Essay on Mechanism of Stomatal Transpiration

Transpiration is the process of loss water in from of water vapour from the aerial part of plant body. A large quantity of water is absorbed by plants; on the other hand some amount of water is also produced during metabolic reaction. But in comparison to the amount of water absorbed and synthesized, very little amount of water is utilized by plants. Therefore plants need to eliminate extra amount is utilized by transpiration and gestation.

So as to the magnitude of transpiration is concerned, plants lose considerable amount of water by way of transpiration. It has been estimated that a bunch of tree may lose water equal to nearly five times the fresh weight of its leaves. On an acre basis, it amount to loss of 300 gallons of water per day.

Types of Transpiration:

Loss of water from plants may occur in three different ways from aerial parts of plant body.

i. Cuticular transpiration:

A small amount of water is lost in form of water vapour through the epidermal cuticle of stem and leaf by simple diffusion.

ii. Lenticular transpiration:

Very minute permanent openings present on the bark of trees and woody fruits called lenticels. Through these pores nearly 0.1% of total water lost is transported.

iii. Stomatal transpiration:

Stomata are natural epidermal openings on leaf thought which 80 – 90% of total transpiration takes place in plants.

There are several types of stomata on the basis of structure and distribution. Stomatal pore is about 3-12u in dimension and about 4u in width and about 26u in length. Stomatal pores are guarded by two kidnesyaped cells called guard cells. These are specially differentiated epidermal cells which are lining and contain nucleus and large number of chloroplasts. Their outer wall is thick and inner wall is this. Guard cells surrounded by adjacent subsidiary cells.

Mechanism of Stomatal Transpiration:

Mechanism of stomatal transpiration involves following steps:

i. Osmotic diffusion of water from xylem to inter cellular spaces through mesophyll cells:

In side the leaf mesophyll cells are in contract with xylem and on the other hand with inter cellular spaces above the stomata. When water saturates the cell wal protoplasm and vacuoles of mesophyll cells by the water supplied by wxlem of leaf, then the cells become turgid. Their diffusion pressure deficit and osmotic pressure decrease with the result that they release water in form of vapours with the result that they release water in form of vapours into the inter cellular spaces close to stomata by osmotic diffusion. In turn the O.P. and D.P.D of mesophyll cells become higher and hence they draw water from xylem by osmotic diffusion.

ii. Opening and closing of stomata:

When the water from mesophyll cells and reach the mercellular spaces above stomata in form of vapour then stomatal movement or closing and opening of stomata is necessary for transpiration. The chief mechanism involved in stomatal transpiration is the mechanism of stomatal movement.

Mechanism of Stomatal Movement:

Stomatal movements are brought about by the changes in the volume and shape of fuard cells. This contraction and expansion of the fuard cells is due to turgidity and flacidity respectively. When guard cells absorb water from the surrounding cells and become turgid a pressure is created that pushes the outer thinner wall to bulga outward. Outward bulging pulls apart the inner thick wall of the fuard cells that caused the opening of the stomata.

Stomatal movements caused due to five different factors and are five different types.

i. Photoactive movement :

Movement that controlled by light due to increase in pH on reduction of Co2 or due to hydrolysis of starch into glucose.

ii. Scoto – active movements :

This is found in succulent plants in which stomata opens at night due to organic acid metabolism.

iii. Hydro -active movement: Loss of water from epidermal cells cause opening and their turgidity cause closing of stomata.

iv. Autonomous movement :

Movement of stomata by rhythemic pulsatory activity due to Co2 concentration or change in cell sap concentration.

v. Passive and active movements :

Stomata open in active state and open in passive state due to change in turgidity.

There are several hypothesis has been proposed to explain stomatal movement.

i. Mohl’s hypothesis :

Von mohal (1856) gave the hypothesis that, the chloroplast present in guard cells manufactures substances which increase the osmotic pressure of guard cells. As a result of which endosmosis takes place and that increases the turgidity in guard cells, consequently cause opening of stomata.

In high concretions of Co2 around stomata would cause opening of stomatal pore, but the pore closes. On the other hand guard cells have feeble role in photosythesis in compared to mesophyll. Hence the phythesis was rejected.

ii. Starch-sugar hypothesis :

This hypothesis was postulated by Lloyd (1908), loft fields (1921) and sayre (1926). These workers noted that, starch content of guard cells is high during night and low during day time. Suyere further observed that, stomata xloses at a pH lower or higher that pH 4.2-4.4.

This hypothesis postulates that:

i. During day time Co2 which released in respiration is utilized in photosynthesis of mesophyll cells. Therefore concentration of Co2 around guard cells and neighbouring cells reduced with rise in pH.

ii. High pH favours conversion of starch into osmotically active teducing sufars which get soluble in cell sap.

iii. In dark Co2 is accumulated in guard cells as photosynthesis stopped. It cause fall in pH of guard cells. At low pH conversion of sugar into starch takes place. Guard cells become flacid and stomata closed. The enzyme phosphorylases present in the chloroplast catalyses this reaction in presence of inoganic phospate, as follow.

iv. Steward’s Hypothesis :

Steward (1964) criticized this above swtarch sugar hypothesis proposed by lloyed and other and pointed out that ,unless glucose 1 – phosphate is furtherer broken down to glucose and inorganic phosphate, no appreciable change occur in the osmotic pressure steward proposed his own scheme, According to which.

i. At high pH the opening of stomata is caused by conversion of starch into glouse.

ii. The closing of stomata requires metabolic energy (ATP), O2 and the enzyme hexokinase which help in conversion of sutars into starch.

Starch-sugar hypothesis is also subhected to criticism in following ground.

i. It fails to explain rise of pH on basis of Co2 concentration.

j. Sugar never noticed in cell sap of fuard cells during opening of stomata.

k. Starch sugar introversion is very slow which does not effect quick stomatal movement.

K+ ion transport mechanism of stomatal movement:

Fujino (1959, 1967) proposed that opening and closing of stomata is directly related to k+ ion conventration of the guard cells. In other words, these phenomena are governed by active transport of k+ ions into the guard cells and out of them.

Rasnchke (1975) sumerised the process as follow:

i. Disappearance of starch from guard cells.

j. Production of organic acids (Malic acid)

k. Migration of H+ ion from fuard cells.

l. Up take of K+ ions into the vacuoles of guard cells.

m. Up take of cl ions into the vaxuoles. Thus stomata opens.

n. Stomatal closure is brought about by outward movementof k+ ion and CL ion from the fuard cells to sorround cells.

Proton transport concept:

Levitt (1974) proposed that the photoactive stomatal opening and closure mechanism and called it as proton transport concept. It is similar to L+ ion transport mechanism.

Transpiration as a Necessary Evil:

i. Transpiration has paramount importance as transpiration pull help in ascent of sap and influence the rate of water absorption by plants.

j. Transpiration cause cooling thus controls the internal temperature of plant body.

k. It helps in gaseous exchange, besides the above importance transpiration cause.

l. Water deficit in plant resulting wilting permanent wilting or death of plants.

m. It causes shedding of leaves.

n. Unnecessary wastage of water from soil and plant body.

o. Structural adoptions required with expense of tissue in xerophyes.

Inspite of above diadvtages plant cannot avoid transpiration, for which curtis (1926) called transpiration as a necessary evil.