when a dynamic
equilibrium is achieved.

17. Water Movement Through a Plant :
To start with the roots:
Most of the water absorption is carried
out by the younger part of the roots.
Just behind the growing tip
of a young root is the
piliferous region, made up of
hundreds of projections of
the epidermal tissue, the
root hairs. Root hairs can be
Absorption mechanism :
All absorption of
water occurs
along gradient
of decreasing
However, the gradient is produced
differently in slowly and in rapidly
transpiring plants. This results in
two absorption mechanisms:
Root pressure:
Roots of plant absorb
water from the soil.
root hairs. Root hairs can be
seen very clearly in newly
germinated seeds, The root
hairs are short lived being
constantly replaced as new
growth takes place.
The narrow walled hairs
greatly increase the surface
area over which water
absorption can take place.
Water in the soil spaces is
taken into the root hairs by
the process of osmosis,
there being a higher water
concentration outside than
within the root hair cells.
of decreasing
water from the
medium in
which the roots
are growing to
the root xylem.
active absorption or
osmotic absorption in
slowly transpiring
where roots behave as
osmometers,
passive absorption in
rapidly transpiring
plants where water is
pulled in by the
decreased pressure or
tension produced in the
xylem sap through the
roots, which function as
passive surfaces. It is
operative in the form of
root pressure, bleeding
and guttation.
Roots of plant absorb
water from the soil.
Water is thus exuded
in the xylem ducts of
the root and stem
under pressure, the
pressure developed
inside the roots due to
absorption of water is
called the root
pressure. It is
believed to be a simple
osmotic process,
caused by
accumulation of
sufficient solutes in
the xylem ducts to
lower the water
potential of the xylem
sap below that of the
substrate.

19. Root system
Length and number Shallow or deep
Active Growth and
metabolism
The number of root
hairs
length of root hairs
length of root hair zone
determine the extent
of water absorbed from
the soil.
Deeper portions of the
roots are less efficient
in the water uptake
Efficient water
absorption through
less deep portions.
The continuous
formation and growth
of root hair facilitate
water uptake.
Also metabolism of
the root hair influences
the amount of water
uptake.

20. Metabolic
state of the
protoplasm,
Nature of
endodermis,
Xylem
vessels and
their location
The rate of water
absorption
directly depends
upon the
resistance to the
passage of water.
Cell wall
permeability,
Distribution
and
diameter.

21. The Pathway of Water
The cytoplasm of root cells — called the symplast — that is, it
crosses the plasma membrane and then passes from cell to cell
through plasmodesmata.
In the nonliving parts of the root — called the apoplast — that
is, in the spaces between the cells and in the cells walls
themselves. This water has not crossed a plasma membrane