water loss

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 injuryTo 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 pressureBeira H.Meressa , 2008

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 Beira H.Meressa , 2008

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 10-20oC higher than the

air because of low transpiration due to

structural modification Beira H.Meressa , 2008

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 Beira H.Meressa , 2008

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 methodWeigh 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 Beira H.Meressa , 2008

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

Incre

ase

in

tu

rgid

ity of g

uard

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 cellincreases negativity of the cell interior

raising its osmotic

pressure of the cell

Entrance 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 25-30oC

Close higher than 30oC due to

higher intercellular CO2 concentration by higher rate of respiration

Decrease in enzymatic activities

3. 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

2. 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

Science

transpiration water

total water

magnitude of water

form of water vapour

form of vapour water

excessive water loss

rate of water loss

leaves transpiration