Transpiration

 

Table of Content

Types of Transpiration Structure of Stomata Mechanism of Opening and Closing of Stomata Factors Affecting Stomatal Opening and Closing Factors Affecting the Rate of Transpiration Significance of Transpiration Guttation

Loss of water in vapour form, from the aerial parts(organs) of living plants is known as transpiration.

a) Only few percentage [1-2%] of absorbed water is used by the plants while remaining [98-99%] of water is lost in atmosphere.

b) Transpiration is an essential evil - by Curtis

c) Transpiration is an unavoidable evil-by Steward.

d) The minimum transpiration is found in succulent xerophytes & no transpiration in submerged hydrophytes.

e) Maximum transpiration is found in mesophytes. 

• Types of Transpiration

Transpiration is of the following three types:

(i) Stomatal Transpiration: Transpiration takes place through the stomata which are present on the leaves of the plants and delicate organs, is called stomatal transpiration. The maximum amount of water is lost by this transpiration. About 80% to 90% transpiration occurs through the stomata.

(ii) Cuticular Transpiration: Loss of water through the cuticle which present on the herbaceous stem and leaves. Cuticle is a wax like thin layer present on epidermis. About 9% transpiration is cuticular.

(iii)  Lenticular Transpiration: Minute pore like structure found on the stem of some woody plants and epidermis of some fruits is called lenticels. Some amount of water lost by lenticels is known as lenticular transpiration.
However, it is approximately 0.1% to 1% of the total water lost.

Foliar transpiration: Total transpiration takes place through the leaves is called as foliar transpiration. Foliar transpiration = Stomatal + Cuticular, from the leaves.

•  Structure of Stomata

Stomata are found on the aerial delicate organs and outer surface of the leaves in the form of minute pores. Stomatal pore is surrounded by two specialised epidermal cells called as guard cell. They are kidney shaped. The number of guard cells are two.

The structure of guard· cells in monocots (Gramineae) is dumbbell shaped.

Guard cells are epidermal cells. But due to presence of chloroplast they are different from that of epidermal cells.

The outer wall of the guard cells is thin and elastic while inner wall is thick and non-elastic.

Guard cells are surrounded by some specialized epidermal cells called subsidiary cells or accessory cells.

Stomata are found on both upper and lower surface. Stomata attached with air chambers and forms a cavity is called sub-stomatal-cavity.

In xerophytic plants position of stomata is deep in the surface of the leaf. Stomata are present in this position are called sunken stomata. 

Types of stomata: On the basis of orientation of subsidiary cells around the guard cells, Metcalfe and Chalk classified stomata into following types:

Anomocytic: The guard cells are surrounded by a limited number of unspecialized subsidiary cells which appear similar to other epidermal cells. e.g., in Ranunculaceae family.

Anisocytic: The guard cells are surrounded by three subsidiary cells, two of which are large and one is very small. e.g., in Solanaceae and Cruciferae families.

Paracytic: The guard cells are surrounded by only two subsidiary cells lying parallel to the guard cells e.g., Magnoliaceae family.

Diacytic: The guard cells are surrounded by only two subsidiary cells lying at right angles to the longitudinal axis of the guard cells. e.g., Acanthaceae and Labiatae families.

Actinocytic: The guard cells are surrounded by four or more subsidiary cells and which are elongated radially to stomata.

Daily periodicity of stomatal movement: Loftfield (1921) classified the stomata into four types, depending upon the periods of opening and closing.

Alfalfa type (Leucerne type): The stomata remain open throughout the day but close during night, e.g., Pea, bean, mustard, cucumber, sunflower, radish, turnip, apple, grape.

Potato type: The stomata close only for a few hours in the evening, otherwise they remain open throughout the day and night e.g., Cucurbiia, Allium, Cabbage, Tulip, Banana etc.

Barley type: These stomata open only for a few hours in the day time, otherwise they remain closed throughout the day and night, e.g., Cereals.

Equisetum type: The stomata remain always open through out the day and night e.g., Amphibious plants or emergent hydrophytes. 

• Mechanism of Opening and Closing of Stomata

1. Photosynthesis in guard cell hypothesis:

This theory was proposed by Schwendener & 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 & stomata will open.

Objection:

(i) In CAM plants stomata open during dark/night.  

(ii Chloroplast of monocot guard cells are non­functional (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 increases during the day time & starch in night.

Detail study of this change was done by Sayre & give starch hydrolysis theory. According to Sayre, starch converts into sugars during day time when pH or guard cell is high. Sugar changes into starch during night at low pH in guard cells (Supported by Scarth). Sayre classified that CO₂ reacts with water during night. Due to accumulation of H₂CO₃ pH in guard cell decreases.
Hanes: Stated that this change takes place by phosphorylase enzyme.

Yin & Tung reported the presence of phosphorylase enzyme in guard cells.

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)

Steward give stomatal mechanism as follows. 

 

Objections:

(i) Starch is absent in GC of some monoccts like onion.

(ii) Formation of organic acids is observed during stomatal opening.

3. Active K⁺ H⁺ exchange theory or active proton transport mechanism.

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

(i) Carbohydrates

Closing of stomata: Plant hormone ABA-acts on guard cells, which interfere the exchange of K⁺  H⁺ ions in guard cells, results in reverse of rxn. of opening of stomata, hence stomata closed. pH of guard cells decreases during night, which favours stomatal closing.

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 Desliva & Cowman (1985). This is modern explanation of stomatal closing only.

Ramdas & Raghvandra suggested that A TPs for stomatal movement comes from cyclic ETS.

Bowlings: Malate switch hypothesis.

Raschke: K⁺ ions in guard cells comes from subsidiary cells.

Stomata opens during night in succulent plants and closes during the day. This nature of stomata in Opuniia is called scotoactive stomata.

In CAM plants organic acid is formed during night which is broken down during day & CO₂ is liberated is used in photosynthesis.

Stomatal opening in succulent plants (Scotoactive stomata): The stomata in succulent plant or CAM plants (like Opuniia, Bryophyllum etc.) open during night (darkness) and remain closed during the day time and is found in lower surface. This type of stomatal opening is called 'Scotoactive type' and the stomata which open during day are called as photoactive. Stomata is closed and opened due to the activity of water. This types of stomata is known as hydroactive stomata. The opening and closing mechanism of scotoactive stomata was explained by Nishida (1963). In succulent plants, during night, there is incomplete oxidation of carbohydrates and accumulation of organic acids (e.g., malic acid) without release of CO₂, During day time the accumulated organic acids breakdown rapidly releasing excess amount of CO₂ for photosynthesis as well as to keep the stomata closed. 
During night: C₆H₁₂O₆ + 3O₂ → 3C₄H₆O₅ + 3H₂O
During day: C₄H₆O₅ + 3O₂ → 4CO₂ + 3H₂O

• Factors Affecting Stomatal Opening and Closing

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 causes stomatal opening.

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

CO₂ concentration:

Stomata are sensitive towards the internal CO₂ conc. in the leaves.

It is internal leaf CO₂ cone rather than the atmospheric CO₂ conc. that ditactes stomatal opening.

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

Growth Hormones:

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

While ABA stimulate the stomata for closing. This hormone oppose the induction effect of cytokinins,

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.

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 is stops but stomata remain completely open.

•  Factors Affecting the Rate of Transpiration

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

(A)    External factors (Environmental factor)

(B)    Internal Lactors

(A)    External factors:

Atmospheric humidity: 

Tr - 1/Atmospheric 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, decreasing the rate of transpiration.

Therefore, the rate of transpiration is high during the summer and low in rainy season. 

Temperature:

T_r µ Temperature

The value of Q₁₀ for transpiration is 2. It means by increasing 10°C temperature, the rate of transpiration is approximately double. (By Loftfield)

Water vapour holding capacity of air increases at high temperature, resulting in increase in rate of transpiration.

On contrary vapour holding capacity of air decreases at low temperature so that the rate of transpiration decreases.

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 at their full capacity in the blue light.

Wind velocity: T_r µ Wind velocityTranspiration 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 - 35 km./hour] But latter on it cause closure of stomata due to mechanical effect and therefore transpiration decreases.

Atmospheric Pressure:The speed of the air increases at low atmospheric pressure, As a result rate of the diffusion increases which in turn increases the rate of transpiration.

The rate of transpiration is found maximum in the high intensity of light at high range of hills.

Transpiration 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 for C₃ plants (500-1000). It means C₄ conserve water with efficient photosynthesis.

CAM plants passes minimum TR (50-100)

Anti transpirants: Chemical substances which reduce the rate of transpiration are known as antitranspirants. Anti transpirants are as follows:

Phenyl mercuric acetate (PMA], Aspirin (Salicyclic acid), Abscisic acid [ABA], Oxi-ethylene, Silicon oil, CO₂ and low viscous wax.

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 of following types:

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

Anatomical characteristics of leaf and leaf orientation:

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

(i)     Leaves modified into spines.

(ii)    Leaves transformed into needle e.g. Pinus.

(iii)    Folding and unfolding of leaves by Bulliform cells. e.g. Amophilla, Poa etc.

(iv)   Small size of the leaves.

(v)    Presence of thick waxy layer on the leaves. e.g. Banyan tree.

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) =I= S/E+S

SI = Stomatal index,

S = No. of stomata/unit area

E = No. of epidermal cells in same unit area. 

(4)    Leaf-orientation

(5)    Water status of leaves

(6)    Root: Shoot Ratio: 

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

Significance of Transpiration

In regulation of temperature: Cooling effect on the surface of leave is produced by the process of transpiration due to which the temperature remain constant of the plants.

In mineral absorption

In ascent of sap

In water absorption

Distribution of absorbed salts

Gaseous exchange

Control of hydrological cycle

•  Guttation

Loss of water from the uninjured part or leaves of the plant in the form of water droplets is called guttation.

The term Guttation was coined by Burgerstein –

Exuded liquid of guttation along with water contains some organic and inorganic (dissolved) substances. It means it is not pure water.

Normally, guttation process is found in hearbaceous plants like Grasses, Tomata, Balsum, Nausiertium, Colocasia,-Sexijraga and in some of the plants of Cucurbitaceae family.

Guttation occurs from the margins of the leaves through the special pore (always open) like structure are called Hydathodes or Water stomata.

Generally guttation occurs during night or early morning.

Parenchymatous and loose tissue lies beneath the hydathode which is known as epitherm or transfer tissue.

The process of guttation takes place due, to the root pressure, developed in cortex cells of root.

NOTE:

Cobalt chloride test: This method is used for the comparison of transpiration from the both surface of the leaves. It is first of all shown by Stall.

The main reason of osmotic pressure of the opened stomata is the potassium chloride or potassium malate.

Parameter is used to find out the area of the stomata on the leaf.

Transpiration measuring instrument is called potometer. The rate of absorption of water is measured through this instrument. In potometer rate of water absorption is proportional to the transpiration.

Stomata covers 1-2% of total leaf area. Size of stomata is

10-40 m (length) × 3-12 m (width).

The photophosphorylation process in the guard cells is a energy metabolic process, not CO₂ metabolic process.

(Cyclic photophosphorylation)

The rate of transpiration of C₄ plants is less as compared to C₃ plants. In CAM plants minimum transpiration occurs.

Manometer is used to measure root pressure.

Cryoscopic osmometer measures the osmotic potential of solution by measuring its freezing point.

Osmotic pressure is maximum in noon. At this time water contents in the cell are minimum.

Psychorometer is used for measuring relative humidity as well as transpiration.

In Saxifraga, the rate of guttation is high during flowering.

Maximum opening of stomata occurs at about 10:00 AM and 3:00 PM (At 12:00 noon, partial closure of stomata occurs).

In C₃ plant the rate of transpiration is high.

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