Water plays critical roles in plants and soils. It has unique molecular properties like its dipole character and high dielectric constant. In plants, water maintains turgor pressure, acts as a solvent for photosynthesis, and transports minerals. It also cools plants and soils. In soils, water is essential for nutrient solubility and transport, tilth, and acts as a buffer against temperature fluctuations. Lack of water especially during reproductive stages can cause major yield reductions in important pulse crops like chickpeas, pigeon peas, and common beans.
3. CONTENTS
A) Molecular characteristics of water
B) Important properties of water
C) Functions of water in plants
D) Functions of water in soils
E) Research paper
F) Conclusion
G) References
4. A) Molecular characteristics of water
Water is a substance of unique electro-physical and
electro-chemical properties which are arise from its
peculiar asymmetrical atomic configuration.
Water molecule has dipole character.
Water has a high dielectric constant of 81.
The angle between the imaginary lines joining the
two H nuclei with the O nucleus is found 104.50
6. B) Important properties of water
1) Specific heat
2) Heat of vaporization
3) Cohesion and surface Tension
4) Adhesion
5) Spontaneous Transport
6) Water potential
(i) In plants
(ii) In soil
(iii) In air
7. 1. Specific heat
• Heat required to increase the temperature of 1g of a
substance by 1°C is called specific heat.
• The specific heat of water is 1 calorie/gram °C which
is higher than any other common substance helps it to
maintain relatively stable internal temperatures in
plants and soils despite large fluctuations in
atmospheric temperatures.
9. 2. Heat of vaporization
Heat of vaporization of water is very high, being 540
to 580 cal/g of water.This leaves a cooling effect on
the water evaporating surfaces of soils, plants and
other bodies.
10. 3) Cohesion and surface Tension
• The ability of two water molecules to cohere to each
other (by H-bonding) rather tenaciously is called
cohesion.
• Surface tension could be defined as the property of
the surface of a liquid that allows it to resist an
external force, due to the cohesive nature of the water.
11. Surface tension at liquid-air interface, is higher than the
surface tension within interior of the liquid.
12. 4) Adhesion
The attraction between water molecules and other
bonding surfaces known as adhesion.
Water molecules hold firmly to solid substances rich
in O and N atoms, e.g. soil colloids, cellulose,
proteins and glass.
A soil colloid particle holds the first molecular layer
of water by force of ‘adhesion’ and its subsequent
layers by ‘cohesion’.
13. 5) Spontaneous transport
Movement of energy from its higher level to its lower
level, or of any substance from its higher energy state
towards its lower energy state without the aid of any
external energy or force, when there is no hindrance
to do so.
In which donor loses energy and the recipient gains it.
15. Water potential
Spontaneous movement of water from one region to
another is explained in terms of free energy of water.
The free energy is the principle of thermodynamics.
It deals with energy change.
The tendency of molecules moves depend upon the free
energy and that is called chemical potential. And the
chemical potential of water is nothing but water potential.
16. Cont…..
Water potential is inside plant and in soil is expressed
by the Greek letter Ψ (psi).
Measured in bars (one bar = 0.987 atm).
The term Water potential was coined by Slayter and
Taylor (1960).
The Water potential of pure water at atmospheric
pressure is zero.
17. Definition of water potential
Water potential can be defined as the difference
between the free energy of water molecules in pure
water and the free energy of water in a solution (or
cell sap).
18. i) Water potential in plants
Water potential (Ψw) in plants is deals with the
osmotic relations of plant cells.
Components of Water potential in plants
Matrix potential (matric potential) (Ψm)
Osmotic potential (solute potential) (Ψs)
Pressure potential (Ψp)
Ψw = Ψ m+ Ψ s+ Ψ p
1
19. Matrix potential (matric potential) (Ψm)
• Matric is the term used for the surface (such as soil
particles, cell wall, protoplasm etc) to which water
molecules are adsorbed.
• It has negative value.
• In case of plant-water relationship ; it is not
considered because it does not play a significant role
in movement in water.
Ψw = Ψ s+ Ψ p
2
20. Osmotic potential (solute potential) (Ψs)
• About three-fourth water present in plants is in the
form of solution of variable solutes of different
concentrations, called cell sap.
• The quantum of water potential lowered by the
presence of osmotically active solutes in the plant
tissues called solute potential.
• In the normally growing crop plants the solute
potential in leaf cell vacuoles are found to vary from
-10 to -20 bars.
22. • In plants turgid cell possess outer turgor pressure
which is equal to inward cell wall pressure.
• It has positive value.
• In the normally growing plants the crop leaves are
found to possess pressure potential of +3 to +5 bars
during daytime and up to +15 bars during night time.
(why ?)
Cont…..
23. ii) Water potential in soil
• There are three main sources causing reduction in
water potential in soil below zero bar.
• These sources are
solutes present in the soil
colloidal soil matrix And
force of gravity
24. Osmotic potential (Ψs)
• Water potential reduced due to the presence of the
solutes called osmotic potential.
• In salt affected soil the osmotic potential may become
a very significant component of soil water potential,
which may lowered to the extent that the plant roots
may fail to absorb water from it, even when it was
sufficiently moist.
25. Cont…..
• Sometimes in extreme cases, water may even move
out from plant roots into the soil by process of
reverse osmosis. Desiccation of plants may then be
very rapid, except halophytes.
• In all normal agricultural soils the osmotic potential
as a component of water potential is ineffective and
can, therefore, be ignored.
26. Matric potential (Ψm)
• The adhesion of water molecules to soil matrix
lowers the water potential in proportion to the
intensity of the adhesion.
• In soils, the matric potential values are very high in
the wet range (-0.1 to -0.31 bar) and low in the dry
range (-0.7 to -15 bars).
• It determines the movement of water within the soil,
as well as from soil into the plant roots.
27. Gravitational potential (Ψm)
• It is importance only in saturated soils where free
water loses its energy and moves in response to
gravity as drainage water.
• In unsaturated soils where water is held against
gravity by forces of adhesion and cohesion,
gravitational potential of water can be easily
overlooked.
28. Cont….
• Reviewing the three components of water potential in
soils it is clear that in normal agricultural soils the
osmosis and gravitational potentials do not
influence the soil water and plant relations.
• Here matric potential alone determines the availability
of moisture to the plant roots.
Ψw = Ψ m
29. iii) Water potential in Air
• In atmosphere the relative humidity (RH) of its air
determines the water potential prevailing at a given
time.
• Water potential at R.H. of around 100%,is near zero
bar. but at 50%RH,it drops to -1000 bars and at 10%
R.H. to about -3200 bars.
• Lower the water potential in atmosphere, greater is
its strength to induce transpiration in plants and
surface evaporation in soils.
30. C) Functions of water in plants
Water maintains turgidity in plant cells and tissues.
only in turgid cells normal biochemical functions
can takes place. because water acts as common
place in these reaction and turgidity makes room in
cells for these activities.
31. Cont…..
The very basic physiological function of plants,
called photosynthesis, is dependent upon water as
source of hydrogen for the purpose.
32. Cont…..
Water is a major constituent (75-85%) of cytoplasm
which is the basic substance of life. Upon
dehydration, cytoplasm as well as nucleoplasm(=
protoplasm) are denatured.
cytoplasm
33. Cont…..
Water movement from root to top of plant and create
transpiration pull due to cohesion force of water
molecule.
34. Cont…..
Water is the solvent in which mineral and organic
solutes are translocated to respective sinks in plants.
Water maintains temperatures in plants to within their
physiological limits.
35. D) Functions of water in soils
Plants absorb water mainly from soils through their
roots. so to say, the presence of adequate amount of
water in soils is primary for plant growth.
Soil moisture acts as buffer in preventing fast changes
in soil temperatures with changing atmospheric
temperatures.
36. Cont…..
Water acts as solvent of mineral nutrients which is
necessary for conducting several chemical reactions
and for maintaining nutrients in their ionic forms for
their rapid absorption by plant roots.
Soil moisture is necessary to enable proper tillage of
the soil to obtain desired tilth.
37.
38. E) Research paper
Turner N.C., Wright G.C., Siddique K.H.M. Nayyar
et al. (2006) Adaptation of grain legumes (pulses)
to water-limited environments, Adv. Agron. 71,
123–231.
39. Economic yield reduction by unavailability of water
in some representative pulse crops
Crop Growth stage Yield
reduction
References
Chickpea Reproductive 45–69% Nayyar et al.
(2006)
Pigeon pea Reproductive 40–55% Nam et al.
(2001)
Common
beans
Reproductive 58–87% Martínez et al.
(2007)
Soybean Reproductive 46–71% Samarah et al.
(2006)
Cowpea Reproductive 11–60% Ogbonnaya et
al. (2003)
41. G) References
Water in relation to soils and plants- O. P. GUPTA
Boscoss Biology II PUC textbook –Dr. K. Rajendra,
Precilla D’ silva, Anita Fernandes.
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