CROP WATER RELATIONS

2. Most important and plentiful chemical
Chemical formula : H2O
Universal solvent

3. Covalent bond vs. Hydrogen bond

4. PROPERTIES OF WATER
1 Colourless and Odourless
2 Molecular weight 18 g mol-1
3 Freezing point 0℃
4 Boiling point 100℃
5 Density 1 g ml-1
6 Polar molecule

5. 7
Excellent and
Powerful Solvent
Due to small size
and polarity
8 High Specific Heat 1 Cal (4.2 Jg-1C-1)
9
High Latent Heat of
Vaporization
597.3 cal g-1 at
0℃
10
High Latent Heat of
Fusion
79.71 cal/gram
at 0℃
11
Cohesion, Adhesion and
Surface Tension
12
Capillary and high
tensile strength

7.  Main constituent of protoplasm: 90-95%
 Forms 90% of plant body by green or fresh weight basis
 Acts as a solvent
 Transpiration carrier of nutrients
 Translocation of organic and inorganic solutes
 Maintain turgidity of cells
 Essential for germination of seeds, multiplication of soil organism
 Medium for biochemical reactions
 Thermal regulation against high temperature
 Constituent for organic compounds
 Participates directly in metabolic process
 Helps in chemical, physical and biological reaction in soil

8. IMPORTANT TERMS
Solute : A substance dissolved in a solution
Solvent : Substance that dissolves the solute
Solution: A homogenous mixture of two or more substances

11. Equilibrium –
When the molecules are
even throughout a space.
Concentration gradient -
a difference between
concentrations in a space.

12. Gravitational water
 Excess water in soil pores
 Drains out due to gravitational force
 Not available for plant growth
Capillary water
 Water left out in capillary pores after excess water has drained
 Held by surface tension- cohesive force 1/3-15 atm.
 Available to plants
Hygroscopic water
 Water absorbed by a oven dry soil when exposed to moist air
 Held at high tension- tightly held by adhesion force- 31 atmp.
 Not available to plants

14. Soil water proportions which dictate whether the water
is available or not for plant growth
Saturation field capacity : Water content of soil when
all pores of the soil are filled with water. Soil moisture
tension almost equal to zero.
Field capacity (FC): Water retained by an initially
saturated soil against the force of gravity.
At FC, macropores of soil are drained off, but water
is retained in the micropores.
Soil moisture tension at field capacity varies from
1/10 (clayey soils) to 1/3 (sandy soils) atmospheres.

16.  Soil water beyond which plants cannot recover from water
stress
 Still some water present in soil but not enough to be of use to
plants
 Water content corresponding to soil water potential of -15
bars.
Temporary wilting point
Denotes the soil water content at which the plant wilts at day
time, but recovers during night or when water is added to the
soil.
Ultimate wilting point
The plant wilts and fails to regain life even after addition of
water to soil.

19. Water held in the soil between field capacity
and permanent wilting point
‘Available’ for plant use
Available water capacity (AWC)
AWC= FC-PWP

21. The movement of materials in and out of
the cells in plants takes place in solution or
gaseous form by
 Diffusion
 Osmosis
 Imbibition

22.  The process by which molecules spread from areas of
high concentration, to areas of low concentration.
 It is simply the statistical outcome of random motion.
 As time progresses, the differential gradient of
concentrations will drop until the concentrations are
equalized.
 Molecules will always move down the concentration
gradient, toward areas of lesser concentration.
Eg: Think of food coloring that spreads out in a glass of
water
Air freshener sprayed in a room

25. Osmosis is the process of diffusion of water across a
semipermeable membrane.
 Water will move in the direction where there is a
high concentration of solute.
 Water molecules are free to pass across the cell
membrane in both directions, either in or out, and thus
osmosis regulates hydration, the influx of nutrients and
the outflow of wastes, among other processes.
Eg: Salt is a solute, when it is concentrated inside or
outside the cell, it will draw the water in its direction.
This is also why we get thirsty after eating something
salty

27. IMBIBITION
Absorption of water molecules or any liquid
molecules by a substance of the cell making them to
swell
The substance which imbibe water is called
Imbibants

28. Hypertonic Solutions - High concentration
Hypotonic Solutions - Low concentration
Isotonic Solutions - Equal concentration

29. • The word "HYPER" means more
• There are more solute molecules outside the
cell, which causes the water to be sucked in
that direction.

30. • The word "HYPO" means less,
• There are less solute molecules outside the
cell, water will move into the cell.
• The cell will gain water and grow larger.

31. • If the concentration of solute is equal on
both sides of membrane, the water will
move back in forth, but it won't have any
result on the overall amount of water on
either side.
• "ISO" means the same.

32. Endosmosis
Living plant cell is placed in
hypotonic solution
Water enter in to the cell
Exosmosis
Living plant cell is placed in
hypertonic solution
Water comes out from the cell

35. Potential - Capacity or energy to do work
Chemical potential of water- Ѱ – Psi
Pure water – Zero – Maximum water potential
Definition
The difference between the free energy of water
molecules in pure water and the free energy of water
molecules in any other system

36. Water potential Ψ= Ψs+ Ψm+ Ψp + Ψg
Where, Ψs = Solute osmotic potential (symbol π)
Ψm= Matric potential (symbol T)
Ψp= Pressure potential (symbol P)
Ψg = Gravitational potential (symbol G)
For plant-water relations
Ψ= Ψs+ Ψp

37. The osmotic potential, Ψs
(or π) is the component
produced by the solute
dissolved in the cell sap,
chiefly vacuolar sap.

38. The matric potential Ψm refers to water held in
micro capillaries or bound on surfaces of the cell
walls and other cell components.

39. The pressure potential Ψp (or P) is the turgor
pressure produced by diffusion of water into
protoplasts enclosed in walls which resist
expansion.

40. The effect of gravity, Ψg (or G) is a term of
negligible importance within root or a leaf but
becomes important in comparing potentials in
leaves at different heights on trees and in soils.

41. Water potential of any solution??
Plant water potential?
Soil water potential?
Always less than ‘0’
i.e.. Negative value

42. Importance of water potential
Water potential is the diagnostic tool to measure
water deficits & water stress in plant cells & tissues.
The lower the water potential in a plant cell or tissue,
the greater is its ability to absorb water.
Conversely, the higher the water potential- the
greater is the ability of the tissue to supply water to
other more desiccated cells and tissues.
Leaf water potential in well watered soils = - 2 to -8
bars
Leaf water potential under reduced soil moisture = >-
8 bars
Plant tissues will cease growth at -15 bars

44.  Uptake of water by plant is called
absorption of water.
 Plant absorb water from soil through
root hairs.
 Water is called ‘Liquid Gold of Life’.
 Plant are capable of absorbing water
from soil solution.
 Mainly plants absorb capillary water.
 Plants also absorb dissolved nutrients
along with water.

45.  Water is mainly absorbed- root
hairs.
 Root hairs- just above root cap.
 Area rich in root hairs called as
root hair zone
 Billions of root hairs- in root
system of a plant.
 They are tubular hair like
projections of epidermal cells.
 Each root hair is single cell.
 Vacuole filled with cell sap whose
water potential is more negative
(low Ψ) than soil solution.

47. Active absorption
Active absorption- The
absorption of water by the plant
with the use of energy is known
as active absorption.
Root cell play active role in the
absorption of water.
Water is absorbed by the
operation of osmotic forces by
the use of energy.
In osmotic active absorption,
water moves from hypotonic
solution to hypertonic solution.
Root
hairs
Pericycle
cells
Cortical
cells
Passage
cells
Xylem
Leaves

48. Apoplastic and Symplastic movement
• Water move through apoplastic pathway
(through intercellular pathway).
• Symplast pathway through plasmodesmata.
• Transmembrane pathway through aquaporins.

49. Passive absorption
 Intake of water by plants due to transpiration pull is called passive
absorption.
 Water absorbed due to transpiration activity in the top of the plants.
 Root hair cell has no role in absorption. It functions as an absorptive
surface.
 Water absorbed through roots.
 Transpiration increase- concentration of cell sap and DPD in the leaves.
 Water moves from xylem vessels into the mesophyll cells of leaves.
 Water in the xylem vessels is in the form of a column.
 Hence, pulls water column and result in a tension in the root hair cells.
 Water moves through the apoplastic pathway, symplast pathway and
transmembrane pathway.
 Greatest amount of water pulled by passive absorption

50. Passive absorption

51. Stomata was discovered by Pfeffer & name
‘stomata’ was given by Malphigii.
Stomata cover 1-2% of leaf area.
It is minute pore present in soft aerial parts of the
plant.
Algae, fungi and submerged plants do not possess
stomata

52. CHARACTERISTICS OF STOMATA

53. Charecteristics of stomata
(a) Stomata are minute pores of eliptical shape, consists of two specialized epidermal
cell called guard cells.
(b) The guard cells are kidney shape in dicotyledon and dumbell shape in
monocotyledon.
(c) The wall of the guard cell surrounding the pore is thicken and inelastic due to rest
of the walls are thin, elastic and semi-permeable.
(d) Each guard cell has a cytoplasmic lining, central vacuole. Its cytoplasm contains
single nucleus and number of chloroplast. The chloroplast of guard cell is capable
of very poor photosynthesis, because the absence of RUBISCO enzyme.
(e) Guard cells are surrounded by modified epidermal cells, known as subsidiary cells
or accessory cells, which supports in the movement of guard cells.
(f) The Size and shape of stoma and guard cell vary from plant to plant. When fully
open, the stomatal pore measures 3-12 μ in width and 10-40 μ in length.
(g) In many gymnosperms and xerophytic plants (plants growing in desert), the
stomata are present embedded deeply in the leaves, so that they are not exposed to
sunlight directly. Such deeply embedded stomata are called sunken stomata. This is
an adaptation to check excessive transpiration in these plants.

54. Types of stomata
I. Depending upon the distribution and arrangement of stomata in the leaves five
categories of stomatal distribution have been recognized in plants
1. Apple or mulberry (hypostomatic) type:
Stomata are found distributed only on the lower surface of leaves, e.g., apple, peach,
mulberry, walnut, etc.
2. Potato type:
Stomata are found distributed more on the lower surface and less on its upper surface,
e.g., potato, cabbage, bean, tomato, pea, etc.
3. Oat (amphistomatic) type:
Stomata are found distributed equally upon the two surfaces, e.g. maize, oats, grasses,
etc.
4. Water lily (epistomatic) type:
Stomata are found distributed only on the upper surface of leaf, e.g., water lily,
Nymphaea and many aquatic plants.
5. Potamogeton (astomatic) type:
Stomata are altogether absent or if present they are vestigeal. e.g., Potamogeton and
submerged aquatics.

55. II. Types of Stomata based on Movement
Loftfield (1856) classified three main groups of
stomata in accordance with their daily movement:
1. Alfalfa Type: The stomata remain open
throughout the day and closed all night, eg., peas,
bean, mustard etc.
2. Potato Type: The stomata will open throughout
the day and night except for few hours in the
evening, eg., Allium, cabbage, pumpkin, etc.
3. Barley Type: The stomata open only for a few
hours in a day, eg., Barley and other cereals.

56. III. Types of Stomata based on Behavior
1. Photo-active movements: Light directly or indirectly controls
stomatal movements. Such stomata remain open during day time
and closed in nights (dark).
2. Skoto-active movements: Stomata remain closed during day time and
open during night. Such cases are found in succulent plants and
other CAM Plants.
3. Hydro-active movements: In some cases, stomata open due to
excessive loss of water from the epidermal cells and close due to
turgid conditions of epidermal cells. This is usually found during
mid-day.
4. Autonomous movements: In certain cases, stomata open and close at
a rate of 10-15 minutes showing diurnal or rhythmic pulsation.
5. Passive and Active movements: Opening of stomata is considered as
active process and closing is the passive process and this is caused by
the turgor changes in the guard cells.

57. MECHANISM OF STOMATAL
OPENING AND CLOSING

61. TRANSPIRATION
Transpiration
• Evaporation of water
from the leaves
Transpiration stream
• Movement of water
through the plant

62. SIGNIFICANCE OF TRANSPIRATION
1. It creates suction force and helps in the ascent of sap.
2. It helps in the absorption of water and minerals by roots.
3. It helps in evaporating excess amount of water from moist soil.
4. It plays a role in translocation of food from one part of the plant
to the other.
5. It brings opening and closure of stomata which indirectly
influences the gaseous exchange for the processes of
photosynthesis and respiration.
6. It helps in dissipating the excess energy absorbed from the sun,
which will otherwise raise the leaf temperature.
7. It maintains suitable temperature of leaves by imparting a
cooling effect.

63. Disadvantages
• In some soils where water availability is in
scarcity the excess transpiration may even kill
the plant.
Anti-transpirants
• Substances which reduce transpiration rate by
causing stomatal closure partially.
• Examples - Colourless plastics, silicone oil, low
viscosity waxes, abscisic acid etc

64. METHODS OF MEASURING WATER
STATUS IN PLANTS
There are two general ways to describe the water status or internal water
balance of plant and plant tissue:
A. The first one is based on the energy associated with water in the plant
tissue.
1. Liquid immersion method
2. Vapour equilibration
3. Pressure chamber
B. The second way to describe water status is to measure the quantity of
water in a tissue i.e. its water content and to express it in relation to a
selected reference.
Three of these methods are
1. Fresh weight method
2. Dry weight method and
3. Relative water content (RWC) method.