soil structure, types of soil water, soil water, xylem transport, root water absorbtion

1. Plant water transport
Dr. Baljinder Singh Gill
Panjab University
Chandigarh

2. Plant Transport
• How does water get from
the roots of a tree to its
top?
• Plants lack the muscle
tissue and circulatory
system found in animals,
but still have to pump
fluid throughout the
plant’s body

3. Plant Transport
• Water first enters the roots and then
moves to the xylem, the innermost
vascular tissue
• Plants need water
– As a starting product for photosynthesis
– As a solvent to dissolve chemicals
– For support
– To ‘pay’ for water lost by transpiration (97%)

4. WATER IN PLANT LIFE
constitutes 80 to 95% of the mass of growing plant tissues.
Wood, has a lower water content; sapwood, contains 35 to 75%
Seeds, with a water content of 5 to 15%, are among the driest of plant
tissues, yet before germinating they must absorb a considerable amount of
water
Water is the most abundant and arguably the best solvent known. greatly
influences the structure of proteins, nucleic acids, polysaccharides, and
other cell constituents. Water forms the environment in which most of the
biochemical reactions of the cell occur

5. THE STRUCTURE AND PROPERTIES OF WATER
for molecules such as sugars and proteins that contain polar —OH or —
NH2 groups. Hydrogen bonding between macromolecules and water
reduces the interaction between the macromolecules and helps draw
them into solution.

6. Thermal properties
The extensive hydrogen bonding between water molecules results in
unusual thermal properties, such as high specific heat and high latent
heat of vaporization. Specific heat is the heat energy required to raise
the temperature of a substance by a specific amount. When the
temperature of water is raised, the molecules vibrate faster and with
greater amplitude. To allow for this motion, energy must be added to the
system to break the hydrogen bonds between water molecules. Thus,
compared with other liquids, water requires a relatively large energy input
to raise its temperature. This large energy input requirement is important
for plants because it helps buffer temperature fluctuations.

7. LATENT HEAT OF VAPOURIZATION
the energy needed to separate molecules from the liquid phase 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 kJ mol–1—the highest value known for any liquid.
Most of this energy is used to break hydrogen bonds between
water molecules. The high latent heat of vaporization of water enables
plants to cool themselves by evaporating water from leaf surfaces,
which are prone to heat up because of the radiant input from the sun.
Transpiration is an important component of temperature regulation
in plants.

8. COHESIVE AND ADHESIVE PROPERTIES OF WATER
Water molecules at an air–water interface are more strongly attracted to
neighboring water molecules than to the gas phase in contact with the water
surface. To increase the area of an air–water interface, hydrogen bonds must be
broken, which requires an input of energy. The energy required to increase the
surface area is known as surface tension. Surface tension not only
influences the shape of the surface but also may create a pressure in the rest
of the liquid. As we will see later, surface tension at the evaporative surfaces of
leaves generates the physical forces that pull water through the plant’s vascular
system.

9. The extensive hydrogen bonding in water also gives rise to the property known
as cohesion, the mutual attraction between molecules. A related property,
called adhesion, is the attraction of water to a solid phase such as a cell wall
or glass surface the degree water attract to solid phase can be quantified by
contact angle. CONTACT ANGLE DEFINE SHAPE OF OF AIR –WATER
INTERFACE . THETA
Water Has a High Tensile Strength
Cohesion gives water a high tensile strength, defined as the maximum force
per unit area that a continuous column of water can withstand before breaking.
We do not usually think of liquids as having tensile strength; however, such a
property must exist for a water column to be pulled up a capillary tube. We can
demonstrate the tensile strength of water by placing it in a capped syringe

11. Minerals
H2O
H2O
CO2 O2
Sugar
Light
CO2
O2

12. Soil-water system
Soil is a heterogeneous mass consisting of a three phase system of solid, liquid
and gas. Mineral matter, consisting of sand, silt and clay and organic matter
form the largest fraction of soil and serves as a framework (matrix) with
numerous pores of various proportions. The void space within the solid
particles is called the soil pore space. Decayed organic matter derived from the
plant and animal remains are dispersed within the pore space. The soil air is
totally expelled from soil when water is present in excess amount than can be
stored.
when the total soil is dry as in a hot region without any supply of water either
naturally by rain or artificially by irrigation, the water molecules surround the soil
particles as a thin film. In such a case, pressure lower than atmospheric thus
results due to surface tension capillarity and it is not possible to drain out the
water by gravity.

13. The salts present in soil water further add to these forces by way of osmotic pressure.
The roots of the plants in such a soil state need to exert at least an equal amount of
force for extracting water from the soil mass for their growth.
Soil properties
Soil texture:
This refers to the relative sizes of soil particles in a given soil. According to their
sizes, soil particles are grouped into gravel, sand, silt and day. The relative
proportions of sand, silt and clay is a soil mass determines the soil texture.
Figure 1 presents the textural classification of 12 main classes as identified by
the US department of agriculture, which is also followed by the soil survey
organizations of India.

15. According to textural gradations a soil may be broadly classified as:
• Open or light textural soils: these are mainly coarse or sandy with low
content of silt and clay.
• Medium textured soils: these contain sand, silt and clay in sizeable
proportions, like loamy soil.
• Tight or heavy textured soils: these contain high proportion of clay
Soil structure:
This refers to the arrangement of soil particles and aggregates with respect to
each other. Aggregates are groups of individual soil particles adhering together.
Soil structure is recognized as one of the most important properties of soil
mass, since it influences aeration, permeability, water holding capacity, etc.
The classification of soil structure is done according to three indicators as:-

16. Type: there are four types of primary structures-platy, prism-like, block like and
spheroidal.
• Class: there are five recognized classes in each of the primary types. These
are very fine, fine, medium, coarse and very coarse.
• Grade: this represents the degree of aggradation that is the proportion
between aggregate and unaggregated material that results when the
aggregates are displaced or gently crushed. Grades are termed as structure
less, weak, moderate, strong and very strong depending on the stability of the
aggregates when disturbed

17. Soil classification
Soils vary widely in their characteristics and properties. In order to establish the
interrelation ship between their characteristics, they need to be classified. In
India, the soils may be grouped into the following types:
Alluvial soils: These soils are formed by successive deposition of silt
transported by rivers during floods, in the flood plains and along the
coastal belts. This group is by for the largest and most important soil
group of India contributing the greatest share to its agricultural wealth.
Though a great deal of variation exists in the type of alluvial soil available
throughout India, the main features of the soils are derived from the
deposition laid by the numerous tributaries of the Indus, the Ganges and
the Brahmaputra river systems. These streams, draining the Himalayas,
bring with them the products of weathering rocks constituting the
mountains, in various degrees of fineness and deposit them as they
traverse the plains. Alluvial soils textures vary from clayey loam to sandy
loam. The water holding capacity of these soils is fairly good and is good
for irrigation

18. Black soils: This type of soil has evolved from the weathering of rocks such as
basalts, traps, granites and gneisses. Black soils are derived from the Deccan
trap and are found in Maharashtra, western parts of Madhya Pradesh, parts of
Andhra Pradesh, parts of Gujarat and some parts of Tamilnadu. These soils
are heavy textured with the clay content varying from 40 to 60 percent.the soils
possess high water holding capacity but are poor in drainage.
• Red soils: These soils are formed by the weathering of igneous and
metamorphic rock comprising gneisses and schist’s. They comprise of vast
areas of Tamil nadu, Karnataka, Goa, Daman & Diu, south-eastern
Maharashtra, Eastern Andhra Pradesh, Orissa and Jharkhand. They also are in
the Birbhum district of West Bengal and Mirzapur, Jhansi and Hamirpur districts
of Uttar pradesh. The red soils have low water holding capacity and hence well
drained.

19. Laterites and Lateritic soils: Laterite is a formation peculiar to India and
some other tropical countries, with an intermittently moist climate.
Laterite soils are derived from the weathering of the laterite rocks and are
well developed on the summits of the hills of the Karnataka, Kerala,
Madhya Pradesh, The eastern ghats of Orissa, Maharashtra, West Bengal,
Tamilnadu and Assam. These soils have low clay content and hence
possess good drainage characteristics.
• Desert soils: A large part of the arid region, belonging to western
Rajasthan, Haryana, Punjab, lying between the Indus river and the Aravalli
range is affected by the desert conditions of the geologically recent
origin. This part is covered by a mantle of blown sand which, combined
with the arid climate, results in poor soil development. They are light
textured sandy soils and react well to the application of irrigation water.

20. Classification of soil water
Gravitational water: A soil sample saturated with water and left to drain the
excess out by gravity holds on to a certain amount of water. The volume of
water that could easily drain off is termed as the gravitational water. This
water is not available for plants use as it drains off rapidly from the root
zone.
• Capillary water: the water content retained in the soil after the
gravitational water has drained off from the soil is known as the capillary
water. This water is held in the soil by surface tension. Plant roots gradually
absorb the capillary water and thus constitute the principle source of water
for plant growth.
• Hygroscopic water: the water that an oven dry sample of soil absorbs
when exposed to moist air is termed as hygroscopic water. It is held as a
very thin film over the surface of the soil particles and is under tremendous
negative (gauge) pressure. This water is not available to plants.

21. Soil water constants
For a particular soil, certain soil water proportions are defined which dictate
whether the water is available or not for plant growth. These are called the soil
water constants, which are described below.
• Saturation capacity: this is the total water content of the soil when all
the pores of the soil are filled with water. It is also termed as the
maximum water holding capacity of the soil. At saturation capacity, the
soil moisture tension is almost equal to zero.
• Field capacity: this is the water retained by an initially saturated soil
against the force of gravity. Hence, as the gravitational water gets drained
off from the soil, it is said to reach the field capacity. At field capacity, the
macro-pores of the soil

22. are drained off, but water is retained in the micropores. Though the soil moisture tension
at field capacity varies from soil to soil, it is normally between 1/10 (for clayey soils) to
1/3 (for sandy soils) atmospheres.
• Permanent wilting point: plant roots are able to extract water from a soil matrix,
which is saturated up to field capacity. However, as the water extraction
proceeds, the moisture content diminishes and the negative (gauge) pressure
increases. At one point, the plant cannot extract any further water and thus wilts.
Two stages of wilting points are recognized and they are:
• Temporary wilting point: this denotes the soil water content at which the plant
wilts at day time, but recovers during right or when water is added to the soil.
• Ultimate wilting point: at such a soil water content, the plant wilts and fails to
regain life even after addition of water to soil.

23. Plant Transport
• Water movement (transport) occurs at
three levels:
– Cellular
– Lateral transport (short-distance)
– Whole plant (long-distance)

24. Tissue organization in roots of soybean (Glycine max
‘Labrador’) (A) and maize (Zea mays) (B) plantlets

25. WATER TRANSPORT PROCESSES
From soil water enter (cell wall, cytoplasm, membrane, air spaces), and the
mechanisms of water transport also vary with the type of
medium. For many years there has been much uncertainty that how only
diffusion can resulted in observed rates of water movement across
membranes
Uncertainty overcome by discovery of Aquaporins are integral membrane
proteins that form water-selective channels across the membrane. Because
water diffuses faster through such channels than through a lipid bilayer,
aquaporins facilitate water movement into plant cells

26. Aquaporin Proteins and Water
Transport
• Aquaporins are transport proteins
in the cell membrane that allow
the passage of water
• Aquaporins do not affect water
potential.
• they facilitate the transport of
water and/or small neutral solutes
(urea, boric acid, silicic acid) or
gases (ammonia, carbon dioxide)
• Plasma membrane intrinsic
proteins (PIP), tonoplast intrinsic
proteins (TIP)

27. WATER TRANSPORT PROCESSES
From soil water enter (cell wall, cytoplasm, membrane, air spaces), and the
mechanisms of water transport also vary with the type of
medium. For many years there has been much uncertainty that how only
diffusion can resulted in observed rates of water movement across
membranes
Uncertainty overcome by discovery of Aquaporins are integral membrane
proteins that form water-selective channels across the membrane. Because
water diffuses faster through such channels than through a lipid bilayer,
aquaporins facilitate water movement into plant cells

28. Diffusion
the rate of diffusion is directly proportional to the concentration gradient (Δcs/Δx)
—that is, to the difference in concentration of substance s (Δcs) between two
points separated by the distance Δx. In symbols, we write this relation as Fick’s
first law:
The rate of transport, or the flux density (Js), is the amount of substance s
crossing a unit area per unit time (e.g., Js may have units of moles per square
meter per second [mol m–2 s–1]). The diffusion coefficient (Ds) is a
proportionality constant that measures how easily substances moves through a
particular medium. The diffusion coefficient is a characteristic of the substance
(larger molecules have smaller diffusion coefficients) and depends on the medium
(diffusion in air is much faster than diffusion in a liquid, for example). The negative
sign in the equation indicates that the flux moves down a concentration gradient.

29. Diffusion is faster over short distance
The average time needed for a particle to diffuse a distance L is equal to L2/Ds,
where Ds is the diffusion coefficient, which depends on both the identity of the
particle and the medium in which it is diffusing. Thus the average time required for
a substance to diffuse a given distance increases in proportion to the square of
that distance. The diffusion coefficient for glucose in water is about 10–9 m2 s–1.
Thus the average time required for a glucose molecule to diffuse across a cell with
a diameter of 50 μm is 2.5 s. However, the average time needed for the same
glucose molecule to diffuse a distance of 1 m in water is approximately 32 years.

30. Pressure-Driven Bulk Flow Drives Long-Distance Water Transport
If we consider bulk flow through a tube, the rate of volume flow depends on the radius (r) of
the tube, the viscosity (h) of the liquid, and the pressure gradient (ΔYp/Δx) that drives the
flow.
This equation tells us that pressure-driven bulk flow is very sensitive to the radius of the tube.
If the radius is doubled, the volume flow rate increases by a factor of 16.
Pressure-driven bulk flow of water is the predominant mechanism responsible for long-
distance transport of water in the xylem. It also accounts for much of the water flow through
the soil and through the cell walls of plant tissues. In contrast to diffusion, pressure-driven
bulk flow is independent of solute concentration gradients, as long as viscosity changes are

31. Osmosis Is Driven by a Water Potential Gradientselectively permeable;
that is, they allow the movement of water and other small uncharged
substances across them more readily than the movement of larger solutes
and charged substances
The direction and rate of water flow across a membrane are determined not
solely by the concentration gradient of water or by the pressure gradient, but
by the sum of these two driving forces.
The chemical potential of water is a quantitative expression of the free
energy associated with water. In thermodynamics, free energy represents the
potential for performing work.
Water potential is a measure of the free energy of water per unit volume (J
m–3).

35. Effects of Differences in Water
Potential
• To survive, plants must balance water uptake and loss
• Osmosis determines the net uptake or water loss by a cell
is affected by solute concentration and pressure
• Water potential is a measurement that combines the
effects of solute concentration and pressure
• Water potential determines the direction of movement of
water
• Water flows from regions of higher water potential to
regions of lower water potential

36. How Solutes and Pressure Affect
Water Potential
• Both pressure and solute concentration affect
water potential
• The solute potential of a solution is proportional
to the number of dissolved molecules
• Pressure potential is the physical pressure on a
solution

40. Addition of
solutes
0.1 M
solution
Pure
water
H2O
ψ = 0 MPa ψP = –0.23 MPa
ψP = 0
ψS = –0.23
The addition
of solutes
reduces water
potential

41. Applying
physical
pressure
H2O
ψ = 0 MPa ψP = –0 MPa
ψP = 0
ψS = –0.23
Physical
pressure
increases
water
potential

42. Applying
physical
pressure
H2O
ψ = 0 MPa ψP = –0.07 MPa
ψP = 0.30
ψS = –0.23

43. Negative
pressure
H2O
ψP = –0.23 MPa
ψP = 0.30
ψS = –0.23
ψP = –0.30 MPa
ψP = –0.30
ψS = –0.23
Negative
pressure
decreases
water
potential

44. • Water potential affects uptake and loss of
water by plant cells
• If a flaccid cell is placed in an environment
with a higher solute concentration, the cell
will lose water and become plasmolyzed
• If the same flaccid cell is placed in a
solution with a lower solute concentration,
the cell will gain water and become turgid
• Turgor loss in plants causes wilting, which
can be reversed when the plant is watered

45. Transport of Water Across the
Root• Water is absorbed from the soil by osmosisosmosis
• Water moves down thedown the ΨΨ gradientgradient
• Water only enters the root near the root tip
• Here there are root hairsroot hairs which increase the surface areaincrease the surface area for osmosis
• Water potential is higher in the epidermal cells than in the central cells
• Water moves across the cortex down thedown the ΨΨ gradientgradient to xylem vessels,
• Water can move via the symplastsymplast or apoplastapoplast routes

46. Diagram of
transverse root
section epidermis with root hairs
cortex
endodermis
xylem
phloem
pericycle

47. Water is transported
across the root by two
routes
Apoplast routeApoplast route SymplastSymplast
routeroute
between the cells viabetween the cells via
the cell wallsthe cell walls
cell cytoplasm to cellcell cytoplasm to cell
cytoplasmcytoplasm

48. The Symplast RouteThe Symplast Route
• Through the cytoplasmcytoplasm
• Water enters the root hair cells across the partially permeable
membrane by osmosis
• Water moves from higher Ψ in the soil to the lower Ψ in the cell
• Water moves across the root from cytoplasm to cytoplasm
down the Ψ gradient
• It passes from one cell to the other via plasmadesmataplasmadesmata
• Water moves into the xylem by osmosis
• The only way across the endodermisThe only way across the endodermis
• Normally the most important pathway

49. The Apoplast RouteThe Apoplast Route
• Water moves through the cellulose cell wallcellulose cell wall and intercellularintercellular
spacesspaces
• The permeable fibres of cellulose do no resist water flow
• Water cannot pass the endodermis by this route
• Because the Casparian strip in the endodermis cell wall is
impermeable to water
• Due to the waterproof band of suberin
• So all water must pass the endodermis via the cytoplasmSo all water must pass the endodermis via the cytoplasm
• Therefore it is under cellular control
• Apoplast route is important when transpiration rates are high as it
is faster and requires no energy

50. The Casparian strip acts as an
apoplast blockThe Casparian strip is made of
suberin, which is impermeable to
water
Water is unable to pass through the
endodermis by the apoplast route
The endodermis actively transports
salts into the root xylem
Lowering the Ψ in the xylem, so
water moves in down the Ψ gradient
by osmosis
Water moves up the stem in the
xylem vessels

52. Water uptake in the roots
• Root hairs increase surface
area of root to maximize
water absorption.
• From the epidermis to the
endodermis there are three
pathways in which water can
flow:
• 1: Apoplast pathway:
• Water moves exclusively
through cell walls without
crossing any membranes
– The apoplast is a continuous
system of cell walls and
intercellular air spaces in
plant tissue

53. Water uptake in the roots
• 2: Transmembrane pathway:
• Water sequentially enters a
cell on one side, exits the cell
on the other side, enters the
next cell, and so on.
• 3: Symplast pathway:
• Water travels from one cell to
the next via plasmodesmata.
– The symplast consist of the
entire network of cell cytoplasm
interconnected by
plasmodesmata

54. Water uptake in the roots
• At the endodermis:
• Water movement through the
apoplast pathway is stopped
by the Casparian Strip
– Band of radial cell walls
containing suberin , a wax-like
water-resistant material
• The casparian strip breaks
continuity of the apoplast and
forces water and solutes to
cross the endodermis through
the plasma membrane
– So all water movement across
the endodermis occurs through
the symplast

55. Water transport through xylem
• Compared with water movement across root
tissue the xylem is a simple pathway of low
resistance
• Consists of two types of tracheary
elements.
– Tracheids
– Vessile elements – only found in angiosperms,
and some ferns
• The maturation of both these elements
involves the death of the cell. They have no
organelles or membranes
– Water can move with very little resistance

56. Water transport through xylem
• Tracheids: Elongated spindle-
shaped cells –arranged in
overlapping vertical files.
– Water flows between them via pits –
areas with no secondary walls and
thin porous primary walls
• Vessel elements: Shorter &
wider. The open end walls provide
an efficient low-resistance
pathway for water movement.
• Perforation plate forms at each
end – allow stacking end on to
form a larger conduit called a
vessel
– At the end there are no plates-
communicate with neighboring
vessels via pits