Lect-2

CROP WATER RELATIONS

BY
SADRAS BHAVANA
DEPARTMENT OF CROP
PHYSIOLOGY
• Almost every plant process is affected directly or indirectly by the water
supply.
• Decreasing water content is accompanied by loss of turgor and
wilting, cessation of cell enlargement, closure of stomata, reduction in
photosynthesis and interference with many basic metabolic processes.
• Eventually, continued dehydration causes disorganization of the
protoplasm and death of most organisms.
 2.1 PHYSIOLOGICAL IMPORTANCE OF WATER:

• The importance of water can be summarized under the following general headings

a) Constituent of Protoplasm
• Water is important quantitatively and qualitatively, constituting 80 to 90 % of the fresh
weight.
• Important as a part of the protoplasm as the protein molecules, changes in structure if
the water content is dropped below certain level.

b) Solvent
• A second essential function of water in plants is as the solvent in which gases, minerals
and other solutes enter plant cell and move from cell to cell and organ to organ.
• The permeability of most cell walls and membranes to water in a continuous liquid phase
extending throughout the plant in which translocation of solutes of all kinds occurs.
c) Reagent
• Water is a reactant or reagent in many important processes, including
photosynthesis and hydrolytic process such as the hydrolysis of starch to
sugar.
d) Maintenance of Turgidity
• Another essential role of water is the maintenance of the turgidity which is
essential for cell enlargement and growth, and for maintaining the form of
herbaceous plants.
• Turgor is also important in the opening of stomata and the
movements of leaves, flower petals and various specialized plant structures.
• Inadequate water to maintain turgor results in an immediate reduction of vegetation
growth.
Turgidity: When water moves into a plant cell, the vacuole gets
bigger, pushing the cell membrane against the cell wall. The
force of this increases the turgor pressure within the cell making
it firm or turgid . The pressure created by the cell wall stops too
much water entering and prevents cell lysis.
e) Different organic constituents (carbohydrates, fats, proteins, nucleic acid,
enzymes) lose their physical and chemical properties in absence of water.
f) Water is source of H+ ions for CO2 reduction in photosynthesis.
g) Helps in translocation of solutes, mobility of gametes, dissemination of
spores and seeds.
h) In tropical plants water plays important role in thermal regulation.
i) Carrier of nutrients from the soil.
j) Water is regarded as “Elixir of life”.
 Fundamental processes involved in movement of water.
DIFFUSION:
• Derived from latin word “diffuses” means ‘to spread’, noticed first by a scotish botanist
ROBERT BROWN.
DEFINITION: The movement of molecules of gas, solids and liquids from an area of
higher conc. to an area of lower conc. due to the internal or external forces.
FACTORS:
• Temperature: directly proportional
• Conc. Of medium: indirectly proportional
• Size and mass of particles: directly proportional
• Solubility of solutes: directly proportional
• Density of diffusing particles: rate of diffusion of any gas particles is inversely
proportional to the square root of its density (Graham’s law of diffusion)
r=1/ √d r=rate of diffusion, d=density of gas.
OSMOSIS:
• When two solutions of different conc. are separated by a semi-permeable membrane,
the diffusion of water or the solvent molecules takes place from solution of low conc.
Towards the solution of higher conc.
• Special type of diffusion.
Types:
• Endosmosis: when water or solvent molecules enter the cell through plasma
membrane from the outer medium.
• Exosmosis: when plant cell is placed in a conc. solution, the water from the cell mves
out into the conc. medium via the plasma membrane.
Factors:
• Conc. Of solution: direct proportional
• Temperature: direct proportional
• Light: indirect role, in sunlight---photosynthesis---increase in conc. Of protoplasm---
leads to increase in OP of cell sap.
IMBIBITION:
• Derived from latin word “ imbibere” meaning ‘to
drink’
• It is movement of water molecules into the
substances like wood, seeds, agar-agar, gelatin
which swell or increase in volume as a result.
Factors:
• Temperature: directly propotional
• Pressure of solutes: indirectly proportional
• Soil reaction pH: more in neutral pH compared to
acidic or basic pH.
Importance:
• Seed to germinate
• Activation of enzymes and bursting of seed coat
2.2 WATER POTENTIAL AND ITS COMPONENTS:
• Water potential or chemical potential of water is a quantitative
expression of the free energy associated with water.
• symbolized by the Greek letter Ψ (psi)
• It is defined relative to the water potential of pure water, which is zero.
Hence the value of Ψ is always negative. The units of water potential are
mega Pascal (MPa).
• It is a relative quantity and depends on concentration, pressure and
gravity at the same temperature.
• Water potential as the sum of component potentials which may be
written as
Ψ = Ψs + Ψm + Ψp + Ψg
• Where Ψs = Solute or osmotic potential (symbol π)
Ψm = Matric potential (Symbol T)
Ψp = Pressure potential (Symbol P)
Ψg = Gravitational potential (Symbol G)
Osmotic potential
• The osmotic potential, Ψs (or π) is the component produced by
solutes dissolved in the cell sap, chiefly vacuolar sap.

Matric Potential
• The matric potential, Ψm (or T) refers to water held in micro capillaries or
bound on surfaces of the cell walls and other cell components.

Pressure potential
• The pressure potential Ψp (or P) is the turgor pressure produced by
diffusion of water into protoplasts enclosed in walls which resist
expansion. In the xylem of transpiring plants Ψp is usually negative and in
guttating plants it is positive as a result of root pressure.

Gravitational Potential
• 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.
• for herbaceous plants and annual field crops of a short vertical height
(less than 10 m) the values of the matric potential and gravitational
potential are small and are commonly omitted. Thus
Ψ = Ψs + Ψp
• Water always moves from less negative water potential to more
negative water potential.
 2.3 IMPORTANCE OF WATER POTENTIAL:
• diagnostic tool that enables the plant scientist to assign a precise value to the
water status in plant cells and tissues.
• The lower the water potential in a plant cell or tissue, the greater is its ability
to absorb water.
• the higher the water potential, the greater is the ability of
the tissue to supply water to other more desiccated cells and tissues.
• used to measure water deficit and water stress in plant
• As a general rule, leaves of most plants rooted in well watered soils are likely
to have water potentials between about -2 and -8 bars. With decreasing soil
moisture supply, leaf water potential will become more negative than -8 bars
and leaf growth rates will decline. Most plant tissues will cease growth
completely ( i.e., will not enlarge) when water potential drops to about -
15bars.
 2.4 UPTAKE OF WATER
• The way in which water is entered in to the root hair and the
• precise mechanism of water absorption is has been explained by two
different approaches.
(a)Active uptake:
• Water is absorbed as a result of activities in the root itself and does
not concern with any process in shoot.
(b)Passive uptake of water:
• The governing force of water absorptionoriginates in the cells of
transpiring shoots rather than in root itself.
 ACTIVE ABSORPTION:
• Although the absorption of water by roots is believed to be a
passive,
• pressure driven process, it is nonetheless dependent on respiration.
Respiratory inhibitors (such as cyanide),anaerobic conditions
(waterlogged condition) decrease in the hydraulic conductance of
most roots .These are some supporting points for active absorption of
water. How ever the exact role of respiration and active uptake is
not clear.
PASSIVE ABSORPTION:
• it is believed that uptake of water is a Passive process.
• Tension or negative pressure originating at the actively transpiring leaf
surface creates a pulling force for water movement in xylem.
(Cohesion-tension theory of Dixon and Jolly)
• The movement of water inside the plant is driven by a reduction in
free energy, and water may move by diffusion, by bulk flow or
by a combination of these fundamental transport mechanisms.
• Thus, water potential difference (i.e solute potential and pressure
potential) across the cells starting from root hairs to xylem plays an
important role in uptake and transport of water.
• The Apoplast method: via the cell wall
• The transmembrane method: via the plasma membrane
• The Symplast method: via the cytoplasm through the
plasmodesmata.
2.5 METHODS OF MEASURING WATER STATUS IN PLANTS:

• The water potential is measured by

(1) liquid immersion method (dye method) or Chardakov’s Falling
Drop Method
(2) Vapor equilibration method (Thermocouple Psychrometer) and
(3) pressure chamber method.
• Investigators make a measurement by placing a piece of tissue
sealed inside a small chamber that contains a temperature
sensor (in this case, a thermocouple) in contact with a small
droplet of a standard solution of known solute concentration
(known Ψs and thus known Ψw).
• If the tissue has a lower water potential than that of the
droplet, water evaporates from the droplet, diffuses through
the air, and is absorbed by the tissue. This slight evaporation of
water cools the drop.
• The larger the difference in water potential between the tissue
and the droplet, the higher the rate of water transfer and
hence the cooler the droplet.
• If the standard solution has a lower water
potential than that of the sample to be measured, water will
diffuse from the tissue to the droplet, causing warming of the
droplet.
• Measuring the change in temperature of the droplet for several
solutions of known Ψw makes it possible to calculate the water
potential of a solution for which the net movement of water
between the droplet and the tissue would be zero signifying
that the droplet and the tissue have the same water potential.
• A relatively quick method for estimating the water
potential of large pieces of tissues
• by P. Scholander and coworkers.
• The pressure bomb is a device that is used to determine
the plant moisture stress and the water potential of a leafy
shoot and in based on the assumption that the water
column in a plant is almost always under tension because
of the pull exerted by the osmotic influences (water
potential) of the living cells of the leaves.
• If the tension is high, the water potential of the leaf cells is
very negative. When a stem is cut, the water column (in
xylem) is disrupted and because the water column is under
tension, it will recede back into the stem toward the
leaves. The shoot is placed in a chamber, with the cut end
protruding through an airtight hole. Pressure is increased
within the chamber and the water column with in the
twig are forced back to the cut surface.
• pressure in the chamber is then carefully recorded.
• pressure required to force the water to appear at the cut
surface is equal to the tension (but with the opposite sign)
of the water column at the time the shoot was cut.
ASSIGNMENT:
• Write long answer for the questions stated below
1. What is water potential. Give the components of measuring water potential in
plants and explain in detail.
2. Give the physiological importance of water.
3. Give the methods for uptake of water in plant. Explain the passive movement of
water in detail.
4. Explain the pressure chamber method to method water potential in plants.
5. Explain the working of Thermocouple Psychrometer in detail.

• Write in A4 sheets
• Date of Submission will be told to you later