Water Loss

Water is the most abundant constituent of plant
tissue
 Except in dry seeds
Large amount of water is absorbed by the roots
Carried to the top of the plant
Lost by the aerial part in the form of water vapour or
rely in the form of liquid
The loss of water from the living tissue of aerial parts
of the plant in the form of water vapour is termed
TRANSPIRATIN
Beira H.Meressa , 2008

Transpiration is due to the anatomical features of plants
Those of leaves
Transpiration is the phenomenon most responsible for
excessive water loss from plants
Other process are also involved
Guttation
Secretion – loss of solution from glands
Bleeding - the slow exudation of water solution from a cut made
in a plant tissue

GUTTATION
Explains the relationship of root pressure to the rate of
water loss from the top of the plant
It the exudation of water from plants in the form of liquid
along the margin or tip of the leaf blade
The amount of water lost by this process is negligible

Factors favouring Guttation
High water absorption
High root pressure
Low or no transpiration
Under these conditions water absorption greatly
exceeds transpiration
Water is forced up the xylem ducts and out through
endings in the leaves
Does as a result of hydrostatic pressure developed in
the sap of xylem duct

Guttation occurs
Through hydathodes – specialized pores at the
extreme tip of the leaf
As exudation directly through the epidermis
 Herbaceous plants
 Laminal guttation
Through leaf scar and lenticels (from stem)

Guttation fluid is not pure
Chemical compassion
 Mineral salts
 Sugar
 Amino acids
 Enzymes
 Vitamins
Hence considerable injury
To leaves when salts are concentrated on the leaves
surface when the guttated liquid evaporates rapidly
To messopyll tissues as minerals left by guttation are
dissolved and drawn back in to the interior of the leaf

FEATURES
Usually small and occurs in the night and early in the
morning
Also during day time
 In plants growing in humid air and moist warm soil
Lost in the form of liquid
Guttated water is not pure
Uncontrolled process
Mostly occurs through special pores called
hydathodes
Expression of positive root pressure

Does not occur in plants growing in:
Cold or dry soil
Soils with a high concentration of solute
Soils with poor aeration
Soil with mineral deficiency

Transpiration
Loss of water vapour from living plants
From any part of the plant exposed to the air
However the leaves are the principal organs of
transpiration
Incidental due to the structural arrangements of
plants for exit and entry of gasses
 Photosynthesis–transpiration paradox

General feature
 Occurs during day time
 The water is lost in the form of vapour
 Water lost is pure
 Takes place through stomata, lenticels and cuticle
 Controlled by the activity of guard cells
 Only living cells are involved in the process

Magnitude of water lost by transpiration
Is very high
Zea maize (ml)
Water occurring as constituent...............1872 ml
Water used in metabolic activities............250 ml
Water transpired ..................................202,106 ml
Total water used in growing season.....204,228 ml

Is transpiration vital or lethal phenomena ?
‘‘ is a necessary evil’’

Dangerous
 Plants die because transpiration exceeds the amount
of water absorbed
 Consumes energy of the plant
 Causes un necessary absorption of excess water by
plant root
PLANTS SERVE AS A PIPE CONECTING THE SOIL
AND THE AIR

Significance
It is the ‘engine’ that pulls water up from the root
 helps in the ascent of sap
Brings mineral salts and water from the root
Relief to the plant : evaporates excess amount of water
Translocation of food from one portion of the plant to the
other
Affects the opening and closing of stomata : influences
the process of photosynthesis and respiration

Maintains a suitable temperature for the leaves
 Leaves exposed to direct sun light absorb large
quantities of radiant energy which, unless dissipated in
some other way, will be converted in to heat energy and
rise the temperature of the leaves
 But since transpiration is energy consuming process, the
loss of water from leaves dissipates most of the excess
energy absorbed by the leaves
 Eg. Euphorbia remains 10-20o
C higher than the air
because of low transpiration due to structural
modification

Kinds of Foliar transpiration
I. Stomatal transpiration
 Stomatal pores are involved
 Controlled by guard cells
 Maximum transpiration of water vapour takes
place through stomata ......80-90 % of total loss

II. Cuticular transpiration
 Cuticles are wax-like layer covering on the epidermis
of leaves and herbaceous stem
 Its thickness varies from plant to plant
 Some cracks on the layer increases water loss..20%
 Transpiration is high when the layer is thin
 Pores are not involved
 Herbaceous stem
 Flower parts
 Fruits

III. Lenticular transpiration
 Pores with uncontrolled opening and closing are
involved called lenticels
 Lenticels are areas in the bark which are filled with
loosely arranged cells: complementary cells
 Contributes about 0.1 % of the total loss
o Negligible in comparison to total loss by the whole plant
o Fruits
o Tubers

The amount of water lost through cuticular and
lenticular transpiration is significant only under dry
conditions
WHY ???

Measurements of transpiration
The amount of water vapour transpired by a plant can
be measures by several methods:
Weighing method
Potometer
The water vapour given off by plants may be collected,
condensation and weighted
Cuvette method

Weighing method
Weigh a potted plant at the beginning of the experiment
Cover the soil surface and wrap with some water repellent
material such as Al foil to retard evaporation from the
surface other than the plant
Weigh the potted plant at the end of a prescribed period
of time
Then, the loss of weight by the plant over short period of
time will be due to transpiration

Mechanisms of Stomatal movement
Stomata are microscopic pores and are bordered by
two specialized epidermal cells
Guard cell control the opening and closing of stomata
The immediate cause is a change in the turger of the
guard cells

When turger develops with in the two guard cells
flanking each stoma, the thin outer walls bulge out
and force the inner walls in to a crescent shape that
opens the stoma.
When the guard cell lose turger , the elastic inner
walls regain their original shape and stoma close

Behaviour of Stomatal movements

Factors affecting stomatal
movements

Factors affecting Stomatal movements
1. Light
 CARBOHYDRATE FORMATION
Sugar
formation
 Osmotic pressure
of cell sap
Endosmosis
Increasein
turgidity
ofguard
cells
Stomata
open
Photosynthesis
Hydrolysis
of starch to
sugar
pH
rise
 CO2

 Potassium uptake
ATP synthesis
proton (H+)pump in the plasma
membrane of the guard cell
increases negativity of the cell
interior
raising its osmotic
pressure of the
cellEntrance of
water in to the
guard cell
guard cell
turgidity
increases

2. Temperature
 An increase in temperature causes an increase in stomatal opening up to 25-
30o
C
 Close higher than 30o
C due to
 higher intercellular CO2 concentration by higher rate of respiration
 Decrease in enzymatic activities
2. CO2 concentration
 CO2 of the leaves intercellular space controls stomatal movement
 Higher CO2 concentration above that fond in the air causes stomata to
close
 But exposure to light causes the stomata to open
 Why ????
 Response of stomata of variegated leaves Vs green leaves

4. Water deficit and ABA accumulation
 Water deficit is created in the plant when transpiration
exceeds absorption
 Stomata closes to reduce further loss of water
 ABA accumulates in the leaves of stressed plants
Triggers closing of the stomata
 How ???

The receptor activates several
interconnecting pathways
ABA binds to receptor
at the surface of
plasma lemma
membrane of the
guard cells
A rise in pH of the cytosol
Transfer of CO2 from the
vacuole to the cytosol
Stimulates the loss of Cl-
and organic ions from cell
Blocs the uptake of K+
in to the guard cell
Reduce the
osmotic pressure
of the cell
Reduce turgidity
of guard cell and
cause stomata to
close

Plant factors affecting rate of transpiration
1. Root –shoot ratio
 Efficiency of the absorbing surface and evaporating surface control the rate
of transpiration
 Increase in root–shoot ratio increases the rate of transpiration
1. Leaf area
 The greater the leaf area , the higher will be the magnitude of water loss
 Small plants transpire at higher rate than do larger plants on a per unit area
basis
 Pruning
 Root system of pruned trees provides greater amount of water to a small number of
leaves thus increasing transpiration rate
 if so what is the importance of pruning fruit trees ???

3. Leaf modifications
 Plants native to dry habitat exhibit structural modification in their
leaves
 The leaves of xerophytic plants possess:
 Thick cuticle-transpiration is inversely related to cuticle thickness
 Thick cell wall
 Needle like leaves
 Well developed palisade parenchyma
 Sunken stomata surrounded by hairs –reduce diffusion rate
 Presence hydrophobic compounds (gums, mucilage)
 Orientation of leaf
 Reduced number of stomata per unit leaf area

Environments factors

Water Loss

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