Transpiration Pull & Stomata | Biology A-Level - A Level PDF Download

TRANSPIRATION

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

• Only few percentage [1-2%] of absorbed water is used by the plants, while remaining [98-99%] of water lost atmosphere. "Transpiration is an essential evil"- by Curtis " Transpiration is an unavoidable evil" -by Steward .

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

• 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 is occurs through the stomata.

• Stomata are absent in algae, fungi and submerged aquatic plants.

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

[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 called lentic els . Some amount of water is lost by lenticels is known as lenticular transpiration. However it is approximately 0.1% to 1% of the total water lost.  

STRUCTURE

• 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 dumbel 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

(A) Based on distribution :

(1) Apple and Mulberry type :- When the stomata present on the lower (Dorsal/abaxial) surface of the leaf e.g.- Oxalis , Peach, Nasturtium, Morus etc.

(2) Potato type :- When stomata present mainly on the lower surface but some stomata are present on the upper surface (Adaxial ) also . e.g. - Tomato, Brinjal, Cabbage, Pea etc.

(3) Oat type :- When stomata are almost equally distributed on both surface of the leaf. e.g. Monocots

(4) Water lily type :- When stomata present only on t he upper surface of the leaf. (Aquatic plants with floating leaves)

(5) Potamogeton type :- The stomata in this type are either absent or rudimentary or functionless. e.g. Submerged hydrophytes.

(B) Based on time of opening & closing - By Loftfield

(1) Alfa-alfa type :- Stomata are opened in day and closed in night. Ex. – Mesophytes as Pea , Bean, Radish, Grapes , Apple etc.

(2) Potato type :- Stomata always open except evening time. Ex. – Onion , Potato , Cabbage, Banana etc.

(3) Equisetum type :- Always opened stomata. Amphibious plants.

(4) Barley type :- Stomata always closed except few hours in day time. Wheat, Maize etc.

(5) Scotoactive opening :- Stomata closed in day and opened in night. Ex- Succulents - Opuntia .

(C) Stomata based on structure and number of accessory cells :

(1) Anomocytic :- Subsidiary cells - 5 or 6 and same in structure.
E x. – Family – Ranunculaceae.

(2) Anisocytic:- Subsidiary cells - 3 and one cell smaller than two.
Ex. – Cruciferae

(3) Paracytic :- Subsidiary cells - 2 and parallel to guard cells.
Ex. – Rubiaceae

(4) Diacytic :-Longitudinally situated and 2 accessory cells.
Ex. – Caryophyllaceae.

Mechanism Of Opening And Closing Of Stomata Or Stomatal Movement And Mechanism Of Transpiration

Stomata generally open during the day & closed during the night with few exceptions. The important theories of stomatal movements are as follows –

(1) Photosynthesis in guard cell hypothesis : This theory was proposed by Schwend en er & Von mohl . According to this the or y 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.
Objections –

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

(ii) Chloroplast of 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 & starch in night.

• Detail study of this change was done by Sayre & given starch hydrolysis theory . Acco . to Sayre , starch converts in to sugars during day time when pH of guard cell is high. Sugar changes in to starch during night at low pH in guard cells (Supported by Scarth ). S ayre 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 cell

Stewards modification –

Acc. 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 gave stomatal mechanism as following -

(i) Starch is absent in GC of some monocots 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 movement.

• First of all Fuji no 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. Acc. to him stomata opens by following mechanism-

K+¾ H + Ion Exchange Theory

• 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 reaction of opening of stomata, hence stomata closed. pH of guard cells is decrease during night, which favours stomatal closing.(4)

• Ca–AB A second messenger model – Given by Desilva & Cowan (1985) this is modern explaination of stomatal closing  only.

• Ramdas & Raghvendra Suggested that ATPs for stomatal movement come s from cyclic ETS .

• 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 & CO₂ is liberated which is used in photosynthesis.

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 [Q 10 ] = 2

[3] CO₂ concentration : -

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

• CO₂ is anti transpirant gas.

[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 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 Factors

[A] External factors :-

• 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 :- T_r µ Temperature

• The value of Q 10 for transpiration is 2. It means by increasing 10⁰C temperature, the rate of transpiration is approximately double. (By Loftfield) Water vapour holding capacity of air inc reased 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 f as t er in blue light than that of red light. Because stomata are completely opened as their full capacity in the blue light.

[4] Wind velocity :- T_r µ Wind velocity

• Transpiration is less in constant air but if wind velocity is high the rate of transpiration is also high, because w ind 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 transpiration decrease.

[5] Atmospheric Pressure :-

• The speed of the air increase at low atmospheric pressure, due to this rate o f the diffusion increase which increase the rate of transpiration.

• The rate of transpiration is found maximum at high range of hills.

• By carrying a plant from Kota, to hill station, rate of transpiration increased.

[6] Anti transpirants : - Chemical substances which reduce the rate of transpiration are known as antitranspiants .

Anti transpirants are as follows :-Phenyl Mercuric Acetate [PMA], Aspirin, (Salicylic 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 :-

[1] Transpiring 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.

[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 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.e.g. Amophilla, Poa etc.

(iv) Small size of the leaves.

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

 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 plants are protected from the burning of heat due to transpiration. Evaporation of water produces cooling effect.

[2] Translocation of minerals in plant body :

• 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.

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

• The term " Guttation " was coined by Burgerstein .

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

• Normally, guttation process is found in herbaceous plants like grasses , tomats, balsam, Naustertium, Coloca , Sex if raga and in some of the plants of Cucurbit aceae 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 mid night or early morning.

• Parenchym atous and loose tissue are lie be neath the hydathode, which are known as epithem or transfer tissues .

• The process of guttation take place due to root pressure , develops in cortex cells of root.

BLEEDING
Fast flowing of liquid from the injured or cut parts of the plants is called bleeding or exudation.

• This process takes place due to high root pressure.

• Sugar is obtained from the Sugar maple by this process.

• The highest bleeding is found in Caryota urens (Toddy palm) (about 50 liter per day).

• Bleeding is important in economic biology, because Opium, Latex of rubber is obtained by this.

• Dropping of soft parts of the plants due to loss of turgidity in their cells is called wilting. Wilting is caused due to high rate of transpiration during mid-day or deficiency of water in soil and also in prolonged drought condition.

• Wilting may be temporary or permanent.

• Incipient wilting : This is the starting of wilting without any external symptom is called incipient wilting.

SPECIAL POINTS

• The main reason of osmotic pressure for stomata is potassium chloride or potassium alate.

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

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

• Cobalt-chloride test : This method is used for the comparison 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 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.

• Distribution of Stomata on leaf surfaces :