Transpiration

CHAPTER: 7(B)

Transpiration

Transpiration:

•      Only a small fraction of the absorbed water is utilized by the plant. The bulk of the water absorbed is not retained and is evaporated into the air from the leaves and other  aerial parts of the plant. The loss of water in the form of vapor from the aerial parts of the plant is known as transpiration.

Significance of Transpiration:

1. Absorption of water: Transpiration influences the rate of absorption of water from the soil.

2. Water movement: By transpiration, water moves upwards and as it passes into the cell vacuoles, it makes the cells turgid. This gives form and shape to cells and plant as a whole.
3. Mineral salt transport: The water stream moving upwards carries dissolved minerals with it. Transpiration also helps in distributing these minerals throughout the plant.
4. Cooling: The evaporation of water during transpiration cools the leaves.
5. Protection from heat injury: Some plants like cacti, retain water by reducing transpiration. This saves the plants from high temperatures and strong sunlight.

Transpiration as a necessary evil:

 Transpiration is a necessary evil because of the following facts:

(i) A large amount of absorbed water is lost during transpiration which is harmful to plants.

(ii) Unnecessary wastage of energy takes place during the process of water absorption which is lost due to transpiration.

(iii) When the rate of transpiration is high in plants growing in soil deficient in water, an internal water deficit develops in plants which may affect metabolic process.

(iv) Many xerophytes plants undergo structural modifications and adaptations to check transpiration.

Ø  Considering both the beneficial and harmful effects of transpiration, it may be concluded that it is definitely advantageous in spite of its harmful features.

Types of Transpiration:

•      Most of the transpiration occurs through foliar surface or surface of the leaves. It is known as foliar transpiration.

•      Foliar transpiration accounts for over 90% of the total transpiration.

•      Young stems, flowers, fruits, etc. also transpire a lot.

•      Mature stems transpire very little. Transpiration from stems is called cauline transpiration.

•      Depending upon the plant surface transpiration is of the following four types:

1. Stomatal Transpiration
2. Cuticular  Transpiration
3. Lenticular Transpiration
4. Bark Transpiration

1. Stomatal Transpiration:

•      It is the most important type of transpiration. Stomatal transpiration constitutes about 50-97% of the total transpiration. It occurs through the stomata. The stomata are found mostly on the leaves. A few of them occur on the young stems, flowers and fruits. The stomata expose the wet interior of the plant to the atmosphere.

•      The internal air, therefore, becomes saturated with water vapours. The outside air is seldom saturated with water except just after rains. Water vapours, therefore, pass outwardly through stomata by diffusion. More water evaporates from the internal cells to replace the outgoing water vapours. The stomatal transpiration continues till the stomata are kept open.

2. Cuticular Transpiration:

•      It occurs through the cuticle or epidermal cells of the leaves and other exposed parts of the plant. In common land plants cuticular transpiration is only 3-10% of the total transpiration. In herbaceous shade loving plants where the cuticle is very thin, the cuticular transpiration may be upto 50% of the total. Cuticular transpiration continues throughout day and night.

3. Lenticular or Lenticellate Transpiration:

•      It is found only in the woody branches of the trees where lenticels occur. The lenticular transpiration is only 0.1% of the total transpiration. It, however, continues day and night because lenticels have no mechanism of closure. The lenticels connect the atmospheric air with the cortical tissue of the stem through the intercellular spaces present amongst the complementary cells.

4. Bark Transpiration:

•      This type of transpiration occurs through corky covering of the stems. Bark transpiration is very little but its measured rate is often more than lenticular transpiration due to larger area. Like cuticular and lenticular types of transpiration, bark transpiration occurs continuously during day and night.

Factors affecting rate of transpiration:

A.Environmental Factors:

1. Light

•      Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata. Light also speeds up transpiration by warming the leaf.

2. Temperature 

•      Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.

3. Humidity

•      The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases. When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.

4. Wind

•      When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air.

5. Soil water

•      A plant cannot continue to transpire rapidly if its water loss is not made up by replacement from the soil. When absorption of water by the roots fails to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.

6. CO₂ Concentration:

•      Reduced CO₂ concentration enhance the stomata opening and Increased CO₂  leads in closing of stomata.

2. Plant Factors:

1. Stomata:

•      The greater the number of stomata, the greater is the degree of stomatal opening  and hence greater will be rate of transpiration.

2. Leaf Structure:

•      Leaves with thick cuticles transpire less in comparison to  thin cuticle on their leaf epidermii.

3. Leaf Area:

•      There is no exact mathematical relationship between the leaf area and rate of transpiration. But, reduced leaf area markedly shows prevention of transpiration as found in xerophytic plants.

4. Root-Shoot Ratio:

•      Plants with a higher proportion of roots can compete more effectively for soil water and nutrients, while those with a higher proportion of shoots can collect more light energy. To maintain proper rate of transpiration there must be balanced root-shoot ratio. High root-shoot ratio increases rate of transpiration where as low root-shoot ratio decreases the rate of transpiration.