Transpiration, Chapter Notes, Class 11, Biology PDF Download

TRANSPIRATION

INTRODUCTION

Though large quantities of water is absorbed by the roots from soil, but only 2-5% of it is utilized by plant and rest 95-98% is lost in form of transpiration.

Definition :-

The loss of water from the aerial parts of plant in the form of vapours is called transpiration.
The instrument used for measuring transpiration is potometer.
“Transpiration is an essential evil” - by Crutis.
“Transpiration is an unavoidable evil” - by Steward.
The minimum transpiration is found in succulent xerophytes and no transpiration in submerged hydrophytes.
Maximum transpiration is found in mesophytes.

Significance of transpiration :

[1] In regulation of temperature :

Cooling effect on the surface of leaves is produced by the process of transpiration, due to which temperature remains constant in plants.
The plant are protected from the burning of heat due to transpiration. Evaporation of water produces cooling effect.

[2] In mineral absorption :

Mass flow of water is found during the passive absorption of water. Hence it is assumed that minerals enter the roots through the water.

[3] In ascent of sap

[4] In water absorption

[5] Distribution of absorbed salts

[6] Gaseous exchange

[7] Control of hydrological cycle

TYPES OF TRANSPIRATION

(A) Lenticular transpiration :

Loss of water through lenticles (pores found in epidermis of mature stems, some roots & some fruits) It results in 0.1% of water loss out of total transpiration.

(B) Cuticular transpiration :

10% of total transpiration occurs from surface, outer wall of epidermis of aerial parts.
The outer wall has cuticle which affects diffusion of water. It is inversely proportional to thickness of cuticle & amount of water.
Cuticle minimize stomatal transpiration.

(C) Stomatal transpiration :
Loss of water through stomata (small opening on the epidermis of leaves).
90% of total transpiration occurs through stomata.
The water lost from leaves is called foliar transpiration.


STOMATA

The name ‘stomata’ was given by Malpighie.
Stomata was discovered by Pfeffer.
Stomata cover 1-2% of leaf area.
Algae, fungi and submerged plants do not possess stomata.

Structure of stomata :

Stomata are small elliptical pores found on the epidermis of plant leaves.
The size of the pore varies from plant to plant for e.g. - In maize the size of pore is 4μ wide to 26μ long.
This pore is surrounded by kidney shaped or bean shaped epidermal cells called guard cells.
Guard cells are living & have nucleus, chloroplast, vacuole and cytoplasm.
Chloroplast is functionally active only in dicots. In monocots it is either rudimentry or absent or functionally inactive.
Inner wall is thick & less elastic and outer wall is thin & more elastic in guard cells.
Guard cells are surrounded by specialized epidermal cells called subsidiary or accessory cells.
The pore opens or closes by the movement of guard cells.
In the inner tangential walls of guard cells micro filaments of cellulose are present in circular form called as radial micellation.
The air cavities to which the stomata opens are called as substomatal cavity.

Distribution of stomata :

In monocots, stomata are uniformly distributed and are present on the upper and lower surface of the leaf.
In dicots, stomata are unevenly distributed and are present more on the lower surface.
In dicots guard cells are kidney shaped, reniform or bean like.
In monocots, guard cells are elliptical or dumbell shaped which are called as Graminaceous stomata.
In xerophytes stomata are situated in a groove and are thus called as sunken stomata.

Types of Stomata :

On the basis of distribution : 5 types

Stomata in Gymnosperms :

(i) Syndetochielic - When subsidiary cells & guard cells originate from single cell.
(ii) Haplochielic - Both cells arises from separate cells.

MECHANISM OF TRANSPIRATION

This involves 3 steps :

Osmotic diffusion of water in the leaf :
Inside the leaf mesophyll cells are in contact with xylem & also with inter cellular spaces above the stomata.Mesophyll cells draw water from the xylem which makes the cells turgid. Their diffusion pressure deficit and osmotic pressure decreases and in turn they release water in the form of vapours in intercellular spaces close to stomata by diffusion.
After releasing water the O.P. & D.P.D. of mesophyll cells increases & hence they draw water from xylem again.

Opening & closing of stomata :

When guard cells becomes turgid stomatal pore opens, while when they becomes flaccid stomatal pore closes.
Stomata generally open during the day and closed during the night except in CAM plants. The important theories of stomatal

movements are as followsTranspiration

(1) Photosynthesis in guard cell hypothesis :

This theory was proposed by Schwendener and Von mohl. According to this theory guard cell chloroplast perform photosynthesis during the day time. This produce sugars in guard cell which increases the O.P. of GC, compared to adjacent epidermal cells (subsidiary cells). Water enters in guard cells form subsidiary cells by endosmosis, due to this guard cells become turgid and stomata will open.

Objections -

(i) In CAM plants stomata open during dark/night.
(ii) Chloroplast of monocot guard cells are non-functional (inactive) photosynthetically.

(2) Starch  Sugar interconversion theory :

This theory was proposed by Sayre (1926). First of all Lloyd stated that amount of sugar in GC is increases during the day time and starch in night.
Detail study of this change was done by Sayre & given starch hydrolysis theory. According to Sayre, starch converts into sugar during day time when pH of guard cell is high. Sugar changes into starch during night at low pH in guard cells (Supported by Scarth). Sayre clarified that CO2 reacts with water during night. Due to accumulation of H2CO3, pH in guard cell is decreases.
Hanes - Stated that this change takes place by phosphorylase enzyme.
Yin & Tung reported the presence of phosphorylase enzyme in guard cells.
Starch + ip Day ph 7.5
NPihgohst pphHo-r y4l.a5s-e5
Glucose-1, P conc. of GC increased  Entry of H2O in GC
GC - Turgid Stomata open.

Stewards modification - 

According to Steward (1964) appreciable change in O.P. of GC is possible after the conversion of glucose - 1 P into Glucose & ip (inorganic phosphate)

(i) Glucose-1, P M utase Glucose-6P
(ii) Glucose-6, P  P hos pha ta se Glucose + iP

Conc. of GC increased
osmotic entry of H2O  GC-Turgid  open.

(iii) Glucose + ATP H ex okin a se Glucose-1, P  Starch  Stomata closed

Objections -

(i) Starch is absent in GC of some monocots like onion.
(ii) Formation of organic acid is observed during stomatal opening.

(3) Active K+  H+ exchange theory or active proton pump mechanism-

Given by Levitt (1973-74). This is modern and most accepted theory for stomatal opening and closing.
First of all Fujino observed that influx of K+ions in guard cells during stomatal opening. (Supported by Fisher and Hsiao). Detail study of this phenomenon was done by Levitt, who proposed this theory.

According to him stomata opens by following mechanism.
(i) Cabohydrates Enzymes  PEP + CO2 PEP
Carboxylase OAA

(ii) O.A.A.  Malic Acid
H+ K+  ATP Malate–

(iii) Malate + K+  K-malate  Conc. of GC increased

Entry of H2O in GC  GC turgid  Stomata open

Closing of stomata :-

Plant hormone ABA-acts on guard cells, which interfere the exchange of K+ pH of guHa+r dio cnesl lisn igsu draedc rceealslse, rdeusruinltgs ninig rhetv, ewrsheic ohf fraxvno. uorfs ospteonminagta ol fc slotosimnga.ta, hence stomata closed.

The ions accumulate in the vacuole of guard cells, lowering the water potential and thereby increasing water uptake and subsequently opening the stomata (PEPcase = Phosphoenol pyruvate carboxylase) High concentration of K+ ions in guard cells is electrically balanced by uptake of Cl– and malate ions in guard cells.

(4) Ca-ABA second messenger model - Given by Desilva & Cowan (1985) this is modern explanation of stomatal closing only.
Ramdas & Raghvandra suggested that ATPs for stomatal movement comes from cyclic ETC.
Bowlings : Malate switch hypothesis.
Raschke : K+ ions in guard cells comes from subsidiary cells.
Stomata opens during the night in succulent plants and closed during the day. This nature of stomata in opuntia is called Scotoactive stomata.
In CAM plants organic acid is formed during night which broken down during day and CO2 is liberated which is used in photosynthesis.

FACTORS AFFECTING RATE OF TRANSPIRATION

Factors affecting stomatal opening and closing :
[1] Light :

In most of the plants stomata open during the day except succulent xerophytic plants and close during the dark. Opening of stomata completes in the presence of blue and red light. Blue light is most effective and causing stomatal opening.

[2] Temperature :

Loft Field show temperature quotient of opening of stomata is [Q10] = 2

[3] CO2 concentration :

Stomata opens at low concentration of CO2 while closed at high concentration of CO2.

[4] Growth Hormones :

Cytokinin hormone induce opening of stomata. It increase the influx of K+ ions and stimulate the stomata for opening.

While ABA stimulate the stomata for closing. This hormone oppose the induction effect of cytokinin.
ABA effects the permeability of the guard cells. It prevent the out flux of H+ ions and increase the out flux of K+ ions. Because of this pH of the guard cells decreased.
Cl– ions also plays important role in stomatal movement. Above mentioned effects also found in high amount of CO2 ABA is formed due to high water stress in chloroplast of leaves.

[5] Atmospheric humidity :

Stomata opens for long duration and more widen in the presence of humid atmosphere, while stomata remains closed in dry atmosphere or partial opening at higher atm. humidity transpiration will be stop but stomata remains completly open.

Factors affecting the rate of transpiration :

Factors affecting the rate of transpiration are divided into two types :

[A] External Factors (Environmental factor)
[B] Internal Factors

[A] EXTERNAL FACTORS (Environmental factor)

[1] Atmospheric humidity : Relative humidity

This is the most important factor. The rate of transpiration is higher in low atmospheric humidity while at higher atmospheric humidity, the atmosphere is moistened, resulting decreasing of rate of transpiration. Therefore, the rate of transpiration is high during the summer and low in rainy season.

[2] Temperature : Temperature

The value of Q10 for transpiration is 2. It means by increasing 10ºC temperature, the rate of transpiration is approximately double. (By Loftfield)
Water vapour holding capacity of air increased at high temperature, resulting the rate of transpiration increased.
On contrary vapour holding capacity of air decreased at low temperature so that the rate of transpiration is decreased.

[3] Light :

Light stimulates, transpiration by heating effect on leaf.
Action spectrum of transpiration is blue and red.
Rate of transpiration is faster in blue light than that of red light. Because stomata are completely
opened as their full capacity in the blue light.

[4] Wind velocity : Tr  Wind velocity

Transpiration is less in constant air but if wind velocity is high the rate of transpiration is also high,because wind removes humid air (saturated air) around the stomata.
Transpiration increases in the beginning at high wind velocity [30-50 km./hour] But latter on it cause closure of stomata due to mechanical effect and transpiration decrease.

[5] Atmospheric pressure :

The speed of the air increase at low atmospheric pressure, due to this rate of the diffusion increase which increase the rate of transpiration.
The rate of transpiration is found maximum in the high intensity of light at high range of hills.

Traspiration ratio (TR) : Moles of H2O transpired/moles of CO2 assimilated.
Ratio of the loss of water to the photosynthetic CO2 fixation is called TR.
TR is low for C4 plants (200-350) while high of C3 plants (500-1000). It means C4 conserve water with efficient photosynthesis.
CAM plants passes minimum TR(50-100)

[6] Anti transpirants :

Chemical substances which reduce the rate of transpiration are known as antitranspirants. Anti transpirants are as follows-
Phenyl Mercuric Acetate (PMA), Aspirin, (Salicylic acid), Abscisic acid (ABA), Oxi-ethylene, Silicon oil, CO2 and low viscous was.
PMA closed the stomata for more than two weeks partially.
Antitranspirants are used in dry farming.

[B] INTERNAL FACTORS :

These factors are concerned with structure of plants. These are following types :

[1] Transpiration area :

Pruning increase the rate of transpiration per leaf but overall reduce the transpiration.

[2] Anatomical characteristics of leaf and leaf orientation :

Several structures of leaf effect the transpiration as follows :-

Stomatal characteristics :

Transpiration is effected by the structure of stomata, position of stomata, distance between the stomata, number of stomata per unit area and activity of the stomata.

By Salisbury - Stomatal Index (SI) = E S

SI = Stomatal index S = Number of stomata/unit area
E = Number of epidermal cells in same unit area.

[3] Water status of leaves

[4] Root - Shoot Ratio :

The rate of transpiration decreases with decrease in root-shoot ratio.
The rate of transpiration increases with increase in root-shoot ratio.

The following characteristics are found in leaf to reduce the transpiration.

(i) Leaves modify in spines.
(ii) Leaves transformed into needle e.g. Pinus
(iii) Folding and unfolding of leaves by bulliform cells. eg., Amophilla, Pea etc.
(iv) Small size of the leaves.
(v) Presence of thick waxy layer on leaves. eg., Banyan tree.

:: SPECIAL POINTS ::

Transpiration in old stems and fruits occurs through lenticels.
The loss of water through transpiration in increasing unit dry weight of the plant is called transpiration ratio.
The quantity of water transpired by unit area of leaf surface in unit time is called transpiration flux.
The diurnal periodicity was diagrammatically represented by Von mohl.
Fresh weight of a plant would be maximum in the morning and minimum in the afternoon.
The main reason of osmotic pressure for stomata is potassium chloride or potassium malate.
Porometer is used to find out the area of stomata on the leaf.
Transpiration measuring instrument is called Potometer. The rate of absorptioon of water is measured
through this instrument. In potometer rate of water absorption is proportional to the rate of transpiration.
Cobalt-chloride test : This method is used for the comparision of transpiration at both the surface of the
leaves. It is first of all shown by Stall.
Stomata covers 1-2% of total leaf area. Size of stomata is 10-40 (Length) × 3 – 12 (width)
The photophosphorylation process in the guard cells is a energy metabolic process, not CO2-metaboic
process.
The rate of transpiration of C4 plants is less as compared to C3 plants. In CAM plants minimum transpiration occurs.

Distribution of stomata on leaf surface :

Auxins which increases metabolic activities of the cells stimulate absorption of water.
Wilting Coefficient : The amount of water expressed as percentage of dry weight of soil, which is left in soil at the time of permanent wilting is called wilting co-efficient.
Value of Tensile strength of xylem sap is upto 300 atm.
The maximum value of transpiration pull has been found to be 20 atm. 1 atm force can raise water upto 10 m height. Thus ascent of sap can occur upto 200 meters height by transpiration pull.
Besides the translocation of water upwards, water is also translocated radially and downwards. Radial translocation is more common.
Cytokinin enhances opening of stomata while ABA induce closing of stomata.
In moist environment, stomata open more widely and for longer time & opposite is the case in dry environment.
The OP of guard cells when stomata open is 30-40 atm.
During wilting rate of photosynthesis also decreases.
The no. of stomata per unit area of leaf is called stomatal frequency. It depends on position of leaf,
external enviromment ect. In a leaf, it increases from base to tip and from midrib to lateral sides.