transpiration dr.rajesh sahu

8/3/2019 Transpiration Dr.rajesh sahu

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TRANSPIRATION

A small fraction (less than 5%) of the total water is used up by the plant and the remaining

amount is lost in the environment in vapor form or liquid form.

The loss of water in the vapour form from aerial organ of living plant is known as

transpiration where as the loss of water in the liquid (droplet) form from aerial uninjured

organ of living plant is known as Guttation.

Rate of transpiration can be measure by

Potometers while total pore area (stoma)

measure by Porometer. Psychrometer is

used to measure relative humidity and rate of

transpiration. The loss of water is so great that

it reduces water level in the soil and can lead

to the death of plant, but transpiration is said

to be necessary for water and mineralabsorption, ascent of sap and lowering the

temperature (cooling effect). So,

transpiration is called as necessary evil

(Curtis) or unavoidable evil (Steward).

Types of transpiration: -

Bases on the plant parts or structure

involved, following types can be recognized.

(i) Stomatal transpiration

(ii) Cuticular transpiration

(iii)Bark transpiration

(iv) Lenticular transpiration.

(i) Stomatal transpiration/foliar transpiration: Surface of leaves possess many

minute pores called stomata, water vapour diffuse out of these stomata in to the

atmosphere. It is called stomatal transpiration. It is the major form of transpiration, because

50-97% water is lost by stomatal transpiration. It however occurs only when stomatas are

open. For performing stomatal transpiration water absorbed by root hairs reaches thexylem vessels & tracheids through the root cortex & then to xylem vessels & tracheids of

the leaf due to which turgor pressure of its cells becomes more than of mesophyll cells. The

intercellular spaces in mesophyll cells are filled with air. By transpiration water vapour

enters the intercellular space and then passes on into the atmosphere through stomata.

(ii) Cuticular transpiration: - It is the loss of the water in the vapour form from the

general surface (leaves & young stem) through the layer of cuticle, but the quantity of loss

will depend up on the thickness of the cuticle, means thicker the cuticle less the water loss

MAMS ACADEMY FOR CAREER EXCELLENCE Page 1


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(as in xerophytes) and thinner the cuticle more the water loss as in hydrophytes. Cuticular

transpiration take place continues through out day & night however it is approximately 3 to

10% of total water loss but in herbs and fern it may be 50%.

(iii)Bark transpiration: - This occurs through the bark of woody stem. It is

approximately 1% of total lost.

(iv) Lenticular transpiration: -This is loss of water in vapour form from lenticels or

aerating pores present in the bark of stems or fruits etc. Through lenticels some water is

lost continuously, however it is approximately 0.1 % of the total water lost.

Structure of stomata: - are minute pore complexes which occur on the soft aerial parts

of the plants, especially on the leaves. Each stomata

is surrounded by two kidney shaped (dicots) or dumb-

bell shape (monocot as Gramineae and cypraceae)

epidermal cells called guard cells. These cells are

living having nucleus, chloroplast, and cytoplasm.

Kidney shape guard cells have outer wall thinner andmore elastic while the inner-wall thicker and less

elastic. Dumb-bell shapeguard cells have thin walled

ends and thick walled middle region. These guard cells are surrounded by some specialized

cells called subsidiary cells or accessory cells which support in the movement of guard

cells. The size, shape, number and position of stomata and guard cells vary from plant to

plant. When fully open, the stomatal pore measure 3-10 in width and 10-40 in

length.

* Scotoactive stomata: - are special types of stomata which open in dark & close

during the day time so they prevent excess loss of water during transpiration. They occur in

succulents xerophytic plants. E.g., Cactus, Bryophyllum, Pineapple etc. (Generally

stomata are photoactive).

Number and distribution of stomata: -In most plants there are 50300 stomata

per sq. mm. or 1000-60000/sq. cm. of leaf area are present. (The least number is

reported is 14 & max. no. 1038/sq.mm.). The number of stomata usually not equal on both

surfaces of leaf and varies plant to plant. Xerophytes possess larger number of

stomata than mesophytes, and higher in trees and shrubs than grasses. Stomata occupy

1-2% of total leaf area.

According to Salisbury there is a co-relation between number of stomata and number of

epidermal cells per unit area which is called stomatal index (I).

I= S x 100 where S=no. of stomata /unit area.

S+E E=no. of epidermal cells/unit area.

Plants are divided into the following categories on the basis of the no. of stomata and its

distribution-



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1. Apple type (mulberry type): -These plants have stomata only on the lower

surface of leaves. E.g., apple, peach, mulberry, walnut etc. such leaves are

hypostomatic type.

2. Potato type: - have stomata on the both surfaces of leaf but more stomata on

the lower surface (multistomatic) and less stomata on upper surface of leaf (pauci

stomatic), such leaves are amphi stomatic and aniso stomatic type. E.g., Potato,tomato, pea, cabbage, bean and many other dicot plants.

3. Oat type: - Plants have equal number of stomata on the both surfaces of the

leaf. E.g.,. oat, wheat, maize, grasses and many other monocots. Such leaves are amphi

stomatic and iso stomatic type.

4. Water lily type: - Have stomata on the upper surface of leavesonly because lower

surfaces of leaves make contact with water surface. E.g.,. Water lily, nymphaea & most

floating plants. Such leaves are epistomatic type.

5. Potamogeton type: - stomatas are absent or vestigialbecause the entire body ofplant dip in water so gaseous exchange take place through general body surfaces. E.g.,

Potamogeton, hydrila, valisnaria and other submersed plants. Such leaves are called

astomatic.

Loftfield has classified stomata into following main groups on the basis of their daily

opening and closing movement-

(i)Alfa alfa type- the stomata remain opens through out the day and closed all night.

E.g., pea, bean, mustard, radish, turnip, apple, grapes etc.

(ii)Potato type- the stomata close for a few hours in the evening. E.g., Onion, potato,

banana, cabbage etc.

(iii)Barley type- the stomata open only for a few hours in a day. E.g., Barley, maize,

wheat and other cereals.

(iv)Equisetum type- stomata remain always open.

Mechanism of opening & closing of stomata: - It depends upon the turgid or

flaccid state of the guard cells. When guard cells are in turgid state the stomatal aperture

become open and when guard cells are in flaccid state the stomatal aperture become

closes.

The inner wall of guard cell is thick while outer wall is thin. When the turgor pressure of the

guard cells is increased the outer thinner wall of the guard cell is pushed out due to which a

tension is created on the inner thicker wall thus pulling the inner thicker wall towards

periphery thus leading to the opening of stomatal aperture & water vapours go outside.

When the guard cells are in a flaccid state the outer thinner wall of guard cells returns to

original position or move towards pore, due to which tension on the inner thick wall is

released which also returns to its original position and stomatal aperture gets closed again.



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Theories explaining opening and closing of stomata: -

(i)Active K+ ion uptake theory (Levit) or Importance of K+ in opening &

closing of stomata: -Acc to Imamura & Fujino (1967) When a leaf is exposed to

sunlight, pH of guard cells rises due to assimilation of CO2 in photosynthesis and uptake of

H+ ions by chloroplast and mitochondria from cytoplasm. At this higher pH starch contents

of the guard cells are converted to PEP. PEP combine with CO2 with the help of

PEPcase and form OAA which get changed into malic acid which dissociate in to

malate anion & H+. In exchange for H+, K+ are taken into the guard cells, some Cl- ions are

also taken into guard cells to neutralize a part of k+. As H+ ions leave the guard cells

(efflux), K+ions enter the guard cells (influx). These K+ ions react with malate anions and

form potassium malates which migrate into the vacuole of guard cells. Due to presence of

potassium malate in the guard cells, the water potential with in the guard cells become

decreased so water from adjoining epidermal cells enters in to guard cells as a result ofwhich their turgor pressure is increased & the stomata open.

During stomatal closing, reversal of H+ & K+ pump occurs where by the K+ ions lost from the

guard cells in to the surrounding cells resulting in increased water potential within the

guard cells so water comes out due to exosmosis & the guard cells become flaccid there by

promoting the closure of stomata. Stomatal opening is an active process where as

closing is a passive process.

In succulent plants during night there is incomplete oxidation of carbohydrate

(respiration) and accumulation of organic acids, the CO2 is therefore not released and as a

result stomata remain open in dark. During night(dark)

2C6 H12 O6 + 3O2 3C4 H6 O5 + 3H2 OMalic acid

Where as during day time, the accumulated organic acids break down rapidly releasing

excess amount of CO2, sufficient for photosynthesis as well as to keep the stomata closed.

During day time (light)

C4 H6 O5 + 3O2 4 CO2 + 3 H2 OMalic acid

Light formation of malic acid from starch in guard cells dissociation of malic acid in to

malate anions & H+ ions influx of K+ & efflux of H+ formation of potassium malate and

enters into the vacuoles water potential decrease in to the guard cell resultant

endosmosis of water into guard cells increase of T.P. in guard cells Stomata opens.

Photosynthetic theory (Von Mohl and Schwendener): -Theory proposes that in

the morning as soon as light is available chloroplast of guard cell start photosynthesis as a

result sugars are produced, which increase the osmotic concentration of guard cells, water



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comes in, guard cells become turgid and stomata are open. This theory is not accepted

because photosynthetic activity of guard cell chloroplast seems to be negligeable and sugar

does not occur in detectable quantity in guard cells.

Starch Sugar Interconversion hypothesis (Classical theory): - The theorywas proposed by Sayre and Scarth and was modified by Steward on the basis of activity of

phosphorylase enzyme.

Starch + iP pH=7 Glucose -1-phosphatePhosphorylase enzyme

According to this theory guard cells contain starch and phosphorylase enzyme, which

causes conversion of starch into glucose at higher pH which is developed by utilization of

CO2 in photosynthesis. Glucose increases osmotic concentration of guard cells, raising the

turgor pressure and causing opening of stomata. In the night, CO2 accumulate in the cell

and intercellular spaces, thus lowering the pH at which phosphorylase causes conversion of

glucose to starch. Osmotic concentration of guard cells decreases water moves out

therefore, stomata are closed. This theory is not accepted because in some families starch

is absent as in onion. Glucose does not appear in detectable quantity in guard cell ofopen stomata rather malic acid accumulates.

Transpiration ratio/water requirement/efficiency of transpiration: -It is the

amount of water transpired by a plant for the synthesis of a unit dry matter. The value gives

an idea of the requirement of water by crops, shrubs and trees. The ratio varies from one

plant species to another and from one condition to another. Desert plants have highest

transpiration ratio.

Guttation (term by Burgerstein): -Loss of water in the liquid state from uninjured parts of

plant is known as Guttation. It usually occurs from tips & margins of leaves during night

or early morning when there is high atmospheric humidity as during wet seasons.Guttation occurs in some plant only (345 genera). E.g.,. Amorphophallus (max.), Oat,

Garden Nasturtium, Saxifraga, Cucurbits, potato, tomato, colocasia & many

grasses etc. In the regions of Guttation, the leaves possess special pores called

hydathodes. Hydathodes/water pores are permanently open pores as their guard cells

are immobile. Hydathodes are present at the tips of veins in leaves. A hydathode consists of

a pore in the epidermis followed by large intercellular spaces and loosely arranged

parenchyma called epithem and blindly ending xylem elements. Root pressure is

probably the main cause of Guttation.

Gutteted water contains minute quantities of both inorganic & organic salts and is not pure.

Guttation is observed frequently during warm-humid night but normally it occur

maximum during day and minimum during night. In moist and humid conditions, the

rate of absorption of water greatly exceeds than transpiration.

Factors effecting transpiration: - are of two types

(i) External factors (ii) Internal factors

(i) External Factors: -are of followings



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1. Light: - light is very important factor of transpiration. In most plants stomata open

during the day. Blue light is most effective in causing stomatal opening followed by Red

light.

2. Relative humidity (R.H.) of atmosphere: - R.H. is the ratio of actual amount of water

vapour present in the atmosphere and the amount of water vapour required to saturate it

completely. If R.H. of a place is low then the transpiration rate become high and if R.H. ishigh then the rate of transpiration become low. R.H. decreases with increasing temperature

and increases with decreasing temperature. Psychrometer is used to measure relative

humidity and rate of transpiration.

3.Wind: - If wind velocity is high the rate of transpiration become also high because wind

removes humid air from around the leaves.

4. Temperature: -temperature directly effects the transpiration. Transpiration increases

with increasing temperature.

5. Available water: - If available water in the soil is low the transpiration is also low.

With the deficiency of available water leaves wilt & stomata are closed thus decreasing the

rate of transpiration.

6. CO2 concentration: - increasing the CO2 concentration around the leaves should lead to

wide opening of stomata but instead partial closure occurs.

7. Sayre (1926) observed that stomata remain open in neutral or alkaline pH (in the

atmosphere of ammonia vapors) even in dark and closed in acidic pH (in the atmosphere of

acetic acid vapors) even in light.

(ii) Internal factors: -

Structure of leaves: -Transpiration will decrease if on the leaves thick cuticle, waxy

coating, sunken stomata, covering of dead hairs etc. present. These adaptations are found

in xerophytes.

Significance of transpiration: -Transpiration is an unavoidable evil or necessary evil

(Curtis-1926), because it often produces water deficit in plants which check photosynthesis,

reduce growth and if too severe may cause death from desiccation. It is probable that plant

are injured & killed by excessive transpiration than by any other cause.

Transpiration is considered necessary for the plants because of its supposed role in cooling

the plant, in upward conduction of water, in absorption & transported minerals.

Plant anti-transpirants: -

These materials applied to plants for retarding transpiration. E.g.,. Colourless plastics,

waxes, silicon oil, aspirin, abscisic acid and a fungicide phenyl mercuric acetate.



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transpiration dr.rajesh sahu

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