Transpiration

Transpiration 

Transpiration is the process of loss of water in the form of vapours from the aerial portions of the  plant.  Less than 5% of total water absorbed by plant is used for the development and metabolic processes.  Remaining 95% of water is released into the atmosphere as water vapour.

Mechanism of Transpiration

The roots absorb water and conduct upwards through the xylem vessels.  From the stem water enters into the leaves through the xylem elements of petiole, veins and veinlets.

The mesophyll cells of the leaves absorb water from the xylem elements of the veins and saturate the veinlets .  When sunlight falls on the leaves, the water of mesophyll cells evaporate and saturate the intervening air spaces with water.

The dry air of the atmosphere has a higher diffusion pressure deficit. The water vapours saturate the intercellular spaces of the transpiring organs. When the stomata are open, the water vapours are drawn from the substomatal cavities to the outside air .

This increases the diffusion pressure deficit of the substomatal air which draws more water vapours from the intercellular spaces.  The intercellular spaces in turn get water vapour from the wet walls of mesophyll cells.  Stomatal transpiration continues till the stomata are open.

Transpiration depends primarily on two factors:

1. Difference in vapour pressure between the inside and outside atmosphere of leaves.
2. Degree of stomatal opening.

Normally, there is always a difference in vapour pressure between the inside and outside atmosphere of leaves but stomata are not always open because of various reasons.  Hence, the rate of transpiration depends upon the number of stomata and the degree of stomatal opening.

Types of transpiration

Loss of water can take place from any exposed part of the plant as it is a physical process.  Transpiration is of three types, stomatal, cuticular and lenticular.

Stomatal transpiration:

Stomata are minute pores on the epidermis of leaves. They form a connecting link between internal tissue and surrounding atmosphere.  Therefore, they are the main passage through which water vapour escapes.  Stomatal transpiration accounts for 80%-90% total water loss of the plant.

Cuticular transpiration:

Cuticle is a wax-like layer of cutin that covers the epidermis of leaves and stems.  Cuticles present on the surface of the epidermis reduces the direct evaporation of water.  If the cuticle is thin, up to 20% of total transpiration may take place through it.

But as its thickness increases, the extent of water vapour loss is significantly reduced.  Cuticular transpiration accounts for about 10% of the total amount of transpiration from leaves under ordinary condition.

Lenticular transpiration:

Lenticels are small openings in the cork of woody stem, twigs and fruits.  Water vapours are lost through these openings.  Lenticular transpiration is the loss of water through lenticels present on woody stem and fruit.  It accounts for 1% of total water loss of a plant.  However, in deciduous trees, which shed their leaves during autumn, lenticular transpiration can be significant.

Stomatal transpiration

Water absorbed by root is translocated to the mesophyll cells of the leaves through the xylem elements.  These cells have large intercellular spaces.  Water evaporated from the surface of mesophyll cells accumulates in the intercellular spaces and then escape to the outer atmosphere through stomata.  This is the process of stomatal transpiration.

Structure of stomata:

Stomata are tiny pores or openings present in the epidermal surface of leaves and young stems.  A typical stomata usually consists of two kidney-shaped or bean-shaped guard cells, which differs from other epidermal cells, mainly in two respects.

• Guard cell contain chloroplast and perform the phenomena of photosynthesis.
• Their walls next to stomatal pores are thick as compared to inner walls.

The guard cells are also characterised by accumulation of starch during night and decrease during day.  Guard cells are usually smaller in size as compared to other epidermal cells.  Therefore, they are rapidly affected by small change in turgor.  The opening and closing of stomata depends upon the turgidity of guard cells.  Modified epidermal cells, called subsidiary cells or accessory cells border the guard cells.

Types of stomata (according to their distribution)

1. Apple type or mulberry type: These types of stomata are present on abaxial or lower surface of the leaves.  For example, apple, peach, walnut etc. The leaves  which have stomata  in the lower surface only are termed as hypostomatic.
2. Potato type: This type of stomata is present on abaxial or adaxial side but more on the abaxial surface of the leaves.  For example, potato, tomato, brinjal and other members of the family Solanaceae.  Such leaves with stomata on both the surfaces are termed as amphistomatic.
3. Oat type: Both the leaf surfaces possess such type of stomata. They are equally distributed on both the surfaces, for example, wheat, rice, grasses etc.  These leaves are termed as amphistomatic.
4. Water lily type: This type of stomata is found only on the upper surface of the leaves, for example, water lily.  The leaves of such plants floats on water surface.  These leaves could be termed as epistomatic.
5. Potamogeton type: In submerged aquatic plants, the stomata are generally absent or if present, they are non -functional , for example, Potamogeton, Hydrilla,Vallisneria. 

Types of stomata (according to the daily movements)

Loftfield classified stomata into following four types on the basis of their daily movement:

1. Alfalfa type: The stomata remains open throughout the day and remains closed throughout the night.   All the thin leaved mesophytes, for example, alfalfa, pea, bean, radish, mustard, turnip, grapes etc posses such type of stomata.
2. Potato type: The stomata remains open throughout the day and night except for a few hours following sunset.  For example, onion, potato, cabbage, bananas etc.
3. Barley type: The stomata remains open only for a few hours during the day and remains closed for the rest of the period.  For example, barley, maize, wheat and other cereals.
4. Equisetum type: The stomata remains open throughout the day and night.  For example, Equisetum.

Type of stomata (depending on their association with subsidiary cells)

1. Atropa type:In this type, stomata does not remain surrounded by any subsidiary cells.
2. Sedum type: In this type, stomata remain surrounded by three subsidiary cells of unequal type.
3. Maize type: Guard cells are dumb-bell in shape. Paired subsidiary cells surround the guard cell.
4. Allium type: Guard cells remain associated with two subsidiary cells.