Diffusion of water vapour to atmosphere

 The last step in the mechanism of transpiration is the simple diffusion of water vapors from

the intercellular spaces to the outer atmosphere through open stomata. This is because the intercellular

spaces are more saturated with moisture in comparison to the outer

atmosphere in the vicinity of the stomata.

According to this theory, Potassium ions (K+) have been found to play a crucial role in stomatal

movements. The opening and closing of stomata are the results of the active transport of potassium

ions into the guard cells and out of them. The adjacent epidermal cell acts as ion storage cells for

guard cells.

(dark) (Light)

The closure mechanism involves the participation of an inhibitor hormone, Abscisic acid (ABA)

which functions in the presence of CO2. In dark, ABA inhibits potassium uptake by changing the

diffusion and permeability of the guard cells. The potassium moves out of the subsidiary cells.

Then, ABA induces the process of acidification in the guard cells, which results in lowering of pH.

At low pH, starch is synthesized leading to the closure of stomata.

Besides, there are some other theories also available to explain the mechanism of stomatal

opening and closing. They are :

1. Theory of photosynthesis in guard cells (Von Mohl,1856)

2. Theory of starch ⇔ sugar interconversion (Sayre, 1926)

3. Theory of glycolate metabolism (Zelitch, 1963)

4. Theory of Proton Transport and Hormonal Regulation (Levitt, 1974):

Significance of Transpiration

It is often said that “transpiration is a necessary evil. It is a vital and unavoidable phenomenon

of plants”. Therefore, the significance of transpiration can be discussed in the following

aspects :

a. Transpiration is unavoidable

As long as there is a need for ingress and egress of oxygen and CO2 during metabolic processes

like, photosynthesis and respiration, transpiration is also considered

unavoidable.

b. Transpiration is advantageous

Transpiration is incidental due to the structural arrangements of leaves. The loss of water does

not serve any good purpose in plant life. Millions of plants die every year because the transpiration

exceeds the rate of absorption of water through the roots of plants. Besides, transpiration also

consumes a lot of energy, which is not available for other metabolic activities of the plant.

c. Transpiration is advantageous

Transpiration is also regarded as a boon for many reasons and its advantages to the plants

are detailed below :

1. It plays an important role in the translocation of food from one portion of the plant to the other.

2. It maintains an optimum temperature for the leaves.

3. It brings about the opening and closing of the stomata, which indirectly influence the rate of

photosynthesis and respiration.

4. It creates suction force and helps in the ascent of sap.

5. It influences the absorption of water and minerals by roots.

6. It helps in the evaporation of an excess amount of water which otherwise would cause suffocation.

d. Transpiration is necessary

The leaves absorb an enormous amount of radiant energy. There are several ways in which the

excess energy can dissipate away. Transpiration is one of the chief ways for the dissipation of excess

energy, which the plant receives from the sun.

Shull (1930) estimated that approximately 0.8 cal of energy is received upon each square cm of leaf

surface per minute, of which about 10% is reflected and 25% is transmitted. The remaining 65% (0.52 cal)

will increase the temperature of the leaves very rapidly. If the weight of the leaf tissue is 0.02g/cm2 with the

specific heat of 0.879, then the temperature rise would be at 320C per minute.

With this rate of increase in temperature, the plant will be killed in less than two minutes, if there is no

dissipation of energy. Transpiration plays a significant role here. It helps in dissipating this excess energy

which will otherwise raise the temperature. Therefore, transpiration is considered the necessary

process for the plant.

Transpiration is a necessary evil!

It is often said that transpiration is a necessary evil. It is a vital and unavoidable phenomenon of

plants. The loss of water does not serve any good purpose in plant life. Besides, transpiration also

consumes energy and causes unnecessary absorption of excess water by roots. Nevertheless, the internal

structure is basically meant for the exchange of gases during photosynthesis and respiration. Therefore, it

cannot check the evaporation of water. Hence, many workers like Curtis (1926) have called transpiration a necessary evil!